~~
Table
I.
Anodic Stripping Analysis of Mercury(l1)"
Electrolysis Av. quantity time, t, Concn., C*, &/C*t of electricity, &, (moles/liter ) minutes pcoulombs x 10--6 0.33 1 . 0 x 10-4 345 10.4 1 . 0 x 10-5 2.0 206 10.3 1 . 0 x 10-6 10.0 108 10.8 1 . 0 x 10-7 20.0 20.7 10.4 1 . 0 x 10-8 20.0 2.35 11.7 4 . 0 x 10-9 30.0 1.35 11.2 Data refer to five replicate determinations at each concentration
3, 4, and 5 . Since the amount of material deposited should be proportional to the bulk concentration, C*, and the electrolysis time, t , a measurement of the number of coulombs, &, involved in the stripping peak(s) should reflect this dependency. Thus, assuming all deposited material is removed in the stripping step, &/ t should be proportional to C*. The mercury(I1) concentration was varied from 1.0 X 10-4L\f to 4.0 X l o - 9 ~ , and coulometric data are given in Table I. The quantity &/C*t remains constant over this large concentration range within 3~6.4%. This amount of deviation is not unusual for stripping analysis when the concentration is
Re]. std.
dev., Yo b3.4 2.1 4.1 5.8 4.9 3.4
varied over several orders of magnitude (9). The reproducibility a t a single concentration is somewhat better (Zk2.1 to 5.8%). Rased on limiting current data obtained a t 1.0 X 10-4Alf, the calculated constant, &/C*t, is 11.1 Zk 0.3 pcoulombs-l./mole-min. This quantity is in good agreement with the experimental data, indicating that all of the mercury deposited is subsequently stripped off, regardless of the deposit size. Although extremely sensitive, the technique has the primary disadvantage that, at intermediate concentrations (10-6 and lO-'M), multiple-peaked stripping curves are obtained (Figures 2 and 4). Thus, analysis of mixtures would be complicated. One possible
solution to the problem would be to use more dilute solutions when multiple peaks might interfere with a determination. The problem of multiple-peaked stripping curves is not unique with mercury. The stripping of micr.odeposits of silver from graphite (6) and of nickel from platinum ( 5 ) give similar behavior. Thus, it would be helpful to know more about the nature of the multiple peaks, and further studies are being carried out in this laboratory. LITERATURE CITED
(1) DeMars, R. D., Shain. I.. A N ~ L CHEM.29. 182.5 119.57), (2jIsrae1, Y.,Ibid.,31, 1473 (1959). (3) Jacobs, E. S., Zbzd., 35, 2112 (1963). (4) Korbl, J., Pribil, R., Chemist-Analyst, 45. 102 119561. (~, (5'1 5 ) r; Nicholson, M.>I., ANAL.CHEM.32, 1058 (1960). (6) Perone, S. P., Zbid., 35, 2091 (1963). (7) Perone, S. P., Mueller, T. R., Zbid., 37, 3 lQfi.5) 2 (1965). Perone, S. P., Oyster, T. J., Zbid., (8) F 36. 235 119641. (9) Shain, "Treatise on Analytical Chemistr;"' I. M. Kolthoff and P. J. Elving, eds., Part I, Section D-2, Chap. 50, Interscience, Yew York, 1963. RECEIVEDfor review March 8, 1965. Accepted April 16, 1965. \ - - -
An Electrochemical Study of Nitrite and Oxide in Sodium Nitrate-Potass um Nitrate Eutectic Melts H. S. SWOFFORD, Jr., and P. G. McCORMlCK Department of Chemistry, University of Minnesota, Minneapolis, Minn.
b The quantitative electrochemical determination of nitrite ion in fused alkali metal nitrate melts is discussed, and current-voltage curves are presented to demonstrate the presence of substantial residual nitrite in freshly fused melts. A method for removal of nitrite is also described. A wave has been observed in the eutectic melt at 250" C. which is attributed to the oxidation of oxide ion. Oxalate is proposed as a desirable species for the production of oxide ion in the melt in a concentration range suitable for electrochemical study. The literature regarding the subjects of nitrite and oxide in eutectic melts is also discussed in light of the present work.
