71, 72] Estimation of Chlorine 47 



.*. -0108 Ag precipitates '00355 CI, but 1 c.c. AgN0 3 



10 



contains *oio8 Ag ,\ we multiply the number of c.c. of — - 



10 



AgN0 3 used by '00355, which will give the number of grams 

 of chlorine in the 25 c.c. of the NaCl solution; this multiplied 

 by 10 gives the total weight of chlorine, and this multiplied 

 by 100 and divided by the weight of NaCl taken gives the per- 

 centage of CI in the NaCl solution. 



VOLUMETRIC ESTIMATION OF IRON 



71. This estimation depends upon the fact that ferrous 

 salts may be oxidised by either K 2 Mn 2 8 or K 2 Cr 2 7 in the 

 presence of acids to ferric salts by one of the equations : 



ioFeS0 4 + K 2 Mn 2 8 + 8H 2 S0 4 = 5Fe 2 (S0 4 ) 3 + 2MnS0 4 + K 2 S0 4 + 8H 2 0. 

 6FeS0 4 + K 2 Cr 2 7 + 7H 2 S0 4 = 3Fe 2 (S0 4 ) 3 + Cr 2 (S0 4 ) 3 + K 2 S0 4 + 7H 2 0. 



From these equations it will be seen that one molecule of 

 K 2 Mn 2 8 will oxidise ten of FeS0 4 or 316 grams of K 2 Mn 2 8 

 will oxidise 10 x 56 grams of iron from the ferrous to the 

 ferric state. Thus a normal 1 solution of K 2 Mn 2 8 would con- 

 tain 31*6 grams per litre. In the same way it may be calcu- 

 lated that a normal solution of K 2 Cr 2 7 will contain 49 grams 

 per litre. 



72. Preparation of Standard Bichromate of 

 Potash. — This solution, although slightly more troublesome 

 to use than permanganate, has the advantage of permanency. 

 It may be kept in a well-stoppered bottle almost indefinitely, 

 without suffering any change of strength. 



Weigh out exactly 4*91 grams of powdered K 2 Cr 2 7 which 



1 It will be noticed that the word normal is here used in a manner which 

 is not included by the definition given on page 38, as K 2 Mn 2 8 is neither 

 an acid nor an alkali. 



