STUDY O F
1721
THE TELLUIRIDE I O N SYSTEM
uncertainty were to occur a t these extreme values, the spread of bond energies would be from 44 to 81 kcal. Thus the magnitudes of the bond energies change by from 1 to 7 kcal. These bonds are still much more easily split homolytically, and this conclusion stands even when the uncertainties in the thermochemical quantities are allowed. to run to their maximum values. Because each of the iron atoms have six coordinate bonds, the uncertainties in the bond energy of any one
bond will be only one-sixth of the uncertainty in the bonding energy for the entire complex. The method presented here can be used to obtain bond energies for the heterolytic cleavage process if the ionization potentials of the ligand anions can be determined. Acknowledgment. We wish to thank the U. S. Atomic Energy Commission for its financial support of this work.
A Study of the ‘Telluride Ion System
by Armand J. Panson Westinghouse Research LCkbOTatOTieS, Pittsburgh 86, Pennsylvania
(Received September 28,1963)
The valence with which tellurium cathodes dissolve was studied as a function of pH. It was found that the major product below pH 9 is Te-2 and above pH 12 is Te2-2. Polarographic analysis of solution composition and electrode weight loss measurements were used in the study. The potential of tellurium electrodes in contact with telluride ion solutions was measured and interpreted. An equation relating valence to pH was derived.
Introduction In the course of developing an electrolytic method for preparing metal tellurides from solution, a study of the pH dependence olf the valence oE cathodically dissolving tellurium waE; initiated. The results obtained in this study are reported here. Cathodic dissolution of tellurium in acid solution gives colorless H2Te1 while in alkaline solution red Tes-2 is obtained. No pH limits for the existence of the monotelluride and ditelluride ions are given in the literature; therefore, it was the purpose of this investigation to determine precisely the limits of stability for these two slpecies. Experimentally, this was done both by electrode weight loss rneasurements and by polarographic analysis for monotelluride and ditelluride ions in solution. Thus two independent methods were used. Measurements were made of the potential of tell~iriumelectrodes in contact with telluride ion solutions. From these data the solution
composition was calculated and found to agree with the polarographic analysis and electrode weight loss measurements. It was found that tellurium cathodes dissolve mainly as Te-2 below pH 9 and mainly as Te2-2 above pH 12. The results agree with an equation which was derived relating valence to pH.
Theory The composition of solutions obtained by cathodic dissolution of tellurium has been calculated from the following equilibria and associated equilibrium constants: the electrolytic reduction reaction
Te
+ ne- + 2(n - 1)Hf -+ 2 - n (n - 1)H2Te + -2
Tez-2 ( 1
5
n
I 2)
(1) M. DeHlasko, Bull. Acad. Pol. Sei., Ser. A , Sci. Mat., 73 (1919).
( 2 ) E. Muller and R. Novakowsky, 2 . Elektrochem., 11, 931 (1905).
Volume 68,Number 7 July, 1964
ARMANDJ. PANSON
1722
the disproportionation reaction Tez-,
Te
+ TeW2,K 3
and the acid dissociation equilibria HzTe
+ HTe-, K I H + + Te-,, K ,
H+
HTe-
The valence, n, per Te atom of the dissolving tellurium is 2 in acid and l in alkaline solution. At the intermediate pH range, the solutions obtained are mixtures of Te-, and Tez-, ions and the ratio of the two species is a function of the apparent valence, n , which has a value between 1 and 2. The following expression for n was derived from the above equilibria
n=
+
2(1 h) 2+h
Polarographic analysis of solutions containing mixtures of mono- and ditelluride ions was used to determine n. The ditelluride ion gives a single wave of equal anodic and cathodic current^.^ I n addition, the anodic wave is indistinguishable from that of TevZ and the diffusion current constant for Tez-, is about twice that of Te-2. Using these experimental observations, it is possible in polarograms of solutions containing monoand ditelluride ions to express the ratio of the cathodic and anodic currents, ic/ia,by the concentration ratio CTez-s/(CTe-z CTe,-,). If the stoichiometric coefficients shown in the electrolytic reduction reaction are substituted for the concentrations
+
2 - n 2
2 - n
where
I n deriving this expression for n, it was assumed that H2Tez is a much stronger acid than HzTe, since no acid dissociation constants are available for HzTe2. The fact that Hz02is a stronger acid than H 2 0 is not inconsistent with the assumption that HzTez is a stronger acid than H2Te. The equation for n has the proper limits of 2 in acid and 1 in alkali. The limit in acid solutions is clear, in alkali lim n = ( 1 cH+
+
-3
0
+
Ks)/(l K 3 / 2 ) . The standard free energy for the disproportionation of Tez-z recently was found to be 5.1 f 1.4 kcal. This gives a value of K 3 = 2 X and therefore lim n = 1 in agreement with the results CH+ + 0
of Yovakowsky.
