METHODS FOE QUANTITATIVE DETERMINATIONS. 419 



piece of glass tubing, to which is attached a piece of rubber tubing in which 

 is cut a vertical slit about one inch long and which is closed at the upper 

 end by a piece of glass rod ; gas or steam generated in the flask may escape, 

 while atmospheric air cannot enter, the ferrous solution being thus protected 

 from oxidation. 



The iron solution, obtained from the 0.2 gramme of iron, is cooled and 

 diluted with about 300 c.c. of water, and then deci-normal potassium perman- 

 ganate solution is added with constant stirring until the solution is tinged 

 pinkish. 



As 1 c.c. of deci-normal permanganate solution corresponds to 0.00555 



FIG. 67. 



Flu.sk for dissolving iron for volumetric determination. 



gramme of metallic iron, the 0.2 gramme of iron wire used will consume 36.03 

 c.c. of the solution. 



Permanganate is often used in determinations of iron and iron compounds. 

 Many of the latter contain iron in the ferric state, which must be converted 

 into ferrous compounds before titration. This conversion is accomplished by 

 heating the solution of a weighed quantity of the ferric compound with nascent 

 hydrogen i. e., with metallic zinc and dilute sulphuric acid in a flask 

 arranged as the one spoken of above, and shown in Fig. 67. 



A very much quicker reduction of the ferric into a ferrous compound may be 

 accomplished by adding very slowly with constant stirring a saturated solution 

 of sodium sulphite to the boiling, acidified iron solution contained in the flask 

 until the liquid becomes colorless. All excess of sulphur dioxide is expelled 

 before titrating, by boiling the solution (which should contain a sufficient 

 quantity of sulphuric acid to decompose all sodium sulphite) for about ten 

 minutes in a flask, arranged as the one mentioned above. 



Equivalents. The equivalent of 1 c.c. normal KMnO 4 for the 

 various substances which can be oxidized by it must be deduced 

 from the equations of reaction, just as in the case of acids and 

 alkalies. 



