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).
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- + H3O+
chlorine + water <=> hypochlorous acid + chloride ion + hydronium ion
In dilute solution and pH > 4, the equilibrium displaces to the right, and little Cl2 exists in solution.
The chlorine added to swimming 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 swimming pool water.
H2O + HOCl <=> H3O+ + OCl- water + hypochlorous acid <=.> hydronium ion + hypochlorite ion
Chlorination of swimming pools
: Distribution of HOCl and OCR- in water at pH levels
Most swimming pools are sanitized with chlorine-based compounds.
When chlorine compounds are dissolved in water, hypochlorous acid forms, 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 swimming pool water.
Each will do its function without causing objectionable tastes, odours or colours in the water, if properly applied.
Swimming swimming pool chlorine may be calcium hypochlorite or sodium hypochlorite.
Chlorine gas in swimming pools
Chlorine is 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 swimming pool water it acts as an oxidizer, sanitizer, disinfectant and biocidal agent.
Chlorine gas is referred to as having 100% 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.
Hypochlorites including calcium hypochlorite, sodium hypochlorite and lithium hypochlorite are used as a disinfectant, sanitizer, bactericide, algicide and oxidizer in swimming pool water.
Calcium hypochlorite is also used as a disinfectant in drinking water.
Calcium hypochlorite, Ca(OCl)2
Calcium hypochlorite, Ca(OCl)2, 65% available chlorine, white granular powder or compressed into pucks, 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 swimming pool.
Ca(OCl)2 + H2O Ca2+ + 2OCl- + H2O
calcium hypochlorite + water calcium ion + hypochlorite ion + water
Chlorinated isocyanurates, cyanuric acid, (CNOH)3
Chlorinated isocyanurates, stabilized chlorine, are a group of chlorine swimming 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 swimming 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 swimming 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.
: Trichloroisocyanuric acid, TCCA, C3Cl3N3O3, white crystalline powder, disinfectant, bleaching agent
"Dichlor" is the common name for dichloroisocyanuric acid, C3HCl2N3O3, or the active ingredient in dichlor cleansing powder sodium dichloroisocyanurate C3Cl2N3NaO3.
Dichlor 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 over stabilization and chlorine lock.
If dichlor is used, a monthly check of the cyanuric acid level is recommended, to prevent over stabilization and chlorine lock.
18.104.22.168 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 swimming pool by hand broadcasting or it can be pre-dissolved and added as a hypochlorite 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 swimming 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 swimming 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 swimming pool, is also needed for optimum swimmer comfort.
Depending on swimming pool conditions, secondary treatment might include decolorizing, additional algicide, and chemicals to adjust mineral levels (hardness) or retard evaporation.
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 algicide 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.
Hypochlorous acid, HOCl
Hypochlorous acid, HClO, is the active form of chlorine in swimming pool water.
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.
Free chlorine in waters.