As we have seen pH measurement and control is essential in any pool or spa to maintain the value within the desired range. For heavily used pools, the pH value should be measured continuously and adjusted automatically, for other pools it is sufficient to measure the pH value regularly and adjust it if necessary.
pH measurements in these cases are by colorimetric indicator and the one which is used world-wide is Phenolred.
This has a good colour change from yellow to red over the pH range 6.8 – 8.4 which makes it ideal for the monitoring of pool and spa water, which should be around the middle of this range.
Testing can be carried out with phenol red tablets or liquid, but in the case of the latter it is necessary to use a separate dechlorinating/debrominating liquid to prevent the disinfectant reacting with the indicator and changing it’s colour. The tabletted reagent has this dechlorinator/debrominator as an ingredient in the formulation. Phenol red in tablet form is also much more stable than the liquid and is easier to use:
A freshly rinsed cell is filled to the 10 ml mark and a phenol red tablet is added.
This is crushed and mixed thoroughly to dissolve, using clean stirring rod.
The colour produced is matched either visually or in a photometer to give the pH
value of the sample.

Note. If the colour produced is purple when testing a brominated water sample this is usually an indication that the bromine concentration is above 10 mg/l.

For disinfectants based on chlorine to work properly and efficiently the pH value of the pool or spa water is critical. The normal recommendation is that the pH value should be maintained at between 6.8 and 7.6 (EN 16713).
For pools and spas using bromine-based disinfectants a wider pH range is acceptable (6.8 to 7.8). This is due to the fact that the efficiency of the disinfection is maintained over this range.