WATER CHEMISTRY FOR SWIMMING POOLS

pH

pH is the single most important element in swimming pool water chemistry. It affects every other chemical balance in pool water.

pH is a measure of hydrogen ion (H+) concentration in water. It 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.

In pools a slightly alkaline pH of 7.4 to 7.6 is most desirable because this range is most comfortable to the human eye and provides for optimum use of free chlorine while maintaining water that is not corrosive or scale forming.

If pH is too low (below 7)

• Water becomes acidic
• Chlorine residuals dissipate rapidly
• Eye irritation occurs
• Plaster walls are etched
• Metal fittings, pump impeller, heater core may corrode
• Dissolved metals may leave stains on walls
• Rapid Loss of alkalinity

If pH is too high (above 8)

• Chlorine activity is slowed and inefficient
• Scale formation and discoloration of pool walls
• Water becomes cloudy
• Filter is overworked
• Eye irritation may occur

pH Adjustment
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.

 

	GALLONS IN POOL
pH	1,000	5,000	10,000	15,000	20,000	25,000	50,000
7.2-7.4	2/3 oz.	3 oz.	6 oz.	9 oz.	12 oz.	1 lb.	2 lbs.
7.0-7.2	3/4 oz.	4 oz.	8 oz.	12 oz.	1 lb.	1 1/4 lbs.	2 1/2 lbs.
6.6-7.0	1 1/4 oz.	6 oz.	12 oz.	1 lb.	1 1/2 lbs.	2 lbs.	4 lbs.
Under 6.7	1 1/2 oz.	8 oz.	1 lb.	1 1/2 lbs.	2 lbs.	2 1/2 lbs	5 lbs

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," do as you oughta, add the acid to the water" (never add water to acid)
NOTE: 10 lbs. sodium bisulfate is roughly the same as 1 gal. muriatic acid.

Total Alkalinity (TA)

Total alkalinity is closely associated with pH but rather than a measure of hydrogen ion concentration it is a measure of the ability of a solution to neutralize hydrogen ions. 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.

If total alkalinity is too low:
pH changes rapidly when chemicals or impurities enter the water. pH may drop rapidly, causing etching and corrosion.

If total alkalinity is too high:
pH becomes difficult to adjust. High pH often occurs causing other problems such as; cloudy water, decreased disinfectant effectiveness, scale formation and filter problems.

Raising total Alkalinity - Total alkalinity can be raised by the addition of bicarbonate of soda (sodium bicarbonate, baking soda). 1.4 lbs. bicarbonate of soda per 10,000 gallons will raise total alkalinity 10 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 and is cheaper than sodium bicarbiante. The problem with using soda ash to increase alkalinity is it 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. The product sodium sesquicarbonate or sodium hydrogen carbonate affects total alkalinity more than sodium bicarbonate, but does not cause quite as much increase in pH as soda ash does.

Lowering total Alkalinity - 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:

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.

Because of the special hazards associated with the use of gaseous chlorine, its use has been prohibited at public swimming pools in North Carolina.

Calcium Hypochlorite: granular or pelletized 65% available chlorine

Advantages:	Disadvantages:
Relatively cheap	Not stabilized - may lose strength if not tightly covered.
Can be mixed into solution for feed pumps	Does not dissolve completely - leaves residue
Can be used in some specially designed erosion feeders	Does not dissolve completely - leaves residue
	High pH (11.7) raises pH of pool
	Highly reactive - may cause fires

WARNING - do not use in closed tablet feeders designed to use other forms of chlorine

Sodium Hypochlorite: Liquid bleach 12.5% available chlorine

Advantages:	Disadvantages:
Next to gas is the cheapest chlorine available	Bulky and heavy
No dissolving required - no residue	Not stabilized-loses strength rapidly
Can be used with chemical feed pumps	High pH (10-13) raises pH of pool

Trichloroisocyanuric Acid: sticks or tablets 90% available chlorine

Advantages:	Disadvantages:
Stabilized - chlorine doesn't dissipate	Cost slightly higher
Easy to handle	Lowers pH - pH 2.8
Low cost, low maintenance erosion feeders	Lowers total alkalinity
Highly concentrated - 90% available chlorine	May elevate cyanuric acid levels
Dissolves completely - very little residue	Not suitable for supeprchlorination

Other forms of chlorine less commonly used are lithium hypochlorite, potassium dichloroisocyanuric acid, and sodium dichloroisocyanuric acid.

Free Chlorine Residual is the 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.

Combined Chlorine is chlorine in the pool which has reacted with substance other than water and is no longer available in its free state. Some combined chlorines are bactericides but they contribute little to the disinfection process. Chlorine combined with ammonia produces 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.

Total Chlorine residual is the concentration of free chlorine plus combined chlorine. To determine combined chlorine residual test for free chlorine and total chlorine.

Breakpoint Chlorination is the process by which combined chlorine and some organics are "burned out" of the pool 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.

Superchlorination: In order to prevent buildup of chloramines in the pool 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 and on 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.

Suggested NSPI Standards - Spas

ALGAE CONTROL

Algae are tiny plants that bloom and grow in swimming pools if nutrients are present and a sufficient level of free chlorine is not maintained. Below are descriptions of the three most common algae problems in swimming pools.

Green Algae The most common algae in swimming pool floats in water and coats pool surfaces. Left unchecked green algae will very quickly turn the pool water pea green.

Mustard Algae settles on pool walls and causes a slimy yellow film.

Black Algae appears in "buds" or clumps attached to tile grout, corners, steps and pool surfaces.

Solution:
Green Algae - is very susceptible to chemical treatment. Superchlorinate with 10 to 20 ppm chlorine in the evening. Keep the filter running and brush the pool walls and bottom. Periodically check chlorine and maintain above 3 ppm until water clears. Using an algicide containing quaternary ammonia the next morning will help prevent the return of green algae.

Mustard Algae - is much more resistant to chemical treatment and clings more tightly to pool walls than green algae. Adjust pH and superchlorinate as for green algae then brush diligently. Later vacuum the pool, check chlorine and superchorinate again if necessary. Mustard algae will generally return unless treated with a special mustard algicide or a copper based algicide. Algicide should be added in the morning to treat algae in daylight - its most active period.

Black Algae - is very difficult to get rid of. It can be controlled to some extent by frequent superchlorination and diligent brushing with a stiff brush. Spot treatments can be made by turning off the recirculation pumps and pouring granular chlorine directly on recently brushed spots. Trichlor tablets can also be rubbed on recently brushed areas to spot treat. Black algae can usually be controlled with the use of strong alicides and maintenance of relatively high free chlorine residual, but complete removal of black algae may require draining and cleaning the pool.

Note: Algae blooms are a problem best avoided. Maintaining proper water quality and frequent brushing of pool walls will deprive algae of the opportunity to get started.