Difference between revisions of "Glauber's hydrochloric acid synthesis"

Revision as of 16:32, 2 February 2016

Glauber's hydrochloric acid synthesis was the method through which concentrated hydrochloric acid (spiritus salis) was first prepared. It is still commonly used in labs.

The reaction

NaCl + H₂SO₄ -> HCl^ + NaHSO₄

It is not recommended to use excess NaCl, because bisulfate is a more useful side product than sulfate. Also it is not recommended to use iodinated table salt as the reagent: the resulting acid may come out yellowish due to traces of iodine (sulfuric acid first displaces hydroiodic acid from iodide, then oxidizes it to form elemental iodine).

The setup

Glauber's original setup was a retort, the "beak" of which was submerged into water which absorbed hydrogen chloride. If you use such a setup, use a retort with a "beak" as wide as possible, and do not submerge it too deep. It is good protection agaist water suckback, which hydrogen chloride loves to do: the wider the nose of the retort compared to the neck of the receiving flask, the quicker lowers the water in the flask, letting air in and stopping the suckback.

If you use a modern setup with a flask and thin glass tubing, use a funnel trap to prevent suckback.

The reagents

First, you need sulphuric acid, with a concentration no less than 60% (chamber acid). If you do not have sulfuric acid of this strength, concentrated phosphoric acid (80-85%) will do in a pinch (but remember that this synthesis involves heating, and hot phosphoric acid slowly attacks glassware).

Second, you need sodium chloride. It is not that hard to obtain this salt in any supermarket, but non-iodinated salt should be preferred.

Step by step guide

1. Measure the stoichiometric amounts of reagents.
2. Put the salt in your boiling flask or retort.
3. Pour the acid into your boiling flask or retort.
4. Assemble the setup, put the receiving flask on its place.
5. Start to heat the boiling flask or retort gently.
6. Soon you will see air bubbling in your receiving vessel. The bubbling gradually slows down as air in the reaction flask is displaced by hydrogen chloride. When pure HCl will start to flow through the tube/neck, bubbling will cease altogether, water will rise through the funnel / retort's nose and start to make jittering motions, quickly going up and down. In the water, "squirming glassy worms" will be visible. This means, hydrogen chloride is dissolving in water without exiting the apparatus. This is exactly what you want to do.
7. During the reaction, the receiving vessel will be heated significantly. If you want to prepare fully saturated acid (36-40%), cool the receiving vessel with ice.
8. After bubbling in the reaction flask ceases, turn off the heating, cool down and disassemble the apparatus. Proceed to clean out the "goat" of bisulphate or dihydrophosphate from your boiling flask or retort.

Alternative method

Also used by Glauber, this method allows obtaining azeotropic hydrochloric acid (21-22%). Use dilute sulfuric acid, the Bat (36%) will do. The reaction does not start right away, you have to distill the mixture. The distillate comes out as the desired acid. This method is simpler if an acid of this concentration is desired, but not recommended if you want very concentrated (fuming) hydrochloric acid. If you need that, use the azeotropic acid as the starting liquid in the process of saturating it with hydrogen chloride.