the Compounds of Tin with Arsenic. 291 



By adding to these numbers the respective quantities of 

 oxygen required for their maximum oxidation, we arrive at the 

 formula — 



AsO'H2Sn02, 

 which I'cquires — 



Tin . . 44-53 Arsenic . . 28-31 



Dried in the stove at 120° C. the loss amounted to 25-3 per 

 cent., corresponding to 10 equivalents. 



The formation of an insoluble arseniate of tin is nothing new 

 of itself, for it has long been known that metalloidic acids form 

 insoluble precipitates with certain insoluble metallic acids. I only 

 point out the favourable circumsta.ice by which a constant com- 

 pound is formed, namely, by taking care to operate with an excess 

 of the metalloidic acid. Phosphoric acid in excess gives with 

 peroxide of tin, — 



PO^2SnO2 + 10HO. 



M. Levol, in seeking for an easy method to ascertain the 

 quantity of arsenic in the metals and their alloys, operated in all 

 his experiments with an excess of the metallic oxide, by which 

 he obtained but imperfect precipitates of tin with arsenic (or 

 antimony with arsenic). This led him to abandon the hope of 

 determining this element by precipitation ; he says, "the increase 

 of weight which arsenic acid obtains by its combination with hy- 

 drated peroxide of tin (or antimony) did not appear to me a 

 sufficient warrant for determining the arsenic from it in a 

 rigorous analysis." 



M. LevoFs combining proportion varies from 1 of arsenic to 

 15, and 20 of tin : but operating under different circumstances, 

 I am able to combine 12-72 of the metalloid with 20 of the metal. 

 The above result may be applied in quantitative analysis, when 

 a mere calculation will replace the delicate and somewhat com- 

 plicated operation of M. Levol. I turned this formation of the 

 constant compound As0^2SnO^ to account in the analysis of 

 the commercial stannates of soda, containing large admixtures of 

 arseniate of soda. 



To a known weight of a commercial stannate I add a known 

 quantity of arseniate of soda, boil with an excess of nitric acid, 

 collect the insoluble arseniate of tin AsO^ 2SnO^, and from its 

 weight calculate the amount of arsenic acid and peroxide of tin. 



The liquid filtrates are treated with sulphuretted hydrogen, to 

 throw down as sulphuret of arsenic the excess of arsenic acid 

 added intentionally. The amount of arsenic acid found, and the 

 amount added intentionally being known, it will be easy to cal- 

 culate the original quantity which the stannate contained. 



