Sulphuric acid as pool acid

[James Watson]

Increase in TDS is low, but as with any chemical, Muriatic, Bleach, Trichlor, Dichlors, Cyanuric Acid, etc. your increasing TDS over time.

Sulfuric is used widely in commercial pools in Florida with great success and is actually more expensive than Muriatic. At a concentration of 38% Sulfuric reduces the pH level the same as 20 degree baum Muriatic Acid without reducing the Alkalinity as much as Muriatic.

The company we purchase Sulfuric Acid from has scientific reshearch to back this up.

Your use of the term "great success" makes it seem that you have a bias and an agenda. How is it a "great success"? All you're doing is lowering the pH.

I suspect that you are the person selling the sulfuric acid. You are probably also selling a lot of baking soda as well. By keeping the pH too low and the TA too high, all you are doing is wasting money and chemicals.

In addition, not all Total Dissolved Solids are equal. Chloride is a lot more benign than sulfate.

Post the scientific research that you claim exists.

[chem geek]

About 0.16 % of the sulfate ion concentration in ppm will contribute to the TA. So, technically, they are correct. But, realistically, they will both reduce the TA by the same amount. Perhaps they are counting the sulfate ion as contributing a significant part of the TA.

I'm not sure how you are getting this number. The pKa for the first dissociation constant for sulfuric acid (going from H2SO4 to HSO4-) is -3 while the pKa for the second dissociation constant (going from HSO4- to SO42-) is 1.92. At a pH of 7.5, the HSO4- concentration is:

[HSO4-]/[SO42-] = 10^(-7.5)*/10^(-1.92) = 10^(-5.6) = 0.00026%

so far lower than 0.16% and is essentially negligible. Essentially, one can assume that all of the sulfuric acid in pool water becomes sulfate ion, SO42- (and 2H+ that lowers the pH). It will have no measurable effect on TA itself (that is, the TA is lowered from acid addition and not increased by the resulting sulfate).

[James Watson]

About 0.16 % of the sulfate ion concentration in ppm will contribute to the TA. So, technically, they are correct. But, realistically, they will both reduce the TA by the same amount. Perhaps they are counting the sulfate ion as contributing a significant part of the TA.

I'm not sure how you are getting this number. The pKa for the first dissociation constant for sulfuric acid (going from H2SO4 to HSO4-) is -3 while the pKa for the second dissociation constant (going from HSO4- to SO42-) is 1.92. At a pH of 7.5, the HSO4- concentration is:

[HSO4-]/[SO42-] = 10^(-7.5)*/10^(-1.92) = 10^(-5.6) = 0.00026%

so far lower than 0.16% and is essentially negligible. Essentially, one can assume that all of the sulfuric acid in pool water is sulfate ion, SO42-. It will have no measurable effect on TA itself (that is, the TA is lowered from acid addition and not increased by the resulting sulfate).

The TA is titrated down to a pH of 4.5. Therefore, you have to determine the HSO4- concentration at a pH of 4.5. I was using a pKa of 1.99 for the HSO4-. 1.99 - 4.5 = -2.51

10-2.51/(1 + 10-2.51) = 0.3081 % in the form of HSO4-.

1.9195 ppm of sulfate is equal to 1 ppm of calcium carbonate. 0.3081/1.9195 = 0.16 %.

I don't think that your calculation for determining the percentage of acid is correct. It works out to be pretty close in this example but it would not work when the pH was closer to the pKa.

[chem geek]

OK, I see what you are doing and it's correct; thank you for helping out and correcting me.

For the purposes of knowing the effect on measured TA where the color of the indicator dye in the TA test changes at around a pH of 4.5, one needs to calculate how much acid needs to be added to get from the starting pH (which I assumed to be 7.5) to this color change pH of 4.5. So the contribution from sulfate/bisulfate will be the difference in the amount of bisulfate at 7.5 that I calculated (which was essentially negligible) and the amount at 4.5 that you calculated -- this difference being how many hydrogen ions were accepted by sulfate ions to become bisulfate ions. The pKa for the second dissociation constant varies by source where some say 1.92 and some others say 1.99.

So 1000 ppm sulfate would increase TA by about 1.6 ppm. So while there is an effect on TA, it is small and less than the +/- 10 ppm measurement error in the TA test. As you and I both noted, the claim that sulfuric acid doesn't lower the TA as much as hydrochloric acid is, for practical purposes, wrong. One would not be adding anywhere near 1000 ppm or more of sulfate in any single treatment using sulfuric acid. In the example I gave earlier lowering the pH from 7.5 to around 7.1 and the TA by 10 ppm, the sulfates increase by 9.6 ppm when using 38.5% sulfuric acid and by 20.6 ppm when using dry acid (93.2% sodium bisulfate).