Van Name and Hill — Solution of Silver in Chromic Acid. 55 



entering- the solution quickly precipitated all the chloride 

 in the body of the liquid, and so rendered it innocuous. 

 On the other hand a larger amount of chloride (1 cm 3 of 

 strong hydrochloric acid in 600 cm 3 of solution) soon 

 produced a coating of silver chloride on the disk, and the 

 reaction velocity dropped to zero in a few minutes. Fol- 

 lowing are the velocity constants obtained in an experi- 

 ment under these conditions, the duration of the reaction 

 periods being ten minutes each, and the normal value of 

 k about 1-95. 



k = 1-08 0-09 —0-04 —0-02 0-12 —0-02 0-02 



Further evidence against the hypothesis that coatings on 

 the disk were responsible for the irregularities noted in 

 experiments under normal conditions, was obtained by 

 removing the disk after each reaction period and 

 immersing it for several minutes in strong ammonia 

 before replacing it. This procedure was employed in a 

 number of experiments, but produced no appreciable 

 change in the velocity constants. 



The possibility that the depression of the reaction 

 velocity might be caused by the accumulation of silver 

 salt in the solution was also investigated. Such an effect 

 could not, of course, be due to a simple displacement of 

 the equilibrium, as in the case of silver in ferric sul- 

 phate, 3 because the reaction between chromic acid and 

 metallic silver is not reversible, but might be produced 

 in some other way. It was found, however, that no 

 measurable change in the reaction velocity was produced 

 by dissolving in the solution at the outset two grams of 

 silver sulphate, an amount considerably larger than that 

 produced during an average experiment. Three succes- 

 sive experiments which illustrate this point are recorded 

 in Table I. 



Table I. 



Silver (Sample A) in Chromic Acid. 



Sulphuric acid 5 molar. 



Exp. 1. k = 220 211 1-88 1-83 208 1-92 195 Av. 2-00 



Exp. 2. k — 2-27 207 1-78 190 1-79 1-95 1-88 Av. 1-95 



Exp. 3. Two grams of Ag„S0 4 added at outset. 



k — 2-12 1-88 1-90 202 1-93 1-97 1-99 Av. 1-97 



At an early stage of the work it became clear that the 

 cause of the variations lay in the metal itself. Two 



3 Loc. cit. p. 310. 



