Pulse Voltammetry Janet Osteryoung State University of New York at Buffalo, Buffalo, NY 14214
Voltammetry concerns the measurement of current which flows through an electrode held at a known potential. In general the experimental conditions ensure that the amount of chemical reaction, or the total charge passed during one experiment, is negligible in comparison with the total amount of material present. Consider for example the reduction of Cd2+ a t a mercury electrode to form the amalgam, Cd(Hg):
+
Cd2+ 2e-
-
Cd(Hg)
Another factor is background currents unrelated to the reaction of the material of interest. A common example in voltammetry is current which flows because the electrode is a capacitor. This current interferes with the direct relation between current and concentration, that is, instead of I = k c we have 1 = kC b, where b is the background current. Although in some cases b can he estimated and subtracted from the measured current, I , to obtain a quantity proportional
+
Because we require conservation of material and charge, the rate of disappearance of Cd2+= rate of appearance of Cd(Hg) = 112 rate of disappearance of electrons But the rate of disappearance of electrons is just the current, and therefore the current is a direct measure of the reaction rate. Electrochemists use voltammetry for two uuruoses, to . . n~udyrhr nature idelw~rochemicslreaction,, :lnd I C Irnw,urr cunrrnrr;~timsoiuecit.a in suh~titm.The 1n;lin cl~arncreristica of reaction rates ale that they vary over many factors of ten and that they are very sensitive to the exart conditions of the experiment. Therefore, measurements of current in voltammetry provide useful information about electrochemical reaction mechanisms and very sens~tivedeterminations of the concentrations of reactants in solution. Whether the purpose of the measurement is mechanistic or analytical, two factors limit the quality of the information. One factor is the senslt m t v of the measurement. that is, the s l o ~ e(k) of the calir ,~micentratiunr( , o r m,!Inuiun I I I ~ \ .uf I ,currrnt ~ i VPISUG terial ai\.ina r:se I,, the current. 'l'hc crwrtr thi. :i'lis~t~(.it~, the small& t h e concentration required t o obtain a current which can be measured with the required precision.
296
Journal of Chemical Education
to hC it is always desirable to make the value of b, or rather the ratio bli, smaller. The smaller b, the lower the value of i that can be related to C. The purpose of pulse voltammetry is to give large values of k and small values of b. In pulse voltammetry,'-3 the potential is held at some value Eo and then stepped instantaneously (pulsed) to a second value, El. Some time after the pulse is applied (at t,), the current is measured. This sequence is shown inFigure 1. When the pulse is applied, a large capacitative current flows; however, this current decreases with time much faster than the current due to the electrochemical reaction. By making t , large enough, the background current can he made very small. But on the other hand, the current due to the electrochemical reaction also decreases as t, increases, hut at a slower rate. The trick to making k large and b small is to choose t, at some optimum intermediate value. Typical values oft, which work well in routine experimental situations are in the range of a few thousandths of a second to a few tenths of a second (2-200 ms). The short times, t,, required to ohtain the best measurements create a practical problem, one of the complexity and
I )APPLY
PULSE AT THlS TlME
2 1
4
SAMPLE
.CURRENT
INITIAlE A NEW
1MEASURE CURRENT
" I
L
I
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n
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Figure 2 Timing pulses to control pulse application and current measurement. The large black dot indicates the period of current measurement. M o m ond choracterirttcr
Norm01 pulse b- 5Omr r
APPLY PULSE
AEp/r
-
.
-
Is
2 mV/r
Figure 1. Potential-timesequence and resulting current-time response in pulse voltammetry. expense of the instruments required. Slower experiments require inexpensive instruments which are simple to operate, hut both the expense and difficulty of the experiment increase as the value oft, decreases. The most widely used forms of pulse voltammetry solve this problem in a clever way, which is illustrated in the timing sequence of Figure 2. A series of timers are used. each of which delivers a voltaee of a , . sienal .. wrtaiii duratim and rrpetit~~m r:nt!. Tlit.it. ignnl; tell thr rr;t of the i~~.~ruinent what 11. d c ~ l ' h etimer, srr wlucnczd r ~ g i v c short pulae and current rnv;l,urenit 111 t1lnt.Z rt i l ~ ~ i 1%) r d.,I)timize rht. c.rrrrent : w u l 11111no iiidi\.:d~~i~l tillwr trr iliitr~tment function has to operate at a high repetition rate. For example, in Figure 2 the current is sampled for 16.7 ms, but there is an interval of 0.5 s between sampling periods in which the measured current can be transferred to some output device, such as a simple strip chart recorder. Thus, the widespread use of pulse voltammetry arises from good sensitivity and detection limits, but also from ease of application and low cost. The general idea of pulse voltammetry can be elaborated by employing a sequence of pulses and a variety of current measurement schemes. Some common choices. the resultine ~.~lrrtni-pote~ilial respun;?, and the nnm?: rltey hwe heell rive~iare illuatr3ttd in Picure :I. 'l'he r~,,niti.mi l r the \,cAt3rnmetric response on the potential axis, characterized by the potential at half the limiting current (E1/12), or the potential at the maximum current (Ep),is determined by the properties of the substance undergoing reaction. The magnitude of the limitine current (11) or the oeak current (1") is nrouortional to the concentration of the reactant in solnti&. lfihe'ohjective of the exoeriment is analvtical. . . conditions are carefullv chosen and controlled to avoid complications and to maintain astrict nrooortionalitv between current and concentration over the widest possihie range. Then concentrations in unknown samples can be determined by comparison of the currents with
Square r o v e r- 5mr. A E p - 2 5 m V .
AEs
-
lOmV
Figure 3. Common pulse techniques. those arisine from standards of known comuosition. On the
a
concentration produces significant change in the position or magnitude of the current-potential response. Analytical Applications Figure 4 and the Table show a differential pnlse voltammogram and calibration data for As(II1). The mechanism of electrochemical reduction of As(II1) is fairly complex, but the first peak, which corresponds to reduction of As(II1) to Ado), is line? in concentration over a very wide range, and therefore can be used analytically. The conditions of Figure 4 give a detection limit of about 0.2 wgL-'As(III), or about 3 X M. A further interesting point is that this method of determination is specific for As(II1). Arsenic(V), the other valence state commonly found in biological and environmental samples, is generally not reduced a t this potential. Thus, the electrothan those necessary for toxicity studies Mechanistic Applications Figure 5 shows a set of square wave voltammograms for the reduction of Zn(II), The shape and position of the voltamVolume 60
Number 4 April 1983
297
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