鉛8-ヒドロキシキノレート沈殿の性質

Many kinds of metal ions precipitate quantitatively with 8-hydroxyquinoline and are analyzed easily by the gravimetric method. Lead 8-hydroxyquinolate has not been recommended, however, because of its indefinite composition. The authors obtained the precipitate of lead 8-hydroxyquinolate quantitatively from homogeneous solution using 8-acetoxyquinoline, and found also that lead 8-hydroxyquinolate had the dimorphism. In this paper the formation of precipitate and the crystal chemistry of lead 8-hydroxyquinolate are described.<BR>2 to 50 mg of lead contained in 100 ml were precipitated by mixing with 8-hydroxyquinoline solution at pH 8.512.0 followed by aging for 4 hours by the conventional method. 5 to 50 mg of lead in 100 ml were precipitated as 8-hydroxyquinolate from homogeneous solution by the slow hydrolysis of 8-acetoxyquinoline at pH 7.512.0 followed by standing over night.<BR>The solubility of lead 8-hydroxyquinolate in water and in the buffer solutions was measured by the atomic absorption method and the solubility product of lead 8-hydroxyquinolate was calculated as K_sp=10^-23.5. The concentrations of various species of lead 8-hydroxyquinolate at various pH's were calculated as shown in Fig. 1 and the solubilities measured were almost agreed with the calculated values as shown also in Fig. 1. The solubility increased remarkably with increasing the temperature as shown in Table I. The solubility loss was decreased by aging of the precipitates as shown in Fig. 3. The solubility loss was increased by addition of large excess of 8-hydroxyquinoline as shown in Table II. The effect of pH on the precipitation of lead 8hydroxyquinolate was shown in Table III. The precipitate of lead 8-hydroxyquinolate had 2 moles of water of crystallization, having the composition of Pb(C₉H₆NO)₂· 2H₂O, and the water was removed by drying at 105110°C. From these results the quantitative precipitation of lead 8-hydroxyquinolate was performed as described before and it could be weighed after drying at 105110°C as Pb(C₉H₆NO)₂. The precipitate of lead 8-hydroxyquinolate can also be dried by means of the high frequency waves in an electronic range. The water of crystallization of lead 8-hydroxyquinolate can be removed by treating the precipitate in an electronic range more conveniently than the usual electric oven as shown in Table V.<BR>It was found that the crystalline nucleous of lead 8-hydroxyquinolate was composed of 2 molecules by the measurements of the relation of the induction period with the initial concentration.<BR>Lead 8-hydroxyquinolate has the dimorphism such as crystallized as needle and hexagonal lamellar particles as shown in Fig. 5. During the precipitation process the hexagonal particles appeared at the initial stage. The needle crystals were more stable than the hexagonal crystals and all of the particles changed completely to the needle crystals during the aging of the precipitate in the mother liquid. The transformation of particles seems to be promoted by dissolution of particles as shown in Fig. 6. The hexagonal particles have fine structure on their surface as shown in Fig. 5c. The small particles seem to be nuclei which grow to the needle crystals. The crystal structure of the hexagonal particles is considered as the isomorphous with that of copper 8-hydroxyquinolate, but such particles could not be prepared without containing needle crystals and, therefore, the crystal structure of the hexagonal particles of lead 8-hydroxyquinolate could not be analyzed. The needle crystals were analyzed by the X-ray diffraction method and the unit cell of these crystals was tetragonal with a₀=b₀=12.1 and c₀ = 13.7 Å.