High FC and Taylor Acid Demand Test

[sfbayjay]

I have a pool with way-too-high CYA (175 ppm), an issue inherited from the prior owner of my new house/pool. I am planning to do a drain/refill in the near future to correct this issue.

In the meantime, I am running with intentionally high FC, between 15 and 17 ppm to counter the effect of high CYA.

I've been running like this for about a month now, making regular chlorine additions to hold levels steady, but I'm handcuffed when it comes to maintaining proper pH. I know that my Taylor pH test is likely to give false high readings when FC is above 10. So, after much reading in the forum here, I've tried using a 50/50 dilution with distilled water for the test, and even neutralizing undiluted pool water with a drop of Taylor thiosulfate reagent.

Here's a rundown of results using these three approaches a couple of days ago:

Undiluted pool water: pH = 7.6 (likely wrong due to interference from high FC - I seem to get this result EVERY time I test with undiluted water)
50/50 pool water/distilled: pH = 7.6 (again, likely wrong)
Undiluted pool water, 1 drop thiosulfate reagent: pH > 8

I am pretty sure my pH is quite high, as I've added very little acid to the pool since taking over about a month back.

So finally, my question: Is the Taylor ACID DEMAND test still valid when used with high FC (15 - 17 ppm) water?

With an undiluted sample that reads (false) 7.6, I added the acid demand reagent and found I needed 2 drops to hit a target pH of 7.5 and 3 drops to hit 7.4. That would suggest an addition of 20 to 30 fluid ounces of muriatic acid for my pool.

BUT - I don't know if these numbers are valid given my high FC level. Using PoolMath "backwards" I can determine that if I really needed to add 20 oz of acid to hit 7.5, then my actual pH must be 8.2 - this can be done by trial and error by changing the current pH value until the required acid addition is 20 oz.

So - can I believe the acid demand test, even though I think the pH reading of 7.6 is wrong due to high FC?

 

[JoyfulNoise]

7.6 is most likely accurate.

You CAN NOT add thiosulfate (R-0007) to the water sample to dechlorinate, it is not a pH neutral reaction. There are several different chlorine/thiosulfate chemical reactions (at least 4 of them) and all of them cause changes in pH.

Diluting the sample with DISTILLED water (NOT bottled water), works best because the alkalinity of your pool water will resist changes to pH from adding the distilled water (pH ~7.0 and 0 alkalinity) while simultaneously cutting the FC in half.

And finally, an FC of 15ppm can still be read on the pH test. The Taylor R-0004 phenol red reagent is blended with a formulation of chlorine reducers to achieve a net zero pH change. While those reactions top out at 10ppm FC, there is still sufficient reduction in FC levels to make the test readable all the way up to 20ppm. The reaction between chlorine and phenol red takes about 30 secs or so to affect the color of the test up to 20ppm FC. At 25ppm FC, the reaction is too fast and the test generally can't be read quickly enough.

So, I would believe the 7.6 values you are getting from the undiluted and diluted pool water. Also, while bleach does initially raise pH, the chlorine oxidation and sanitation reactions are all acidic and will lower the pH. The amount of excess lye in a gallon of bleach is so small that it will not have any dramatic effect on pH. You'd have to add many, many gallons of bleach all at once to spike the pH high enough to be of any concern. As long as you get your FC high enough to maintain a good FC/CYA ratio, then all you are adding each day should be what is lost to sunlight (2-4ppm per day) which isn't enough bleach to significantly affect pH.

 

[sfbayjay]

JoyfulNoise - thanks for the quick reply! Any and all insights much appreciated.To help with any further discussion, here is a complete rundown of current pool chemistry - results from Taylor kit:

FC 15
CC 0
pH 7.6
TA 110
CH 440
CYA 170
Temp 74

7.6 is most likely accurate.

