by Glen Brizius

Created on: October 08, 2009

Hydrochloric acid, or HCl in chemists shorthand, is commonly known as muriatic acid, which lends clues to its most ready method of production for the practicing lab-scale chemist. Muriatic is an old term, meaning concerning brine (sodium chloride).

Muriatic acid is, surprisingly, commercially available. You can find it at any well-stocked hardware store. It has many uses, although caution must be used as it is extremely corrosive; exposure to skin or mucous membranes requires immediate and extensive flushing with water (not an 'antidote' such as a solution with a basic pH such as bicarbonate; the resulting exothermic, or heat-releasing, acid-base reaction will cause even more damage to the effected area).

Many are confused by the fact that the most common form of hydrochloric acid they see in a laboratory (so called concentrated hydrochloric acid the same as the muriatic acid you might buy from a commercial vendor) is a colorless liquid. HCl is actually a gas a white, misty, toxic, corrosive gas. Unlike other acids, such as sulfuric acid (which is an oily liquid at room temperature) pure HCl is not a liquid. However, it dissolves to an appreciable extent in water, up to a maximum concentration of about 37% HCl. Bubbling more HCl through the acidic water at that point has no value, the gas will simple bubble up to the top of the container and escape into the air.

In the laboratory, it would make no sense to generate your own aqueous HCl (37% in water). It's cheap and readily available in high grades of purity. However, HCl gas is a whole another story. It's expensive; it requires pressure vessels, gas regulators, pressure gauges, metallic hoses, and connection fittings all of which must be corrosive resistant. One pinprick leak in the setup and you'll be fleeing the scene, probably for your life.

It turns out that the production of small quantities of anhydrous (lacking water) HCl gas is critical for many industries. They don't need it all the time, so there's no sense in investing in the expensive equipment and safety training necessary to have a compressed cylinder of HCl gas on-site. Thankfully, muriatic acids name (pertaining to brine) gives us the clue we need to make small quantities of HCl gas for laboratory use.

A small amount of sodium chloride (aka regular table salt) is placed in the bottom of a sealed flask, with an inlet for liquids and an outlet connected to a length of rubber hose, which runs to the solution you wish to expose to the HCl

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