T
determination of nitrite ion in fused alkali metal nitrates has been discussed by very few workers. Novik and Lyalikov ( I S ) reported that the oxidation of nitrite could be observed in a current-voltage curve following the addition of iodide to HE QUANTITATIVE
970
ANALYTICAL CHEMISTRY
NaN03-KN03 eutectic melts, nitrite not being seen before the halide addition. Oxide ion has received more consideration in the literature. Delarue (S,4has observed a wave in LiC1-KC1 eutectic a t 400" C. which he suggests is attributable to oxide; no other references have been found for the voltammetric determination of this species. Many methods, however, for adding oxide ion to nitrate melts have appeared in the literature (6, 7 , I S , 17-20); thus far no simple direct method has been suggested which does not leave some question as to the stoichiometry involved. This paper proposes oxalate as a species which is easily weighable and produces oxide in the NaN03-KN03 eutectic melt at 250' C. in the desired concentration range. Also described is a wave which has been observed in the eutectic as being attributable to oxide. Because of substantial residual nitrite, observed during the course of the
present work, an extensive electrochemical study was made of this species in the eutectic melt. A method for removal of residual nitrite is also described. EXPERIMENTAL
Equipment. The equipment used was similar t o that described by Swofford and Laitinen (21) with only minor modifications. The electrolytic cell had a capacity of 250 ml.; investigations were carried out in the bulk melt, rather than in compartments, unless otherwise noted. Temperature control was accomplished by manual adjustment of a variac in the heater circuit. A thermocouple immersed in the melt and connected to a dial-reading pyrometer was used to monitor the temperature; control to & 3 O C. was possible using this arrangement. All current-voltage curves were recorded on a Sargent Model XV Recording Polarograph using a potential scan rate of 0.20 volt per minute. Constant current was obtained from a
,,,trite
COnC
(~~10-3~
a.0.217
b.0362 c = 0.449
Figure 1 . Typical current-voltage curves obtained following successive additions of N a N 0 2 to the melt Electrode a r e a = 0.0770 sq. crn.
Sargent Coulometric Current Source, Model IV. Electrodes. The reference electrode was a spiral of pure silver wire dipping into a solution of silver ion (0.0731 in fused eutectic) contained in a borosilicate glass tube separated from the bulk solution by a medium porosity fritted disk. The indicator electrode was prepared by sealing a length of platinum wire (20 gauge, B. and S.) in soft glass tubing, cutting the exposed wire to a length which produced the desired surface area, and bending this section over a t right angles to the glass tubing a short distance from the sealed end. Three electrodes prepared in this manner, all with areas of approximately 0.1 sq. em., were used in the course of this work. The electrodes were rotated a t 600 r.p.m. by means of a Sargent Synchronous Rotator. Reagents. Reagent grade sodium nitrite, recrystallized twice from water, oven dried a t 110' C., and stored under vacuum over magnesium perchlorate, was used. The purified
I
0.2
I
I
0.4 0.6 NITRITE CONC (mM)
material was assayed using standard titrimetric procedures ( 1 4 ) , and was shown to be 99.95-100.0270 pure K'ah'Oz. Anhydrous potassium oxalate was prepared from the reagent grade monohydrate by heating in an oven at 110' C. for 24 hours. The material was asqayed using standard titrimetric procedures (15) and was 100.O~opure KaC204. All other chemicals were of reagent grade quality, and were oven drled and stored in a desiccator until needed. Eutectic Preparation and Purification. The solid eutectic mixture was prepared by weighing out the proper amounts of the reagents (45 mole % NaN03 and 55 mole % KNO,), mixed by pulverizing in a ball mill for 24 hours, dryed, and stored in tightly closed containers. Before each experiment, the desired quantity of eutectic (240 grams) was weighed out, dried again in an oven for 24 hours, and purified. The details of the purification process have been previously described (21). Techniques. Current-voltage studies were run in the usual manner, scanning in the anodic direction in order to present a reduced platinum surface to the melt a t all times. NaNOl was found to be readily soluble in the melt, while K2C20r was more slow to dissolve. To facilitate the dissolution of the latter, the melt was rapidly stirred with a magnetic stirring bar introduced directly into the flask and rotated by means of a stirring motor positioned beneath the heating mantle. The alnico stirring bar was previously conditioned in a small quantity of molten eutectic. A thin coating of oxide formed on the bar initially, but no further reaction was observed, and no anomalies were seen in current-voltage curves recorded following introduction of the bar into the melt under study. All stirring was discontinued during electrochemical experiments. NOa gas was introduced into the melt by passing the Nz stream (used for purification and stirring) through a tube
