Maryland D. Kemp
U.S. Army Engineer Research and Development Laboratories Fort Belvoir, Virginia
H.s.
I
A Safe Method for Preparation of Uncontaminated Hydrazoic Acid
Booth's "Inorganic Synthesis"' reports that hydrogen azide can be prepared by the reaction of strong sulfuric acid (40 per cent) upon a sodium aside solution containing sodium hydroxide. Sodium hydroxide is used as a moderator of the reaction since hydrazoic acid vapor in high concentrations is spontaneously explosive. In the procedute, the solution of sodium azide and sodium hydroxide colrtained in a flask connected through a condenser is brought to boiling and the acid is added to the hot boiling solution. During efforts to make pure aqueous solutions of hydrazoic acid in connection with the preparation of some pure alkali metallic azides it was observed that during the precipitation and crystallization of potassium azide small amounts of potassium sulfate were always present. Potassium sulfate is less soluble in water than potassium azide, and during attempts to grow potassium azide single tetragonal crystals, the orthorhombic sulfate form was always present. Sulfuric acid in the preseuce of its salts or more volatile acids tends to fume a t temperatures much below the boiling point, or a t about 160°C. During the addition of the sulfuric acid to the boiling solution in this procedure, it is obvious that the exothermic reaction between the strong sulfuric acid and the solution causes premature decomposition of the sulfuric acid, and fumes carried into the receiving flask are subsequently transformed, in this case t o potassium sulfate.
I42 /
Journal of Chemical Education
In an effort to eliminate the sulfate contamination this laboratory has modified the procedures somewhat. The same reagents are used as the source of the hydrazoic acid. The procedure is as follows: A saturated solution of sodium azide is placed in the round bottom flask and a stream of nitrogen is allowed to flow through for a t least thirty minutes. One and ?,f0 times the required stoichiometric amount of sulfuric acid is added dropwise, cold, and the mixture heated slowly to boiling and boiled 5 to 10 minutes. The evolved hydrazoic acid is received in a flask surrounded with an ice bath. The yield is almost quantitative, and any dilutions desired in the prepared hydrazoic acid solution are made by adding water to the receiving fiask. Nitrogen is allowed to flow through the system during the entire run although during the boiling it could be dispensed with. In 40 to 50 preparations of hydrazoic acid not once did any explosions occur although solutions 75 to 80 per cent in hydrazoic acid were often prepared. This modification of the procedure eliminates the hazard of high concentrations of hydrazoic acid in the vapor phase and also yields a product free of the sulfate impurity.
' AUDRIETH,L. F., AND GIBBS, C. F., I n o ~ g m i cSynthesis, McGraw-Hill Book Company, New York, 1939, Val. 1, p. 7"
8 , .
Presented for publication with permission of the Department of Defense.
