sanger and black. — quantitative determination of arsenic. 319 



The Procedure in Presence of Arseniates. 



It is well known that the reduction of an arseniate solution to arsine 

 goes on more slowly than that of an arsenite. This is provided for in 

 the Marsh procedure by continuing the reduction for a longer time when 

 arsenic acid is present ; fully an hour, or, if small amounts are present, 

 still longer. The deposition of the mirror being in a comparatively 

 small compass, its size and appearance are not appreciably changed, 

 within the range of the standards, by the slower accumulation of the 

 arsenic particles. In the Gutzeit procedure the case is different for 

 two reasons. The formation of the color bands is over a greater sur- 

 face and the standard set is based on the deposition of the color in a 

 short time, which, in turn, depends upon a comparatively quick reduc- 

 tion of the arsenious acid. Not only will some arsenic escape reduction 

 during this time, if arsenic acid is present, but the slower congregation 

 of the particles will result in a shorter band. Hence, from a given 

 amount of arsenic as arseniate, the reading of the color after thirty min- 

 utes is invariably low. The subsequent reduction may be studied frac- 

 tionally for sixty to ninety minutes, with successive strips, although the 

 colors from the last fractions may only be shown by the 2 mm. strips (see 

 below). The proportion of color within thirty minutes has been shown 

 by us from repeated trials to be reasonably definite. It is rarely over 

 50 per cent of the standards, rarely under 40 per cent, and the bands 

 formed are somewhat denser in appearance. This implies that the band 

 from an arseniate, though shorter, contains more arsenical substance 

 than a band of the same length from an equivalent amount of arsenite, 

 and this is borne out by the fact that the subsequent color estimations 

 from the continued reduction do not apparently carry the total per- 

 centage of arsenic to more than 80. 



There are two ways of approximately estimating the value of the 

 color bands derived from arseniates. We may either make a series of 

 standards from known amounts of arsenic as arsenic acid, with which 

 the test band from an arseniate may be compared, or we may multiply 

 the reading of the ordinary standards by 2 or 2.5. Either of these 

 alternatives will answer, more simply the latter, — though both are 

 obviously inexact, — if one's object is only to get a rough idea of the 

 amount of arsenic present. The estimation can be made, however, 

 within the ordinary limits of the method, if the arseniate is converted 

 to arsenite before reduction to arsine. 



Before arriving at the procedure finally adopted, we studied the 

 effect on the reduction of an increase of temperature and also that of 

 various catalyzers. A solution containing 10 mmg. of arsenious oxide 



