Producing Hydrochloric Acid Gas on a Laboratory Scale its use for Binding to Amines

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 gas. Pure, concentrated sulfuric acid is added slowly to the mound of table salt through the liquid inlet. A rapid foaming reaction takes place, generating some heat, and HCl gas pure of impurities and water immediately begins to bubble through the rubber tubing and into your second flask, the one containing the material of interest.

Why is this so important? It turns out that many pharmaceuticals of importance contain nitrogen groups known to organic chemists as amines. Have you ever smelled a rotting fish? That smell is amines, being given off by the decaying matter. These pharmaceutically important amines are often oils or waxy, pasty solids, that reek of rotting fish hardly something you can package into an easy to take tablet, let alone market to a discriminating public. However, by bubbling HCl gas through a solution of your pharmaceutical in some suitable inert, organic solvent, a wonderful change takes place. The HCl binds to the amine in an acid-base reaction, and the pharmaceutical molecule precipitates out of solution as a white, crystalline, odorless, often tasteless, powder. A quick filtration and drying later, and you as the chemist manufacturer are primed to begin marketing your medication as your drug can now be accurately weighed, mixed with inert ingredients, and pressed into hard tablets.

Which would you prefer to ingest a foul smelling waxy paste or a tasteless, odorless, innocuous white pill? Yep, me too! And the real kicker of this whole story is that it makes absolutely no difference to your body. If you were to somehow choke down the non-HCl bound amine medication, once it hit the hydrochloric acid that is naturally present in your stomach, it would form the precise same odorless, tasteless, white crystalline salt. So why put yourself through all of that horrible process of swallowing something so foul?

You can easily tell which medications have been processed in this way, simply by looking at the active ingredients on the back of the medication container. A prime example is Sudafed. The active ingredient is an amine-containing molecule called pseudoephedrine, yet what do you find labeled as the active ingredient on the rear of the box? Pseudoephedrine hydrochloride. Pseudoephedrine has been treated with HCl gas to make the medication more palatable, easier to dispense, and it doesn’t change the chemistry of the product one bit.

Practically anyone who has ever taken one of hundreds of different medications owes a debt of gastrointestinal gratitude to this lowly, but effective, method of producing hydrochloric acid. It’s a simple trick, but so very effective.