I
0.8
Figure 2. Linearity of current due to nitrite oxidation as a function of added NaNOg
I
containing Pb(NOa)2which was being decomposed by heating, and then bubbling the gas mixture directly into the melt. In this way the melt was kept saturated with NO, while maintaining a dense atmosphere of the gas above the surface. The exit gas was passed through two washing towers containing strohg alkali solution. RESULTS AND CONCLUSIONS
Nitrite. Freshly fused and purified melts produced well behaved currentvoltage curves with negligibly small residual currents (defined as 0 to 2 pa. on an electrode of area approximately 0.1 sq. cm.) in the region -0.60 t o +0.20 volt us. Ag/,4g(I). I n the anodic region between f0.20 and +l.OO volt a well defined anodic wave appeared whose size depends on the temperature a t which the melt has been fused. This wave can be attributed to residual nitrite in the eutectic melt. That this is the case was demonstrated by adding weighed portions of solid SaNO, to the melt and rerunning the current-voltage curve. Typical curves for such a study are shown in Figure 1. The current increases linearly with added N O 2 - as shown in Figure 2. The residual nitrite is due to reduction of the melt by organic impurities, and not nitrite impurities in the salts. This was shown by qualitatively testing the salts before melting. They gave negative results with a test shown to indicate nitrite a t a level of O . O O l ~ o . Recrystallized materials give melts with very low residual currents, of the order of 4 pa. on an electrode of about 0.1 sq. cm. To further establish this wave as being due to nitrite, it was decided to duplicate Swofford's work (21) by cathodically reducing the NO3- of the eutectic itself. Since nitrite has been established as a reduction product of nitrate, this seemed a likely method for adding known quantities of the ion. For these experiments, the compartment arrange-
, 0.44
, 0.48 E
0.52
k o l t s w Ag/Ag(di
0.56
I
0.60
Figure 3. Typical wave analysis plot of nitrite wave such as shown in Figure 1 VOL. 37, NO. 8, JULY 1965
971
ment (fritted sealing tubes) described by Swofford and Laitinen (21) was used. Current-voltage curves were run after each increment of constant current generation. Good agreement was achieved with the results described by Figure 1 and 2. Finally, to demonstrate that nitrite is the sole species responsible for the wave, the eutectic was saturated with NOZ gas, and current-voltage curves were run. A very slight (3 pa.; electrode area -0.1 sq. cm.) enhancement of the nitrite wave was observed, and this was completely removed with a few minutes of bubbling with Nz. No other electrochemical activity was observed in the entire range covered by our studies [-1.00 to +1.00 volt VS. Ag/Ag(I)]. I t would appear that NOz gas is practically insoluble in the melt since, though the melt acquired a distinct yellow-brown color due to dissolved gas, the color was completely removed in just a few minutes by bubbling with Nz. The lack of any distinct change in the polarogram when the melt was saturated with NO2 seems to indicate that the gas exhibits no significant electrochemical activity. The residual nitrite can be removed, or reduced to a negligible level by controlled potential electrolysis. Excellent results have been obtained in the present work by poising the working electrode (consisting of a Pt gauze electrode of large surface area) a t +0.90 volt vs. Ag/ Ag(1). A massive P t electrode (area -2 sq. cm.) in a fritted sealing tube served as counter electrode. During electrolysis, NOz gas is seen to be evolved a t the electrode surface, suggesting that the anodic process proposed by Lyalikov and Novik (ff)viz., NOz- = NOz e
+
is correct. This proposition was further supported by placing a silver coulometer in series with the controlled potential electrolysis apparatus during nitrite removal. The number of mmoles of silver transferred agreed within 5y0 with the number of mmoles of nitrite removed, assuming the above mechanism. The nitrite wave, both residual and that due to added nitrite, is well behaved and a plot of E vs. log i/(& - i) produces a straight line as shown in Figure 3. The slope of this line, 0.097 volt, is indicative of a oneelectron oxidation with an Ellz of f0.476 volt vs.Ag/Ag(I). A more thorough discussion of the probable electrode mechanism will appear in a forthcoming paper (26). Oxide. The cathodic reduction of the melt described above produced, besides the nitrite wave, another anodic wave between -0.20 and f 0 . 2 0 volt, whose magnitude is also directly proportional to the generation current. 972
0
ANALYTICAL CHEMISTRY
oxalate conc ( ~ ~ 1 0 - 3 ) a:0.404
b=0.753 c.1.043
-80
/
Ill
Figure 4. Typical current-voltage curves obtained following successive additions of K&04 to the melt Electrode a r e a = 0.1 3 8 7 sq. cm.
