School Science Lessons
Topic 18a Swimming pool chemistry
2012-04-24 SPwp
Please send comments to: J.Elfick@uq.edu.au
18.7.0 Swimming pool chemistry
18a1 Chemicals used in swimming pools
18a2 Chlorine used in swimming pools
18a3 pH of swimming pools
18a4 Swimming pool management
3.9.1 Swimming pools, checklist of daily routine for the pool operator
3.9.2 Swimming pools, pool test ranges
18a5 Tests for swimming pools
18a1 Chemicals used in swimming pools
18a2 Chlorine
18.7.19 Bromine products
18.7.2.2.1 Calcium hypochlorite, Ca(OCl)2
16.2.4.3.2 Chloramines in swimming pools
18.7.14 Cyanuric acid (CNOH)3, conditioner, stabilizer
18.7.2.4 Dichlor, sodium dichloro isocyanuric acid
18.7.2.2 Inorganic hypochlorites
18.7.11 Oxidizing agents
18.7.22.1 Sodium bicarbonate, Use of sodium bicarbonate
18.7.2.2.2 Sodium hypochlorite, NaOCl
18a2 Chlorine used in swimming pools
18.7.2.3 Chlorinated isocyanurates, stabilized chlorine
18.7.12 Measure chlorine levels
18.7.2 Swimming pool chlorination
18.7.1 Chlorine and water
18.7.5 Available chlorine
18.7.17 Cost of chlorination
18.7.6 Dissolve chlorine in pool water by electrolysis
18.7.2.1 Chlorine gas
18.7.21.2 Chlorine in swimming pools
18.7.13 Chlorine lost from swimming pools in sunlight
18.7.4 Measure the free chlorine in water
18.7.2.5 Pool chlorinating concentrates
18.7.8 Superchlorination, shock treatment, breakpoint chlorination
18.7.8.1 Reactions of chlorine with ammonia and ammonia compounds
18a3 pH of swimming pools
18.7.10 Adjusting the pH of pool water
18.7.7 Pool water and pH
18.7.3 The effect of pH
18a4 Swimming pool management
18.7.21.1 Acid demand of swimming pools
18.7.15 Algaecides, control of algae in swimming pools, chelated copper algaecides
3.9.1 Checklist of daily routine for the pool operator
3.9.3 Expired air resuscitation (EAR) and Cardiopulmonary resuscitation (CPR)
18.7.20 Filters, flocculent, coagulants
3.9.2 Pool test ranges
18.7.18 Stabilized and unstabilized pools
18.7.22 Starting to use a filled pool
18.7.23 Swimming pool terminology
18a5 Tests for swimming pools
18.7.9 Alkalinity, total alkalinity and buffer capacity
18.7.21.6 DPD test for swimming pools, diethyl-paraphenylene diamine
18.7.21.3 Methyl orange test for chlorine in swimming pools
18.7.21.5 OTO test for swimming pools, orthotolidine
18.7.21.0 Test kit for chlorine levels in swimming pools, available chlorine, free available chlorine,
residual chlorine
18.7.16 Total dissolved solids (TDS) water hardness, scale
3.9.1 Swimming pools, checklist of daily routine for the pool operator
1. Principals should:
1.1 ensure that approved guidelines are provided to, and observed by, operators of school pools, and
1.2 ensure that only one person has responsibility for the operation of the school pool.
2. Operators should:
2.1 ensure that the water quality is both biologically and chemically acceptable,
2.2 follow the daily checklist,
2.3 maintain the pool within the pool test ranges.
3. Morning
Check flow gauge. Quick check of pumps, motors and filter. Test pool water for free and total chlorine
(DPD 1 and 3.) Test pool water for pH. Record test results. Adjust sodium hypochlorite feed rate if
necessary. Check level of hypochlorite and alum dosing tanks. Adjust if necessary. Tanks may have to be
cleaned with hot water. Adjust pool pH if necessary before swimmers enter. Remove leaves and floating
matter using a skimmer. Vacuum the pool before swimmers enter, or backwash filters and commence
dosing alum, or clean and disinfect lint filter and strainer basket, or clean tiles around pool surface and
scum gutter.
4. Midday
Test pool water for free and total chlorine (DPD 1 and 3.) Test pool water pH. Once per week test total
alkalinity. Record all results. Adjust hypochlorite feed dose rate if necessary. Stop alum dosing pump if
filter was backwashed that morning.
5. Afternoon
Check flow gauge. Tour the pool complex, check general conditions of equipment. Record any
deficiencies. Stocktake weekly. Report problems to the Principal. Clean dosing pump equipment if
necessary, or attend to pool grounds, or special jobs such as removal of black spot algae, or repairing
tiles. Wash out and disinfect change rooms, toilets and showers. Test pool water for free and total
chlorine (DPD 1 and 3.) and pH. Test pool water cyanuric acid and hardness when necessary. Record
results. Adjust hypochlorite dose rate if necessary. Adjust pH, cyanuric acid, alkalinity and hardness if
necessary after pool is closed. Empty bins. Tidy plant room including chemical storeroom. Check gauges,
flow rate and pump motor. Hose down concourse and disinfect if necessary. Leave pool complex
securely locked, especially the plant room and chemical store.
3.9.2 Swimming pools, pool test ranges
The operator should maintain the pool within the following ranges:
1. Free chlorine (DPD 3.): 3.5 to 3.0 mg / L
2. Total chlorine (DPD 3.): 3.5 to 3.0 mg / L
3. pH: 7.5 to 7.8 Optimum is pH 7.6 to 7.7
4. Total alkalinity: 60 to 120 mg / L Optimum: 80 to 110 mg / L
5. Hardness: 150 to 250 mg / L Optimum will depend on pool water balance.
6. Cyanuric acid: 25 to 50 mg / L Optimum: 25 to 38 mg / L.
16.2.4.3.2 Chloramines in swimming pools
Chloramine (NH2Cl), dichloramine (NHCl2), trichloramine, (NCl3)
Organic chloramines, RNHCl, are quite stable and useful sources of chlorine for bleaching, disinfection
and oxidation when necessary to kill bacteria. However, chloramines can be eliminated from pool water
only by using activated carbon filters. In pure form it is an oily liquid that is highly reactive and explosive.
Some chemists who first discovered it or first used it were badly injured. It may be formed in swimming
pools when disinfecting. If the pH of swimming pools is < 6.8, urea forms chloramines. Chloramines
react with urea from swimmers' urine to irritate mucous membranes.
4NH3 + 3Cl2 --> NCl3 + 3NH4Cl
ammonia + chlorine --> nitrogen trichloride (trichloramine) + ammonium chloride
In hot water ammonia forms
NCl3 + H2O --> NH3 + 3HOCl
nitrogen trichloride (trichloramine) + hot water --> ammonia + hypochlorous acid (chloric (I) acid)
There is no chemical which changes colour when someone urinates in a swimming pool. There are dyes
which could cloud, change colour, or produce a colour in response to urine, but these chemicals would also
be activated by other compounds, producing embarrassing false-positives.
Ammonia (NH3) quickly combines with chlorine to form bad smelling chloramines. The chloramines are
undesirable smelly compounds formed when insufficient levels of free available chlorine react with
ammonia and other nitrogen containing compounds (swimmer waste, sweat, urine). Chloramines are a
threat to human health and are very poor sanitizers. Chloramines can be destroyed by superchlorination.
Since all chlorinated sanitizers react with water to produce HOCl, chlorine consumption depends on the
amount of contamination that is present. Enough sanitizer must be added to meet the chlorine demand of
the water before a measurable residual can be maintained. This amount depends on the amount of
contamination present in make up water, plus whatever is added by swimmer loading, rain, dust and other
external sources, e.g. nitrogenous wastes from swimmers' bodies. Whether they are as simple as
ammonia in urine, or as complex as the components in perspiration or saliva, they present special
problems when they accumulate in pool water. These contaminants react with HOCl to form compounds
called chloramines, or combined chlorine. The combined chlorine reaction begins with one unit of
ammonia, combining with one unit of HOCl to form monochloramine (NH2Cl). This reacts with another
unit of HOCl to form dichloramine and finally with a third unit of HOCl to produce trichloramine (NCl3).
It takes a fourth unit of HOCl to finally convert the original molecule of ammonia into harmless nitrogen
gas (N2) water and chloride ion (Cl-) and a fifth unit of HOCL before a free available chlorine residual
can be measured. These chloramines cause trouble in pool water because they are stable and persistent.
The monodichloramine and trichloramine from this first unit of ammonia will survive and accumulate with
the chloramines formed from subsequent units of ammonia. This is actually chlorine consumption, because
HOCl combined with ammonia forms chloramines. Chloramines have very poor sanitizing power, so algae
and bacteria can grow. In fact, they have such poor pool sanitizing power that they would be rated at only
0 to 10 on a relative activity scale with HOCl rated at 10 000. Chloramines could provide germ fee water
if they were present at a concentration of at least 25 to 50 ppm. However, this concentration would create
additional problems in a swimming pool, because chloramines are very pungent and irritating, causing eye
irritation and chlorine odours at very low concentrations.
