hy the Action of HydroehloriG Acid, 375 



of the first action weakened by boiling. Plainly two methods 

 of bringing about the re-treatment with strong acid are open : 

 either the weak acid remaining after the boiling process may 

 be wholly removed by evaporation and replaced by concen- 

 trated acid, or it may be cooled and re-saturated with gaseous 

 acid. Table II contains the record of results obtained in both 

 these ways. 



These experiments show very clearly that several successive 

 treatments with strongest hydrochloric acid, consisting either 

 in the addition of the concentrated aqueous acid to the dry 

 residue in successive portions, or in passing gaseous acid through 

 the residual liquid cooled in ice-water, finally bring the residue 

 to a condition of reduction in which the recharging of tlie 

 liquid residue with gaseous acid produces no brown or green 

 color but a clear blue. When this condition is reached the 

 Holverscheit procedure produces no appreciable further reduc- 

 tion. The holding of the blue color when the solution cooled 

 in ice-water is thoroughly charged with the gaseous acid, appears 

 to be the best evidence of the complete reduction of YJd^ to 

 Yfi^ ; it is not sufficient that the boiled solution, in which the 

 acid has been weakened, should be blue. 



In the several manipulations of the repeated processes there 

 is likely to be some mechanical loss of chlorine. For this 

 reason, the direct titration of the residues' by potassium per- 

 manganate in presence of a manganous salt was combined, as a 

 control, with the determinations of the iodine set free by 

 chlorine in the distillate, as shown in the experiments of 

 Table III. 



In these experiments we have again evidence tliat it is pos- 

 sible to effect the reduction of vanadic acid to within a few 

 per cent of the amount present by a single treatment with con- 

 centrated hydrochloric acid, and that the amount of the reduc- 

 tion may be determined by titrating the residue with potassium 

 permanganate. It is plain, therefore, that Gibbs' application 

 of this mode of reduction and titration to the determination 

 of the small proportions of vanadium pentoxide found in many 

 of the complex salts studied by him is also justified. 



The experiments show also that when the action of hydro- 

 chloric acid is sufficiently continued, ordinarily large amounts 

 of vanadic acid may be completely reduced to the condition of 

 the tetroxide ; and the method of reduction is of special 

 advantage in those cases v\^hich call for titration of the tetrox- 

 ide by permanganate, since in such cases the use of Holver- 

 scheit's admirable method of reduction by hydrobromic acid is 

 precluded. 



Am. Jour. Sci. — Fourth Series, Vol. XIV, No. 83. — November, 1902. 

 26 