In view of Swofford and Laitinen's establishment of oxide as the other product of nitrate reduction, it is postulated that this wave may be attributed to the oxidation of oxide. I t was necessary to find a method of adding oxide to the melt in exactly determinable amounts, without adding nitrite as well. Shams El Din (27-20) has added both KOH and N h O z to molten KNO3 a t 350' C., and claims to have obtained stoichiometric amounts of oxide in both cases. Kust and Duke ( 7 ) have electrochemically reduced oxygen gas a t a Pt electrode and also claim to have obtained stoichiometric concentrations of oxide ion. Kust (6) added oxide via carbonate ion, but showed the decomO+ position--,e., cos-' = COZ to be too incomplete for our purposes. Adding oxide via metal oxides limits the obtainable concentrations to the (relatively low) solubilities of metal oxides in the melt. In an extensive study of various cement systems by workers a t the NBS, potassium oxide was added to limealumina samples as the corresponding oxalate, which decomposed a t high temperature yielding the desired oxide. Based on the work of Brownmiller (b), Taylor states ( S 4 ) , "Potassium oxalate was used as the source of KzO since it is not hygroscopic and weighing difficulties are eliminated by its use." I t was decided to attempt the use of oxalate in this work as a source of oxide ion. Due to the lower solubility of sodium oxalate in this melt, it was decided to prepare and use pure potassium oxalate. Addition of KzCZ0,to the melt produced a wave identical in appearance and a t the same potential as the wave obtained by direct reduction of the melt.
+
Successive additions of oxalate increased the wave height, producing current-voltage curves typified by those shown in Figure 4. The limiting current was linear with added oxalate as is demonstrated in Figure 5. A plot of E vs. log i/ (id - i) is shown in Figure 6 to be linear with slope 0.12 volt and E l / z of +0.024 volt vs. Ag/ Ag(1). The wave analyses for the constant current generation of oxide and that for oxide added as oxalate give the same slope. This value for the slope would appear to indicate the possibility of an irreversible electrode oxidation. I t is interesting, however, to note that if one assumes reversibility, the slope is more indicative of a one-electron transfer than of the expected two-electron transfer. This phenomenon has been reported by others in the literature (8, 17'). A possible explanation for this fact and a proposed mechanism for the electrode reaction is contained in a forthcoming paper (92). DISCUSSION
The subject of nitrite determination in melts has been discussed by Lyalikov These workers and Novik (9-13). used extremely concentrated solutions (15-200 mg. of nitrite/30 grams melt) and a dipping Pt needle indicating electrode in their investigations. Their wave appeared a t roughly the same potential as observed in the present work (values corrected for the difference in reference potentials), and showed a temperature dependence of 0.8%/degree (fa). The Russian work, carried out in a temperature range of 260" C. to 400" C. showed no decomposition of the melt up to400" C. (ff), and the nitrite wave to be absent in the supporting electrolyte before addition of iodide (13). We have been unable to confirm these observations in our investigations. It was found in these investigations that the nitrite wave is indeed present in the residual current-voltage curve of the melt, and grows rapidly with increasing temperature. I t would seem that a t temperatures as high as those used by IVovik and Lyalikov the melt should have contained large quantities of residual nitrite in the voltammetric sense. The only explanation for their not seeing it would appear to be the use of a relatively insensitive current measuring device; their electrode area was approximately half that used here. It is also interesting to note that work done in this laboratory by Propp (23) with dissolved halides has shown that the anodic oxidation wave of bromide occurs a t almost the same potential as that for nitrite at 250" C., and that for iodide is only slightly displaced from nitrite. Under the conditions of the experiments as carried out by Novik and Lyalikov, it seems remarkable
