Voltammetric Behavior and Analytical Detection of Oxygen, Superoxide, Hydroxide, and Water Present Together in an Ionic Solvent Pier Giorgio Zambonin Istituto di Chimica Analitica, via Amendola 173, Universitd di Bari, Bari, Italy
Molten salt solutions of superoxide(02-) were unstable in the presence of traces of water. In the (Na-K)NOa eutectic at -500 OK the disproportionation reaction 4 0 2 - 2 H20=40H-+302prevailsand itsequilibrium lies mainly to the right. A voltammetric study, performed on the reacting system, permitted to record a composite current-potential profile characterized by four well-defined limiting-current domains due to the reagents and to the products involved in the stoichiometric equation. The results are discussed in terms of the possible quantitative detection of the species 02,02-,OH-, and H 2 0 present together in the molten solution.
+
THERE IS an increasing interest in the behavior of the superoxide ion (02-) in molten salts, since it has been found that such a species can play a primary role in ionic solvents. In alkali nitrate melts, 02-is the final product of the oxidizing action (1-6) of NO3- on the other electronated oxygen species, oxide (02-) and peroxide (0z2-), according to
and 02'-
+ 2 NOa-
= 2 NOz-
+2
02-
(2)
Analogously molecular oxygen can oxidize 02- to OZ2and/or 02-in molten hydroxide (7), chlorides (8), and nitrates (1-6) according to the Reactions 0'-
+ 0.5
022-
+
=
0 2
0 2
=
02'-
(3)
2 02-
(4)
Furthermore a species, identified as 0 2 - , has been found to exist in molten fluorides (9) up to -700 OK, where it also seems to present a surprising stability under vacuum. O n the basis of results obtained in molten hydroxides (7) and nitrates (I-3), superoxide can be oxidized or reduced according to the polarographically reversible processes =
O2
+e
=
02-
02-
+e
(5)
02*-
(6)
(1) P. G. Zambonin and J. Jordan, J . Amer. Chem. SOC.,89, 6365 (1967). (2) Zbid.,91, 2225 (1969). (3) P. G. Zambonin, J. Elecfroanal. Chem., 24, 365 (1970). (4) Ibid.,App. 25 (1970). (5) P. G. Zambonin and A. Cavaggioni, J. Amer. Chem. SOC.,93, 2854 (1971). (6) J. Jordan, W. B. McCarthy, and P. G. Zambonin, in "Molten Salts," G . Mamantov, Ed., Marcel Dekker, New York, N. Y 1969. (7) J. Goret and B. Tremillon, Bull. SOC.Chim. Fr., 1966,67. ( 8 ) E. P. Mignonsin, L. Martinot, and G . Duyckaerts, Znorg. Nucl. Chem. Lett., 3, 511 (1967). (9) F. L. Whiting, G. Mamantov, and J. P. Young, J. Amer. Chem. Soc., 91, 6531 (1969).
