Basic Water Chemistry for a Pool

Keeping your swimming pool water properly balanced is very important.  Chemical balance in the pool water is essential for health and sanitation purposes.  A wide range of problems can appear if the water is not properly balanced.  These problems can include, but are not limited to, algae growth, scaling, etching, cloudy water and corrosion of the pool surface and/or your equipment.

Check your water regularly to prevent these problems from arising. Using a test kit or test strips check your pool water balance a minimum of  once a week during the swim season and at least every two weeks the rest of the year (unless you close your pool for the winter).   Following are general chemical ppm (parts per million) levels that should be maintained in your pool:

Chlorine/Bromine	1 – 3 ppm
pH	7.2 – 7.8
Alkalinity	80 – 120 ppm
Calcium Hardness	250 – 450 ppm
Total Dissolved  Solids	250 – 1500 ppm

ppm = parts per million

Chlorine is the most widely used sanitizer in swimming pools for disinfecting.  It comes in many forms such as tablets, gas, liquid, granular or a salt generator that produces chlorine in a gas form.  It does not matter which form you use, the same active sanitizer known as “Free Chlorine” is added to the water. This is the form of sanitizer that is responsible for the disinfection of the water.  As the free chlorine combines with the water it mixes with algae, a fungus, bacteria, waste, etc. in the water and becomes “Combined Chlorine”. “Total Chlorine” is the combination of free chlorine and combined chlorine. Total Chlorine can be measured using a pool chemical test kits or test strips. This reading should be between 1-3 ppm.  It is also very important to keep the pH in the proper range (7.2 – 7.8) to allow the chlorine to be the most effective.  Lower pH will be detrimental to the pool surface and equipment, while higher levels of pH will render the chlorine ineffective as a sanitizer.

Bromine and Chlorine are both members of the same chemical family known as Halogens. Bromine has become more popular recently especially in the hot tub and spa application as the hot water temperatures do not affect bromine’s sanitizing abilities.  Bromine is available in three forms: tablets, sticks and caplets.  Bromine is less dependent on the pH than chlorine is to be an effective sanitizer.  Bromine is not an oxidizer, so when using bromine you must also put in an oxidizer or a small amount of chlorine (some bromine tablets have small amounts of chlorine in them – check the label to see if you use bromine).

pH is the term used to measure the degree of acid or base in the water.  It is the most important chemical factor in swimming pool water. pH can be effected and changed by the use of a sanitizer (usually chlorine or bromine) in the pool.   Since the sanitizer is the most frequently added chemical to the pool, it has a powerful impact on the pH.  When pH is low (7.2 or lower) damage can occur on the plaster such as etching, it can also cause damage to the metals in heat exchangers.  Chlorine is much less stable when the pH is low.  High pH (above 7.8) will increase the tendency for scale and cloudy water.  High pH will also reduce the effectiveness of the chlorine, therefore requiring higher levels of chlorine to achieve maximum sanitation, thus more chemical expense.  pH is lowered by adding muriatic acid or sodium bisulfate to the pool water.

Total Alkalinity is the measure of how much of the alkaline substances there are in the water. The main purpose of Alkalinity is to help manage the pH.  When the alkalinity is below 80 ppm, the water can become aggressive and the pH can easily swing too far upward or too far downward. If the value is above 120 ppm the water can become cloudy and form scale and the pH will drift upward.   To lower alkalinity you can use small amounts of acid (liquid or dry). This needs to be done slowly.  If you need to raise alkalinity, sodium bicarbonate can be added to the water.   When the pH is low and alkalinity is high, raise the pH first to normal levels (7.2 – 7.8) then lower the alkalinity.  When the pH is high and alkalinity is low, raise the alkalinity first then lower the pH. To do this in proper order will save you money on chemicals.

Calcium Hardness is the sum of all the calcium dissolved in the water.  Calcium is important since high calcium levels are more unstable if the pH or total alkalinities are above normal levels.  These imbalances can result in cloudy water and/or scale.  If the calcium is too low, calcium chloride (hardness increaser) can be added to the water.  If the calcium is too high the only way to combat this is by diluting the water with water that has lower calcium levels.  An alternative to diluting the water is to minimize the effects of the high calcium with a sequestering agent.  The sequestering agent  will bond chemically with the calcium and other minerals in the water and make them more soluble.  They are still in the water but are less likely to cloud the water or form scale.

Total Dissolved Solids (TDS) are the least worrisome factors in pool chemistry.  TDS is the sum of all materials dissolved in the pool.  There is no low limit for TDS, if TDS gets above 1500 ppm you may want to drain some of the water out of the pool and re-fill.  High TDS may lead to cloudy or hazy water, difficulty in maintaining water balance, reduction of sanitizer activity and foaming.  In a saltwater pool, the TDS may read a little high due to the salt, so this needs to be taken into consideration because it will raise the TDS levels but it will not cause the normal problems associated with high TDS.

**Always refer to the manufacturer’s recommendations for chemical doses when adding chemicals to your pool water.  You should also know how to figure the gallons of water your pool holds: See How to Calculate Pool Size

**Treat all chemicals with respect and care. Always add chemicals to water NOT water to chemicals.  NEVER mix chlorine and acid.