Unfortunately, the chlorine odours generated by chloramines lead many people to think that too much
chlorine has been added. So, they stop adding chemicals and problems grow worse. These symptoms are
a signal to test and adjust pH, and add enough chlorine to oxidize all the chloramines, establishing a free
available chlorine residual. It is often very difficult to convince a pool owner that insufficient chlorination is
the cause of chlorine odours, eye burn and algae because the owner probably tested the pool when a
problem was noticed, and got a very positive chlorine test according to the test kit. This is the most
confusing problem caused by combined chlorine. Some test methods measure it as part of a total chlorine
residual.
Recent studies have focussed attention on “combined chlorine” in swimming pools, a common name for
mono-, di- and trichloramine.
Chloramines and trihalomethanes, with chloroform, as the best known compound in this group, are
generally characterized as “disinfection by-products”, because they emerge as secondary pollutants out
of the reaction between chlorine disinfectants and organic pollutants in water.
Chloramines are formed in water by reaction between nitrogen hydrocarbons, more especially ammonia
compounds like urea (NH2)2CO and chlorine following reactions (1) – (5)
1. NH4+ + H2O <--> H3O+ + NH3 (decomposition of urea from urine, sweat)
2. NH3 + Cl2 --> NH2Cl monochloramine
3. NH2Cl + Cl2 --> NHCl2 dichloramine
4. NHCl2 + Cl2 --> NCl3 trichloramine
5. NH2R + Cl2 --> NHRCl alkylmonochloramine
Trichloramine (NCl3), formed in pool water at pH < 6, is very volatile and responsible for a pungent, acid
smell which is generally confused with a strong chlorine odour and which is the main cause for eye
irritation in covered swimming pools.
Chloramines can be prevented forming by using CO2 as a pH corrector to limit the low pH needed for trichloramine formation,
or by using alternative disinfection products, bromine, ozone or chlorine dioxide (in pure form or as a complex
compound, e.g. Hydroxan, all with a low chloramine forming potential.
Once formed, chloramines can be reduced with
1. active charcoal in granular form in filters, in powder or as “hydroanthracite” in double layer filters)
2. breakpoint chlorination, with chlorine at least ten times above the chloramine concentration in pool
water.
As chloramines can hardly be found in open air pools because of the almost immediate destruction by
incident sunlight, the idea grew years ago to use artificial sunlight as a chloramine reduction tool. The
“chloramine separator” of P. Kaas used ultraviolet and infrared light and was used with changing success
from the years 1975-80 on. The use of UV light was promoted as “best available technique” in water
treatment of swimming pools by the Flemish Technology Research Institute VITO.
Recently, new UV systems became available on the market using low or medium pressure UV lamps. Low
pressure lamps contain mercury vapour at about 10-3 bar and emit monochromatic light of 254 nm while
the medium pressure type operates at mercury vapour pressures of 0,1 – 10 bar and emits polychromatic
light, including the 254 nm wavelength. This is near the optimum wavelength of 245 nm at which
monochloramine – the precursor of dichloramine and trichloramine – breaks down by photochemical
oxidation. Thus low pressure lamps focus on the breakdown of monochloramine, while medium pressure
lamps attack dichloramine and trichloramine.
18.7.0 Swimming pool chemistry
A swimming pool system lacks the three purifiers that protect water quality in natural bodies of water:
1. Aeration, the addition of oxygen to the water from the continuous flow of water through lakes, streams
and rivers.
2. Dilution of sediment from continuous water flow.
3. Prevention of contaminant build-up by water flow, movement and dilution, and biodegradation by
aquatic organisms.
As a swimming pool lacks these purifiers, it is subject to rapid stagnation. Also, it is usually contaminated
with bacteria, algae, dust and dirt, and organic materials from swimmers wastes, sweat, urine and even
faeces. A swimming pool must be disinfected as part of a total system to remove bacteria, algae and
organic contaminants leaving water with acceptable clarity and colour.
Volume of a swimming pool length × width × depth. If length 25 m, width 10 m, depth 1 m to 2 m
(average 1.5 m) then volume 25 × 10 × 1.5 375 cubic metres 375 000 litres.
18.7.1 Chlorine and water
When chlorine is added to water, a mixture of hypochlorous acid, HOCl, the active sanitizing species,
and hydrochloric acid, HCl, forms within seconds at room temperature.
Cl2 (aq) + 2H2O <=> HOCl + Cl- + H2O+
In dilute solution and pH > 4, the equilibrium displaces to the right, and little Cl2 exists in solution. The
chlorine added to pool water does not produce a concentrated solution of a strength to yield such a low
pH. However, the oxidizing property of the added chlorine is in the HOCl formed and produces the main
disinfecting action of added chlorine solutions. Hypochlorous acid dissociates almost instantaneously into
hydrogen and hypochlorite ions. The reaction is reversible. The dissociation depends on the pH and
temperature. Hypochlorous acid exists in a pH dependent equilibrium with hypochlorite ion (OCl-) in pool
water.
H2O + HOCl <=> H3O+ + OCl-
18.7.2 Swimming pool chlorination
See diagram 13.4.2: Distribution of HOCl and OCL- in water at pH levels
Most pools are sanitized with chlorine-based compounds. When chlorine compounds are dissolved in
water, hypochlorous acid is formed which does the actual sanitizing. In most cases, the non-chlorine part
of the chlorine compound serves no other purpose than to hold the chlorine until the product dissolves.
The three categories of sanitizer contain chlorine in different forms. All three sanitizers are compatible and
effective with other chemicals in pool water. Each will do its function without causing objectionable
tastes, odours or colours in the water, if properly applied.
18.7.2.1 Chlorine gas is referred to as having 100% available chlorine (see later for a discussion of
available chlorine) is relatively low cost and is used in public swimming pools and most bulk drinking and
waste water treatment systems. However, it is a gas that must be delivered in bulky metal cylinders and
has to be applied to the water through sophisticated metering systems operated by trained personnel. It is
highly corrosive, toxic, and very acidic because of the H3O+ and Cl-. Operators of regulated public
swimming pools may be required to install separate feeding equipment to add soda ash to neutralize the
acidity from the chlorine gas. If chlorine gas were the only chemical available to disinfect water, there
would be few home swimming pools.
18.7.2.2 Inorganic hypochlorites
Hypochlorites including calcium hypochlorite, sodium hypochlorite and lithium hypochlorite are used as a
disinfectant, sanitizer, bactericide, algaecide and oxidizer in swimming pool water. Calcium hypochlorite is
also used as a disinfectant in drinking water.
18.7.2.2.1 Calcium hypochlorite, Ca(OCl)2, 65% available chlorine, white granular powder, pH 11.8,
also contains 5 to 8% of insoluble material, which can cause cloudy water. A by-product of this reaction
is the calcium ion (Ca2+) a major component of water hardness, and a contributor to scaling tendencies in
the pool.
Ca(OCl)2 + H2O < = > Ca2+ + 2OCl- + H2O
18.7.2.2.2 Sodium hypochlorite NaOCl liquid, at 12-15% available chlorine, often called “liquid
bleach”. Laundry bleach only has about 5% available chlorine. Sodium hypochlorite, with a pH of 13 is
relatively low in available chlorine concentration so more is required to maintain the disinfectant residual in
a pool. Because of the bulk of its liquid form and its poor storage stability, it must be purchased frequently
throughout the pool season. Although it does not add hardness to the pool, its high pH can contribute to
scaling tendencies in hard water areas. It is generally cheap, but difficult and dangerous to handle. It also
loses its potency rapidly and is usually only used in large commercial pools.
NaOCl + H2O < = > Na+ + OCl- + H2O
Both forms have a high pH, and may require frequent additions of acid to maintain pool water in the
proper pH range for chlorine sanitizing efficiency, equipment longevity and swimmer comfort. The
hypochlorite ions also establish equilibrium with hydrogen ions, depending on the pH. The same relative
amounts of HOCl and OCl- exist at equilibrium at a given pH if either chlorine gas or hypochlorites are
used. Chlorine decreases the initial pH, and hypochlorites increase the initial pH. Neither product
provides protection against the destructive effects of sunlight on a chlorine residual, so frequent chemical
additions and adjustments are necessary to maintain satisfactory water quality in outdoor swimming pools.
18.7.2.3 Chlorinated isocyanurates, stabilized chlorine, are a group of chlorine pool sanitizers that
contain stabilizer (cyanuric acid or isocyanuric acid) as the granular form dichlor 56% available chlorine
and the tablet or stick form trichlor 90% available chlorine, the latter usually used in a chlorine feeder.
Chemical feeder: a device that dispenses chemicals into the pool water at a predetermined rate. Some
provide chlorine or bromine while others add Cyanuric acid, (CNOH)3: also called conditioner and
stabilizer. A granular chemical added to the pool water which provides a shield to chlorine for protection
from the UV radiation from the sun. It is also found in dichlor / trichlor products.
