Diffusion Coefficients of Silver Ion in Lino3-Csno3 and KNO3-Csno3 Mixtures*

Diffusion Coefficients of Silver Ion in LiNO3-CsNO3 and KNO3-CsNO3 Mixtures* By Kazutaka Kawamura** The diffusion coefficients of silver ion in molten lithium nitrate-cesium nitrate and potassium nitrate-cesium nitrate mixtures have been determined by chronopotentiometry in the temperature range from 260 to 380℃. The diffusion coefficient of silver ion in lithium nitrate-cesium nitrate mixture increases with the increase of concentration of cesium nitrate from 0 to 20 mol % and decreases with the increase of concentration of cesium nitrate from 20 to 100 mol %, indicating a maximum value at 20 mol % of cesium nitrate. The diffusion coefficient of silver ion inthe potassium nitrate-cesium nitrate mixture decreases monotonously with the increase of the concentration of cesium nitrate. The activation energy for diffusion of silver ion has a minimum value at about 20 mol % of cesium nitrate in the lithium nitrate-cesium nitrate mixture and increases monotonously with the increase of concentration of cesium nitrate in the potassium nitrate-cesium nitrate mixture. By comparing the determined diffusion coefficients of silver ion in alkali nitrate mixtures with those in pure alkali nitrates, it is found that the circumstances around the silver ion diffused at the concentration of about 20 mol % ofcesium nitrate in the lithium nitrate-cesium nitrate mixture approach that in pure sodium nitrate. Such a finding may explain the above concentration dependence of the activation energy for diffusion of silver ion in the lithium nitrate-cesiumnitrate and potassium nitrate-cesium nitrate mixtures. (ReceivedAugust 15, 1973) materials by filtering the melt through the fritted silica Ⅰ.Introduction disc under dry nitrogen atmosphere followed by bubbl- ing dry nitrogen gas for about 2 hr through the filtered Although much efforts have been expended in the melt in order to remove small amounts of moisture measurements of the chronopotentiometric diffusion in the melt. The details of this method have been coefficients† of silver ion in pure molten alkali ni- reported previously(10). A small amount of silver trates(1)～(5), only a few attempts have been made to nitrate from reagent grade material was then added. examine how the chronopotentiometric diffusion coef- After the diffusion experiment, the solidified salt ficient of silver ion in binary molten alkali nitrate mixtures were dissolved in 22 of distilled water. Fifty ml mixtures(6) varies with the concentration of alkali of the solution was transferred to the electrolytic cell nitrate. in order to determine the concentration of silver ion It is thus the purpose of this work to determine the electrolytically. The remaining solution was also chronopotentiometric diffusion coefficients of silver electrolyzed to remove the silver ion and evaporated to ion in molten lithium nitrate-cesium nitrate and potas- dryness. The obtained dry salt mixture was reused sium nitrate-cesium nitrate mixtures. Moreover, the for the measurement of chronopotentiometric dif- obtained values of the chronopotentiometric diffusion fusion coefficients. coefficients, together with our previously obtained values of chronopotentiometric diffusion coefficients of Ⅲ. Results silver ion in molten lithium nitrate-potassium nitrate (7) and sodium nitrate-potassium nitrate(8) mixtures, are The chronopotentiometric diffusion coefficients of explained by the values of chronopotentiometric dif- silver ion, together with the activation energies of the fusion coefficients of silver ion in pure molten alkali diffusion(calculated mainly fbr the temperature range nitrates. from 260 to 380℃ for lithium nitrate-cesium nitrate and from 315 to 380℃ for potassium nitrate-cesium Ⅱ. Experimental nitrate), over the almost entire range from pure lithium nitrate to pure cesium nitrate and from pure The apparatus for chronopotentiometry described in previous papers(1)(7)～(9)was used. The lithium potassium nitrate to pure cesium nitrate are shown in Tables 1 and 2, respectively. The probable errors are nitrate-cesium nitrate and potassium nitrate-cesium also listed in these tables. nitrate mixtures were purified from the reagent grade The chronopotentiometric diffusion coefficients of * Presented on 6th Fused Salt Chemistry Conference silver ion in lithium nitrate-cesium nitrate and potas- , Nov. 1972, at Okayama. sium nitrate-cesium nitrate mixtures are summarized ** Research Laboratory of Nuclear Reactor , Tokyo Institute in Figs. 1 and 2, respectively, together with the pub- of Technology, 2-12-1, Ookayama, Meguro-ku, Tokyo lished data on diffusion coefficients of silver ion in 152, Japan. pure lithium nitrate(7)and potassium nitrate(7)～(9)(11). † It is noted that the chronopotentiometric diffusion It can be seen from