T H E COLOR OF SILVER CHROMATE ny
FLORENCE BUSH
Hunt, in 1845, published a paper’ concerning the effect of light on solutions, stating that metal chromates precipitated from bichromate solution exposed to light, differed many shades in color from that of a precipitate obtained in a solution made and kept in the dark. As an example he mentioned silver chromate, describing the precipitate from the actinized solution as having a “much more beautiful color”. Unfortunately the investigator did not state his procedure and conditions. An effort was made to repeat the results. A potassium bichromate solution was made up, one portion exposed t o sunlight for several weeks, another kept unexposed. Equal volumes of the solutions were treated with the same volume of silver nitrate solution. In each case the silver chromate precipitated was brick-red and curdy, of identical appearance. Here, at least, the exposure of pure bichromate solution to sunlight had no effect on a precipitate subsequently formed in the solution. I n an attempt t o find what Conditions would produce a difference in the appearance of the precipitate, the effect of the following factors was examined: I) Presence of chromium ions 2) ” ” free acid (small amounts) 3) Temperature increase 4) Rate of mixing solutions
Experimental cc. dilute chromium sulphate solution was added to I O cc. of both actinized and unexposed potassium bichromate solution and silver chromate was precipitated with silver nitrate solution. A brick-red, curdy deposit came down, similar to that first obtained. Varying the amount of chromium salt added, produced no change in the type of precipitate obtained. A portion of bichromate solution was acidified with ?U’HNO3 and 2) silver nitrate added. The silver chromate formed in lustrous, plate-like crystals, many shades darker than from neutral solution. 3) A solution containing the curdy precipitate was warmed. On cooling, the presence of the dark, lustrous crystals wm observed. 4) Bichromate solution was added to silver nitrate solution a t different rates. The slower the addition, the darker colored and more noticeably lustrous were the crystals formed. I)
I
Conclusions From the observations made, it may be stated that it is the rate of precipitation, and not the action of sunlight, which causes a difference in the appearance of the precipitate. Mern. Chem. Soc., 2, 316 (1845).
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FLORENCE BUSH
Silver chromate has a low solubility in water, and under ordinary condition of precipitation in fairly concentrated solutions, extensive supersaturation is rapidly attained. The precipitate comes down in very finely divided form. Under conditions xhere supersaturation is reached more slowly, precipitation is not so rapid, the crystals tend to grow relatively more slowly and larger. Silver chromate has a higher solubility in acid and in hot aqueous solutions than in aqueous solutions at room temperature. I n such solutions supersaturation is not so readily attained, the crystals are larger and fewer in number. When the curdy precipitate was heated, the silver chromate dissolved to some extent. Instead of forming the amorphous deposit again, the chromate precipitated in such a way as to increase the size of the crystals already formed. When the potassium bichromate solution is added very slowly to the silver nitrate the tendency is toward a low degree of supersaturation in respect to silver chromate. .A smaller number of crystal nuclei form initially and slow addition of the bichromate causes an increase in the size of the crystals already formed, rather than an increase in the number of crystals present. These results corrcspond, at least qualitatively, with the theory of the dispersion of a solid phase worked out by von Keimarn' originally. He stated that for crystalline precipitates of low solubility the degrce of dispersion of the solid phase depended on the solubility of the substance, the amount of solid n-hich must be deposited to reduce the concentration of the precipitating substance to its solubility value, and on the viscosity of the solution. He formulated his law thus:
s
d =-pn
s,
Where d = dispersion coefficient; S, = amount of deposited solid; S, = solubility of the solid; n = viscosity of liquid. The ratio Kp#lS,he terms the degree of supersaturation. I n general, small degree of supersaturation favor the formation of large crystals, and precipitation from solutions having high degrees of supersaturation favors thc formation of very finely crystalline or amorphous precipitates. As a possible explanation for the color differences noted by the original observer in the silver chromate precipitated from actinized and unexposed solutions of bichromate, it might be that he had varied his addition rates for the two solutions without appreciating the effect this would have on the physical condition of the precipitate formed. Any heating due to the light would also have an effect. Summary I) Precipitates of silver chromate from actiniwd potassium bichromate solutions have the same appearance as silver chromate precipitated from solutions unexposed to sunlight. The rate a t which the precipitating solution is added t o the bichromate 2) solution causes a difference in the color and physical characteristics of the silver chromate formed. Corneil Cnii'ersily Washburn: "Physical Chemistry", p. 367
