Barium sulfate and barium chromate crystallize in the orthorhombic system, the lattice constants of the former being a = 8.85 X cm., b = 5.43 X and c = 7.14 X 10-8, and of the latter being a = 9.14 X cm., b = 5.54 X G = 7.35 X From the close agreement in lattice constants one would expect that barium sulfate and barium chromate can form a complete series of mixed crystals. In the present study the distribution coefficient of the chromate ion between an aqueous solution and barium sulfate has been determined under conditions of homogeneous distribution of the chromate through the solid phase. Upon shaking barium sulfate with a solution containing sulfate and chromate ions the following exchange occursThe chromate ions replace part of the sulfate ions in the barium sulfate, and the barium chromate formed is incorporated in the solid phase in the form of mixed crystals. The mixed crystals thus formed are heterogeneous, and become homogeneous, and in thermodynamic equilibrium with the liquid phase, only upon repeated recrystallizations of the solid phase. In previous studies3 it has been shown that fresh lead sulfate precipitated from 0.1 M lead nitrate and potassium sulfate is highly imperfect and perfects itself by repeated recrystallizations when left in contact with the mother liquor. It was expected that, qualitatively, barium sulfate would behave in an identical way and, therefore, that shaking of freshly precipitated barium sulfate with solutions containing sulfate and chromate ions for sufficient periods of time would yield homogeneous mixed crystals of barium sulfate and barium chromate in equilibrium with the liquid phase. Scholtz and Abegg4 in their study of the system (1) (1) Part of a thesis submitted by G. E. Noponen to the Graduate School of the University of Minnesota in partial fulfilment of the requirements of the degree of Doctor barium sulfatebarium chromate did not realize the importance of obtaining equilibrium conditions. This explains why no constant value of the distribution coefficient is obtained from their results.When the mixed crystals are in equilibrium with the liquid phase, the following relation holds in which "a" denotes activity, the subscript S the component in the solid phase, and L the component in the liquid phase. The ratio of the activity coefficients of the sulfate and chromate ions in the solution will remain constant over a wide range of ionic strengths. If this ratio in the solid phase is also constant over a wide range of varying compositions of the mixed crystals, expression (1) can be written in the following way in which the symbols between parentheses represent mole fractions or the number of moles. If the entire system were ideal the distribution coefficient K should be equal to the ratio of the solubility products of barium sulfate and barium chromate.6 Experimental Experiments at Room Temperature (26 * lo).-To 50 ml. of 0.10 M sodium sulfate in a volumetric flask of 250 ml. was added 25 ml. of 0.10 M barium nitrate and 25 ml. of 0.1000 M sodium chromate. The suspe...
higher than integral, the accuracy claimed being 0.01 per cent. In converting from the physical to the chemical scale, if the higher factor of Mecke and Childs, 1.00022, is used, the atomic weight of radium lead falls between 205.96 and 206.00. However, if the latter is always accompanied by actinium lead, Pb207, in the proportions found in Katanga lead by Aston4 (93.3:6.7) the average atomic weight of uranium lead should be 0.07 unit higher, a difference far outside the apparent accuracy of our experiments. This suggests the possible presence in uranium lead of lead isotopes of lower atomic mass than 206. An alternative is that the atomic weight of lead as found by us is too low, or that the packing fraction of lead determined with the mass spectrograph is incorrect.The experimental result of this research is that the atomic weight of uranium lead from a specimen of cyrtolite is 205.92 ± 0.02.(4) Aston, Nature, 129, 649 (1932).
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