30 PRACTICAL ORGANIC AND BIO-CHEMISTRY 



The nature of the halogen may be determined by treating a little 

 of the acidified solution with chlorine water, and then testing with 

 starch solution for iodine, or by extracting with carbon bisulphide or 

 chloroform for bromine. 



(c) Heating with Lime. Halogens are best detected by heating 

 with quicklime. The substance is finely powdered and mixed inti- 

 mately with lime (if liquid, e.g. chloroform, the lime is moistened with 

 the substance) and then heated strongly. When cool, water is added 

 and the lime dissolved in nitric acid. On adding silver nitrate, a pre- 

 cipitate of silver halide is obtained if halogen be present. 



4 Sulphur. 



(a) As sodium sulphide. A small portion of the substance is 

 heated with metallic sodium as described under 2 (b). The hot tube 

 is broken in a little water, the contents are filtered and tested for 

 sodium sulphide with (i) lead acetate, (2) sodium nitroprusside. 



(b) As sulphate. A small portion of the substance (dried fibrin) 

 is fused in a crucible with three times its quantity of fusion mixture 

 {2KNO 3 + Na 2 CO 3 }. The mixture is heated cautiously at first round 

 the edge and the heating is continued after the fusion until all charred 

 particles have vanished. The mass, when cool, is extracted with hot 

 water and the filtered solution is tested for sulphates with barium 

 chloride in the presence of mineral acid (HC1 or HNO 3 ). 



5. Phosphorus. 



(a) Some caseinogen is fused with fusion mixture as described for 

 sulphur, the fused mass is extracted with hot water, and the solution 

 is divided into two parts. To the one part is added excess of nitric 

 acid and ammonium molybdate : a yellow precipitate on warming 

 indicates "phosphoric acid; to the other part excess of ammonia is 

 added and phosphates are precipitated with magnesia mixture. 



(ft) A small quantity of caseinogen in a small flask is covered with 

 5-10 c.c. of concentrated sulphuric acid and an equal volume of con- 

 centrated nitric acid is added. The mixture is heated gently over 

 a small flame (in the draught crnmber) until the mixture becomes 

 colourless. If it becomes brown, it is cooled, more nitric acid is 

 added and it is heated again. When it is colourless, it is allowed to 

 cool, water and a little ammonium nitrate solution are added and it 

 is heated nearly to boiling; on adding ammonium molybdate solution, 

 a yellow colour or precipitate indicates the presence of phosphoric 

 acid (Neumann's method). 



