Communications to the Editor
424
value of 3.2 i0.7 x 1021~ m calculated - ~ by Minton from his dipole liittlce sum. Agreement is satisfactory in view of the slow convrrgence of the direct dipole summation used by Minton Our technique for calculating dipole lattice sums uses lxansformations to rapidly convergent summations over the lattice and reciprocal lattice. We conclude t h a t our representation of the local electric field in ice I indicates that the birefringence is due mainly to the anisotropy of the local field. However, we predict a small anisotropy in the effective molecular polarizability of water molecules in ice I, which we suggest is due to interactions between, the water molecules and their anisotropic environment.
Acknowledgment Valuable correspondence with Dr. A. P. Minton is acknowledged, and one of us (P. G. C.) thanks the Ur uted Kingdom Science Research Council for a Research Studentslnip. Department of C'hemi'sfry The University Sheffieid S3 7 N F . U . X.
P. G. Cummins D. A. Dunmur"
8eceived July 7, 7972
Temperature Dependence of the Magnetic Susceptibility of Mercury Tetrathiocyanatocobalt Pubiication costs assisted by Kernforschungsaniage TulichZentra1i;ibliothek
Sir: Magnetic susceptibilities are generally measured by the Gouy or Faraday method. Since both methods are relative ones, calibration substances with accurately known TABLE I
n
100
reemperature
Reciprocal of the measured mass susceptibility vs. temperature. Figure 1.
ibrated our system at room temperature with conductivity water (xg = -0.720 x emu/g) and mercury (xg = -0.1667 X emu/g). Since calibrations with water can be falsified by dissolved oxygen due to its high paramagnetic susceptibility, we drove it off quantitatively by bubbling helium through it for some time. Hereafter the water was enclosed in small quartz bulbs, which served as sample holders. Our vacuum balance (Sartorius) allowed i s to observe changes in weight of 1 pg. With our magnetic system, fields up to 15 kG could be applied. The mercury tetrathiocyanatocobalt was prepared as recommended,l and introduced into quartz sample holders whose diagmagnetic susceptibility showed no temperature dependence. We measured eight different samples weighing between 2 and 25 mg and found no field dependence at room temperature os a t 5°K. Above 15°K the reproducibility of the different measurements was always better than 1%, whereas below this temperature we observed larger deviations due to temperature fluctuations.
-_ Tempexitwe, Susceptibility, Temperature, Suszeptibility,
"K 106 emu/g
"K IO6 emu/g
293 16.45 66 79.8
227 21.85 43 121.3
susceptibilities have to be used. As a very convenient calibration agent HgCo(SCN)4 has been suggested1 and has found wide application.2 Mercury tetrathiocyanatocobalt has a gram susceptibility of 16.44 (k0.08) x 10-6 emu/g a t 20". The temperature dependence has been measured down to 80"M3 and follows within this range the CurieWeiss law ;yg = C / T - 8. By extrapolation to l / x g = 0 the authors find for the Weiss constant a value of 8 = .- 10°K. Since this substance might also be useful for calibrations a t low lemperatures we extended the investigations to 5°K. We used a commercial helium flow type cryostat (Leybold) t o establidi temperatures between 5 and 300°K. Helium gasl a t a pressure of 10 Torr served as heat exchanger between the sample and the cold plate of the cryostat. The temperature was measured by means of a calibrated carSon resistor and a gold (+0.03% Fe)-alumel thermocouple. We employed the Faraday method and calThe Journal of physical Chemistry, Vol. 77, No. 3, 1973
203 24.54 36.5 144.1
177 28.47 26 206.1
157 32.40 16.5 341.7
117 43.60 7.2 879.7
80 63.3
5.8 1237
The results of a typical run are given in Table I. In Figure 1 the reciprocal of the measured mass susceptibility is plotted against temperature, and is found to follow the Curie-Weiss law down to the lowest temperature. For room temperature we find the same value as the abovementioned authors;l however, the Weiss constant determined is 8 = +2"K, indicating that the former extrapolation from 80°K has lead to an erroneous result. (1) B. N. Figgis and R. S. Nyholm, J. Chem. Soc., 4190 (1958). (2) L. N. Mulay, Anal. Chem., 34, 343 (1962); W. Bronger and H. Schuer, Chem. Ing. Tech., 40, 961 (1968); H. Kessler and M.-J, Sienko, J. Chem. Phys., 55, 5414 (1971). (3) B. N. Figgis and R. S. Nyholm, J. Chem. SOC..338 (1959).
lnstitute for Physical Chemistry 517Julich 1, Germany Received September 28, 7972
H.-St. Rade