In particular, the existence of Reactions 3, 4, and 5 has recently permitted the attempt (4) of a unitary interpretation of potentiometric findings relevant to the oxygen electrode behavior in molten salts. In all cases the described superoxide chemistry and electrochemistry has been found strictly related to the presence of perfectly dry reaction media. This paper reports a voltammetric study of the interactions between superoxide and water in molten alkali nitrates. The research, other than by the interest it can have per se, has been suggested by the fact that water is a very difficult-to-eliminate impurity of several molten systems. Knowledge of its masking effects can, perhaps, help to rationalize results obtained under conditions of imperfect drying, which is often unavoidable when working with very hygroscopic solvents. EXPERIMENTAL Chemicals. The solvent was a n equimolar mixture (100150 grams) of reagent-grade sodium and potassium nitrate. Potassium superoxide (supplied by Alfa Inorganics) was used without any purification. When necessary, the commercial high-purity nitrogen was further purified by contact with: Ascarite (for COZ); copper wool at -500 "C (for 0,); Dryerite, magnesium perchlorate, and Molecular Sieves, a t -80 "C (for moisture). Apparatus and Procedures. The work was performed using the Rotating Disk Electrode (RDE) cell previously described (IO). The aluminum block thermostat, used for the cell, held a rotating magnet suitable for stirring the solution when necessary. The R D E voltammograms (obtained employing a three-electrode system) are referred t o a Ag/Ag+ (0.07 rn) reference half-cell, connected to the solution via a n asbestos wick. When required (see Figure 1) water was added to superoxide solutions by subliquid-level injections (2-3 seconds) via a long Teflon (Du Pont) needle. This water certainly reached the melt as a vapor stream at the pressure of about 1 atmosphere and because of its (11) high solubility (8 X lo-' mole Kg-I T o r r 1 at 510 OK) could be dissolved t o a large extent. The water concentration was always in relatively large excess compared t o the concentrations of the other species present in solution. During the short injection time, the melt was gently stirred with the rotating magnet. After the injection, the only stirring was due t o the rotating electrode. A slow flux of dry nitrogen was usually maintained over the melt during these experiments. When KO2 was introduced in a partially wet melt (see Figure 3), a constant water concentration was maintained in the solvent by flowing nitrogen, containing water vapor a t a known partial pressure (-15 mm Hg). During experiments such as that described in Figure 2, gas volumes were sampled in the atmosphere immediately over the melt and analyzed for the presence of oxygen. (10) P. G. Zambonin, ANAL.CHEM.,41, 868 (1969). (11) P. G. Zambonin, V. L. Cardetta, and C. Signorile, J. Electroanal. Chem., 28, 237 (1970).
ANALYTICAL CHEMISTRY, VOL. 43, NO. 12, OCTOBER 1971
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l
lh81
POTENTIAL Pt ELECTRODE GREFERENCE
(Volt)
Figure 2. Sequential voltammograms recorded, under the same experimental conditions as Figure 1, after the specified lapses of time ( t = 0 corresponds to the injection of water)
-l,o POTENTIAL Pt ELECTRODE &REFERENCE
(Volt)
Figure 1. Curve A : voltammogram recorded in (Na-K)NOa eutectic containing 2.6 X rn KOz and 1.0 X 10-3 m NaOH Curve B; voltammogram recorded about 1 min after the injection of N 0.1 mi of water on the system represented by curve A Experiment performed under continuous, moderate flowing of dry nitrogen over the melt surface. RDE area 0.017 cm4 rotated at 600 rpm; potential scanning 3 V/minute; T = 510 O K . Curves corrected for residual current; zero current axis arbitrarily shifted Voltammograms on systems containing 0 2 were preferentially recorded scanning the potential from negative to positive values. In this way it is possible to avoid the formation of a small maximum (approximately corresponding to the limiting current hs in Figure 1) present scanning in the opposite direction. RESULTS Injection of Water in Solution of Superoxide. The effects of the introduction of an excess of water in a solution containing 02-are apparent from the experiment, described in Figures 1 and 2, which is a representative example of results obtained at superoxide concentrations between 8 X lo-‘ and 4 x in the temperature range 502-520 OK. Profile A in Figure 1 is characteristic of a solution containing, superoxide (02-)and hydroxide (OH-) together in the absence of water. The symmetric wave hl and hz are due (2, 3 ) to Reactions 6 and 5 , respectively, and ha to the oxidation of OH- according (12) to (12) M. Francini and S. Martini, Electrochim. Acta, 13, 135 (1968).
