The pH level of swimming pool water has a direct effect on the effectiveness of the disinfectant, typically chlorine, in the water. The ideal pH level of the water is a slightly basic 7.2, and when the water's pH is too high, the chlorine will not effectively kill microorganisms and algae in the pool. A pool's pH fluctuates naturally, but the presence of chemical compounds such as hydroxides, carbonates and bicarbonates in the pool help to hold the pH level steady and keep it within the optimal range.
A pool's pH level tends to rise over time, but alkaline substances in the water act as a buffer to natural pH shifts, and the higher the concentration of these substances, the more resistant the water is to a change in pH. In general, the ideal total alkalinity level, which is a measure of alkaline substances in the water and can be determined using most pool water test kits, is between 80 and 120 parts per million.
When total alkalinity is too high, the pool's water may be cloudy, and its pH will likely drift upward, resulting in ineffective disinfection. When total alkalinity is too low, however, the pool is vulnerable to unstable pH, corrosion and etching of pool equipment and walls, a green tint in the water, and irritation of swimmers' eyes and skin.
Raising Total Alkalinity
The water's total alkalinity level can be raised with the addition of sodium bicarbonate, a compound commonly called baking soda, or sodium carbonate, which is commonly referred to as soda ash.
If you're using sodium bicarbonate to raise total alkalinity, add 1.4 pounds for each 10,000 gallons of water to raise alkalinity by 10 parts per million. Adding 7 pounds per 10,000 gallons will raise alkalinity by 50 parts per million. Sodium bicarbonate raises the water's total alkalinity without significantly altering the pH.
To use baking soda, add a maximum of about 2 pounds per 10,000 gallons, and then retest the water after about 24 hours. If total alkalinity is still too low, make another application and retest, repeating the process until the TA reaches the desired level.
Sodium carbonate will raise alkalinity more dramatically and is cheaper than sodium bicarbonate. Adding just 14 ounces will raise the alkalinity of 10,000 gallons by 10 parts per million, and adding 4.4 pounds will raise the level by 50 parts per million. But sodium carbonate will also raise the water's pH, often to an unacceptably high level, so it should only be used if the pool's pH level is also initially too low.