Chronopotentiometric Study of the Oxidation of Iodide on Platinum Surfaces in Fused Sodium Nitrate-Potassium Nitrate Eutectic at 250 "C R. B. Fulton and H. S. Swofford, Jr. Department of Chemistry, Unioersity of Minnesota, Minneapolis, Minn. 55455 PREVIOUS VOLTAMMETRIC studies involving the oxidation of iodide at platinum electrodes in the KNO8-NaNO3 eutectic melt at 250 "C(I) have shown the first wave to be a reversible oxidation of iodide to iodine (21- = Iz 2e-). The present work, employing the chronopotentiometric technique, supports and extends the previous work and provides further characterization of the nature of this process.
+
+0.6C
EXPERIMENTAL Apparatus. A multi-purpose instrument, constructed from solid state operational amplifiers, following the basic design presented by G . Lauer ( 2 ) , was used in these investigations. This instrument provided constant currents for the chronopotentiometric studies and also served as a means of monitoring or controlling the potential of the platinum indicator electrode. Chronopotentiograms were recorded with either a Houston HR-97 X-Y recorder or a Tektronix 564 storage oscilloscope used in conjunction with an associated Polaroid camera. Electrodes. The indicator electrodes were prepared by sealing 20-gauge (B & S) platinum wires into the ends of softglass tubes. The resulting cylindrical electrodes were boiled in concentrated nitric acid, dried before use, and examined for cracks. The geometric areas of these electrodes were determined by physically measuring their dimensions with a travelling microscope. The usual Ag/Ag(I) (0.07M) reference electrode was employed in all cases (3); the auxiliary electrode used was a large platinum flag electrode, Medium-porosity sinteredglass sealing tubes were used to isolate the reference and auxiliary electrodes from the bulk melt. Small platinum gauze electrodes of large surface area were used for the massive electrolyses of nitrite when necessary. A detailed description of the melt preparation and cell design appears in previously published works (3, 4). Reagents. Reagent grade chemicals, oven dried and stored in desiccators until needed, were used in all cases. Standard KI Solutions. Before preparing a potassium iodide solution, the residual nitrite in the melt was removed by a prior controlled potential electrolysis [+0.85 V cs. the Ag/Ag(I) reference (511. The various concentrations of iodide were achieved by adding weighed amounts of solid potassium iodide directly into the melt (volume of melt 125 ml). Instrument and Electrode Check. To insure that the instrument was functioning properly, it was checked by the chronopotentiometric determination of the diffusion coefficient for ferrocyanide in aqueous solution. The diffusion coefficient was calculated from the Sand equation and found (1) H. S. Swofford, Jr. and J. H. Propp, ANAL.CHEM.,37, 974 (1965). (2) G. Lauer, Ph.D. Thesis, California Institute of Technology, 1967. (3) H. S. Swofford, Jr. and H. A. Laitinen, J . Ekctrochem. SOC., 110,814 (1963). (4) H. S. Swofford, Jr., Ph.D. Thesis, University of Illinois, 1962. ( 5 ) H. S . Swofford, Jr. and P. G. McCormick, ANAL.CHEM., 37, 970 (1965).
t 5 0
@.*a
L Y
+OP(
I
I
I
1.00
2.00
T
1 s.
(SEC)
Figure 1. A typical chronopotentiogram showing the method of measuring T for the oxidation of iodide
C
= 5.00 mM, A = 0.176 Ern* and i =
196 +A
to be in agreement with published values (6, 7). Because the indicating electrodes used in the chronopotentiometric experiments were cylindrical in shape, they were expected to show deviations from predicted behavior at the longer transition times (8). Convective mass transfer resulting from heat convection in the melt was also expected to cause significant deviations at longer transition times. The magnitude of these effects was examined by determining the diffusion coefficient for silver(1) in the eutectic melt; transition times of about one second or less gave a value of D for silver(1) in good agreement with the value reported by Inman and Bockris at 250 "C (9). At longer transition times significant deviations were observed as expected. Hence, in the work presented in this paper, all transition times were kept to less than 1.240 seconds. RESULTS AND DISCUSSION Table I presents typical data obtained for the oxidation of iodide at platinum electrodes in the fused eutectic melt. Both current density and concentration were varied over a wide range to ensure the reliability of the data. The average value of the chronopotentiometric constant (i71i2/AC)was found (6) P. J. Lingane, ANAL.CHEM., 36,1723 (1964). (7) H. A. Laitinen and I. M. Kolthoff, J . Am. Chem. SOC.,61, 3344 (1939). (8) D. G. Peters and J. J. Lingane, J . Elecrroanal. Chem., 2, 1 (1961). (9) D. Inman and J. O'M. Bockris, ibid., 3, 126 (1962). VOL. 40, NO. 8, JULY 1968
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to be 220 =t5.8. By use of this value in conjunction with the Sand equation, the diffusion coefficient for iodide at 250 "C was calculated to be 6.6 X 10-6 cmz/sec. Assuming reversible behavior, one may write the following Nernst expression for the oxidation 21- = I2 2e- at 250 "C (2.3 RT/F is 0.104 V at 250 "C),
+
Table I. ChronopotentiometricConstants Obtained from the Oxidation of Iodide to Iodine
AC iT1/2
C(mM)
A(cm2) 0.176
i(@)
ra(sec) 0.0165 0.0240 0.0520 0.106 0.183 0.273 0.332 0.405 0.492 0.620 0.836 1.120 0.0266 0.0575 0.0870 0.144 0.230 0.339 0.418 0.552 0.782 1.220 0.0284
304 228 226 249 224 168 218 115 218 87.0 216 71.0 220 65.0 220 59.0 219 53.4 220 47.6 224 41.8 225 36.2 216 706 0.244 2.18 490 221 393 218 305 218 236 213 214 196 211 174 155 216 213 128 215 104 220 1146 0.176 5.00 0.0500 221 872 0.166 220 476 224 0.298 362 0.448 224 295 0.700 226 238 230 1.067 196 228 1.240 181 0.0302 209 2224 7.58 0.244 0.0618 208 1552 214 0.119 1147 213 0.205 872 0.312 213 706 0.448 214 592 215 0.568 527 0.798 219 456 0.944 220 418 0.0203 224 0.176 2760 9.94 0.0336 232 2220 0.0540 223 1680 0.116 223 1146 227 0.209 872 229 0.324 706 229 0.465 590 229 0.715 476 Each value listed is the average of at least two determinations.
0.970
Table 11. Chronopotentiometric Current Reversal Ratios C = 9.94 mM, A = 0.176 cm2 ibA) Tforward@) Treverse(SeC) ratio ( ~ d r r ) 600 0.350 0.130 2.69 600 0.255 0.090 2.84 lo00 0.135 0.043 3.14 lo00 0.110 0.035 3.14 2400 0.022 0.0072 3.06
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ANALYTICAL CHEMISTRY
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