Aqueous Hydrazoic Acid Solutions

Thrush [42] made small amounts of hydrogen azide by bubbling nitrogen through an aqueous hydrazoic acid solution the gases were dried with P2O5. 

Deuterium azide, DN3, was made from deuterophosphoric acid and sodium azide, either by dropping the acid onto the azide [43,44], or by adding dropwise a solution of sodium azide in deuterium oxide to the deuterophosphoric acid [45]. The Guenther-Meyer reaction, using deuterostearic acid, was also used [46]. 

Concentrated solutions of hydrazoic acid are not safe to handle as they explode on mild mechanical shock and may also explode spontaneously. Dilute 

Like hydrogen azide, the aqueous hydrazoic acid solutions are made by decomposing sodium azide with an acid, which is followed by separating the product by any of the following methods. One of them [47,48] makes use of the volatility of HN3 with water vapors  

Sodium azide (15 g) is dissolved in 150 ml water and placed in a flask fitted with a dropping funnel and a Liebig condenser the receiver contains 100 ml water. After the solution has been brought to boil, 90 ml sulfuric acid (40%, v/v) is added dropwise. To obtain a 3% hydrazoic acid, distillation is continued until 50 ml remains in the flask. It is important to add the acid to the already boiling solution, as otherwise concentrated hydrazoic acid vapors build up in the reaction flask and cause explosion. 

---

In summary, aqueous hydrazoic acid solutions up to 1 molar are conveniently made by cation exchange and are obtained chemically pure from anion pure (nitrate) sodium azide and a prepurified resin. Purification of the latter by washing the heavy metals out as chloro complexes is simple in principle, but time-consuming. For projects requiring numerous hydrazoic acid batches the approach has its advantages, but for single preparations distillation methods are preferable. 

Hydrazoic acid is unique in that both the aqueous and gaseous phase are explosive hazards. Detonations of hydrazoic acid solutions are capable of causing serious injury. It is possible to work with gaseous HNs.if an inert diluent gas is used. 

Kramer [48] calculated the equilibrium constant to be 3.5 X 10" and the pressure of hydrazoic acid is 1.9 X 10- mm at a carbon dioxide pressure of 1 mm. Aqueous hydrazoic acid in equilibrium with the gas phase at 25°C would be 2.3 X 10" M with respect to hydrazoic acid. The equilibrium constants were measured [48] for militaiy-grade azide in 50 50 alcohol-water solutions and are summarized in Table I. 

It was made by dissolving finely ground cobalt carbonate in ethereal hydrazoic acid. After 3-4 days the clear solution is evaporated to obtain reddish-brown crystals. The product, which retains some water tenaciously, is best stored protected from light under ethereal hydrazoic acid [156]. In another approach, cobalt carbonate was dissolved in aqueous hydrazoic acid and the solution concentrated to obtain a basic azide. This was again dissolved in strong hydrazoic acid to produce the reddish-brown crystals. The product was considered to be Co(N3)2 [157] but was not analyzed. 

N4H4 is made by simple metathetic reaction in liquid, solid, or gaseous media which may involve distillation or precipitation. For example, equimolar amounts of ammonium chloride and sodium azide may be distilled with an equal quantity of water. At 160°C pot temperature, the product volatilizes with water vapors and solidifies in the condenser tube which should, therefore, be at least 1 inch wide [305]. Equally clean and safe is a gas-phase reaction which requires, however, the preparation of hydrazoic acid gas. The reaction takes place in a long, 1-inch-wide glass tube which has two inlet tubes with orifices 20 inches apart, and a vent. The HN3 gas, carried with nitrogen, and excess ammonia stream in and precipitate the product as fine needles [39]. N4H4 is also precipitated when ammonia gas is bubbled into an ethereal hydrazoic acid solution [86]. The product stays in solution when HN3 vapors, carried with nitrogen, are bubbled into aqueous ammonia [306]. 

Pure hydrazoic acid is a colourless liquid, b.p. 310 K. It is very ready to detonate violently when subjected to even slight shock, and so is used in aqueous solution. It is a weak acid, reacting with alkali to give azides, which contain the ion Nj. 

Silver Azide. Silver a2ide, AgN, is prepared by treating an aqueous solution of silver nitrate with hydrazine (qv) or hydrazoic acid. It is shock-sensitive and decomposes violendy when heated. 

