Water Chemistry

The most common chemical used in the treatment of swimming pool water is chlorine. It not only eliminates bacteria and algae through a disinfecting (killing) action, it also oxidizes (chemically destroys) other materials such as dirt and chloramines.

Chlorine exists as a solid, a liquid and a gas. The strength of each type is determined by the amount of chloride within the material that is effective both as a disinfectant and an oxidizer.

1. Liquid chlorine (sodium hypochlorite) contains 12-16% available chlorine. (Household liquid bleach generally contains about 5% available chlorine, and is, therefore, of a lesser strength than pool chlorine.)
2. Solid Chlorine (powder or granules)
   1. Calcium Hypochlorite - contains 65-75% available chlorine.
   2. Chlorinated Isocyanurates - contains varying percentages of available chlorine.
   3. Lithium Hypochlorite - contains 30-35% available chlorine 3.
3. Gaseous Chlorine contains 100% available chlorine. This substance is highly toxic and, therefore, requires special procedures when it is used.

State codes require that chlorine be injected into the pool water through the use of an approved chlorinator. When chlorine (in any form) is added to water, a weak acid called Hypochlorous acid is produced. It is this acid, not the chlorine, which gives water its ability to oxidize and disinfect. Proper chlorination and filtration gives pool water its clear, sparkling appearance.

As chlorine is introduced into pool water, a portion is always consumed during the processes of disinfection and oxidation. That portion of available chlorine consumed is referred to as chlorine demand. The hypochlorous acid left after the chlorine demand has been satisfied is referred to as the free chlorine residual. This is what we read when we test the pool water for chlorine. State codes require that the free chlorine residual be at least l.0 ppm throughout the pool. Maintaining a higher residual l.5 - 3.0 is recommended, though, to insure that the residual level does not drop below the minimum.

Some of the available chlorine will react with nitrogen-containing compounds to form chloramines (combined chlorine). A common source of nitrogen is ammonia, produced from perspiration and urine. Chloramines give off a strong chlorine odor and are irritating to the eyes. When these two conditions are present, most people think that the chlorine residual is too high, however, just the opposite is true. In order to eliminate chloramines, the pool operator must raise the chlorine residual to 5 to 10 times the normal level. This procedure, called superchlorination, oxidizes the chloramines leaving only free chlorine. Depending on the swimmer loads, it is recommended that a pool be superchlorinated as often as once a week in hot weather and once a month during cooler weather in order to control the formation of chloramines.

pH is measure of the acidity or basicity of water and directly affects some of the chemical reactions that occur in swimming pool water. It is measured numerically on a scale from 0 to 14 with 0 being very acidic, 7 being neutral, and 14 being very basic.

From 7 (neutral) down to zero acidity increases, while from 7 up to 14 solutions become increasingly basic. The ability of chlorine to oxidize matter and kill micro-organisms is directly affected by pH. As the pH rises (water becomes more basic) this ability is adversely affected. At a pH below 7.0 - irritation to the eyes and mucous membranes of swimmers will occur due to the acidic condition. In addition, metal parts of a pool system and the plaster finish of a pool may become damaged due to the corrosive action of the acidic water. At a pH above 8.2 - scaling (precipitation of mineral components) and cloudy water may result.

It is recommended that the pH be held within a basic range of 7.4 to 7.6. The addition of muriatic acid (hydrochloric acid) is necessary to lower the pH from a basic condition to an acceptable level. (Note: It is recommended to add the desired amount of acid to a plastic bucket FULL of water before adding to the pool. Acid added directly can stain the pool wall.) The addition of soda ash (sodium carbonate) will raise the pH from an acidic condition to an acceptable level.

Alkaline (basic) substances are present in all water. Within the 7.2-7.8 pH range, alkalinity exists as bicarbonate material. Total alkalinity, as determined with a testing kit, is a measurement of the amount of these substances in parts per million within pools and indicates the water’s capacity to withstand changes in pH. Therefore, total alkalinity is a measure of the buffering ability of pool water. The recommended level for swimming pool water is 80-120 ppm. When pool water is found to be low in alkalinity (below 80 ppm) it is recommended that this level be increased. This can be accomplished with the addition of baking soda using the following formula. (1- l/2 lb. baking soda will raise 10,000 gallons of water 10 parts per million).

Free chlorine is decomposed by the ultraviolet rays of the sun. This decomposition can be greatly reduced by “stabilizing” the chlorine through the addition of cyanuric acid (a crystalline weak acid C3N3(OH)3). The level of cyanuric acid in a pool should be measured with a test kit routinely. The recommended level is 25-50 ppm and levels over 100 ppm will not be accepted. If levels over 100 ppm are found, a portion of the pool water should be removed and fresh water added. The stabilizing effect of cyanuric acid does not increase as the level increases. Instead the chlorine becomes “locked in” at levels over 100 ppm and may not be as effective in killing bacteria and algae.

Often times, pools reach excessive levels of cyanuric acid without the pool operator’s knowledge. This is because much of the powdered chlorine sold today contains cyanuric acid. If this type of chlorine is used over a long period of time, level of cyanuric acid will become too high. Pool operators should check to see if the chlorine they are using contains stabilizer. Powdered chorine that contains cyanuric acid will have one of the following active ingredients:

1. Potassium dichloro-s-triazinetrione (potassium dichloroisocyanurate).
2. Sodium dichloro-s-triazinetrione (sodium dichloroisocyanurate)
3. Trichloro-s-triazinetrions (trichchloroisocyanurate)

It has been determined that bacteria in the presence of cyanuric acid are destroyed at a slower rate. Therefore, when a pool has been stabilized, the free chlorine residual must be maintained above l.5 ppm minimum to offset this phenomenon.

Care should be taken in the handling and storage of all swimming pool chemicals. The materials should be stored separately from one another and should never be mixed, in order to prevent harmful chemical reactions from occurring. (Pool chlorine and pool acid, if mixed, will produce chlorine gas, which if inhaled, could be fatal.) Chemicals should never be added directly into the pool while it is open for use! Time should be allowed for the chemicals to circulate throughout the pool before allowing swimmers to enter.

Note: This rule does not apply to chlorine added to the system through a chlorinator.

It is generally recommended that the pool water be maintained between 78 to 82 degrees Fahrenheit, although this level is not acceptable to everyone. It should be remembered, however, that at levels above this range, chlorine dissipates faster, algae grows better, and the formation of scale (calcium carbonate deposits) is more likely to occur. State regulations do not require the pool water to be heated.