POLAROGRAPHY OF THE DITELLURIDE 10s
Oct., 1963
Studies have been made of the dichroism of the LIBM complexes produced by orienting dilute suspen-
2 177
APPENDIXA MOLARABSORBANCY INDICES (a X OF AQUEOUS METHYLENE BLUESOLUTIONS AT 25” AND VARIOUB (TOTAL) COXCENTRATIONS
sions of the lamellar particles in pulsed electric fields. These have given the information that monomeric and associated bound dye molecules are oriented with transition moments approximately parallel to the particle surfaces. The new experimental and theoretical findings on electric dichroism are of general interest in macromolecular structure studies, and a separate article is being prepared.20
X
_--_------Concentration (i.) 9.80 16.5 1 9 . 6 31.8
5000 5500 5875 6000 6125 6250 6375 6500 6640 6750 6875 7000
Acknowledgments,,-Financial support of this research by a (Grant-in-Aid of the Petroleum Research Fund, administered by the American Chemical Society, is gratefully acknowledged. Jl‘e wish to thank Dr. D. F. Bradley and hlrs. N. Stellwagen for helpful comments in reading the inanuscript before publication.
0.35 0.75 2.30 3.45 4.24 4.40 5.18 6.95 8.45 6.50 2.31 0.70
0.31 0.73 2.48 3.72 4.48 4.49 5.22 7.37 8.30 6.31 2.31 0.76
0.34 0.81 2.47 3.72 4.54 4.43 5.21 6.90 8.21 6.06 2.33 0.74
of 3IB (c X loe)------5 9 . 3 107.6 143.2
0.31 0.88 2.64 4.09 4.88 4.63 5.09 6.73 7.93 6.01 2.24 0.82
0.34 0.96 2.87 4.22 4.89 4.38 4.65 5.90 6.91 5.19 1.97 0.77
0.40 1.07 3.16 4.49 4.96 4.23 4.27 5.18 6.05 4.79 2.01 0.91
0.38 1.12 3.27 4.67 5.17 4.27 4.10 4.94 5.67 4.39 1.86 0.84
215
0.36 1.17 3.35 4.93 5.18 4.15 3.77 4.65 5.07 3.31 1.72 0.84
POLAROGRAPHY OF THE DITELLURIDE ION BY ARMANDJ. PANSON Westinghouse Research Laboratories, Pittsburgh 36,Pennsylvania Received iMarch IS,1965 A polarographic investigation of the ditelluride ion, Tez-2, has been made for concentrations up to 0.25 mM. Singie well defined waves were obtained with anodic and cathodic portions of equal height. The waves were Te-*. rlnalyses interpreted in terms of a disproportionation of Tez-2 a t the mercury electrode as Te2+ + Te 2II+ of half-wave potentials us. p H indicate that the potential-determining step is the reaction H2Te+ Te 2e-. This analysis gives a new value for the second acid dissociation constant of HzTe, Kz = 5 X The 2e- was found to be 0.51 v. us. the standard hydrogen standard potontial for the reaction H2Te = Te 2 H f Te-2 was calculated to electrode. From these data AFO for the disproportionation reaction Tez+ e Te be 5.1 kcal. These values differ markedly from the previous accepted values of Kz = lo-”, E o = 0.72, and AF’ = 14.0 kcal.
+
+
Introduction Recent interest iii tellurides arises from their importance as thermoelectric materials. This paper reports results obtained in the course of a study of telluride ion solutions. The purpose of the study was to gain understamding and control of the chemistry involved in preparing telluride compounds from solution by electrolytic methods. I n the study, polarography was used to analyze solutions for Te-2 ions. The polarographic waves of the T e 2 P ion were interpreted for the first time and these results are reported here. Apart from the very interesting polarographic information obtained, a key result of this paper is the determination of equilibrium constants which are essential in defining the pH limits for the preparation reactions. Polarographic investigation of the ditelluride ion, Te2-2j has not been reported previously in the literature. Lingane and Niedrach’ found that Tec2 ions give well defined anodic waves stemming from the oxidation of HzTe to Te a t the dropping mercury electrode 2H+ (d.m.e.) according to the reaction HzTe = Te 2e-. These authors concluded that the Te formed in the reaction is insoluble in the mercury electrode. The current investigation reveals that the Te2+ ion disproportionates at the microelectrode according to the reaction Te2+ S Te Tec2. Interestingly, the zero-valent tellurium is reduced a t the dropping mercury electrode and the Te -2 is oxidized. The polarographic
+
+
+
(1) J J (1948).
