The recommendations for pools as laid out by PWTAG (Pool Water Treatment Advisory Group) is to keep pH between 7.2 and 7.4 to achieve the best results. This is important to maintain the correct free chlorine levels in the pool.
Chlorine levels are very dependent on pH as the graph below shows:
The graph shows how pH dramatically affects the amount of free chlorine which is dosed. At pH 7, 80% of the chlorine dosed is available and effective, by 7.5 this has dropped to 50%. By pH 8 this is down to 25%, above 8 it drops off to under 20%, and by 8.5 it is less than 10%.
This means that if water coming into the pool is mostly pH 8 or higher then you are only getting 23-10% of chlorine added to the system. This is wasteful and also has the potential to create nasty by-products.
Free chlorine reacts with whatever is available not only bacteria but organic material. The main by-product (chloramines) are created by chlorine reacting with bacteria, skin, hair or other dirt carried on the body. This is a reaction between chlorine and protein that is first degraded to amino acids by the chlorine to then form the chloramines. These produce the classic ‘chlorine’ smell that many people associated with swimming pools, but are in fact a sign of dirty water.
For the best levels of free chlorine, pH should be as low as possible but not below 7.2. pH’s below 7 will lead to other problems such as damage to pool furniture, steps, tiles, equipment by corrosion, breakdown of tile cement etc. This can also be affected by a build up of chlorides in the pool due to insufficient turnover of water, not something that typically affects private pools. This will happen when you backwash the filter to drain and refill with fresh water to replace what has been lost.