1572
A (equal to B in Figure l), t N 1 min; B , t t Z 1 0 m i n ; D,t=15min; E , t = 2 h r
OH- =
‘Ir O2 +
+e
H20
N
5 d n ; C,
(7)
Separate experiments have shown that h3 was proportional to OH- irrespective of the presence of superoxide. Under these experimental conditions (vide infra) the ‘limitingcurrents can be expressed by the Levich (13) relations
where [02-],and [OH-], are the molal concentration of 02- and OH-, respectively, and the other symbols have their usual electrochemical significance. Curve B in Figure 1 is the new voltammetric profile recorded after an injection of water on the system represented by curve A . As apparent from Figure 2, it slowly changed as a function of time. The phenomenon was accompanied by development of little bubbles of gas identified as oxygen. The gas evolution decreased with time and was completely absent as soon as the situation represented by Curve E was reached. Introduction of KOz in Water-Containing Melts. A curve qualitatively similar to B in Figure 1 was, also, obtained by introducing superoxide in a wet nitrate melt. This is illustrated in Figure 3, which is an example of experiments performed at water concentrations between 5 X lo-’ and 2 x 10-2rn. Curve A represents (11,14) the wave of reduction (13) B. Levich, “Physicochemical Hydrodynamics,” PrenticeHall, Englewood Cliffs, N. J., 1962. (14) H. S. Swofford and H. A. Laitinen, J. Electrochem. Soc., 110, 814 (1963).
ANALYTICAL CHEMISTRY, VOL. 43, NO. 12, OCTOBER 1971
r
v+nc-= Product
I
40K
I
f - - -_---E
.-..
;9E
Y
a
-1P
-a5
RO
5:
POTENTIAL Pt ELECTRODEsREFERENCE
(Volt)
Figure 4. Current-potential curve recorded in a molten (Na-K)NOa equimolar mixture, 1.5 X 10-4m in hydroxide, bathed with wet oxygen (PO, = 1 atmosphere; PH,O = 0.16 Torr) Experimental conditions as in Figure 1 ,O
-1.0
POTENTIAL Pt ELECTRODE
I
s.REFERENCE (Volt)
The separation between the curve A and B in Figure 4 indicates an irreversible voltammetric behavior of Reaction 7.
Figure 3. Current potential curves recorded under the same experimental conditions as Figure 1, but with the melt under a flux of nitrogen at a partial pressure of ~ ~ mm 1 H 5g
DISCUSSION AND CONCLUSIONS Interpretation of Voltammetric Findings. The modification
of the current-potential curve A of Figure 1 after the addition
Curve A and B recorded, respectively, before and after the introduction of solid KO? corresponding to a theoretical superoxide concentration 5 X lo-' m
of water can be accounted for by the following over-all reaction
of water; curve B the voltammogram recorded after introduction and complete dissolution of a solid sample of potassium superoxide. In a few minutes hr completely disappeared, while h3 increased up to a top value exactly as in the experiment reported in Figure 2. The limiting current hs disappeared (in a considerably longer time) at the end of the gas evolution. Contemporary Presence of 02,HzO, and OH-. For a better understanding of the results reported in Figures 1-3 the polarographic behavior of the electrochemical process, Reaction 7, has been investigated under the experimental conditions of the present work. Figure 4 reports the currentpotential curve of a solution 1.5 X lO-4m in hydroxide bathed with a flux of wet oxygen. The waves A and C are due to hydroxide and water, respectively (see Curves A in Figures 1 and 3). Such waves were not substantially modified by flowing wet nitrogen in place of oxygen; on the contrary, in the absence of oxygen, curve B completely disappeared. In the presence of an excess of water, oxygen can be electroreduced (15) according to the one-step process 0 2
+ 2 HsO + 4 e = 4 OH-
(10)
ix.,the reverse of Reaction 7. By comparison, the theoretical voltammetric waves are reported (dashed-curve) for the reduction of the same concentration of oxygen, to superoxide (Os-) and peroxide (OZ*-),in the complete absence of water (3) (Curves D and E). A further reduction of peroxide to oxide ( 0 2 - ) is possible at more negative potentials, but it can be detected only in solvents presenting a very high cathodic decomposition potential, for instance in molten hydroxides (6). (15) P. G. Zambonin and F. Paniccia, unpublished data.
