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What is pH control in the context of Swimming Pool Water Quality?

pH is an abbreviation from the words “potential(or power) of Hydrogen” and is simply a measure of the number of hydrogen molecules that have been dissolved into a litre of liquid.

In numerical terms there is a very big range of hydrogen ion concentration and so the more user friendly logarithmic scale called pH is used to measure this solution quality.

 It always lies between the numbers 0 and 14 and is normally calculated to one place of decimals. At a pH value of 7.0, water is neither acidic nor alkaline and is “Neutral” and is the pH scale equivalent of 1 divided by 10 to the power of 7. It is only water that has a Neutral value of 7 because other liquids will be neutral at different pH levels.

What does pH measure?

So a pH below 7.0 is a measure of the total acidity of the water and will include all the various types of acid that are present. It simply represents the capacity of the acidic water to react with an alkaline solution. Similarly total alkalinity represents the capacity of alkaline water to react with an acidic solution.

What are the Practical Implications with regard to the Management of pH levels in Swimming Pool water?

So what are the practical implications of all this insofar as pool water is concerned?

The pool water needs to be maintained in a slightly alkaline state for reasons explained below, If the alkalinity is too low the pH levels become very volatile and it is difficult to maintain a stable pH value and this is important because wide variations in pH whenever small amounts of acid or alkali are added to the pool. If the alkalinity is too high the water becomes cloudy and carbonate deposits occur. Total alkalinity is best when between 80 and 150 parts per million.

pH and its effect on swimming pool chlorination

When Chlorine is added to water it forms hypochlorous acid and hypochlorite. The hypochlorous acid is known in the pool industry as free chlorine and it is this “free chlorine” that carries the disinfection of the pool water.

When the pH value of water is 5.5 all the chlorine added to water turns into “free chlorine” but at this level the water is far too acidic to swim in.

At a pH level of 7.0 about 80% of the added chlorine turns into “free chlorine” – but algae growth can also occur.

At a pH level of 7.2 about 50% of the added chlorine turns into “free chlorine” and this is also the pH level of our eyes.

At a pH level of 8.0 about 20% of the added chlorine turns into “free chlorine”  and so 5 times as much chlorine is required as for a pH of 5.5.

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