Tdanta, Vol. 30, No. 7, pp. 515-518, 1983 Printed in Great Britain

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0039-9140/83 %3.00+0.00 Pergamon Press Ltd

RAPID-SCAN PULSE VOLTAMMETRY

LEON ASHLEY and SOLOMON L. LEVINE* IBM Instruments, Inc., P.O. Box 332, Danbury, CT 06810, U.S.A.

(Received 26 March 1982. Revised 24 August 1982. Accepted 26 January 1983)

Summary-In pulse voltammetry slow scan-rates (l-5 mV/sec) are generally used, so long scan-times are needed to cover reasonable potential ranges. Data are presented to show that fast pulse voltammetry with stationary electrodes and the dropping mercury electrode can provide useful analytical data which fit the theory of conventional pulse voltammetry. The possibility of reducing the capacitive currents and increasing the apparent faradaic current in cyclic pulse voltammetry is also shown.

Since the introduction of pulse polarography’ and its evaluation for use in trace analysis,2 the pulse tech- niques have been used in a wide variety of analyses, and use made of their many and well-discussed advantages. However, one disadvantage of pulse techniques is that the scan-rates generally range from 1 to 5 mV/sec, so relatively long times are needed to scan a reasonable potential range.

To increase the scan-rate, the entire potential range can be applied to a single mercury drop, or stationary electrodes can be used [as in differential pulse volt- ammetry (DPV)] or square-wave voltammetry.~’ DPV has also been used with a single mercury drop6 and reports of fast DPV have been presented,‘-I2 besides at least one on differential pulse stripping voltammetry” and one on fast alternate-drop pulse polarography.14 Combination of rapid scanning with pulse techniques requires short drop-times or pulse repetition times. In all the works cited above, the instruments used were either home-made, or com- mercial models modified to provide drop or repeti- tion times down to 100 msec.

Data are presented here to show the effects of rapid-scan differential pulse and alternate-drop volt- ammetry and polarography at scan-rates of up to 100 and 900 mV/sec, respectively, with use of unmodified analogue instrumentation. It is also noted that cyclic pulse voltammetry can be used. A primary benefit of rapid-scan pulse voltammetry is the substantially reduced analysis time, an important factor when the scan constitutes a major fraction of the total analysis time.

EXPERIMENTAL

All voltamperograms were recorded with an IBM Instru- ments EC/225 Voltammetric Analyzer and 7424MT X-Y

*Present address: IBM Corporation, 1701 North Street, Endicott, NY 13760, U.S.A.

recorder. The differential pulse and alternate-drop pulse potential voltamperograms were recorded at rates from 5 mV/sec to 1 V/set, with drop and pulse-repetition rates of l-10 per set and pulse amplitudes from 5 to 115 mV.

An IBM Instruments voltammetric cell with an SCE reference and platinum-wire coil counter-electrode was used. The working electrodes used were a dropping mercury electrode (DME) with a drop-knocker (mercury flow-rate 4.55 mg/sec for 0.1 set drop-time and 4.38 mg/sec for 1 .O set drop-time), an IBM Instruments Model A6011 glassy- carbon electrode and a Metrohm E410 hanging mercury- drop electrode (HMDE). The glassy-carbon electrode was used as received, and occasionally repolished with 0.2~pm alumina.

Systems with well-understood electrode processes were studied to determine the effects of fast scanning, and all the solutions were prepared from reagent-grade or ultrahigh- nuritv chemicals. These included 0.2 mM solutions of ferro- &me -and IO-methylphenothiazine (K and K Laboratories) in a pH-4.8 methanolic acetate buffer prepared by dissolving 13.6 g of sodium acetate trihydrate in 50 ml of water, adjusting the pH with glacial acetic acid (-5.8 ml) and diluting to 1 litre with methanol. Cadmium, lead and zinc solutions were prepared from either the pure metal dissolved in the appropriate acid (hydrochloric or nitric) or from J. T. Baker “Dilut-it” standards. The supporting electrolyte was 0.1 h4 potassium nitrate.

The nitrogen used for deoxygenation of solutions was piped to the laboratory from a reservoir of liquid nitrogen and used without further purification. It was passed through distilled water before the solution in the polarographic cell and was passed over the solution surface while the volt- amperograms were recorded.