Lingane and L I T Niedrach, J
A m Chem. S o c , 70, 4115
+
+
+
+
wave thus consists of an anodic and a cathodic part of equal heights. The anodic portion of the polarographic wave of Te2c2thus is identical with the anodic waves obtained by Lingane and Niedrach from Te-2 solutions. Experimental Ten+ ion solutions were prepared by cathodic dissolution of Te directly in the cell used for polarograph measurements. A Pt anode used for the electrolysis was isolated by means of a KCI-saturated agar salt bridge. Tellurium electrodes, 2.5 em. 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+ %). Solutions were degassed with purified Sn prior to electrolysis. The cathodic dissolution of tellurium was performed at constant currents of 2.5 or 5 ma. (0.6 or 1.2 ma./cm.z) and the concentrations of the solutions R ere determined from the number of Faradays passed on electrolysis. Gelatin (0.003%) was added to the solutions t o suppress polarographic maxima. Polarograms were obtained with a Sargent Model XJ’ polarograph using a dropping mercury microelectrode and saturated calomel reference electrode. In the study of halfwave potential vs. pH, S a O H solutions of 1,0.1, and 0.01 J 4 were used for the pH 14, 13, and 12 experiments. Solutions of 1 M NHdCI with sufficient S H 4 0 H added to adjust the p H were used for the lower p H experiments. The reaction was tested and found to be diffusion controlled by measurements of current us. height of the mercury column. The current in diffusion controlled reactions is proportional to the square root of the head of mercury while kinetic currents are independent of the mercury head.2 Table I presents the data. (2) P. Delahay, “New Instrumental &lethods in Electrochemistry,” Interscience Publishers, Inc., New York, N.Y . , 1954, p. 90. (3) I. M. Kolthoff and J. J. Lingane, “Polarography,” Vol. 1, 2nd Ed., Interscience Publishers, Ino., New York, N. Y., 1952, p. 86.
ARMAND J. PSSSON
2178
> -
T'ol. 67
a t the d.1n.e. In additim, aiialysis of the half-wave potential us. pH curve has given new values for thermodynaniic constants of the telluride ion system. The standard electrode potential for the reaction H2Te = Te 2H+ 2e- was found to be 0.31 v. us. the standard hydrogen electrode. The second acid dissociation constant for H2Te was found to be 6.9 X From Te-* was calcuthese results AFO for Te2-2 = Te lated to be 5.1 kcal, These results differ significantly from the published results of E o ~ z ~ = e / 0.72 ~ e v.,~ K2 = 10-11,1 and AFO = 14.0 kcal.6 The results reported here, hoiTever, have been used successfully to predict theoretically the pH dependence of the apparent valence of cathodically dissolved tellurium. These results will be reported in a future paper. The following section gives details of the experiments. Diffusion Current vs. Concentration.-Lingane and Niedrach found the diffusion current constant, I = i ~ ' ( C m z / 3 t ' /to o )be . 3.5 f 0.1 for Tec2 in 1 N KaOH. For Te2-* solutions, T should then be twice that found for Te-? solution or 7.0 because in the disproportionTe-*, one mole of Tez-2 ation reaction Te2-2 + Te furnishes one mole of Te-* which is oxidized in the anodic part of the ~ i a v eand one mole of Te which is reduced in the cathodic part of the wave. Neasuremeiit of diffusion current as a function of concentration gives a value of 6.8 for I which is in excellent agreement with the theory. Table I1 gives the data obtained in this experiment.
