While the names “muriatic acid” and “hydrochloric acid” are often used interchangeably, chemically they are identical. Both refer to an aqueous solution of hydrogen chloride (HCl) gas dissolved in water. The distinction between the two is not a matter of different chemical compounds, but rather a difference in purity, concentration, and context of use. Hydrochloric acid is the modern, scientific name, while muriatic acid is the historical and commercial name for a less refined grade. This difference in grade affects how the substance looks, where it is used, and the safety precautions required for handling it.
The Shared Chemical Identity
Both muriatic acid and hydrochloric acid are classified as strong mineral acids, meaning they completely dissociate in water. The underlying chemical is hydrogen chloride (HCl), a diatomic molecule that is a gas at room temperature. When this gas is dissolved in water, it forms hydrochloric acid.
In the aqueous solution, the hydrogen chloride molecules break apart, producing chloride ions and hydronium ions. This abundance of hydronium ions is responsible for the corrosive nature and low pH of the acid, typically less than 1.0 in concentrated forms. Because both names refer to this exact chemical structure, they share the same fundamental acidic properties and chemical reactivity.
Muriatic Acid: Purity and Commercial Grade
The primary difference between the two terms is the level of refinement and the historical context of the name. Muriatic acid is the common name used for the industrial or commercial grade of hydrochloric acid. This grade is less pure than the laboratory-grade hydrochloric acid used in scientific settings.
Commercial muriatic acid often contains trace amounts of impurities, such as iron, residuals from the manufacturing process. The presence of iron compounds, specifically ferric chloride, gives muriatic acid its characteristic pale yellow to slightly greenish color. Highly refined, laboratory-grade hydrochloric acid is colorless.
The name “muriatic” is a historical term, derived from the Latin word muria, meaning “brine” or “saltwater.” This name originated because the acid was historically produced using common salt. The commercial product is typically sold at a concentration between 20% and 35% hydrochloric acid.
Handling and Practical Uses
Muriatic acid is widely used in consumer and construction applications due to its potent cleaning and dissolving power. One frequent use is in pool maintenance, where it is added to lower the water’s pH and total alkalinity. It is also a standard product for cleaning and preparing masonry, such as etching concrete before sealing or painting, and removing mineral deposits or efflorescence from brick and tile.
Given its strength, even the commercial grade requires caution during handling. The acid is corrosive and emits strong, pungent fumes that can irritate the respiratory system. Personal protective equipment, including chemical-resistant gloves, eye protection, and a face shield, is necessary to prevent skin and eye contact.
Work should always be done in a well-ventilated area or outdoors to minimize the inhalation of the harmful vapors. When diluting the acid, slowly add the acid to water, never the reverse, to control the heat generated by the reaction. Muriatic acid should never be mixed with chlorine bleach or ammonia, as this combination releases highly toxic gases.