When chlorine products are added to water they react with it to produce two different compounds. One of these compounds is very efficient at killing bacteria while the second is around 100 times less efficient. The higher the PH of the water the larger the amount of the second, less useful product is produced and therefore the less effective the chlorine is.
The PH level must not be reduced to far as an acidic water is not good for bathers, liners or pool equipment in general even though it produces much more of the useful chlorine compound. Therefore, a PH level of 7.2 to 7.4 is considered to be the best at producing the most amount of useful chlorine from water that is pleasant to bathe in and doesn't damage pool equipment.
PH should be tested every time your maintenance company visits as it can constantly change. The total alkalinity of the water dictates how stable it is and how much the PH will fluctuate. We have come across many pools where the PH is far below acceptable levels which leads us to believe that quite a few maintenance operatives simply chuck in a couple of litres on every visit in the assumption that the PH has risen. If the water is stable then the PH will remain stable and adding litre after litre of acid will just keep on dropping the PH to unacceptable levels.