pH reading with high FC - invalid?

AnnaK

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#1
I've read here that the pH reading is invalid when FC is high. How high is "high"??

Using the TF kit I get FC=23, CC=0, pH=7.2, TA=90. The pool is in its final recovery phase after the swamp event. Water is once again crystal clear.

I am not presently adding any chlorine but am monitoring with the FAS-DPD test every three days. The filter runs 8 hours at night.

I'm wondering now whether the pH is really that low or whether the test itself is influenced by the high FC. Since TA is at a good level I'm hesitant to mess with the chemistry just yet, you know?

Anna
 

ktdave

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#2
Anna,
FC 23 is high. You can rely on the pH test when FC gets to around 10. pH test with high FC tends to read high, so you may actually be lower than 7.2. I would add some soda ash (or whatever you use to up the pH) to get your reading to about 7.6 just to be sure you're not too low. HTH
 

AnnaK

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#3
Thanks!

I've used baking soda in the past to increase pH but that'll raise the TA as well. So now my question is: for the vinyl liner, which is worse: low pH or high alkalinity?

Is washing soda the better choice for raising pH?

Anna
 

duraleigh

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#4
Anna,

Don't be in a big rush to raise your pH. (with 20 mule team borax, BTW).

If your CYA level is at all reasonable, your FC should come down to a manageable level in a day or two and you can then confirm your pH (it MAY currently be more accurate than you think).

I believe any induced error in pH reading will be small enough for you to let FC come down and then test......with no harm to anything.
 

chem geek

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Mar 28, 2007
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#6
AnnaK said:
Thanks!

I've used baking soda in the past to increase pH but that'll raise the TA as well. So now my question is: for the vinyl liner, which is worse: low pH or high alkalinity?

Is washing soda the better choice for raising pH?

Anna
Arm & Hammer Baking Soda is used to raise the TA and should not generally be used to raise the pH since it only does that a little. Arm & Hammer Washing Soda (careful: not the laundry detergent) is the same thing as Soda Ash and is what is in most pH Up products and can be used to raise the pH, but it will also raise the TA. 20 Mule Team Borax will raise the pH without raising the TA as much -- you use twice as much by weight as you would Washing Soda / Soda Ash.

For a liner, low pH below 7.0 is definitely far worse than high TA. High TA doesn't matter unless you've also got either high pH or high CH in which case you could get some scaling, but usually in a vinyl pool the CH is low so scaling is not usually an issue. The only other problem with high TA is that it will make the water tend to rise in pH over time due to greater carbon dioxide outgassing. Pool water is intentionally over-carbonated, just like a carbonated beverage (though not as carbonated), and a higher TA means more such carbonation.

Richard
 

JasonLion

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#7
Taylor says the PH test is good up to a FC level of 15. There have been reports of apparent errors in the PH reading for FC levels as low as 11, though this has not been verified. With FC somewhere in the 20s, I am not sure exactly where, the PH test starts being way way off. In all cases, when there is a problem because of high FC the PH reads higher than it actually is. [Your PH test solution can't have expired yet, since the TFKit hasn't been around for that long. If the PH test solution was several years old there could be errors in either direction because it was old.]

So your PH reading is either 7.2 (as tested) or lower than that. Also, as the FC gets used up, and the FC level goes down, the PH will go down (consumption of FC is acidic). I would raise the PH somewhat now, to be sure it doesn't get below 7.0 (the point where problems start being possible). That should be safe because any FC related errors in the reading would mean your PH was even lower.

Borax is the best thing to use to raise PH if you don't want to raise alkalinity. The alkalinity will go up a little, but much less than if you used washing soda/soda ash.
 

AnnaK

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#8
You people are terrific!

Every time I come here I learn something new. Richard's and Jason's posts on the different products for increasing pH, TA, both, or either are invaluable. I've printed them for my pool log book.

The CYA tested at ~60, possibly a tad lower. It's really a very subjective test.

I'll go find some 20 Mule Team Borax; it's the one "B" I don't yet have in my collection of Bs.

Thanks a ton, people!

Anna