T H E COLORS O F COBALTOUS H17DKOSIDE BY CHARLES K . STILLWELL
\Vhen an excess of caustic potash is added to a solution of a cobaltous salt a voluminous, deep blue precipitate is formed which later turns to rose cobaltous hydroxide. This change was probably first studied by Winkelb1ech.l He also noted the appearance of green flakes if the precipitation !vas made in dilute solution. Winkelblech's conclusion is that the blue is a basic d t of cobalt and that the green is the same thing partially oxidized. I n 1912 Hantzsch showed? that the blue precipitate is also cobaltous hydroxide but that it adsorbs strongly the cobaltous salt from which it is precipitated, or a bwie salt of cobalt formed during the precipitation. He postulates /OH the former being stabilized hydrate isomers, blue C o 0 . H 2 0and red Co-OH, by the adsorbed salt. Weiser3 reviem the pros and cons of the matter, casting aside the explanation of Hantzsch and offering evidence to indicate that the blue may be due to finely divided particles and the red to larger particles: or that we may be dealing with two allotropic forms of cobaltous hydroside. The present work \vas undertaken to determine which, if either, of thew explanations is the true one. Many worker. hare endeavored to explain the change in color of a cobalt Folution from red to blue as concent,rated acid is added, and it was thought that this controversy might throw light on the present problem. Hantzsch,4 htudying absorption spectra, concludes that in the red solution the cobalt atom is associated with six other groups, while in the blue the cobalt is associated Tvith four. Howellj reached the same conclusions independently by a comparison of the absorption spectra of the solutions vr-ith the absorption spectra of solids whose crystal structure had been determined by X-ray More recent papers seem to support this general idea.6 If these conditions hold for the cobaltous hydroxides we should expect a difference in the crystal structure of the two, and there is further evidence to substantiate this. Small amounts of nickelous hydroxide precipitated with the cobaltous hydroxide will retard the change from blue to rose.' This could be explained bi- assuming that the crystal structure of the blue more closely corresponds to that of the nickelous hydroxide than does the crystal struc' . i n n . . 13, ~ j i183j). j 2. anorg. Chem.. 73, 304 1 1 9 x 2 ~ . "The Hydrous Oxides," 147. ' 2. anorg. Chem., 73, 309 (1912). j Phil. l I a g . , (61 48. 833 (1924). This paper contains a complete bibliography of earlier vork. H o m l l : J . Chem. SOC..1927, 158, 2039, 2843; Hantzsch: Z. anorg. Chern. 159, 273 (192;): 166,237 (1927). ~ , (1504;; Prcc. IndianaAmd. Fci., 3 4 , 163 (1924). Benedict: J. .Am. Ckem. C C C . , ~69j,
1248
CHIZRLES W
STILLFELL
ture of the rose. Several similar cases are known: notably red and green chromic oxide with alumina’, red and yellow mercuric iodide with mercuric bromideJ2and yellow and orange silver iodide with silver bromide.3 Bef3i-e investigating the crystal structures of the blue and rose cobaltous hydroxides, the conditions necessary for their preparation were studied in more detail. When potassium hydroxide is added to cobaltous chloride the first precipitate is always deep blue and gelatinous, laminar in appearance, regardless of the relative proportions of reactants used. If the precipitate subsequently changes t o green, it has a noticeable greenish tinge immediately after mixing; that is, although predominately blue it is not as pure a blue as those which subsequently undergo change one or t v o in the chart below. If concentrated solutions are used, say two molar, a stiff gel is actually formed and later experiments show that the original deep blue color can only be preserved in the stiff gel. As soon as the gel breaks down, aging and a change of color set in. The gel may be stabilized and the deep blue color maintained by adding glycerine, as well as by using more concentrated solutions. Under normal conditions this deep blue may change either t o green, or through violet to rose, or may remain blue-a lighter blue-depending upon the relative amounts of reactants used. Under certain conditions the green changes on long standing first to blue and then to rose. The following diagram indicates all these changes: Dark green gelatinous
I
+
Deep blue (greenish) gelatinous
3
L
1 2
.c
rose I granular
light blue I granular Aged light green granular
5
+
Aged light blue I1 granular
6
-+
Aged rose I1 granular
It is shown below that the blue and rose prepared directly are different from the same colors obtained by the aging of the green precipitate, and the former are therefore separate and are designated “blue I” and “blue 11” although superficially the two blues appear identical as do the two roses. The aged precipitate and changes four, five and six are discussed in section 11. The immediate discussion deals with changes one, two and three. Stillwell: J. Phys. Chem., 30, 1441 i(1926). Mellor: “A Comprehensive Treatise of Inorganic and Theoretical Chemistry,” 3, 427. Mellor: Ibid., 4, 504.
7 HE COLORS
OF COBALTOUS HYDROXIDE
1249
The color changes of the deep-blue precipitate and the rate of change depend on several factors. The deep blue t o rose change is accelerated by I . Heating' 2 . Increasing the concentration, keeping the molar ratio of reactants constant. 3. Increasing the ratio moles KOH to moles CoC12, keeping the concentration constant'. This change in color is accompanied by a visible settling of the precipitate and a change from gelatinous laminae to granules. The deep-blue to green change is accelerated by I . Decreasing the concentration, keeping the molar ratio constant. 2 . Increasing the excess CoC12 added, keeping the concentration constant. The green precipitate remains gelatinous and there is no evidence of settling. The deep-blue to light-blue change depends on the molar ratio of the reactants. The light-blue is a granular precipitate and settles to about half t'he volume of the original precipitate, occupying about the same volume that an equal weight of rose precipitate would occupy. The following series of experiments was run in order to determine more accurately the conditions cont,rolling these changes and to explain the part played by the cobaltous chloride in escess. To fixed amounts ( I O cc.) of 0.8311 c'oC12 varying amounts of I . j j l I KOH were added, the resulting precipitate shaken well and small amounts of the mother liquor filtered off and tested for cobalt. The remaining precipitate TTas allowed to stand in the test tube for several days and the supernatant liquid was again tested for cobalt. The results are given in Table 11.
TABLE I1 So.
Val. IiOH added
Molar ratio
CoCl?:I