18.7.2.4 Dichlor
See diagram 16.13.8: Trichloroisocyanuric acid, TCCA, C3Cl3N3O3, white crystalline powder,
disinfectant, bleaching agent
Dichlor is the common name for sodium dichloro isocyanuric acid. It is a quick dissolving chlorine
compound made up of chlorine and cyanuric acid (stabilizer) and has a pH of 6.9. Shock treatment with
dichlor is not recommended as it may result in overstabilization and chlorine lock. If dichlor is used, a
monthly check of the cyanuric acid level is recommended, to prevent overstabilization and chlorine lock.
pH adjusting chemicals. They protect the chlorine from the UV rays of the sun.
Chlorinating tablets, chlorinating concentrate
Pool chlorinating concentrates provide the effectiveness of HOCl, the ease and convenience of
concentrated solids, and the benefits of stabilization, to provide outstanding water quality with minimum
effort and expense. Granular compounds react with water to produce the same active sanitizing species,
hypochlorous acid. Therefore, it is an effective bactericide and algaecide that will oxidize organic
contaminants. Tablets react similarly, but produce three units of hypochlorous acid. Both the granular
compounds and tablets have a by-product, cyanuric acid which is supposed to stabilize free chlorine
residual without interfering with its sanitizing effectiveness. However, the binding action of cyanuric acid
reduces the concentration of free available chlorine and demonstrably reduces sanitizer efficiency. Pool
owners who use cyanuric acid must have higher free residual chlorine to compensate for this fact so that
sanitizer efficiency is maintained. Some Australian States have banned the use of cyanuric acids in
commercial installations. So to describe cyanuric acid products as a “conditioner” is misleading.
18.7.2.5 Pool chlorinating concentrates
Both the granular and tablets forms are based on cyanuric acid (CNOH)3, the central structure of which
is composed of alternating carbon and nitrogen atoms. In the granular form two atoms of chlorine are
added, giving an available chlorine of 56%. Because this is a sodium salt, it has excellent solubility at
26.1% w / v and a nearly neutral pH of 6.7. It may be added directly to the pool by hand broadcasting
or it can be pre-dissolved and added as a hypochlorinator solution. The tablet form contains three atoms
of chlorine, giving it 90% available Cl2. It has a relatively low pH of 2-3. Because of its high available
chlorine content, much less needs to be added. So it, too, has a minimal impact on pH. Its low solubility
of 1.2%. w / v makes it ideal for use in tablet form in continuous feeding systems.
This trichloro product should not be added directly to the pool in either its tablet or granular form. Its high
available chlorine, slow solubility and acidic pH give it the potential to bleach, or pit any pool surfaces that
it contacts. Both products are free of insoluble residues, produce a minimal impact on pH and do not
contribute to water hardness or scaling. After the chlorine has been consumed in performing the sanitizing
functions, the cyanuric acid remains dissolved in the water to provide maximum stabilization for the free
chlorine residual.
Secondary chemical treatment with substances that control pH and buffer the pool, is also needed for
optimum swimmer comfort. Depending on pool conditions, secondary treatment might include
decolorizers, additional algaecide, and chemicals to adjust mineral levels (hardness) or retard evaporation.
18.7.3 The effect of pH
Hypochlorous acid is a weak acid and dissociates poorly below pH 6, so it exists mainly as HOCl at low
pH. Between pH 6.0 and 8.5, a very sharp change occurs from undissociated HOCl to almost complete
dissociation. At 20oC and pH greater than 7.5, hypochlorite ions, OCl-, predominate. HOCl is about 100
times more effective as a sanitizer than OCl- because the negative charge on the OCl- ion hinders it
entering living cellular structures and oxidizing the contents.
18.7.4 Measure the free chlorine in water
Free chlorine is the amount of chlorine available to do its job of sanitizing the water. The standard method
for determining free chlorine is to measure the amount of oxidant by its ability to liberate iodine from
acidified iodide solution. Titrate a water sample with a standard iodide solution and detect the iodine
released by the blue colour formed with a fresh starch indicator. Find the amount of iodine released by
back titration with sodium thiosulfate, hypo. The reagent is only oxidized to tetrathionate by the iodine.
With chlorine it is oxidized to sulfate.
HOCl + 2I- + H3O+ ---> Cl- + I2 + 2H2O
I2 + 2S2O32- ---> S4O62 -+ 2I-
Devices called rapid test meters can be used to test for chlorine and pH.
18.7.5 Available chlorine
When chlorine gas dissolves in water, it forms hydrochloric acid and hypochlorous acid. Since only the
hypochlorous acid sanitizes, only half the chlorine added to the water is usable. Chlorine gas is defined as
100% available chlorine, so compounds for which all the chlorine in solution is active will have
percentages twice the value based on composition. Ca(OCl)2, which is 99.2% available chlorine for the
pure material may be quoted as 100% available chlorine. It produces two moles of active chlorine
compared to only one mole from Cl2. However, it has more than twice the molecular mass, a ratio of
143: 71. So, on a mass basis, both chlorine gas and calcium hypochlorite are equally effective. Materials
releasing other oxidizing agents when dissolved in water have available chlorine defined in the same
manner.
Approximate percentage available chlorine of substances added to swimming pool water are as follows:
Cl2 chlorine gas 100% (by definition)
Bleaching powder (chloride of lime) 35%
Ca(OCl)2 calcium hypochlorite 99%, commercial products 70%
NaOCl, sodium hypochlorite 95% (solution 100%)
Commercial bleach (industrial product) 12%
Commercial bleach (household product) 3%
ClO2, chlorine dioxide 263%
NH2Cl, monochloramine 138%
NHCl2, dichloramine 165%
CONClCONClCONCl, C3Cl3N3O3, trichloroisocyanuric acid 91%
CONClCONClCONH, dichloroisocyanuric acid 72%
CONClCONClCON- Na+, sodium dichloroisocyanurate 64%
18.7.6 Dissolve chlorine in pool water by electrolysis
Add common salt to water at concentration of approximately 4000 ppm to generate hypochlorous acid
continuously with an electrolysis cell. The water will gradually become basic so you must add an acid to
keep the pH between 7.2 and 7.8 for optimum swimmer comfort and to maintain sanitizer efficiency.
Cathode (-) reaction: 2e- + 2H2O < = > 2OH- + H2 (g)
Anode (+) reaction: 2Cl- < = > Cl2 (g) + 2e-
Overall Reaction: 2Cl- + 2H2O < = > Cl2 (g) + H2 (g) + 2OH-
18.7.7 Pool water and pH
Lower the pH of the pool below pH 7 to increase the oxidizing strength of the HOCl but more acidic
solutions will corrode many components For tiled swimming pools, the recommended pH range is pH
7.4-8.0 and you must add about 200 mg / L calcium chloride to offset removal of calcium salts from the
grouting between the tiles. A test for chlorine in water may measure the free residual chlorine or the free
residual chlorine plus the chloramines which are termed combined chlorine. The lower the pH, the more
readily chloramines form. Above pH 7 few chloramines form, so keep the pH above 7.
18.7.8 Superchlorination, shock treatment, breakpoint chlorination
See diagram 13.4.2: Chlorine dosage and residual chlorine for breakpoint chlorination
When chlorine in its various forms is added to water, it is used up in oxidizing any material for which it is
a sufficiently strong oxidizing agent, e.g. iron II, sulfide, nitrite. Next, chloramines, called combined
chlorine, form from reactions of chlorine with organic nitrogen compounds. The breakpoint when further
addition of chlorine is not used to oxidize nitrogenous compounds, but remains as free available chlorine.
When you superchlorinate or “shock treat” your pool, the goal is to reach a high enough level of free
chlorine to break apart all molecular bonds, specifically the combined chlorine molecules, ammonia or
nitrogen compounds and to completely oxidize all organic matter. If you add enough chlorine to achieve
this breakpoint chlorination, chlorine added after that point will be free available chlorine.
Superchlorination refers to further additions of chlorine that will remain in the water as residual chlorine to
react with any material later added to the pool water. Maintain a free chlorine residual at all times to
achieve sparkling clear, sanitary pool water by periodic super chlorination of the pool water.
When chloramines are removed, better efficiency of chlorine is achieved. More of the chlorine residual can
then exist as the free or active form, rather than as the less effective combined form. You can use
potassium monopersulfate for strong oxidation reactions to oxidize any chloride ion back to chlorine.
Superchlorination is the remedial action that should be taken when the DPD test detects the presence of
0.2 ppm or greater of combined chlorine in the pool. Superchlorination simply involves the addition of
5 to 10 × normal daily dose of chlorine. Super chlorination should be routinely performed on very hot
days under heavy loads after heavy rains or if DPD test shows combined chlorine in excess of 0.2 ppm
combined chlorine. Superchlorination is a remedial action and is recommended to remove gross amounts
of chloramines, correct eye and nasal discomfort, or destroy a visible algae growth. It also eliminates
waiting, vacuuming, scrubbing and back washing to remove algae that will have appeared while
chloramines accumulated. However, although it works well, eventually if you keep loading chemicals and
contaminants in a pool you will have problems.