RESULTS AND DISCUSSION

Figures la and lb show differential pulse volt- amperograms of ferrocene and IO-methylphenothi- azine scanned at 5 and 100 mV/sec respectively, the glassy-carbon electrode being used. In both cases, the peaks are well-defined and analytically usable, but the rapid scan was recorded in only 9 set, whereas the slow scan took 180 sec. With a pulse-repetition rate

515

516

' 0'6 ' 0'6 '

lb

-!I ' 0!2 ' 0 0'6 ' 016 ' 0'4 ' 012 ' 0 ' 0'6 ' 0'6 ' 0'4 ' 0'2 '

+ E vs. SCE (VI

Fig. 1. Vohamperograms of 0.2 mM ferrocene and IO-methylphenothiazine in methanolic acetate buffer (glassy-carbon electrode). a, DPV: scan-rate = 5 mV/sec; l.O/sec pulse-repetition. b, DPV: scan-rate = 100

mV/sec; lO/sec pulse-repetition. c, ADV: scan-rate = 500 mV/sec; lO/sec pulse-repetition.

of 10 per set, 100 mV/sec is the fastest scan-rate that can be used without distortion of the peaks.

The theoretical relationships for conventional (slow) differential pulse polarography are also valid for this rapid scanning technique. Plots of current and peak-width at half peak-height as functions of pulse amplitude for fast scanning compare favour- ably with those for slow scanning, as shown in Figs. 2 and 3, and in general with the theoretical plots.2 The similarity is further shown by comparing the fraction of maximum current obtainable (as a function of pulse amplitude) with that theoretically predicted from the (6 - 1)/(6 + 1) term of the equation for differential pulse current. 2~‘5 Table 1 shows these data normalized to a pulse amplitude of 100 mV for these

i (pA)

AE ( mV)

Fig. 2. Current (i) US. pulse amplitude (AE). Rapid-scan DPV: scan-rate = 100 mV/sec; lO/sec pulse-repetition. Con- ventional DPV: scan-rate = 5 mV/sec; lO/sec pulse- repetition. a, Ferrocene (conventional); b, ferrocene (rapid- scan); c, IO-methylphenothiazine (conventional); d,

IO-methylphenothiazine (rapid-scan).

l-electron transfer reactions. All the peak potentials in the rapid scans were shifted from the half-wave potential by the predicted’ AE/2, within 2-3 mV.

Alternate-drop pulse polarography is a relatively new technique designed to compensate completely for the capacitive backgrounds arising from extrusion of the mercury drop from the DME.16 In this technique, alternate drops are subjected to a constant-amplitude pulse superimposed on the ramp; intervening drops are held at the pulse potential for the life of the drop. The current output is the difference between the currents for the pulsed and non-pulsed drops. With the solid electrodes used in this portion of the work, the measurement is made on alternate pulses and the pulse is applied at alternate pulse-repetition times; hence the name alternate-pulse (AP) voltammetry. A

0 20 40 60 80 100

AE (mV)

Fig. 3. Peak-width (W,,,) at half-height, as a function of pulse amplitude (AE). a, Ferrocene (conventional); b, ferro- cene (rapid-scan); c, lo-methylphenothiazine (con-

ventional); d, IO-methylphenothiazine (rapid-scan).

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Table 1. Fraction of maximum current obtainable, as a function of pulse amplitude (normalized to 100 mV)

Ferrocene IO-Methylphenothiazine Pulse

amplitude, Theoretical i, % i, % i, % i, % i, % mV i, % (Slow DP)* (Fast DP)t (Fast AD)$ (Slow DP) (Fast DP)

10 12.9 16.1 14.8 15.3 15.6 25 31.9 31.6 33.4 41.8 29.2 34.9 50 60.4 - 61.0 60.4 59.1 80 86.9 87.4 87.9 86.9 87.8 92.6

100 100.0 100.0 100.0 100.0 100.0 100.0 115 107.6 110.0 -

*Drop-time 1.0 set; scan-rate 5 mV/sec. tDrop-time 0.1 set; scan-rate 100 mV/sec. §Drop-time 0.1 set; scan-rate 500 mV/sec.

similar compensation is obtained for the capacitive contribution to the background currents. Figure lc shows an AP voltamperogram scanned at 500 mV/sec, showing the well-defined peaks and some- what lower background current at the more positive potentials. This technique could be used at scan-rates up to 900 mV/sec before the peak distortion became large enough to make the curves unusuable. The distortion manifests itself as skewed, ill-defined peaks, because the current-follower and sample/hold circuitry is not fast enough to keep up at the higher scan-rates. All other effects were similar to those in fast-scan differential pulse work, as stated above.

Reduction processes were followed by both slow and fast pulse polarography, with various concen- trations of the three metal ions and use of both the DME and HMDE. Polarograms obtained for 10m4M solutions of the three metal ions at lOO-mV/sec scan-rate and O.l-set drop-time and lO/sec pulse repetition were analytically usable, but the currents obtained were only 20-25% of those obtained from more conventional polarograms (5 mV/sec, l.O-set drop-time and l/set pulse repetition). The lower currents are due to the smaller electrode areas ob- tained with the short drop-times. With the HMDE, currents obtained for the slow and fast DPV scans were comparable; in the AP scans, the currents were smaller, as expected.