Hmmm - OK, but I thought I read somewhere in the forum that interference from the high FC would basically always cause the pH test to read around 7.6. I am finding it hard to believe that my pH has not changed in almost 6 weeks. My (limited) understanding is that, left to it's own devices, a swimming pool's pH will slowly increase to some maximum value as it equilibrates with the atmosphere. I'm not concerned with pH changes due to chlorine additions.

You CAN NOT add thiosulfate (R-0007) to the water sample to dechlorinate, it is not a pH neutral reaction. There are several different chlorine/thiosulfate chemical reactions (at least 4 of them) and all of them cause changes in pH.

Yes - I understand that adding thiosulfate changes the pH of the sample. Using a single drop was suggested elsewhere on the forum as a potential, albeit imperfect, workaround for testing pH in high FC water. I did this as an experiment only to see how much different the pH reading would be.

Diluting the sample with DISTILLED water (NOT bottled water), works best because the alkalinity of your pool water will resist changes to pH from adding the distilled water (pH ~7.0 and 0 alkalinity) while simultaneously cutting the FC in half.

Yes, I used distilled water for the dilution as recommended here in the forum. I found the results to be suspect simply due to another quick appearance of pH 7.6, even using the 50% dilution.

And finally, an FC of 15ppm can still be read on the pH test. The Taylor R-0004 phenol red reagent is blended with a formulation of chlorine reducers to achieve a net zero pH change. While those reactions top out at 10ppm FC, there is still sufficient reduction in FC levels to make the test readable all the way up to 20ppm. The reaction between chlorine and phenol red takes about 30 secs or so to affect the color of the test up to 20ppm FC. At 25ppm FC, the reaction is too fast and the test generally can't be read quickly enough.

I have generally been trying to read the pH within 5 to 10 seconds of agitating the water/indicator solution (44 mL water, 5 drops reagent). It seems to immediately lock on to 7.6 or 7.7. Again, this value has never changed since I started maintaining the pool 6 weeks ago.

So, I would believe the 7.6 values you are getting from the undiluted and diluted pool water. Also, while bleach does initially raise pH, the chlorine oxidation and sanitation reactions are all acidic and will lower the pH. The amount of excess lye in a gallon of bleach is so small that it will not have any dramatic effect on pH. You'd have to add many, many gallons of bleach all at once to spike the pH high enough to be of any concern. As long as you get your FC high enough to maintain a good FC/CYA ratio, then all you are adding each day should be what is lost to sunlight (2-4ppm per day) which isn't enough bleach to significantly affect pH.

Got it - I'm not concerned with pH changes resulting from chlorine additions, just from atmospheric equilibration or other mysterious causes. I suppose I'm just under the impression that, all else equal, pool pH typically rises over time and acid is required to knock it back down to optimum levels. Since I'm not adding any acid, I am struggling to understand how the pH has remained constant?

And that loops me back to my original question - can I "believe" the acid demand test results from my Taylor kit used with undiluted pool water, even with FC = 15ish? The problem for me is that the acid demand charts don't seem to agree with values I get from PoolMath for my water. Here's an example, using chemistry from a few days ago, when I did the acid demand tests:

FC 14.5
CC 0
pH 7.6
TA 110
CH 440
CYA 170
Temp 72
Volume 11,000 gallons

Target pH = 7.5
Taylor Acid Demand = 2 drops = 20 oz 31.45% Muriatic Acid
PoolMath = 4.6 oz 31.45% Muriatic acid

So which one is "valid"?

 

[JoyfulNoise]

Hmmm - OK, but I thought I read somewhere in the forum that interference from the high FC would basically always cause the pH test to read around 7.6. I am finding it hard to believe that my pH has not changed in almost 6 weeks. My (limited) understanding is that, left to it's own devices, a swimming pool's pH will slowly increase to some maximum value as it equilibrates with the atmosphere. I'm not concerned with pH changes due to chlorine additions.