18.7.8.1 Reactions of chlorine with ammonia and ammonia compounds
The reactions of chlorine with ammonia and ammonia compounds formed from organic waste to form
chloramines are often written as follows:
Formation of monochloramine: NH3 + HOCl < = > NH2Cl + H2O
Formation of dichloramine: NH2Cl + HOCl < = > NHCl2 + H2O
The reactions of monochloramine with dichloramine: NH2Cl + NHCl2 ---> N2 (g) + 3HCl
Total equation: 2NH3 + 3HOCl ---> N2 (g) + 3HCI + 3H2O
Further addition of chlorine forms nitrogen trichloride, the "smell of chlorine", that leaves "agitated" water
and may cause eye irritation:
NHCl2 + HOCl ---> NCl3 + H2O
18.7.9 Alkalinity, total alkalinity and buffer capacity
See diagram 13.4.2: Titration of sodium carbonate with hydrochloric acid
See 12.3.0.5 Ionization reaction of carbonic acid
Alkalinity is the total amount of dissolved alkaline compounds in swimming pool water. Alkalinity is used
a measure of the buffer capacity of pool water, i.e. the resistance to change in the pH of water when strong
acids or bases are added. If the alkalinity is too low, pH control is difficult, because the pH is sensitive to
small amounts of acid and base.
Calculate how much 0.1 M hydrochloric acid would be needed to change the pH by one unit if there is no
buffering capacity.
The pH 8.4 of sodium bicarbonate in water lies between the pK values of the first and second dissociation
constants of carbonic acid, 3.60 and 10.25. Carbon dioxide constitutes about 0.033% of the atmosphere
and dissolves in rainwater to form carbonic acid and produce theoretical pH value pH 5.65. Carbonic acid
has two acidic hydrogens and hence has two dissociation constants, PKa1 and PKa2.
H2O (l) <--> H+ (aq) + OH- (aq)
2H+ (aq) + CO32- (aq) <--> H2CO3 (aq) carbonic acid
CO2 + H2O <--> H3O+ + HCO3-
HCO3- + H2O <--> H3O+ + CO32-
CO2 + H2O ---> H2CO3 carbonic acid
H2CO3 ---> HCO3- + H+
Ka1 2.5 × 10-4 mol / L, Pka1 3.60
HCO3- ---> CO32- + H+
Ka2 5.61 × 10-11 mol / L, PKa2 10.25
Total equation: CO2 + H2O ---> HCO3- + H+
Total equation KA = 4.30 × 10-7, PKa = 6.36, but call this value the acidity constant.
Total alkalinity is a measure of the buffering capacity of pool water, i.e. resistance to change in pH of the
water when acid is added. It is usually expressed as carbonate alkalinity, the total amount of alkaline
materials present in the water, principally hydroxides, carbonates and bicarbonates. Control of pH can be
simplified by maintaining total alkalinity in the range of 80 to 150 ppm. Total alkalinity is composed of
carbonates, bicarbonates, and hydroxides, and functions as a buffer to help keep pH in the proper range.
Total alkalinity is easily measured with a test kit, and can be adjusted with alkalinity control or acid
according to label directions. When you measure the total alkalinity, only measure the carbonate
alkalinity level. The term "alkalinity" comes from the common use of sodium bicarbonate as a buffer and
its alkaline pH of 8.4. However, you can also use acid buffers and so "buffer capacity" may be a better
term than "alkalinity". If the pool has an unstable pH, the total alkalinity should be tested and adjusted.
The ideal total alkalinity range for pools containing either calcium or sodium hypochlorite is between 60
and 120 mg / L with the optimum between 80 and 100 mg / L.
Total alkalinity too low
A pool with low alkalinity, below 60 mg / L, will require the addition of a little acid to cause a rapid
reduction in pH. A constant pH in low alkalinity water is difficult to maintain after rain. Low buffering
capacity causes corrosion and pH bounce, i.e. the pH resumes previous levels soon after adjustment
because the buffering capacity is too low. Raise total alkalinity by addition of an alkalinity control base,
e.g. sodium bicarbonate, that in turn increases the buffering capacity of the pool. To raise alkalinity with
sodium bicarbonate, the dosage calculation is based on tables prepared for the swimming pool industry.
For example a 25 000 L pool would need 445 g of 57% sodium bicarbonate to increase alkalinity by
10 mg / L. You can add 80 to 120 mg / kg of the white powder sodium bicarbonate as a buffering agent
to raise the total alkalinity of pool water without having much affect on pH. It will only increase pH up to
8.4, regardless of the quantity used. Avoid adding large quantities at one time.
Total alkalinity too high,
A pool with high alkalinity, over 120 mg / L, requires the addition of large quantities of acid to produce a
drop in pH. It will tend to retain pH in most conditions and cause staining, scale deposits and difficulty in
adjusting pH. High buffer capacity requires large amounts of acid or base to change the pH of the
solution. If the pH rises rapidly even after the addition of large amounts of acid, the buffering capacity is
too high. Reduce total alkalinity to 80 ppm by addition of granular addition of PH minus or hydrochloric
acid (muriatic acid) or sodium bisulfate (sodium bisulfate, sodium hydrogen sulfate) granular dry acid, a
dry white crystal that is safer to handle than hydrochloric acid. To lower alkalinity with hydrochloric acid
or sodium bisulfate, the dosage calculation uses tables to find the volume of hydrochloric acid required to
lower alkalinity. The weight of sodium bisulfate, dry acid, required is found by multiplying the volume of
hydrochloric acid by 1.2.
18.7.10 Adjusting the pH of pool water
Measure the pH with an indicator that changes colour at a suitable pH. The indicator is a weak acid that
shifts from one colour to the other just as hypochlorous acid shifts from HOCl to OCl-. Use an indicator
with the same pKa as hypochlorous acid. The pH of natural water is about pH 5.6 because dissolved
carbon dioxide from the air forms carbonic acid that lowers the pH. The continuous addition of
hypochlorite powder will raise the pH, so you will have to add "pool acid" after some time, e.g. the solid
"pool acid" sodium hydrogen sulfate, NaHSO4. You can add hydrochloric acid to salt water pools with
electrolyzers. A neutralizer is a chemical used to deactivate or destroy chlorine or bromine used in better
test kits to increase the accuracy of the pool water tests. Base demand is a titration test used to find the
amount of a base (alkali) required to increase the pH to the correct level. Base demand determined by a
titration to find the amount of a base (alkali) required to increase the pH to the correct level. Sodium
bicarbonate is used as a base to raise the pH, but sodium carbonate is more effective and more commonly
used. The adding of sodium bicarbonate to water gives pH 8.4, but adding sodium carbonate to water
gives pH 11.6. The addition of acid changes the pH according to the titration curve diagram. The pH of
pool water may be changed from the desired range of pH 7.2 to 7.8 by rain, dust, swimmer wastes and
the addition of the sanitizer. However, a stabilized granular form of sanitizer, with a pH of between 6 and
7, will obviously have little effect on the pH of the pool. Stabilized pool chlorine tablets, although low in
pH, have a minimal effect on pH because their high available chlorine and stabilization means far less is
needed to provide effective sanitation. Chlorine gas, calcium hypochlorite, and liquid bleach all require
greater pH adjustment because their pH is far from the desired operating range of pH range 7.2 to 7.8.
This pH range must be maintained because of its impact on sanitizer efficiency, swimmer comfort,
corrosion and scaling. Since total alkalinity affects the amount of pH adjusting chemical which must be
added, alkalinity should first be adjusted to the 80-125 ppm range. After making any adjustments to the
pool chemistry, the pH balance should be checked.
18.7.11 Oxidizing agents
See: Standard electrode potential, electrode potential, reduction potential, E0
See diagram 13.4.2: Reduction potential of chlorine in water and pH
The ability of a material to oxidize is measured by the standard half cell reduction potential, in volts. This is
an equilibrium value given for very specific conditions and so gives only a general indication for a practical
situation. The larger the half cell reduction potential, the stronger the oxidizing agent. The standard
electrode potentials, E0 are reference to a standard electrode, usually the hydrogen electrode, which is
arbitrary defined as 0 V.
The oxidizing agent HOCl is stronger than OCl-.
HOCL + H3O+ + 2e- < = > Cl- + 2H2O, E0 = 1.49 V
ClO- + 2H2O + 2e- < = > Cl- + 2OH-, E0 = 0.94 V
It is dangerous to store hypochlorite powder pool chlorine near materials that can burn because it is a
strong oxidizing agent. If the material comes in contact with something that can be oxidized with chlorine,
e.g. brake fluid, then a fire can occur. Chlorine gas is released when solid hypochlorite powder comes in
contact with moisture at any pH.
