Muriatic acid is the common name for a diluted solution of hydrochloric acid (HCl), a strong mineral acid used widely in residential and industrial applications. Since this substance is an aqueous solution, meaning hydrogen chloride gas is dissolved in water, the liquid will eventually freeze just like pure water. However, the freezing point is significantly lower than water’s 0°C (32°F), requiring extremely cold temperatures to solidify. The specific freezing temperature depends on its concentration, which is the amount of hydrogen chloride dissolved in the water.
How Concentration Affects the Freezing Point
The presence of dissolved hydrogen chloride molecules changes the physical properties of the water solvent, a phenomenon known as freezing point depression. When the acid is mixed into water, the HCl molecules dissociate into hydrogen ions and chloride ions. These solute particles interfere with the water molecules’ ability to organize themselves into the fixed, crystalline structure of ice.
This disruption means a lower temperature is required for the water molecules to form a solid. Consequently, a higher concentration of the acid results in a lower freezing point. The more concentrated the muriatic acid, the colder the temperature must be for it to freeze.
Specific Freezing Points of Commercial Solutions
Commercial muriatic acid concentration typically falls within a range of 20% to 32% hydrogen chloride by weight. This variance results in freezing temperatures far below what most regions experience. For a 20% solution, the freezing point can be as low as approximately -59°C (-74.2°F).
A more concentrated solution, such as 32% hydrogen chloride, has a slightly higher freezing point, typically around -43°C (-45.4°F). Despite the differences, the temperatures remain extremely low across the entire commercial range. These temperatures are rarely reached outside of specialized industrial settings or the coldest, most remote environments.
Safe Storage and Handling in Cold Weather
Although the freezing point is very low, protecting muriatic acid from prolonged exposure to extreme cold prevents physical damage. Freezing causes the acid solution to expand, creating pressure that may rupture storage containers. If a container cracks, it releases the corrosive chemical and its toxic fumes, creating an immediate hazard.
For storage, keep the acid in its original, tightly closed, corrosive-resistant plastic container. Store it in a cool, dry, and well-ventilated location, protected from temperature extremes and direct sunlight. The storage area must be isolated from incompatible materials, especially metals, as corrosive fumes can cause rust and dangerous reactions.
If the acid has been exposed to temperatures near or below its freezing point, handle it with caution upon thawing. Frozen or partially frozen acid may experience stratification, where the solution separates and the concentration shifts. Before use, allow the acid to return to ambient temperature and gently mix it to ensure a uniform concentration.