+
+
+
+
Fig. 1.-Polarograms of Tez-2 in 1 M NaOH 0.0035 gelatin at 25" I residual current; a, 0.0498 mM; b, 0 0995 mM; c, 0.248 inJi
TABLE I HEIGHTOF & l E R C U R Y RESERPOIR 0.1 I& Te2-2 in 1 S XaOH 0 O03yGgelatin, t = 25' h corrected , I i d , iia -' 2 p a c m -' C U R R E l T US
91 9.1 84 82 68 76 58 04 51 24 50.83 43 07 3-1 59
+
1 438 1 385 1 215
1 120 1.045 1 020 0 935 0 810
Surface tension back pressure - 1.52 cm See ref. 3. a
+
0 1500 1504 1465 1470 .1439 1431 1425 1377 Av. 0 1402 i0 0056 correction: - 3.l/(mt)'/a
2
TABLE I1 DIFFLSION CURRENT us. CONCESTRATIOS Tez+ in 1 A' XaOH O.O030l, gelatin m2/at'/o = 1.87,t = 250
+
=
Results and Discussion Solutions of Te2? ions prepared by cathodic disS a O H show single well defined solutioii of Te in 1 pDlarographic waves at E1 = 1.19 v. us. s.c.e. (Fig. 1 illustrates the waves obtained.) Significantly, the waves consist of an anodic and a cathodic portion which are equal in height. This indicates that equivalent amounts of an oxidized and reduced form of Te react a t the d.m.e. The ditelluride ion therefore must disproportionate a t the d.ni.e. according to the reaction Te2-2 + Te $- Te-2. The anodic wave then is identical with the anodic waves found by Lingane and Kiedrachl in solutions of Tec2 ions. I n 1 N XaOH these authors found E = 1.19 v. us. s.c.e., which is identical with the value found for Tez-2 solutions in this work. At concentrations greater than 0.3 mX, the anodic wave found for Te-* solutions shows distortions typically associated with film formation; L e . , a step is formed of constant height independent of concentration. The anodic part of the ditelluride wave s h o w the beginnings of a step a t 0.13 mJI. This study was confined mainly to concentrations no greater than 0.23 niJ1 in order to avoid working with film-distorted waves. However, some further observations on the wave distortion can be made but will be reserved for a later section. Studies were made of the diffusion current as a function of concentration and the pH dependence of the half-wave potential. Results of these experiments support the interpretation that Te2-2 disproportionates
C, mmole/l.
ad, pa.
0.0498 ,0747 .0995 ,124 ,248
0.609 0.938 1 266 1.615 3 270
I
Av.
6.54 6.72 6 80 6 96 7 05 6.81 & O 15
Half-Wave Potential us. pH.-The pH dependence of the half-wave potential, E,/,, of Te2-* solutions has been measured. Figure 2 shows that potentials found in this work are identical with those found for Te-2 solutions by Lingane and Kiedrach.l This is convincing proof that the electrode reaction H2Te Te 2H+ 2e- is involved in polarography of the Tez-2 ion at the d.ni.e. The study was made in solutions of pH 14 to 4.5 into which Te was cathodically dissolved. The solutions obtained between pH 14 and 12 are coinposed almost entirely of Te2-2. This is indicated by the fact that the anodic and cathodic currents are equal. Below pH 12, mixtures of Tec2 and Te2-* ions are obtained by cathodic dissolution of Te. This is indicated by an increase in the anodic wave relative to the cathodic ware. Below pH of about 8.5 the solutions are composed almost entirely of Te-2 ions since only an anodic wave is obtained. An analysis of the apparent valence of the dissolving Te has been made in terms of the ratio of cathodic to anodic current and was found to compare well with the yalues obtained from electrode weight loss measurements. The results of these experiments will be presented in a future paper.
