Ap pI ication of Derivative Techniques to Stationary Electrode Polarography S. P. PERONE Department of Chemistry, Purdue University, lafayette, Ind.
T. R. MUELLER Analytical Chemistry Division, Oak Ridge National laboratory, Oak Ridge, Tenn. Derivative measurement techniques have been applied to typical reversible electrodepositions at the hanging mercury drop electrode using electrolysis with linearly varying potential. The theoretical and analytical characteristics were investigated for first, second, and third derivative measurements. The accuracy and reproducibility of first derivative measurements are comparable to voltammetric results, but a t least an order of magnitude increase in sensitivity is observed. In addition, ihe utility of the method is enhanced considerably by the ability to demask the reduction wave due to a small amount of one species in the presence of an excess of a more easily reducible species.
F
OR THE COMBIrCED ADVANTAGES Of
simplicity, sensitivity, and precision in a directly applicable electroanalytical technique, electrolysis with linearly varying potential (LVP) probably is unexcelled (15). The technique is not the most sensitive, homever, the limiting interfering factor being charging current. Other techniques, such as second harmonic a . c. polarography (3, 19) or squarewave polarography (1, 2 ) minimize charging current interference, but these techniques require more complex instrumentation without an appreciable increase in sensitivity. The technique of stripping analysis ( 8 ) is considerably more sensitive, but it is not directly applicable, since a time-consuming preelectroly+ step is required. I n an attempt to enhance the sensitivity of the LVP technique, while preserving its simplicity and direct applicability, a straightforward approach aimed at minimizing the charging current interference was adopted. This approach involved recording the derivative of the conventional voltammetric curve. The theoretical effects of concentration and other parameters on the nature of the derivative curves can be predicted readily from the rigorously derived equations for the corresponding conventional voltammetric case. In the 2
ANALYTICAL CHEMISTRY
work reported here the theoretical relationships for the reversible case a t a spherical electrode were computed, using the equations derived by Xicholson and Shain (12). First, second, and third derivatives were considered. Comparisons were made between theoretical and experimental shapes of derivative curves to verify the theory as well as the experimental approach. The experimental dependence of the derivative peak heights on concentration of electroactive species and on potential scan rate was determined for typical reversible depositions at the hanging mercury drop electrode. In addition, the applicability of the firbt derivative technique to multicomponent systems was investigated. EXPERIMENTAL
Instrumentation. Several methods for obtaining derivatives of linear potential sweep current voltage curves have been employed ( 5 , 10, 1 4 ) . The general purpose instrument used in this work has been described previously ( I S ) ; it is based on the operational features of analog computer amplifiers, and uses some of the ideas suggested by Booman (4)and DeFord (6). To supplement this instrument for the measurement of second and third derivative curves a Heathkit operational amplifier unit (KO.EUW-19h) was used also. A three-electrode arrangement was eniployed such that the potential between the working electrode and reference electrode was controlled to within 1 mv., while the current in the cell passed between the working electrode and an auxiliary counter electrode. The circuit arrangement used to obtain the voltammetric current-voltage curves, as well as the first, second, and third derivative curves, is shown in Figure 1. The signal generator (S.G.) was either an operational amplifier integrator module, which provided sweeps from 10 mv. per second to 3.0 volts per second, or the sawtooth output of a Tektronix Model 536 oscilloscope with a Type T plug-in unit. The sawtooth was used for scans from 100 mv. per second on up to hundreds of volts per second; the oscilloscope controls were set such that only a single sweep was obtained and the