Fig. 1 that the chronopotentio- coefficient of silver ion can only be measured for the dilute solution of silver ion in the pure molten alkali metric diffusion coefficient of silver ion in the lithium nitrate or alkali nitrate mixture. nitrate-cesium nitrate mixture increases with the Trans. JIM 1974 Vol. 15 414 Kazutaka Kawamura Table 1 Diffusion coefficients and activation energies for silver ion in LiNO3-CsNO3(D=×10-5cm2/sec, ED=kcal/ mol). Table 2 Diffusion coefficients and activation energies for silver ion in KNO3-CsNO3(D=×10-5cm2/sec, ED= kcal/mol) Fig. 2 Diffusion coefficients and activation energies for silver ion in KNO3-CsNO3. Fig. 1 Diffusion coefficients and activation energies for silver ion in LiNO3-CsNO3. increaseincrease of concentration of cesium nitrate from 0 to 20 mol % and decreases with the increase of concentration of cesium nitrate from 20 to 100 mol %, showing a maximum value at 20 mol % of cesium nitrate. On the other hand, it is seen from Fig. 2 that the chronopotentiometric diffusion coefficient of silver ion in potassium nitrate-cesium nitrate mixture decreases monotonously with the increase of the concentration of cesium nitrate. Since the variations of chronopotentiometric diffusion ceofficients of silver ion in the lithium nitrate-cesium nitrate and potassium nitrate-cesium nitrate mixtures with temperature are given by the Arrhenius relation, the activation energies of diffusion process can be calculated from the slope of the Arrhenius plot. As an example, the typical variation of the chronopotentiometric diffusion coef- Fig. 3 Arrhenius plot of ln D versus 1/T for silver ion ficient of silver ion in lithium nitrate-cesium nitrate in LiNO3-CsNO3. Diffusion Coefficients of Silver Ion in LiNO3-CsNO3 and KNO3-CsNO3 Mixtures 415 with temperature (Arrhenius plot) is shown in Fig. 3. The activation energy of the diffusion process decreases with the increase of concentration of cesium nitrate from 0 to about 20 mol % and increases with the increase of concentration of cesium nitrate from about 20 to 100 mol % in the lithium nitrate-cesium nitrate mixture, indicating a minimum value at about 20 mol % of cesium nitrate. In the potassium nitrate- cesium nitrate mixture, the activation energy of diffusion process increases monotonously with the increase of concentration of cesium nitrate. Ⅳ. Discussion Since the chronopotentiometric diffusion coefficient of silver ion is identical with the self-diffusion coefficient of silver ion and the interdiffusion coefficient for the dilute solution of silver ion(12)(13), the term "diffusion coefficient of silver ion" is used hereafter Fig.4 Diffusion coefficients(at 360℃)and activation instead of the chronopotentiometric diffusion coeffi- energies for silver ion in LiNO3-KNO3 and NaNO3-KNO3(7). cient of silver ion. Now, let us determine the relation between the obtained diffusion coefficient of silver ion in the binary with the diffusion coefficient of silver ion in pure alkali nitrate mixture and that in pure alkali nitrate. sodium nitrate as shown in Fig. 4(7). The diffusion coefficients of silver ion in various pure A possible explanation of the maximum value of alkali nitrates at 360℃ are shown in Table 3, in which the diffusion coefficient of silver ion in the lithium the diffusion coefficient of silver ion in pure cesium nitrate-cesium nitrate and lithium nitrate-potassium nitrate at 360℃ is obtained from the extrapolation of nitrate mixtures is that the circumstances around the the diffusion coefficient of silver ion at 80 mol % of silver ion diffused at the concentration of about 20 cesium nitrate in the lithium nitrate-cesium nitrate and mol % cesium nitrate in the lithium nitrate-cesium potassium nitrate-cesium nitrate mixtures to that at nitrate mixture and at the concentration of about 100 mol % of cesium nitrate as shown in Figs. 1 and 2. 50 mol % of potassium nitrate in the lithium nitrate- It can be seen from Table 3 that the diffusion coeffi- potassium nitrate mixture approach that in pure cient of silver ion in pure sodium nitrate has the molten sodium nitrate. Qualitatively, such an explana- highest value among those in various pure alkali ni- tion may be supported by the fact that in the periodical trates and the activation energy of diffusion process table the sodium has the position between lithium and of silver ion has the lowest value in pure sodium ni- cesium or has the position between lithium and potas- trate. In other words, the silver ion is more mobile sium. In other words, since the values of the ionic in pure sodium nitrate than in other pure alkali ni- radius and the polarizability of sodium ion, which are trates. considered to play an important role in the dynamic It is evident from Fig. 1 that the maximum value process,

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