CAIIV: DF/rECTlON OF ARSENIC, ANTIMONY ANT) TIN. 261 



For exactly the same reasons, the sulfids of tin may be hindered 

 from precipitating by making the solution highly acid. Hydro- 

 chloric acid of the concentration indicated is sufficiently strong to 

 entirely prevent the precipitation of antimony or tin while the 

 arsenic is promptly thrown. On account of its slight degree of 

 dissociation, hydrogen sulfid is practically as soluble in hydro- 

 chloric acid as in water, and therefore in diluting the solution with 

 water saturated with hydrogen sulfid, the concentration of the 

 hydrogen sulfid can be treated as a constant, and from the equation 



H2S=2H + + S= 

 ^ 1 ^ 2 ^ 3 

 KCj=C|C3 or since C,=const. 

 Ki=KC'=C8C3 



C| ^ 



it follows that the concentration of the sulfur as ion will vary inversely 

 • as the square of the concentration of the hydrogen, and therefore, 

 if the solution be diluted one-half, the concentration of the sulfur 

 as ion will be nearly four times as great as before. Since the 

 dissociation of the antimony "chlorid or tin chlorid increases very 

 rapidly with the dilution owing to the decrease in the concentra- 

 tion of the chlorine as ion it follows that a point must be reached 

 when one or both of them will be precipitated. This point will of 

 course be that at which the product of the concentration of the 

 antimony or tin as ion and if the sulfur as ion has reached the 

 precipitation value. 



That substances having the smallest precipitation value will 

 appear first then that having the next larger. The values for the 

 sulfids of tin and antimony stand in this order, 



Sn S>Sn S2>Sb2 S3. 



Therefore the antimony sulfid appears first, then the stannic, and 

 last the stannous sulphid. 



One-tenth of a milligram of arsenic is easily detected even in 

 the presence of a gram of antimony. The limit for antimony and 

 tin is naturally somewhat higher 



If other metals precipitated by hydrogen sulfid are present they 

 are all precipitated together and the arsenic, antimony and tin 

 dissolved by yellow ammonia sulfid and represented by dilute 

 hydrochloric acid and filtered off and redissolved in concentrated 



