SANGER AND BLACK. — QUANTITATIVE DETERMINATION OF ARSENIC. 801 



per cent of lead and not more than 0.013 per cent of iron. The hydro- 

 chloric acid is obtained of the Baker and Adamson Company of Easton, 

 Pennsylvania, and has been shown by careful analysis to contain not 

 over 0.02 milligram of arsenious oxide per liter. The dilution em- 

 ployed, one part of acid to six of water, is equivalent to a normality of 

 about 1.5. The quantity of diluted acid used in the analysis would 

 not contain over 0.00004 mg. of arsenious oxide, an amount beyond 

 the practical limit of the delicacy of the method. 8 No evidence of 

 sulphur, phosphorus, antimony, or arsenic has been obtained from these 

 reagents when used in long continued blank tests. 



Procedure. Three grams of carefully and uniformly granulated zinc 

 are placed in the bottle, and a strip of sensitized paper is slipped into 

 the deposition tube to a definite distance, the paper being wholly within 

 the tube. Fifteen cubic centimeters of diluted acid are then added 

 through the thistle tube, and the evolution of hydrogen is allowed to 

 continue for at least ten minutes. At the end of this time the rate of 

 flow of the gas has become as regular as possible, and the atmosphere 

 in the deposition tube has a nearly definite degree of saturation with 

 aqueous vapor. On these two conditions depends chiefly the uni- 

 formity of color bands from equal amounts of arsenic. In this time, 

 also, the absence of arsenic in reagents and apparatus is assured, in the 

 great majority of cases, by the nou-appearance of color on the sensitized 

 paper, but the blank test may be as long continued as circumstances 

 demand. 



The solution to be tested is then introduced, either wholly or in 

 aliquot part, which may be determined by weighing or measuring. In 

 the former case we use a side-neck test tube of about 30 c.c. capacity, 

 and weigh to the second decimal place. Unless the amount of arsenic 

 be exceedingly small, it is not necessary to add the whole of the solu- 

 tion, but in that case the volume must be obviously not over 15 c.c, 

 on account of the capacity of the bottle. 



After introduction of the solution the color appears upon the paper 

 in a few minutes and the deposit reaches its maximum within thirty 

 minutes. The band of color thus obtained is then compared with a set 

 of standard bands. From the amount of arsenic as estimated from the 

 comparison, and the amount of solution from which the band was 

 obtained, the calculation of the arsenic in the entire solution is 

 simple. 



8 "We are also indebted to the Baker and Adamson Company for a preparation 

 of hydrochloric acid containing a still smaller quantity of arsenic, the use of 

 which will be later explained in the discussion of the absolute delicacy of the 

 method. 



