CHROME LEATHER MANUFACTURE 151 



amount of Cr in the same volume of liquor. A chromium 

 estimation is therefore also necessary and is most readily 

 done by evaporating a portion of liquor to dryness, igniting 

 the residue and oxidizing the chrome to chromate by 

 heating in a muffle furnace with magnesia and sodium 

 carbonate in equal parts, or fusing in a blowpipe with 

 sodium and potassium carbonates in equal parts. The 

 oxidized residue is dissolved in hydrochloric acid and 

 titrated with thiosulphate as described for the two-bath 

 process. 



Another attempt to determine the practical value of 

 a chrome liquor is the empirical test suggested by 

 McCandlish, in which 10 c.c. of the liquor is titrated with 

 standard alkali until the precipitation point is reached and 

 a turbidity appears. The figure thus indicates approxi- 

 mately the degree of nearness to the precipitation point 

 and the amount of free acid in the liquor. The author 

 has found this a useful test taken in conjunction with the 

 basicity determination. It is best expressed in the same 

 units, e.g. grams SO 4 per 52 grams Cr. 



Another method is the determination of the hydrion 

 concentration of the liquor. This has useful possibilities 

 for research work, but is usually too laborious for rapid 

 commercial control. The results, moreover, are not less 

 empirical, for the hydrion concentration of the liquor 

 indicates but imperfectly the electrical condition of the 

 particles of the tanning sol. 



In classifying one-bath liquors into types, it is best 

 to take together those in which the usual " basicity " and 

 " acidity " determinations have at any rate approximate 

 comparative value, and this is determined in the main 

 by tlu method by which the liquor is manufactured. 

 Broadly speaking, there are three types of chrome liquor : 

 (i) those made from chromic salts by adding suitable 

 amounts of alkali ; (2) those made from sodium dichroniate 

 by reduction with organic matter ; and (3) those made 

 from sodium dichromate by reduction with sulphurous 

 acid or its salts. 



