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 approximately 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 the method by which the liquor is manufactured. Broadly speaking, there are three types of chrome liquor: (1) those made from chromic salts by adding suitable amounts of alkali; (2) those made from sodium dichromate by reduction with organic matter; and (3) those made from sodium dichromate by reduction with sulphurous acid or its salts. Of the first type the most common is that in which chrome alum (a bye-product of the dyeing industry) is the starting-point. To a solution of this a solution of washing soda is gradually added, with constant stirring, until the salt corresponding with the formula Cr(OH)SO{4} is obtained. Now:-- K{2}SO{4}Cr{2}(SO{4}){3}·24H{2}O + Na{2}SO{4} \___________________________________________/ 998 + Na{2}CO{3}·10H{2}O \________________/ 286 = 2Cr(OH)SO{4} + K{2}SO{4} + CO{2} + 33H{2}O