1914
L. G. LONGSWORTH
T'ol. 64
575' impractical, and extensions to higher temperaApparently the disparity in conductance behavtures were not possible. We infer from Fig. 1 that ior at the compositions 2PbC12.KC1 and PbC13. the two sets of curves would resolve themselves into 4KC1 reported by Bloom and Heymanns was due to single lines a t the upper limits of the temperature the relative differences between the melting points range investigated. of the compounds and the temperatures a t which This work was primarily concerned with those the measurements were made. It is likely that a temperatures between the melting point and the conductance minimum would also be observed at lowest temperatures used by Bloom and H e ~ m a n n . ~the composition 2PbCl2-KC1 a t temperatures In this range, it seems that the lead encounters nearer the melting point. However, the relatively some hindrance to diffusion while the chlorine is low diffusion coefficient of the lead, with its attendunaffected. The lead is increasingly hindered in ant low transference number, would probably its diffusion as the composition approaches that of moderate the decrease in conductance. Thus, the the compound 2PbCl2-KC1,but this hindrance be- minimum a t the composition 2PbC12.KC1 should comes less as the temperature is raised. not be so pronounced as that a t the composition In order to explain the inapplicability of the PbC12.4KCl. Stokes-Einstein equation to the pure, molten PbCl, More interesting here is the lack of any evidence system it was postulated that the lead was held that changes in composition influence the diffusion more immobile than the chlorine.1° This might be of the chlorine. Heretofore, explanations of the due to the greater mass, the influence of the greater various types of behavior encountered in this syscharge in electrostatic interactions, or to other tem have always assumed the presence of one or factors. It would seem that the same mechanism more complex ions involving both the chlorine and is operative here, since the diffusion coefficient of lead. It would appear that any complexes or other the lead remains smaller than that of the chlorine in PbC12-KC1 mixtures. On the other hand, for the aggregations which might be present a t these temthree compositions investigated, the activation peratures have little or no influence on the difenergy of diffusion is greater for the lead than for fusion of the chlorine but mainly involve the lead. the chlorine. This is contrary to the behavior of It may be that close to the melting point the bepure lead chloride. Since the activation energy of havior is influenced more by the presence of some Pb2l0varies with composition and is a maximum cation aggregation, while at higher temperatures, a t that potassium content corresponding to 2PbC12. these cation structures may vanish and the behavior KC1, it would appear that the potassium ion has an may become dependent upon the presence of the adverse influence upon the mobility of the lead in complex ions previously postulated. The trends the vicinity of the compound composition. It is to shown by these data, however, would indicate that, be expected that this influence is mutual and that the potassium ion also encounters a hindrance to its at the higher temperatures, the behavior of the diffusion. Such an effect has been indicated in the lead and the chlorine becomes independent of comresults of Duke and Fleming3 and Tubandt and position and this would rule out the presence of complex ions involving the lead and chlorine. Reinh~ld.~
THE MUTUAL DIFFUSION OF LIGHT AND HEAVY WATER BY L. G. LONGSWORTH Rockefeller Institute, New Y o r k , N . Y . Received June 9.8, I960
With the aid of a new diffusion cell and Rayleigh interferometry the mutual diffusion of light and heavy water has been measured at 5,25 and 45' over the entire range of composition. Paralleling rather closely the fluidity of HzO-DzO mixtures, and also the chloride ion mobility therein, the diffusion coefficient exhibits small negative departures from a linear decrease with increasing mole fraction of DzO. The effect of temperature on diffusion in this system is compared with that of large solutes in aqueous solution.
Using a diaphragm cell Adamson and Iranil observed a pronounced minimum in the diffusion of light and heavy water at a mole ratio near unity. With the aid of a porous frit Baur, Garland and Stockmayer2 were unable to confirm this result but their measurements did not indicate the slight dependence on the mole fraction that had been observed3 with an optical method in H20(1) A. W. Adamson and R. R. Irani, J. Am. Chrm. Soc., 79, 2967 (1967). (2) M. E. Baur, C. W. Garland and W. H. Stookmsyer,
