TA question

[survey77777]

I'm new here and have yet another question. I started another thread about my success? with clearing up algae using BBB and of course found myself to be such an expert that I've taken on a friends pool too to help them clear the same sort of algae mess we had (I'm kidding on the expert part)
So when I tested their water for them last night their chlorine was off the chart for my test kit as it had just been shocked. PH 7.6 but the question comes with the TA. With my test kit I titrate until it goes from blue/green to red (just in case this isn't the case with all). When I tried to test their's, the sample instead turned yellow at about 140 and never left that yellow colour when I added another 7 drops. Hoping that someone here can clue me in on what this indicates. Their pool is 90000L vinly with sand filter.
Thanks again...so glad to have found you all.

 

[JohnT]

Blue to yellow is what happens when chlorine is high. The test is still valid, the colors are just different.

 

[survey77777]

thanks

Thank you...now I know
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[chem geek]

Taylor has a list of interferences with the K-2006 test kit and that also applies to the TF-100 kit and may be found here. Though they talk about the high chlorine interference on TA that you saw, they don't talk about the specifics of the high chlorine interference on pH, but the following is from their "Pool & Spa Water Chemistry -- A Testing & Treatment Guide":

FALSE READINGS: high levels of chlorine (usually >10 ppm) will quickly and completely convert phenol red into another pH indicator (chlorphenol red). This new indicator is a dark purple when the water's pH is above 6.6. Unfortunately, some pool operators mistake the purple color for dark red and think the pool water is very alkaline and wrongly add acid to the pool.

When a sanitizer level is not extreme, only some of the phenol red may convert to chlorphenol red. However, purple+orange (for example, pH 7.4) = red. This error is more subtle as no purple color is observed and the operator does not suspect that a false high pH reading has been produced. Some operators neutralize the sanitizer first by adding a drop of chlorine neutralizer (i.e. sodium thiosulfate). However, thiosulfate solutions have a high pH and, if heavily used, may cause a false higher sample pH.

The Taylor pH test reagent has some thiosulfate in it and the color comparators are set accordingly (to account for the basic solution of the pH drops) but it's not clear how well this resists the problems described above. For those who operate pools at higher chlorine levels, perhaps due to higher CYA levels, then testing pH would seem to be a problem. This is one of those experiments that if I had a lot of time, I would do (i.e. see the color differences at different chlorine levels -- one can use acid/base demand drops to shift the pH to see the range of colors).

Richard

 

[survey77777]

Thanks for that info Richard. I actually do have a lot of time. Never know I may actually spend some of it doing just such an experiment once I get my pool back to sparkling again.
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