+
2 0,- H20
f r
2 OH-
+ 1.5
0 2
(11)
It is well known that this occurs between solid NaOz or KO2 and water vapor and, in the reverse direction, between hydroxide and oxygen in particularly dry reaction media (see references 7 and 16). All reagents and products of Reaction 11 were found involved in the electroprocesses responsible for the current domains h3', h4, h6, h~ in Figure 1. Domain ha'. This current was readily ascribed to the oxidation of OH- (compare curve h a and h3' in Figure 1). As noted in the previous section, h3 is proportional to the hydroxide concentration irrespective of the presence of 02-. At the same time (see Figure 1) the symmetry (2, 3) of hl and h2 remains unaltered in presence of OH-. These facts indicate that process (r) in Equation 11 is not very fast, (and/or the equilibrium constant is not very low); otherwise, part of the s u p h x i d e oxidized to oxygen according to Reaction 5 could be regenerated, This should give origin to a kinetic component for h2 destroying the symmetry between h~ and h2, and the linearity of h a with the hydroxide concentration. The absence of kinetic currents makes valid (as anticipated in the previous section; see Equations 8 and 9) the applicability of the Levich relation to both limiting currents of hydroxide and superoxide. For ha' one can write ha'
=
1.2 X lo-' F A D 2 / 3 ~Y ~- ~- Iu'/~[OH-], '
(12)
where [OH-], is given by [OH-],
=
[OH-],
+ [OH-]r
(1 3)
in which [OH-], and [OH-], represent the concentration of (16) I. I. Vol'nov, "Peroxides, Superoxides and Ozonides of Alkali and Alkaline Earth Metals," A. W. Petrocelli, Translation Ed., Plenum Press, New York, N. Y., 1966.
ANALYTICAL CHEMISTRY, VOL. 43, NO. 12, OCTOBER 1971
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Table I. Determination of Concentrations of the Species Involved in Reaction 11 on the Basis of Limiting Current Values Obtained from Current-Voltage Profiles such as Those Reported in Figure 1 (Curve B), Figure 2, and Figure 3 (Curve B ) Species Calculation of concentrations From the current value h’a introduced in the calibration OHcurve a in Figure 5 From the current value h, introduced in the calibration Ozcurve b in Figure 5 From the current value h5” (obtained from Equation 0% 20) introduced in the calibration curve c in Figure 5 From Equation 23 where [OZ-] and [OZ] are calculated Hz as given above and [HzO],is obtained by introducing the current value he in the calibration curve d in Figure 5 C i i t m r i i i I[ i i t irtcifiti itriiiiiitii I W K(-’I
Equations 10 and 16, ha can be ideally divided into two components (see Figure l), i.e.,
Figure 5. Plot of limiting current us. concentrations of specified solutes
hs
=
ha’
+ ha”
(17) For the two components, the Levich equation can be written in the following form
The curve relevant to Ozis dashed to indicate that it has been traced out using only one point other than origin (see text). Experimental conditions as in Figure 1
hs’ = 1.2 X lO-’n’ F A D z ’ S ~ ~ - ~ - ’ ’ b ~ 1 ’ 2 [ 0 z ~(18) ]
hydroxide produced by the reaction of superoxide with water and that which was originally present in solution, respectively. The calibration curve: limiting-current us. [OH-], determined in a parallel experiment, is reported in Figure 5. Domain h4. This limiting current was related to the oxidation, according to process 5, of the unreacted superoxide h4 = 1.2
x
10-3 FAD2/30*-y-l16w1/2[Oz-]
(14)
In all experiments it was verified the relation
where (see Figure 1, and Equations 8, 11, and 12) hz and (ha’ - h3) are proportional to the initial concentration of 0 2 - and to the concentration of the hydroxide produced by chemical reaction, respectively. Dol- and D O H -are the diffusion coefficients (2, 3, 11) for 02- and OH-. The validity of Relation 15 indicates, as required by Equation 11, the existence of a 1 :1 ratio between the superoxide reacted and the hydroxide formed. A calibration curve: limiting-current us. [OZ-] obtained in a separate experiment is reported in Figure 5. Domain hs. This current plateau was related to the reduction of oxygen in the presence of water according to Equation 10 which occurs (see Figure 4) in this region of potentials. At the same time, this wave must be comprehensive of a contribution due to the reduction of the superoxide still present in solution, according to
