r
= = AE = 4a =
t
=
radius of wherical electrode time shift of potential during decay variation of charge density on electrode upon application of charging pulse at t = 0 decay constant LITERATURE CITED
(1) Delahay, P., ANAL. CHEY. 34, 1267
(1962).
( 2 ) Ibid., p. 1161.
( 3 ) Delahay, P., Anal. Chini. Acta 27,
90 11962). (4)Ibid., in press. ( 5 ) Delahay, P., “Kew Instrumental Methods in Electrochemistry,’’ p. 213, Interscience, New York, 1954. (6) Grahame, D. C., Parsons, R., J . 4 m . Chem. Soc. 83, 1291 (1961). (7) Meites, L., “Polarographic Techniques,” p. 277, Interscience, Sew York, 19.5.5. _...
(8) Ibid., p. 294. ( 9 ) Reilley, C. S . , Cooke, W. D., Fur-
man, X. H., ANIL. C m x 23, 1030 (1951). (10) Reinmuth, W. H., Zbid., 32, 1509 119601. ( 1 l ) Reinmuth, IT7. H., Kilson, C. E., Ibid., 34, 1159 (1962). (12) Ross, J. D., DeMars, P. D., Shain, I., Ibid.,28, 1768 (1956). RECEIVEDfor review July 27, 1962. Accepted August 22, 1962. Research supported by grant G-10006 from the Kational Science Foundation.
Mercury-Film Electrode for Precision Voltammetry STEPHEN A. MOROS Research Service Departmenf, Organic Chemicals Division, American Cyanamid Co., Bound Brook, N . 1.
b An electrode, useful in various voltammetric procedures, i s prepared simply and rapidly b y a novel abrasion method for coating platinum with mercury. The potential-sweep amperometric response can be successfully observed with commercial recording polarographic equipment. The peak height was sufficiently sensitive, reproducible, and linear with concentration to permit determination of nitrobenzene in aniline down to 0.07 p.p.m. (0.6 pF). Measurements at the 200-p.p.m. (1.6 mF) level with a relative standard deviation of &0.67% were feasible using a single electrode whose calibration remained constant for at least 2 0 months. Preliminary work on various cations, anions, miscellaneous organic compounds, and oxygen, as well as stripping methods for metals and halide ions, indicated the versatility of the electrode; it should also be useful for chronopotentiometry and microcoulometry.
hanging drop, and plated electrodes for voltammetry are reviewed by Bruckenstein and Sapai (,2), Seeh ( I I ) , and Shain and Polcyn (15). While these types have advantages in special cases, a stable, reproducible electrode which could be prepared simply from commercially available material.. and used in conjunction i~ith standard polarographic equipment seemed to be a desirable supplement to the electrodes previously described, particularly for control allplication. This paper records the preparation. properties, and application of an electrode IT ith the..? characteristics. ERCURT POOL,
EXPERIMENTAL
Preparation of t h e Electrode. The niercury-film electrode (MFE) consists of a commercial platinum-in-
1584
ANALYTICAL CHEMISTRY
glass inlay (Beckman S o . 39273) with the metal surface completely covered with mercury. T h e platinum can be covered either b y abrading the surface while i t is submerged in mercury or by means of sodium amalgam. Abrasion is the simpler of the techniques, but not applicable to irregular surfaces, in which case the platinum may be treated by dipping i t into sodium amalgam under a layer of ethanol and alternately raising and lowering it into the amalgam until the platinum surface is completely oovered; excess sodium is then removed by immersing the electrode in water. -4flat-surfaced electrode, such as that employed in all of the work described here, is readily covered with the aid of S o . 600 abrasive paper glued to the narrow end of a rubber stopper; this s e n es as the bottom of a container for mercury. The platinum inlay is twisted against the abrasive surface until it is completely c o ~ e r e dwith an adherent layer of mercury. Excess mercury is removed by tapping the electrode against the edge of a polyethylene beaker which serves as a waste mercury receptacle, and the electrode is ready for use. Film electrodes thus prepared retained, on the alerage, a 10-micron coating of mercury; this limit was estimated from the combined weight (31 nip.) of 10 films squeezed off the platinum by rolling a short length of plastic tubing over the surface of an electrode in which the platinum disk had a diameter of 5.58 mm. Because the abrasion method is instantaneous, it is preferable to electroplating procedures often recommended for preparing t h r electrode or repairing the film if it becomes discontinuouq for any reason. Electrodes prepared bv this technique have been stored in contact with a drop of mercury for month. without changing characteristics. Equipment. I n use the film electrode rrplaces the dropping mercury electrode; only the conventional equipment for recording polarograph! is required. I n this work, a n H-cell containing a sintered glass plug and a saturated KC1-agar bridge was used