The decrease in current due to the small drop areas limits the analytical sensitivities of fast scan with the DME to about 10-6M for these ions as opposed to about lo-‘M with slower scans. Figures 4a and b show typical fast DP scans with the DME; Fig. 3b was scanned with a 0.1 -set time-constant filter on the output. Although the curve is smoothed, there is also the expected shift in peak potentials.

The DP and AP techniques are both designed to reduce the background, particularly the capacitive current, in d.c. polarography. Reducing this current should result in better defined peaks with a higher

i (PA)

0.1 pA !

I 1

0.2 0.4 0.6 0.8 1.0 1.2

-E vs SCE (V)

Fig. 4. Polarograms of IO-“.&4Pb*+, Cd*+ and Zn2+ in 0.1M KNO, (DME). Conditions: AE = 50 mV; scan-rate = 100 mV/sec; drop-time 0.1 set and pulse repetition lO/sec. a, No

output filter; b; O.l-set time-constant output filter.

a&b 5rA t

C b a I I I1 1 I I I I I I, I I I , 1 I I I I I, I I I,,

.8 .7 .6 .5 .4 .3 .2 .l .O .8 .7 .6 5 .4 .3 .2 .l .O .8 .7 .6 .5 .4 .3 .2 .I .O

+ E vs. SCE (VI

Fig. 5. Rapid-scan cyclic pulse voltamperograms of 0.2 mM ferrocene in methanolic acetate buffer (glassy-carbon electrode). a, d.c., scan-rate 100 mV/sec; b, AP, scan-rate 500 mV/sec, pulse repetition

IO/set; c, DP, scan-rate 100 mV/sec, pulse repetition IO/WC.

518 SHORT COMMUNICATIONS

apparent current in cyclic voltammetry. Cyclic DP and AP voltammetry at the faster scan-rates both proved feasible, as shown in Fig. 5 for ferrocene. The backgrounds for voltammetry of these organic com- pounds on the glassy-carbon electrode are lower than those obtained in conventional d.c. cyclic volta- mmetry. This should allow the use of lower analyte concentrations in cyclic voltammetry.

The data presented indicate that rapid-scan pulse polarography and voltammetry show promise as time-saving tools; scans over reasonable potential ranges can be accomplished in seconds as opposed to a few minutes at the more conventional slower rates. However, the rapid scans require higher concen- trations than the slower scan-rates do. With the DME in particular, the practical detection limit is relatively high (10~4-10~6M) because of the increase in background and the smaller currents caused by the small drop areas obtained in rapid scans.

With cyclic voltammetry, the background signal obtained with solid electrodes is lower than that of d.c. positive-potential cyclic scans with a glassy- carbon electrode. The pulsed cyclic scans give easier current measurements and lower detection limits. More work is necessary, however, to refine these techniques and utilize their advantages.

2.

4.

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8. 9.

IO. 1 I. 12.

13.

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15.

16.

REFERENCES

G. C. Barker and A. W. Gardner, AERE Harwell, C/R 2291, 1958. E. P. Parry and R. A. Osteryoung, Anal. Chem., 1965, 37, 1634. L. Ramaley and M. S. Krause, Jr., ibid., 1969, 41, 1362. J. H. Christie, J. A. Turner and R. A. Osteryoung, ibid., 1977, 49, 1899. J. A. Turner, J. H. Christie, M. Vukovic and R. A. Osteryoung, ibid., 1977, 49, 1904. H. Blutstein and A. M. Bond. ibid.. 1976. 48. 248. K. C. Burrows, M. P. Brindle and M. C. Hughes, ibid., 1977. G. D. Christian, J. Elecfroanal. Chem., 1969, 22, 33. S. C. Rifkin and D. H. Evans, Anal. Chem., 1976, 48, 1616. Idem, ibid., 1916, 48, 2174. K. C. Burrows, Ph.D. Thesis, Lehigh University, 1979. M. C. Hughes. E. F. Blaedel. Jr. and J. Grossman. Presented at 198 1 Pittsburgh Conference on Analytical Chemistry and Applied Spectroscopy, Atlantic City, March, 1981. P. Valenta, L. Mark and H. Rutzel, J. Electroanal. Chem., 1977, 82, 327. J. A. Turner and R. A. Osteryoung, Anal. Chem., 1978, 50, 1496. A. J. Bard and L. R. Faulkner, Electrochemical Meth- ods, pp. 190-196. Wiley, New York, 1980. J. H. Christie, L. L. Jackson and R. A. Osteryoung, Anal. Chem., 1916, 48, 242.