Posted by on Saturday, December 17, 2011 Under: Pool Water Chemistry
If your total alkalinity levels are too high, or too low; you will have problems controlling the pH levels. High total alkalinity causes 'pH lock', low total alkalinity causes 'pH bounce'. 

As pool plant operators, we all know that the pH of the swimming pool water is a critical factor. If the pH is too high, the disinfectant properties of the chlorine will be minimised. If the pH is too low, the swimming pool water will be too corrosive. Controlling the pH levels is usually a simple process which involves adding an acid (low pH value) in order to reduce the pH levels.

Sometimes, problems controlling pH occur. One of these problems is called 'pH lock' and the result is that, try as you might, you can't seem to bring the pH down, no matter how much acid you add, the pH levels are locked. This is usually caused by high levels of total alkalinity. By total alkalinity, we're referring to the levels of alkaline salts dissolved in the water and NOT the pH level of the swimming pool water.You measure total alkalinity (or should be) every week, along with calcium hardness, TDS etc. 

Another problem is 'pH bounce'. This is the opposite problem to the one described above. Now, rather than not being able to shift the pH level, you find that it shifts too readily and is extremely reactive; you add a little acid to bring the pH down one minute, and the next, you're adding an alkali in order to get the pH back up.

What's happening here is that either the swimming pool is not 'buffered' enough (pH bounce), or it is 'buffered' too much (pH lock). Buffering solutions are used all the time in chemistry in order to keep pH levels stable. In our scenario, the swimming pool water itself is the buffer solution, but it's important to realise that water by itself doesn't have this buffering effect; it has to be given these buffering properties via the addition of certain chemicals. The chemicals that provide this buffering effect are referred to as the 'total alkalinity family' and are comprised of:

  • carbonates
  • bicarbonates
  • carbonic acid  
Sound familiar? They should. You probably have a chemical called sodium bicarbonate on your site. You would add this if you wanted to increase the total alkalinity. Be careful though, if you don't calculate the correct amount to add you'll either not add enough of the chemical and the pH will continue to bounce (because the water is not buffered enough). Or, if you overdo things and add too much, you'll buffer the water too much and the pH level will be over-stabilised (or 'locked). If this happens, you need to dilute with fresh water. If the source water itself has high total alkalinity levels, you may need to look at switching to a different disinfectant.

In : Pool Water Chemistry 

Tags: "total alkalinity" "total alkalinity family" "sodium carbonate" "sodium bicarbonate" "carbonic acid" "ph" "buffer" "ph lock" "ph bounce"