18.7.12 Measure chlorine levels
Free chlorine must remain in the swimming pool after all the organic material has been oxidized, so
measuring free chlorine is necessary. This is usually done with commercial test kits, e.g. using OTO,
ortho-tolidine (4,4'-diamino-3,3'-dimethylbiphenyl) or DPD. A chemical reagent used to test the total
chlorine level in water at normal temperatures. It can measure free available chlorine if the water is first
cooled to 1oC. Its results depend on pH, time and concentration of chlorine. because of its toxicity, the
use of OTO is restricted or prohibited in many western countries. It is chemically similar to benzidine
without the methyl groups, a bladder carcinogen.
The colour test with chlorine works well with laboratory solutions of chlorine in water, the reagent also
reacts slowly with chloramines, the compounds which chlorine forms when reacting with nitrogenous body
waste products in swimming pools. So the free chlorine must be measured above what has reacted with
waste otherwise under dosing of chlorine may cause ear, eye and throat infections increases because
insufficient free chlorine exists in solution. Breakpoint chlorination occurs when free chlorine and combined
chlorine are simultaneously minimized. Addition of further chlorine gives free chlorine. Beyond which point
nitrogenous compounds have been oxidized, principally to nitrogen gas.
The methyl orange test measures free residual chlorine (HOCl + OCl-) but the OTO commercial kit
measures the combined chlorine as well but you must do the test quickly. Add chlorine solution to
ammonia to form chloramines. Make a stock solution of 0.2 g of Ca(OCl)2 per litre (1.78 × 10-3 M) as
stock solution. Dilute by a factor of five for use (3.5 × 10-6 M or 25 mg / kg Cl2 equivalent). Add the
stock solution in 20 mL aliquots to 1 mL of 0.015 M NH3 solution (255 mg / kg). A concentrated
ammonia solution keeps the volumes almost constant. Test with indicators. Methyl orange will not bleach
until about 2 mole equivalents of chlorine have been added (about 80 mL) and free HOCL is present.
OTO, on the other hand, will show off-scale from the beginning, indicating reaction with chloramines as
well. Note the time taken by the oxidation reactions. Measure a chlorine solution as a control to monitor
any loss of chlorine
Chlorine meters do not measure HOCl concentration. They measure ORP and via an algorithm
approximate HOCL concentration. The reading may be unreliable.
18.7.13 Chlorine lost from swimming pools in sunlight
See diagram 13.4.2: Ultraviolet absorption spectrum of pool chlorine
Reports suggest that in strong sunlight, up to half of the HOCl is destroyed within 17 min. Chlorinated
sanitizer will produce HOCl, but up to 97% of that residual can be lost in 2 hours so when 4-5 ppm of
free available chlorine is added as calcium hypochlorite at 6 am, it will be completely gone by 12 noon
whether or not anyone uses the pool. This same wasteful chlorine consumption occurs with chlorine gas
and sodium hypochlorite. The free available chlorine residual could be maintained by testing the water at
noon, adding another 4-5 ppm residual for safe afternoon swimming, then repeat the process at 6 pm to
use the pool in the evening or install a chemical feeding system to constantly add sanitizer.
18.7.14 Cyanuric acid, conditioner, stabilizer
Cyanuric acid can stabilize chlorine in swimming pools by protecting chlorine in the water against the
effects of the UV rays from the sun. Cyanuric acid is made by heating urea or some of its derivatives. It is
a selective herbicide, very toxic to barley and radishes. It reacts with chlorine to give dichloro(iso)cyanuric
acid in a chemical equilibrium. Stabilize pool chlorine by using cyanuric acid at a minimum concentration of
30 parts per million, ppm. (1 ppm = 1 mg per litre.) In the stabilization process, a portion of the chlorine
residual is temporarily bonded to the cyanuric acid molecule and protected from the destructive effects of
sunlight. The nature of this bond is such that the chlorine demand is imposed upon the system and
continues to be released as long as a demand exists. As OCl- or HOCl is used up in the pool more OCl-
is released from the dichloro(iso)cyanuric acid to re-establish the equilibrium to maintain a constant
amount of chlorine in the swimming pool. Other commercial chemicals are trichlorocyanuric acid and
sodium dichlorocyanuric acid. The amount of cyanuric acid is kept constant in the pool between 30 and
80 mg / L by the initial addition of any of the three compounds then only hypochlorite should be added to
keep the level of free chlorine at 2 mg / L. The equilibrium in the reaction stabilizes the concentration of
OCl-. If excess cyanuric acid is used, the chlorocyanuric acids do not absorb the ultraviolet light from the
sun.
To find the level of cyanuric acid in the swimming pool a reaction with melamine forms a salt that
precipitates and scatters light. The turbidity is proportional to the amount of cyanuric acid. The turbidity is
measured by the depth of solution required in a standard Nessler tube to just obliterate an object at the
bottom of the tube. Anionic surfactants are measured by a similar salt formation chlorine using methylene
blue. The salts formed are sufficiently lyophilic to transfer into organic solvents and to be estimated by the
depth of colour in the organic solvent.
During a pool season, the conditioner may decrease from leakage, swimmer activity causing splash out or
drag out, and maintenance operations, e.g. vacuuming and filter back washing. Because of the slightly
acidic pH of conditioner, addition of a small quantity of PH plus or acid may be necessary to maintain pH
at the desired level. If the conditioner concentration is too high, it can be lowered by dilution with fresh top
up water. The CYA stabilizer reduces the free chlorine, which is what does the sanitizing, and hence more
must be added to maintain free chlorine and an ORP within the range 650 mV to 750 mV. Pools not
exposed to the sun, e.g. indoor pools should not be treated with CYA. Some Australian State
governments have banned the use of CYA in commercial swimming pools.
18.7.15 Algaecides, control of algae in swimming pools, chelated copper algaecides
If a pool has an algae problem first vacuum and backwash, then breakpoint chlorinate. Use an algaecide
as a last resort. Algae is the most common fouler of pools because it is propagated by air-borne spores.
When conditions favour their growth, e.g. heavy rain, intense sunlight, and presence of nitrogenous
materials, they can cause black and green spots on pool walls and form an algae bloom in the water
resulting in a sharp rise in pH, as the algae consume carbon dioxide in the pool water. If algae bloom is
present, use superchlorination followed by an algaecide to control it and prevent its reoccurrence.
Algaecides should be used as a backup to a routine sanitation program. Maintain a free chlorine residual
in the pool by sanitizing with stabilized pool chlorinating concentrates and adding algaecide according to
the directions on the label. Algae do not cause disease, but may provide an ideal substrate for bacteria.
Algaecides as liquids or granules include copper and silver compounds, Quat compounds, chlorine
enhancer and herbicides. They may be obtained formulated for a specific type of algae, e.g. green algae,
mustard algae, blue-green algae, black algae (black spot) the hardest to treat, and be a bactericide. Pink
algae or red algae-like organisms may be a bacteria. Chelated copper algaecides that contain an
ingredient to prevent the copper from staining the pool surfaces or producing coloured water but do not
affect the ability of copper to kill algae. Copper may also be used in the equipment and plumbing in
swimming pools. High levels of copper and chlorine from cheaper copper-based algaecides, overuse of
These algaecides or corrosive water may cause green hair, blue fingernails, stained pool surfaces and green,
brown or blue water. copper (II) sulfate was one of the original copper algaecides as with aluminium
sulfate it also provides a flocculent. However, it may harm some aquatic creatures in natural pools and
stain solids in swimming pools. Small concentrations of copper or silver ions produced by electrolysis can
be effective. Materials similar to cationic surfactants are also used as algaecides. In addition to getting rid
of algae, algaecides extend the effectiveness of chlorine residual. While chlorine is an algaecide, add
additional quantities as a backup, a maintenance dose.
18.7.16 Total dissolved solids (TDS) water hardness, scale
Total Dissolved Solids, TDS, are a measure of all the dissolved chemicals in the water, whether they are
natural components of source water, residues of treatment chemicals, swimmers wastes, or wind and rain
borne atmospheric pollutants. They stay in the water and may cause staining, scaling and reduced chlorine
efficiency, if allowed to accumulate beyond recommended limits.
All pool water contains total dissolved solids. Stabilizing and using chloroisocyanurate produces the least
amount of TDS. It is estimated that TDS should be maintained at less than 1500 ppm. Cyanuric acid
causes no ill effects in pool water but that it may, by accumulation, indicates the onset of problems
because of TDS in the pool. The only practical way to remove dissolved solids from a pool is to remove
a portion of the water in which they are dissolved. The recommended rate of water removal per week is
1 to 3%. In a 10 000 gallon pool, this represents 100 gallons per week
Hardness, total hardness is a measure of the calcium and magnesium content of the water. All water
contains some natural hardness. The amount will vary regionally, and from source to source. Calcium
hardness refers to the calcium content of the water.
