The Total Alkalinity Family

Posted by on Saturday, December 17, 2011 Under: Pool Water Chemistry
If you carry out your weekly pool water balance test and find that the total alkalinity levels are low, you're going to need to add some sodium bicarbonate. This chemical will introduce some alkalinity into your pool without having too much of an effect on the pH level. The sodium bicarbonate that you add will go through various chemical reactions and some of it will convert into different chemicals according to what the pH level is at any given time. If the pH is high, some of the sodium bicarbonate will convert to sodium carbonate. If the pH is low, then some of the sodium bicarbonate will convert to carbonic acid.

So, when we're dealing with total alkalinity levels and its relationship with pH, we're actually dealing with three chemicals, not one, because some the chemical you add to alter the total alkalinity (sodium bicarbonate) will change into either sodium carbonate or carbonic acid, depending on what the pH level is. These three chemicals are collectively referred to as the alkalinity family.

This chemical reaction process is important for pool plant operators because it is this process that has the stabilising (buffering) effect on the pool water's pH level. Without this chemical reaction going on, the pH levels would be very difficult to control and would either be swinging from low to high and back again (pH bounce), or won't budge at all (pH lock). What's happening is that hydrogen ions are either being released, which will cause pH levels to come down (because hydrogen ions are acidic), or they are being mopped up, which will cause the pH levels to rise.

Let's look at a practical example to make some sense of this:

pH is high, so you add some acid to bring it down. But then it goes down too low, so you add some sodium carbonate (after figuring out ho much to add; download manual dosing calculator) to try and bring the pH up again. But then it goes too high again. 

The pool water is not 'buffered'. This is because the pool water does not contain enough alkalinity. Without enough alkalinity, the release, or mopping up process described above is not happening. If there was enough alkalinity, the pool water would be buffered and what would have happened when you added the acid in order to get the pH down is:

The sodium carbonate would have 'mopped up' some  of the acidic hydrogen ions as it went through the chemicals process described above.

Get that pool water buffered! Think of unbuffered pool water as being an extremely reactive substance. Get some sodium bicarbonate in there in order to 'put the breaks on' as it were and give you some control back. If you get the total alkalinity levels back into the correct range (80 - 120 if using calcium hypochlorite, 120 - 150 if using sodium hypochlorite), you will find that the pH levels aren't as reactive and are therefore easier to control.

In : Pool Water Chemistry 

Tags: ph "total alkalinity" "the alkalinity family" "sodium bicarbonate" "sodium carbonate" "carbonic acid"