ion.
The total c e l l reaction i s
TABLE 4.20. ACTIVITY RATIO IN NaF-ZrF4 MIXTURES A T 650’C
The electromotive force of the c e l l may, therefore, be expressed as
It i s reasonable t o assume that the fluoride ion in such a system i s rather completely complexed by the ZrF, t o form complex ions o f the type ZrF,--, etc. Since current transported by ZrF,-, such large ions should be very small compared w i t h that carried by small simple ions, i t seems l i k e l y that the transport number o f N o t i s near unity. The vapor pressure o f ZrF4 over NaF-ZrF, melts has been measured w i t h considerable If the ratios o f vapor pressures for precision. ZrF, a t the concentrations shown above are assumed t o represent the a c t i v i t y ratios o f ZrF, and if t i s assumed t o be unity, the a c t i v i t y ratios for N a F i n the various combinations can be obtained directly from the emf equation. The results of such calculations are shown i n Table 4.20. From the calculations, i t appears that the rapid and uniform addition of ZrF, t o molten Na,ZrF, educes the activity of the NaF u n t i l the 50-50 mole % composition i s reached; further additions
Cell No. 2
64.0/58.2
2.4
2.1
Cell No. 1
58.2/50
3.4
3.0
Cell No. 4*
50/44.8
1.1
1 .o
*Cell No. 4: c 1 = 50.0 ZrF4; E6,,-,oC = 0.010 V.
male %
ZrF,;
1
c1
= 55.2 m o l e %
o f ZrF, do not alter the NaZrF, complex. C e l l s with Structural Metal Fluorides. Cells consisting of metallic nickel electrodes immersed i n solutions of NiF, in molten NaF-ZrF4 mixtures showed potentials o f 1 t o 3 mv when the NiF, concentrations in the half c e l l s were varied; the NiF, concentrations were, i n a l l cases, sufficiently high t o afford a saturated solution. Similar Fe/FeF, cells in which the half c e l l s contained equal concentrations of FeF, showed potentials of 1 t o 10 mv. When the half c e l l s contained differing FeF, concentrations but more FeF, than that required for saturation, potentials were obtained which varied from about 40 mv a t 550’C t o 10 mv a t 700OC. Exact voltages varied from c e l l t o cell, but the decrease in emf w i t h temperature seemed quite reproducible.