as the electrolysis veqsel. -1 convenient form of replaceahle reference electrode (Figure l ) , conqisting of a Corning S o . 39533-12C coarse porosity gas-dispersion tube containing calomel (0.5 gram pressed into the pores with a glass rod which fits loosely into the sintered cylinder) and filled ivith mercury, was immersed in saturated potassium chloride solution in the reference compartment of the cell. -1Sichrome n ire inserted through the rubber medicine dropper bulb into the mercury provided the external electrical contact. (This form of SCE has an extremely low resistance; two such electrodes immersed in saturated KC1 solution and connected in series-opposition have a resistance of only 0.4 ohm as indicated by a model RC 16B Industrial Instruments, Inc., conductivity bridge.) The qolutions and cells were kept at 25.0’ C. by means of a thermostated water bath. Curves were recorded by means of a Leeds &- Sorthrup Type E Electrocheniograph or by means of a separate recorder attached to a Fisher Elecdropode bearing the motor drive accessory; both have voltage scanning rates of 200 mv. per minute. PROPERTIES A N D APPLICATIONS
The fundamentals of potential-sweep amperometry with stationary planar electrodes have been treated extensively ; earlier treatments were reviewed by Delahay ( S ) , Reinmuth (13) and Nueller and Adams (10). I few of the practical aspects of analytical interest will be reviewed here. After a brief description of the behavior of the electrode, the details of a practical analytical application-ie., the determination of nitrobenzene in anilinen ill illustrate the advantages and limitations of the method. Finally, applications currently under invectigation and other areas of potential applicability nil1 be considered. General Procedure and Typical Response. The 1 I F E is immersed in
the deaerated sample solution and
Re'ere n:e
Electroee'
Figure 1 . low resistance reference electrode and mercury-film electrode
connected to the lead ordinarily employed for the dropping mercury electrode. With t h e reference electrode in position. the output voltage of t h e polarograph is connected, b u t without turning on the voltage scanning motor. The initial current surge noted (Figure 2 ) falls exponentially with time ( k = A log i/A t was 1.0 min.-' under the conditions recommended for the analysis of aniline) and after a few minutes the backgound current becomes constant. In this case the low resistance (170 ohms) and lorn current involved mean that controlled-potential conditions were attained in fact, if not in principle, despite the use of a controlled applied voltage source. While often neglected, the importance of this controlled potential preelectrolysis step cannot be overemphasized. It succeeds in bringing the electrode and adjacent solution into a n equilibrium or steady state. I n the process, the double layer is charged and traces of electroactive material in the mercury, on its surface, and in the diffusion layer are converted to forms stable a t the potential of the preelectrolysis. While this process is rapid and constitutes only a minor drawback in practical analytical applications, it is not instantaneous, and, if ignored, produces results which are, at best, difficult to interpret. Subsequently, when the potential sweep is initiated, the charging component is observed as a small increase in the background current. The reduction of the electroactive species of interest is manifested as a peak in the potential-sweep amperogram. As the voltage is scanned, the electrolytic process ensues and the current rises. I-Iowever, in the absence of stirring or drop renen-all the concentration of electroactive material in the layer of solution immediately adjacent to the electrode surface is depleted by the electrode process; the result is a decreasc in current. The current observed thus depends on the extent of electrolysis (which can be expressed in terms of the rate of variation of voltage) in addition to the more familiar parameters of electrochemistry.