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POLAROGRAPHY OF THE DITELLURIDE Im
Oct., 1963
2179
Ai1 analysis of the theoretical equation relatiiig Ellg and pH focuses attention on the question of the formation of a n insoluble film of Te on the Hg drop during the aiiodic reaction. Lingaiie and Niedrachl analyzed their data iii terms of the equation E,;,
=
RT E" - - 111 Z 2F
C + RT hi 2F 2 -
-
+
where 2 = ( C H - ) ~-t K ~ C H - KIK2, C is the total divalent Te concentration, K1 and KZ are the first and second acid dissociation constaiits for H2Te, and E" refers to the reactioii HzTe = Te 2H+ 2e-. This equation mas derived with the assumption that a film of Te with uiiil, activity forms on the Hg drop. If 110 film formation occurs, which is the case for the cathodic reaction, or if the activity of the deposited Te equals the activity of Te in solution at the Hg-solution interface, the equation obtained
+
El,,
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0
(4) W. M. Latimer, "The Oxidation States of the Elements a n d Their Potentials in Aqueous Solutions," Prentice-Hall Inc Sei7 York N Y , 1938,p. 77. ( 5 ) &I. de Hlasko, J. c h m p i y ~ 20, 167 (1923). (6) W. M. Latimer, "The Oxidation States of the Elenients and Their Potentials in Aqueous Solutions, ' 2nd E d Prentioe-Hall, I n c , Ken P o r k , N. Y , 1952, p. 86. (4 tvpographical error gl>es F H ~ re T ~= - 0 72 1 The correct value 13 0 72 1 ) (7) S. A. Awad, J P h u r Cirem , 6 6 , 890 (1962)
8
6
IO
12
1
Fig. 2.-Half-u-nve potentials us. pH: 0,this work, Te-2 TQ-~ solutions; O, Lingene and n'iedrach, Te-2 solutions.
E o - - In 2 2F
Therefore, when the anodic wave involves the formation of a Te filni of unit activity, it will be displaced from the cathodic wave by h'TI2F In C/2 volts or -0.04 v. a t 0.1 mM Since wch displacement is not observed at concentrations up to 0.13 niX, either films are not formed a t the low coiicentratioiis or the activity of the film i s equal to the activity of Te in solution at the electrode interface. At this point, we caii analyze the E l / ,us. pH curves in terms of the theory. To calculate the theoretical curve Lingane and Kiedrachl used E" = 0.69 v. on the Hzscale estimated by Latimer4 from thermal data, and K1 = 2.27 X determined by Hlasko6 from conductance measurements. A value of K z = was required to fit the experimental data. Latimer,6 however, concluded that either K z was too small or that the magnitudie of E" was too great since these values lead to a value of 14.0 kcal. for the free energy of disproportionation of Te2-2. Latimer considered this value to be too large and a value of 3.6 kcal. to be inore reasonable. Recently, Awad7 estimated that E" = 0.50 v. us. the standard hydrogen electrode on the basis of over-potential measurements. I n Awad's paper, stationary potentials measured against a hydrogen electrode were assumed to correspond to the standard electrode potentials E o ~ n ~ e and E o ~ , T e z / T e . The justification for this assuniptioii is iiot clear since neil her the activities or conceiitratioiis of &Te and lH2Te2 involved in the electrode reactions are defined in the experiments. I n view of the uncertainties in Kz and E o ~ , T , / T e , it is of interest to reexamine the experiin~entalcurve. An analysis of the Ell2us. pH data gives a value of 0.51 v. vs. the standard hydrogen electrode for E", which agrees with Awad's estimate. The analysis was performed as follows. Z is stroiigly
4
PH
D T 1L1
=
2
14
+
dominated by KICH+ in the pH range 11 to 5. I n this range E,,, ,- E" - R T / B F la CH+- l z T / 2 F In K1. The slope of the curve is therefore R T / 2 F and the intercept at pH 0 is E" - RT/2F In K1. Figure 2 shows that a line of slope R T / 2 F fits the experimental data well in this pH range and gives an intercept of 0.83 v. us. s.c.e., or 0.59 v. us. the standard hydrogen electrode. From this intercept, E" is calculated to be 0.51 v. us. the standard hydrogen electrode. = Above pH 14, 2 is dominated by KiKzand E" - RT/2F 111 KIKz. Observe that this is the value of E" for the reaction Te-2 = Te 2e-. The data indicate that this potential is 1.19 v. ts. s.c.e. or 0.95 v. 2's. the standard hydrogen electrode. From