The Care & Feeding of Pool Waters
pH

The proper chemical treatment of pool waters depends upon many factors. Some of these factors are inherent to the water used to fill the pool, such as pH and hardness. pH is an inherent condition in water.

 

pH is the way we measure the intensity of the acidic or alkaline character of a water. It is measured on a scale of 0 to 14 Standard Units - 0 is very acidic and 14 is very alkaline (another term for alkaline is basic). 7.0 is neutral.

 

Swimming pool and whirlpool waters should have a slightly alkaline pH, 7.2 to 7.8. The ideal range is 7.4-7.6. Most pools have a test kit using the chemical Phenol Red to measure the pH. This chemical can only read from 6.8 to 8.2. If the reading is near the ends of the readable range (less than 7.0 or more than 8.0) other indicators or a pH meter have to be used to obtain an accurate reading.

 

A high pH can cause turbidity (cloudiness) in a pool, scaling on pool surfaces and clogging up of pipes.  A low pH can cause corrosion and damage to pool surfaces, eating away of pipes and the dissolving of mortar.  Very high or low pH can cause irritation and injury to swimmers. pH also affects the effectiveness of chlorine.

 

When you first fill up your pool the water will show its natural pH. Most waters in New Jersey have a relatively neutral pH, 6.5-8.5. Some waters in south Jersey have even lower pHs. This should be adjusted to 7.4-7.6. You adjust the pH by adding either acidic or alkaline chemicals to the water.

 

The most common acidic chemicals are muriatic acid, a liquid, and sodium bisulfate, a powder. Muriatic acid is a dilute solution of hydrochloric acid and has to be handled with extreme caution. Sodium bisulfate is less dangerous to handle so might be a better choice for smaller pools.

 

The common alkaline chemicals used are caustic soda, a liquid, and soda ash, a powder. Caustic soda is a liquid solution of sodium hydroxide and has to be handled with extreme caution. Soda ash is sodium carbonate and is less dangerous to handle so this, too, might be a better choice for smaller pools.

 

All chemicals should be handled with care. The liquids are especially hazardous and can cause severe burns if they come in contact with the skin or eyes.  Hazardous chemicals should only be handled by qualified personnel.

 

Both liquid chlorine (sodium hypochlorite) and powder chlorine (calcium hypochlorite) will raise the pH. Liquid chlorine will raise the pH more than powder chlorine. Either muriatic acid or sodium bisulfate can be used to neutralize these alkaline compounds.

 

Stabilized chlorines are usually acidic in nature and have to be neutralized. Some brands of stabilized chlorine already have soda ash added to them so they have a neutral pH. You have to read the label to find out what you might be using.  Chemicals should never be mixed together. They should always be added to the water and you should wait until one compound is dispersed in the water before adding another.

 

pH also affects the effectiveness of chlorine in killing bacteria and algae. When free chlorine is present in pool water it exists in two states, in a molecular state as hypochlorous acid (HOCl) and in an ionized state as the hypochlorite ion (OCl-). The hypochlorous acid (HOCl) is a far better bactericide. As pH rises the ratio of HOCl to OCl- falls.

At pH 7.0 the chlorine is 72% molecular (HOCl).

At pH 7.5 the chlorine is 50% molecular (HOCl).

At pH 8.0 the chlorine is 21% molecular (HOCl).

At lower pHs the chlorine dissipates rapidly. At higher pHs it kills the bacteria less effectively because it is mostly in the ionized form.

 

A pH of 7.5 is just right because you have a good deal of HOCl present to kill the bacteria, and enough OCl- to act as a reservoir of chlorine to provide for more as the HOCl is used up killing bacteria.

 

Total alkalinity is a measurement of the amount of alkaline material in the water. It is related to, but not the same as pH. It is an indication of the buffering capacity of a water. This is the capacity of water to consume acid compounds without a large shift in pH. The total alkalinity of pool water should be about 80-150 parts per million (ppm), although in some instance you might want up to 200 ppm.  You can increase your total alkalinity by adding an alkaline substance such as caustic soda or soda ash. These will raise both your pH and alkalinity.

 

In some cases you might want to raise just your alkalinity and not your pH. You can do this by adding sodium bicarbonate, which is also called baking soda. This is not usually needed in waters in New Jersey and should only be done after consulting with a knowledgeable chemist.  Sodium bicarbonate is only needed when you want to raise the alkalinity and not the pH.  We do not recommend adding baking soda to your pool on a regular basis, as adding too much baking soda can do damage to the pool.

 

The pH and free chlorine of pool water should be tested once every 2 hours that the pool is in operation. Changes in pH should be made slowly to avoid shooting past the desired pH.  Phenol red liquids do expire, so only use fresh reagents if you are using a liquid testing kit. The phenol red tablets should be good for at least 1 year if kept in the foil wrap.

 

Please see our webpage on the comparisons between liquid and tablet test kits.