METHODS OF ANALYSIS. 23 



large amount of bromin the following method devised by Gooch and 

 Whitfield was used: 



An aliquot portion of the alkaline filtrate is evaporated to dryness 

 and extracted with 90 per cent alcohol, as in the qualitative examination 

 just described. The alcohol extract is evaporated to dryness, acidu- 

 lated with dilute sulphuric acid, mixed with a ferric sulphate solution, 

 and distilled from a retort which is joined to a condenser, sealed by a 

 U-tube filled with water and carbon bisulphid. If iodin is present, it 

 colors the carbon bisulphid and is titrated with sodium thiosulphate. 



After the distillation has been continued long enough to be sure that 

 all iodin has been volatilized, crystals of potassium permanganate are 

 added and the distillation continued as before, except that the U-tube, 

 acting as a seal, is now filled with water and chloroform. The con- 

 tents of the tube are treated with sodium hydroxid and zinc in a 

 beaker and the chlorid and bromid solution so formed acidified with 

 nitric acid and precipitated with silver nitrate. The precipitate is 

 dried and weighed. It is then dissolved in potassium cyanid and the 

 silver precipitated by electrolysis. 6 In this way data on the weight 

 of the combined silver chlorid and bromid and the weight of silver in 

 same is determined and the weight of the bromin calculated. 



After the authors had used the above quantitative method on a 

 number of waters the need of a shorter method became apparent, both 

 on account of the time element involved and because of the fact that 

 in evaporating large volumes of heavily mineralized waters the large 

 quantities of salts other than iodids and bromids are very difficult to 

 handle. A colorinietric method was therefore devised by the senior 

 author (based on the usual qualitative method of examination) which 

 was used in subsequent examinations when bromids and iodids were 

 present in moderately small quantities. Where bromids and iodids 

 were present in very large quantities, however, the Gooch and Whittield 

 method was still followed. The procedure for the colorinietric 

 method is as follows: 



An aliquot portion of the alkaline filtrate (p. 22) is evaporated to 

 dryness and extracted with 90 per cent alcohol just as in the qualita- 

 tive examination above. The residue is dissolved in a little water, 

 acidified with sulphuric acid (1 to 5) only using about 3 or 4 drops of 

 acid in excess, and transferred to a small flask. Four drops of 2 per 

 'ent potassium nitrite solution are now added along with about 5 cc of 

 carbon bisulphid freshly purified by distillation. The whole is shaken 

 up till all iodin is extracted. A filter is now moistened and the acid 

 solution filtered off from the carbon bisulphid. The flask, filter, and 

 contents are washed with cold distilled water and the carbon bisulphid 



U. S. Geological Survey, Bui. 47. 

 &Whitfield, Amer. Chem. J., 1886, 8: 421. 