/a I
0.5
1.0 OXALATE CONC. (mM)
1.5
Figure 5. Linearity of current due to oxide ion oxidation as a function of added K&04
that they could have observed distinct waves for these species. Finally, their published currentvoltage curve for iodide (IS) appears to have points plotted in the region beyond +2.0 volt us. Pt reference electrode. Since the melt is oxidized anodically a t 1.2 volts us. Ag/ Ag(1) and the potential of the Pt reference electrode is roughly 0.2 to 0.25 volt more positive than Ag/Ag(I), it seems unlikely that a voltage of +2.0 volts is attainable. The literature contains a great deal of ambiguity concerning the subject of oxide ion in fused salt solvents. In their study of iodide, Novik and Lyalikov (13) postulated the production of both nitrite and oxide in equal amounts from the reaction of iodide with nitrate. They did not, however, report seeing the oxide wave even though they reported a wave for nitrite. Furthermore, investigations in this laboratory by Propp (83) have shown that the reaction as proposed by Novik and Lyalikov does not occur to any appreciable extent. I t would appear under the conditions of their experiments that the residual nitrite wave and the wave produced by iodide are indistinguishable from one another. There is also serious disagreement among reports in the literature regarding the methods used for the addition of oxide ion to these solvents. Shams El Din and Gerges have published extensively (17-20) concerning their potentiometric acid-base studies in molten KNO3. As a source of oxide they have used KOH and, more recently, Na202. Their calculations indicate that they postulate complete reaction and stoichiometric oxide ion production from both species. Our work, conversely, shows that KazOzis only slightly soluble in the melt, solid material being visible after saturation has been achieved. Moreover, current-voltage curves of fusions containing added Na2OZshow a wave in the cathodic region distinct from that of oxide which increases with added
+
Figure 6. Typical wave analysis plot of oxide wave such as shown in Figure 4
peroxide and diminishes with nitrogen bubbling. I t seems likely that a reductant other than oxide results from the addition of peroxide. Work is continuing on this matter. Bennett and Holmes (I) have added hydroxide and peroxide to fused alkali metal nitrate to stabilize complexes of manganese. Their work, involving spectrophotometric studies, assumes that hydroxide added to the melt remains as such. Though their work was done a t 260' C., it seems hard to believe that the 100' difference between this temperature and that used by Shams El Din (360" C.) could effect complete conversion from hydroxide to oxide. Another point which Shams El Din has failed to account for is the thermally dependent equilibrium between the various oxides of chromium. I t has been shown by x-ray examination that higher chromium oxides (in chromatedichromate melts) are rapidly transformed into the corund modification of Cr203a t temperatures below 400' C. (6). Furthermore, extensive studies (16) of the thermally-dependent interconversion of various dissociation products of CrO3 has shown significant decomposition to occur a t 240" C. I t would be necessary, therefore, for Shams El Din to demonstrate that all the dichromate added to his melt is available as such for neutralization before the applicability of his derived relationships can be accepted. Finally, Shams El Din himself has been inconsistent as to disposal of the water formed in his various neutralization reactions. At one point (19) he states that in titrations of dichromate, the water formed remains dissolved in the melt, and his calculations take into account its concentration. In titrations of phosphate species (again with Na2Oz) he states (18) that water formed during titration continuously evaporates during the course of the reaction. It would seem that resolution of this point is necessary before one can estab-
