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There are some differences between muriatic acid (HCl), dry acid (sodium bisulfate, NaHSO4) and sulfuric acid (H2SO4) in the treatment of swimming pool water. We will attempt to review and clarify these three types of acids.

When added to water, all three of these acids increase the amount of hydrogen ions (also known as hydronium ions) present in water. These hydrogen ions immediately react with alkaline components in pool water such as bicarbonate (alkalinity) ions and convert them into carbonic acid, thus reducing the alkalinity in the water and lowering the pH. Also, when added, these three acids add a byproduct to the water. With muriatic, it is the chloride ions (Cl), with dry acid and sulfuric acid, the bi-product is sulfate (SO4) ions.

Muriatic acid (also known as hydrochloric acid) is probably the most common and most used type of acid for lower pH and alkalinity in pool water. The strength that is purchased for pool water use is normally 31.45%, but sometimes is marketed at 20% and even 10% in supermarkets and hardware stores.

Dry acid (sodium bisulfate) has become more popular in recent years, probably because it is a little easier and safer to use. Its strength is usually about 93-94%. But don’t let that fool you. While sodium bisulfate may have a higher strength percentage, it still costs more to use than muriatic acid. For every gallon of muriatic acid, it requires about 10.5 lbs. of sodium bisulfate to do the same job. While the cost varies depending volume purchased and other factors, if muriatic acid is about $3 to $4 per gallon, that would be compared to 10 lbs. of dry acid that costs about $8 to $12, perhaps more.

Some service techs use sulfuric acid to lower pH and alkalinity and also for acid washes. There is a general understanding that sulfuric acid may remove copper stains more effectively than does muriatic acid. The typical strength for sulfuric acid purchased in one gallon containers is around 38%. While the percentage strength of sulfuric acid (38.5%) is higher than muriatic acid (31.5%), they are nearly equal in pH and alkalinity reduction gallon for gallon. So the price per gallon between the two acids may be a deciding factor when considering which to use.

There is the possibility of purchasing a higher strength sulfuric acid in bulk containers which has the strength of 93% or higher. This high strength acid is about 3.5 times stronger than muriatic acid. So price wise, using this high strength sulfuric acid would save money. However, one problem with using 93% sulfuric acid is that it is more hazardous to use. It is extremely corrosive, more so than muriatic acid and sodium bisulfate.

Also, other potential problems to consider when using sulfuric acid (including sodium bisulfate) is that they add sulfates to the water. In time, the content of sulfate increases in the water, which then can combine with calcium ions and form a precipitate crystal. Calcium sulfate deposits can be particularly difficult to remove from pools. Regular acid washes do not have much of an effect on removing this deposit. High sulfate contents in pool water can also be harder (more corrosive) to cement finishes.

The following are formulas on how much of the three acids to add for lowering alkalinity.

For muriatic acid, it is Volume (of the water) divided by 125,000 then multiplied by the ppm desired reduction in alkalinity which then gives the amount of QUARTS to add.

For sodium bisulfate (dry acid), it is Volume divided by 47,000 then multiplied by the ppm desired reduction in alkalinity which then gives the POUNDS to add.

For 38.5% sulfuric acid, it is Volume divided by 128,000 then multiplied by the ppm desired reduction in alkalinity which then gives the QUARTS to add.

For 93% sulfuric acid, it is Volume divided by 432,000 then multiplied by the ppm desired reduction in alkalinity which then gives the QUARTS to add.

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Comment by Kim Skinner on September 1, 2011 at 11:49am

Hold on. I just read in a ACI manual (212.3R-10) that resistance to sulfate attack is decreased when concreate (plaster) mixes contain calcium chloride. So that leaves in question what I read earlier and commented on. 

Comment by Kim Skinner on August 29, 2011 at 4:07pm

Gee thanks, Richard.  You think I don't have enough to do? Trying to get me in trouble?  You could get your hands messy and make some plaster coupons too. ;-) 

A very interesting issue; sulfate solutions are corrosive to cement, and yet can precipitate out as calcium sulfate scale. And one other thing, calcium sulfate (gypsum) is added to cement to regulate the hardening process. Complicated?  Perhaps some day we can get a handle on this.  

Comment by Richard A. Falk on August 27, 2011 at 9:33am

Looks like an experiment for OnBalance to try with plaster coupons and various sulfate and chloride levels :-)

 

Suflate also lowers the saturation index due to the calcium sulfate ion pair.  At 300 ppm CH, 400 ppm sulfate lowers the saturation index by 0.11 so this is not a big effect unless the levels get extraordinarily large.

 

This link says that sulfate can increase the likelihood of scale in a saltwater chlorine generator cell though I'm not sure why (the phosphorous makes sense due to precipitation of calcium phosphate at the high pH at one of the plates).  This paper and this paper talk about calcium sulfate precipitation (and calcium carbonate sulfate combinations), but it seems that the concentrations have to get pretty high for that to occur so maybe the sulfate accelerates calcium carbonate precipitation which is the primary scale in salt cells.  This paper shows that magnesium sulfate has a rather high salt recrystallization pressure which would mean that splash-out with evaporation (especially multiple cycles of this) could be harmful to hardscape surfaces.

