pH question

[Carolina]

I'm in NC, and I have an 11,000 gallon inground gunnite pool. It's been running over the winter, but I've been "neglecting" it the past month or so. I've always used a Taylor kit to check my water.

I recently went back to work after years as a stay at home mom. I work in an environmental laboratory, and have access to all sorts of fun equipment. Yesterday, I decided to bring a water sample to work for a pH check. My meter read 7.4, which seems a bit low given the fact that my pool's pH has a tendency to creep up if I don't stay on top of it.

I did a comparison using my Taylor kit, and that reading was 8.2. This is a huge difference.

I'm stumped! Any thoughts?

[Goolsbymd]

How long was the water sitting in the sample bottle for also was everything clean before adding the water to the test bottle, kit, and so forth? Also when did you buy your Taylor kit?

[Carolina]

Taylor kit is approximately 8 months old. I tested the water via Taylor immediately. At work, there was about a 10 - 15 minute lag time between sampling at my home and testing at work. In both scenarios, equipment, vials, etc are either unused/ new or cleaned thoroughly.

The only other variable might be that I didn't let my sample at work equilibrate long enough, such that the reading wasn't actually stable. I dunno, though.

[kelemvor]

When was the last time your pH meter was calibrated? I ask because I've seen admins here repeatedly mention that unless electronic pH testers are calibrated "very frequently" (although I don't know exactly how often that means) that they can be quite unreliable.

My k-2006 is a year old now and seems to give reliable results (it jives with what the guys at the pool store generally test at). You could always just get a new reagent for your taylor, or even a "basic" kit from taylor or lamotte for under 10 bucks.

Also, what are your other readings for the pool? Things like high CL can affect the pH test.

[Carolina]

The meter is calibrated immediately before testing, with a 3 point curve - buffers 4, 7, and 10. Calibration is also verified with a second source standard, as well as a QC sample with a known value. I'd not trust a value unless it had been calibrated within 24 hours or so. That being said, the value would only be off probably by 0.1 or so... my home kit is based on a color comparison, which is more subjective.

Maybe my home kit has gone bad.

[kelemvor]

Given the price, you could probably just replace the phenol red reagent. I believe it's this one: http://www.amazon.com/Taylor-Replacement-Reagents-pH-Indicator/dp/B0002IXIL8 for $2

[CarlD]

Did you neutralize the chlorine before running the lab test? Though, high chlorine usually makes pH look high.

Is Taylor still using the self-neutralizing phenol reagent, or is it the non-neutralizing agent? If your chlorine levels are high try adding a drop of the neutralizer to the Taylor test--I THINK it's bottle #7 and see if the results are more consistent.

Also, this is a rare case where I'd double check with either a Hach or LaMotte brand test strip and see which it confirms...if you have the strips around.

[chem geek]

Taylor's phenol red solutions have a special balance of neutralizers in them that are somewhat pH neutral. They do not recommend adding thiosulfate solution since that can raise the pH itself. If you have a water sample with more than 10 ppm FC, then you can measure its pH by diluting the sample with unbuffered (distilled or deionized) water. Do not use tap water or other buffered water as it will affect the pH.

[PoolDoc]

Carolina, please retest the pool with both the K2006 and the lab meter. But, when you carry the sample into the lab, make sure there's no air space in the sample bottle. And, dilute your POOL sample 50:50 with distilled water before testing.

Also, tell us what your free chlorine level is -- phenol red doesn't usually deteriorate, but if you convert it to chlorophenol red by very high FC (>10ppm) levels, you can get false readings.

This chart:
http://isbchem1.pbworks.com/f/FG15_13.jpg
from http://isbchem1.pbworks.com/w/page/9206118/pH%20Review shows the different ranges -- but the colors are not very accurate.

You can compare the actual end-point colors in these pictures from
http://www.titrations.info/acid-base-titration-indicators-preparation

Chlorophenol red:
http://www.titrations.info/img/chlorophenol-red.jpg

Phenol red:
http://www.titrations.info/img/phenol-red.jpg

In my experience, the chlorophenol red endpoint (reached before 7.0!) is more violet than shown.





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