Tech Talk: What is this Alkalinity? Part II
What do we use for this necessary weak acid in the pool? We all agree that it wouldn't be too nice to swim in a pool containing vinegar or a fertilizer. We want something that is odor-free, inexpensive, and without negative or harmful side effects. Mother Nature complies very nicely by providing CO2 and carbonic acid – something that fulfills all of the above requirements.
Let's first look what happens when you dissolve the colorless and odor-free gas carbon dioxide (CO2) in water. More than 90% if the gas simply dissolves, just as sugar dissolves in hot tea. But some of the CO2 reacts chemically with the water:
CO2 + H2O => H2CO3 (called carbonic acid) |
If you put a lot of CO2 into the water (this natural gas dissolves quite easily), you get carbonic acid at a pH of approximately 4.5. If that number sounds vaguely familiar, it's because you might remember that soda-pop has a pH of 4.5. Why? Because it contains a lot of CO2.
The graph shows the dissociation of carbonic acid. Because H2CO3 has two hydrogen atoms (it is a di-protic weak acid), we have three curves. The left one shows that up to a pH of 4 the acid is not split up at all; it is undissociated. Starting at pH 4, it gradually - with rising pH - lets go of one hydrogen atom:
The anion HCO3- is called the bicarbonate ion. If that one sounds familiar it is because you know that to increase the alkalinity you add "bicarb" (sodium bicarbonate: NaHCO3) to the water, where it dissolves into Na+ and HCO3-. We reach the halfway mark at a pH of 6.35; 50% of the carbonic acid is in the original form, while the other 50% is floating around as bicarbonate.
If we continue to increase the pH, the second "H" is reluctantly released as well.
HCO3- => H+ + CO3-2 CO3-2 is called the carbonate ion.
This second dissociation starts just above pH 8. We'll come back to the importance of this point (relating to scaling) at a later time. At pH 8.5 the original carbonic acid is completely gone, and all that's left are bicarbonate and carbonate ions. pH 10.33 is the 50-50 point for those two, but that's already way above of what the pH of your pool water should be therefore of no interest to us.
In indoor pools and un-stabilized outdoor pools, the so-called total alkalinity is all based on carbonic acid and bicarbonate – there are no other alkalinity contributors at all. To be proper it should, really, be called bicarbonate alkalinity.
Here's one to remember: Of all the virtues and critical assignments attributed to a pool's total alkalinity that you've ever learned from pool schools or books or test-kit pamphlets, the pH-control factor described above is the only legitimate one. All the rest are, well, baloney x 102!
Now if you use cyanuric acid as a stabilizer in your outdoor pool (and ONLY in an outdoor pool!), CYA, too, contributes to the total alkalinity. The rule of thumb that you can use to calculate your bicarbonate alkalinity is as follows:
| Bicarbonate Alkalinity = Total Alkalinity - 1/3 (cyanuric acid in ppm) |
The total alkalinity is what you measure with your test kit. If there is no CYA in the water, your total and your bicarbonate alkalinity are the same. To calculate the calcium saturation index (Langelier Index), you must use only the bicarbonate part of the total alkalinity. This again has to do with scaling, which will be discussed in the next column – after we've all had a brew.
~wh
© 2002 Professional Pool Operators of America
