262 ANALYTICAL CHEMISTRY. 



solution until it is no longer decolorized. In the same manner the 

 titer of the stronger solution is determined. 



Having ascertained the strength of each solution, the proportions 

 for mixing them are ascertained by using the formula: 



Stronger solution. Weaker solution. 



(W 10) S. + (10-S.) W. 



By W are indicated the c.c. of weaker solution, by S the c.c. of 

 stronger solution required to decompose 10 c.c. of deci-normal oxalic 

 acid. 



For instance: Assuming that 9.5 c.c. of the stronger (S) and 10.4 

 c.c. of the weaker (W) solution had been required, then, substituting 

 these values in the above-given formula, we obtain : 



(10.410) 9.5 + (109.5) 10.4, 

 or 



3.8 -f 5.2, 



making 9 c.c. of final solution. 



The bulk of the two solutions is now mixed in the same proportion, 

 say 380 c.c. of the stronger and 520 c.c. of the weaker solution 



After the mixture is thus prepared, a new trial should be made, 

 when equal volumes of the solution prepared and of deci-normal 

 oxalic acid solution should exactly decompose one another. Instead 

 of working with 10 c.c. of the solutions it is advisable to use larger 

 quantities, say 20 or even 50 c.c., whereby the errors made by reading 

 are diminished. 



Instead of using oxalic acid for standardizing permanganate solution, 

 metallic iron may be used, and the operation should be conducted as follows : 

 0.2 gramme of pure, thin iron wire is dissolved in about 20 c.c. of dilute sul- 

 phuric acid (1 acid, 5 water) by the aid of heat, and in a flask arranged a& 

 in Fig. 37. The flask is provided, by means of a perforated cork, with a 

 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 0.005588 

 gramme of metallic iron, the 0.2 gramme of iron wire used will consume 3'5.7 

 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 



