Measuring pH Balance in Swimming Pools

In measuring pH balance, advantage is taken of the fact that certain dyes, known as indicators, change their colour in definite and reproducible manner and degree, according to the pH balance of the solution with which it is mixed.

To cover the pH balance range that is likely to be encountered in swimming pools, the following indicators and corresponding discs are available -

The simplest and best test known is litmus paper, which is red in acid solution and blue in alkaline solution. However, to determine to what degree a solution is acid or alkaline, resort is made to different indicators with varying colour changes at different pH balance ranges.

The same Lovibond comparator is used for measuring pH of the sample water.

Indicators for the above may be obtained as a solution, or in tablet form. In the case of Phenol Red, which is the most common indicator used, a dulling screen must be used with the colour disc.

When determining the pH of water containing a high residual of free chlorine, it is necessary to destroy this free chlorine before adding the appropriate quantity of indicator solution. If this is not done, them the strong oxidising power of the free chlorine will alter the colour of the indicator solution and give a false reading. In the case of Phenol Red, a free chlorine residual will gradually intensify the colour and indicate a higher pH.

To destroy the chlorine present it is recommended that analytical grade (A.R.) of sodium thiosulphate be used.

One only small crystal of sodium thiosulphate should be added to the beaker of sample water after the chlorine tests have been carried out, and stirred thoroughly with a glass rod.

As sodium thiosulphate is an alkaline substance, excess should be avoided otherwise a higher reading may be recorded. It is preferable to make up a solution of sodium thiosulphate A.R. grade containing 1.3 grams per litre of distilled water. One drop of this solution will destroy 5 ppm of free chlorine in 15mL of water without affecting the pH.

Procedure for measurement using Phenol Red.

1. Fill one tube with sample water and add to left hand side of comparator.
2. To another cell add 0.5mL of Phenol Red solution from a graduated stopper.
3. Fill up to the 10mL mark with water sample previously de-chlorinated with sodium thiosulphate. Stir with glass rod.
4. Place in right hand compartment of the comparator and record reading.

NOTE: Make sure that the dulling screen has been inserted for the Phenol Red disc.