E vs SC E Pre-elecrrolys s
,olts
>ale?- a
5LLeep
Figure 2. Typical potential-sweep amperograms obtained when the recommended procedure was applied to aniline Upper curve. Aniline containing 1 4 8 p.p.rn. of nitrobenzene lower curve. Aniline containing no nitrobenzene
For
3
reversible process:
i, = 2728
nalzvl/2D1/2C
(1)
There i, is the peak current (in pa.) observed with an electrode area A sq. cm. immersed in a solution containing C moles per liter of an electroactive material (whose diffusion coefficient is D in sq. cm./sec.) undergoing an electrolysis involving n faradays/mole; v is the rate of variation of potential in volts per second. For a homogeneous reversible reduction, the relation between peak potential, E,, and the polarographic half-wave potential, El,*, has been expressed as: E, = E1,z
-
1.1 RT/nF
(2)
I n this case, as well as for the reversible deposition of a solid and for the irreversible electrode process, the peak current is proportional to concentration. I n all cases, the peak potentials are related to the half-wave or standard potentials, and, in principle, they can serve for qualitative purposes. Only when a solid product is involved is the peak potential a function of concentration and the qualitative utility of this method impaired. Application to t h e Determination of Traces of Nitrobenzene in Aniline. The method of preparing the sample is similar to that of K'ovack (1%'). The sample of aniline is mixed with concentrated hydrochloric acid. The aniline hydrochloride formed redissolves in the aniline where it performs the dual role of supporting electrolyte and buffer. Details of the recommended method follow. Rinse the sample compartment of a polarographic H-cell with distilled water and with ethanol; remove the alcohol b y aspirating air through it. Pipet 25
ml. of the aniline to be tested into the sample compartment. Place the cell into a constant temperature bath. Add 5.00 ml. of concentrated ACS hydrochloric acid. Pass a moderate stream of nitrogen gas through the mixture for 10 minutes. Rinse the mercury-film electrode with alcohol and wiue i t drv with filter Dauer. Rinse the elecirode su"rface with 2'drops of clean mercury and t a p the electrode to remove excess mercury. The disk should appear to be a round, shiny mirror-like spot without discontinuities in the mercury film or ragged edges. If these are visible, rub the electrode surface with filter paper or repeat the abrasive treatment; rinse with mercury and remove the excess. When the sample has been deaerated, divert the gas stream over the surface of the solution and insert the electrode below the surface of the solution, taking care to avoid trapping gas bubbles below the electrode. Connect the electrode to the lead ordinarily used for the dropping mercury electrode. Insert the saturated calomel reference electrode into the reference compartment of the polarographic cell. Adjust the polarograph to provide an output of -0.35 volt and, for the purest aniline samples, a sensitivity of 1 pa. (full scale). Connect the electrodes and wait 2 minutes for the current to become constant. Turn on the chart motor and the voltage scanning motor. When the potential reaches -0.6 volt, turn off the chart and scanning inotors and disconnect the electrodes. Remove the film electrode, rinse it thoroughly with alcohol and blot it dry with filter paper. Rinse the sample compartment of the polarographic (,ell with alcohol and with distilled water. Measure the peak current, i,,, as the vertical distancc between the average currents a t -0.40 and -0.56 volt (Figure 2). Calculate the nitrubcnzene content from the peak current corrected for the blank and the calibration factor VOL. 34, NO. 12, NOVEMBER 1962
e
1585
for the electrode previously determined by applying this method to synthetic standard mixtures. RESULTS A N D DISCUSSION
Qualitatively, the reduction of nitrobenzene in the HC1-aniline mixture is characterized by a peak at, -0.56 volt ~ 8 SCE . (Epl*= -0.50 volt) in the entire range (0.3 to 211 p.p.m. or 2pF to 1.7 m F nitrobenzene in aniline) examined and is independent of concentration. The polarographic wave under these conditions is characterized by -El;? = -0.537 volt, El 4 - J;.'3,4 = ,ai mv.; I ( = id/~rn.?'~t''6)= 1.75 for the mixture. Quantitative Response. K i t h the procedure detailed above, eight synthetic mixtures (0.30 t o 211 p.p.ni.) of nitrobenzene in chlorobenzenederived aniline were examined with the Electrochemograph using a n electrode having a n area of 0.25 sq. cm. Instead of measuring t h e current a t -0.40 volt as recommended previously for extremely dilute solutions, the mixtures containing over 50 13.p.m. were measured a t -0.35 volt to avoid encroaching on the foot of the peak. The results are summarized in Table
Table 1.