Comment by Kim Skinner on August 27, 2011 at 9:16am
Thanks Richard for that link and information on sulfate attack.  I have done some reading and while it is true that sulfates are not good for pool plaster, I suspect that the 300 mg/l is a low or very conservative number for pool plaster.  One source I read stated that chloride in plaster inhibits the effect of sulfates. When it comes to chemistry, it seems that there can be occasional exceptions or variables to contend with.
Comment by Richard A. Falk on August 19, 2011 at 6:57pm
Though this is an old blog, this thread gives references on the dangers of high sulfate levels to concrete which might apply to pool plaster as well.  A few references give a 300 mg/L limit on sulfate concentration in the water.
Comment by Kim Skinner on July 29, 2009 at 11:25am
Your additional and beneficial information should be known as # 2 on this ACID blog.
My understanding on the point at which precipitation occurs or the solubility of calcium sulfate is around 1200 ppm. But this can be slightly affected by other variables.

Sulfates are corrosive to cement products, more so than chlorides, but still would require high amounts (high ppm in pool water) over a long period of time to be significantly detrimental.v How much exactly, I don't know.

I agree that 93% sulfuric acid should not be used in our industry because it is too dangerous.
Comment by Al Neumann on July 21, 2009 at 6:45am
Kim,
Very good beginning on a discussion of pool acids, and I do hope this is just a beginning, as the discussion can go far deeper. In our area, in Central Wisconsin, the difference in use of acids is related to the relative safety of use, and fuming issues, which is not only a safety issue, but a corrosion issue as well.

38% Sulphuric Acid
On the plus side, one of the key reason why some promote it’s use is that it is supposed to be less fuming than muriatic acid, so therefore less corrosive to equipment rooms. Another plus at least in our area, is that there is one distributor that bulk delivers this product, and pumps it directly into bulk tanks, taking the handling of it away from the operators.

On the negative side, from what I’ve been told, it is more hazardous to use than muriatic acid due to being a “heavier acid”– (lack of a better term) – in that it is harder to wash off and will likely cause a more severe, longer lasting injury than muriatic would on contact with skin.

Another disadvantage is the one that you mentioned about the little known sulphate by-product issue. Another potential source of sulphate in the water that you didn’t mention was in the regular use of non-chorine shocks. It all adds up.
At least in our area, it costs considerably more than muriatic, and there aren’t that many distributors that carry it, so availability is an issue.

Questions I have on its use would be
At what point does high sulphate becomes a problem? How do they affect the Langelier Index? Are they even considered in the Rhysner Index? How often should they be tested for? The only tester I know that tests for them is the Palintest 25,Photometer. If too high, is dilution the only way to get rid of them?

Why would you even mention 93% Sulphuric acid, and give the formula for its usage. It has to be far too dangerous of a product to use for most individuals in the pool industry. If not for the service companies, then what about the operators, or the potential issues when metering pumps fail and spray product all over. Another issue is that this percentage also generates its own heat, enough to weaken many common types storage containers – making storage an issue. Both are potential liability issues I want to stay a long ways away from.

Muriatic Acid – 31%
There’s little to say abut this acid that you already didn’t cover. It’s the main acid used in most pools…because of its availability and pricing. The disadvantages are that it is high fuming if undiluted, and therefore is a cause of a lot of corrosion in equipment rooms – particularly if partially empty carboys are left open or with loose caps. Same comment can go for storage containers of bleach or acid that have uncapped or oversized holes in them.

Questions I have on its use would be
Most concerns about many acids evolve around safety issues of handling and fuming, and concerns of corrosion. At what minimum dilution level is the fuming level reduced?

Specialty “Safe” Acids
What’s your take on the blended Muriatic and Sulphuric Acids that have become available? These acids are promoted as being relatively non-fuming, and relatively non-reactive to contact with intact skin. In our area, there are 2 types. Acid MagicTM is a base of 20% Muriactic Acid, with it’s own proprietary blend of other reducers and chemicals. Likewise, Crystal ClearTM is a base of 20% Sulphuric Acid, along with there own proprietary blend of reducers and chemicals.

Both claim that amounts needed is pretty much the same as the same as the other acids mentioned above – this being because of the proprietary blend of other reducers in the mix.

Questions I have on its use would be
For many facilities, the issues of safety and being non-fuming, make the switch to either of these products a no-brainer. For them, the added cost is worth the benefit…and on that note, I agree. However, the proprietary blend” is what bothers me somewhat, in that should there be a concern with by-products that we don’t know about. What effect do they have on the pool water in the long term? Anybody have any incite on this?
Comment by Bruce Hudson on July 15, 2009 at 2:39pm
Thanks Kim. Looking forward to Part 2.

We changed from muriatic to sulphuric mainly to avoid killing plants around the pool with the "cloud" during high humidity in South Florida. We also found that our bulk storage area suffered less corrosion to metal fixtures and electric controls. Repairs are nothing though compared to the ill will of killing your customers prized potted plants.

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