pH - A value for the relative acidity or basicity of a substance such as water, as indicated by the hydrogen ion concentration. Expressed on a scale of 0 to 14, 0 being most acidic, 7 being neutral and 14 being most basic.
The pH is a measure of the active acidity in pool or spa water. Technically it is a measure of the concentration of hydrogen ions in the water. The greater the concentration of active hydrogen ions, the lower the pH.
Carbon dioxide (CO2) in the form of carbonic acid (H2CO3) tends to lower the water's pH. But carbon dioxide is pushed out of the water as the water's temperature rises. Jets and blowers in a spa also strip carbon dioxide out of the water. As the acid concentration is reduced, the pH rises.
In a pool or spa, the water's pH should be maintained between 7.4 and 7.6, but should never exceed 7.8 or fall below 7.2. When pH rises above 7.8, especially when combined with high total alkalinity and high calcium hardness, the water can become cloudy and calcium carbonate scale can form on filtration and heating equipment, in sand filters, and on pool surfaces. Under these conditions, chlorine sanitizers are less effective in killing bacteria and algae.
On the other hand, when the water's pH drops below 7.2, especially when combined with low total alkalinity and low calcium hardness, the water becomes corrosive, and can be damaging to vinyl and plaster surfaces, as well as pipes and equipment. Low pH can also result in eye and skin irritation for bathers.
In addition, a proper pH level contributes to water balance.
When pH is too high, add acid to increase the hydrogen ion concentration, and thus lower the pH. When pH is too low, add sodium carbonate (soda ash) to decrease the hydrogen ion concentration, and thus raise the pH. Note that sodium carbonate is not the same as sodium bicarbonate (baking soda).
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