THE SOLUBILITY OF CHROMIC HYDROXIDE IN ALKALIES ROBERT B. COREY, CORNELL UNIVERSITY. ITWCA. NEW YORK In almost every textbook on general chemistry or qualitative analysis that has come to the attention of the writer the solubility of chromic hydroxide in solutions of caustic alkalies has been ascribed to the formation of a chromite for which the formula NaCrOl is generally written. This explanation may possess certain pedagogic advantages in serving to emphasize the analogy between aluminum and chromium in many compounds. However, a review of the literature fails to reveal much reason for belief in the formation of chromites except by fusion methods. No comprehensive survey of the evidence will be attempted, but the results of some of the published experiments in this connection should prove interesting. Herzl noticed that when alkali was added to a solution of a chromic salt a precipitate formed which was readily soluble in the precipitant. However, if the precipitate were filtered off, washed and dried, it became practically insoluble in the same reagent. Later Herz and Fischer2 undertook a study of the nature of the solution of chromic hydroxide in alkali. They found that such solutions became turbid on standing, finally yielding a precipitate which settled out. Precipitation was retarded by excess of alkali, but greatly accelerated by heating. From these observations they were led to the conclusion that the chromium was present in the alkali solution in the form of a colloidal suspension of chromic hydroxide, not in the form of a chromite. To test further their conclusions they treated a chromic chloride solution with an excess of alkali, and subjected the solution thus obtained to dialysis. Alkali passed through the membrane, but chromium did not. Additional confirmation was obtained from experiments upon the electrical conductivity of the mixture. To a solution of chromic chloride an excess of alkali was added and the electrical conductivity was measured. The solution was then heated so as to precipitate the chromium. A redetermination of the conductivity after the removal of the precipitate by filtration gave the same value as before. The authors concluded that all of these experiments were best explained by ascribing a colloidal nature to the solution of chromic hydroxide in alkali. They pointed out that in this respect chromium differs from aluminum which forms a compound under the same conditions. These conclusions were questioned by Kremann,' but the experiments of later investigators have served to confirm them. Hem, Z.anorg. Chem., 28, 344 (1901). a
Herz and Fischer, Ibid., 31,352 (1902). Kremann, Ibid., 33,87 (1902); 35, 48 (1903).
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Nagel4 has shown that from a solution of chromic oxide in alkali the chromium may be precipitated readily by colloidal ferric oxide. I n another experiment he found that by the use of a collodion ultra-filter it is possible to filter out hydrous chromic oxide from its solution in caustic alkali. WeiserQublished a comprehensive review of the literature in his study of hydrous chromic oxide. He concludes that "hydrous chromic oxide is peptized by alkali and that no appreciable amount of chromite is formed within a reasonable time."' A concise review of the chemistry of hydrous chromic oxide is given in a more recent article by Seymour-Jones.' Several authors describe the settling of chromic hydroxide from its solution in alkali, but in no case is mention made of testing the supernatant liquid for chromium. I t occurred to the writer that a small amount of chromite might be detected in this solution if a delicate test for chromium were applied. With this in mind, a series of twelve solutions was prepared by treating chromic chloride with an excess of sodium hydroxide. The resulting solutions were all 0.1 normal with respect to CrC12. The NaOH content ranged from 0.1 to 1.4 normal. They were placed in tightly stoppered test tubes and allowed to stand. One by one, the solutions became turbid and deposited a green precipitate, leaving a clear colorless liquid above. Turbidity and precipitation appeared first in those solutions which were lowest in alkali content. At the end of about eight weeks, settling was sufficiently complete so that samples of the supernatant liquid could be removed with a pipette. They were tested for chromium by first oxidizing with hydrogen peroxide in alkaline solution and then applying the perchromic acid test. In every case the result of the test was negative. Comparison tests on solutions of known chromium content showed that 0.01 mg. of chromium could be detected readily under similar conditions. The writer does not attach any great significance to the results of these experiments. Nevertheless, they are interesting in as much as they appear to be in line with the evidence already cited, the great bulk of which seems to establish the fact that the solubility of chromic hydroxide precipitates in excess of alkali is due primarily to the formation of a colloidal solution, and to leave us little reason for the belief in the formation of a chromite. Nagel, J. Phys. Ckem., 19, 331, 569 f1915).
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Weiser, Ibid 26, 401 (1922). Ibid., 26, 429 (1922). 7 Seymour-Jones,Ind. Eng. Chem , 15, 75 (1923).
