Evan (waterbear),

While I agree that the test is imprecise because +/- even half a drop can be a somewhat large dosage difference, I say again that the tables have nothing to do with alkalinity and are not based on any "normal" alkalinity range. If there was very little alkalinity in the system and therefore adding a drop of acid made a large color change then in fact this is what would also happen in your pool when adding a relatively small amount of acid -- it would lower the pH quite a bit. So the drop test reflects (in theory) what would happen in your pool.

Now, that said, you are absolutely right about the impracticality of the test due to the errors it can introduce. Not only is their the +/- 1 drop error, but the reagents themselves may be in error and oftentimes the adjustment is rather small so 1 or 2 drops is a very small amount of liquid that can get contaminated (by an unclean dropper tip) whereas most other tests uses 5 drops or more so any such contamination errors are a smaller fraction. At any rate, we agree on the test not being accurate -- we just disagree on the technicality of whether the test is "independent" of alkalinity.

As for the colors in the phenol red test, I don't think I made myself clear. I can obviously tell the difference between 7.4 and 7.6, but I have a hard time knowing if the color I am seeing is really a 7.5 or a 7.4, for example. That is when the extra drops of the acid and base demand test come in handy. Then again, I am using the Taylor complete test and not Ben's test and my understanding is that Ben's test uses a different phenol red. Do you know what the difference is? Is it more concentrated to produce a deeper color? If so, then that would probably help me.

It sounds like I'm too overly concerned with hitting the right pH. The main reason I try to get that parameter right is that a 0.1 shift affects water balance much more than the 10 ppm shifts in other parameters (CH, CYA, TA, TDS) so I really wanted to get that right. Of course, there's a rather large +/- 0.3 leeway allowed for water balance, but I figure with all the errors introduced elsewhere I wanted to at least get the pH parameter right. Anyway, like I say, it's probably overkill.

Thanks for your feedback. I'm getting educated.

Richard