start of the sweep triggered the oscilloscopic display. The sawtooth peak output was 142 volts, and this was fed into an attenuator-inverter operational amplifier module to get out either a 1.42-volt or 0.678-volt sweep. Two low-pass filter networks were used in the signal measuring part of the circuit (CljRz and C4,R 5 ) . I n addition, the combinations of R3-C3, Rs-C6, and R&B, function as variable frequency response controls for the three differentiators, and act as low-pass filters. The extensive filtering is necessary because of the inter-electrode noise pick-up, which is amplified by the differentiators, and also because of the high frequency (>1.0 c.11.s.) noise generated internally by the various diff erentiators. Various measuring devices (M0-M3) were used. For most work (scan rates less than 100 mv. per second) a k e d s and Xorthrup Speedomax G, lO-mv., lir-second recorder was used. For faster scan rates, either a Tektronix Model 564 storage oscilloscope, with Types 3hl and 3B3 plug-in units, or a Tektronix Model 536 oscilloscope with Types D and T plug-in units was used. A DuMont No. 2620 Polaroid camera attachment to the hlodel 536 oscilloscope was used to record certain traces, using Polapan Type 47, speed 3000 film; whereas the Model 564 storage oscilloscope was used when only peak height measurements were needed. Calibration factors for the current axes of the various recording devices were calculated from the values of resistance and caiiacitance used in the differentiators. Cells and Electrodes. The cell was a 300-ml. capacity borosilicate glass beaker with a 60/12 f ground-glass mouth. A machined Teflon lid was used which had holes to accommodate the various electrodes, a nitrogen disperser, and a Teflon scoop uaed to transfer mercury drops from the dropping mercury electrode (DME) capillary to the hanging mercury drop assembly. Stirring was provided by the nitrogen disperser, or manually by the Teflon scoop. The cell assemhly was mounted on a ring stand which rested on a thick pile of folded paper which reduced vibration effects considerably. The temperature was controlled for al! experiments at 25.0' f 0.1" C. by a Sargent Theriiionitor water bath.
where a is nFviRT; v is the rate of potential scan in volts per second; TO is the electrode radius in cm.; .1 is the electrode area in sq. cm.; Do is the diffusion coefficient in sq. cm. per second; Co* is the concentration of oxidant in the bulk of the solution expressed in moles per cubic em.; and n, F , R, and T have their usual significance. The functions at) and +(at) are defined in the paper by Xicholson and Shain. Expressions for the first, second, and third derivative behavior were obtained readily by simple differentiation of Equation 1, and the results are given in Equations 2 , 3, and 4.
s
d i / d t = I’
=
nFACo*v
(%)[l/G
(2)q]
amps./sec.
-I; Figure 1 .
+ (2)
Generalized circuitry for voltammetry and derivative voltammetry
R = Reference electrod14
M , to M3 = Potentiometric recorder or oscillo-
W = Working electrodlr
scope for monitoring voltammetric curve, or flrst, second, or third derivative curves, respectively. RL = 5 to 1 OOK., 1 % R i , R7, Rg = 1 OOK., 1 % Rz, R j = 0 to 30K. R g , Re, RR = 0 to 1 5 0 K . RI = I K . to 1 0 0 M e g . CI, C1 = 2 0 mfd. Cl,Cb, C i = 1 .O mfd., Mylar. c3, C6, cs = 0.001 mfd.
C = Counter electrode F = Follower module A = Adder-Controller module CB = Cathode Bias module D 1 , Dz, 03 = Differentiator modules E, = Initial potential source, 0 to 3.0 S. G. = Signal generator
V.
