Closing bleach addition shot up ph

[grottoguy]

I am getting my pool closed tomorrow so I wanted to lower my ph to 7.4 (because I know it will creep up). I tested using the TFT100 kit (and checked it with my ph meter) and got 7.6 with the meter and just over 7.5 with the TFT100. I then added the requisite acid to lower it to about 7.4 But around the same time I added a lot of Clorox regular bleach to get my FC to 19 (was at about 5). The bleach bottle looked the same as last year but it said "with Cloromax technology" on the bottle, and I don't recall if last years bleach said that.

Well after adding a little more than half the bleach I checked the ph a couple of hours later and I was shocked to see it was 8.2. I then added the rest of the bleach, and two hours after that it was 8.5 (TFT test kit showed it greater than 8.2 and my meter (which has been accurate all year) showed it as 8.5.

I recall reading that the bleach should not raise the ph so I was surprised. When I close my pool tomorrow, I would like the PH around 7.4, so I wasn't sure if I should add acid tonight or not because I wasn't sure if the bleach was giving me a false ph reading. I ended up adding acid (but not enough to lower it to 7.4) and will check it tomorrow morning and then add more acid if need be.

Has anyone else experienced this? I don't think this happened to me last year.

If relevant, the rest of my numbers are CH 300 and TA 70. I haven't tested my CYA in a while but it was 40-50 last time I checked. I will test it tomorrow.

Thanks

 

[mknauss]

From the Deep End - a post on chlorine and pH

Technically, bleach does raise the PH right when you add it, but then as the chlorine gets used up the PH comes back down. Over time the PH change from bleach is very nearly zero.

So, if you raised your FC quickly, it does raise pH. Your drop test for pH is effected by the high FC and the reading is not correct above FC 10.
I do not know if the meter is effected.

The Cloromax bleach should not be used. It has polymers in it that we do not know what effect they may have in pool water. Clorox was contacted by a member earlier this year and they stated to not use the Cloromax bleach in a pool.

Take care.

 

[JoyfulNoise]

Bleach, when added to water, will temporarily raise pH. Oxidation and sanitation reactions of chlorine are acidic and will lower pH by the same amount. There is a small, excess lye content to bleach that will slightly raise TA over time.

Your water is probably so clean at this point and the chemical reactions are so slow that you are seeing the temporary increase in pH.

How high is your FC? You can’t reliably measure pH with the phenol red indicator when the FC is over 10ppm. Your pH probe should be more immune to it but high oxidizer levels can mess up cheap pH meters as well.

You could add some acid to bring it down to 7.8 and it will be fine.

 

[Swampwoman]

Just so you know, ph usually does not rise once covered.
When you said this:

I wanted to lower my ph to 7.4 (because I know it will creep up

Is that based on your observations on opening in past years, or were you assuming that based on open water behavior?

 

[mknauss]

Also, check your CSI with cold water. You want a higher pH if you need it for CSI. Cold water drops CSI quite a bit.

Take care.

 

[Divin Dave]

FC above 10 will result in a false high pH test. Your pH is not actually that high.

This is well known on TFP. Here are a couple of links to old discussions on the subject.
This is why when slamming a pool, we recommend to first adjust pH if necessary, and then add chlorine after that and test pH anymore until after the slam is finished and the FC has dropped below 10.

here are a couple of links to old discussions about the pH test being inaccurate after raising the FC
At What FC Level Does the pH Measurement Become Inaccurate, and in Which Direction?
pH at high FC levels