lish the fundamentality of his neutralization reactions and acidity scale. In their work with the oxygen-oxide electrode, Kust and Duke have claimed (7') that they produced oxide by electrochemically reducing oxygen gas. Experiments in this laboratory have failed to reproduce their work. The current due to oxide oxidation remains linear with constant current generation (and also with controlled potential electrolysis) regardless of whether or not oxygen gas is being bubbled over the electrode surface. Two items point to the conclusion that it was electrochemical reduction of the melt itself, and not oxygen, that was producing the oxide : current in the nitrite wave also increased linearly with constant current generation, and the working electrode was observed potentiometrically to be depolarized a t a potential much more cathodic than that proposed as the potential of the oxygen-oxide couple. Our electrode area and current density were somewhat larger than Kust and Duke's, but to firmly establish their contention of 100% current efficiency for this process it would be necessary to show that current in the oxide wave was linearly dependent on the generation current and that current for the nitrite wave remained constant during the electrolysis. As a final point of clarification, the observations of Bennett and Holmes can be easily explained in light of this work. They have noted ( I ) that both Mn(I1) and Mn04- are converted to MnOz in the melt. The oxidation of Mn(I1) by the melt is not surprising since the molten nitrate is a very strong oxidizing medium. Our work with permanganate has shown that both oxide and nitrite are easily oxidized by this species. If both are present we have shown that oxide is preferentially attacked, nitrite concentration remaining constant, and MnOz being the product of manganese reduction. In the p r e s VOL. 37, NO. 8, JULY 1965
973
ence of nitrite alone, YO2 gas and MnOz are the products. ACKNOWLEDGMENT
Thanks are due to J. H. Propp for his very helpful discussions and suggestions.
( 7 ) Kust, R. N., Duke, F. R., J . A m . Chem. Sac. 85, 3338 (1963). (8) Littlewood, R., Argent, E. J., Electrochim. Acta 4 , 114 (1961). (9) Lyalikov, Yu. S., Zh. Analit. Khim. 8 , 38 (1953); C . A . 4 7 , 5 2 7 3 ~(1953). (10) Lyalikov, YII. S., Novik, R. 31.) PolyarograJia Raspluvlen Solei, Akad. Nauk Ukr. SSR, Inst. Obshch. i Neorgan Kham 1962, 41; C.A. 57, 11857h
(1962).
LITERATURE CITED
(1) Bennett, R . M., Holmes, 0. G., Can.
J . Chem. 41, 108 (1963). (2) Brownmiller, L. T., A m . J . Sei. 29, 260 (1935). (3) Delarue, G., J . Electroanal. Chem. 1 , 13 (1959/60). (4) Zbid., p. 285. (5) Flood, H., Muan, A,, Actu Chem. Scund. 4,364 (1950). (6) Kust. R. E.,Znoro. Chem. 3. 1035 (1964):
i l l ) Lvalikov. Yu. S.. Novik. R. hl..
Uch.* Zap. kzshznevsk. Gos. Unzv. 27;
61 (1957), C.A. 54, 22101d (1960). (12) Xovik, R. M., Tr. 1-01 (Pernoi) Kauch. Konf. Rfolodvkh ITchenvkh Maldanii Kishinev 1958, 73; C.A.”55, 19611i (1961). (13) Novik, R . hl., Lyalikov, Yu. S., Zh. Anal. Khim 13, 691 (1958). (14) Rosin, J., “Reagent Chemicals and Standards,” 4th Ed., p . 411, Van Nostrand, Princeton, ?;. J., 1961. (15) Ibid., p. 343.
(16) Ryss, I. G., Selyanskaya, A . I., iicta Physicochim URSS 8, 623 (1938); C.A. 33,6697 \119.14) ^“VV,. (17) Shams El Din, A. AI., Electrochim. Acta 7, 285 (1962). 1181 Shams El Din, A . N., Gerges, A . A. A., Zbid., 9,123 (1964). (19) Zbid., p. 613. (20) Shams El Din, A. AI., Gerges, A . A . A,, J . Electroanal. Chem. 4. 309 (1962). (21) Swofford, H. S., Laitinen, H. A,, J . Electrochem. Soc. 110, 814 (1963). (22) Swofford, H. S., McCormick P. G., University of lIinnesota, unpublished data. (23) Swofford, H. S.,Propp, J. H., ANAL. CHEW37,974 (1966). (24) Taylor, W. C., J . Res. S a t l . Bur. Std. 21, 315 (1938). RECEIVED for review January 18, 1965. Accepted hlay 3, 1965. Work supported by the State of llinnesot,a and the Procter and Gamble Co. I-
A Voltammetric Study of the Oxidation of Iodide and Bromide in Potassium Nitrate-Sodium Nitrate Eutectic Melts H. S. SWOFFORD, JR., and J. H. PROPP School of Chemistry, University o f Minnesota, Minneapolis, Minn.