The working electrode used for all experiments was a hanging mercury drop (HMDE), the assembly for which was obtained from the E. H . Sargent Co., Chicago, Ill., (No. S-29314-30), and was prepared in the usual way (20). The mercury drops were obtained from a conventional D M E assembly. Oneand three-drop electrodes were used in this work, a fresh electrode being used for each run. The size of each was determined by collecting 50 D M E drops, drying and w~ighing,and calculating surface area and drop radius from density and geometry considerations. For a one-drop ele’atrode, .I = 0.0319 sq. cm., ro = 0.05014 cm.; for a threedrop electrode, .I = 0.0662 sq. cm., r0 = 0.0726 cm. The reference electrode was a large saturated calomel electrode, and the counter electrode was a l/r-inch X 3-inch graphite rod immersed in a 1.0.11 KC1 solution. Ultrafine-porosity sinteredglass disks were used to isolate the auxiliary electrode compartments froni the sample solution. Reagents. All solutions were prepared in water purified by distillation and passing over a mixed cationanion exchange resin bed. ;ill chemicals were reagent grade and were used without further purification. High purity nitrogen was passed through a gas-washing bottle containing the inert electrolyte solut,ion and was used to remove dissolved oxygen
by dispersion through a coarse-porosity sintered-glass disk in the solutions. Fifteen minutes were usually sufficient for deaeration. RESULTS A N D DISCUSSION
Theory. T h e theoretical shape of the plot of the first derivative of current with respect to time as a function of time when the electrode potential is varied linearly was predicted by Shekun (18), who solved the boundary-value problem for a reversible system at a plane stationary electrode. However, the theoretical shapes of the derivative curves may be obtained by simple differentiation of the existing theoretical expressions for current-voltage behavior in stationary electrode polarography (‘?‘,I$).For this work, the theoretical plots were obtained from the expressions derived by Nicholson and Shain (12) describing the currentvoltage behavior for a spherical electrode. The theoretical expression for the current-voltage behavior at a stationary spherical electrode is given by ( 1 2 ) :
nFACo*v3
(g)3 [z/zF+
The values of the various derivatives of ut) and +(at) could be obtained from the tables of Xicholson and Shain, taking finite intervals. For the second and third derivatives particularly, these tables are not sufficiently complete. The wave shape was, therefore, recalculated with the aid of a Control Data 1604-h computer according to the method utilized by these authors. The conventional wave and its first, second, and third derivatives were computed in the same operation. The results were printed out and also stored on tape so that currents could be calculated readily by insertion of the experimental parameters n, D , v, A , rolCo*, and T . The electrode radius and area were inserted independently so that curves for a plane electrode could be computed, without alteration of the program, by giving ro a large value-e.g., 10’0. Provisions for plotting theoretical curves were included. Utilization of the computer is particularly valuable for the calculation of theoretical curves for spherical electrodes, since both the peak heights and positions of the peaks on the potential axis are shifted with n, D , v , roIand T . Hand calculation of a single peak is a time-consuming process, whereas the computer requires only about 60 seconds for the calculation of a VOL. 37, NO. 1, JANUARY 1965
3
voltammetric curve (300/n mv. range, T = 25” C.) and its first three derivatives. Some values of at) and +(at) for the first three derivatives of the LVP current-voltage curve are presented in Tables I, 11, and 111. Maxima and minima are underlined. Potential increments are chosen so that theoretical curves can be constructed with errors less than 1%, 1.5%, and 3% for the first, second, and third derivatives, respectively. The uncertainty could be reduced by taking smaller intervals in the integration process but at the
Table
(E
- El,&,
mV 140 130 120 110 99.9 89.9 79.9 69.9 59.9 50.1 40.1 36.0 30.1 25.2 21.1 20.0 18.8 ___ 18.5 18.2 18.0 17.7 17.5 17.0 16.4 15.9
x ’ (at)& 0,0040 0.0061 0.0090 0.0132 0.0192 0.0276 0.0392 0.0547 0.0744 0,0970 0.121 0.130 0.142 0.148 0.151 0.1515 0.1517 0.1517 0.1517 0.1517 0.1516 0.1516 0.1514 0.1513 0.1510
@’ ( a t )
0.0043 0.0063 0.0092 0.0135 0.0197 0,0285 0.0409 0.0581 0.0809 0.109 0.144 0.159 0.181 0.199 0.213 0.216 0.2198 0,2205 0.2213 0.2220 0.2228 0.2235 0.2250 0.2264 0.2278
expense of greatly increased computer time. Figures 2, 3, and 4 illustrate the theoretical shapes of the first, second, and third derivative curves for the reduction of 2.0mM Cd+* in 0.2.11 KC1 a t a hanging mercury drop electrode and a relatively slow scan rate. The diffusion coefficient used for the calculation was obtained by interpolation from the data of Von Stackelberg, Puritan, and Toome (21). For the set of experimental parameters taken for the calculation, the spherical correction term accounts for a displacement of the
I.
