SANGER AND BLACK. — QUANTITATIVE DETERMINATION OF ARSENIC. 811 



with the result that a given volume will not yield the same depth of 

 band as when first prepared, or, in the Marsh process, the same inten- 

 sity of mirror. In more dilute solutions the change is very rapid, and 

 solution IV, containing 0.1 mmg. per cubic centimeter, is of no value as 

 a standard in a day or two. The use of boiled water for dilution 

 greatly retards the change, which would lead to the conjecture that 

 the reaction might be one of oxidation, with formation of arsenic acid, 

 which, as shown below, does not give the same depth of color in a given 

 time as its equivalent of arsenious acid. Yet the treatment of an old 

 solution with sulphurous acid does not increase the amount of arsenic 

 from a given portion of it, as far as we have been able to determine. 

 Solution IV (0.1 mmg.) should be freshly prepared before use; solu- 

 tion II (10 mmg.) will hold its strength for a few weeks, and solution I 

 (1 mg.) should not be used if it has stood for a very long time. 



The deposit of color is of course on both sides of the paper. If the 

 strip exactly bisects the tube and the ilow of hydrogen is the same in 

 both segments, the intensity of color should be the same on each side 

 of the strip. It often happens that there is a slight difference, and in 

 consequence the band may appear on one side greater than the stand- 

 ard, on the other less. The set of standards is also a series of mean, 

 though not greatly varying color densities, and when viewed from one 

 side or the other may not seem regularly graded. The set should be 

 mounted in such a way that both sides of the strip can be examined, 

 and the mean density of the test band should be compared with the 

 mean density of the standard. The judgment is greatly assisted by 

 treating the band or its duplicate with hydrochloric acid or amnionic 

 hydroxide (particularly the latter), and comparing the result with the 

 corresponding standards. 



Treatment of the Bands. Whatever may be the formula of the red 

 compound, it is probable that the reaction is only complete in the 

 presence of an excess of hydrochloric acid. As previously mentioned, 

 the color fades completely on treatment with hot water. Cold water 

 brings about a gradual fading, but this is succeeded by a secondary 

 reaction by which a gray substance is formed. This action of water 

 was further studied by treatment with sodic acetate. A set of stand- 

 ard bands was immersed in half-normal sodic acetate for two hours in 

 the cold. The red color gave place to a uniformly graded light yellow 

 with a tinge of orange. The set, after pressing between filter paper, 

 was then sealed while still moist. In twenty-four hours the yellow 

 had changed to a dull white, with no color except in the higher values. 



