SANGER AND BLACK. — QUANTITATIVE DETERMINATION OF ARSENIC. 321 



acetate paper, on which in some cases there was a slight deposit of the 

 sulphide. 32 



In practice, when the solution contains an arseniate, or when the 

 substance has been oxidized, say by nitric acid, one may add a suffi- 

 cient quantity of sulphurous acid to the entire solution or to the ali- 

 quot portion taken for reduction. In analysis No. 9, Table I, we 

 followed the latter plan, adding 10 c.c. of sulphurous acid in two parts, 

 the second after partial evaporation. The excess of sulphur dioxide 

 is then expelled, but the evaporation must not be carried too far, as 

 chlorides, if present, would cause a loss of arsenic. In testing the 

 residues the lead acetate paper should be used. 



The Absolute Delicacy of the Method. 



For most practical purposes the set of standards from 2 to 70 mmg. 

 is sufficient. Amounts of arsenic between 2.0 and 0.5 mmg. can be ap- 

 proximated by the 4 mm. strip, but in studying the limit of delicacy 

 we have allowed the action to take place within a smaller compass. 

 The ordinary strip is cut in two, and these pieces are again divided 

 lengthwise, giving a piece 2 mm. wide and 35 mm. long. This is in- 

 serted in a tube of slightly more than 2 mm. in diameter, which is 

 fitted into the usual deposition tube by a washer of rubber tubing. 

 With these small strips a series of standards may be made from 

 10 mmg. down. "The yellow color appears definitely, though of course 

 slightly, from 0.5 mmg. Treated with hydrochloric acid, ammonia, or 

 hydrochloric acid and auric chloride, the indication is much sharper, 

 and from this amount up to 10 mmg. the gradation of the 2 mm. stan- 

 dards is well marked. From 0.3 mmg. the yellow color is exceedingly 

 faint, but development with the reagents brings it out. At 0.2 mmg. 

 the formation of yellow is no longer seen, but treatment with hydro- 

 chloric acid gives a faint but definite color, which under the glass is 

 seen to be greater than the effect produced by 0.1 mmg. Development 

 with ammonia or auric chloride is also definite. From 0.08 mmg. a 

 faint fringe of color is visible under the glass after treatment with 

 hydrochloric acid, and the indication is even sharper with ammonia 

 or auric chloride. From 0.05 mmg. no results were obtained. These 

 tests were made on two solutions, prepared at different times. 



Between 0.05 and 0.08 mmg. is clearly the limit at which we have 

 been able to detect any arsenic by the mercuric chloride paper under 

 the conditions of our method. It is safe to set this limit at 0.08 mmg. 



32 We have found tliat the lead acetate paper is more sensitive to hydrogen 

 sulphide than the mercuric chloride. 

 vol. xliii. — 21 