If the calcium hardness level is too low, the water may be corrosive and attack the materials of
construction. So a certain amount of hardness is necessary in water to control its tendency to dissolve.
Hardness treatment will increase low water hardness and prevent etching, pitting and corrosion of surfaces
and metallic components. The soluble white salt calcium chloride is used to raise the calcium hardness or
total hardness level of pool water.
If the calcium hardness level is too high, the water may have a tendency to form scale visible as crusty
grey deposits and cloudy water or deposits in piping that will y not function properly if their diameters are
decreased by scale formation. Scale on pool surfaces is unsightly and unattractive. Scale is formed from
calcium carbonate crystals when the calcium hardness, pH or total alkalinity levels are too high.
18.7.17 Cost of chlorination
Assess the cost at, say, 1.5 KW for 6 hours for 4 months and 1.5 KW for 4 hours for 8 months at
$0.12 per kWh. Amortise the cost of the equipment over, say, 7 years.
18.7.18 Stabilized and unstabilized pools
In swimming pool waters, the free chlorine, HOCl, may be consumed by the following:
1. destroying bacteria and algae introduced by swimmers and by wind and rain borne contamination,
2. reacting with reduced metals such as Fe2+ to produce the oxidized Fe3+ and chlorine ions,
3. the action of the ultraviolet energy of sunlight, which converts free available chlorine to the inactive
chloride ion,
4. oxidizing nitrogenous compounds such as ammonia (NH3) and urea introduced into the water as
components of perspiration, urine and other bodily excretions.
Maintenance of free available chlorine residual at 1 ppm to 3 ppm. Pre-conditioning the pool with 30 ppm
of conditioner, and using a sanitizer to maintain that minimum, automatically provides protection for a pool
against the effects of sunlight. Most overseas countries require commercial pool operators to measure free
chlorine and ORP.
18.7.19 Bromine products
Bromine is a halogen family used as a sanitizer or disinfectant to destroy bacteria and algae in swimming
pools and spas. It is resistant to heat and rapid pH fluctuations and is available as a tablet or as sodium
bromide "bromide", a granular salt. BCDMH (bromochlorodimethylhydantoin) is a bromine sanitizer.
When BCDMH dissolves in water, it produces hypobromous acid, the active form of bromine.
Bromamines are by-products formed when bromine reacts with nitrogenous compounds including
swimmer waste. Unlike chloramines, which are strong smelling and have low sanitizing properties,
bromamines are active disinfectants and do not smell, although high levels are harmful to health. A
brominator is a mechanical or electrical device for dispensing bromine at a controlled rate from a floater
filled with bromine sanitizer tablets.
18.7.20 Filters, flocculent, coagulants
The three main types of pool filters used in pools are sand filters, cartridge filters and diatomaceous earth,
DE filters. Filter aids include the clarifiers, flocculents and coagulants that are the inorganic salts of
aluminium (alum) or organic polyelectrolytes, and diatomaceous earth, called filter powder. Sand filters
use hard sharp silica or quartz or the zeolite clinoptilolite. Flocculents, e.g. alum, and coagulants cause
minute particles to clump together to be trapped by the filter or fall to the bottom as "floc". Flocculent are
also used to clarify muddy pool water with the charged aluminium ion from alum. Aluminium ion is also
used in sticks to coagulate blood from shaving cuts and in antiperspirants to coagulate sweat from pores
under the arm. Alum sinks everything to the bottom to be vacuumed to waste and a small amount of alum
can also be used as a sand filter additive.
18.7.21.0 Test kit for chlorine levels in swimming pools, available chlorine, free available chlorine,
residual chlorine
A test kit is a manual or electrical device used to measure specific chemical residuals, levels or demands in
pool water. Kits usually contain reagents, vials, titrants and colour comparators for the tests. The most
common tests are as follows: pH, free available chlorine, combined chlorine, total alkalinity, calcium
hardness, cyanuric acid and metals. Test strips are small plastic strips with pads attached that have been
impregnated with reagents to test pool water. The strips are dipped into the water and the resulting
colours are compared to a colour scale to find the values. Balanced water is the result when all the
chemical parameters are within the tolerance limits. The most important parameters of water balance are
pH, total alkalinity, calcium hardness and temperature, as measured using the Langelier Saturation Index.
18.7.21.1 Acid demand of swimming pools
Acid demand is the amount of acid required to bring high pH or total alkalinity down to their proper
levels is determined by an acid demand test. The acid demand test is a reagent test usually used in
conjunction with a pH test to find the amount of acid needed to lower pH or total alkalinity. Acid demand
is the amount of acid that needs to be added to swimming pool water to lower the pH and total alkalinity
to acceptable levels.
18.7.21.2 Chlorine in swimming pools
Swimming pool chlorine may be calcium hypochlorite or sodium hypochlorite. Combined chlorine is the
chlorine that has combined with ammonium. compounds or organic matter containing nitrogen to form
chloramines. Available chlorine content is the term used to compare the amount of oxidizing power that
chorine-containing products have when compared to gas chlorine (Cl2). It permits easy comparison of
relative values of chlorine compounds. Available chlorine is the amount of free chlorine that is available to
sanitize or disinfect the water. It is also called Residual Chlorine and Free Available Chlorine.
Total chlorine is the sum of combined chlorine and free chlorine. When chlorine is added to water in a
newly filled pool, some of it is consumed in the process of destroying algae, bacteria and other oxidizable
material in the water. The amount of chlorine consumed in this process is referred to as the chlorine
demand of the water. Once the chlorine demand is satisfied, any additional chlorine added is referred to
as chlorine residual.
Combined chlorine residual is the chlorine combined with simple nitrogen compounds such as ammonia
and urea as chloramine that is non-effective as a sanitizer compared to free available chlorine.
Bactericidal properties of combined chlorine (i.e. chloramines) is only about one hundredth that of a
similar level of free chlorine in water.
Free chlorine (free available chlorine, free residual chlorine) is the chlorine not combined with ammonia
but free to kill bacteria and algae in a swimming pool and is the chlorine available to do its job of sanitizing the
water.
18.7.21.3 Methyl orange test for chlorine in swimming pools
Free chlorine bleaches methyl orange solution quantitatively
Make a stock solution of methyl orange: Dissolve 0.05 g methyl orange in 100 mL of water. Make a
standard solution: Add 0.2 g NaCl to 10 mL of stock solution then dilute to 100 mL. The reagent
appears to be stable in the dark for years. Make a test solution: Add 3 mL of 6 M hydrochloric acid to
24 mL of standard methyl orange solution.
Step 1: Put 0.25 mL of test solution in a test-tube. Add 10 mL of swimming pool water.
1.1 It decolorizes instantly so at least 1 mg / kg chlorine present.
1.2 It does not decolorize instantly so insufficient chlorine in the swimming pool. Add more chlorine to the
pool and repeat the test.
Step 2: Put 0.5 mL of test solution in a test-tube. Add 10 mL of swimming pool water.
2.1 It decolorizes instantly so at least 1.5 mg / kg chlorine present.
2.2 It does not decolorize instantly. The pool contains between 1.0 and present. 1.5 mg / kg of chlorine.
This is the best concentration.
Step 3: Put 0.75 mL of test solution in a test-tube. Add 10 mL of swimming pool water.
3.1 It decolorizes instantly so at least 1.75 mg / kg chlorine present. 3.2 It does not decolorize instantly.
The pool contains between 1.5 and 1.75 mg / kg of chlorine. This concentration is too high.
18.7.21.5 OTO test for swimming pools, orthotolidine
The OTO test kit is seen as unreliable and is no longer used extensively. If you find a yellow colour in the
comparator and a clear liquid reagent, then it is clear that this pool is being tested by the orthotolidine, or
OTO method. OTO This OTO method has some advantages that have made it popular and widely used.
It also has some major deficiencies. The fact that it cannot easily distinguish free from combined chlorine
makes it a very doubtful aid to pool operation. Even worse, it creates a false sense of security, leading to
erroneous diagnosis of pool problems, which delays remedial action. The pool owner could test once,
twice, or three times daily and still have no idea whether enough free available chlorine was present to
protect the quality of the water. A 1 ppm residual measured by OTO will provide far less protection to
pool than a 1 ppm residual measured by the DPD method.
18.7.21.6 DPD test for swimming pools, diethyl-paraphenylene diamine
The DPD test kit is in very common use. DPD will distinguish free from combined chlorine. A DPD
residual will be fee available chlorine, not some combination of HOCL, monochloramine, dichloramine
and trichloramine. It will effectively protect the pool from contamination. The method is simple and rapid.
A DPD NO. 1 tablet is dissolved in a measured amount of pool water to produce a coloured solution.
