1034
THEODORE B. WARNERAND RALPHL. SEIFERT
Electronic Commutator Determination of Eo of Formation and Related Thermodynamic Quantities for Molten Lead Chloride'
by Theodore B. Warner and Ralph L. Seifert Department of Chemistry, Indiana University, Bloomington, Indiana
(Received November 16, 1963)
The electronic comniutator method for deterxining E" of formation of molten halides has been improved and applied to obtain the following equations for E " , AGO, A H " , A S " , and AC," of formation of liquid lead chloride in the temperature range 493-866". The equation for E" reproduces the data within the experimental precision of 0.2 inv. The larger error liiiiits in brackets are based on consideration of maxinium probable temperature inhoinogeneity in the melt and other possible systeniatic errors. The figures in brackets are average standard errors calculated for t = 493-750 and 750-866", respectively; X = (t - 700)/100.
4- 0.833X2 + 0.062X3 + 0.116X4 [*0.8 inv., 3 ~ 1 . 2inv.] + 2.496X - 0.038X2 - 0.0029Xs - 0.0054X4 [ lt0.035 kcal., lt0.053 kcal. ] A H " (kcal./niole) = -77.737 + 0.748X + 0.122X2 + 0.2140X3 + 0.0160X4 [ f 0 . 1 2 kcal., f 0 . 2 4 kcal.] A S o (e.u.) = -24.957 + 0.768X 4- 0.086X2 + 0.2140X3 [=t0.14e.u., ltO.19 e.u.1 AC," (cal./iiiole deg.) = 10.41 + 0.34X - 0.075X2 [*0.47 cal., lt0.76 cal. J = 1158.90 - 54.11X AGO (kcal./niole) = -53.451
E" (niv.)
Introduction
cision, and determine whether it might give more reproducible results and be applicable over a wider tenperature range than the equilibrium cell method. In
Most previous determinations of the E" of forination of molten binary halides have been made using the equilibrium cell method, which, although simple in (1) This work was supported in part by t h e Directorate of Chemical principle, has produced widely varying results. For Sciences, Air Force Office of Scientific Research, and by the U. S. Atomic Energy Commission. example, E" values obtained for the equilibrium cell Pb(l) /PbCI2(1)lC12(g) by a number of i ~ i v e s t i g a t o r s ~ - ~ (2) ~ V. Czepinski, 2. anorg. Chem., 19, 208 (1899). (3) 0. H. Weber, ibid., 21, 305 (1899). vary by as much as 70 inv. In general the precision (4) R. Lorenz and M. G. Fox, Z . physik. Chem., 63, 109 (1908). has been 1 or 2 mv. (0.5 inv. in one case7), and most (5) J. H . Hildebrand and G. C. Ruhle, J . A m . Chem. Soc., 49, 722 studies were restricted to temperatures below 700", (1927). where experimental difficulties were minimized. The (6) R. Lorenz and H . Velde, 2. anorg. allgem. Chem., 183, 81 (1929). (7) A. Wachter and J. H . Hildebrand, J . A m . Chem. SOC.,52, 4655 value of dE"/dT has usually been reported to be con(1930). stant, withir: experiinental error; although the wide ( 8 ) L. Holub, F. Neubert, and F. Sauerwald, 2. physik. Chem., 174, range of reported values (-0.54 to -0.74 inv./"C.) 161 (1935). produces large uncertainty in the derived values of (9) S. A. I'letenev and V. N. Rozov, Acta Physicochim. C'RSS, 7,339 (193i). AHr" and A S f " of lead chloride. Since the precision (IO) B. P. Artamonov, "Collected Works on the Electrochemistry of of measurement has never permitted evaluation of Fused Salts," P. F . Antipina, E d . , State Scientific-Technical Pubd2E"/dT2,(AC,")l could not be calculated. lishing House for Chemical Literature, Leningrad, 1940, pp. 31-39. (11) S. I. Rempel and I. N. Ozeryanaya, Zh. Fiz. Khim.. 25, 1181 The electronic commutator neth hod'^ developed in (1951). this laboratory was used in the present study of lead (12) M. F. Lantratov and A. F . Alabyshev, J . Appl. Chem. U S S R , chloride to test the method further, increase its pre26, 235 (1953). The Journal of Physical Chemistry
E"
OF
FORMATION OF ~ I O L T ELEAD N CHLORIDE
the commutal or method, square current pulses are passed through the molten salt to form the constituents of a reversible cell periodically a t inert electrodes. Measurement of cell e.m.f. as a function of time between pulses permits correction for overvoltage and determination of the reversible e.m.f.
Experimental Details of the apparatus and procedure are given el~ewhere'~; they differed in many respects from those used in the earlier work with silver chloride.14 (1) Apparatus. The electrolysis cell, which was enclosed in a fused quartz housing, could be evacuated or filled with prepurified chlorine, hydrogen chloride, or nitrogen, either directly or by bubbling through the melt. The positions of electrodes in the cell could be adjusted and the electrodes were introduced and removed through close-fitting tubes flushed by an outward flowing stream of purified nitrogen. The melt (30 inin. deep) was contained in a high purity alumina crucible. Gas pressure in the cell was adjustable and maintained at 760 torr. The cell housing was held in a massive iron cylinder serving as temperature ballast in a crucible furnace. Furnace temperature was controlled (+0.2') with a proportioning controller and cell temperature was measured with chromel-alumel thermocouples calibrated as before. l 4 ( 2 ) Electrodes. Electrodes were spectroscopically pure graphite rods with platinum wire leads. Quartz tube sheaths were collapsed tightly onto the graphite about 1 c ~ nfrom . the bottom of the rod. The cathode16 sheath was sealed off below the graphite to form a compartment where lead generated during electrolysis could collect. A small hole at the top of the compartment admitted melt. The anode sheath extended below the end of the electrode and was open at the lower end. A small hole in the side of the sheath permitted melt to cover the end of the anode while chlorine could collect about the electrode above the melt. Apparent anode areas varied from 0.1 to 0.4 c m 2 ; cathode areas were much larger. The graphite rods were pretreated by heating to 1000" under vacuum for 12 hr. arid then strongly igniting the electrode end in an oxy-hydrogen flame. l7 After fabrication, electrodes were held under vacuum for 24 hr. at NO",cycled teri times for approximately 20-min. periods between vacuunl and gas at 1 atm. (Clz for anodes, S2 for cathodes), then cooled and stored under gas until used. and Associated Circuitry* The current DUlSiIlg and e.1-n.f. measuring- circuit differed ._ from that previously used14 by providirig greater a wider range Of length and total cycle time and by utilizing a more satisfactory
1035
method to determine cell e.m.f. a t a selected time following a current pulse. Instead of entering a gated amplifier, the difference signal from cell and K-2 potentiometer in series was ainplified with a d.c. amplifier (gain of 100; current drawn 10-8 amp.; drift