8.0
5.9 4.4 3.1 0
-10.0 -20.0 -24.9 -28.5 -28.8 -34.4 -40.1 -50.1 -60.1
x’ (at)v’/?r 0.1507 0.1504 0.1500 0.1496 0.1491 0.1486 0.1480 0.1474 0.1467 0,1455 0.1439 0,1409 0.1364 0.132 0.129 0.118 0.0792 0.0346 0,0141 0.0003 - 0.0006 -0,0193 -0.0342 -0.0515 -0,0591
9’ ( a t )
0.2291 0,2304 0.2317 0,2330 0.2342 0,2354 0.2365 0.2376 0,2386 0.2401 0.242 0.244 0,247 0.249 0.249 0,250 0.241 0.216 0.200 0.187 0.186 0.165 0.144 0,109 0.080
x“
0 0046 0.0061 0,0089 0.0128 0.0182 0.0254 0.0345 0.0451 0.0555 0.0592
9” ( a t ) 0 0042 0 0062 0 0090 0 0135 0 0189 0 0268 0 0374 0 0509 0 0666 0 0736
24.9 20.0 14.9 10.0 4.9 0 -4.6 -10.0 -12.1 -14.4
0.0258 0.0061 -0,0179 -0.0421 -0.0669 -0.0874 -0.102 -0.113 -0.115 -0.1156
6” ( a t ) 0 0899 0 0801 0 0650 0 0464 0 0235 0 0000 -0 0223 -0 0464 -0 0547 -0 0632
50.1 48.0 47.0 46.8 46.5 46.2 46.0 45.5 45.0 44.4 40.1 37.5 36.0 33.9 29.0
0.0622 0.0626 0.0627 0.0627 0.0627 0.0627 0,0627 0.0626 0.0625 0.0624 0.0598 0.0569 0.0547 0.0510 0.0392
0 0820 0 0849 0 0863 0 0867 0 0870 0 0873 0 0876 0 0883 0 0889 0 0895 0 0938 0 0954 0 0960 0 0964 0 0946
-14.6 -14.9 -15.2 -15.4 -15.7 -15.9 -16.2 -16.4 -16.7 -17.0 -19.0 -24.4 -30.1 -33.9 -40.1
-0.1155 -0.1155 -0.1155 -0.1154 -0.1154 -0.1153 -0.1151 -0.1150 - 0 . i149 -0.1147 -0.113 -0.104 -0.0895 -0,0781 -0.0590
-0 -0 -0 -0 -0 -0 -0 -0 -0 -0 -0 -0 -0 -0 -0
~
I
~~
ANALYTICAL CHEMISTRY
( E - El,*)%, mV -64.0 -65.0 -66.0 -66.8 -67.1 -67.3 -67.6 --67,8 -68.1 -68.3 -68.9 -69.9 -70.9 -79.9 -89.9 -99.9 -120.0 -140.0 -160.1 -182.2
x’ ( -0 -0 -0 -0 -0 -0 -0 -0 -0 -0 -0 -0 -0 -0 -0 -0 -0 -0 -0 -0
u t i 4
0601 0602 0603 06026 06026 06025 06025 06024 06022 06020 06016 0600 0599 0571 0523 0468 0364 0283 0223 0176
9’( a t ) 0.0708 0.0684 0.0661 0.0644 0.0639 0.0634 0.0628 0.0623 0.0617 0.0612 0.0601 0.0581 0.0561 0.0409 0,0255 0,0197 0.0092 0.0043 0.0020 0.0008
Values of the Current Functions at) and +”(at)
140.0 130.0 120.0 110.0 99.9 89.9 79.9 69.9 59.9 55.5
X ” (at)&
4
Values of the Current Functions x’(at) and +’(at)
( E - E l d n , mV 15.4 14.9 14.4 13.9 13.4 12.8 12.3 11.8 11.3 10.5 9.5
Table II.