Experimental Cathodic dissolution of tellurium was carried out in a cell of about 500-ml. volume equipped with separate anode and reference electrode compartments. A platinum anode was isolated from the solution by a glass frit and KC1-saturated agar salt bridge. A saturated calomel reference electrode, similarly isolated from the solution, was used for polarographic analysis and also in measurements of potentials at the tellurium electrode. A dropping-mercury microelectrode was placed in the cell for the polarographic measurements. Solutions were purged of dissolved gases and stirred by bubbling purified nitrogen through them. Tellurium electrodes 15 mm. long and 5 mm. in diameter were cast in Vycor tubes under high vacuum. The tellurium used was American Smelting and Refining Co. semiconductor grade (99.999 %).
+
The Journal of Physical Chemistry
The ratio ic/iawas found to be independent of total concentration over the range from 0.25 to 1.00 X M studied. Telluride solutions are very unstable toward oxygen and thus rigid precautions were essential to exclude oxygen.
Results and Discussion Gravimetric and Polarographic Studies. Cathodic dissolution of tellurium was studied over the pH range 4.65 to 14 and the valence of the dissolving tellurium was compared with theoretically calculated values. Polarographic studies of the composition of the solutions and electrode weight loss measurements per faraday were used independently to determine the valence or apparent valence a t which a tellurium cathode dissolves. Tables I and I1 present the gravimetric and polarographic results. A plot of n us. pH is shown in Fig. 1. It was found that in solutions of pH less than 9, HzTe forms as the major product while a t pH greater than 12 the major product is Tez-2. (3) A. J. Panson, J . P h y s . Chem., 67, 2177 (1963).
(4) During the investigation the effects of oxygen on telluride mixtures were observed. A clear deep pink 0.5 m M solution in 1 M NH4OH (a = 1.21) turned deep red-brown when poured from the electrolysis cell in air into a beaker and then returned immediately to the cell. T h e polarographic wave decreased from 6.4 to 2.2 pa. after this treatment. The ratio i C / Lappeared to be unaffected. T h e deep red color and decrease in diffusion current are attributed to the formation of colloidal tellurium. Oxygen was bubbled through the deep red solution for 30 min. and the color was discharged. The colorless solutions, after purging with nitrogen, showed a single cathodic polarographic wave of 11.1 pa. a t = - 1.11 v. %s. s.c.e. This is attributed to the formation of tellurite on further oxidation. Polarographic analysis for TeOa showed that the amount of TeOz in the solution agreed with the amount predicted by electrode weight loss measurements. The analysis was performed by adding TeOs solutions of known concentrations to the unknown solutions, ;.e., the method of standard addition.
STUDY O F THE
TELLKRIDE ION
1723
SYSTElM
I
Table I : Electrode Weight LOSSData
I
I
I
I
I
I 4
I 6
I 8
I 10
12
n Current No. of Te wt. density, faradays loss, ma. passed, PH
4.65 4.65 8.00 8.00 10.00 10.00 10.50 10.50 11.00 11.20 11.60 11.60 11.60 13.0 13.0 13.0 14.0
Buffer solution
0 1 M NH~Ac,NH3 0 . 1 M NH~Ac,NHS 1 M NH4C1, NHa 1 M NHaC1, NH3 1 M NH4C1, NHa 1 M "4C1, NH3 1 M "4C1, NHa 1 M NHdC1, NH3 1 M NHdC1, NIIs 1 M NH4C1, NHa 1 M NH4C1, NH3 1 M ?JH,Cl, NH3 1 M NH4C1, NHa 0 . 1 M NaOH 0 1 M NaOH 0 1 M XaOH 1 OMNaOH
mg.
31.8 31.6 8.1 8.0 9.3 9.4 11.2 11.8 13.4 14.3 15.0 15.9 15.1 15.5 15.1 15.4 14.5
om.-%
4.24 4.24 2.12 2.12 2.12 2.12 2.12 2.12 2.12 2.12 2.12 2.12 2.12 1.06 2.12 2.12 2.12
X 104
n
9.96 9.96 2.49 2.49 2.49 2.49 2.49 2.49 2.49 2.49 2.49 2.49 2.49 2.49 2.49 2.49 2.49
2.01 2.01 1.96 1.98 1.71 1.69 1.42 1.35 1.19 1.10 1.06 1.00 1.05 1.02 1.05 1.03 1.09
5 0 8 0 8 5 9 4 10 0 10 5 11 0 13 0 14 0
Solution
+ HAC + ",OH + NHiOH + NHaOH + NH40H + "40H
1 M NHIAI: 1 M NHaCl 1 M NHaCl 1 M NH4Cl 1 A4 NHdCI 1 M NH4CI 1 M NHiOH 0 1 M XaOH 1 0 M NaOH
io/is
n
0 0 0 0,065 0.194 0.295 0.647 1.oo 0,996
2.00 2.00 2.00 1.93 1.67 1.54 1.21 1.00
1 .oo
Between pH 9 and 12 mixtures of HzTe and Te2+ are produced during electrolysis; a value of n = 1.5 is obtained at pH 10.5. Figure 1 shows that the curve calculated from the theoretical expression for n as a, function of pH approaches the experimental data. The calculations werle made using K ' 1 = 2.27 X determined by DeHhsko' from conductance data and and K 3 = 2 X (AGO = 5.1 i K z = 6.9 X 1.4 kcal.) recently determined by the author in a study of the polarography of the ditelluride ions3 Also included is a plot of the theoretical curve calculated with previously published values of K z = 10-11 and A G O = 14.0 kcal. ( K s = 5.5 X 10--11).5This curve deviates considerably from the experimental results. Since the n us. pH curve calculated from the new values of K2 and K3 approaches the experimental data more closely, superiority of the new dissociation constants is indicated.
