466 Gooch and Peters — Titration of Oxalic Acid. 



From these results it appears that the presence of a suitable 

 amount of manganous salt — either the sulphate (exps. 4-7, 

 13-15, 22-24) or the chloride (exps. 10-12, 16-20) — is capable, 

 either in cold solution (exps. 1-20) or in hot solution (exps. 22-24) 

 of preventing the action of the permanganate upon the hydro- 

 chloric acid. It appears, also, that, for a given dilution and 

 strength of acid, less manganous salt is needed in the cold 

 solution (exps. 4-7) than in the hot solutions (exps. 22-24). 

 Thus, in the hot solution, at a dilution of 145 cm3 to 500 cm3 

 1 grm. of manganous sulphate must be present with 5 cm3 of 

 strong hydrochloric acid, with or without sulphuric acid ; 

 while in the cold solution 0'04 grm. of either the sulphate or 

 chloride is enough to secure adequate protective effect. 

 Experience showed, however, that 0*5 grm. or 1*0 grm. of the 

 mauganous salt should be present in order to push the reaction 

 with reasonable speed in cold solutions. 



Wagner* has made record of the increased evolution of 

 chlorine in oxidations of ferrous chloride by potassium per- 

 manganate in presence of various salts, of which barium 

 chloride was the most active. We have made some experi- 

 ments, therefore, to determine whether such action would 

 appear in the oxidation of oxalic acid in cold solutions con- 

 taining certain salts, and, if so, whether it would be preventable 

 by the presence of the manganous salt under our conditions 

 of working. From the results given in the accompanying 

 table, it is plain that the evolution of chlorine in cold solu- 

 tions is less in the presence of these salts than when hydro- 

 chloric acid is used without them, and that such evolution may 

 be entirely prevented (within the proportions of our work) by 

 the presence of 0*5 grm. to 1 grm. of manganous chloride. 



Finally, it appears as the result of an investigation, that the 

 titration of oxalic acid by potassium permanganate in presence 

 of hydrochloric acid is ordinarily attended with some inaccu- 

 racy due to liberation of chlorine from the hydrochloric acid ; 

 that this tendency may be overcome by the presence of a man- 

 ganous salt — either the sulphate or chloride ; that 1 grm. of 

 the manganous salt is enough to so affect the conditions of 

 equilibrium that titrations in moderate volumes (100 cm3 to 

 500 cm3 ) and in presence of hydrochloric acid (5 cra3 to 15 cm3 of the 

 strong acid) may be conducted with safety and reasonable 

 rapidity, either with or without sulphuric acid, at the ordinary 

 atmospheric temperature. 



* Loc. cit. 



