# Why is HCl the acid of choice for swimming pools?

Why is muriatic acid the go to acid to put into pools? Does it work better than nitric acid, sulfuric acid, or another strong acid? I mean they all will add the same amount of H+ right? Note, we use some cyanuric acid as well, but mainly HCl for adjusting pH.

• The HCl not only increases the acidity of the pool but also increases the concentration of "free chlorine", or as chemists like to call it, hypochlorous acid. This is why most pools smell strongly of bleach. Also, HCl is much cheaper and less dangerous than nitric and sulfuric acid. May 16, 2020 at 1:36
• "muriatic acid" term is not used for many decades, some young chemists may not even know what it is. Hydrochloric acid is the term to use. I see this is not the first post combining the archaic term with pools and the author (you?) was already informed. May 16, 2020 at 4:13
• @Poutnik well I took a few chemistry classes so I know it as HCL. But when marketed as pool supplies, they call it muriatic acid. Even says so on the box
– Ryan
May 16, 2020 at 4:17
• Yes, I have already guessed it would be the case. May 16, 2020 at 4:18

There are several reasons not to consider these acids to be interchangeable for use in swimming pools. A total answer covers aspects of chemistry, biology, safety, availability, and cost. As hydrochloric, nitric, and sulfuric acids are all strong acids and can provide the desired acidification, they are very different when you consider their respective counter ions. Do you want chloride, nitrate, or sulfate in your pool water?

Safety First:

• Hydrochloric, nitric, and sulfuric acid are all strong acids that are capable of causing chemical burns and present inhalation hazard from their fumes.
Additional safety information can be obtained from PubChem.:
• Hydrochloric acid
• Sulfuric acid
• Nitric acid

Chemistry:

• Nitric and sulfuric acids are more corrosive to copper and stainless steel. Copper (I) chloride is significantly less soluble in water than copper salts of nitric and sulfuric acids. Swimming pool heaters are likely to contain copper pipe.
• As an added benefit for salt pools, which use an electrochemical chlorine generator, the Cl- ions are converted to chlorine for disinfecting the pool water.

Biology:

• As pointed out in another response, nitrates can promote algae growth.

Availability and Cost:

• Hydrochloric acid is commonly available as muriatic acid (31.5% HCl, ~10 molar) from home centers, building and construction suppliers, and hardware stores, because it is used in many construction-related activities, with cost ranging from \$5 - 9 (USD) per US gallon. A gallon will contain approximately 37 mole equivalents of H+. • Sulfuric acid is available from auto parts retailers as battery acid (37% H2SO4) and costs approximately \$10 (USD) per US quart ($40 per gallon). A gallon of battery acid will contain approximately 36 mole equivalents of H+ (adjusting for normality). Sulfuric acid costs 4-8 times more than hydrochloric acid for equivalent pH reducing capacity. • Nitric acid is less readily available to the consumer market. One on-line supplier offered reagent grade (70%) for \$125 (USD) per liter.

Cyanuric acid is a weak acid and serves as a stabilizer. Quoting from PubChem:

Cyanuric acid is used in swimming pools to lower the rate of photochemical reduction of chlorine, hypochlorous acid and hypochlorite ion.

• About the price of H2SO4 versus HCl, I have always thought the latter is generally more expensive with H2SO4 being the cheapest mineral acid. Is it possible that the "accumulator grade" H2SO4 is extra expensive, because like any product aimed to cars is often much more expensive than similar product not aimed on cars ? At least I have such experience. May 18, 2020 at 10:45
• @Poutnik Price and availability may depend upon country. I'm speaking for US markets and don't know what the situation is in Europe or other locations, which may be very different. For the US, sulfuric acid is also found in some drain cleaners, but those products contain additives, like gels and "metal inhibitors," that seem undesirable for a swimming pool. Muriatic acid remains the most cost effective. Acid concentration and additives are also treated as trade secrets for some products. May 18, 2020 at 12:10
• I had rather meant acid itself may be much cheaper than final products aimed to particular customers. May 18, 2020 at 12:13
• Comparing prices from Sigma-Aldrich (now Millipore), reagent grade H2SO4 (95-98%) costs \$85 (USD) for 2.5 L versus reagent grade HCl (37%) which costs \$108 (USD) for 2.5 L. Sulfuric acid is cheaper than hydrochloric acid, but readily available muriatic acid (31.5% HCl) is still less expensive for consumer markets. May 18, 2020 at 12:28
• Reagent grade is too good for pools, I guess. May 18, 2020 at 12:30

The equivalent amounts of hydrochloric, sulphuric or nitric acid are interchangeable, acidity-wise. But there are other properties that may lead to prefer one over the others:

Nitric acid would provide nitrates as food to algae or other microflora.

Sulphuric acid is unpleasant for manipulation, as even tiny droplets create during the time holes in clothes. Additionally, but not sure if it happens, calcium rich water, when mixed with the acid, may locally and temporarily create a haze of calcium sulphate due its limited solubility, what could raise questions if anything is wrong.

Cyanuric acid is a weak acid ( weaker than acetic acid ), not well suitable for pH adjustment. It is rather used as the chlorine stabilizer.

Cyanuric acid is used as a chlorine stabilizer in swimming pools. It binds to free chlorine and releases it slowly, extending the time needed to deplete each dose of sanitizer.