PHOTOLYSIS OF THE AZIDE ION IN AQUEOUS SOLUTION

I The photolysis of the azide ion in an aqueous medium purged with argon is found to generate ammonia and nitrogen whereas oxygcn purged solutions generate nitrite and nitrogen. Quantum yield measurements are given and possible reaction mechanisms are suggested.


Introduction
The kinetics of the decomposition of inorganic azides in solid state have received conslderable l 9 In vacuum all azides decompose into metal and nitrogen, the decomposition can be brought about by heat, light, shock or ionizing radiation.Heavy metal azides in solid state show absorption edges in the vicinity of 300-400 nm.The photochemical decomposition of heavy metal azides is believed t o originate from band gap excitations which generate electron-hole pairs.9However, in the case of alkaline metal azides the excitonic absorption in the region 140-150 nm is known t o cause the decompo~ition.~According t o Beckman and ~ikinson,' the photolysis of hydrogen azide solution yields gaseous nitrogen, ammonia, hydrazine and hydroxylamine.Nosaka et a11 l 6 have reported that photolysis of azide ion (aqueous medium) in the presence of semiconductor catalysts produce ammonia and nitrogen.However, they have not compared the yields in the absence and presence of semiconductor catalysts.We have carefully examined the photolysis of azide ion in the presence and absence of semiconductor catalysts and found that the photodecomposition of azide ion in aqueous medium is not influenced by semiconductor catalysts.The only detectable products in the photolysis of argon purged solutions were ammonia and nitrogen.Photolysis of oxygen purged solutions yielded nitrite ion and nitrogen.

~x ~e & n e n t a l Method
Photolysis experiments.were carried out in a double walled photochemical reactor (Applied Photophysics) made from borosilicate glass.The central axis of the cylinder is fitted with a 400W medium pressure Hg lamp.A facility is also provided for the insertion of cylindrical glass filters.350 ml of sodium azide solution was continuously irradiated and aliquots of solution drawn at different intervals of time were tested for ammonia (indophenol blue method3 ) hydrazine (p-dimethylaminobenzaldehyde method8 ) and hydroxylamine (potassium iodate resorcinal reagent2 ).All these estimations were carried out spectrophotometrically. Before irradiation, solution was rapidly purged with purified argon.The gas in the atmosphere of the reactor and in the solution was analysed for nitrogen, oxygen and hydrogen by gas chromatography (Shimadzu model E6-9AM).An International light IC 700 radiometer was used for the measurement of light intensities.The absorption spectrum of the azide solution was recorded using a Shimadzu UV-3000 spectrophotometer.

Results and Discussion
In argon purged solutions the only detectable products of photolysis were ammonia and nitrogen (hydrazine and hydroxylamine were absent or less than 104M).Figure 1 indicates the yield of ammonia as a function of time.
The absorption spectrum of the sodium azide solution shows a sharp edge (Figure 2) at 280 nm.Photodecomposition is seen t o occur only if the irradiation wavelength is 6 280 nm (when a filter that cuts off light of wavelength ?J 300 nm was used, the decomposition did not take place).
The quantum yields of ammonia and nitrogen at X = 265 nm (the significant emission in the lamp at X< 300 nm is 265 nm and the intensity at this wavelength estimated using a rnonochromator is ^(. 4.9 X 10' photons <' ) was estimated t o be 5.3, 18.6 5 % respectively.When the solution was purged with O2 (160 ml min-' ) the products of photolysis were nitrite and nitrogen with barely detectable.amounts of ammonia.The quantum yields of nitrite, nitrogen were found to be 41, '4.7 % respectively.Semiconductor.photocztalysts did not have any influence on yields or the products of photolysis.~~a i h , in both cases (ammonia production .inargon purged solution and nitrite production in oxygen purged solution) were found t o be insensitive t o changes in pH.The photochemical reactions occurring can be summarized as follows: .
-The observed ratios of the quantum y i e l d s ' ( ~~~ :N2 % 1 : 3.5, NO,-: N2 ? 1 : 1.l)agrees with (1) and ( 2)..The in,sensitivity of the reaction t o pH ,and the, presence of semiconductor catalysts suggests that the primary step in photolysis does not involve chaige transfer from azide t o H+ ions, but ins-,tead-the excited azide ion or an intermediate product of its decomposition directly interacts with water molecules or oxygen.