Phenol Red vs Cresol Red for pH

[jeffr47]

Recently I got the Taylor Cresol Red pH test K-1285-6 to help my pH testing. I have a really hard time with the Phenol Red kit, and my pool right now is at the upper end (7.7-8.0). I'm intentionally leaving it a bit high as my fresh plaster cures. The cresol red kit, in my opinion, is easier to read and I've read it is less sensitive to chlorine as well. (My CYA got a bit too high, so I'm having to keep my FC around 7ppm). However, try as I might I can't find the reference for this "fact" - I'm trying to follow up with Taylor to confirm.

Thoughts? One main main issue I see is price: cresol red is about $11 for for 40 tests, phenol red is $2.50 for ~for 100 tests. So cresol red is a much more expensive test, although we are still talking about a different of about $10 a season. (You can get a cresol red refill kit of 112 tests for $13.50.) So apples to apples is closer to $13.50 vs $2.50, once a season.

 

[mknauss]

Looks like a good find if it helps you read your pH.

 

[jeffr47]

I got a quick response from Taylor:

Phenol red indicator measures pH in the range of 6.8 to 8.4.
Cresol red measure pH in the range of 7.2 to 8.8.

Regardless of what pH indicator is used, the presence of a high halogen (chlorine or bromine or ozone), will alter the indicator and produce colors that are not within their normal ranges.

Taylor manufactures four different phenol red indicators for the pool/spa testing side of our company. All four have an inhibitor as part of the recipe for the reagent that prevents any interference by the presence of halogens up to 15 ppm. If the sample goes purple (a classic interference sign of the presence of high halogens), then your chlorine or bromine level is >15 ppm. To counteract this interference, we say to add ONE DROP ONLY of R-0007 Sodium Thiosulfate to a new sample and then add the indicator. One drop is sufficient o remove the interference. Two drops actually changes the pH of the sample and will produce a false high value.

Assuming the instructions are followed properly, they are equally accurate.

 

[JoyfulNoise]

Well certainly price is an issue.

But, beyond price is the color transition. Creasol red has a pKa of 8.5 and a range of 7.2-8.8. Compare that to Phenol red with a pK of 8.0 and a useful range of 6.4-8.0. For pools, the range is better with phenol red since you can access the lower pH levels and do acid-demand tests better. Creasol red works too, but it works better at the higher range and that's not really where most pools want to be.

As for chlorine oxidation, that will happen to most pH indicators which is why you need to get one that is formulated for pools. Taylor knows this and they often formulate their pH tests to handle a normal level of chlorine. You can buy cheaper pH tests that are used for aquariums but, because one does not typically put chlorine in their fish tanks, the indicators are not formulated to handle oxidation. So a cheaper H test will often fail due to the presence of chlorine.

 

[jeffr47]

As for chlorine oxidation, that will happen to most pH indicators which is why you need to get one that is formulated for pools. Taylor knows this and they often formulate their pH tests to handle a normal level of chlorine.

Yes, Taylor confirmed that both their phenol and cresol kits contain sodium thiosulfate to neutralize halides (chlorine, bromine, etc).

 

[an1vrsy]

So, is Taylor indicating that by adding one drop of R-007 to the pH sample under test that accuracy can be attained at higher levels of chlorine? If CYA 30 and SLAM FC at 12, per Taylor, pH could be accurately tested?

 

[jeffr47]

That seems to be a reasonable conclusion...

 

[Richard320]

Yes, Taylor confirmed that both their phenol and cresol kits contain sodium thiosulfate to neutralize halides (chlorine, bromine, etc).

So, is Taylor indicating that by adding one drop of R-007 to the pH sample under test that accuracy can be attained at higher levels of chlorine? If CYA 30 and SLAM FC at 12, per Taylor, pH could be accurately tested?

That seems to be a reasonable conclusion...

Not so. Their website says

A high sanitizer level (>10 ppm) is interfering. Retest, this time adding only one drop Thiosulfate N/10 (R-0007) to the sample first if using a 2000 Series™ comparator. For sureCHECK™ Residential comparators, add one drop of R-0007 to a 50 mL sample; then add that treated sample to the comparator.​

​

Source: Technical FAQs​

Which comparator do you have? What is the volume? Don't know? Better not go adding R-007 so fast. And you better not be telling anyone else to do it unless you are sure which comparator they are looking at. For that matter, I think the vials are only 35 ml, so someone is going to have to get a graduated cylinder or a whole new comparator (and the correct phenol red solution for it) if they have the small one.

Failure to follow things exactly will lead to errors. See the pictures: Accurate pH test during shock levels with R-007?

 

[JoyfulNoise]

The fact of the matter is, the phenol red indicator from Taylor is fairly stable all the way up to 20ppm (I’ve done the experiments). The problem is, you have to be fast about it. Get sample, add the drops quickly, cover, invert to mix, and read. It takes time for the chlorine to react with the phenol red and you’ve about 20-30secs before the chlorphenol red’s purple color starts to break through.

Additionally, if you’re at SLAM FC levels near 30ppm, you can dilute your pool water sample with DISTILLED waster (1:1) and test pH just fine. Most pool water has high carbonate alkalinity and distilled water has almost no alkalinity whatsoever. So diluting the pool water sample won’t change the pH much at all because of the buffering effect. With a 1:1 dilution, you can change a shock level sample from 30ppm to 15ppm and get a good reading (if you do it fast!).

I personally would not add any R-0007. The thiosulfate/chlorine reactions are complicated (there are several) and which one dominates the reaction depends on FC and starting pH. So one drop might be ok, or it might not be depending on FC levels.

 

[an1vrsy]

Which comparator do you have? ... And you better not be telling anyone else to do it unless you are sure which comparator they are looking at...

I have the 2000 comparator which comes with the K-2006C. In most cases, I don't give water chemistry advice, that's for the mods and more experienced pool operators/owners. I use this information for my pool care.

...
I personally would not add any R-0007. The thiosulfate/chlorine reactions are complicated (there are several) and which one dominates the reaction depends on FC and starting pH. So one drop might be ok, or it might not be depending on FC levels.

I don't have to worry about getting FC levels above 10 ppm. So I personally won't be adding any R-007. Following TFPC my FC targets for CYA 40 is 5 - 7. So, my pH readings are all within normal test parameters.

 

[jeffr47]

Well, for what it's worth, Taylor's rep told me one drop of thiosulfate was OK for up to 44 ml. Would be interesting to do a test with a solution of known pH at high FC. Although I'm also not exactly sure what useful info we'd get. Unless you are SLAMing, why would your FC be so high. And in that case, is it truly necessary to test? Isn't Poolmath reliable enough?

 

[Richard320]

Well, for what it's worth, Taylor's rep told me one drop of thiosulfate was OK for up to 44 ml. Would be interesting to do a test with a solution of known pH at high FC. Although I'm also not exactly sure what useful info we'd get. Unless you are SLAMing, why would your FC be so high. And in that case, is it truly necessary to test? Isn't Poolmath reliable enough?

You would be amazed at how many people want to adjust everything at once, even while in SLAM mode with a green pool. Because, you know, the TA is 30 ppm higher than the target someone arbitrarily picked out of the suggested range and therefore the pH must be lowered and the pool aerated. I'm dead serious.