484 M. Carey Lea — Attotrojnc Forms of Silver. 



to determine the ratio between the silver on the one hand and 

 the citric acid on the other, either excluding from the deter- 

 mination, or else removing, that portion of the citric acid 

 which was combined with sodium (sodic citrate being used in 

 excess) or with iron. The first attempt was to exclude with- 

 out removing it, by using Wolcott Gibbs's ingenious method of 

 precipitating the base by hydrogen sulphide, and determining 

 the acid thus set free in a solution originally neutral. It was 

 ascertained by careful experiment on weighed quantities of 

 23ure anhydrous citric acid, that exact titration could be made 

 with the aid of phenolphthalein. The silver was next redis- 

 solved and estimated as chloride. A large number of deter- 

 minations were made, but the method proved unsatisfactory. 

 It was found tha/t portions of the same material operated upon 

 separately gave different (even widely different) results. In 

 fact, this very discordance was in itself a proof that no stcechio- 

 metrical combination existed between the silver and the citric 

 acid. 



The importance of the matter led me to take it up -again 

 with different means, estimating the citric acid by Creuse's 

 method. In this method the solution, after being reduced to a 

 small bulk, is exactly neutralized (with ammonia or acetic acid), 

 is treated with a slight excess of barium acetate and then 

 mixed with twice its bulk of 95 per cent alcohol, let stand a 

 day and filtered and washed with 65 per cent alcohol. In 

 igniting, a few drops of sulphuric acid convert the barium salt 

 into sulphate in which form the estimation is made. A pre- 

 liminary trial with a weighed quantity of citric acid showed that 

 this method gave fairly good results. I was obliged to vary 

 the method somewhat : the precipitate of barium citrate carried 

 down with it enough iron to render it ochrey in appearance. 

 It was, therefore, after thorough washing with 65 per cent 

 alcohol till every trace of barium acetate was removed, dis- 

 solved on the filter with dilute hydrochloric acid (acid 1, water 

 10) in which barium citrate is extremely soluble and washed 

 through. This was followed by still weaker acid and finally 

 with water. From the filtrate, sulphuric acid precipitated 

 snow-white barium sulphate. 



But this method requires that both the sulphates and the 

 excess of sodic and ferric citrate shall first have been perfectly 

 removed. The blue precipitate was therefore washed with 

 dilute solution of ammonic nitrate until this was effected. The 

 necessity for this purification was regrettable as introducing a 

 possibility of a change during the treatment. It was, however, 

 indispensable that the ferrous, ferric and sodic citrates present 

 should be got rid of. The material after this treatment was 

 still freely soluble in water, to a dark-red solution. An ex- 



