Different pH readings w/ different tests during stain trtmt.

[Chuck_Davis]

I'm in the middle of an ascorbic acid (AA) stain treatment. I dropped the CL to zero and the pH to 7.2 and then added the AA.

After adding the AA and letting it circulate overnight my LaMotte ColorQ and Taylor test kits still registered 7.2, but my LaMotte test strips and the local pool store read a pH of 6.8-7.0.

I spoke with LaMotte, and the tech I spoke with said that it was possible that the ascorbic acid might "complicate" the ColorQ and Taylor test kit readings.

I assume that the pH will drop, and the two sets of readings will converge, as I "burn off" the AA with chlorine.

My question is: Which testing tool and pH reading should I use as I try to hold the pH near 7.2 while bringing up the chlorine level?

(If pH readings during an AA treatment do depend on the testing tool being used, TFP might want to add this information to the stain treatment "cookbook.")

 

[duraleigh]

Re: Different pH readings w/ different tests during stain tr

Years of reports here at TFP basically suggest there is little credibility with test strips and not much more in pool stores. Trust your own testing.......it works.

Your pH should remain pretty stable as you add back in FC. Again, trust your own testing....not test strips or pool stores.

 

[Chuck_Davis]

Re: Different pH readings w/ different tests during stain tr

I was surprised that adding the ascorbic acid didn't lower the pH as measured by the ColorQ kit. The TPH stain treatment "cookbook" says that it will. (I didn't start using the Taylor kit until the discrepancy in readings surfaced and I was looking for another "vote.")

I'm not a fan of strips (hence the ColorQ and Taylor kits), but two different bottles of strips that I have (both LaMotte, so probably same tech), plus a test strip at the pool store (unknown brand), all gave readings similar to the (fairly new) test system at the store.

 

[chem geek]

Re: Different pH readings w/ different tests during stain tr

How much ascorbic acid did you add in what volume of pool water? What was your TA level? Do you have borates in the pool?

The recommended amount of ascorbic acid is 1/2 to 1 pound per 10,000 gallons. With 80 ppm TA and no borates, then starting with 7.2 pH one would expect the pH to drop to 7.08 (with one pound) or 7.14 (with 1/2 pound). Also note that at the lower pH the rate of carbon dioxide increases so the pH will be rising over time faster so measurements need to be taken soon after the ascorbic acid is mixed well in the pool water (i.e. not 8 hours later or the next day).

It's when starting from a higher pH that the ascorbic acid addition has a more noticeable pH drop where one pound would go from 7.5 to 7.31. This is because the carbonate buffer system gets stronger as the pH gets lower.

I don't see how the ascorbic acid would interfere with the pH test. Your results are much more consistent with test strips not doing a great job, though usually for pH they are often OK. Obviously that's not true in your situation.