When you're looking for a product to lower the pH of your swimming pool water, you go to the store and buy a container of muriatic acid. You'd probably be squeamish about hydrochloric acid in your pool instead, especially if you're about to go swimming, but in fact, that's essentially what you're doing. Muriatic acid is one of the names of hydrochloric acid, and it was the most common name until French chemist Joseph Louis Gay-Lussac coined the term hydrochloric acid in the early 19th century. Modern chemists make a differentiation between muriatic and hydrochloric acid based on concentration and purity. They both have the chemical formula HCl.
Both muriatic and hydrochloric acid consist of hydrogen chloride (HCl) dissolved in water. Hydrogen chloride is a gas at room temperature, and in times gone by, the most common way to produce it was to react a salt, such as sodium chloride (NaCl) with an acid. That's where the word "muriatic" comes from. It refers to brine or salt. Reacting salt with sulfuric acid is still a common way to produce HCl gas, which is then dissolved in water to produce hydrochloric or muriatic acid.
If you have reservations about using HCl acid around the house, you're right to be concerned. Muriatic acid may be one of the most effective stain removers you can find, and it's the best product for lowering pool alkalinity, but it's dangerous and must be used with care. Don't be too worried about it, though. You're carrying HCl around in your body as a primary component of stomach acid. If it wasn't there, you wouldn't be able to digest your food.
Production of Muriatic Acid
Chemical companies produce muriatic acid by dissolving hydrogen chloride gas in water. The concentration determines whether they label the product muriatic acid or hydrochloric acid. While there is no definitive standard that governs the difference, generally any solution with a concentration of more than 31.5 percent HCl by mass qualifies as hydrochloric acid, and anything less is muriatic acid. Many muriatic acid solutions are diluted to somewhere between 14.5 and 29 percent.
The most common method for producing hydrogen chloride gas is mixing common table salt with sulfuric acid. The reaction proceeds in two stages. In the first, the products are sodium bisulfate and hydrogen chloride:
NaCl + H2SO4 → NaHSO4+ HCl
Sodium bisulfate is an acid salt that also reacts with sodium chloride to produce sodium sulfate and hydrogen chloride, but this reaction only occurs at higher temperatures and in the absence of excess water.
NaCl + NaHSO4 → Na2SO4 + HCl
If the reaction is conducted with a strong sulfuric acid solution, hydrogen chloride gas is emitted and can be captured in a distillation flask. If the sulfuric acid solution is weak, which means more water is present, the hydrogen chloride dissolves in the water while the salts precipitate out.
While the final concentration of hydrogen chloride – or the density of the HCl solution – determines whether the product gets labeled as hydrochloric or muriatic acid, the purity of the solution is also important. Hydrochloric acid is generally free of contaminates and is a clear-colored liquid. Muriatic acid frequently contains impurities that give it a pale yellow hue. The main impurity is usually iron, which is responsible for the yellow color, but other minerals may also be present. These minerals usually have no effect on the strength of the acid.
Some Uses of Muriatic Acid
Historically, muriatic acid became famous in the search for the philosopher's stone – a substance capable of transforming a base metal into gold or silver. At the beginning of the Industrial Revolution, it became an important ingredient in steel production. It effectively dissolves rust, so steel producers use an 18 percent concentration to "pickle" steel. Muriatic acid is also a primary ingredient in the production of polyvinyl chloride (PVC), a type of plastic with multiple uses. It's also used in gelatin production and leather processing. Pouring muriatic acid over limestone is a way to produce calcium chloride, a salt used for de-icing roads.
Around the house, the most common use of muriatic acid – besides regulation of swimming pool acidity – is for cleaning. Because of its ability to dissolve mineral salts, muriatic acid is the go-to product when you want to remove mineral stains from masonry, ceramic or porcelain. For example, when basement walls get discolored by efflorescence, which are ground minerals that seep through porous concrete, you remove them by scrubbing with a dilute muriatic acid solution. When a toilet bowl gets discolored by iron and manganese stains, muriatic acid is the most effective cleaning agent.
When using muriatic acid for cleaning, you typically spray or pour it on the surface you're cleaning, give it a few minutes to work and then scrub. When the stain is gone, flush with plenty of clear water. In some cases, it's a good idea to neutralize the surface with baking soda (sodium bicarbonate), a strong base.
How to Use Muriatic Acid Safely
Muriatic acid is one of the strongest chemicals you can buy without a license, and proper handling is essential. If you use it improperly, you can suffer skin burns. If you mix it with certain other chemicals, it can release toxic gas that can damage your respiratory system and, in extreme cases, even kill you. Because it is so hazardous, you should follow these important safety guidelines:
- Always wear protective clothing, gloves and eyewear. This is important even if you're simply pouring muriatic acid in your pool, because a sudden gust of wind could blow the liquid back in your face. If you get muriatic acid on your skin or in your eyes, flush with plenty of pure water. In severe cases, neutralize with baking soda before flushing with water.
- Always add acid to water – never the other way around. If you pour water into muriatic acid, a violent reaction occurs that causes the solution to bubble up and spray acid in all directions.
- Do not mix muriatic acid with other chemicals, especially bleach (sodium hypochlorite) or potassium permanganate (KMnO4). Combination with these chemicals in particular produces toxic chlorine gas.
- Dispose of muriatic acid responsibly. When using it to clean a toilet, don't simply flush it into the plumbing system where it can corrode pipes and pollute the waste system. Either neutralize the bowl water with plenty of baking soda or transfer the water to a bucket for disposal as hazardous waste.
- Store muriatic acid in plastic or glass containers. It corrodes metal, so you should never keep it in a metal container, such as an old paint can.