Would pH change over time?

[doncaruana]

I'm trying to make sure I have decent readings and I got a sample in a plastic bottle with a known pH value. Like an idiot, I didn't test it when I first got it and when I did (almost three weeks later), I got a slightly lower value than I was supposed to. Would sitting in the plastic bottle too long do that?

Also, does anyone have any suggestions on where to buy waters with known concentrations (ph, cya, alkalinty, whatever) for this purpose?

 

[JasonLion]

If you have a sample specifically designed to be a calibration solution then it should still be good several weeks later. If you got a sample somewhere else, tap water, pool water, etc, then the PH is likely to have changed. Calibration solutions are specifically designed to have stable readings even after a fairly long time in storage, but other sources of water will not have that properly.

Taylor makes various calibration solutions. TFTestKits.net carries several of them, as do various places on the Internet.

 

[doncaruana]

Thanks Jason for the speedy reply! I got a pH of 7.4 and the bottle was supposed to be 7.57 with a range of 7.5-7.7. Alkalinity read 88 (and 84, second reading) and was supposed to be 80 (range of 75-85). CH was waaay off, but I know the reagents are old and frankly, with my vinyl liner, my learnings here tell me not to worry too much about CH anyway.

This is of course with my "ColorQ", but the big question is: Am I being overly anal about this and these numbers are close enough that I should quit worrying about it?

 

[JasonLion]

I wouldn't worry about it at all. The tests are all only approximate to begin with and you can be +-20% and still do alright.

Only the PH is far enough off to be outside the expected precision of the test. The TA test is +-25, CH +-30 (or +-50 depending how high it is), and so on. PH is +-0.1, so that is the only one that could be considered "wrong", and PH is by far the most likely number to have changed during storage.

 

[doncaruana]

Thanks. I've been rapidly coming to the conclusion that I'm overthinking all this. I guess I needed a sanity check and level set.