Arsenomolybdic acid reagent

Arsenomolybdate reagent—Dissolve 125 g (NFD6M07 024 4 HzO in 2.25 liters of distilled water, and add 105 ml of concentrated H2S04. Dissolve 15.0 g of Na2HAs04 7 H20 in 125 ml of distilled water, and add to the above acid molybdate solution. Store in a brown bottle for 24 hr at 37°C. This reagent should be yellow with no green tint. 

Ammonium molybdate solution when the reagent and nitric acid are added in considerable excess to a solution of an arsenate, a yellow crystalline precipitate of ammonium arsenomolybdate, (NH4)3 AsMo, 2 40 is obtained on boiling (distinction from arsenites which give no precipitate, and from phosphates which yield a precipitate in the cold or upon gentle warming). The precipitate is insoluble in nitric acid, but dissolves in ammonia solution and in solutions of caustic alkalis. 

Arsenates and silicates with ammonium molybdate yield the ammonium salts of arsenomolybdic, H3[AsMo12O40] or H3[As(Mo3O10)4], and silico-molybdic, H4[SiMo12O40] or H4[Si(Mo3O10)4], acids respectively these complex acids and their salts react similarly with benzidine. However, phosphates may be detected in the presence of arsenates and silicates by preventing the formation of the corresponding molybdo-acids by the use of a tartaric acid-ammonium molybdate reagent which does not react with arsenic and silicic acids but does react with phosphoric acid when the reaction is carried out on filter paper. 

Air dry chromatograms and hang in ammonia chamber for IS sec to neutralize acid. Expose to UV for 30 sec. Spray with arsenomolybdate reagent and heat 10-15 min at 90°C. Greenish spots result... 

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