Why does my pH measurement increase the longer I wait? Is that normal?

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hawkmoon7

Well-known member
Nov 27, 2020
138
Florida
When I add the pH indicator, and take a reading, I might measure something like 7.4. I noticed if I don't rinse out the test vial quickly, but leave it alone while performing other tests, it will gradually turn more and more magenta. After say, 5 minutes, it's off the chart magenta.

I'm curious if this is normal behavior and if so, why? What chemically is changing in the vial during those 5 minutes to affect the reagent?
 
The reading is correct for the first 30 seconds to a few minutes (the exact time varies) and after that it will often drift.

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.

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.
 
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PoolStored said:
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.
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Learn something every day! Makes my sealed-top free Leslie's bottle more useful.
 
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PoolStored said:
The reading is correct for the first 30 seconds to a few minutes (the exact time varies) and after that it will often drift.

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.

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.
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Thank you!
 
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