Sodium Carbonate → Dichromium tricarbonate

500 g of a 10% aqueous solution of sodium carbonate, and 85.4 g of a 35% aqueous solution of chromium chloride (manufactured by Nippon Chemical Industrial Co., Ltd.) were each placed in a container and prepared. Next, the aqueous solution of sodium carbonate was adjusted to 20° C., and the aqueous solution of chromium chloride was adjusted to 20° C. The aqueous solution of sodium carbonate and the aqueous solution of chromium chloride were simultaneously added into pure water adjusted to 20° C. The addition speed was 10 g/min for the aqueous solution of sodium carbonate and 1.7 g/min for the aqueous solution of chromium chloride. The addition was continuously performed. The addition was performed for 50 minutes. During the addition, the pH of the reaction liquid was maintained at about 7. During the addition, the temperature of the reaction liquid was maintained at 20 to 30° C. During the addition, the reaction liquid was stirred (300 rpm) so that the amount of trivalent chromium was not locally excessive with respect to the amount of sodium carbonate. A precipitate produced by the reaction was filtered and washed with water at 30° C., until the conductivity of the filtrate was 1 mS/cm, to obtain a chromium (III) carbonate cake. This chromium (III) carbonate cake was dried in a vacuum at 40° C. for 48 hours. A light blue chromium (III) carbonate powder having a water content of 18.6% was obtained in this manner. The MV and D and L*a*b* of the obtained chromium (III) carbonate were as shown in Table 1. The solubility when the chromium (III) carbonate corresponding to a Cr content of 1 g was added to 1 liter of an aqueous solution of hydrochloric acid having a pH of 0.2 at a temperature of 25° C. (immediately after the production of the chromium (III) carbonate, and after storage for 90 days, the number of revolutions of a magnetic stirrer: 200 rpm) was as shown in Table 1.