pH color changes with time

[Richard320]

This doesn't happen every time, but often enough that it makes me wonder what's going on.

Yesterday, FC = 9, pH=7.5+ a little. Added 2 cups MA, pH should have gone down to 7.2

Today I scooped up my sample, and checked pH. Perfect match to 7.2. I would have expected it to be just a little above 7.2, knowing the way my pool runs. FC= 8. Go to empty the two tests out and pH is now 7.5. Not even 2 minutes has elapsed, all I did was the FAS-DPD test.

I could see things changing if it sat in the sun for an hour or something, but not that fast. And as I said, it has happened before a few times. When it does happen, I treat it as the later reading, and so far, it has reacted the way it should. The reagent is fresh, no more than a couple months old.

Does this happen to anyone else? Does anyone (chemgeek??) have a clue why it is so?

 

[JasonLion]

Various things that can shift the PH happen far far more quickly in the vial than they do in the pool. CO2 and chlorine both outgas far more quickly and the contents of the vial are much more sensitive to sunlight.

 

[tlwilson]

It happens to me too, especially when I put water in the sample bottle. If I dip the testing tube in the water and rinse it really well, then dip it down into the water the length of my arm, the test is pretty consistent. When I use the sample bottle, the first time I get 7.2 to 7.4 and then subsequent tests I get 7.6 to 7.8. I'm getting tired of testing twice and using double the reagant every time I test pH, so I've started dipping directly out of the pool.

 

[Vette]

I actually find it easier to use the bottles!

 

[JasonLion]

This is the reason it is important to do the PH and FC tests first when using a bottle of water from the pool. PH and FC will both tend to start to drift as soon as the water is removed from the pool. You want to test them right away before the drift becomes significant.

 

[linen]

If I may hijack Richard320's thread just slightly...this brings up a question I have had.

When using the K-1000, it is recommended to read the ph first when doing the tests together. Is this because the OTO test needs a little time to stabilize (react with any CC that is present), is it because ph can start changing from it's real value if it sits (quicker than the OTO test), or something else?