SANGER AND BLACK. — DETERMINATION OF ARSENIC IN URINE. 331 



with the gaseous hydrochloric acid and meets the concentrated nitric 

 acid, no loss is to be feared from the dilution of the nitric acid by the 

 acid distillate. Comparatively little organic matter is distilled, and 

 this is entirely destroyed by the subsequent procedure. 



Treatment of the Distillate. To the distillate are added 25 c.c. con- 

 centrated nitric acid, in order to decompose completely during evapora- 

 tion any excess of hydrochloric acid and thus guard against loss of 

 arsenic. The mixture is then evaporated to a small bulk, three to 

 five cubic centimeters of concentrated sulphuric acid added, and the 

 evaporation continued until the nitric acid is expelled. To destroy the 

 slight amount of organic matter which usually remains, a few drops of 

 nitric acid are added, and the heating is continued until only the small 

 residue of sulphuric acid is left, which must be colorless. The residue 

 is then diluted with water to a measured volume of about 25 c.c, or, 

 if preferred, to a quantity which is weighed in a side-neck test tube to 

 the second decimal place. 



Determination of the Arsenic. The subsequent procedure, as in the 

 paper above referred to, follows closely the method of Sanger ^^ for 

 determining small amounts of arsenic, except that the capillary tube 

 should be cooled at the deposition point of the mirror, as advised by 

 Gautier,^^ Thomson,^"^ Lockemann,^* and others. An aliquot portion 

 of the ultimate solution, accurately measured or weighed, is introduced 

 into the Marsh flask, the entire apparatus having been in action for a 

 sufficient time to show absence of arsenic. This time varies according 

 to the importance of the test in hand, but should not be less than twenty 

 minutes. If, after the addition of the solution, a mirror does not make 

 its appearance in the capillary tube within ten minutes, a larger portion 

 or the whole of the solution is added. After the appearance of the 

 mirror the heating of the tube is continued for a sufficient time to 

 insure the complete deposition of the arsenic, which usually occurs 

 within an hour. During this time the flow of hydrogen is regulated 

 by the constant generator, so that the height of the flame at the end 

 of the heated tube is about one millimeter, the regular deposition of 

 the mirror being dependent on this condition. The mirror obtained 

 is compared with a set of standards, which is prepared as explained in 

 the paper referred to. From the amount of solution used and the 



" Loc. cit. 



" Bull. Soc. Chim., [2] 27, 1030 (1902.) 



" Chem. News, 88, 228 (1003). 



" Zeitschr. f. angew. Chem., 18, 416 (1905). 



