NOTES
July, 1957 extrapolated at 21OOOK. gives a value of p-10 atm.: and the equation of Hsiao and Schlecten, from data collected between 704 and 1006”
-
13,981 l o g p = ___
+ 8.979 ( p mm.)
extrapolated at T = 2100OK. gives p = 209 mm. The fact that the melting point of ZnS as determined by us is, within the limits of experimental error, in good agreement with the value reported by Tiede and Schleede a t 100-150 atm. would indicate that the m.p. is only slightly dependent on pressure. This conclusion, however, must be accepted with caution. Acknowledgments.-We wish to thank Dr. J. S. Saby, Lamp Research Laboratory, and Mr. D. E. Elmendorf, Lamp Engineering Research Laboratory, for much support given to the research and for criticism in the preparation of this paper.
THE COMPLEXING OF CERIUM(I~I), LANTHANUM(II1) AND GADOLINIUM(II1) BY FLUORIDE IONS IN AQUEOUS SOLUTION1
1021
ide to a phenolphthalein end-point. A stock solution of lanthanum perchlorate in perchloric acid was prepared by dissolving a known weight of lanthanum oxide in standard perchloric acid. Spectrographic analysis showed that the lanthanum oxide contained no other rare earths and less than 0.01% of Al, Ca, Fe or Si. The gadolinium perchlorateerchloric acid stock Bolution was prepared by dissolving a Enown weight of gadolinium oxide (Research Chemicals, Inc.-O.€i% Sm and 0.1% Dy) in standard perchloric acid.
TABLE I EXPERIMENTAL CONDITIONS A N D CALCULATED EQUILIBRIUM QUOTIENTS p = 0.50, T = 25.00°, Fe(C104)a 5 X M , Fe(C104)a 5 x 10-4 M
-
-
Metal
Cerium
Run
“2101,
,+I
M(Cl0i)s
&I
X 10, M
During the course of experiments in this Laboratory on the solubilities of rare earth fluorides, it became necessary to measure the extent of complexing of cerium (111), lanthanum(II1) and gadolinium(111) by fluoride ions. The experimental technique employed is based upon the competition between the rare earth ion and iron(II1) for fluoride ions in a ferrous-ferric concentration cell (“ferri” method). 3-4 Experimental The two concentration cells, the potentiometer circuit, the experimental procedure and the preparation and analyses of the stock solutions of sodium fluoride, sodium perchlorate, perchloric acid and ferrous perchlorate-ferric perchlorateperchloric acid have been described previously.b*6 A stock solution of cerous perchlorate in perchloric acid was prepared in the following manner. Analytical reagent “ammonium hexanitrato cerate” was dissolved in distilled water. This solution was added with vigorous stirring to 2 M ammonium hydroxide. The light yellow precipitate was filtered and washed with distilled water. This hydrous ceric hydroxide was dissolved in perchloric acid and reduced with analytical grade 35% hydrogen peroxide. The resulting solution was boiled, and after several days was filtered through sintered glass. The cerium(II1) concentration was determined in a manner similar to that described by Smith.? After adding an excess of sodium fluoride to precipitate all the cerium, the perchloric acid concentration was determined by titrating with carbonate-free sodium hydrox-
0 1.5 1.7 1.4 1.7 Av. 1 . 6
I I1 I11 IV
0.0566 .0987 .0507 .0561
4.21 4.34 1.79 8.29
2.54 3.62 2.72 1.87
I
.0545 .0996 ,0562 .0527
8.88 8.88 1.28 4.65
1.59 2.22 1.52 2.00
Lanthanum
I1 I11 IV
BY J. W. KURY,Z Z. HUGUS,JR.,AND W. M. LA TIMER^ Contribution f r o m the University of California Radiation Laboratory, and the Department of Chemistry and Chemical Engineering, University of California, Berkeley, California Received April 8, 1967
NaF3
X loa,
Av. Gadolinium
I
.0498
2.77
6.71
0.62 .60 .55 .57
.59 2.9
The initial concentrations and the highest total fluoride concentration reached in the rare earth half-cell for each run are presented in Table I. In each run anywhere from 5 to 11 e.m.f. determinations were made at different fluoride concentrations. A t the highest fluoride concentration reached in every run, the potential of the rare-earth-rontaining cell began to drift downward due to a slow precipitation of the rare earth fluoride. The noticeable onset of this downward drift approximately coincided with the appearance of a Tyndall effect in the rare earth half-cell.
Discussion The difference in potential (in the region where no Tyndall effect is observed) between the two cells with and without rare earth perchlorate at equal total fluoride concentrations indicates the presence of one or more complex fluorides. It is possible to interpret satisfactorily the experimental results by assuming the presence of only the simplest complexed species, MF+2 (M = cerium, lanthanum or gadolinium). The method used to calculate the equilibrium quotient, &, for the reaction M+S
+ H F = MF+* + H +
from the variation of cell potentials with total fluoride concentrations has been described previously.6pgThe resulting equilibrium quotients are presented in Table I. Acknowledgment.-The authors wish to express their thanks to Professor Loren G. Hepler for help in carrying out some of the experiments.
(1) This work was performed under the auspices of the U. 9. Atomio Energy Commission. (2) Deceased. T H E HEAT OF COMBUSTION OF HOLMIUM’ (3) C. Brosset and J. Orring, Suensk. R e m . T i d . , 66, 101 (1943). (4) H . W . Dodgen and G. I