No, that is wrong. Chlorine interacts with phenol red and forms chlorophenol red. Chlorophenol red is an acidic indicator dye that operates in a range from 4.8 to 6.7 and is yellow at a pH of less than 4.8 and purple at a pH higher than 6.7. Therefore, when FC reacts with the phenol red indicator it starts to add purple color to the typical yellow/red transition of phenol red. This always causes the phenol red indicator to look like a pH value higher than it is. It does not "lock" the pH indicator at a particular color but blends purple with whatever color the pH from the phenol red is.

Yes - I understand that adding thiosulfate changes the pH of the sample. Using a single drop was suggested elsewhere on the forum as a potential, albeit imperfect, workaround for testing pH in high FC water. I did this as an experiment only to see how much different the pH reading would be.

Even a single drop will shift the pH. Thiosulfate reductions of chlorine have huge impacts on pH (raises pH A LOT) without much impact on alkalinity. That's why one can use it in the TA test because it removes the chlorine interference and the pH rise is easily removed with a small amount of strong acid (the R-0009); the thiosulfate adds little to alkalinity so has no real impact on the measurement.

[EDIT]

Just as a side note - the reduction of 3ppm of chlorine using thiosulfate with water starting at a pH of 7.5 can have the pH rise to well over 9.0 because of the creation of hydroxide anions (OH-). This why even one drop of thiosulfate is enough to shift the pH test color well above purple.

[END-EDIT]

Yes, I used distilled water for the dilution as recommended here in the forum. I found the results to be suspect simply due to another quick appearance of pH 7.6, even using the 50% dilution.

Why would the speed of the test matter? When I test pH, the results are immediate. The phenol red indicator will almost immediately take on the appropriate color in the 7.2 to 7.8 range. It is more typical for color speed to be noticed with acidic solutions as it takes the phenol red a few seconds for the acidic yellow color to develop.

I have generally been trying to read the pH within 5 to 10 seconds of agitating the water/indicator solution (44 mL water, 5 drops reagent). It seems to immediately lock on to 7.6 or 7.7. Again, this value has never changed since I started maintaining the pool 6 weeks ago.

There are many people who have pools that can go almost an entire season without ever needing a pH adjustment. Mine certainly does need lots acid additions throughout the season BUT my pH will tend to hang at 7.7-7.8 for well over a week.

Got it - I'm not concerned with pH changes resulting from chlorine additions, just from atmospheric equilibration or other mysterious causes. I suppose I'm just under the impression that, all else equal, pool pH typically rises over time and acid is required to knock it back down to optimum levels. Since I'm not adding any acid, I am struggling to understand how the pH has remained constant?

Pools generally have rising pH because the dominant component of that is the TA and the outgassing of CO2 to the atmosphere. Pool water is over-carbonated relative to atmospheric levels of CO2 and there is always a driving force for CO2 to leave the water. When CO2 outgasses, the bicarbonate alkalinity shifts to resist that change and, in the process, consumes a proton (H+) to convert bicarbonate into dissolved CO2. It's THAT PROCESS which dominates your acid demand above any other source of pH rise. From your test results, your TA is WAY TOO HIGH for a plaster pool and it is your high TA which is leading to your acid demand.

And that loops me back to my original question - can I "believe" the acid demand test results from my Taylor kit used with undiluted pool water, even with FC = 15ish? The problem for me is that the acid demand charts don't seem to agree with values I get from PoolMath for my water. Here's an example, using chemistry from a few days ago, when I did the acid demand tests:

FC 14.5
CC 0
pH 7.6
TA 110
CH 440
CYA 170
Temp 72
Volume 11,000 gallons

Target pH = 7.5
Taylor Acid Demand = 2 drops = 20 oz 31.45% Muriatic Acid
PoolMath = 4.6 oz 31.45% Muriatic acid

So which one is "valid"?

No, the acid demand test is not valid because the indicator is compromised by chlorophenol red. It will take a much larger amount of the titrating acid demand reagent to drop the pH far enough so that the test is no longer showing some purple color. The chlorophenol red goes purple above a pH of 6.8 and so you'll need a lot of extra drops of the R-0005 to remove that color interference.