This colour is compared with the colour standards in the comparator to find the amount of FAC that is
present. The amount of chloramine that is present can easily be determining by adding a DPD NO. 3
tablet to the same test sample and determining whether any additional colour develops. The difference
between this total chlorine measurement and the FAC test result equals the chloramine content of the
water. Example: DPD No. 3 minus DPD No. 1 Chloramines 1.5 ppm to 1.3 ppm 0.2 ppm chloramines
Total Free Chloramine residual This test method indicates precisely when remedial action is necessary to
prevent the accumulation of chloramines in pool water. DPD (Di ethyl-paraphenylene di amine). Just
remember what to do next. Bleaching out occurs when at above 10 ppm free available chlorine in a pool,
a DPD test kit often indicates zero chlorine because the reagent is being destroyed. If you observe an
initial pink colour which then rapidly fades, you probably have far too much residual chlorine in the water.
18.7.22 Starting to use a filled pool
1. Adjust the water level.
2. Check pump, filter, dosing equipment and gauges, and replace or repair if faulty.
3. Check for algae infestation on walls and floor.
4. Superchlorinate until free chlorine residual of 10 milligrams per litre is obtained. This may be achieved
by the addition of calcium hypochlorite or by running the sodium hypochlorite dosing pumps while the
recirculation system is running.
5. Start up the pump and filter system.
6. Run for at least 48 hours before allowing swimmers into the pool.
7. Test pool and make adjustment to pH water balance if required.
18.7.22.1 Use of sodium bicarbonate
A correspondent from New Zealand reports that he got great results, sparking water, excellent control
and minimal chemical use by raising the total alkalinity to 200 ppm + using sodium bicarbonate and in the
process got a stable pH of 8.3 which he almost never had to test for. Also, the chemical was safe to use.
He compensated for the higher pH by running the pools with a FAC of around 4 ppm and not letting it
drop below 1 ppm. He only used sodium hypochlorite diluted to 8% to give a good shelf life. This
procedure was satisfactory for about 20 pools.
In this procedure the ORP is above 650 mV at pH 8.4 and 4 ppm free chlorine, but less than 750 mV.
650 mV to 750 mV is the recommended range for effective sanitation. Assuming he did not use stabilizer
or did not need to, he probably needed to top up the bicarbonate relatively frequently to keep it at around
200. Perhaps a lower alkalinity and a weekly does of acid, to give a lower pH and hence a higher ORP
could allow free chlorine to be reduced to about 1 ppm.
18.7.23 Swimming pool terminology
Biguanides: The name for a certain class of sanitizers using the polymer PHMB, the only non-halogen
sanitizer available for pool use. Soft Swim and Baquacil are manufacturers of this sanitizer. Biguanides are
NOT compatible with the Pool Wizard.
Chlorinator: A mechanical or electrical device for adding chlorine to a pool at a controlled rate. Most
often a floater filled with tablets of chlorine or an in-line feeder.
Chlorine: A member of the halogen family of sanitizers. Its use in swimming pools is in the form of a gas, as
a liquid, in granular or tablet forms. When added to water it acts as an oxidizer, sanitizer, disinfectant and
biocidal agent.
Chlorine, combined: The measure of chlorine which has attached itself to other molecules or organisms,
typically ammonia or nitrogen compounds. Most of these compounds are present as unwanted
chloramines.
Chlorine, free available: Free available chlorine is active chlorine and is not combined with any other
molecule. A portion of the free available chlorine is present as hypochlorous acid, which reacts to destroy
organic material in the pool water.
Chlorine, total: The sum of combined and free available chlorine levels. With a DPD test kit, DPD1 shows
free available chlorine and DPD3 shows total chlorine. The difference, if any, is the level of combined
chlorine.
Chlorine demand: The amount of chlorine necessary to oxidize all organic matter (bacteria, algae,
chloramines, ammonia and nitrogen compounds) in the pool water.
Chlorine enhancer: A chemical compound used in conjunction with chlorine, which makes the chlorine do
better as an algaecide.
Chlorine generator: An electrical device that generates chlorine from a salt solution. The salt solution may
be in a separate tank or may be in the pool itself.
Chlorine lock: If the level of cyanuric acid (stabilizer) in the water is much over 80 ppm, the chlorine
becomes trapped and is unable to oxidize effectively. Despite being able to measure normal chlorine
levels, the Redox potential is very low, indicating a lack of oxidizer. The only way to fix this is to drain
some of the water and refill the pool. Care should be taken when using stabilized chlorine products
(dichlor or trichlor) to avoid the level of cyanuric acid increasing too much.
Chlorine neutralizer: A chemical used to deactivate or destroy chlorine. It is used in better test kits to
prevent the bleaching effect of the chlorine and so to increase the accuracy of the tests.
Chlorine residual: Also called Free Available Chlorine. The amount of chlorine left in the pool water after
the chlorine demand has been satisfied.
Contaminants: The general name for any microparticle or organism which reduces water clarity, quality or
presents health hazards. Filtering, oxidizing and sanitizing are necessary to destroy the contaminants.
Corrosion: The effects of an acidic pool environment when the pH and / or alkalinity are very low.
Corrosion in the form of etching, pitting or erosion of pool equipment and surfaces is the result. Corrosion
may also be caused by misuse of acid or by soft water.
Defoamer: Also called anti-foam. A chemical added to the water to destroy the foam. These products do
not remove the source of the foaming. Shocking and superchlorination may help prevent foaming.
Controlled use of certain of the cheaper algaecides can prevent their resulting in foaming.
Disinfectant: Chemicals, elements or processes which destroy vegetative forms of micro-organisms and
other contaminants. Examples are chlorine, bromine, ionizers, ozonators and copper and silver algaecides.
DPD: Chemically, NN Diethyl-p-phenylene diamine sulfate. An indicator reagent used to measure free
available chlorine (DPD1) and total chlorine (DPD3) bromine, ozone and other oxidizers in water. Far
superior to OTO.
The efficacy of chlorine is affected by many factors, including the sun, temperature, water balance and the
chlorine demand of the water.
Fill water: The water used in filling or topping up the swimming pool.
Foam: A froth of bubbles on the surface of the water. Usually comes from overuse of algaecide but may
also be caused by soaps, oils or other contaminants carried into the water by swimmers. Enzymes may be
used for foam control.
Free available chlorine: The amount of free chlorine in the pool water that is available to oxidize, sanitize or
disinfect the water. The level can be measured using a DPD1 test kit. It is also called residual chlorine or
available chlorine.
Only chlorine and bromine are used as oxidizers, disinfectants and sanitizers in swimming pools.
Hard water: Water that is high in calcium, magnesium or other salts, which makes it difficult for soap to
lather. Hard water also has a tendency to form scale.
Hardness, calcium: The amount of calcium dissolved in the water. It is usually measured as calcium
carbonate.
Hardness, total: The amount of calcium, magnesium and other salts dissolved in the water.
Hydrogen peroxide: An unstable, colourless liquid which is used as an antiseptic in the home. It can be
used as an oxidizing agent in pools. It is NOT compatible with the Pool Wizard.
Hypobromous acid: The active form of bromine in water.
Hypochlorous acid: The active form of chlorine in water.
Ionizers: A water sanitizer that uses electricity to generate metal ions, usually copper and silver. It works
by passing a current through a set of electrodes. The copper is an algaecide and algaestat, while the silver
is a bactericide. Ionizers can significantly reduce chlorine
consumption. If the ion levels get too high, problems with staining or discoloration of the water occur.
Iron: Iron is a natural element that can cause the water to become clear brown or green in colour. It can
also result in staining of the pool surfaces. Iron can be controlled by the addition of a suitable sequestering
or chelating agent.
Stabilized chlorine should be used with care, to avoid problems such as chlorine lock that may be caused
by overstabilization.
Langelier saturation index: Also called Langelier index or saturation index. This index can be used to find
water balance according to the levels of pH, Total alkalinity, calcium
Hardness and water temperature. When all the parameters are in balance, the water will neither be
corrosive nor scaling.
Liquid acid: Also called hydrochloric acid or muriatic acid. It is used for lowering pH, total alkalinity and
for acid washing.
Lithium hypochlorite: A dry granular chlorine compound with 35% available chlorine and has a pH of
10.7. It dissolves quickly and can be used to superchlorinate vinyl liner pools, painted pools and fibre
glass pools.
Magnesium hardness: A measure of the amount of magnesium dissolved in the water. It is part of total
hardness.
Make-up water: Also called top up or refill water. It is the water used to replace water lost to
evaporation, splash out, leaks and back washing.
Marbelite: Also referred to as "plaster". It is a mixture of white cement and white marble dust used as an
interior finish of a pool. It can be given a colour or it may be left white.
Micron: A unit of length equal to 1 millionth of a metre. Microns are used to describe the pore size of
filter media. Sand filters have openings of 25 to 30 microns, cartridge filters have openings of 8 to 16
microns, and DE (diatomaceous earth) filters have openings of 1 to 5 microns. Zeolite used in sand filters
have openings of 3 to 6 microns.
Substances such as calcium, manganese, magnesium, nickel, copper, silver, zinc, iron,
Cobalt or aluminium. Their presence in high non-chelated concentrations can lead to stains or scale
formation. The measure of water hardness is dependent on these minerals.
