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 determined by (1) 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 formulæ 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 radicals 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 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.