strange pH results

I started shocking my pool last night to try and once and for all get rid of some mustard algae. Before starting, I checked my pH using my k-2006 kit.

It was surprisingly high (> 8). Since I have been keeping a close eye on it lately, I expected somewhere between 7.6-7.8. Anyway, I added enough acid to bring it from 8 to 7.4 (32 oz). Tested again in an hour and it was still over 8.

This morning, I tested again with the same results. So just as a sanity check, I used my other OTO kit (uses R-0014 reagent), and it showed 7.2. This is much more in line with my expectations, since I really questioned my first test last night and put in a fair amount of acid.

So my question is which test to trust? If it really is 7.2, I don't want to put more acid in. But if it is over 8, I want to right away! The R-0014 is somewhat old (a couple years) but it has been responding lately, going up and down as I would expect.

I would trust the K-2006. How high was your chlorine when you first ran the pH test last night? If it was 10 or above, your pH test will give you a falsely high reading.

You might also try doing the acid demand test in the K-2006 and see what that shows you.

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Originally Posted by Watermom

If it was 10 or above, your pH test will give you a falsely high reading.

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I can't say on the first test, but the second test last night and the one this morning were both taken with FC at 20. So if it gives falsely high readings that should explain it. That doesn't affect the R-0014 results?

The acid demand test suggested I add exactly the amount I added last night. I guess I should wait it out now until I'm done shocking since I may not be getting accurate readings.

Thanks!

I would wait it out--better to have the pH a little high than too low where it can do damage to your pool.

Janet

I would test both your pH reagents with the acid and base demand reagents in the K-2006 to see how quickly the color changes.. It is not unusual for phenol red to go bad. How old are the reagents? I have had them go bad after one or two seasons and when they do they read around 7.8 or 8.0 no matter what the pH is. Also, is there any contamination in the comparator of the k-2006? You might want to clean it well, rinse it with pool water and then retest.

I would test again and take a sample to the pool store and see what they come up with.I suspect it will confirm one test or the other. For the one that is "off" get new reagent and clean the comparator well.

I suspect that your R-0014 in the K-1000 kit is newer than the R-0004 in the K-2006 and is the one to be trusted.

Also, since the K-2006 uses the same comparator for the CH and pH (and TA) tests and the CH test is done at a high pH cross contamination could also be the cause.

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Originally Posted by waterbear

I suspect that your R-0014 in the K-1000 kit is newer than the R-0004 in the K-2006 and is the one to be trusted.

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Actually, it is just the opposite. I got the R-0014 with one of Doc's kits a few years ago. I got a new k-2006 this year. The old one kept rising and falling through the shock period, the new one shot through the roof.

Once the shock period was over, the R-0004 has gone back to normal and is now reading similar to the R-0014. I certainly don't understand it, but those are the results I witnessed.

Your theory of cross-contamination could explain it though. Before I shocked, I was testing both CH and TA for a couple days with both pool and source water. I probably didn't clean it like I should have, so there could have been residuals that affected it.

That or the very high FC I maintained for a few days. :)

Thanks for all the help from you all, I'm starting to get a handle on things finally!