The scientific definition is 'the negative logarithm of the Hydrogen ion concentration'. OK - don't give up - its a very important concept. Lets get through to that definition in every day terms.
First accept pH is a scale measuring the acidity or alkalinity of a solution. Squeezed lemon and vinegar are sour or acidic. If we drank them we would take something alkaline like bicarbonate of soda or magnesia to neutralise the acidity in our stomachs - in other words raise the pH. The pH scale runs from 0 (highly acidic) to 14 (highly alkaline) with distilled water being neutral at pH 7.
Now instead of sour, lets use the term 'hydrogen ions', and instead of alkaline lets use the term 'hydroxyl ions'. Vinegar has many more hydrogen ions than hydroxyl ions. Conversely, soda ash and bicarbonate, being alkaline, possess more hydroxyl ions than hydrogen ions. In summary, acids produce hydrogen ions, alkalis produce hydroxyl ions. pH is the power (German 'potenz') of a solution to yield hydrogen ions [H+].
One more step to go. The scale between 0 and 14 is logarithmic (pH 8 is 10 times more alkaline than pH 7 and pH 9 is 100 times more alkaline than pH 7)
Now we are back to the scientific explanation of pH as 'the negative logarithm of the hydrogen ion concentration'. Negative because the more hydrogen ions, the lower the pH.
The pH value affects the amount of hypoclorous acid (free available chlorine) that is formed, and therefore determines the effectiveness of the chlorine as a killer of bugs.
Unfortunately you cannot run your pool at pH 6.5 - it would acidic enough to corrode the metal fittings in your pool circulation system and it is too far from the human body's pH of 7.4 to be comfortable to bathe in. The compromise is 7.2 to 7.6, preferably midpoint of 7.4. Remember, if you let the pH drift out of this range, you will have to use more chlorine to get adequate disinfection.
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