SA:!fGER AND BLACK. — QUANTITATIVE DETERMINATION OF ARSENIC. 309 



Although we have not tried it for ourselves, it would seem obvious 

 that the electrolytic reduction of the solution could be employed if 

 desired. 



Reagents. From the delicacy of the method, as discussed below, the 

 zinc used by us is evidently highly sensitive. Indeed, the amount of 

 iron present, to which metal, from the work of Chapman and Law,20 

 Parsons and Stewart, ^i and others, may be attributed the insensitive- 

 ness of most samples of zinc, is about one-seventh of the amount in a 

 zinc which Chapman and Law show to be sufficiently sensitive in the 

 Marsh process. 



We have also in this connection studied the effect of the presence 

 of other metals on the sensitiveness of the zinc. The retention of 

 arsenic by the addition of platinic chloride or cupric sulphate, con- 

 firmed by one of us ^^ (S) several years ago, is well known. With a 

 bright platinum foil in contact with our zinc and using either sulphuric 

 or hydrochloric acid, we have never noticed any loss of arsenic in the 

 Marsh procedure. Similarly, there is no diminution in the delicacy of 

 our method when platinum foil is used. The use of zinc carefully 

 covered with copper after the procedure of Lockemann ^3 makes no 

 difference whatever in the results, nor does the addition of tin or lead 

 salts to the solution during the reduction. 



The zinc is granulated by pouring the metal, melted in a porcelain 

 casserole, from a height of six feet through a hot porcelain sieve into 

 two feet of cold water. 



The estimation of the arsenic in the hydrochloric acid was made on 

 samples of 100 c.c. in two ways. The acid was distilled to half its 

 volume, a treatment which we have shown in the following paper ^4 to be 

 sufficient to expel all the arsenic. The distillate was collected in 

 35 c.c. nitric acid and evaporated with a small amount of sulphuric 

 acid. Again, the acid was allowed to drop slowly into hot nitric acid 

 and the mixture was then evaporated. Several residues obtained by 

 both of these procedures from lots of 100 c.c. gave closely agreeing re- 

 sults, both from the reading of the Marsh mirrors and the Gutzeit 

 color bands. The mean of all determinations was 0.002 mg. for 100 

 c.c, or 0.02 mg. per liter. 



20 Analyst, 31, 3 (19(X)). 



21 Jour.'Amer. Chem. Soc, 24, 1005 (1902). 



22 These Proceedings, 26, 21 (1891) ; Amer. Chem. Jour., 13, 431 (1891). 



23 Zeitschr. f. ansjew. Chem., 18, 41G (1905). 



24 These Proceedings, 43, 327 (1907) ; Jour. Soc. Chem. Ind., Vol. 26 (1907); 

 Zeitschr. f. anorg. Chem., Vol. 56 (1907). 



