322 PROCEEDINGS OF THE AMERICAN ACADEMY. 



(0.00008 mg.) of arsenions oxide, "which is equivalent to 0.00006 mg. 

 of metallic arsenic or one seventeen-thousandth of a milligram. 



In the above tests, on quantities under 10 mmg., the hydrochloric 

 acid containing 0.004 mg. arsenious oxide per liter was used. This, in 

 15 c.c. of the diluted acid, assured a quantity of arsenic far below the 

 above limit, while blank tests of over an hour's duration gave negative 

 results. The deposits from these small amounts were formed within 

 thirty minutes, and each reduction was continued thirty minutes 

 longer. 



Although the method is a very delicate one, as shown by the above 

 tests, we are far from claiming that 0.08 mmg. of arsenious oxide can 

 be recognized by it with certainty under the varying conditions of 

 analytical practice. We are not so much concerned with the absolute 

 delicac}', however, as with the amount which may be considered a 

 practical limit, the recognition of which is definite under all conditions, 

 and which, when obtained from an aliquot portion of a solution, may 

 safely be used as a factor in the quantitative determination of the 

 arsenic. In this particular we agree fully with Chapman and Law,'^^ 

 who have expressed the opinion that in the Marsh method 5 mmg. 

 should be taken as a practical limit, and that one's efforts should be 

 directed tov/ard recognizing this amount with certainty. We consider, 

 therefore, that 1 mmg. (0.001 mg.) of arsenious oxide may be set as 

 the practical limit of our method, although less than one tenth of this 

 amount may be recognized under favorable conditions. The color 

 produced on the large or small strip by 1 mmg. need not be confused 

 with that from hydrogen sulphide, stibine, or phosphine, if these are 

 unavoidably present, while the more minute traces of color, though 

 not easily confounded with those from the first two, are similar in 

 appearance to that from the last. We have found by trial that 0.1 

 mmg. of arsenious oxide, if present as arseniate, can be recognized after 

 reduction with sulphurous acid. 



Previous estimates of the delicacy of the Gutzeit test have not been 

 under 0.1 mmg., so far as we know, with the exception of that made 

 by Dowzard,^* who states that one fifteen-thousandth to one twenty- 

 thousandth of a milligram can be recognized by the modification 

 described by him. This figure is practically the same as ours. 



The Use of the Method. 



The method naturally suggests comparison with the Marsh in the 

 present accepted form of the latter. In the modification described by 



33 Zeits. f. angew. Chem., 20, 67 (1907). 34 Lqc. prim. cit. 



