pH measurements super high AFTER adding acid

Jun 6, 2017
12
Los Altos
I use a Taylor K-2006 test kit on my 32K gallon pool. I can's seem to make sense of the pH. I know it tends to run high because of the SWG, but I've been unable to get a sensible reading with my kit all year.

I just took these readings today:

FC = 13ppm (I keep it high all winter. No one is swimming anyway, and it keeps the water crystal clear. No reason not to, that I know of, esp. with SWG)
CC = 0
pH = 8.2, although this requires a guesstimate because it is more magenta than the 8.0 comparator color, max on this kit
TA = 120 ppm (this seems very stable regardless of the pH all year)

Using a pool calculator, I planned to lower the pH to 7.2 or so, using the aerating effect of the SWG to slightly lower TA. It told me to add 1 gallon of 10º Baumé muriatic acid, which I carelessly misread. I added 1 gallon of 20º Baumé acid. Oops. But when I took a new measurement, instead of the resulting color being towards yellow/brownish red, it was a vivid purple. VIVID.

Now, obviously I made a mistake, theoretically making the pH way too acidic for awhile, although with the SWG still running it will rise quickly (I don't let the water get below 56, and run SWG ~50%). But even when I haven't added too much acid, but just what was called for, I have not been able to get a 7.5-7.8 reading all summer using my kit, even when the Pool Service company where I buy acid tests it at 7.7. It just always looks redder than the 8.0 no matter what I do.

So I wonder:
Even after reading Pool School for the umpteenth time, I still don't understand the importance of acid balance, other than comfort for swimmers. Since no one is swimming in my 56º pool, what difference does it make if the pool's pH is too high? too low? Is it bad for pool machinery? SWG? Heater? Or is it affecting chlorine effectiveness?

Thanks,

Bill
 

JJ_Tex

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You want your pH to be in the 7's for your equipment, but the pH test is also invalid once your FC is above 10. I would let it drift down to 9 or so and retest pH. Also, what is your CYA?
 

mknauss

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The SWCG does not appreciably effect pH. Follow what JJ states to accurately test your pH.

Your fill water TA will effect pH along with aeration.

You say you do not let your pool water temperature drop below 56F. How do you achieve that? If you have a pool heater (not shown in your signature), you are not to use it unless the pool water is heated to above 68F.
 

ajw22

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You say you do not let your pool water temperature drop below 56F. How do you achieve that? If you have a pool heater (not shown in your signature), you are not to use it unless the pool water is heated to above 68F.

... because of the corrosive effects of heater condensation...

 
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ajw22

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A pH above 8 can lead to water chemistry that causes scale to form on your pool and in your SWG cell.

Low pH below 7 is acidic and can damage equipment and plaster.

What is your CH? CYA?
 

brendio

Active member
May 18, 2020
30
Queensland, Australia
As JJ has said, the phenol red pH test is invalid above FC of 10. You don't say what your CYA is. At high FC, chlorine reacts with the phenol red indicator to form a red/purple chlorophenol red.

If your CYA is low, adding acid will have increased the oxidising power of the chlorine and this reaction will be increased, leading to an increased red/purple colour that you are interpreting as higher pH.

Long story short, at FC 13, we do not reliably know what your pH is unless you use a pH meter, which is not affected by FC in the same way.
 
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Jun 6, 2017
12
Los Altos
Thank you for all the answers. I truly appreciate the explanations!

My CYA is at 55. I know TFP recommendation is a little higher for SWG. My understanding is that if the CYA is too low, then the FC drops too quickly with sunlight. How else does it affect this? I assume, the higher the CYA, the lower the pH reading, but I would like to know how much of a factor it is and what conclusions to draw. (I have low confidence in the accuracy of my CYA measurements as I imagine I can see that dot for a long time; I usually compare my reading to ones I get at the supply store and they are usually off by up to 20 units.

My heater is new this summer, a Raypak 336A digital. I've added it to my signature. Thanks for making me aware of the minimal temperature requirement.

So because I've added way too much acid, and I can't read it myself, I have to guess that it is at 7.0 or lower. How urgent is it to adjust this? No one is swimming, obviously. So it is a question of how corrosive it is to the pool equipment, and how quickly it will rise naturally. With the heater turned off, the Intellichlor will stop operating, so I'm not sure what other influences will raise the pH over time.

Thanks again!

Bill
 

Bama Rambler

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Just to give you a wag at it, use Pool Math 'Effects of Adding Chemicals' section and add the amount of acid you used and see if the TA somewhat matches the value you have now and see what it says the pH should have dropped. Then subtract that from the starting pH and see what it is. pH calculations aren't very accurate, but at least it'll give you a guesstimate at it.