this value, K z is calculated to be 6.9 X The complete theoretical curve relating Ell2to pH calculated from these data is shown in Fig. 2. AF O for the disproportionation reaction Tez+ = Te Te+ was calculated to be 5.1 kcal.; this value is in agreement with Latimer's estimate of 3.6 kcal. This calculation was niade using E" = 0.95 v. found in 2H+ 2e- and E" = this work for H2Te = Te 0.84 v. for the reaction TezP2= 2Te 2e- which was determined by Karsaiiomsky8 from direct cell ineasuremeats. The correspondiiig value of 1.1 and 0.74 v. published by dmad7 leads to a AF" of 17 kcal., n-hich is too large. Summary ~ e This study has led to a consistent and reasonable set of thermodynamic data for the telluride ioii system. The results are summarized as follows. (1) Te2-2was shown to disproportionate at the d.ii1.e. according to the reaction Tez+ + Te Te-2. Anodic and cathodic waves result from the reaction HzTe Te 2H+ 2e-. ( 2 ) Ell22)s. pH measurements demonstrate that E" for the reaction H2Te = Te 2H+ 2e- = 0.51 v. us. the standard hydrogen electrode in agreement with the estimate published by Asvad7 based on overvoltage studies and in contrast with the previous estimate of 0.72.6 (3) Kz, the second acid dissociation constant for H*Te, is 6.9 X l O - I 3 in contrast with the previously accepted value of 1 x 10-l1.6
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/
+ +
+
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(8) J KarsanoasBS, Z a n o i g allgem. Giiem
+
, 128,33 (1923)
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GIDEOXCZAPSKI AKD B m o x H. J. BIELSKI
2 180
(4) The standard electrode potential, E O , for the 2e- is 0.95 T-. z’s. tlie standard reaction Tec2 = TeO hydrogen electrode in contrast with previous estimates of 1.14 1 7 . ~ and 1.1v.’ (5) A F o for the disproportionation reaction Tez? S Te Te-2 is 5.1 kcal., in contrast to tlie previous published value of 14.0 kcal. Acknowledgments.-The author is pleased to acknowledge helpful discussions with many colleagues at
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Vol. 67
the Westinghouse Research Laboratories. He is especially grateful to Dr. Joseph Veissbart for stimulating discussions. Professor Paul Delahay’s suggestion that a test of diffusion control be made is gratefully acknowledged. bfr. Paul Sassoon gave able assistance in the experimental work. The author wishes to thank Dr. Robert Nadalin for the loan of his polarograph instrument. This sTork was supported in part by a S a v y Bureau of Ships Contract.
THE FORMATION AXD DECAY OF H203 L4ND HO, I N ELECTRON-IRRADIATED AQEEOUS SOLUTIONS1 BY GIDEON CZAPSKI~ AND BESOXH. J. BIELSKI Chemistry Department, Brookhaven h-ational Laboratory, Upton, Long Island, S e u : York Receicecl March $7. 2963 The kinetics of the reaction between perhydroxyl radicals, HOZ,have been studied a t 23”. The radicals were generated in rapidly flowing water containing dissolved oxygen by an electron beam from a Van de Graaff accelerator, and the quantity remaining at any time after irradiation was determined by rapid mixing with a solution containing tetranitromethane, which reacts with HOa radicals. The rate constants found were k ~ - o~0~~ = 2.2 x 106 M-1 see.-’ and koa- + 02- = 1.5 X 10’ -1f-l sec.-l, and the p K of HOa was found to be 4.4 2c 0.4. hnother intermediate, believed to be HZ03 on kinetic grounds, was found to exhibit pseudo-first-order disappearance, the first-order rate constant being a function of acidity. This intermediate was found using ferrous sulfate as the scavenger solution. The half-life of H203 reaches a maximum of 2 sec. in 0.02 M (H+),decreasing to less than 0.1 8ec. in 1 and 10-4 A‘ (H-), Tlit kinetics are shown to agree well with acid- and baee-catalyzed 0.. decomposition of HZ03 to form H,O
+
