DETECTION OF SULPHUR 5 



the phosphide is decomposed with the production of 

 phosphine, PH3, readily detected at the mouth of the 

 tube by its characteristic smell. 



{b) About 2 grams of potassium nitrate are fused 

 in a test-tube, and small quantities of the substance 

 under examination dropped into the fused mass from 

 time to time. Rapid oxidation of the organic sub- 

 stance, accompanied by reduction of the potassium 

 nitrate, takes place, and oxides of nitrogen and carbon 

 dioxide are evolved. Any phosphorus present is 

 oxidised to potassium phosphate, the presence of 

 which is detected by the production of the character- 

 istic yellow precipitate of ammonium phospho-molybdate 

 when the cooled product in the test-tube is dissolved in 

 water, the filtered solution acidified with strong nitric 

 acid, and a solution of ammonium molybdate added. 

 The formation of the yellow precipitate is hastened by 

 the application of heat. 



Phosphorus, which occurs in largest amount in seeds, exists in 

 the plant in combination as certain proteins and as lecithin, or 

 choline distearyl-glycero-phosphate, a substance occurring especi- 

 ally in peas, beans, and certain fungi. 



3. Qualitative Detection of Sulphur. 



{a) Plant products containing sulphur when heated 

 with metallic sodium in an exactly similar way to that 

 described above in 1 ib) give sodium sulphide. This 

 is detected in the product after heating, when the 

 boiled and filtered solution treated with a solution of 

 sodium nitroprusside gives a violet coloration. 



{b) Plant products containing sulphur, when oxidised 

 by fused potassium nitrate according to the method 

 mentioned above in 2 {U)^ give potassium sulphate. 

 This is detected by acidifying the solution of the 

 contents of the test-tube with strong hydrochloric acid, 



A 2 



