320 PROCEEDINGS OF THE AMERICAN ACADEMY. 



as arsenic acid was prepared by evaporating 10 c.c. of solution I repeat- 

 edly with nitric acid and making up to one liter. The bottles were 

 heated during the reduction in an air bath in such a way that all above 

 the necks protruded. At 60° the bands obtained from 3 c.c. of the 

 arseniate solution after thirty minutes of reduction were only about 43 

 per cent of the standard for 30 mmg. of arsenious oxide. Parallel trials 

 with 3 c.c. of the arsenite solution gave bands of the standard length. 

 Another experiment at 90° gave no better results. The bands from 

 the arseniate solution were not over 50 per cent of the standard, while 

 the parallel arsenite reductions gave shorter bands than at ordinary 

 temperature, owing to the larger amount of moisture carried over. 

 That a reduction at the boiling point would cause a practically com- 

 plete conversion to arsine, as claimed by Bird, seems improbable, while 

 the moisture equilibrium would be so disturbed as to invalidate the 

 procedure. 



Returning to the reduction at ordinary temperature, it was found 

 that no increased effect was produced Avithin the standard time by the 

 addition of stannous chloride or potassic iodide. Platinum in contact 

 with the zinc, even when the acid was more concentrated, was of no 

 service, and the use of copper-covered zinc did not help. An appre- 

 ciable increase but not a complete reduction was effected by sesquisul- 

 phate of titanium. It was evident that the use of a catalytic agent did 

 not solve the problem with such small amounts of arsenic, and we 

 were therefore forced to a reduction of the arseniate to arsenite before 

 testing. For this purpose we found sulphurous acid the simplest 

 substance, since comparatively little is needed, no excess of reagent 

 need be left in solution, and it can easily be prepared free from 

 arsenic. 



The sulphurous acid solution was made from pure copper and pure 

 sulphuric acid, and was saturated at 0°. The solution gave no test for 

 arsenic when tested in quantities larger than would be used in an 

 analysis. The tests were made after boiling' out the sulphur dioxide 

 from the samples. 



"We tested the efficacy of the sulphurous acid as follows : Four por- 

 tions of the arsenic acid solution, corresponding to 10, 20, 30, and 40 

 mmg. of arsenious oxide, were evaporated in small glass dishes with 

 6 c.c. of the sulphurous acid until the excess of sulphur dioxide was 

 apparently expelled. On adding the residues to the reduction bottles, 

 the color bands came up quiclcly as in the case of arsenites, and in thirty 

 minutes all the bands were equal to the corresponding standards in 

 length and intensity of color. Subsequent trials conducted similarly 

 confirmed these results. The precaution was taken to use the lead 



