Coprecipitation of zinc and cadmium 8-hydroxyquinolates
1973 Volume 22 Issue 3 Pages 301-306
Details
1973 Volume 22 Issue 3 Pages 301-306
Zinc and cadmium 8-hydroxyquinolates have the same chemical composition, M(C9H6NO)2·2H2O, and precipitate quantitatively at pH values higher than 5 and 5.5, respectively. The solubility products of zinc and cadmium 8-hydroxyquinolates are 10-27.68 and 10-26.24, respectively. The precipitated particles of zinc 8-hydroxyquinolate are long hexagonal lamellar crystals of the monoclinic system. Cadmium 8-hydroxyquinolate in acid medium deposits as particles having the same morphological and crystallographic properites as zinc 8-hydroxyquinolate, but it precipitates from basic medium initially as needle crystals belonging to the tetragonal system which change to long hexagonal lamellar crystals isomorphous with zinc 8-hydroxyquinolate. The ionic radii of zinc and cadmium are 0.83 Å and 1.03 Å, respectively. The coprecipitation of zinc and cadmium 8-hydroxyquinolates, whose properies are almost the same as described above, are discussed in this paper.
Each 5 ml of 0.01 M zinc and cadmium solutions were mixed, and the pH of the solution was adjusted at 5.6 to 11.6. 8-acetoxyquinoline solution in acetone was added to the solution. The final volume of the solution was adjusted to 200 ml, and it was stirred at 25°C. The precipitate of 8-hydroxyquinolate produced was filtered and analyzed polarographycally after being dissolved in hydrochloric acid.
The results of coprecipitation are shown in Fig. 1. The coprecipitation curves were different at different pH's and the distribution coefficient reached the maximum value of unity at pH 8.1 as shown in Fig. 2.
Around pH 5.5 zinc 8-hydroxyquinolate precipitated quantitatively, but cadmium 8-hydroxyquinolate did not, and in acidic medium zinc 8-hydroxyquinolate precipitated prior to cadmium 8-hydroxyquinolate. Around pH 8.1 the solubilities of both zinc and cadmium compounds were practically the same and both compounds precipitated at the same rate. At higher pH cadmium 8-hydroxyquinolate had a tendency to deposit as needle crystals which dissolved and transformed to hexagonal lamellar particles. Thus cadmium 8-hydroxyquinolate seems to be more soluble than zinc compound at the initial stage of the precipitation at higher pH. Therefore, zinc 8-hydroxyquinolate precipitated prior to cadmium compound at higher pH region.
The particles prepared by the coprecipitation at various pH's were similarly long hexagonal lamellar crystals as shown in Fig. 4. At pH 9 the needle crystals of cadmium 8-hydroxyquinolate, which would deposit in a reacting solution containing only cadmium, did not appear in the case of coprecipitation. During the coprecipitation of 8-hydroxyquinolates at pH 9 zinc precipitated prior to cadmium and cadmium 8-hydroxyquinolate deposited as hexagonal lamellar particles with zinc compound. Cadmium 8-hydroxyquinolate was induced by the monoclinic zinc compound to form hexagonal particles and the produced particles seem to be solid solutions of zinc and cadmium 8-hydroxyquinolates.
X-Ray diffraction analysis showed that the particles produced by the coprecipitation at various pH's were the solid solutions.
