Importance of pH
pH is abbreviation for ‘power of hydrogen’ and is a critical factor in the treatment of pool water. The recommended range for the pH is 7.2 - 7.6. The reason 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.
When you add chlorine to the swimming pool water, chemical reactions start to occur. The chlorine reacts with the water and ends up producing the following two substances:
1) hypochlorous acid
2) 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 18% 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 0.18mg/l, which would be too low for adequate disinfection.
You need to bear in mind that the free chlorine reading 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 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:
- Sodium Bisulphate (dry acid)
- Carbon Dioxide
- Sulphuric Acid
- Hydrochloric Acid
These chemicals serve no other purpose than to bring the pH level back down to between 7.2 – 7.6. They make the disinfectant far more effective, but they do not act as disinfectants directly.