pH test discrepancy

[htwooh]

My pool water pH tests as 7.5 with the Taylor phenol red indicator kit or 7.6 with a similar BioGuard kit; however, the same water sample shows a pH of 7.0 when tested with a glass pH electrode and well calibrated pH meter from my research lab. Any idea why this discrepancy is occurring?

Other test results for the pool water are:
Free Cl 5.0 ppm
Combined Cl 0 ppm
Total alkalinity 110
Ca hardness 180
Borates 45 ppm
CYA 30 ppm

The pool is a 25K vinyl-lined pool in the southeastern US. Thanks for any insights!!

 

[JasonLion]

Welcome to TFP!

I've definitely seen some variation between a glass bulb PH sensor and the Taylor PH test, with the difference increasing as FC levels rise. However, I have only seen differences as large as what you describe when the FC level is approaching 20 (give or take).

What test did you use to measure your FC level? Some of the common FC tests will show FC levels well above 5 as if they were 5. I would also be interested to see how much variation you see when the FC level is near zero. You could take a water sample and leave it in the sun for a couple of days to get the FC level down to zero and then measure it both ways.

All of the swimming pool PH tests include something to neutralize chlorine, so that it won't affect the phenol red. However the chlorine neutralizing process is never completely PH neutral. The more chlorine there is, the more the neutralization process will drive up the PH. The Taylor reagent does a better job at minimizing this PH change than the other brands I've seen, but they can't completely eliminate it.

 

[htwooh]

Hi Jason,
Thanks for the welcome and for the information!

I used the TF-100 FAS-DPD test kit to measure the free chlorine the morning that I tested the pH with the kits and pH meter. That evening, after a full day sunshine on the pool, the free chlorine was 2.0 ppm. The pH still tested at 7.5 with the Taylor kit, but I did not retest with the pH meter.

As you suggest, I will set a water sample aside for a few days and test again with the kits and pH meter after the free chlorine drops near 0.

Thanks again.

 

[dschlic1]

I routinely use a glass electronic pH meter to measure the pH in my pool. Every once in a while I will also measure the pH useing the pH test in the TFT100. Readings come out as close as you can read the TFT100 test. I normally run around 5 ppm FC in my pool.