c,
Quantitative Response
i,, pa. 0.059
c/t' P I corr.a 0.30 6,250 1.48 0 220 (7.08) 2 . 9 6 0.476 6.366 7 . 4 0 1.186 6.258 '14.8 1.38 6.245 148. 23.3 6.352 211. 32 3 (6.532) Mean 6.302 p.p.m./pa. Std. dev. & O . 056 Rel. std. dev. =l=0.89?; a i,, corr. is i, - 0.011 pa., the background correction. p.p.m.
Table II.
Precision Study
Preelectrolysis time, i-o36voltj Run min. pa. A 2 1.00 B 2 0.55 0 65 c 2 D 2 0 54 E F G
3.5 2 1
H
1
0.42 0.45
0.66
0 66
i, (corr. for i-ot ~ ~ ' ~ l t , pa.
22.85
23.15 22.95 23.0;
22.95 22.86 22.65 23.01
Mean 22.94 pa. Std. dev. + O . 15 pa. Rel. std. dev. & 0 . 6 j % Aniline containing 148 p.p.m. of nitrobenzene (1.2 mF) repeatedly examined by the recommended method using an electrode which had previously exhibited corrected peak currents of 22.7 (6 months), 22.4 and 22.7 pa. (21 months) prior to the series of measurements
1586
e
ANALYTICAL CHEMISTRY
I ; it is evident that the response is essentially linear over almost three orders of magnitude of concentration when the appropriate correction for the background current is included. The deviation in response a t the highest concentration may reflect the onset of the iR drop effect. Reproducibility. Several other quantitative features of the recommended method are illustrated by the d a t a in Table 11. T h e reproducibility of t h e method and the possibility of repeatedly measuring a particular sample were investigated, as was the effect of time elapsed between the preparation of a fresh mercury film and the measurement of the sample. film electrode (0.244 sq. em. area) vrhich had been stored in contact with mercury for 6 months was washed with se\eral drops of mercury and the excess removed. The electrode was then allowed to remain in contact with air (warm, humid atmoyhere) for 2 hours before being immersed in deaerated hydrochloric acid-aniline mixture. Eight amperograms were recorded; a stream of nitrogen bubbles impinging on the mercury film was used to rinse electrolysis products away from the electrode surface between runs. Table I 1 includes all the values obtained in the experiment as well as the results obtained with this electrode-and-solution combination 6 months and 21 months prior to this eyperirnent for comparison. Table I shon s that repeat amperograms are not only possible but also are highly reproducible. This was not the caqe with the pool electrode of Rooney ( I d ) . I t is true only \Then (as in thii race, for example) the products of a homogeneous reaction can be swept away from the electrode surface between measurement.. With the MFE this is accomplished with a stream of nitrogen bubbles which also stirs the bulk of the test solution efficiently. If a reversible electrolytic reaction resulting in the formation of an amalgam or film on the electrode surface is involved, the amalgam or film must be electrolytically stripped prior t o stirring with nitrogen and repeating. The ease with which the measurements can be repeated without replacing the electrode facilitates its application to amperometric titrations and methods involving standard additions. The high precision is unusual in voltammetry; for this reason it is important to note that an instrument providing a t least the precision cited above is required. The relative standard deviation obtained with the Electrochemograph employed i ~ a s found to he ~kO.27, when measuring currents flowing through a purely resistive load. Another factor possibly contributing to the high precision obtained involves the high viscosity of the medium. The
