pH is an abbreviation of ‘power of hydrogen’ and is a critical factor in the treatment of pool water. pH is a scale of acidity to alkalinity from 0 to 14. Substances that aren’t acidic or alkaline (that is, neutral solutions) usually have a pH of 7. Acids have a pH that is less than 7. Alkalis have a pH that is greater than 7.
The recommended range for the pH level to be maintained at is 7.2 – 7.6. The reason that the pH level needs to be kept between these values is that the disinfection efficiency of chlorine falls off significantly at higher pH levels and the coagulant will also not be as effective. At lower pH values, the pool water will be too corrosive.
The effect of the pH level on the disinfection process is an area that many pool plant operators fail to fully understand. Therefore, they don’t take the correct actions and end up with low quality swimming pool water and an excessive yearly spend on chlorine. Let’s take a look at what’s going on with pH and chlorine.
The blue line in the graph below represents the percentage of active disinfectant in the chlorine at different pH values. As you can see, the higher the pH value, the lower the percentage of active disinfectant.
When you add chlorine to pool water it reacts and ends up producing the following two substances:
- hypochlorous acid
- hypochlorite ion
The key disinfectant in chlorine is hypochlorous acid, which is about x100 stronger than the hypochlorite ion, so that’s what we want more of.
The higher the pH level, the higher the proportion of hypochlorite, the lower the pH level, the higher the proportion of hypochlorous acid.
At a pH level of 7.5, you’ve got about 45% of the chlorine as hypochlorous acid, so if your free chlorine reading was 1.0 mg/l when tested, in real terms the amount of active disinfectant would only be around 0.45mg/l. If the pH level was allowed to get to 8.0, then only 25% of the chlorine would be hypochlorous acid, so if the test reading came out at 1.0 mg/l again, the actual amount of active disinfectant would only be 0.25mg/l, which would be too low for adequate disinfection.
You need to bear in mind that the free chlorine reading you get with the DPD1 tablet test includes both the hypochlorous acid and the hypochlorite, but it does not tell you the proportion of each. This is why it’s so important for pool plant operators to understand how and why the pH levels have such a dramatic effect on the disinfection process.
Because both calcium hypochlorite and sodium hypochlorite (the most commonly used chlorine disinfectants) are very high on the pH scale, the pH of the pool water will be pushed upwards when these disinfectants are dosed into the pool. As explained above, this would mean that the effectiveness of the disinfectant would be reduced. What needs to happen is a chemical needs to be dosed that is low on the pH scale (i.e., an acid). Some chemicals that are commonly used for this purpose are listed below:
A highly acidic substance that converts to sulphuric acid when dissolved in water. Supplied as a dry powder and often referred to as ‘dry acid’.
Supplied as a gas and stored on site in several gas bottles or a large bulk container. Converts into carbonic acid when dissolved in water. Is an asphyxiant, so CO2 detectors should be installed to alert staff of leakages.
Liquid substance that is very acid. This is what sodium bisulphate becomes when mixed with water.
Another liquid substance. Very acidic and tend to give off fumes. Not as common as the other pH reducers.
All the above substances will react strongly with chlorine and alkalis in general as well as with oxidising substances. The result can be the production of a highly toxic chlorine gas.
Storage and handling arrangements should be such that the risk of accidental mixing of incompatible substances is reduced to as low a level as is reasonable practicable, or, even better, eliminated completely.