P.S. Make sure you have all your other parameters set in Pool Math as they affect the pH calculation.
 

mgtfp

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Thank you for all the answers. I truly appreciate the explanations!

My CYA is at 55. I know TFP recommendation is a little higher for SWG. My understanding is that if the CYA is too low, then the FC drops too quickly with sunlight. How else does it affect this? I assume, the higher the CYA, the lower the pH reading, but I would like to know how much of a factor it is and what conclusions to draw. (I have low confidence in the accuracy of my CYA measurements as I imagine I can see that dot for a long time; I usually compare my reading to ones I get at the supply store and they are usually off by up to 20 units.

My heater is new this summer, a Raypak 336A digital. I've added it to my signature. Thanks for making me aware of the minimal temperature requirement.

So because I've added way too much acid, and I can't read it myself, I have to guess that it is at 7.0 or lower. How urgent is it to adjust this? No one is swimming, obviously. So it is a question of how corrosive it is to the pool equipment, and how quickly it will rise naturally. With the heater turned off, the Intellichlor will stop operating, so I'm not sure what other influences will raise the pH over time.

Thanks again!

Bill


What brendio was trying to point out was that at very low CYA (basically 0), the oxidizing power of chlorine is very pH dependant. This could increase the effect of high FC (above 10ppm) interfering with the phenol red pH-test, making the red at lower pH (that in case of high FC gets misinterpreted as high pH) even more red.

But at CYA around 60, the pH dependency of the oxidizing power of chlorine is not very significant, so shouldn't be an issue in your case.

Otherwise, CYA is not affecting the pH reading (and even the above mentioned effect is probably very minor, and only an add-on to the high-FC interference, and if noticeable at all, then only at CYA very close to zero). But since CYA is an acid, adding CYA will decrease pH to a certain degree (that's a real effect, not just a test interference). How much, you can estimate in the "Effects of Adding" section in PoolMath.

By the way: Don't bother to guestimate CYA values between the scale marks. If the dot disappears between two marks, then round up to the next ten. In your case, the CYA is 60.

EDIT: I should add that CYA generally decreases the oxidizing power of chlorine (which is why we compensate for that by following the FC/CYA Chart). At CYA=0, you basically have no chance to get a valid pH reading with the phenol-red test above FC=10. With CYA in the water, you might have a few seconds if you are not too high above FC=10 - but you really need to understand the test and how much time you have at a certain FC-level to make that work, which is why TFP doesn't recommend to rely on that. Very advanced, don't do...
 
Last edited:
Jun 6, 2017
12
Los Altos
Thank you Brendio and mgtfp for educating me. I decided one simple thing to do is just get a pH meter. Although I still liked getting an explanation of the wherefore.
 

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HermanTX

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FC = 13ppm (I keep it high all winter. No one is swimming anyway, and it keeps the water crystal clear. No reason not to, that I know of, esp. with SWG)
CC = 0
pH = 8.2, although this requires a guesstimate because it is more magenta than the 8.0 comparator color, max on this kit
Testing pH when FC is greater than 10ppm will give erratic or invalid results. Need to let the FC drift down to 8ppm and retest pH.
Per the Taylor booklet
"False Readings: High Chlorine (>10ppm) and high bromine (>20ppm) can react with phenol red indicators, causing purple color to develop, which is not part of the normal color range for phenol red." Then "...wait until the sanitizer level has returned to a normal level before testing pH."
 

Bvacchiano

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Apr 6, 2018
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Sugar Land, Texas
The SWCG does not appreciably effect pH. Follow what JJ states to accurately test your pH.

Your fill water TA will effect pH along with aeration.

You say you do not let your pool water temperature drop below 56F. How do you achieve that? If you have a pool heater (not shown in your signature), you are not to use it unless the pool water is heated to above 68F.
So I’m confused by this statement. My pool water is currently 58 deg. I have an attached spa as well. Are u saying I can’t use the Spa because the heater shouldn’t be used until water temp is 68 deg? That can’t be right
 

mknauss

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As long as you heat the pool water to above 68F or higher in one session, no issue.
As I stated, " unless the pool water is heated to above 68F ".
 

mknauss

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From the Pentair Mastertemp Owners Manual
"OPERATING THIS HEATER CONTINUOUSLY AT WATER TEMPERATURE BELOW 68° F. WILL CAUSE HARMFUL
CONDENSATION AND WILL DAMAGE THE HEATER AND VOID THE WARRANTY. Do not use the heater to protect
pools or spas from freezing if the final maintenance temperature desired is below 68° F., as this will cause condensation
related problems."
 
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