wave on the potential axis of less than 1 mv.; the ratios of peak heights of successive peaks for each derivative, however, differ by as much as 10% compared with those for a plane electrode with the same area. The shape of the initial portion of each curve is dependent upon the initial potential chosen for the scan. One of the boundary conditions invoked for the solution of the wave equation is that the concentration of reduced species is zero when the scan i q begun ( t = 0 ) . The initial potential chosen corresponds to some finite ratio of
( U t h G
0641 0650 0659 0668 0676 0684 0693 0701 0709 0717 0775 0891 0953 0964 0938
X’’ (at)&
-50.1 -60.1 -66.5 -70.1
-79.9 -89.9 -97.6 -97.9 -98.1 -98.4 -98.7 -98.9 -99.9 -110.0 -120.0 -130.0 -140.0 -150.0 -181.9
-0 0307 -0 0096
-0 +O 0 0 0 0 0 0
0003 0037 0106 0137 0142 0142 0142 0142
0 0142 0 0142
0 0142
0 0134 0 0120 0 0104 0 0090 0 0076 0 0045
9” ( a t )
-0 -0 -0 -0 -0
-0 -0 -0
-0 -0
0820 0662 0555 0505 0374 0268 0205 0203 0201 0199
0198 0196 0189 0131 0090 0062 0042 -0 0029 -0 0008
-0 -0 -0 -0 -0 -0 -0
- -PO} :*
-30
1
-40
"
"
!
100
"
"
"
"
"
"
"
'
PO
60 80
-PO
40 0 ( E - E d , MILLIVOLTS
-100
-60 -40
-80
Figure 3. Theoretical second derivative curve for electrolysis with linearly varying potential a t a stationary, spherical electrode Conditions some os those for Figure 2 40
'
'
'
"
.
'
r
"
'
1
1
C,/CR, and current will flow until the required ratio is established when the control potential is impressed on the electrode. For the initial potential selected for these plots, the current flowing as a result of the concentration adjustment decays to insignificance slightly after the first peak of the third derivative curve is reached. If it is necessary to use this peak for quantitative information, an initial potential more remote from Eli2must be chosen. An alternative procedure is to start the voltage scan after the initial transient has decayed, as usually done in practice. The direct proportionality between derivative peak value and concentration indicated by Equations 2, 3, and 4 is significant for quantitative analytical applications. Since the derivative peaks occur a t potentials successively closer to the foot of the wave, quantitative information may be acquired for systems in which convection makes a significant
Table 111.
( E - Eldn, XI!' ( a t ) & mV 0 003 140.0 0 006 130.0 0 008 120.0 0 0117 110.0 0 0160 99.9 0 0210 89.9 0 0257 79.9 0 0280 69.9 0 0280 69.4 0 0280 68.9 0 0280 68.3 0 0278 67.6 0 0277 67.1 0 0277 66.5 0 0274 66.0 0 0272 65.0 0 0267 64.0 0 0262 62.9 0 0256 61.9 0 0229 58.8 ~
(at)
@I,''
0,0042 0.0060 0,0087 0,0124 0.0173 0.0236 0.0309 0.0379 0.0382 0.0385 0.0388 0,0392 0.0394 0.0397 0.0399 0,0404 0.0408
0.0411 0.0414 0,0417
I00
PO
60
-PO
40 0 (E - h i t ) , MILLIVOLTS
80
-60
-40
-100 -80
Figure 4. Theoretical third derivative curve for electrolysis with linearly varying potential at a stationary, spherical electrode Conditions some as for Figures 2 and 3. Noise represents uncertainty in caicd. wove form. Values of x(ot) in Table 111 token from "smoothed" curve
contribution to the conventional peak current. Thus, the derivative technique should be particularly useful in fused salt systems and other systems where convection is a problem.
