Lower Oxo-Salts

Solid phosphites and hypophosphites both give phosphine on heating, which is readily recognised in very small concentrations by its smell. The thermal decomposition of the solid sodium salts can be represented approximately by Equations 14.5 and 14.6. Powdered zinc, when added to an acid solution of a phosphite or hypophosphite, will also cause evolution of phosphine. 

Solutions of phosphites and hypophosphites with silver nitrate give initially white precipitates of silver phosphite and silver hypophosphite, respectively. Both these precipitates decompose to give metallic silver on warming (14.7) and (14.8). This test will distinguish them from hypophosphates or fully oxidised phosphates whose silver salts are relatively stable when precipitated. 

Phosphites and hypophosphites both decolourise acidified potassium permanganate solutions, but only the latter reacts in the cold. Phosphites can be distinguished from hypophosphites by their reaction with copper sulphate solution. The former will give a light blue precipitate of cupric phosphite, while the latter will give a reddish precipitate of copper hydride on warming. 

Phosphides and other unreactive inorganic compounds are best detected and estimated as orthophosphates after suitable oxidation. Insoluble rocks and minerals are usually fused with sodium carbonate or sodium peroxide to secure conversion into soluble orthophosphates. Alternatively dissolution with HF and HCIO4 may be employed. 

Traces of white phosphorus in red phosphorus can be detected by shaking the latter with carbon disulphide and testing with copper sulphate. If any white phosphorus is present, it dissolves in the carbon disulphide and precipitates copper phosphide or copper (good quality commercial red should contain not more than about 0.01% of the white form). 

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