150 ANIMAL PROTEINS 



It will be readily understood, therefore, that there 

 is some difficulty in expressing the tanning power of a 

 chrome liquor. As near as can be yet said this is deter- 

 mined by (i) the concentration of the actual tanning sol, 

 and (2) its nearness to the isoelectric point. Now, these 

 points are not readily determined by analytical methods, 

 and the best that can yet be done is to determine the 

 conditions which have large influence upon these points. 

 Thus the degree to which the liquor is " made basic " by 

 adding alkali is known, and can be expressed in formulae 

 by assuming that the acid neutralized by this alkali is 

 replaced in the chrome salt by hydroxy groups. Chromic 

 chloride, Cr 2 Cl 6 , with the addition of soda to correspond to 

 half the acid formed upon complete hydrolysis, would be 

 considered then to be a solution of the salt, Cr 2 (OH) 3 Cl 3 . 

 This has given rise to the conception of the " basicity " 

 of a chrome liquor, which may be expressed in many ways, 

 the most common of which in practice is the number of 

 grams SO 4 still combined with 52 grams Cr. Thus the 

 salt corresponding to the composition Cr(OH)SO 4 is said 

 to have a basicity of 96. The practical importance of such 

 determinations of basicity has been much exaggerated, 

 for they are but a rough guide to the degree of hydrolysis 

 of the chrome and to the extent to which the sol is positive. 

 Thus if the chrome salt be actually a sulphate, a liquor 

 of basicity 96 has about the same practical value as a 

 chloride liquor of basicity 72, and in each case the figures 

 are of little significance if many organic substances be 

 present. If, however, as is usual in practice, there be 

 approximately the same acid radicles throughout the 

 tannage and about the same relative proportion of organic 

 matters or of inorganic salts, then these determinations 

 have some practical value for comparative purposes. 

 The determination is itself simple : a portion of liquor 

 is titrated direct with caustic soda. The titration is at 

 boiling-point, and is continued until a permanent pink 

 is obtained with phenolphthalein. The amount of SO 4 

 corresponding to the soda required is then relative to the 



