Sulphuric acid as pool acid

[Petra]

We have a chemical company trying to sell us sulphuric acid as "pool acid" instead of muriatic. Has anybody heard of this for public pools

[Pool Balance]

Sulfuric acid has been used in swimming pools, though it tends to increase the level of Total Dissolved Solids quite rapidly. It also adds to unwanted sulfates in the pool water.

It is used in pools where a few cents in the maintenance budget makes a difference but is NOT recommended, especially for public pools.

[Jelmo]

Sulfuric acid has been used in swimming pools, though it tends to increase the level of Total Dissolved Solids quite rapidly. It also adds to unwanted sulfates in the pool water.

It is used in pools where a few cents in the maintenance budget makes a difference but is NOT recommended, especially for public pools.

Do you know what side effects sulphuric acid in pool water might have, particularly on the skin?

[Pool Balance]

Sulfuric acid, in low concentrations typical of swimming pool water, have no known effects on the skin, skin sensitisation or allergic reactions.

The danger from sulfuric acid is from the liquid and fumes in its concentrated form, and from the sulfates left behind in the water which increase the TDS value of the water.

The EPA guidelines for drinking water give a limit for sulfates of 250ppm, based on the aesthetic effects it has on taste and odor. It falls into the category of secondary standards, having no proven adverse health effects.

[Joey]

Does sulphuric acid increase the sodium hypochlorite demand?

[chem geek]

Not directly, but any acid that lowers the pH will have the chlorine breakdown a little faster in sunlight. [EDIT] I got this backwards -- hypochlorite ion breaks down faster in sunlight than hypochlorous acid so it's actually higher pH where chlorine will breakdown somewhat faster in sunlight. [END-EDIT] The main side effect of sulfuric acid, as well as with dry acid (sodium bisulfate) is an increase in sulfates. High sulfate levels can hurt saltwater chlorine generator (SWG) cells and splashed out magnesium sulfate can be very harsh to stone or cement (it has one of the highest salt recrystallization pressures). Unfortunately, I don't know at what level sulfates start to be a problem in these two areas (i.e. how high is "high").

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.

[FlPoolPro]

Sulfuric acid has been used in swimming pools, though it tends to increase the level of Total Dissolved Solids quite rapidly. It also adds to unwanted sulfates in the pool water.

It is used in pools where a few cents in the maintenance budget makes a difference but is NOT recommended, especially for public pools.

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.

[FlPoolPro]

Does sulphuric acid increase the sodium hypochlorite demand?

No. Your sanitizer level will increase as your pH is lowered. Sunlite is a huge factor on sanitizer levels. for additional information pick up a K2004 Taylor water test manual at a local pool store. Aquacheck also has a great book on pool water chemistry with their test strips.

[FlPoolPro]

Not directly, but any acid that lowers the pH will have the chlorine breakdown a little faster in sunlight. The main side effect of sulfuric acid, as well as with dry acid (sodium bisulfate) is an increase in sulfates. High sulfate levels can hurt saltwater chlorine generator (SWG) cells and splashed out magnesium sulfate can be very harsh to stone or cement (it has one of the highest salt recrystallization pressures). Unfortunately, I don't know at what level sulfates start to be a problem in these two areas (i.e. how high is "high").

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.

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.

[chem geek]

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.

This is pure bunk. Please provide a link or contact phone number to the company that claims to have scientific research showing that different strong acids result in differing amounts of Total Alkalinity (TA) reduction. I will then contact them.

Adding 25-1/2 fluid ounces of full-strength Muriatic Acid (31.45% Hydrochloric Acid) or 34.4 ounces weight of Dry Acid (93.2% Sodium Bisulfate) or 26 fluid ounces of 38.5% Sulfuric Acid in 10,000 gallons all result in a lowering of Total Alkalinity of 10 ppm regardless of starting TA or pH (and also results in a pH drop from 7.5 to 7.07 if the starting TA is 100, though the drop in pH depends on starting pH and TA).

Adding a strong acid always lowers both pH and TA just as adding a strong base always raises pH and TA. If one wants to lower only the TA, then one can do so efficiently using the procedure described in this post. One essentially accelerates the process of driving carbon dioxide out of the water through a combination of aeration of the water at low pH. Pools are intentionally over-carbonated both to provide a pH buffer and to saturate the water with calcium carbonate to protect plaster surfaces. Just as stirring a carbonated beverage vigorously or blowing bubbles through a straw causes it to go flat faster, the same is true for pools via aeration. Lower pH speeds up the process as well as more of the carbonates in the water are in the form of carbon dioxide. The factor by which there is more carbon dioxide in pool/spa water than there is with equilibrium in air is shown in this chart.

[chem geek]

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]

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]

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]

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]

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.