2
I 14
PH
Figure 1. Plot of valence, n, us. pH. Solid line gives the theoretical curve for K1 = 2.27 X 10-8, Kz = 6.9 X 10-13, and Ks = 2 X Dashed line gives n calculated from older data: KZ = lo-" and K3 = 5.5
Table I1 : Polarographic Data PH
1 0
x
10-11.
The limits shown on the calculated curve of n us. pH are due to the uncertainty in K3. +he curve would fit the experimental data if K1 were increased by about one order of magnitude. As K2 is determined from the product KlK2,Kz would then be decreased by a factor of 10. A redetermination of K,, by pH titration, would be interesting. The original determination of the constant was made from conductance data and this perhaps is not as good a method as the titration would be for an acid of this strength. Potential Measurements. A further independent determination of the valence n as a function of pH was made by measuring the e.m.f. of tellurium electrodes in contact with telluride ion solutions of various pH values. The Nernst expression for the electrode reaction, -,2Te 2e-, is E = E o - RT/2F In a2Te/ aTe,-%, If we define Ct as the sum of the molar concentrations of the species in solution, then Ct = CTe,-% C T ~ - ~ C H T ~ - C H ~ T ~Then . from eq. 1,
+
+
+ + Te + ne- + 2(n - 1)H+
--f
( 5 ) W. M. Latimer, "The Oxidation States of the Elements and their Potentials in Aqueous Solutions," 2nd Ed., Prentioe-Hall, Inc.,
New York, N. Y . , 1952.
Volume 68, Number 7
J u l y , 1964
ARMAND J. PANSON
1724
C T ~ ~=- % [(2 - n ) / n ]Ct. We now have, assuming UT^ = 1 in the Nernst expression and substituting concentrations for activities
E
=
E"
T + R2F -1n-
2 - n Ct n
(2)
Potential measurements were made as a function of total telluride ion concentration for solutions of pH ranging from 9.8 to 13. The solutions were generated by cathodic dissolution of tellurium as in the other studies. Concentrations were determined from the total coulombs passed on electrolysis. An additional tellurium electrode was introduced into the cell so that e.m.f. measurements could be made with a nonpolarized electrode. A saturated calomel electrode was used for reference. The e.m.f. us. log total concentration data were linear with the theoretical slope RTIBF.
From these data, the e.m.f. for solutions of Ct = 1 was determined. These potentials are plotted against pH in Fig. 2. The potentials approach a limit of -0.845 v. us. the normal hydrogen electrode a t pH 13. The limiting behavior of the e.m.f. is predicted by eq. 2 since at high pH, n approaches unity as a limit. The limiting potential then corresponds to E o = -0.845, which compares fairly well with the value - 0.836 v. at 20" reported by Kasarnowsky.e The Nernst expression ( 2 ) gives
n(E)=
0 P
1 2F exp[(E Ct RT
-
- E")] + 1
Using E" = -0.845, n was calculated from the potentials given in Fig. 2 and plotted us. pH in Fig. l. These results are in excellent agreement with the polarographic and gravimetric data.
Summary This study shows that cathodic dissolution of tellurium in solutions of pH less than 9 gives HzTe as the major product, while mainly Tead2forms above pH 12. The following are additional results. ( 1 ) An equation was derived which relates to pH the relative amounts of ions of two valence states formed by electrolytic dissolution of an electrode. The equation was found to describe the tellurium electrode successfully. (2) Potentials were measured a t a tellurium electrode in solutions containing mixtures of Te-2 and Tez-z, The data followed a Nernst expression derived for the system.
9
10
12
11 DH
Fig. 2. Plot of e.m.f. us. pH; Ct
T h e Journal of Physical C h e m i d r y
=
1
13
Acknowledgment. The author is pleased to acknowledge the technical assistance of Paul Sassoon, W. Kramer, and D. Sestrich. Dr. R. Nadalin kindly furnished the use of his polarograph instrument. This work was supported in part by a contract with the U. S. Navy Bureau of Ships. (6) J. Kasamowsky, 2. anorg. allgem. Chem., 128, 33 (1923).
