The pH level in pools


Pool owners often get confused over pH in swimming pools. I have had customers say that their pool has gone cloudy because the pH is wrong. Whilst this is possible it is very unusual and highly unlikely. As I said in last week’s post about chlorine, the most important chemical for keeping your pool clear and safe is your sanitiser (chlorine or bromine). Most pools go cloudy because they have not got any chlorine or bromine in them or the filter is not working properly.

What is pH?

  • The term pH is a scale of the concentration of hydrogen ions in water.
  • It is expressed as a scale from 0 to 14.
  • The pH test determines if a substance is acidic, neutral or basic.
  • A substance with a pH of 7.0 is neutral, neither acidic nor basic.
  • Those with a pH of less than 7.0 are acidic (orange juice has an acidic pH of 4.2).
  • A pH above 7.0 is basic (ocean water is basic with a pH of about 8.0).

So why do we bother to control the pH of a pool?

The first and most important reason is that chlorine does not work very well at pH above 7.6. Chlorine breaks up into two parts (dissociates) when it is in water. One part kills micro-organisms (hypochlorous acid) whilst the other part does almost nothing (hypochlorite ion). The higher the pH, the less hypochlorous acid there is and therefore the chlorine’s killing power is reduced (see graph below). This is not so important if you are using bromine as it works well between pH 7.2 to 8.2 (I will deal with this in a later blog on bromine).

The effectiveness of chlorine and bromine at the different pH levels.

Secondly, if pH is too low it will corrode pool equipment and also dissolve any grout or cement the water comes into contact with. If the pH is too high it can cause scale to be deposited on the scum line and pool.

Finally, a low or very high pH level can irritate swimmers eyes or skin.

What makes the pH of pool water change?

The pH level varies in a pool, but not as much or as quickly as the chlorine level. The pH level changes due to the different chemicals we put in the pool. For example:

  • Stabilised chlorine tablets (trichloro iso cyanuric acid) are acidic so they reduce the pH. This is why they are a good chemical to use in hard water areas where the incoming water has a high pH.
  • Sodium hypochlorite has a very high pH (12+) so it significantly increases the pH.
  • Calcium hypochlorite has a high pH (11) so it increases pH.

The pH level is also affected by the water added to the pool from the mains supply or by rainwater.

What should the pH level be?

The ideal pH for pool water treated with chlorine is slightly basic – between 7.2 and 7.6. This range protects the pool equipment, and allows chlorine to work efficiently. If bromine is used the pH can be more basic – between 7.2 and 8.2.

How do we adjust the pH?

To reduce the pH level you need to add an acid:

  • Hydrochloric acid –ideally below 12% strength for safety reasons.
  • Sulphuric acid – not recommended as it is such a dangerous material.
  • Sodium bisulphate (dry acid).
  • Carbon dioxide – not for hard water.

To increase the pH level you need to adjust the alkalinity then if necessary add an alkali:

In general it is only necessary to increase pH if you are using chlorine gas or stabilised chlorine tablets.

How do we test the pH level?

A phenol red tablet indicator is used to provide a distinct pH colour reaction which varies from yellow to deep red. It measures pH from 6.8 to 8.4. An alternative way to test the pH level is by using test strips.

Test Hints

  1. If using a pH test kit (phenol red tablet) it is very important to wash out the test apparatus thoroughly. For example, if a phenol red test is carried out after a chlorine DPD1 test the pH will always appear to be low as the DPD 1 is acidic. As a general rule, wash it out with tap water and then with the water that is to be tested.
  2. A reading of 6.8 means the pH level is 6.8 or less (it could be much lower ie 5).
  3. Similarly a reading of 8.4 means the pH level is 8.4 or more.

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