Quote Originally Posted by kaybinster
It is not possible to tell how much the pH would drop by adding the acid unless you know the alkalinity of the pool. If you recall high school chemistry you may remember something about buffers - they tend to hold pH at their current level and its only when you exceed that level does the pH change. Thus, the higher the alkalinity the lesser impact you will have on the pH.
Kay, you're right, but most people never had high school chemistry, and most of those no longer remember what a buffer is. So, you need to include more info, with an answer like that.

Everybody else . . . Kay's answer is both correct, and relevant. Mike's Calculator has a embedded caution that acid doses are very approximate. But, I've learned -- repeatedly -- that what seems like a large warning to me, is still overlooked by about a zillion of the folks who see it.

The bottom line is that it's not possible to know, from pH alone, how much a given dose of a particular acid or base will change the pH in your pool. More than that, you can't even predict the change accurately if you have BOTH the pH level and the alkalinity level. After all, the pool alkalinity test is ACTUALLY nothing more than a measurement of how much acid it takes to move your pool's water from it's current pH, to a pH of 4.6!

Taylor's K2005/6 testkits included vials of acid and base, so you could perform "Acid Demand" and "Base Demand" tests which -- in theory -- allowed users to accurately determine exactly how much acid or base to add. In theory . . .

But, in practice, those tests caused no end of problems. The problems began with a systematic error in one of the Taylor charts (now corrected -- I forget which one it was). But, there were all sorts of other problems as well. Just to mention one: few people have the capability to accurately measure doses of acid or base BY WEIGHT. But, if you are going to use the AB demand test, that's just what you have to do.

These problems, and others, meant that users who tried to use the AB rarely got it right, and often overshot their target, creating a new problem that had to be corrected!

As a result, the very first change I made in the K2006, when I began modifying it, was to dump the AB tests, and add an extra vial of R-13 reagent.

Folks, I've said it before: if you are selling chemicals to pool owners, you need to be able to calculate one shot treatment doses. Your customer won't want to return for three more tests, and small purchases of, say, acid. But, if you are a pool owner, there is NO benefit to trying to adjust the pH all at once. You should be testing your pool frequently, anyhow -- use small repeated doses, and you'll get there without causing more problems.

Good luck!

Ben

To return to the original poster's ? -- CYA does lower pH, but the effect is usually not noticeable, among results of all the other things that affect pH. The change noted came from the added acid, not the CYA