Acid/Base Demand Test

[waterbear]

Evan (waterbear),

The acid and base demand tests actually do not make any assumptions about the alkalinity which is why such tests are in fact reasonable. The tests simply add real acid or base to your existing pH measurement sample and you note the color change. The tables simply convert the number of drops into the amount of acid or base needed in scaled-up pool volumes.
In actual practice I have seen these tests overdose and underdose pools. If the TA is not a factor coming into play then it is the tables themselves introducing error when trying to change a minute quatity such as a drop into pints and pounds of chemical. These demand tests, in my experience, are not very worthwhile. I do use them when testing water and usually recommend putting in half of the amount suggested and retesting. In the majority of cases (at least with lowering pH) this is sufficient if the TA is not way out of line. If TA is low I even lessen the amount and if it is high I recommed retesting and reajusting pH until the goal is reached!
If your alkalinity is very high, then adding the drops will only make the color change a little bit because the pH won't move much. If the alkalinity is very low, then the drops will make the color move a lot.
My point exactly. Lower TA values will make pH shift quickly and high TA values will keep pH from changing and then it changes very fast with just a small addional amount pH adjusting chemical added. I am not sure but I believe the tables are based on a 'normal' alkalinity range and since an imprecise drop in a very small sample is being extrapolated into a sample of thousands of gallons of water any error introduced is magnified greatly!
The tables have nothing to do with alkalinity and are simply volume conversion tables from your drops into the amount needed for pool volumes (plus a conversion to the type of acid or base you use if that is different than what is in the drops).

I have a spreadsheet that does the calculations for me so I don't use nor need the acid/base demand test for figuring out how much acid/base to add. However, I do use the drops to make the color move when I'm in that hard to discern range of pH between 7.4 and 7.6.
The colors produced by phenol red are very distinct and a comparator is not needed at all if your are familiar with them. In fact, it is very easy to differentiate between a pH of 7.4 (orange color) and a pH of 7.6 (pinkish red color) without a comparator!
The phenol red color range is as follows:
purple 8.2 or above
purplish red 7.8- 8.0
red (actually pinkish) 7.6
orange 7.4-7.5 (this is the color to shoot for when adjusting pH)
yellow orange 7.0-7.2
yellow 6.8 and below
If you use your demand reagents to take the indicator through all of its colors (IMHO, the best use for them) you wil soon learn to recognize them without the comparator scale, especially the orange color of pK[a] at pH of about 7.4 for phenol red (sorry, there are no subscripts so I substituted brackets.) (Actually, I believe the actual pK[a] for phenol red is 7.3, certainly close enough for balancing a pool!) You can even double check with a pH meter but you will find if you do the colors of the indicator and pH are extremely close, if not spot on, to what I say above!
In actual practice my customers have done much better at maintaining a proper pH in their pools when I tell them to just make the color orange with their inexpensive 2 way (oto and phenol red) testers! We use a LaMotte Waterlink Express colorimeter at work and when I test their water with it I find that they are in the range of 7.4-7.5.
By adding a drop one way and then a drop or two the other way, I can verify that I am indeed straddled between two of the colors.

Richard

Understand that I come from the point of view of actually adjusting pH in pools, my own and my customers who tend not to be as versed in chemistry as I am.