PREPARATION of CRYSTALS of SPARINGLY SOLUBLE SALTS W. CONARD FERNELIUS AND KENNETH D. DETLING The Ohio State University, Columbus, Ohio
Fair-sized crystals of many slightly soluble salts may be prepared in pure condition by the s h mixinx of solutions of soluble salts supplying the requisite ions to generate the slightly soluble salt. This slow mixing is accomplished through the agency of diffusion.
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OR a number of years Professor W. E. Henderson has included in a course of Inorganic Preparations in This Department an experiment of unusual interest; i . e., a method for obtaining sparingly soluble salts in well-crystallized condition.* As ordinarily prepared by mixing aqueous solutions containing the requisite ions these substances are in the form of fine precipitates. Crystals of these same substances, however, will grow to much larger size provided the two solutionsbe brought together slowly enough. This slow mixing may be readily accomplished by utilizing the natural process of diffusion. The procedure as applied to barium thiosulfate, BaSzOa.HzO,is described below. The necessary apparatus includes a rather deep crystallizing dish of about 25 cm. diameter (or a large beaker-two-liter capacity or larger--or a battery jar) * Cf. JOORNSTON, J., I.Am. Chem. Soc., 36, 16-9 (1914).
and two small crystallizing dishes (or beakers withont lips) of about 75 cc. capacity. Fill one of the small dishes about three-fourths full with a saturated solution of sodium thiosulfate, adding some crystals of the solid salt. Similarly, add to the second dish a saturated solution and crystals of barium chloride or nitrate. Place the two dishes 5 to 10 cm. apart in the large dish and carefully fill them with water withont stirring. By means of a piece of rubber tubing connected to a supply of distilled water, let water flow into the large vessel to a height of a t least 4 cm. above the tops of the small dishes (see figure). Great care must be taken in this operation to prevent the premature mixing of the two reagents. Then pour a layer of melted paraffin over the surface of the water to prevent evaporation of the solvent and to protect the contents of the vessel from all disturbances that might produce convection currents. Now set the vessel away in a place where it can remain undisturbed for several (2-6) weeks at as nearly constant temperature as possible. The crystals f o m very slowly as diiusion brings the reagents into contact. When a sufficient crop has been secured, carefully remove the paraffin, siphon off the solution as well as possible, and lift out the small
dishes with their contents. Collect the crystals and dry them on paper. Almost invariably sizable,crystals of barium thiosnlfate are obtained by this method. They f o m in rods or plates, the latter sometimes being as large as 16 X 8 X 0.25 mm. In general, the place where the crystals f o m will depend upon the relative rates of diiusion of the two ions concerned in the precipitation. Oftentimes, however, for lack of the necessary nuclei to start crystallization, crystals will make their first appearance a t most unusual places such as the edges of the small dishes or wherever there may be a scratch on the glassware. When the rates of diffusion of the two reacting ions are very different the top of one of the small dishes is apt to become entirely crusted over with precipitated material. Consequently, it is desirable to have these dishes as large in diameter as possible. In order to test the adaptability of this method, numerous assignments were made to students regularly enrolled in the inorganic preparations course. The results of these experiments are recorded below. Calcium hydroxide (generated from CaC4 and NaOH) was obtained as well-formed semi-transparent hexagonal columns &15 mm. long and 1.5-2 mm. in diameter. On another occasion the crystals were larger and clearer. Variations in temperature interfere greatly in the growth of good crystals of calcium hydroxide. The interdiiusiou of lead nitrate and sodium chloride gave a good crop of uniform, needle-shaped crystals, the larger ones measuring 10-18 X 2 X 1 mm. There were many crystalline aggregates of larger size and also a marked development of good crystalline faces. Lead bromide was obtained similarly in the form of small white needles, the largest of which were 10 mm. long and slightly more than a millimeter in thickness. 'The more difficultly soluble lead iodide formed very thin golden plates as large as 5 mm. on a side. These plates were frequently matted together. The sulfates of the alkaline earth metals and of lead can be prepared in satisfactory crystalline form by the method of diffusion. Calcium sulfate precipitated in the f o m of very fine needles 3 4 mm. in length. A very few small separate crystals of strontium sulfate [from Sr(NO& and Na?SOn]were formed, although there resulted many small crystal aggregates, 2-3 mm. on a side. The interdiffusion of barium hydroxide and sodium sulfate resulted in the formation of several clear square plates 1.5 mm. thick and 5 mm. on a side as well as many about one-quarter this size.