CAUTION All azides, particularly low molecular weight acyl and alkyl azides, are explosive, and great care should be taken while preparing and handling these materials. In addition, hydrazoic acid, which is liberated from unbuffered aqueous solutions of sodium azide, is highly toxic and all operations involving its use should be carried out in an efficient fume hood. 

C17-0062. For a 1.50 M aqueous solution of hydrazoic acid, HN3, do the following (a) Identify the major and minor species, (b) Compute concentrations of all species present, (c) Find the pH. (d) Draw a molecular picture illustrating the equilibrium reaction that determines the pH. 

Insensitive to impact, it decomposes, sometimes explosively, above its m.p. [1], particularly if heated rapidly [2], Although used in aqueous solutions as a preservative in pharmaceutical preparations, application of freeze-drying techniques to such solutions has led to problems arising from volatilisation of traces of hydrazoic acid from non-neutral solutions, condensation in metal lines, traps or filters, and formation of heavy metal azides in contact with lead, copper or zinc components in the drying plant [3,4],... 

An aqueous solution containing hydrazoic acid results when a solution of nitrous acid reacts with a hydrazinium salt. 

When thiocyanate ions are added to nitrous acid in water, a pink colouration develops which is believed to be due to the formation of nitrosyl thiocyanate (equation 34), which is too unstable to be isolated but which can be used as a nitrosating agent in aqueous solution. Because the equilibrium constant for ONSCN formation81 is quite large (30 dm6mol 2) at 25 °C, thiocyanate ion is an excellent catalyst for aqueous electrophilic nitrosation. The well established82 series is Cl- < Br < SCN < (NH2)2CS. Thiocyanate ion is also a sufficiently powerful nucleophile to react in acid solution with nitrosamines in a denitrosation process (equation 35), which can only be driven to the right if the nitrosyl thiocyanate is removed by, e.g., reaction with a nitrite trap such as hydrazoic acid. 

The reaction is monomolecular. The half-life at 330°C is 12 min. In a dilute aqueous solution hydrazoic acid is stable and not liable to decompose even on long boiling (Curtius [46]). However, a 17% aqueous solution of hydrogen azide can probably detonate [47]. 

The salts of hydrazoic acid, the azides, have solubilities similar to those of the corresponding chlorides. Sodium azide dissolves in water. Silver azide does not dissolve in water or in nitric acid, but dissolves easily in an aqueous solution of ammonia. Lead azide, like lead chloride, is sparingly soluble in cold water but more soluble in hot water it is also soluble in ammonium acetate. 

Curtius and Rissom [41] prepared cupric azide by the action of an aqueous solution of sodium azide on an aqueous solution of cupric sulphate, obtaining the salt in a hydrated form. The anhydrous salt was prepared by Straumanis and Ciru-lis [125] in the form of dark brown, reddish sediment by reaction of lithium azide on cupric nitrate in an alcohol solution. Another method described by Curtius consists of reacting hydrazoic acid with metallic copper in an aqueous medium. 

Angeli35 obtained a white precipitate of insoluble silver azide by mixing saturated solutions of silver nitrite and hydrazine sulfate and allowing to stand in the cold for a short time. Dennstedt and Gohlich 36 later procured free hydrazoic acid by the interaction of hydrazine sulfate and potassium nitrite in aqueous solution. 

TABLE 15. Products from reaction of benzaldehyde with hydrazoic acid in aqueous solution of sulphuric acid n 11250 p 3750 /- Ui ,i 225 nm... 

A. HYDRAZOIC ACID (AQUEOUS SOLUTION OF HYDROGEN AZIDE)... 

A 3 per cent solution of hydrazoic acid (synthesis 26A) is neutralized with an aqueous solution of pure potassium hydroxide. The resulting solution of potassium azide is concentrated on the steam bath to incipient crystallization. The solution is then made slightly acid with hydrazoic acid to replace the hydrogen azide lost by hydrolysis. A volume of ethyl alcohol twice that of the solution is added, and the solution is cooled in an ice bath. Since the solubility in alcohol of the alkali and alkaline earth azides is very slight (see table below), precipitation in the form of a white microcrystalline salt takes place readily. From 90 to 95 per cent recovery of the theoretical quantity of potassium azide can be effected. The precipitated azide is filtered on a Buchner funnel and washed with cold absolute alcohol and then with ether. Any traces of adhering solvent may be removed in a vacuum desiccator. In a typical run, 300 ml. of a solution of hydrazoic acid containing 8.5 g. of HN3 was neutralized with potassium hydroxide, and the isolation of potassium azide effected as indicated above. Yield 14.7 g. (91.5 per cent) KN3. 

