144 



ANIMAL PROTEINS 



chroming baths in common use on a practical scale are 

 given below under the heading of their type : 



All the figures are percentages of the weight of pelt. As 

 K 2 Cr 2 O 7 has a molecular weight of 294, and Na 2 Cr 2 O7,2H 2 O 

 a molecular weight of 298, in practice they may be con- 

 sidered as interchangeable, weight for weight. The sodium 

 salt is cheaper and more often used. The corresponding 

 amount of chromic acid, 2CrO 3 , has an equivalent weight 

 of 200, hence any weight of dichromate may in practice be 

 substituted by two-thirds the weight of commercial chromic 

 acid. Equivalent weights of commercial sulphuric acid 

 are sometimes used in place of hydrochloric. The quantity 

 depends upon the strength of the sulphuric acid used. 

 Aluminium sulphate, Al 2 (SO 4 ) 3 i8H 2 O (mol. wt. 666), may 

 be replaced by ordinary potash alum, K 2 SO4,A1 2 (SO 4 ) 3 - 

 24H 2 O (mol. wt. 948). In practice 7 parts of the former 

 and 10 parts of the latter may be considered equivalent. 

 It should be remembered that both these salts are hydro- 

 lyzed in solution, and therefore increase slightly the amount 

 of free acid present. Their presence decreases the amount 

 of chrome taken up, and as little or no alumina is found 



