D. L. Randall — Titration of Mercurous Salts. 139 



For practical purposes the application of the method to mer- 

 curous nitrate is of much greater importance. In Schneider's* 

 method for the determination of manganese, permanganic acid is 

 titrated with hydrogen peroxide in the presence of nitric acid. 

 Ibbotsonf and Brearley in their modification of this process 

 recommend the use of standard ferrous ammonium sulphate 

 instead of hydrogen peroxide. Blair;}: also studied this process 

 and recommends the use of ferrous ammonium sulphate. In 

 the absence of any data concerning the quantity of nitric acid 

 that can safely be present in the titrations of ferrous sulphate 

 .the determinations in Table II were made. The nitric acid 

 used was rendered free from the lower oxides of nitrogen by 

 the prolonged passage of a current of air. The acid used in 

 all the subsequent experiments was purified in a similar man- 

 ner. 







Table II. 











H 2 S0 4 





Approx. 



?eS0 4 



Dil. 



1 :1 



HN0 3 



N/10 KMnO 



cm 3 . 



cm 3 . 



cm 3 . 



cm 3 . 



cm 3 . 



25 



200 



5 







13-37 



25 



200 



5 







13-39 



25 



200 



5 







13-41 



25 



200 



5 







13-38 



25 



200 



5 







13-38 



25 



200 







3 



13-38 



25 



200 







5 



13-40 



25 



200 







5 



13-41 



25 



200 







5 



13-40 



25 



200 







5 



13-38 



25§ 



200 



5 







13-47 



25 



200 



5 







13-50 



25 



200 



5 







13-49 



25 



200 







5 



13-53 



25 



200 







10 



13-50 



25 



200 







20 



13-51 



20 



200 







30 



13-00 



When more than 10 per cent by volume (20 cm3 in 200 cm3 ) of 

 the concentrated acid was present the oxidation of the ferrous 

 salt by the nitric acid was evident in the change of color, of 

 the solution, low results, || and uncertain end reaction. This 



*Ding. Pol. Jour., cclxix. 224. 



fChem. News, lxxxiv, 247. 



\ Blair, Jour. Amer. Chem. Soc, xxvi, 793. Also, Chemical Analysis of 

 Iron, vi, Ed., p. 121. 



§ Changed FeS0 4 solution. 



|| As nitrous acid is at least in part oxidized by direct titration with potas- 

 sium permanganate, and completely when an excess is used (method of Kin- 

 necut and Nef, Amer. Chem. Jour. , 5, 388), only a part of the loss due to 

 oxidation of the ferrous salt by the nitrous acid would appear in the result. 



