SANGER AND BLACK. — QUANTITATIVE DETERmNATION OF ARSENIC. 305 



led to adopt the suggestion of Panzer, ^^ as applied to the standard 

 Marsh mirrors, for the preservation of our standards. A clean, dry, 

 glass tube, about 5 ram. in diameter, is sealed at one end, at which is 

 placed a small quantity of phosphorus pentoxide covered by a bit of 

 dry cotton wool. The strip is then inserted, colored end down, fastened 

 by a drop of Canada balsam, and the tube is sealed. The set of stand- 

 ards prepared in this way can be used for several months, although the 

 brilliancy of the color is lost after a few weeks. 



The color band may be developed by treatment with rather concen- 

 trated hydrochloric acid, of a normality of about 6 (one part acid to 

 one of water). This is done in a small test tube, at a temperature not 

 exceeding 60° and for not over two minutes, else, with this concentration 

 of acid, the paper is likely to become disintegrated. The strip is then 

 thoroughly washed with running water and dried. The color on the 

 wet strip is a brilliant dark red in the higher values, while the smaller 

 amounts show a deeper yellow than in the initial set. The length of 

 the bands is considerably greater than that of the original. On drying, 

 the color becomes duller. These bands must also be sealed as above 

 with phosphorus pentoxide, and are somewhat more permanent than 

 the initial set. Figure 2 (Plate 1) represents the set obtained by 

 development of the initial set with hydrochloric acid. 



If the original color band is treated for a few minutes with normal 

 ammonic hydroxide, a dense coal black color is produced, of slightly 

 gi'eater length than the original. This color is far more permanent 

 than the others, but it is nevertheless necessary to seal the dry strips 

 in glass, using fresh, powdered quicldime instead of phosphorus pent- 

 oxide. Figure 3 (Plate 2) shows the set obtained by development of 

 the initial set with ammonia. 



General Precautions. 



As far as concerns the reduction of the arsenic, no other precautions 

 are necessary than those which must be observed in the proper conduct 

 of the Berzelius-Marsh method when applied to small amounts. The 

 solution to be reduced should contain no interfering organic matter, 

 nor any metals which prevent or retard the formation of arsine. Sul- 

 phur in any form reducible to hydrogen sulphide should be absent. It 

 is well known that small amounts of hydrogen sulphide interfere with 

 the Gutzeit reactions, and it is the custom of most analysts to pass the 

 arsenical hydrogen over paper or cotton wool containing plumbous 

 acetate, or even through a lead solution, before it reaches the mercuric 



18 Zentralbl., 74 (1), 821 (1UC3). 



VOL. XLIII. — 20 