b Both I- and Br- exhibit reversible 1-electron anodic waves at a platinum electrode in the KN03-NaN03 eutectic melt at 250” C. Formal potentials for the 1-/12 and Br-/Brz couples are presented from these waves. Asecond wave, previously ascribed to the formation of NOS- upon addition of I-, is discussed as arising from a continued oxidation of 12.
T
a conspicuous lack in the literature of work involving the oxidation of halides a t platinum surfaces in oxy-anion melts. The Russian investigators, R. M.Novik and Yu. S. Lyalikov (2-4) appear to be among the few workers who have studied this problem. Their interpretation of experimental results, however, is questionable, and the present work deals with an extension of their studies as well as a different interpretation of the experimental findings. HERE IS
EXPERIMENTAL
Apparatus. ,111 current-voltage curves were recorded with a Sargent, hlodel XV, polarograph. Potential measurements were made with a n L & N, Type K-3, Universal Potentiometer. Constant current for electrolysis was obtained using a Sargent, Model IV, coulometric current source. Electrodes. A rotating platinum microelectrode (area -0.074 sq. cm.) 974
ANALYTICAL CHEMISTRY
55455
served as the indicating electrode in all cases, and a Sargent Synchronous Rotator was used to rotate the electrode a t a constant 600 r.p.m. The electrode was prepared by sealing a piece of platinum wire in the end of a soft glass tube and then bending it a t a right angle to the tip. The electrode was polished with jeweler’s rouge and boiled in ”03 before use. The usual Ag/Ag(I) (0.07M)reference electrode was employed (6). Mediumporosity sintered glass sealing tubes were used to compartmentalize the melt when necessary. Small platinum gauze electrodes of large surface area were used in the massive electrolysis work. A more detailed description of the above mentioned instrumentation appears in previously published work ( 6 ) . Reagents. Reagent grade chemicalq, oven dried and stored in a desiccator until needed, were used in all cases. RESULTS A N D DISCUSSION
Table I represents a standard addition study involving the limiting anodic currents observed for the oxidation of both iodide and bromide a t a rotating platinum electrode in the fused eutectic mixture for various additions of K I and KBr. In both cases the current values fall on straight lines passing through zero with slopes of 2.92 X lo4 pa./M and 3.27 x IO4 p a . / M for KI and KBr, respectively. This work supports that of Swofford ( 5 ) who has also shown
iodide and bromide to be stable in the eutectic melt a t 250’ C. by potentiometrically titrating these species with electrochemically generated Ag(1). Upon addition of K I to the melt, it is observed that the anodic wave resulting from the oxidation of residual S O 2 - ( 7 ) is increased in magnitude. Novik and Lyalikov (4) have attributed this to the follou-ing reaction, 2KI
+ KNOs
=
Iz
+ KNOz + K,O
For the above reaction to be correct with the equilibrium lying to the right, an additional wave for oxide should appear in the current-voltage curve, and the anodic iodide wave resulting from the addition of iodide to the melt should be diminished. To ascertain if additional NOz- is really produced, or whether the increased current is due to some other cause, controlled potential electrolyses were carried out. Reference to Figure 1 shows that before the addition of KI the current-voltage curve for residual NOZ- exhibited a current of 20 pa. a t +0.70 volt. Addition of KI produced a limiting current of 15 pa. at f0.25 volt for the oxidation of I-, while the limiting current for the NOsoxidation increased to 76 pa; thus, it would appear that 56 pa. of S O z - were produced. The iodide was then removed by controlled potential oxidation a t t0.30 volt, and the product, I?, bubbled out of the melt. Following the