disruptive influence of vibratioii aid density gradients on the diffusion layer is minimized under these circumstances. The addition of thickening agents may, in general, provide sufficient improvement in the precision of amperometry and chronopotentiometry to offset the inevitable loss of sensitivity; this effect will be examined further. The relatively high precision of measurement is observed only when the controlled-potential preelectrolysis step is employed. While this is only a secondary factor a t the highest concentration levels, the background current predominates a t low concentrations, and deviations from linear i-C behavior are observed. ,4t the 1.5-p.p.m. ( 1 2 p F ) level. for example, current peaks of only half the evpected height are observed when the preelectrolysis step is omitted. In addition, poor precision results from variations in the background current in the absence of the preelectrolysis step. I t is primarily the reproducibility (rather than the magnitude) of the background current which determines the limit of sensitivity of the method. Thuy the magnitude of the background current (determined using aniline prepared from chlorobenzene) nas of the order of 0.2 pa. (0.8 pa./sq. cm.) in the -0.4 t o -0.6-volt region. However, the average of 25 values of i-o.jg recorded on 5 different days over a one-month period was 0.011 pa. with a reproducibility (standard deviation of a single measurement) of L0.009 pa. From the value of the calibration factor for the electrode. this corresponds to 0.069 f 0.057 p.13.m. of nitrobenzene (0.57 i 0 . 4 7 p F ) . Other Electrode Characteristics. I n addition to convenience, high sensitivity. low background current. wide range of linear response, high precision. and applicability to repeated measurements, other factors t h a t determine the usefulness of a n electrode include the useful range of potentials, its selectivity, R pplicsbility t o yarious solvent media. and versatility. The cathodic limit of applicability was found, as eupected, to be p H dependent. As examples of these limit., the tangent potentials (us. SCE) in aqueous media a t low sensitivitie-4 are -1.9 volts in 1F S a O H , -1.5 volts (i = 4 pa,/sq. em.) in 0.1F KCI. - 1.2 volts in pH 4 (acetate) buffer. and -0.8 volt in 0.1F HCI. The anodic limit in O.IF H S 0 3 u a s f 0 . 8 volt. I n 1F ammonium acetate, 1F ammonia, it was f 0 . 9 volt (i = 0.4 pa. 'sq. em. a t +0.8 volt); in other media t h e limit xould depend primarily on the presence of a particular anion and its concentration. The background current observed in the aniline analysis m-as previously considered. I n several other cases background currents of 0.05 pa.
(0.2 pa./sq. em.) were observed over wide regions of potential. Other analytically significant properties of the electrode became evident from brief examinations of other potential-sweep amperometric applications. These involved the reduction of oxygen (in F/10 NaOH E,,, = -0.13 volt, -1.04 volts; the distinction between degrees of irreversibility is clearly illustrated), the reduction of various nitro compounds and quinones, as well as the deposition and stripping of lead and various anions. As an indication of the resolution of the method under favorable circumstances, clearly defined peaks representing reduction of 3,5-dinitrobenzoic acid in acetate buffer were observed at -0.27 and a t -0.39 volt us. SCE. The separation is governed primarily by the degree of irreversibility of the electrode processes and by the relative concentrations of t’he components. The stripping of lead from the electrode (after preelectrolysis of a 1 p.p.m. Pb solution in 0.18’ HC1 at -0.45 volt for 2 minutes without stirring) was characterized by a 0.42 z+= 0.01 pa. peak a t E, = -0.407 =t 0.004 volt us. SCE; E,/* = -0.435 =t 0.002, -0.383 0.002 volt. The effects of interniet,allic compound formation on potential-sweep amperornetry have been \\-idely noted (4, 6 , 7 , 9, 16) and some of their causes have been discussed and described ( 5 ) ; w c h effects are t o be
*
expected. Stirring during preelectrolysis increases the sensitivity t o a n extent depending on the efficiency and duration of stirring. This was also the case in the reduction of mercurous chloride and iodide films deposited on the AIFE b y preelectrolysis a t +0.4 volt. Upon sweeping to more negative potentials, the chloride and iodide exhibit peaks a t potentials which depend on concentration; they occur a t +0.09 and a t -0.07 volt, respectively, at the 50 p F level in p H 5 acetate medium. In this case the method can possibly be made selective for iodide a t disparately high ratios of chloride to iodide b y employing a preelectrolysis potential selected to avoid the formation of mercurous chloride along with