+
02- 2 H?O
+ 3 e = 4 OH-
(16)
In fact, as clearly shown in Figure 4, the presence of water completely conceals Curves D and E present in a dry melt. In a suggestive way, one can say, that the presence of water “anticipates” the processes of reduction of oxygen and superoxide to hydroxide (Le., to the hydrated form of oxide) and that the reduction occurs in only one step. The separation of waves ha and h6 is in agreement with the polarographic irreversibility of the Process 10. The high value of hs is explainable on the basis of the large number of electrons involved in Relations 10 and 16. On the basis of 1574
hs” = 1.2
x
10-3 n” FAD*/30,-v-1~ew1/2[02]
where n’ and n“ indicate the number of in Equations 16 and 10, uiz., three and Then the individual values of hb’ and ha” on the basis of the experimental values Equations 14 and 18 lhs’l =31h4l and, by introducing Equation 20 in 17
(19) electrons involved four, respectively. can be evaluated h4 and hs. From (20)
(hs”l = ( h t / - 31hr( (21) An unknown oxygen concentration, in a solution containing together Oz and 02-, can be obtained by introducing the value hs” in a limiting-current us. [OZ] calibration curve. An example of such a curve is reported in Figure 5. It has been obtained by combining the current value of Curve B in Figure 4 and the oxygen concentration calculated from the relevant (15) Henry’s coefficient. More work (15) on the electrochemical behavior of oxygen and its analytical detection in melts is in progress. Domain hs. This wave was readily identified as the Swofford and Laitinen (14) water wave, by considering its location on the potential scale and its dependence on water concentration, The electrode reaction for this process can be indicated H 2 0 ne = Products (22)
+
where the value of n (comprised between 1 and 2) is dependent (11) on the water concentration. It must be noted that in the presence of Oz and 02not all the water, which diffuses to the electrode, is directly reduced according to Equation 22 ; it is in part involved in the Processes 10 and 16. When water is present in excess (as in all our experiments), it can be supposed that the 100 of O2and 02is reduced according to Reactions 10 and 16. So, [HzO],, the concentration of water reduced according to Equation 22 will be, see Equations 10 and 16
+
[HzOIr = [HDI - 2{[02l [OZ-I} (23) where [H20], [OJ, and [02-] are the bulk concentrations of water, oxygen, and superoxide. A calibration curve limiting-current us. [HzO], determined
ANALYTICAL CHEMISTRY, VOL. 43, NO. 12, OCTOBER 1971
(in absence of O2 and 0;) in separate (11) experiments, is reported in Figure 5 . Table I summarizes the way to treat the polarographic data for the quantitative resolution of systems containing together OH-, 0 2 - , 0 2 , and HzO. All these species are certainly present whenever moisture and oxygen are in contact with molten solutions containing oxides, see Equations 1-4 and 11. Sometimes, of course, one or more of them can be voltammetrically undetectable; this is, for instance, the case of superoxide ion in the experiment presented in Figure 4. In this case the absence of detectable traces of 0 2 - (even after prolonged oxygen flow) indicates that Equilibrium 11 lies mainly to the right. In effect, successive potentiometric findings have confirmed (17) this qualitative finding; an
equilibrium constant K = 1 X lov3 Kg1l2 could be calculated for Reaction 11 at 503 OK. Then the timedependent decreasing of superoxide concentration, apparent from the curves reported in Figure 2 doesn’t seem due (or at least, not entirely due) to the slow rate with which oxygen, produced by Reaction 11, leaves the solution. Slow chemical steps must be involved in the forward process of Reaction 11. Preliminary kinetic findings have confirmed this supposition. A study (made possible by the analytical tool presented in this work) about the influence of reagents and products on the rate of Process 11 and on its reaction mechanism is in progress in our laboratory.
