62 Gooch and Blumenthal — Use of Selenic Acid. 



grm. of sodium selenate with 3 cm3 of sulphuric acid (1 : 1), is 

 diluted with 20 cm3 of water and put into the separatory funnel. 

 Into the relay flask are put 2'5 cm3 of selenic acid and 10 cm3 to 

 15 cm3 of water, or 0*2 grm. to 0*3 grm. of the selenate with l cm3 

 to 2 cm3 of the (1 : 1) sulphuric acid and 10 cra3 to 15 cm3 of water. 

 The relay flask is heated in the oil bath kept at 115° to 120°. 

 The receiver is charged with 3 grm. to 4 grm. of potassium 

 iodide dissolved in 200 ctn3 to 250 cm3 of faintly acidified water, 

 and set in a jar of cold water. The apparatus is connected (as 

 shown in fig. 2), the dilute sulphuric acid is allowed to run into 

 the reaction flask, and a slow current of carbon dioxide replaces 

 the air in the apparatus. The reaction flask is heated until 

 the liquid boils gently, while the oil bath which heats the relay 

 flask is kept at 115° to 120°. The reaction mixture is boiled 

 gently until diminished to a volume of 10 c,a3 to 15 cm3 , when the 

 source of heat is withdrawn and carbon dioxide is passed in a 

 current of sufficient rapidity to prevent back suction of the 

 liquid in the receiver, but regulated to the lowest convenient 

 limit, in order that iodine may not be swept from the receiver 

 and lost to estimation. After the flasks are cool enough to be 

 handled conveniently the apparatus is disconnected, the con- 

 tents of the receiver and safety bulb are united, and the free 

 iodine is titrated with standard sodium thiosulphate and taken 

 as the measure of the bromine evolved. The entire operation 

 need not occupy more than fifty minutes, and, with practice, 

 may be shortened to forty minutes. 



With selenic acid as the active agent it is possible to deter- 

 mine, under the conditions defined, the bromine of 0*25 grm. 

 of bromide in association with 0*15 of chloride in the haloid 

 salts. With sodium selenate and sulphuric acid to bring about 

 the reaction, the separation is shown to be regular and fairly 

 accurate for 0*07 grm. of bromine associated with 0*05 grm. 

 of chlorine in the haloid salts. What the accuracy of the 

 process may be when larger amounts of the salts are handled 

 remains to be determined. 



