Odd pH Base Demand Test Behavior

[Shocking]

Recently purchased a Taylor K-2006 Test Kit, and can't say enough good things about it. As a newbie, I feel like I have a million questions, but the one thing I don't have to wonder about is the quality and consistency of test results.

Tried testing my make-up water today. TH (30 ppm) and CH (20 ppm) tests behaved as expected. The pH testing was a bit strange. The pH test itself is consistently off-scale low, which on this kit means less than 7.0. i.e., the color is bright yellow. Then when I attempted the base demand test, the solution would become bright pink when I first added the base demand solution -- appearing off-scale high -- but after a minute it would calm back down and the color would be in range. I have not noticed this behavior with the pool water -- solution goes more directly to the in-range color without initially appearing so bright pink.

Is my make-up water base-demand test accurate? Perhaps the make-up water is too far outside the intended 7.0-8.0 range?

Just want to make sure I'm interpreting the test correctly.

Thanks!

 

[Richard320]

Low TA can make the pH adjustment quite volatile. And really, that's all you're doing with the acid and base demand tests - adding known strength acids and bases to get pH in the right range.

It doesn't behave that way with pool water, right?

 

[Shocking]

Correct. With pool water it behaves as expected. For example, a couple days ago pH was low and I did a base demand test.

FC 3.6 ppm
CC ~0
pH 7.1, 3 drops base demand
TA 60 ppm

Base demand test behaved as expected. As I understand it, TA = 60 ppm is low but it's not as low as the make-up water test where TA = 30 ppm. So your TA idea might explain the odd behavior in the make-up water test.

Oh, wait, I think I see what you mean! TA is all about stabilizing pool pH, but in the microcosm of the test vial it's also stabilizing the pH test itself. That would explain the temporary pH spike. Hmmm. Well, after a minute or two it settles back down to an appropriate color. And you're helping me see that that color reading is indeed valid -- I just have to be patient and let it complete its reaction.

Thanks!