(17) P. G. Zambonin, J. Electroanal. Chem., in press.
RECEIVED for review March 5,1971. Accepted May 26,1971.
Analytical Evaluation of a Cyanide-Ion Selective Membrane Electrode under Flow-Stream Conditions Bernard Fleet and Henning von Storp Department of Chemistry, Imperial College, London, S . W.7., England Evaluation of a silver sulfide-silver iodide membrane electrode as a continuous monitor for cyanide ions has shown that it is well suited for the measurement of cyanide in the concentration range to 5.10-5M. Serious anion interferences are encountered only from iodide and sulfide ions, and a detailed study of the values obtained for the interference from iodide indicates that the potential-determining mechanism is more complex than the simple replacement reaction; Agl 2CNAg(CN)zIand involves a contribution from the direct displacement reaction AgCN IAgl CN-
Ion-selective electrodes are ideally suited for continuous monitoring, for example, in effluent analysis or in the computer control of industrial chemical processes. The use of these devices in continuous analysis has been reviewed by Light (6). At the present time, however, one of the major limitations to the routine use of these electrodes is the lack of sufficient reliable data on their performance and interference characteristics. The aim of the present work was to evaluate the Orion cyanide member electrode (Model 94-06) under continuous analysis conditions. Selectivity ratios for a range of interfering ions have also been measured.
THE RANGE of solid membrane electrodes originally introduced by Ross et al. ( I ) have found increasingly wide application both for the direct potentiometric determination of anions and cations and also as end-point sensors. Cyanide monitoring is currently of importance in several fields (2), atmospheric and water pollution control, analysis of plating baths and cyanide oxidation systems, and in biological measurement of cyanide-containing plant glycosides (3). Two main types of cyanide electrode are at present commercially available. One of these is manufactured by Orion ( 4 ) and consists of a solid membrane of mixed silver sulfide and silver iodide. The electrode developed by Pungor and coworkers ( 5 ) consists of a silver iodide impregnated silicone rubber membrane.
EXPERIMENTAL
+
+
+
+
(1) M. S. Frant and J. W. Ross, Science, 154, 1553 (1966). (2) L. S. Bark and H. G. Higson, Analyst, 88, 751 (1963). (3) B. Gyorgy, L. L. Andre, L. Stehli, and E. Pungor in “Proceedings of the International Measurement Confederation on Electrochemical Sensors,” Veszprem, Hungary, 1968, p 111. (4) Orion Research Inc., Bulletin 94-06, 11 Blackstone Street, Cambridge, Mass. ( 5 ) K. Toth and E. Pungor, “Proceedings of the International Measurement Confederation on Electrochemical Sensors,” Veszprem, Hungary, 1968, p 35.
Chemicals. All chemicals used were of analytical reagent grade. Stock solutions of 0.1M NaOH and 0.1M NaCN were stored in polythene bottles. Standard cyanide solutions were standardized argentometrically using Liebig’s method. Apparatus. The Technicon AutoAnalyzer was used, the main modules required being a pump I1 and a sampler 11. An Orion Model 801 digital pH meter was used for potentiometric measurements. The output from the potentiometer was displayed on a Servoscribe recorder (Model R E 511 A, Smiths Industries, London). A backing-off circuit was incorporated to increase sensitivity. The electrode (Model 94-06) was obtained from Orion Research Inc., (Cambridge, Mass.) and was modified for continuous analysis by fitting a flow-through cap on to the membrane face (Figure 1). The flow-through cap also contained an agar junction connected to the saturated calomel reference electrode. Procedure. The flow system is shown in Figure 2. Solutions of adjusted ionic strength buffer and sample are fed as
(6) T. S. Light in “Proceedings of Symposium on Ion Selective Electrodes,” Nut. Bur. Stand. (US.), Spec. Publ., 314, 1969, p 349.
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