Wear face shield, goggles, laboratory coat, and nitrile rubber gloves. Cover spill with a 1 1 1 mixture by weight of sodium carbonate or calcium carbonate, clay cat litter (bentonite), and sand. Using a plastic shovel, scoop into a pail of water in the fume hood (about 66 mL/g). Cautiously add aqueous 5.5% ceric ammonium nitrate (4 volumes per volume of aqueous solution) and stir for an hour. If the solution remains orange, an excess of ceric ammonium nitrate is present and the azide has been completely destroyed. The solution can be washed into the drain with at least 50 times its volume of water.6,7 The solid residue is treated as normal refuse. A spot test for checking if azide is completely destroyed is as follows Place a drop of the test solution in the depression of a spot plate and treat with 1 or 2 drops of dilute hydrochloric acid. Add a drop of ferric chloride solution and gently heat the spot plate. A red color indicates hydrazoic acid and incomplete decomposition. 

The compound hydrazoic acid, HN3, is 98-percent nitrogen. In the pure state it is, as expected, explosive but it can be studied in aqueous solution. Numerous salts and organic derivatives (both called azides) have been made. The sodium salt can be prepared by reaction of nitrous oxide with the strong base sodamide. Under the anhydrous conditions used for the reaction, the adduct (or perhaps its tautomer) loses water to form the azide ion as shown ... 

Silver azide, AgN8.—Hydrazoic acid reacts with silver nitrate, forming the azide.13 It is also produced by the interaction of silver nitrite and hydrazine sulphate in aqueous solution ... 

Hydrazoic acid is a volatile, endothermic liquid of unbearably acrid odor. Some of its properties are given in Table 18. Hydrazoic acid and its salts are very toxic. Inhalation of small quantities causes the blood pressure to drop and damages the mucous membranes. Exposure to HN3 vapors can be recognized by reddening of the eyes. Pure, liquid hydrazoic acid is very sensitive and explodes with slight mechanical stress. It is completely miscible with water and soluble in alcohols and ethers. Tests have shown that aqueous solutions with less than 20% hydrazoic acid do not detonate. Diethylether, which has almost the same boiling point (307.7 K) as HN3 can be used as a blanketing fluid. 

Among other kinetic studies in aqueous solutions are those of the reaction of chlorate with vanadium(II) and with chromium (II) °. The reactions of bromate with hydrazoic acid" and with hypophosphite" have been examined. 

Two German patents by Muller describe preparative methods which avoid the use of hydrazoic acid by heating sodium azide with ammonium chloride or sulfate in aqueous solution. A recent Japanese patent describes a reaction between sodium azide and ammonium carbamate in liquid ammonia to give ammonium azide. Evans, Yoffe, and Gray have suggested the reaction of sodium azide with ammonium chloride in iV,iV-dimethylformamide. A modification of the last method has been made it involves the reaction of sodium azide and ammonium sulfate in dimethyl-formamide to give anhydrous ammonium azide of high purity. 

According to Bagal 1] aqueous solutions of hydrazoic acid can detonate 70% aqueous HNj showed the rate of detonation 7300 m/. Anhydrous hydra-zoic (liquid) gave 8100 m/s. 

Misiti et al. found that in coned, sulfuric acid at 0° hydrazoic acid reacts with some alkylated quinones in an entirely different manner to give 2,5Af-2,5-azepindiones such as (5). A solution of the quinone (I) in coned, sulfuric acid is treated at 0° with 1 equivalent of sodium azide, added in portions. When evolution of nitrogen ceases the mixture is poured into ice and water and the precipitate is crystallized from aqueous ethanol. Yields are in the range 75-85%. NMR data for (5) show that the NH proton is directly coupled to the vinylic proton and hence that these groups are adjacent. 

Reaction with quinones. Addition of an aqueous solution of sodium azide to a solution of a-naphlhoquinone in acetic acid is attended with rapid evolution of nitrogen and separation of orange-ied needles of 2-amino-1,4-naphlhoquinone in 87% yield. The reaction is interpreted as involving 1,4-addition of hydrazoic acid... 

---