J. D. VAN NORMAN, J. S. BOOKLESS, AND J. J. EGAN
1276
A Spectrophotometric and Chronopotentiometric Study of the
Lead-Lead Chloride and the Zinc-Zinc Chloride Systems1
by J. D. Van Norman, J. S. Bookless, and J. J. Egan Brookhaven National Laboratory, U p t o n , N e w York
(Receioed November 16, 1965)
The dissolution of lead and zinc in their respective molten chlorides was studied using both spectrophotometric and chronopotentiometric techniques. It was found that the amount of lead which dissolved in lead chlorides in contact with liquid Au-Pb alloys at 518 and 530" varied directly as the activity of lead in the alloy indicating formation of a soluble Pb2+ species. The solubility of pure lead in its chloride at 518" was evaluated chronopotentiometrically and was found to be 0.0057 mole %. The diffusion coefficient of Pb?+ was evaluated at 1.8 x 10-5 cm2/sec. In a similar manner it was found that zinc dissolved in its chloride at 500" to form the Znz2+species. The solubility of zinc in its chloride was 0.019 mole % and the diffusion coefficient was 7.6 X cm2/sec. The results of this investigation are compared with previously published results.
Introduction The mode of dissolution of lead in its own molten chloride has been studied previously by two methods. First, Karpachev, Stromberg, and Jordan2*studied the emf of the cell PblPbClz
+ Pb(sat)IIPbClz + PblC
at 700". When the amount of lead in the right-hand compartment was varied, a Nernst plot yielded an n value of 1 corresponding to the electrode reaction Pb2+
+ le = Pb+
Thus the mode of dissolution was taken to be the formation of P b + by the reaction PbO
+ PbClz = 2PbC1
(1)
Topo12bhas tried to repeat these results without success. Because of' the low solubility of lead in molten lead chloride, his results were inconclusive but tended more to a n n value of 2 instead of 1. Secondly, an anodic polarographic technique has been used3 to measure the concentration of subhalide as a function of lead activity which was adjusted by the use of a series of liquid gold-lead alloys of known lead activity in equilibrium with the lead chloride mixture. I t was found that the concentration of the subhalide varied linearly with the lead activity. This T h e Journal of Physical Chemistry
mould be the case if lead dissolved in lead chloride according to the reaction
If, however, the subhalide was formed by reaction I, the concentration would have depended upon the square root of the activity. Reactions I and I1 may also be written in the ionic form without altering the conclusions. Because of the discrepancy existing between the two experiments, it was thought worthwhile to carry out further research on the Pb-PbC1, system. Thus both chronopotentiometry and spectrophotometry have been employed in this work to follow the concentration of dissolved subhalide with varying activities of lead. It is known that solutions of lead dissolved in lead chloride take on a very dark coloration. The optical absorption of these solutions can be used as a measure of subhalide concentration. Assuming Beer's law, the law of mass action, and Henry's law to hold for (1) This work was performed under the auspices of the U. S. Atomic Energy Commission. (2) (a) S. Karpachev, A. Stromberg, and E. Jordan, Compt. Rend. Acad. Sci. U S S R , 36, 101 (1942); (b) L. E. Topol, J . Phya. Chem., 67, 2222 (1963). (3) J. J. Egan, ibid., 65, 2222 (1961).
A SPECTROPHOTOMETRIC AND CHRONOPOTENTIOMETRIC STUDY OF Pb-PbClz
1277
COVER
COOLING COILS
CELL COMPARTMENTS
HEATING COILS LIGHT PATH CONTROL THERMOCOUPLE llGHT PATH
--
-THERMOCOUPLE WELL
SANTOCEL INSULATION
ALIGNMENT NOTCH
LEVELING SCREWS
Figure 1. Furnace used for high-temperature spectrophotometry.
dilute solutions of subhalide, one obtains eq 1 for reaction I and eq 2 for reaction 11, where A , is the ab-
(5)
small known amounts of lead to lead chloride while monitoring the concentration of subhalide chronopotentiometrically. For each addition of lead, transition times are measured at various current densities. From eq 3 it can be seen that the product iorl/’ is directly proportional to the concentration. Thus, a plot of this product vs. added lead will be a straight line until saturation is reached, at which point the product i0r1/’ becomes constant. Corbett and von Winbush6 have measured the solubility of lead in its molten chloride at different temperatures, permitting a comparison of results. The diffusion coefficient, D , for the subhalide can be calculated from the slope of the previous plot. The mode of dissolution of zinc in its chloride has never been determined, Topo12bwas unable to secure stable potentials in emf cells. Corbett, von Winbush, and illbers’ measured the solubility of zinc in its chloride. Therefore, the Zn-ZnC12 system was studied
for reaction 11. This technique was used previously to study the Mg-MgCls systeme4 The solubility of lead in its molten chloride can also be determined using the chronopotentiometric technique developed for the Ag-AgC1 and the Ag-AgBr systems.5 This technique involved the addition of
(4) J. D. Van Norman and J. J. Egan, J . Phys. Chem., 67, 2460 (1963). (5) J. D. Van Norman, J . Electrochem. Soc., 112, 1126 (1965). (6) J. D. Corbett and 5. von Winbush, J . Am. Chem. Soc., 77, 3964 (1955). (7) J. D. Corbett, S. von Winbush, and F. C. Albers, ibid., 79, 3020 (1957).
