School Science Lessons
 Topic 18a Swimming pool chemistry
 2012-01-26 SP
 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
18a5 Tests used in 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
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 used in 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
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 reacts 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)
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 either be prevented to form with
1. CO2 as a pHcorrector, limiting the low pH needed for trichloramine formation,
2. 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 x width x depth. If length 25 m, width 10 m, depth 1 m to 2 m (average 1.5 m) then volume 25 x 10 x 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
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 you should 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 X 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.
 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
 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 (what is agitated water?) 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 X 10-4 mol / L, Pka1 3.60
HCO3- ---> CO32- + H+
Ka2 5.61 X 10-11 mol / L, PKa2 10.25
Total equation: CO2 + H2O ---> HCO3- + H+
Total equation KA = 4.30 X 10-7, PKa = 6.36, but you should 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 X 10-3 M) as stock solution. Dilute by a factor of five for use (3.5 X 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%.