Sulphuric acid as pool acid

Problems relating to pH and total alkalinity.
Increase ph, increase TA. Reduce pH, reduce TA.
pH chemistry advice and techniques for the pool.
chem geek
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Sulphuric acid as pool acid

Postby chem geek » Thu 11 Nov, 2010 21:07

FlPoolPro wrote:I sure would like to see some technical data on how Sulfuric hurts salt cells. The local Hayward rep has been saying that for two years but can't produce any data to back it up.

I can't find scientific research on this, but this link from the Salt Institute implies that sulfates may increase the rate of scaling and, of course, scaling on salt cells reduces their life.

There are scientific papers regarding calcium carbonate and calcium sulfate scale such as this one and this one . Straight calcium sulfate precipitation, however, would normally only occur with extremely high sulfate levels. 300 ppm calcium would require around 3000 ppm sulfate to saturate the water above which scaling might occur.

Another issue with sulfate levels as well as high salt levels is described in this paper where salt recrystallization pressure can be high, most especially for magnesium sulfate. Splash-out of salt can cause damage to soft hardscape/stone materials and the worst effect would be with hard water (so typically high in magnesium, not just calcium) that is also high in sulfates (say, from using sulfuric acid or sodium bisulfate dry acid regularly).


James Watson

Sulphuric acid as pool acid

Postby James Watson » Thu 11 Nov, 2010 21:10

chem geek wrote:A sulfate level of 220 ppm lowers the saturation index by 0.1 so high sulfate levels need higher pH, TA or CH to compensate in order to prevent dissolving of plaster/gunite/grout surfaces.

Why would the sulfate level affect the CSI?
chem geek
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Sulphuric acid as pool acid

Postby chem geek » Thu 11 Nov, 2010 21:28

James Watson wrote:Why would the sulfate level affect the CSI?

Because of the formation of ion pairs. Specifically for sulfate, there is significant formation of the neutral CaSO40 ion pair so sulfates effectively reduce the calcium ion concentration in the water. There are other ion pairs as well including CaHCO3+, CaCO30, CaOH+, CaCl+, CaB(OH)4+, but the only one large enough to significantly affect the saturation index is calcium sulfate.

The calcium bicarbonate ion pair lowers the CSI by about 0.02 so is ignored by most saturation index formulas. Calcium chloride at the 3000 ppm salt level of saltwater chlorine generator pools lowers the CSI by about 0.04, but this pales in comparison to the ionic strength effect which lowers the CSI by about 0.22. At 300 ppm Calcium Hardness (CH), 100 ppm sulfate lowers the CSI by about 0.03 so not a big deal unless sulfate levels get quite high -- 400 ppm sulfate lowers the CSI by about 0.11
James Watson

pH in pool water

Postby James Watson » Thu 11 Nov, 2010 21:57

FlPoolPro wrote:
Pool Balance wrote: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.

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.
James Watson

pH in pool water

Postby James Watson » Thu 11 Nov, 2010 23:20

FlPoolPro wrote: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.

If it is more expensive, then why is it your preferred acid?

Sulfuric acid is not a good choice for lowering pH. Muriatic acid is a much better choice. If you are using lot of acid, then you are keeping your Total Alkalinity too high.

Also, there is no research necessary. It's basic chemistry. The alkalinity at pool pH is lowered the exact same amount for either acid.
James Watson

pH in pool water

Postby James Watson » Fri 12 Nov, 2010 00:04

FlPoolPro wrote: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
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pH in pool water

Postby chem geek » Fri 12 Nov, 2010 02:35

James Watson wrote: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

pH in pool water

Postby James Watson » Fri 12 Nov, 2010 08:51

chem geek wrote:
James Watson wrote: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
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Sulphuric acid as pool acid

Postby chem geek » Fri 12 Nov, 2010 11:51

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).

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