Can't stabilize ph

[JimK]

Sorry, I wasn't clear. When I say to take a reading immediately, I mean immediately after adding and mixing the pH reagent with the water sample. If I let it sit after adding the reagent, it will change to purple /false high; the higher the FC level, the faster the color change to false high occurs.

I've certainly never had a problem with the pH test at FC levels below 10 and am surprised you're having trouble unless you're adding the pH reagent and letting it sit.

[polyvue]

Sorry, I wasn't clear. When I say to take a reading immediately, I mean immediately after adding and mixing the pH reagent with the water sample. If I let it sit after adding the reagent, it will change to purple /false high; the higher the FC level, the faster the color change to false high occurs.

I've certainly never had a problem with the pH test at FC levels below 10 and am surprised you're having trouble unless you're adding the pH reagent and letting it sit.

Wow. I see what you mean.

I had noticed in the past that in a treated sample the apparent pH would change during the first few seconds of observation (at higher levels of chlorine) but hadn't fully appreciated the rapidity since I would routinely discard the result ("not tested") or retest using a dilute mix.

I performed a test this morning to explicitly compare the two approaches: '1 - Immediate Reading' and '2 - Dilute Sample Before Testing'. A quart of sample water supplied both tests. FC 6.8 ppm before and after, from original sample cup.

For the 'Dilute Sample' test, I decanted 1/2 C. sample water into a rinsed 2-Cup glass measure, then added 1/2 C. distilled water, swirling to mix. Thereafter, this was used in the same manner as the sample water for the 'Immediate Read' test, first used to rinse the test cell and then fill it to the calibration mark. In each test, a .5 mL pipet of Taylor R-1003J pH indicator solution (Phenol Red) was added to 11.5 mL sample in the test cell, capped and upended quickly to mix. Immediately, the sample color was compared to the liquid standards and recorded.

I was dumbstruck with how fast the sample in the first test changed color... in little more than a couple of seconds after noting the initial color it was already changing, and was out of range before I could count past three (seconds). In contrast, the Dilute Sample test was uneventful. I don't have an arbitrator to conclude that my testing method or result is correct. Time to get a pH meter?

Immediate Reading

~ 1 second - 7.7 (intermediate between 7.6 and 7.8, light purple)

~ 3 seconds - 8.2+ (clearly exceeded test range, deep purple)

~ 1 minute - 8.2+ (no change, remained deep purple)

Dilute Sample

~ 1 second - 7.5 (intermediate between 7.4 and 7.6)

~ 3 seconds - 7.5 (no change, remained pinkish orange)

~ 1 minute - 7.5 (no change, remained pinkish orange)

The sample water used for the pH test was only FC 6.8 ppm CC < .2 ppm (34 drops FAS-DPD titration reagent, 25 mL sample, tested before pH tests and a minute or so afterward). I didn't think to test FC on the dilute sample before discarding! Other parameters: 85 degrees F., 10 ppm borates (est.), 720 ppm salt, 250 CH, 125 TA, 35 CYA

Are chemgeek or waterbear still contributing here?

My sincere apologies to the OP for taking over your thread.

[JimK]

Interesting. I noticed the reagent you're using is different from the one I'm using, Taylor R-0014. Perhaps this explains the difference in our experiences?

Under 10ppm FC, I can take my time reading the result. It doesn't change near as fast as it did for you until my FC gets above 15ppm FC or so.

It would be great if Chemgeek would pop in and give his input.

To the OP - my apologies also for the sidetrack, though it may relate to what you are seeing. How long to you let the pool circulate after adding chlorine before you check pH? What's the FC level at the time you check pH? As you can see by our discussion elevated FC can give a false high pH reading.