Values of the Current Functions
(E - Edn, mV 55.0 48.0 45.0 38.0 34.9 27.0 20.0 15.9 14.1 12.1 11.0 10.0 9.0 ~
8.0
6.9 5.9 4.9 3.9 2.8 1.5
X"' ( a t ) &
0,0183 0.0033 -0,0057 - 0.0317 -0.0461 -0.0837 -0.113 -0.124 -0.127 -0.128 -0.128 -0.127 -0,127 -0.125 -0.123 -0.120 -0.118 -0.114 -0.110 -0.105
x"'
(at) and
(at) 0.0410 0.0354 0,0307 0,0142 0,0040 - 0.0293 - 0,0633 -0.0831 -0,0911 - 0.0996 -0.104 -0.107 -0.110 -0,114 -0.116 -0.119 -0.121 -0.124 -0.124 -0.125 $I"'
Certain characteristics of the derivative technique are indicative of the increased sensitivity obtainable. One of these is the more pronounced scan rate dependence of the derivative peak
4"'
(at)
(E - Edn, mV 0 -1.0 -2.8 -8.0
-13.9 -14.9 -20.0
-27.0
-30.1 -33.4 -36.0 -37.5 -45.0 -50.1 -54.0 -59.1 -65.5 -75.5 -89.9 -97.6 -120.0 -181.6
(at)& -0 098 -0 091 -0 081 -0 046 -0 004 +O 003 0 035 0 065 0 073 0 078 0 080 0 081 0 073 0 063 0 059 0 046 0 033 0 018 0 003 0 0 -0 0035 < -0 0035 XI''
$I", ( a t )
-0.125 -0.125 -0.124 -0.114 -0,092 -0.088
-0.063 -0,029 -0.015 -0.002 +0.008
0.013 0,031 0.038 0.041 0.042 -___ 0.040 0 034 0,024 0,019 0,009
is uncertain and is being investigated imrrently. CONCLUSIONS
The voltammetric first derivative technique has sevrral advantages over the conventional voltammetric method. The preci4on obtained a t the 0.4-11-12 level with cadmium indicates a t least an order of magnitude increase in sensitivity ; closely spiced reduction waves can be resolved readily; and the masking effects due to the presence of an excess of more easily reducible sub-
stances are minimized. Furthermore, the basic instrumentation involved is not any more complicated than much of that already used for conventional voltammetric work. Several further studies need to be carried out, however, to develop the derivat,ive method along promising lines. For example, the use of fast scans or second and third derivative techniques have not been investigated extensively here. This was primarily because further refinements in the instrumentation are needed before these techniques can be utilized readily, since background noise is a serious interference in these cases. Kit,h improved instrumentation further enhancement in the sensitivity of the derivath-e allproach should be attemllted by utilizing faster scans and:'or second and t'hird derivative techniques. Ailso,it might prove advantageous to apply derivative techniques to theoretical voltammetric studies where fast scans are utilized, since the charging current interferences would be minimized. LITERATURE CITED
(1) Barker, G. C., Anal. Chim. Acta 18, 118 (1958'1.
(2) Barker, 'G. C., Jenkins, I. L., Analyst 77, 685 (19.52). (3) Bauer, H. H., J . ElectroanaL C'hem. 1, 256 (1960). (4) Booman, G. L., AXAL.CHEM.29, 213 (1957). (5) Davis, H. AI., Seaborn, J. E., Electronic Engineering 2 5 , 314 (1953).
( 6 ) IleFord, I). D., 133rd lleeting, ACS, Sati Francisco, Calif., April 1958.
( 7 ) llelahay, P a d , "Sew Iirstrrimental Alethods i n Electrochemistry," 2nd ed., p. 119, Interscience, New York, 1954. ( 8 ) De Xlars, 11. U., Shain, Irving, ANAL. CHEM.29, 1825 (1957). (9) Frankenthal, 11. P., Shairi, Irving, J . -4m.Chem. SOC.78, 2069 (1Oj6). (10) Kelley, A I . T., Joties, H. C., Fisher, I). J., ASAL. CHEM.31, 1475 (1959). (11) Alartin, K . J., Shain, Irving, l b i d . , 30, 1808 (19.58). (12) Sicholsori, R . S., Shain, Irving, I b i d . , 36, 706 (1964). (13) Perorre, d. P., Oyster, T. J., Ibid., 36, 235 (1064). (14) Pozdeev, N . AI., "1)ifTerential AIethod of Oscillographic Polarography," (Akad. Xaitk. 1 o.59.
