WATER CHEMISTRY FOR SWIMMING POOLS

• Proper water chemistry is essential to maintaining safe and consistent swimming pool operation.
• Chemicals used in swimming pools include:
  • Disinfectants to destroy objectionable organisms;
  • Alkalinity and pH Adjusters to maintain
    • a consistent acid-base relationship
    • acid buffering capacity;
  • Chlorine Stabilizer to prevent unnecessary loss of chlorine;
  • Algicide to kill and prevent algae,
  • Filter Aids to help remove foreign material.
• This page will discuss
  • The various factors which affect water chemistry,
  • How these factors affect swimming pools
  • How to use pool chemicals to restore a properly balanced water chemistry.

• pH is the single most important element in swimming pool water chemistry.
• pH affects every other chemical balance in pool water.
• pH is a measure of hydrogen ion (H+) concentration in water.
• pH indicates the relative acidity or basicity of pool water.
• pH is measured on a scale of 0 (strong acid) to 14 (strong base) with 7 being the neutral pH.
• pH range for pools of 7.4 to 7.6 (slightly alkaline)
  • Is most comfortable to the human eye
  • provides for optimum use of free chlorine while maintaining water that is not corrosive or scale forming.

• To avoid the problems listed above, pH must be maintained between 7.2 and 7.8.
• The most desirable level for pH is between 7.4 and 7.6.

• If pH is too low
  • run alkali demand test if available.
  • Raise pH by adding soda ash (sodium carbonate).
  • Never add more than 2 lbs per 10,000 gallons in a single treatment.
  • Be sure the pump is running when chemicals are added.
  • Allow to recirculate then retest to determine if further treatment is necessary.
  • Caustic soda (sodium hydroxide) is sometimes used with chemical feed pumps to raise pH.
  • If problems with low pH persist, it may be necessary to raise total alkalinity to stabilize the pH.

• If pH is too high
  • run acid demand test if available.
  • pH is lowered by adding muriatic acid (hydrochloric acid) or sodium bisulfate.
  • Carefully add acid at the deep end of the pool.
  • Try not to pour acid near pool walls or fittings.
  • Remember: When using or diluting acids," add the acid to the water" (never add water to acid)
  • 10 lbs. sodium bisulfate is roughly the same as 1 gal. muriatic acid.

• Total alkalinity is closely associated with pH
• Total alkalinity is a measure of the ability of a solution to neutralize hydrogen ions rather than a measure of hydrogen ion concentration
• Total alkalinity is expressed in parts per million (ppm)
• Total alkalinity is the result of alkaline materials including carbonates, bicarbonates and hydroxides - mostly bicarbonates.
• This acid neutralizing (buffering) capacity of water is desirable because it helps prevent wide variations in pH whenever small amounts of acid or alkali are added to the pool.
• Total alkalinity is a measure of water's resistance to change in pH.
• Total alkalinity should be maintained in the range of 80 to 150 ppm.

• In some cases, soda ash can be used to raise total alkalinity.
• Pound for pound, soda ash raises alkalinity 60 percent more than sodium bicarbonate
• Soda ash is cheaper than sodium bicarbiante.
• Soda ash also drastically increases pH.
  • This can cause cloudy water and scale formation.
  • Soda ash should only be used to increase total alkalinity if you also need to increase the pH or if only small increases in alkalinity are needed.

• Chemical manufactures are now marketing a total alkalinity increaser which combines the effects of sodium bicarbonate and soda ash.
• Product names
  • sodium sesquicarbonate
  • sodium hydrogen carbonate
• These products
  • affect total alkalinity more than sodium bicarbonate
  • do not cause quite as much increase in pH as soda ash does.

• Total alkalinity can be lowered by adding muriatic acid or sodium bisulfate.
• Acid may be added in doses of up to 1 quart per 10,000 gallons.
• Total alkalinity tests and further required additions of acid can be made every 2 hours

• Calcium hardness is a measure of the dissolved calcium salts in water.
• Under normal conditions this should not be a problem in properly operated swimming pools.
• Estimates of the proper range of calcium hardness vary widely but the ideal level for plaster pool is generally considered to be about 250 ppm.
• If calcium hardness is very low then water may leach calcium from pool walls causing pitting of the plaster surface.
• Very high calcium hardness may contribute to scale formation and clouding of the water.
• To raise calcium hardness - add calcium chloride.

• To lower calcium hardness anhydrous trisodium phosphate may be used.
• One pound trisodium phosphate per 10,000 gallons will lower calcium hardness 11 ppm.
• Use in small increments or clouding may occur.
• Another method of lowering calcium hardness is to drain off part of the pool water and dilute the remaining water with fresh make up water.

