At What FC Level Does the pH Measurement Become Inaccurate, and in Which Direction?

[Michael1]

I searched the forum for answers to this, and couldn't find one.

At what FC level does the pH measurement become inaccurate?

At high FC levels, does the pH read higher or lower than it really is?

Can you have a CYA level that demands so much FC that you will never get an accurate pH measurement?

Does high CYA extend the FC level in which pH measurement is undisturbed?

Thanks!

 

[chem geek]

For the Taylor pH test, which has special chlorine neutralizers in it, the pH is reliable up to 10 ppm FC. Above that in a pool with CYA the pH can be accurate if read quickly but the higher chlorine level will over a fairly short time (tens of seconds) oxidize the pH indicator giving it a false purple color that can look too high in pH. As Taylor notes in their Potential Interferences for the K-2006 kit:

Sanitizer levels > approx. 10 ppm may cause a blue-purple color resulting in false high readings. Wait for sanitizer level to decrease to normal levels and retest to assure an accurate reading.

This normally prevents reading the pH level during a SLAM, but for normal chlorine levels it should be OK for usual CYA levels up to 80 ppm. For high CYA levels, it would be more of a problem, but as I wrote if you look at the pH quickly it may still be accurate but if you wait longer then the high chlorine can create a falsely high reading.

 

[cowboycasey]

All great questions

At or above 10 FC the PH begins to be off.. By how much and high or low I am not sure...

Any CYA above 20 requires the SLAM to be over 10 fc, that is why its always recommended to adjust your PH before you start the SLAM..

 

[Michael1]

Thanks to both of you! I appreciate the input.

 

[oldguy70]

I have a high CYA and keep pool at 13 - 15 FC and have found that my pH reading seems reliable. Have tested numerous times letting chlorine go to 9-10 for a brief time and pH tests read what was expected.

A month ago (neighbor works for a paint mfg. gave me 2 digital pH testers to use) tested for a week period and pH reading were within reason.... Although both digital testers read to .00 and varied .05 at most....I kept a log will post sometime, calibrated both units every 2nd day using a 7.01 solution and kept both in a storage solution, not anything scientific...just my personal tests.

 

[JesseWV]

I can only guess, but either:

a) The effectiveness of the chlorine neutralizing agent in Taylor's pH test varies. Sometimes it may be able to neutralize FC levels slightly above 10ppm, other times not, depending on the batch?

b) Taylor's neutralizing agent is "new and improved." It's now able to work at higher FC levels (up to 14ppm) as evidenced by several user's experiences here on the forum?

 

[JesseWV]

This gets me thinking about doing some experiments. Does anyone know in what volume of water would a single drop of 8.25% bleach increase the FC by 1ppm?

This is assuming it really IS 8.25%. I understand varying strength is inevitability.

Also, ChemGeek if you're following, did anything ever come of the distilled water idea in this thread:

How to test pH with a high chlorine level?

 

[chem geek]

If the dropper tip used for the "single drop" were that from Taylor tests, then that is 24 drops per milliliter in which case one drop of 8.25% bleach in 0.95 gallons would be 1.0 ppm FC. In 1 gallon it's 0.95 ppm FC. What would probably be easier is 1/8th of a teaspoon of 8.25% bleach in 1 gallon is 14.1 ppm FC.

As for the Taylor test for pH and the 10 ppm, first of all it's not a hard limit in that the error doesn't all of a sudden happen at 10 ppm FC (we've said this for quite some time as in this post). Second, as I mentioned, the chlorine combining with phenol red to form chlorophenol red is not instantaneous so is dependent on the FC/CYA level. So at normal levels it takes time for the pH test to turn more purple. This isn't new. The rule about not trusting the pH test above 10 ppm is done because it is simple and will apply even if there is no CYA in the water.

As for testing pH at high FC levels reliably, one can definitely dilute the water sample with distilled or deionized water (i.e. the water must be unbuffered) and that will work because diluting buffered water still has sufficient buffering to not affect the pH by enough to be noticed in the test, but dilution does lower the FC concentration so that it no longer interferes with the pH indicator dye.

 

[JayG]

Apologies for resurrecting an old thread, but this now explains why my pH reading appears higher if I wait 10 minutes or so. I've been maintaining a high FC while opening my pool and do the pH test first and make a note of the reading within a few seconds of adding the indicator. I've found it curious that after doing all the tests, when i'm ready to dump the pH cuvette, the reading now looks different (e.g., higher). Now it all makes sense. The first and fastest reading is the accurate one. Right?

 

[Kiss4aFrog]

Now it all makes sense. The first and fastest reading is the accurate one. Right?

You are correct

 

[Saturn94]

To a point that is correct. But at some point the color shift towards a false high reading will happen too quickly. For example, I've successfully tested pH at about 13-14ppm FC (CYA 70-80) using the quick read method, but when I tried this at about 20ppm, the color shifted to purple/false high too quickly to read. I don't know where the tipping point is.

Having said that, it's probably "best practice" to stick with the guideline of testing pH with FC not exceeding 10ppm. If you must test pH with FC above 10, then the dilution method is probably a better bet than the quick read method.

 

[JayG]

My FC has been in the 20-28 range since opening (at opening I bumped the FC to 28 b/c of all of the debris) and slowly decayed to 20 over time. The curious thing is that I have never seen a "purple" shift in my tests done within a few minutes. But, I turned on my heat pump 3 days ago which has warmed the water to 80°F and now that my FC is 17. Now when I measure pH, it has dropped from 7.6 to 7.2. Maybe I was fooling myself previously. I did add 50 ppm of boric acid when the FC was 20, but that is not supposed to influence pH, right? Now that I've added the boric acid, pool math says I need to add ~13 lbs of borax to bump my pH back up to 7.6. That is not a big deal, but should I wait for my FC to drift down into the maintenance range before adjusting pH again? BTW, I'm working with a CYA of 70.

Thanks.