pH Drop Test and Meter Conflict

[steveoFresno]

Hi all,

Hoping a chemistry guru can help me make sense of something. I’ve been tinkering with a pH meter because I have a hard time differentiating the colors on the drop test, even after switching to a larger 2000-series (44 mL) comparator and R-0004 reagent.
I calibrated the meter today at 6.86, 4, and 9.18. It wasn’t far off at either step, but I calibrated it anyway. Then I tested my pool water and it said it had a pH of 7 while the colorimetric test indicated about 7.5 (blindly confirmed by three people).
I went back to the standards, and the meter was within a few hundredths of matching all three. I tested my tap water and the meter and colorimetric tests agreed at 8. I then tested the 6.86 standard and the meter and colorimetric test agreed on that too. Back to the pool water, the meter read 6.99 and the colorimetric test was clearly somewhere in the 7.4-7.6 range. What gives? Is there something in my water interfering with one method or the other? Here’s what I know I have:
FC 10
CC 0
TA 80
CH 400
CYA 60
Salt 4400
Borates 40
Temp 78 (the meter is supposed to correct for this).
Anything else in the water is unknown. I’ve never used algaecide, clarifier, or anything except chlorine, acid, CYA, CaCO3, NaHCO3, borates and salt.

Thanks in advance!!

 

[proavia]

I believe the pH will read falsely high with FC above 10 when using the drop test to check.

 

[mknauss]

Depending on how long you let the drop test sit, a FC of 10 ppm can effect the result, to the high side. The test should be done and read within a few seconds.

 

[steveoFresno]

Thanks. I read them all immediately. I was also thinking along those lines, so I just tried again after adding 2, 5, and then 10 drops of sodium thiosulfate. Interestingly, this did not change the colorimetric results, but each brought the meter readings progressively closer to those of the phenol test. So it appears that the meter is somehow inaccurate in the presence of “high” chlorine levels. Appreciate your response.

 

[mknauss]

Meters are not influenced by chlorine levels.

Not sure what is going on -- lets ask @JoyfulNoise for his input.

 

[JoyfulNoise]

You can’t add thiosulfate without affecting the pH, the chlorine neutralizing reactions are not pH neutral. The R-0010 reagent has chlorine neutralizers in it that yield a pH neutral reaction when the FC is 10ppm or lower. No need to add anything else.

It’s hard to know from your post but not all pH meters are constructed the same. Depending on how they make it, a pH meter can be affected by the ORP of the test water if the water contains a powerful oxidizer. And chlorine is a powerful oxidizer. You’ll likely need to contact the probe manufacturer and ask them if your specific probe can be used when chlorine sanitizer is present.

 

[steveoFresno]

Will do. Thank you both for your time and the info. I really appreciate it.

 

[Saturn94]

You can’t add thiosulfate without affecting the pH, the chlorine neutralizing reactions are not pH neutral. The R-0010 reagent has chlorine neutralizers in it that yield a pH neutral reaction when the FC is 10ppm or lower. No need to add anything else.

It’s hard to know from your post but not all pH meters are constructed the same. Depending on how they make it, a pH meter can be affected by the ORP of the test water if the water contains a powerful oxidizer. And chlorine is a powerful oxidizer. You’ll likely need to contact the probe manufacturer and ask them if your specific probe can be used when chlorine sanitizer is present.

Please pardon the off topic question, but does this also apply to salt meters? I’m thinking of trying one, but would like to know if things like FC, CYA, etc can interfere with the meter readings.

Thank you.