Did Polyquat jack up my pH... or is it interfering with R-0004/T-9056??

[JamesW]

"The test is based on the reaction of the cationic polyquat with an anionic polyelectrolyte using Toluidine Blue O as the indicator. The color change is blue to purple. Levels are determined using the drop count method. Range and Sensitivity is 1 drop = 1 ppm Polyquat". Polyquat Test Kit - Lamotte Water

QAC (Alkyl dimethyl benzyl ammonium chloride) Two methods are available.

A masked bromphenol blue indicator is added to the sample and turns green.

Sodium tetraphenyl-boron is added to complex the QAC and the color changes to red.

This method is best suited to higher QAC concentrations.

A poly-electrolytic titration, like the one used for polyquat, is used for low to high concentrations.

https://www.cec-online.com/wp-content/uploads/2023/04/lamotte_water_quality_testing_products_catalog_2017-2018.pdf

 

[Sampo]

Hi @JamesW -- This is great stuff! Your well-sourced replies are much appreciated. I don't doubt that the Chlorophenol Red driven mechanism of interference might have happened in some of the prior threads that you've cited. And I also don't disagree that it could be a secondary contributor in my case. But I'm pretty confident in my observations -- and I don't think that the chlorine level alone can explain what I saw, based on the following reasons:

• My chlorine level before and after the addition of Polyquat didn't change (FC=12.5). The only thing that I added between the pre and post pH/FC tests was the Polyquat. And the pH artifact appeared on the very next test afterwards (a few hours after adding Polyquat, following a thorough circulation of water).
• I've previously had even higher levels of FC this summer (up to about FC=15.0) when my SWG got a little out of hand. Yet I didn't see this artifact then.
• The only other time that I've encountered the same purple color was last fall at closing: techs from my PB winterized my pool, using Polyquat and various other additives. I took a post-close sample out of pure curiosity and was horrified to see the purple pH, thinking that they'd added something bad. But last year, I was forced to leave things alone as my pump was already off (which was a good thing in retrospect, since I now know that the pH reading was false). Of note, my closing chlorine last year was FC=9.0.
• I just ran a phenol red test this morning, but added a drop of R-0007 to my 2000 series T-9056 comparator, as the Taylor video that you linked suggested (my FC is currently 10.5). Admittedly I'm not seeing the purple artifact right now, so this isn't such a great experiment -- more of a baseline test to see if there is a difference at this FC level. At least at this FC level, my pH reading with/without the R-0007 was the same. Of course, that's not surprising based on what you say about the phenol red having built-in reducing agents up to FC=10. But this level also isn't that far from the FC=12.5 where I saw the dramatic purple artifact. And it's higher than the FC=9.0 from last year when the same thing happened.

I really value the insightful analysis from you and @JoyfulNoise -- especially since I'm not qualified to debate the "Deep End" part of this discussion. I can only give experimental data. But my instincts suggest to me that the Polyquat is probably the primary driver of the artifact that I saw. The mechanism that JoyfulNoise proposed certainly seems consistent with what happened -- though perhaps the chlorine is further potentiating the Polyquat/Phenol red interaction, as he also suggested.

Anyway, I'll still recommend the term "Polyquat Purple Artifact" for this interference... though perhaps that terminology won't ultimately catch on.

Sampo

 

[JamesW]

Maybe it is a thing that can happen, but Poly 60 is a very commonly used chemical, and I I think that it would have a lot of reports if it was a common occurrence.

At this point, I would say that it might occur in some circumstances, but probably not in most.

Maybe there is correlation, but that does not prove causation.

Anecdotal attributions are useful, but not conclusive unless you get enough examples.

In my opinion, if you follow the dosing guidelines and allow the water to circulate completely before testing again, the interference probably will not happen.

If anyone can find a reference from a source like Taylor Technologies or other test kit makers, that would be helpful.

 

[JamesW]

This indicates a possible interference in the colorimetric TA test, but not a pH test.

Many algaecides contain quats (quaternary ammonium cations) or polyquats.

High levels of these can cause low readings in colorimetric alkalinity tests.

Inhibitors are added to minimize this interference but over dosing with these algaecides can cause errors in alkalinity readings.

Very high levels of biguanide have also been known to cause a similar interference.

https://www.phta.org/pub/?id=081f4f68-1866-daac-99fb-1e280f51ea42

Test Killers: Common Water Testing Interferences and How to Avoid Them

Common water testing mistakes that can mess up your results and lead you astray.

www.aquamagazine.com