How does a swimming pool test kit measure pH levels?

[barryj]

TL;DR

What is the chemistry of determining the pH of a swimming pool?

Swimming pool test kits combine a few drops of an unknown chemical with about 10 mL of pool water and then check the color of the reaction.
I would like to know the chemistry behind this. I assume that the H+ and/or the OH- ions combine with some cheemical to produce the color to be checked. What is the unknown chemical? It is not stated in the test kit. and how does it woek?

 

[Borek]

These are typically solutions of pH indicators: https://en.wikipedia.org/wiki/PH_indicator

 

[barryj]

After looking at the site you indicated, i see that Resazurin has a color that changes from red to blue with a pH change from 7 to 8. This would be a good chemical to test the pool pH it seems. However, how do I determine the concentration of resazurin and how much to add to get a color that is visible? Maybe a better question is what amount of what molar solution of resazurin should be added to How much test water?

 

[Borek]

The lower the concentration, the better (indicators are acids/bases themselves, so they can change the pH), but the actual value depends on the particular indicator. You want the lowest concentration that is reasonably well visible. In most cases that actually means traces. Compare http://www.titrations.info/acid-base-titration-indicators-preparation - these are solutions that you will use in amounts of few drops per 200 mL flask.

Note that not all of these indicators are easy to find - some of them are quite popular and easy to get, some are rarely used, so they can be only bought at specialized chemicals suppliers that won't sell to individuals.

 

[barryj]

Thanks for the info. I have found on the Internet that Phenol Red for example is made into a 0.02% solution for pool testing. Now that I have a start I will continue to learn about these. Thanks for getting me going.

 

[Tom.G]

Just an observation:
I have seen pool service technicians pour a bit of their pH test solution directly into the pool. They then just visually evaluate the resulting color!

Not very accurate unless you have calibrated eye eyeballs.

Cheers,
Tom