ASIA,” = apb’’’ Aa/A,”
(1)
(2)
= UPb
sorbance and A,” is the absorbance of a solution at unit activity of lead. In anodic chronopotentiometry, it is the quantity iorl” which is proportional to concentration, since
(3) where io is the current density, T the transition time, D the diffusion coefficient, and C the concentration. Consequently, eq 4 holds for reaction I and eq 5 holds ior1’2/iorapb11/2 = apb’” ior1/2/iorapb-11’2 =
UPb
(4)
Volume 70, IVumber
4
April 1966
1278
J. D. VANNORMAN, J. S. BOOKLESS, AND J. J. EGAN
chronopotentiometrically and spectrophotometrically at 500" in the same manner as the Pb-PbCl2 system. Experimental Section
Materials The lead chloride used in this investigation mas reagent grade and was further purified by passing HC1 gas through the powder while slowly raising the temperature above the melting point. Once the lead chloride was molten, HC1 was bubbled through it for an additional 3 hr. The salt was then filtered through a fine quartz frit to remove any insoluble material. This purified salt was contacted for 24 hr a t 520" with a piece of pure gold foil. The lead metal used was of !39.99% purity. The zinc chloride used in the investigation was prepared by passing HC1 over the metal (99.99% purity) a t 700" and distilling the ZnC4 off. The distilled ZnClz was rollected in the molten state in a crucible and was later subjected to a stream of chlorine gas while in the molten state to purify the salt further. The zinc chloride so prepared was then contacted with liquid bismuth metal to remove any traces of dissolved zinc metal. Apparatus and Measurements. The spectral measurements were taken using 2-mm quartz optical cells on a Cary 3Iodel 14 spectrophotometer with a special furnace compartment as shown in Figure 1. The entire assembly fitted into the sample compartment of the spectrophotometer. The steel core was 1.75 in. in diamcxter by 5.25 in. in height and could be rotated to obtain the spectra of two samples. The spectrum of pure PbC12 contacted with gold was taken as a background. Spectra were then taken of PbClz contacted with various gold-lead alloys. The chronopotentiometric measurements were taken in a cell similar to that used by Van ?;rormana5 I n the present investigation, an iridium electrode in contact with the molten salt served as the reference electrode. For some of the measurements on the zinczinc chloride system, a special Pyrex cap fitted over the quartz crucible to cut down on the amount of zinc chloride lost through volatilization. Small amounts of lead were added to the purified lead chloride; after each addition, anodic chronopotentiograms were taken by passing a constant current between the two iridium electrodes while measuring the potentid between them as a function of time. Once saturation was achieved, a large excess of lead was added and small weighed amounts of gold were successively added to vary the alloy composition. At the completion of an experiment the salt used was weighed: about 450 g. I n the case of the zinc-zinc chloride system, bismuth The Journal of Phuslsical Chemistrfi
1.6 -
1.4 -
-
1.2 W
g
1.0-
4
m
5: 0.8m a
-
0.6
-1
PbCI, + A u
t
Oa2
01 4000
I
I
I
5ooo 6000 WAVELENGTH, 8
7000
I
Figure 2. Absorption spectra a t 530" of pure PbCL equilibrated with Au and of PbClz equilibrated with Pb. The curve labeled subhalide is the difference of the two. 1.0
0.8
t / 3'
0.4
0.2
r/
0 0
I
I
0.2
I
I
0.4
I
I
0.6
I
I
0.8
I
I.0
OPb
Figure 3. The relation between Pb activity and absorption due to subhalide in PbCl2. Curve A is the theoretical curve for the formation of Pb+, and curve B is for the formation of PbZ2+.
metal was used to remove initially any zinc dissolved in the pure zinc chloride. The solubility was then measured by successive addhions of small amounts of zinc. A large excess of pure bismuth metal was added, and the composition of the Bi-Zn alloy was varied by adding known amounts of zinc to the system. The amount of zinc chloride used was about 300 g.
1279
A SPECTROPHOTOMETRIC AND CHRONOPOTENTIOMETRIC STUDY OF Pb-PbCh
I
I
I
I
I
I
I
I
81.2 I.0ma/cm2
/
-
SATURATION
5
0
0
0.5
1.0
1.5 2.0 TIME - S E C O N D S
2.5
Figure 4. Typical anodic chronopotentiograms in the Pb-PbClz system a t 518".
Results and Conclusions Figure 2 shows the absorption spectra at 530" of pure PbC12 and of PbC12 equilibrated with lead. The curve labeled subhalide is the difference of the two. Similar spectra were obtained at various activities of lead in lead-gold alloys. Activities of lead were calculated from emf measurements published by I