In one experiment lead sulfate [from and Pb(NOa)e] was obtained largely in the f o m of needles, many being 3-5 mm. long; while in another experiment [NaeSOr and Pb(CeHaO&] plates 3 mm. on a side predominated. Barium chromate formed a few needle crystals (3-5 mm. long) but the majority of the product was a crystalline powder. The strontium and lead chromate products were peculiar fluffy masses containing many soft needle-like growths 3-5 mm. long. No crystals of calcium chromate formed, probably because this substance is too soluble in water. The interdiffusion of ammonium oxalate and of barium chloride produced a large quantity of uniform needle crystals, many of which were 8-9 mm. in length. Very small crystals of strontium oxalate were obtained, while only a h e precipitate of calcium oxalate resulted. A splendid crop of yellow-green rod-shaped crystals (many 6 mm. long) of silver nitrite resulted from silver nitrate and sodium nitrite. The precipitate of thallous iodide was definitely crystalline although the individual crystals were very small. A small portion of a mercuric sulfide preparation was in the form of small hexagonal tablets, about a cubic mm. in size. These could not be duplicated. Attempts to prepare mercurous chloride, silver iodide, calcium silicate, calcium carbonate, cupric and zinc sulfides, cupric hydroxide, and ferric ferrocyanide in crystalline form were unsuccessful. The diffusion crystallization method as described above is limited to those sparingly soluble salts which are not extensively hydrolyzed. If now the medium through which the diffusion takes place be treated in such a way as to overcome hydrolysis, the usefulness of this method of crystallization is greatly extended. Thus, in the preparation of crystalline magnesium ammonium phosphate and arsenate the soluble salts are permitted to diffuse through ammonium citrate or ammoniacal ammonium chloride solutions. The first of these compounds was obtained, on one occasion, in the f o m of crystal blades of such dimensions as 11 X 3 X 2 mm. and 17 X 3 X 1 mm. with larger crystalline aggregates and, on another occasion, in the form of clear columns and plates (typical dimensions: 18 X 2 X 2 mm., 17 X 5 X 3 mm). The similar arsenic compound grew in the form of thin, narrow blades reaching a maximum length of 20 mm. In a dilute nitric acid diffusion medium, large, dark-green metallic plates of silver dichromate resulted from the interdiffusiou of silver nitrate and potassium dichromate. In one interesting case these plates formed a crude helix growing down into the dichromate dish. A situation somewhat similar to hydrolysis prevention is encountered in the case of manganous sulfite (from NazSOs and MnCL). Here it is necessary to add sulfur dioxide to the diffusion medium (adding a little acetic acid to the sodium sulfite solution is sufficient) in order to prevent oxidation. Wben one attempts to correlate the success of the
diffusion method of crystallization with the solubility of the corres~ondingsalts. one finds that the method is not successful^ when-the salt under consideration is extremely insoluble. The compilation of solubility product TABLE 1 SOLUBILITY PRODUCT Cr~slol Srrl~m Rho=. Rhom. Rhom. Rho=. Hex. Mono. Rho=. Hex. Rho",. Rho=. Mono.
D*l*
Solubility Pmdurf
Trie. Tet. 1ao.
Rhom. Mono. Rhom.
*
Subtan- are arranged approximately in the order of the size of crystal obtained, the largert being first.
data for the various substances mentioned above (Table 1) will make clear the implication of this statement. Finally, i t should be mentioned that substances pre-
pared by diffusion methods are likely to be much more pure than the same materials prepared by ordinary precipitation methods.* The larger, well-developed crystals prepared by the first method possess a much smaller relative surface than fine precipitates and consequently will absorb a much smaller amount of impurities from the solution in which the crystals are formed.
* Cf. JOHNSTON, J . . JOG.cil. LITERATURE CITED
AmENRIETH AND WINDAUS,Z. anal. Chem., 37,295 (1898). BWE, ibid., 49, 557 (1910). SEUBERTAND ELTEN,ibid., 4,81 (1893). BOTTCER, Z. pkysik. Chem., 46, 521 (1903). LICHTY, 1.Am. Chem. Soc., 25, 469 (1903). KOHLRAUSCH, Z. physik. Chem., 64, 154 (1908). MELCHER, J. Am. Chem. Soc., 32, 54 (1910). SHIPLEYAND MCHARPIE.J. SOL. Chem. Ind.. 42, 3214T (1972)~ - - -- ,. (9) ABECGAND PICK,Bn.,38,2571 (1905). Z. anorg. allgem. Chem., 130, (10) HERZAND HELLEBRANDT, 188 (1923). GADENNE,AND NIEMANN,Ber., 60,1514 (1927). (11) SCHOLDER, Landolt-Bornstein, 11, p. 1184. (12) PLEISZNER, Z. physik. Chem., 64, 163 (1908). (13) KOHLRAU~CH, J. Am. Chem. Soc., 29, 1674 (1907). (14) SEERRILL, (15) KOHLRAUSCR, 2. pkysik. Chem., 64, 129 (1908). 16) BOTTGER. i W . . 46, 568 (1903). iW., 64, 158 (1908). 17) KOKLRAUSCH, 18) FRESENNS,Z. anal. Chem., 30, 672 (1891). 19) KOHLRAUSCH, Z. physik. Chem.. 64, 152 (1908). COPPREY. AND BISBEE.Z. anorg. Chem., 28, 85 20) RICHARDS, (1901). 21) BRuNER AND ZAWADZKI. ibid.. 67,455 (1910). Z. Eleklrochem., 10, 303 (1904). 22) LEYAND HEIMEUCKER, 23) HILL,J. Am. Chem. Soc., 30, 14 (1908). ibid.. 38, 982 (1916). 24) JOHNSTON AND WILLIAMSON,
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