316 Andrews and Farr — Determination of Arsenic. 



Art. XXY. — The Volumetric Determination of Small 

 Amounts of Arsenic; by Latjncelot W. Andrews arid 

 Henry Y. Farr. 



The problem of the analytical determination of small amounts 

 of arsenic, present as an impurity in products of the most varied 

 character, is a constantly recurring one. The comparison of 

 mirrors formed in the Marsh test in its several modifications 

 and a few colorimetric processes have solved the problem for 

 the special case in which the arsenic is present in traces only 

 and in which a merely approximate estimation of the amount 

 suffices. We have been led to seek a solution covering a wider 

 range of cases, in which, for example, the arsenic may be 

 present in any quantity from O'l mg., or possibly less, to 100 mg. 

 and in which the accuracy of an ordinary quantitative analysis 

 is required. 



Often, but not always, the distillation of the arsenic, after 

 Fischer as arsenious chloride, furnishes satisfactory results 

 under these conditions as a method of separation, but it 

 always has the disadvantage of being rather time-consuming. 

 It is better adapted for the determination of larger than for 

 smaller amounts, since the distillate contains its arsenic in a 

 highly dilute state and much contaminated with foreign salts, * 

 or else in the form of arsenic acid,f which is not particularly 

 well suited to quantitative determination. A method, in order 

 to be perfectly satisfactory for the purpose in view, should 

 comprise : (A) a process for the direct separation of the arsenic 

 in a concentrated form, applicable to the greatest possible 

 number of cases ; (B) a simple, rapid procedure for determin- 

 ing the arsenic in the form in which it is furnished by step (A). 



It is obvious that none of the existing methods can meet 

 these demands. The well-known Bettendorff test,;); in which 

 the arsenic is precipitated in the metallic state by a mixture of 

 hydrochloric acid and stannous chloride, appeared to us to pos- 

 sess possibilities of development for the process we were look- 

 ing for. This test has been used by E. S. Peck§ for the 

 colorimetric determination of arsenic in metallic iron. Accord- 

 ing to Bettendorff (1. c.) the sensitiveness of the reaction is 

 such that it will show " most definitely " the presence, in one 

 cubic centimeter, of two millionths of a milligram of ammo- 

 nium-magnesium arsenate, that is, of less than a millionth 

 milligram of metal. Our own observations on this point con- 



* Alkali chloride, if the distillate is received in alkali hydroxide solution. 

 f When the distillate is received in nitric acid or other oxidizing agents. 

 JFres. Zeitschr., ix, 105, 1869. 

 §Pharm. Jour. (4), xiii, 130, 1901. 



