450 Gooch and Pulman — Estimation of Molybdic Acid 



die acid undergoes the ideal reduction. When this point is 

 reached dilution with cold water obviates danger of immediate 

 oxidation and gives opportunity for continuing the treatment 

 referred to above. The method yields accurate results, but is 

 tedious on account of the delay of an hour and a half to two 

 hours before the excess of the iodine introduced to reoxidize 

 the molybdic acid can be safely titrated by the arsenite. Tak- 

 ing the matter up again at this point, we find that it is perfectly 

 feasible to obviate the difficulty in question and to secure 

 immediate reoxidation of the molybdic acid by substituting 

 potassium permanganate for iodine as the oxidizer. 



When an excess of potassium permanganate is added to a 

 solution containing hydrochloric and hydriodic acids with 

 molydenum in a condition of oxidation corresponding to the 

 pentoxide, several different effects of oxidation may be expected. 

 There is the immediate liberation of iodine from the hydriodic 

 acid, the production of iodic acid, some slight tendency to lib- 

 erate chlorine from the hydrochloric acid, the formation of 

 representatives of the higher oxides of manganese, and, lastly 

 the reproduction of the molybdic acid, which is the object of the 

 operation. All other effects than the last must be prevented 

 or recorded in order that the estimation of the molybdic acid 

 may be accomplished. Of the secondary actions, the liberation 

 of chlorine may be prevented by adding a manganous salt, 

 according to the well-known proposals of Kessler* and Zimmer- 

 man^ the iodine set free may be converted to hydriodic acid 

 again by the introduction into the acid solution of a sufficient 

 excess of a standard arsenite solution ; the iodic acid may be 

 reduced in the acid solution in the same manner, as we have 

 found by experiment, the value obtained for a solution of iodic 

 acid by adding to it in presence of sulphuric acid a measured 

 amount of standard arsenite, making alkaline, and titrating 

 back with iodine, being the same as that found by acting 

 directly upon the iodic acid with an excess of potassium iodide 

 in presence of dilute sulphuric acid and determining by stand- 

 ard arsenite the iodine liberated ; and the higher oxides of 

 manganese are likewise reduced in the acid solution by the 

 arsenious acid of the standard arsenite. 



Whatever excess of arsenious acid is left over after the 

 reactions described may, obviously, be determined by neutral- 

 izing the solution with an alkaline bicarbonate and titrating 

 with iodine. 



Every difficulty introduced by the secondary oxidations 

 should, it might be supposed, be overcome by adopting the 

 following procedure : the addition of manganous sulphate to 

 the reduced, diluted, and cooled solution ; the introduction 

 of measured standard potassium permanganate until its char- 



* Ann. Phys., cxviii, 48; cxix, 225-226. f Ann. Chem. (Liebig), ccxiii, 302, 