R&, J. \V., De AIars, li. I)., Shain, Irving, SSAL. C f i E h i . 28, 1768 (1%56). (16) Rrrlfs, C. I,., J . .am. f'hem. Soc. 76, 2071 11954). (17) Shairi, Irving, Xlartiii, IC. J., J . Phys. ('hem. 65, 254 i1961). (18) Fhekirii, L. Ya,, Ya.,R i r s s . J . Phys. Chem. 1962). 36. 36, 239 ((1962). (19) Smith, 1). E., I1einrnr1th, \V. H.. ASAL. CHEM.33, 482 (1!961). 120) Cnderkofler, FV. I.., Shain, Irving, Ibid.,33, 1966 11961). (21) T 7 0 n Starkelberg, AI., Priritan, AI.,
(1;)
Toome. Toome, I-.,%. Elektrochem. 57, 342
(1953).
RECEIVED for review AIay 6, 1964. Arcepted Sovember 4, 1964. 1)ivision of Analytical Chemistry, 148th AIeeting, ACS, Chicago, Ill., September 1964. \Vork siipported i n part h>- L-. S. Atomic Energy Comniissioii utider Contract with the Union Carbide Corporation, arid ill part by the .4merieari Caiicer Society.
A pplication of Derivative Tech niq ues to Anod~icStripping Voltammetry S. P. PERONE and J. R. BlRK Department o f Chemistry, Purdue University, lafayette, Ind. Derivative voltammetry has been used in conjunction with anodic stripping analysis a t the hanging mercury drop elecirode. The derivative technique was compared in sensitivity, accuracy, and reproducibility to the conventional stripping technique, which involves direct measurement of the anodic voltammetric curve. A significant enhancement in sensitivity was attainable with the derivative technique. For solutions of cadmium(l1) as dilute as 8 X 10-W, only a 2-minute pre-electrolysis was required for a determination b y the derivative stripping technique, whereas a 10-minute preelectrolysis was necessary to obtain comparable sensitivity using direct voltammetric measurement. Cadmium (11) could b e determined in solutions as dilute as 6 X 10-"M b y the derivative stripping technique.
0
of the most sensitive techniques for trace analysis is anodic stripping voltammetry ( I , 3, 4, ?). Using this technique, heavy metal ions as dilute as 10-951 have been determined with electrolysis times of the order of 60 minutes. Accuracy of the order of +57G and precision of the order of i1 to 4% are obtainable, even though concentrations may vary over 4 or 5 orders of magnitude. The Iirimary disadvantage of the stripping analysis method is that a aizable time delay (electrol required before analyttral data may be obtained. This sacrifice is macle willingly in most rases in order to obtain the necessary semitivity. However, where short analysis times are critically import.ant, stripping analysis may be worthless at the lo-* to .If level. SE
The objectives of this study, therefore, were to develop a more sensitive stripping technique and to the trace level. 'The technique elected for use in the stripping step was deriyative voltammetry, n-hirh recent work ( 5 ) ha> hhon.n to be at least an order of magnitude 1 1 i 0 1 ~ ~>ensitive than conventional \-oltammetry. In addition, it i+ 1c.s sensitive to csharging current intc.rfPrrnce. to the erratic effects of + i d a c e film>. and to other interference> +iir.h a> 1)rrrding rharge transfers. The erlierimental Iirorcrlure for deriI.atiye btri1)1)iiig \-oltaninwtry i,s identical to that u h ~ dill coii\.entional strii>i)ingvoltanmirti~y,t3sccl)t that the drrivativr of the 5tril)l)iIig currentvoltagc r t i r v ~is r c ~ o r r l c d . Quantitative VOL. 37, NO. 1 , JANUARY 1965
9
