THE NATURE OF CHROME LEATHERS 131 



taken, and to such an extent that there can be little doubt 

 that the chrome sol is not far from its isoelectric point. 

 Amongst these " steps " are (i) making the liquor " basic," 

 i.e. adding alkali to neutralize much of the free acid, which 

 involves a considerable reduction in the stabilizing effect 

 of the hydrions ; (2) the adsorption of hydrions by the 

 hide gel when first immersed in approximately neutral 

 condition ; (3) the operation of the " valency rule " that 

 the predominant ionic effect in discharging is due to the 

 multivalent anions. In this case the divalent SO 4 " ions 

 assist materially in discharging the positive charge on 

 the chrome sol ; (4) the final process of neutralization in 

 which still more alkali is added. The operation of the 

 valency rule is the most complex of these factors, for there 

 is also to be considered the stabilizing effect of the kations, 

 especially of the trivalent kation Cr" from the unhydro- 

 lyzed chromium sulphate. It is quite possible also that 

 in the last stages of chrome tanning' there are " zones of 

 non-precipitation " due to the total effect of multivalent 

 ions, and it is quite conceivable that the chrome sol may 

 change its sign, i.e. become a negative sol and thus give 

 also a mutual precipitation with the hide-gel. This is 

 particularly probable where a local excess of alkali occurs 

 in neutralization. However that may be, it is probable 

 that most of the tannage is accomplished by chromium 

 hydrate in acid solution, and it is therefore legitimate to 

 conclude that adsorption and gelation have a relatively 

 greater part in chrome tannage. The operation of the 

 valency rule makes it easy to understand why basic 

 chlorides do not tan so well as sulphates ; the precipitating 

 anion is only monovalent (Cl') and chromic chloride con- 

 tains no substance analogous to the potassium sulphate of 

 chrome alum and hence contains a less concentration of the 

 precipitating anion. Hence also the stabilizing influence 

 of common salt added to a basic alum liquor, the effect being 

 to rq-)lace partially tin- divalent SO 4 " by the monovalent 

 Cl I,\ otrope influence, however, may be here at work. 

 It i- .n^ibic to make out a rather weak case that the 



