Sodium hydroxide mixture, Devarda

During preparation of an oxidising agent on a larger scale than described [1], addition of warm sodium hydroxide solution to warm ammoniacal silver nitrate with stirring caused immediate precipitation of black silver nitride which exploded [2], Similar incidents had been reported previously [3], including one where explosion appeared to be initiated by addition of Devarda s alloy (Al—Cu—Zn) [4], The explosive species separates at pH values above 12.9, only produced when alkali is added to ammoniacal silver solutions, or when silver oxide is dissolved with ammonia [5], The Sommer Market reagent mixture used to identify cellulose derivatives led to a severe explosion [6],... 

Cyanides, thiocyanates, hexacyanoferrate(II)s, and hexacyanoferrate(III)s also yield ammonia under these experimental conditions. The reaction is somewhat slower for these anions up to 5 minutes may elapse before ammonia can be detected from hexacyanoferrate(II)s and hexacyanoferrate(III)s. If these are present, or are suspected as a result of the preliminary tests, particularly that with concentrated sulphuric acid, they must first be removed as follows. Treat the soda extract with excess of nitrate-free silver sulphate, warm the mixture to about 60°, shake vigorously for 3-4 minutes, and filter from the silver salts of the interfering anions and excess of precipitant. Remove the excess silver ions from the filtrate by adding excess sodium hydroxide solution and filter off the precipitated silver oxide. Evaporate the filtrate to about half bulk and test with zinc, aluminium or Devarda s alloy. If cyanides alone are present, they may be rendered innocuous by the addition of a little mercury(II) chloride solution. 

If ammonium is absent, add about 10 mg of Devarda s alloy (or of aluminium as powder or thin foil) and 0-2-0-3 ml sodium hydroxide solution to the reaction mixture from test 5, introduce the filter tube carrying the reagent paper and proceed as above. 

Nitrate in the presence of bromide and iodide The brown ring test for nitrates cannot be applied in the presence of bromides and iodides since the liberation of free halogen with concentrated sulphuric acid will obscure the brown ring due to the nitrate. The solution is therefore boiled with sodium hydroxide solution until ammonium salts, if present, are completely decomposed, and the solution is then cooled under the tap. Powdered Devarda s alloy or aluminium powder or zinc dust is then added and the mixture gently warmed. The evolution of ammonia, detected by its smell, its action upon red litmus paper and upon mercury(I) nitrate paper (see Section III38, reaction 1) indicates the presence of a nitrate. 

If a substance is suspected to contain a nitrate ion (NO3), the presence of the ion can be detected by heating the nitrate with sodium hydroxide soiution and Devarda s alloy. Devarda s alloy is a roughly equal mixture of aluminium and copper, to which a little zinc is added. If a nitrate is present it is reduced and ammonia is evolved. Ammonia can be detected by its choking smell or, because it is an alkaline gas, it turns damp red litmus paper blue. The detection of ammonia confirms that a nitrate was present. 

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