Introduction Rate studies on two unstable hydrogen-oxygen coinpounds are reported in this paper. The first compound is the hydroperoxy radical, H02,and its basic form Os-. The hydroperoxy radical is believed to react with itself to give hydrogen peroxide aiid oxygen. The rate constant of the reaction between 02-and 02- was measured recently by Schmidt using a conductivity rnethodu3 Saito and Bielski generated H02 in a flow system by mixing ceric ions with an excess of HzOzand followed the decay of the radical by e.8.r. measuremeiit~.~ The rate constants mere 1.45 x lo7 M-l sec.-l and 2.4 X lo6 J1-I sec.-l for 02- and HOz, respectively, when defined by -d(HOz),’dt = ~ ? C ( H O ~ ) ~ . We have generated HOn and 02-by flowing oxygensaturated water past a 1.6 RIev. electron beam. The HOz radical was detected by reaction with a solution of tetranitromethane in a mixing chamber shortly after the water had left the electron beam. In the course of these experinleiits another interniediate, longer-living, was found which exhibits a firstorder decay and which reacts with ferrous sulfate solutions. Experimental General Description of Apparatus and its Operation.-The experimental device mas a combination of a flom- system end a Tan de Graaff accelerator. The flom s j stem >vas similar to that emploied by Sutin in fast reaction studies.B T m o motor-driven syringes force two solutions through glass tubing to a mixing chamber and the mixed solutions flow into a beaker One SJ ringe contains the solution to be irradiated. This solution flows (1) Research performed under the auspices of the U. 5 . A t o m i c Energy Commission. (2) The H e b r e n Unirersity, Jerusalem, Israel. (3) K. Schmidt, 2. S a t u T f o r s c h . , 16B,206 (1961). (4) B. H. J. Bielski and E. Saito, J . P h y s . Chem., 6 6 , 2266 (1962). ( 5 ) E. Saito a n d H. H. J. Hielski, J . Am. Chem. Soc.. 83,4467 (1961). (6) N. %tin and B. XI. Gordon, ;hid., 83, 70 (1961).
through a thin-walled, small diameter tube and is irradiated with an electron beam a known distance before the mixing chamber. The other solution contains an excess of reagent which will react with the intermediate surviving a t the mixing chamber (tetranitromethane for the perhydroxyl radical and ferrous sulfate for the H203). The mixing chamber consists of two jets for each solution, the four jets entering tangentially to the discharge tube. To begin the run, a sxitch starts discharging the syringes and turns on the electron beam and a timer simultaneously. When the syringes are empty, the plunger trips a microswitch which stops all three. This use of a flow system differs from the normal use in that the reaction being studied occurs between the electron beam and the mixing chamber instead of occurring after the mixing chamber. The scavenger solution quenches intermediates surviving a t the mixing chamber and a proportionate color change is produced. Analysis is done a t the operator’s leisure with a spectrophotometer. The syringes, flow tubes, and the mixing chamber were made of Pyrex glass, the stopcock plugs were Teflon. No grease was used. The solution to be irradiated WE saturated with oxygen which was purified by bubbling it through 2 A!! NaOH and then twice through distilled water. Flow rates of 2 to 5 cc./sec. were used. The thin-walled flow tubes were 0.1 or 0.175 em. in diameter. Linear flow rates Tvere 1.5 to 6 m./sec. The Tan de Graaff beam was collimated by an aluminum block with a 3-mm. hole mounted in front of the flow tube. The scavenger solution was shielded from stray radiation by aluminum. Intervals between irradiation and mixing were altered by changing the flow rate, the capillary diameter, or the distance between the beam and the mixing chamber. The flow time was calculated from the flow rate, the capillary cross section, and the distance between the mixing chamber and the electron beam. Relative radiation intensities were measured by collecting the electron beam current and putting it through a current integrator. The current integrator readings were calibrated by flowing 5 X A’ FeSOl in On-saturated, 0.8 ‘3’ H&Y.Id solution past the beam and measuring the ferric ion produced. The ferric yield was taken as 15.5 ions oxidized per 100 e.v. absorbed. All experiments were a t room temperature (23 =t2’). Three runs were made for each experiment: (1) a blank run in which the solutions were passed through the system without being irradiated; ( 2 ) an “infinite time” run in which the scavenger was not added to the irradiated solution until a few minutes after the irradiation was stopped (at that time the short-lived inter-
