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, lie between those of lithium ion and cesium of the diffusion coefficient of silver ion in the lithium ion for the lithium nitrate-cesium nitrate mixture and nitrate-cesium nitrate mixture agrees with the diffusion lie between those of lithium ion and potassium ion for coefficient of silver ion in pure sodium nitrate within the lithium nitrate-potassium nitrate mixture, it is 4 % of the observed diffusion coefficient of silver ion. acceptable that the circumstances around the silver Such a behavior seems to be rather predominant in ion diffused in the lithium nitrate-cesium nitrate and the lithium nitrate-alkali nitrate mixtures such as the lithium nitrate-potassium nitrate mixtures approach lithium nitrate-cesium nitrate and lithium nitrate- that in sodium nitrate. This idea is also supported by potassium nitrate, because the maximum value of the the fact that the diffusion coefficient of silver ion in diffusion coefficient of silver ion in lithium nitrate- the potassium nitrate-cesium nitrate mixture varies potassium nitrate at 360℃ is also almost identical monotonously with the concentration of cesium ni-

Table 3 Diffusion coefficients(at 360℃)and activation energies for silver ion in various alkali nitrate melts(7)～(9)(14)

p: Polarographic diffusion coefficient ( ): Estimated value 416 Kazutaka Kawamura trate, since the diffusion coefficient of silver ion in of activation energies for diffusion process of silver pure molten rubidium nitrate is not larger than that ion in the lithium nitrate-cesium nitrate, potassium in pure molten potassium nitrate (14).Moreover, it can nitrate-cesium nitrate and sodium nitrate-potassium be explained by this idea that the diffusion coefficient nitrate mixtures. of silver ion in sodium nitrate-potassium nitrate varies monotonously with the concentration of potassium Ⅴ. Conclusion nitrate(8)(9)(11) It is interesting to note that the concentration, The diffusion coefficients of silver ion in the molten at which the diffusion coefficient of silver ion shows lithium nitrate-cesium nitrate and potassium nitrate- its maximum value for the lithium nitrate-cesium cesium nitrate mixtures have been determined by nitrate mixture and the lithium nitrate-potassium chronopotentiometry in the temperature range from nitrate mixture, is about 20 mol % of cesium nitrate 260 to 380℃. and about 50 mol % of potassium nitrate, respectively. The obtained diffusion coefficient of silver ion in In view of the order off lithium, sodium, potassium, the lithium nitrate-cesium nitrate mixture increases rubidium, and cesium in the periodic table, it seems with the increase of concentration of cesium nitrate reasonable that the maximum value of the diffusion from 0 to 20 mol % and decreases with the increase of coefficient appears at a low concentration of cesium concentration of cesium nitrate from 20 to 100 mol %, nitrate (about 20 mol % of cesium nitrate) in the indicating a maximum value at 20 mol % of cesium lithium nitrate-cesium nitrate mixture and at inter- nitrate. The diffusion coefficient of silver ion in the mediate concentration of potassium nitrate (about 50 potassium nitrate-cesium nitrate mixture decreases mol % of potassium nitrate) in the lithium nitrate- monotonously with the increase of the concentration potassium nitrate mixture, respectively. of cesium nitrate. As for the activation energy for the diffusion of By comparing the obtained and published diffusion silver ion, the consideration on the concentration coefficients of silver ion in various alkali nitrate dependence of the diffusion coefficient of silver ion mixtures with those in pure alkali nitrates, it is found may be applicable. Both the activation energy for the that the circumstances around the silver ion diffused diffusion of silver ion at 80 mol % of cesium nitrate in at the concentration of 20 mol % of cesium nitrate in the lithium nitrate-cesium nitrate mixture and the the lithium nitrate-cesium nitrate mixture and at the activation energies over the almost entire range of concentration of 50 mol % of potassium nitrate in concentration of cesium nitrate in the potassium the lithium nitrate-potassium nitrate mixture approach nitrate-cesium nitrate mixture are determined from that in pure molten sodium nitrate. Such a finding may the diffusion coefficients of silver ion at 340, 360 and explain the concentration dependence of activation 380℃ for lithium nitrate-cesium nitrate and 315, energy for the diffusion of silver ion in the lithium 340 and 360℃ for potassium nitrate-cesium nitrate. nitrate-cesium nitrate, potassium nitrate-cesium nitrate Therefore, there is some uncertainty in the determina- and sodium nitrate-potassium nitrate mixtures. tion of the activation energies. However, it can be Acknowledgment seen that the minimum activation energy for the The author would like to thank National Research diffusion of silver ion in the lithium nitrate-cesium nitrate mixture is almost consistent with the activa- Institute for Metals for the financial support during the course of this work. tion energy for the diffusion of silver ion in pure molten sodium nitrate within 24 % of the obtained REFERENCES value of activation energy, and that the activation (1) K. Kawamura: USAEC Report TID-20866, (1963), energy for the diffusion of silver ion in the potassium p. 13. (2) C. E. Thalmayer,S. Bruckensteinand D. M. Gruen: J. nitrate-cesium nitrate mixture changes monotonously Inorg. Nucl. Chem., 26 (1964),347. with the concentration of cesium nitrate. It has (3) G-A. Mazzoccinand G. Schiavon:J. Electroanal.Chem., previously been observed(7) that the minimum value 38 (1972),229. for the diffusion of silver ion in the lithium nitrate- (4) S. Sternbergand C. Herdlicka:Rev. Roum. Chim., 14 potassium nitrate mixture is consistent with the value (1969).991. of the activation energy for the diffusion of silver ion (5) S. Sternbergand C. Herdlicka:Rev. Roum. Chim., 15 (1970),343. in pure sodium nitrate within 6 % of the obtained value (6) J. E. L. Bowcottand B. A. Plunkett:Electrochim. Acta, of activation energy. Nevertheless, the experimental 14 (1969),363. fact that the concentration dependence of activation (7) K. Kawamura:Denki Kagaku,38 (1970),12. energy is not identical with that of the diffusion coe- (8) K. Kawamura: KinzaigikenWabunhokoku, 9 (1966), fficient remains unexplained. On the other hand, it is 181. (9) K. Kawamura:Yooyuen, 8 (1965),641. favourable for our idea that the published value of (10) K. Kawamura:Denki Kagaku, 38 (1970),258. activation energy in the sodium nitrate-potassium (11) K. Kawamura: Electrochim.Acta, 12 (1967),1233. nitrate mixture(8)(9)(11)increases gradually with the (12) R. W. Laity and M. P. Miller: J. Phys. Chem.,68 (1965), concentration of potassium nitrate. With the excep- 2145. (13) R. W. Laity and J. D. E. Mclntyre: J. Amer. Chem. tion of the concentration dependence of activation Soc., 87 (1965),3866. energy in the lithium nitrate-potassium nitrate mixture, (14) K. Kawamura: Preprint of 4th Fused Salt Chemistry our idea may explain the concentration dependence Conference,Tokyo, (1970),p. 81.