PH testing question WRT higher chlorine levels

[superdave5599]

I keep seeing how we cannot reliably trust the PH drop test when chlorine is over 10 ppm.

But, that is close to where I kept my chlorine most of last year due to CYA levels (I slightly overshot it at the beginning of the season).

So, the question is, does CYA not affect this number? And/or does that number take into account generally recommended CYA levels around here?

 

[jimbethesda]

Not really your question, but you can get an electronic PH meter. I have one, and it works great.

 

[ajw22]

So, the question is, does CYA not affect this number? And/or does that number take into account generally recommended CYA levels around here?

Affect what number?

Chlorine over 10ppm bleaches out the reagent used in the pH drop test and causes it to be a purple color and give false readings.

CYA alone has no effect on the pH test.

 

[JoyfulNoise]

The technical answer is that you can still do the pH test when the chlorine is near 10ppm but you should read the results quickly, do not let it sit.

The Taylor reagents have a very good system of pH neutral chlorine neutralizes added to then but it can only handle about 10ppm FC realistically. Once about that, there’s enough residual FC left to start converting the phenol red into a different pH sensitive chemical called chlorophenol red. Chlorophenol is a low pH indicator that turns purple above a pH of 6.9 and so it causes a false-high pH reading in pool water. The amount of chlorphenol created and the rate at which it is created is directly related to the concentration of FC above 10ppm. So between 10-15ppm, the reaction is fairly slow and you can still get a good pH reading. Above 15ppm, you can start to see false higher readings. Once you are at 20ppm or above, the test is entirely useless.

One way to avoid this problem with your specific water ample is to dilute the pool water 1:1 with distilled water and then measure the pH. The dilution will have almost no effect on the sample pH but it will cut the FC in half.

CYA doesn’t affect the pH measurement at all.

 

[superdave5599]

I guess what I was thinking was, if CYA buffers the chlorine's ability to kill algae or whatever, does it also buffer the chlorine's ability to bleach out the phenol red?

 

[JamesW]

does it also buffer the chlorine's ability to bleach out the phenol red?

It slows it down, but it does not prevent interference.

It does not really bleach the reagent, it converts the reagent into a different reagent with different colors.

 

[JamesW]

 

[JamesW]

The chlorine combines with the phenol red to form chlorphenol red (or chlorophenol red) and this creates different colors at different pH levels.

If you have CYA, the chlorine is slowly released from the CYA, which can give you a brief window to get a more accurate reading if you do the test fast enough.

 

[JamesW]

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