Ph And Temperature

[Yoshi]

I took two samples of water at the same time in two bottles. I put one bottle in the fridge and let the other one in the spa water. After about an hour, I mesured the pH of both sample:

The cold sample had a pH of about 7.2 and the warm sample had a pH of about 7.8

Does the temperature affect the pH of the water or does it affect the test itseft?

[PaulR]

I have wondered the same thing. pH is a measure of the concentration of hydrogen ions, it seems weird that would change with temperature (based on a feeble memory of high school chemistry 30 years ago).

--paulr

[Nitro]

The Pool Calculator

[Yoshi]

The Pool Calculator

Are you trying to say that since temperature affects the CSI, it must affects the pH? But does it also affects the test accuracy?

If the pH is affected by the temperature, then it would be important to test the pH immediatly after taking the sample. But does the test reading accurate at that temperature? If the test accuracy is affected by the temperature, then it would be important to test the pH after the sample has cool down. But does the sample pH now different at room temperature?

I think it's very important to figure that out. Otherwise, what it worth fine tuneing the pH or the CSI. Or what it worth to use drop kit instead of test strips?

[chem geek]

The temperature does affect the pH, but actually in the opposite direction as you saw. Going from 50F to 100F in water with around 100 ppm TA would have the pH drop from 7.5 to 7.2. Or going from 80F to 50F, the pH would rise from 7.5 to 7.7. Basically, all else equal, the pH is higher at colder temperatures. This partially offsets the temperature effect on the calcite saturation index (CSI) which changes by about half the amount of the pH change.

What you were most likely seeing in the test that you did was the effect of carbon dioxide outgassing which occurs faster in a hotter water sample. Try doing the test again, but have very little air above the sample and keep it covered. Or you can try with water that is very low in TA.

When testing for pH you should not let the sample get to room temperature and should not wait too long before testing the sample. It's not a disaseter, however, since the temperature of your water vs. room temperature is likely not going to be more than 30F different so would report a difference in pH of around 0.2.

In the above discussion I am not accounting for any error in pH measurement due to the test itself -- that is, the equilibrium between the different states of the dye at varying pH and temperature. I don't know if there is an effect there -- I suspect it's minor since the "predicted" temperature effect is something that I've seen in my pool going into and out of winter and measuring cool (50F) pool water vs. letting it warm to room temperature (72F).

Richard