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.
In theory, but it does not hold out in actual practice much of the time.
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.
The K-2005 (DPD for chlorine) and K-2006 (FAS-DPD for chlorine) use the phenol red r-0004 which is for use with a 44 ml sample so the acid/base demand tests can be done. Ben's kit (and the Taylor K-1000) use a small comparator tube that looks to be about 9 ml, although I have not measured it, and uses reagetn R-0014. I have both comparators and both reagents and often cross check and the colors produces are almost identical. I assume the only difference is in the concentration of the reagent so the same color intensity will be produced in different sized samples. Taylor also makes other phenol red reagents for use with their other comparators. I am not familier with them but the comparators use liquid standards (much like the LaMotte high end comparators do) and supposedly provide a much more accurate color match.
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.
If you are so concerend about precise pH measurements then perhaps a pH meter might be the way to go. Personally I find them a pain in the a** to use. When I was in High School I was in a special program where I went to school half a day and spent half a day doing research at the University of Miami School of Marine and Atmospheric Science in oceanographic physical chemistry (yeah, I was one of those 'brainy kids') on the nucleation and precipitation of calcium carbonate from various simple seawater like solutions, real seawater samples and Copenhagen standard seawater to investigate how various ions would affect the precipitation time and the form that precipitated. A pH meter was an integral part of the equipment and it seemed I spent more time doing three point calibrations, cleaning the electrodes with conc. H2SO4 with KMnO4 dissoved in it, and replacing defective electrodes than running samples! (Of course I also had to prepare my own buffers and standards). I have never really cared for using pH meters since! I have played around with some of the inexpensive 'pocket meters' with my aquariums and, while they do produce accurate results, they still suffer from the need for constant calibration and making sure the electrode does not dry out that I find them more trouble than they are worth! Perhaps they would work for you.
Thanks for your feedback. I'm getting educated.
Richard
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