pH test bleaching out?

[P00LNerd]

I remember reading about the higher FC levels promoted by TFP making the typical color tests for FC inaccurate, as the reagents can bleach out. So, we have the drop test, which works great.

But can the same happen with pH test reagents?

I'm trying to lower my Total Alkalinity, and of course the recommendation is to lower pH with muriatic acid, which will also drag down TA, then aerate to bring pH back up. But ever since starting to run higher FC levels, my digital pH meter and my Taylor test kits disagree. The meter recently underwent 3-point calibration, which should give it accuracy near ±0.01, but I'm not sure I trust it. I've always favored the Taylor test kit accuracy.

Right now, I'm at 7.4 on the Taylor kit vs. 7.81 on the digital meter. Neither is a problem, but the 7.4 already has me at CSI = -0.48, so I want to be sure I'm getting reasonably accurate pH readings before dragging it down farther to the recommended pH = 7.2.

 

[Rancho Cost-a-Lotta]

From Pool School:

High FC levels will throw off the test results. The Taylor pH reagent is compensated up to an FC level of 10. When FC is beween 10 and 20 the test will read higher than actual, the reading will be off by more at higher the FC levels. At FC levels somewhat above 20 the phenol red converts to chlorphenol red and will read as 8.2 at any plausible pH level.

pH Test

The pH test is a dropped based test using color matching. The solution is dropped … Read more…

www.troublefreepool.com

 

[reggiehammond]

I remember reading about the higher FC levels promoted by TFP making the typical color tests for FC inaccurate, as the reagents can bleach out. So, we have the drop test, which works great.

I think you're misconstruing the advice about the pH test being inaccurate above FC10 with any advice about the chlorine tests.
The OTO Chlorine test (color matching) is not widely used as it lacks the accuracy or range of the FAS-DPD test. But the FAS-DPD is accurate and dependable at any FC level.

 

[JoyfulNoise]

The pH test (phenol red) doesn’t “bleach out” at high FC. The indicator becomes oxidized by high chlorine levels forming a different chemical compound that has a lower color range (measured low pH better) than phenol red. So the oxidized indicator takes on a purple hue which causes a false-high reading at high FC.

However, your FC levels are not high at all. Your PM logs indicate 7pm FC … that’s considered standard operating levels by most people here. The Taylor pH indicator is good all the way up to 10ppm FC and can work reliably above 10ppm if you do the test quickly. It really only becomes problematic when the FC is above 20ppm.

If your FC worries you, then simply dilute the pool water sample 1:1 with distilled water. The dilution will have no effect on pH but it will cut the FC in half.

 

[P00LNerd]

Excellent. Thanks, guys! I think my electronic meter is just garbage. As noted, I'm mostly measuring pH when my FC is 8 ppm or below.