What does it mean when you test your pool water and get a reading higher than pH 7.8? In basic scientific terms, this indicates that your pool is too alkaline. Alkalinity can lead to a number of problems. Most importantly, when the water is too alkaline, the chlorine used to sanitize the pool loses its efficacy. What this means to swimmers is that water that is not kept neutral (between pH 7.0 and 7.8) is not clean and safe enough to swim in.
But why does this happen? There are a number of reasons, which include the following:
When carbon dioxide dissolves in water, it forms carbonic acid. The presence of this in your pool lowers the pH of the water. When you lose carbon dioxide, which happens when the water is disturbed, say by splashing, you also lose carbonic acid and the pH rises.
The chemicals used to sanitize the pool also affect the pH of the water. Granulated or liquid chlorine is alkaline and, therefore, tends to raise the pH level. Bromine is more neutral and has a minimal effect on pH, while chlorine gas is acidic and lowers pH.
Then there are chemicals that can also be added purely to adjust the pH. The most common ones are hydrochloric acid, which lowers pH, and soda ash, which raises it. These are generally used to offset any pH imbalance created by sanitizers. If the balance of these chemicals is not correct, then the pH will reach undesirable levels.
The simple answer is that, when the pool is too alkaline, the thing to do is add something to lower the pH. Hydrochloric acid is usually used for this purpose. Test the water to measure the pH and to calculate the amount of acid you will need to lower it appropriately. It’s all about maintaining balance. Pool water should be tested every day and, if necessary, corrective action taken.
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