Bosworth — Iodometric Determination of Silver. 287 



Art. XXXI. — A Method for the Iodometric Determination 

 of Silver Based upon the Reducing Action of Potassium 

 Arsenite ; by Rowland S. Bosworth. 



[Contributions from the Kent Chemical Laboratory of Yale Univ. — ccii.] 



It is well known that an ammoniacal solution of silver 

 arsenite deposits metallic silver when the ammonia is evap- 

 orated by boiling. During this reaction, by which the silver 

 salt is reduced, the arsenious acid becomes oxidized to the 

 higher condition of oxidation, where arsenic has a valence of 

 five, according to the equation 



2Ag 2 + As 2 3 = As 2 5 + 4Ag. 



The present paper deals with some work done with the purpose 

 of determining whether this reaction was quantitative and of 

 devising a rapid iodometric method for the determination of 

 silver based upon the reduction of the silver salt to the metal. 



Experiments were first performed upon a silver nitrate solu- 

 tion of known strength according to the. following plan : To 

 a definite portion of a standard solution of silver nitrate was 

 added a known volume of a standard potassium arsenite 

 solution in excess of the amount necessary to reduce the silver 

 salt present. Ammonia was then added in sufficient quantity 

 to dissolve the precipitate formed, and the resulting solution 

 was diluted to 100 cm3 and boiled until the escaping vapor gave 

 no test for free ammonia with moistened litmus paper. The 

 solution, out of which metallic silver had then separated, was 

 filtered, cooled, and made faintly acid in order to neutralize 

 any possible trace of ammonia which might remain. After 

 making alkaline with sodium bicarbonate, the excess of potas- 

 sium arsenite was titrated with N/10 iodine. The silver value 

 of the iodine used was subtracted from that of the potassium 

 arsenite originally taken, and the result used as a measure of 

 the silver present. In Table I, A, are given the results of 

 experiments performed in the manner described above. 



The effect was next tried of the use of sodium bicarbonate 

 as a means for producing alkalinity in place of the ammonia. 

 The following procedure was used : To a solution of silver 

 nitrate was added an excess of standard potassium arsenite 

 solution. The mixture was made alkaline by means of 25 cm3 

 of a saturated solution of sodium bicarbonate, diluted to 100 cm3 , 

 and boiled until the precipitate of silver arsenite was converted 

 to metallic silver. The solution was then filtered and acidified, 

 in order to break up the neutral carbonate formed by boiling 

 the bicarbonate. Finally the solution was made alkaline with 

 sodium bicarbonate and the potassium arsenite present was 