mercurous iodide. Other Applications Possible. I n many other voltammetric applications, including differential voltammetry, chronopotentiometry, and coulometry (generating or indicating electrode), the MFE would presuniably duplicate the results obtained with pool, drop, or plated wire electrodes. I n certain cases, however, the use of the MFE, which combines the overpotential properties of mercury with a well defined planar surface area, would circumvent disadvantages of these electrodes. Thus, it should be uniquely well suited for chronopotentiometry [cathodic, or anodic cia the film technique of K u n a n a and Adams (S)] while providing the
choice of up or down orientation required for minimizing the effects of conrection resulting from density gradients (1). LITERATURE CITED
( 1 ) Bard, -4. J., A x . 4 ~ . CHEM. 33, 11
(1961). (2) Bruckenstein, S., Nagai, T., Ibid., 33, 1201 (1961). (3) Delahay, P., “New Instrumental Methods in Electrochemistry,” Chap. 6, Interscience, New York, 1954. ( 4 ) En~elsman.J. J.. Claasens. A. 34. J. JI., .Tuture 191, 240 (1961). ’ (5) Ficker, H. K., Meites, L., Anal. Chim. Actu 26, 172 (1962). (6) Gardiner, K. W., Rogers, L. B., ANAL. CHEM.25, 1393 (1953). (7) ,Kcmula, W., Kublik, Z., Galus, Z., A uture 184, 1795 (1959). (8) Kuwana, T., Adams, R. S.,Anal. Chirn. Acta 20, 51 (1959). (9) Marple, T. L., Rogers, L. B., ANAL. CHEM.25, 1351 (1953). (10) Mueller, T. R., Adams, R. S . ,Anal. Chim. -4ctu 25, 452 (1961). ill) Neeb. R.. 2. Anal. Chem. 171. 321 \
,
79, 6358 (1957); ASAL. CHEX 33,
1793 (1961). (14) Roonep, R. C., Tulanta 2,190 (1959). (15) Shain, I., Polcyn, D. S., J. Phys. Chem. 65, 1649 (1961). (16) Swaay, ?*I. van, Deelder, R. S., .Tatitre 191, 241 (1961). RECEIVEDfor review March 14, 1962. Accepted August 30. 1962. Presented at the Xetropolitan Regional Meeting, ACS, Sew York, January 22, 1962.
Anodic Voltammetry and EPR Studies of Isomeric Phenylenediamines H. Y. LEE and RALPH N. ADAMS Department of Chemistry, University of Kansas, Lawrence, Kan.
b What i s believed to be quite reliable voltammetric data for the isomeric phenylenediamines has been obtained at carbon paste electrodes. Praciical analysis of mixtures of these diamines through peak current measurements at stationary electrodes was unreliable, as shown b y a combination of voltammetry and electron paramagnetic resonance (EPR), because of chemical interactions following the electrode process. Practical analysis of phenylenediamine mixtures b y stationary electrode voltammetry i s not recommended.
T
HE AKODTC VOLTAMMETRY Of the isomeric phenylenediamines has received considerable attention from a number of workers. However, no com-
prehenaive report has been made including both practical analytical information as well as details of the oxidation mechanism. The phenylenediamines are Tvidely used aromatic intermediates and the isomer ratio analysis is of practical significance. While their redox reactions themselves are complex, a fuller understanding of the electrochemical oxidation pathn ays could serve as a model for more complex redox reactions of biological importance. The work reported herein is concerned with the voltammetric characteristics of the individual isomers and mixtures. Electron paramagnetic resonance (EPR) techniques have been combined with voltammetry to elucidate the unusual voltammetric behavior of phenylenediamine mixtures.
EXPERIMENTAL
All voltammetric studies were carried out with a carbon paste electrode (CENjP), a paste of *kcheson graphite and Nujol. -4complete description of these electrodes and their use in anodic voltammetry has been given ( 6 ) . The anodic polarography was done with a Leeds and Xorthrup Electrochemograph a t a scan rate of 200 mv. per minute. Some polarograms were run with a controlled-potentia1 scanner. The polarographic techniques mere conventional in all respects Unless otherwise noted, all polarograms were taken at a quiet electrode and are peak type current-voltage curves. All potentials are referred to the saturated calomel electrode (SCE). The three phenylenediamines were purified by reduction with tin and VOL. 34, NO. 12, NOVEMBER 1962
1587
