pH keeps creeping up

[chem geek]

Chuck,

It is true that buffers work more efficiently within 1 pH unit from the pKa, but it is not true that the don't work outside that range. As I said, having the pH near the pKa means that the buffer has capability in BOTH directions of pH swing. The Borate buffer at a pH of 7.5 is mostly all boric acid and not borate ion. That means it is a good buffer at resisting a rise in pH, but it is not as good and has very little capacity at resisting a lowering in pH.

For example, at a pH of 7.5, the Borate buffer has almost 40 times as much Boric Acid as Borate ion. If I have a TA of 80 and 0 ppm Borates, then if I add 1 cup of Muriatic Acid (31.45% HCl) in 10,000 gallons, the pH gets lowered from 7.5 to 7.29 or a drop of 0.21 in value. If I add 3 ounces of lye, then the pH goes from 7.5 to 7.75 or a rise of 0.25 in value.

If I repeat the above but also have 50 ppm Borates, then adding 1 cup of acid goes from 7.5 to 7.37 or a drop of 0.13 in value. If I add 3 ounces of lye, then the pH goes from 7.5 to 7.61 or a rise of 0.11 in value.

So you can see that the Borates do buffer in both directions, but are a better buffer at preventing a rise in pH. If I repeat this experiment starting with a pH of 9.1 which is the Boric Acid pKa adjusted for ionic strength, then with no Borates, 1 cup of acid has the pH drop from 9.1 to 8.96 or a drop of 0.14 while adding 3 ounces of lye has the pH go from 9.1 to 9.20 or a rise of 0.10 in value. With 50 ppm Borates, 1 cup of acid has the pH go from 9.1 to 9.0773 or a drop of 0.0227 while 3 ounces of lye has the pH go from 9.1 to 9.1198 or a rise of 0.0198 in value.

So yes, the buffers would be more effective if near their pKa, but they still work even away from their pKa. So at a pH of 7.5, the 50 ppm Borates with 80 ppm TA improves buffering of acid by 38% and of base by 56%. At the optimum pH of 9.1, the 50 ppm Borates with 80 ppm TA improves buffering of acid by 84% and of base by 80%.

As for the advice of using Borates to raise pH, this is because Borax IS a base (boric acid, on the other hand, is a weak acid). Borax is sodium tetraborate decahydrate. When dissolved in water, the following occurs:

Na2B4O7•10H2O --> 2Na+ + 4B(OH)3 + 2OH- + 3H2O
Borax --> Sodium Ion + Boric Acid + Hydroxyl Ion + Water

then an equilibrium is established between Boric Acid and Borate Ion:

B(OH)3 + H2O <--> B(OH)4(-) + H+
Boric Acid + Water <--> Borate Ion + Hydrogen Ion
(since the pKa is around 9.1, most of what is in the water is boric acid, not borate ion, so this equilibrium is mostly shifted towards the left)

So Borax in water produces hydroxyl ion which is why this is a base that raises pH. However, once in the water and having raised the pH, it no longer has an effect or movement on making the pH change. That was my point. This is unlike the carbonates that DO have an effect or movement on pH just by being in the water in an over-saturated state relative to air. The pool is over-carbonated. So not only do the carbonates and borates both provide a buffer against changes in pH, but the carbonates themselves FORCE the pH to rise because they are out of equilibrium with the air -- there are too many carbonates in the water relative to the carbon dioxide in the air. The fact that the carbonates are a pH buffer is irrelevant. If carbonic acid were a substance that didn't dissociate (which it's not), then the same pH rise effect would still take place if there was too much of it in the water relative to carbon dioxide concentration in the air.

Richard