Nitrogen: A gas that causes algae to bloom and disables chlorine. It is introduced into the water by rain
and by swimmers. Maintaining proper chlorine levels will prevent nitrogen from becoming a problem.
Superchlorination can destroy nitrogen and nitrogenous compounds.
Non-chlorine shock: A granular form of potassium peroxymonosulfate (potassium monopersulfate,
potassium permonosulfate) used to oxidize materials such as micro-organisms, contaminants (ammonia,
nitrogen, swimmer waste) or chloramines. Organic waste: Also called swimmer waste or bather waste,
refers to the soap, deodorant, suntan lotion, body oils, sweat, spit, urine. introduced into the water by
swimmers, as well as the leaves, dust and insects that end up in the pool. The organic waste may form
undesirable chloramines, which require large amounts of chlorine or non-chlorine shock to be destroyed.
ORP: The abbreviation for Oxidation-Reduction Potential. It is a measurement of the ability of the
oxidizer, e.g. chlorine, to oxidize contaminants versus the ability of the contaminants, e.g. algae, to reduce
the oxidizer. It is an indication of the level of free available ability of the oxidizer in the water. ORP is
generally used with automated dosage systems and can give a fair idea of the sanitation of the water. It is
not a measure of the total or available chlorine. Sometimes called Redox Potential.
Oxidizer: Any compound that removes or destroys organic waste and organic compounds in the water.
Ozonator: An electrical device that produces ozone that is introduced into the water as a sanitizer.
pH: The ideal range for pH in swimming pools is 7.0 to 7.6. The pH of human tears is 7.2 to 7.4.
Phenol red: A chemical reagent dye used to test pH. It can measure pH from 6.8 to 8.4. The tablet form
usually incorporates a chlorine neutralizer for more accurate results.
Polymer: Many coagulants are made from organic polymers as are algaecides and algaestats.
Potassium peroxymonosulfate: See Non-chlorine Shock.
ppm: The abbreviation for parts per million. It is a method of assigning value to concentrations of
chemicals in the water. Many of the common pool water tests, as well as acceptable ranges, are stated as
ppm. 1 ppm 1 mg / L
Precipitation: Material forced out of solution will settle, stain, scale or remain suspended in the water.
Pump strainer basket: A device placed on the suction side of the pump, which contains a removable
strainer basket designed to trap large debris in the water flow without causing restriction. Sometimes
called a Pump Leaf Trap.
Quaternary ammonium compounds: Also called Quats or QAC. A type of algaecide composed of
ammonia compounds. They are also effective algaestats for certain types of algae.
Redox potential: The abbreviation for Reduction-Oxidation Potential. It is a measurement of the ability of
the chlorine oxidizer to oxidize contaminants versus the ability of the contaminant, e.g. algae, to reduce the
oxidizer. It is an indication of the level of free available oxidizer in the water. ORP is generally used with
automated dosage systems and can give a fair idea of the sanitation of the water. It is not a measure of the
total or available chlorine. Sometimes called ORP.
Residual bromine: The amount of free available bromine remaining in the water after the bromine demand
has been satisfied.
Residual chlorine: The amount of free available chlorine remaining in the water after the chlorine demand
has been satisfied.
Sanitize: To kill all micro-organisms, including bacteria and algae, and to remove unwanted contaminants.
Scale: The precipitate that forms on surfaces in contact with water when the calcium hardness, pH or total
alkalinity levels are too high. Scale may appear as grey, white or dark streaks on the plaster, fibreglass or
vinyl. It may also appear as a hard crust at the waterline.
Scum: The foreign matter which floats to the surface of the water and forms a layer or a film. It can also
refer to a residue deposited on the tiles or walls of the pool.
Sediment: The solid material that precipitates out of the water and settles to the floor of the pool.
Sequestering agent: Also called Chelating agent. A chemical or compound that combines with dissolved
metals or minerals in the water to prevent them from coming out of solution, thus colouring the water or
causing stains. Sequestrant: A chemical which holds metals in solution and helps prevent scaling.
Shock treat: Adding large amounts of an oxidizer such as chlorine, hydrogen peroxide or potassium
peroxymonosulfate to the water to destroy ammonia and nitrogen compounds, chloramines and other
contaminants.
Soda ash: Chemically, sodium carbonate. A base that is used to raise the pH of acidic (below pH 7.0)
water.
Sodium bicarbonate: also called baking soda or bicarb. A base that is used to raise Total Alkalinity in
pool water with only a slight effect on the pH. Sodium bicarbonate can only raise the pH of the water to
8.5, regardless of the amount used. Do not add large quantities at one time.
Sodium bromide: a salt of bromine which is used to raise the bromine levels in a pool before using
bromine tablets.
Sodium carbonate: Also called soda ash. A base that is used to raise the pH of acidic (below pH 7.0)
water.
Sodium dichlor: A granular, stabilized organic chlorine compound providing 56% or 62% available
chlorine that has a pH of 6.9. Used for regular chlorination. Should be used with caution for
superchlorination as it can cause the stabilizer level to rise too high, resulting in chlorine lock.
Sodium hypochlorite: Liquid chlorine for use in pools. It usually provides 12% to 15% available chlorine
and has a pH of 13. It is generally cheap, but difficult and dangerous to handle. It also loses its potency
rapidly and is usually only used in large commercial pools.
Sodium monopersulfate: Active ingredient and chemical name of a non-chlorine shock treatment or
non-chlorine oxidizer. See Non-Chlorine Shock.
Sodium persulfate: Active ingredient and chemical name of a non-chlorine shock treatment or non-chlorine
oxidizer. See Non-Chlorine Shock.
Sodium sulfite: A chemical that can be used to neutralize chlorine or dechlorinate pool water.
Sodium thiosulfate: A chemical that can be used to neutralize chlorine or dechlorinate pool water.
Soft water: Water that has a low calcium and / or magnesium content. Soft water can result in the etching
of the pool surfaces, and should be increased with calcium chloride.
Stabilized chlorine: A family of organic chlorine compounds that contain stabilizer (cyanuric acid or
iso-cyanuric acid) to protect the chlorine from the degrading UV rays in sunlight. Most common types are
dichlor and trichlor. The granular form is dichlor and the tablet or stick form is trichlor.
Stabilizer: also called cyanuric acid or conditioner. A granular chemical added to the pool water which
provides a shield to chlorine for protection from UV radiation. Too much can result in chlorine lock.
Stain: A discoloration or a coloured deposit on the walls or bottom of a swimming pool. Stains are
usually the result of metals such as iron, copper or manganese in the water. The stains may be green, grey,
brown or black. They may discolour the water without affecting the clarity. Sometimes a sequestering
agent, chelating agent or commercial stain remover may remove them. If that does not work, the easiest
way to remove the stains is to drain and acid wash the pool.
Stain inhibitor: also called a sequestering or chelating agent. A chemical that will combine with dissolved
metals in the water to prevent the metals from coming out of solution and so avoiding discoloration of the
water or stains.
Superchlorination: Adding 7 to 10 times the normal dose of chlorine to the water to destroy ammonia,
nitrogen, chloramines and other contaminants.
Suspended solids: Insoluble solid particles that either float on the surface or are in suspension in the water
causing cloudiness. They may be removed by filtration, but if the particles are too small a flocculent or
coagulant is necessary to enable the filter to trap them.
TDS: See Total Dissolved Solids
Total chlorine: The total amount of chlorine in the water. It is the sum of free available chlorine and
combined chlorine.
Total dissolved solids: Also called TDS. A measure of everything that has ever dissolved in the water and
all the matter that is in solution. The only way to lower TDS is to drain part of the water and replace it.
Trichlor: A slow dissolving, tablet or granular stabilized organic chlorine compound which provides 90%
available chlorine and has a pH of 2.9. It must be dispensed using a floating feeder or an in-line
chlorinator.
Trichlor contains cyanuric acid that prevents the chlorine from being destroyed by the ultraviolet rays of
the sun. When using trichlor, the cyanuric acid level needs to be checked regularly to avoid chlorine lock.
Turbidity: The cloudy condition of the water because of the presence of extremely fine particles in
suspension that are able to pass through the filter. Adding a flocculent or coagulant will clump the particles
together so they can be trapped in the filter. Devices called turbidity meters can be used to measure
turbidity.
Vinyl liner: also called a liner. The vinyl membrane that acts as the container to hold the water in one type
of pool construction.
Water clarifiers: see coagulant / flocculent.
Zeolite: An alternative to quartz or silica for use in sand filters. It is typically clinoptilolite, the specific
zeolite suitable for pool water conditions. Zeolite can provide filtration down to 3 microns, is able to
absorb heavy metals such as iron or manganese, and absorbs ammonia and nitrogen compounds. Zeolite
can also absorb chloramines and is regenerated by washing in salt. Being less dense than quartz, 60% to
70% of the weight of quartz is used. Zeolite is also claimed to reduce back washing by 50%.