Total Dissolved Solids (TDS)

• After a pool has been in use for a time, dissolved solids may begin to accumulate.
• Normally this is less of a problem with outdoor pools because of rain water and no use during winter months.
• Indoor pools sometimes have a buildup of dissolved solids requiring draining the pool and refilling with fresh water.
• Most pools should be drained after 3 to 5 years.
• Ideally pool water contains under 450 ppm total dissolved solids.

• body wastes
• suntan lotion
• stabilizer
• chlorines
• algicide
• dirt
• pollen
• etc

DISINFECTION OF POOL WATER

• A proper balance of the water chemistry factors will provide water that will not damage pool components and is non-irritating to swimmers.
• It is then necessary to provide for disinfection of the water to
  • prevent the spread of disease organisms from person to person
  • prevent unwanted growth of bacteria and algae in the pool.

• The most common disinfectant for swimming pools.
• Chlorine is a heavy greenish yellow gas which
• Chlorine gas is so toxic that is has been used as a weapon in chemical warfare.
• A number of chlorine compounds have been formulated to provide chlorine in forms that can be handled and used safely by swimming pool operators.

• Other forms of chlorine less commonly used
  • lithium hypochlorite
  • potassium dichloroisocyanuric acid
  • sodium dichloroisocyanuric acid.

• Amount of chlorine in the pool which has not reacted with substances other than water.
• It is the chlorine which is available to disinfect pool water and oxidize organic substances.
• Free chlorine residual should be maintained between 1 and 3 ppm.

• Pool chlorine which has reacted with substance other than water and is no longer available in its free state.
• Some combined chlorines are bactericides but they don't add much to the disinfection process.
• Chlorine combined with ammonia makes chloramines which cause eye irritation and an objectionable chlorine odor.
• For this reason combined chlorine residual should be kept to a minimum preferably below 0.2 ppm.

• The concentration of free chlorine plus combined chlorine.
• To determine combined chlorine residual test for free chlorine and total chlorine.
  

Total chlorine - free chlorine = combined chlorine

• Process which "burns out" combined chlorine and some organics by addition of large amounts of chlorine.
• The reaction of chlorine with ammonia to form chloramines occurs in several stages with free chlorine consumed at each stage.
• If enough chlorine is added to the water the total chlorine residual will rise to a point that forces the reaction of chlorine with ammonia to go rapidly to completion.
• Compounds of nitrogen and chlorine are released from the water and the apparent residual chlorine decreases.
• The point at which the chlorine residual suddenly drops is called the breakpoint.
• When enough chlorine is added to pass the breakpoint, combined chlorine compounds disappear, eye irritation potential and chlorine odors disappear, and the chlorine remaining in the water is all in the free state

• In order to prevent buildup of chloramines, it is necessary to periodically add large amounts of new chlorine in an effort to pass the breakpoint.
• Public swimming pools should be supechlorinated about once a week.
• The amount of chlorine needed to reach the breakpoint will vary depending on
  • the amount of organic material introduced by bathers
  • the level of free chlorine maintained in the pool.
• If the amount of combined chlorine is known then the amount of new chlorine needed is ten times the amount of combined chlorine.
• When combined chlorine residual is not known, superchlorination is accomplished by adding 10 ppm of new chlorine to the pool.
• Ordinarily calcium hypochlorite at a dose of at least 1 lb. per 10,000 gallons is used for superchlorination.
• The chart below shows the amounts of various chlorine compounds which can be used to introduce 10 ppm of chlorine to the pool.

• Several products have been developed which oxidize organics without the use of chlorine.
• Pools which use those products can accomplish the reduction of organics without closing the pool for any longer than it takes to dissolve and distribute the chemicals.
• Those products are more expensive than chlorine but may be preferred where it is necessary to keep a pool open.

• Chlorine reacts with water to form Hypochlorous acid (HOC1).
• The reaction is different for each form of chlorine
• Hypochlorous acid is produced by each of those reactions and is the form in which chlorine serves best as a disinfectant.
• Hypochlorous acid is a weak acid and easily dissociates to an ionized hypochlorite state as shown below.

• Both hypochlorous acid and the hypochlorite ion are counted as free chlorine residual on your test kit
• But only the hypochlorous acid portion is an effective disinfectant.
• The balance between hypochlorous acid and the hypochlorite ion is affected by pH.
  • The higher the pH, the less hypochlorous acid present and the less effective free chlorine becomes.
  • At a pH of 7.2 about 66% of free chlorine is hypochlorous acid.
  • At a pH of 7.8 only about 33% of free chlorine is hypochlorous acid.
• pH control is essential for maintaining the effectiveness of chlorine