462 Gooch and Peters — Titration of Oxalic Acid. 



to act upon the hydrochloric acid, or the chlor-ferrous acid of 

 Wagner. According to Volhard* the reaction of Guyard is 

 favored and hastened by heat and concentration of the solu- 

 tion, while it is delayed by acidity and dilution ; but even in 

 solutions containing very little manganous salt and a consider- 

 able quantity of free acid the faint rose color developed by 

 the careful addition of permanganate ultimately vanishes until 

 every trace of the manganous salt is precipitated. When a 

 considerable amount of the salt is present interaction follows 

 immediately the introduction of the permanganate. Zimmer- 

 mann advocates the use of 4 grams of manganous sulphate 

 uniformly in titrations of a ferrous salt by permanganate, a 

 procedure to which Wagner gives acquiescence, though point- 

 ing out that a ninth of that amount is all that he finds to be 

 necessary. The excess of the manganous salt can do no harm 

 so long as the higher oxide, the product of interaction of the 

 manganous salt and the permanganate, is immediately reduced 

 by even traces of a ferrous salt, and this appears to be the case 

 at least within the limits proposed by Zimmermann and Wag- 

 ner. Thus we find, as shown in results of the accompany- 

 ing table, that so much as five grams of the sulphate may be 

 present in 135 cm3 of the liquid, containing about 5 cm3 of hydro- 

 chloric acid of full strength, without interfering with the regu- 

 larity of the titration ; and the effect is trivial even when the 

 amount of manganous sulphate reaches ten grams. We find 

 also practical regularity of working when manganous chloride 

 is substituted for the sulphate, and in this respect our results 

 accord with those of Zimmermann and differ from those of 

 Wagner: f 



Total volume 













at beginning 



HC1 





KMn0 4 







of titration. 



Sp. gr. 1-09. 



FeCl 2 . 



tVN. 



MnS0 4 . 5H 2 0. 



MnCl 2 . 4H 2 



cm 3 . 



cm 3 . 



cm 3 . 



cm 3 . 



grams. 



grams. 



135 



10 



25 



21-70 



1- 





135 



JO 



25 



21-70 



3- 





135 



10 



25 



21-70 



5- 





135 



10 



25 



21-75 



7- 





135 



10 



25 



21-75 



10- 





145 



20 



25 



21-75 



10- 







175 



50 



25 



21-75 



io- 



_ _ _ „ 



135 



10 



25 



21-70 





1- 



135 



10 



25 



21-70 





2* 



145 



20 



25 



21-70 





2- 



155 



30 



25 



21-75 





3- 



165 



40 



25 



21-70 







4- 



In all cases, however, in which the larger amounts of manga- 

 nous salt are present, the end reaction is marked by the advent 



* Ann. d. Cliem , cxcviii, 318, 18*79. f Loc. cit., p. 104. 



