concentration range. This linearity does not preclude, however, large errors a t low efficiencies. The coulometrically determined CC13F and cc14 concentrations are approximately 25% low over the entire concentration range of Figure 3. As the ionization efficiencies approach 100% these errors become negligible as shown in Figure 4. It follows that accurate coulometric analysis requires conditions favoring maximum ionization efficiencies. From our data, it seems reasonable that, barring irreversible sorption phenomena, accuracy greater than 95% should be routinely obtained a t ionization efficiencies greater than 90%. Errors of 25% and less can be expected a t ionization efficiencies exceeding 50%. According to Equation 6 , a plot of the reciprocal efficiency us. flow rate should be linear. This is confirmed for CC13F, cc14,SFe, and CH3I in Figure 2 . The linearity allows one to determine an efficiency a t one flow rate and extrapolate it to zero flow rate. This has been done for all appreciably ionizable compounds of Table I. Practically, one can determine with such plots the feasibility and possible errors of using coulometry. A large slope indicates poor efficiency which can only be compensated for by very low flow rates. A small slope allows flexibility in the choice of flow rates. However, although one can theoretically achieve 100% ionization efficiency for an ionizable compound by approaching zero flow rate, such flow rates may be incompatible with practical analysis. Table 111 gives the detector response at two flow rates for CCl3F and CH3I. The responses are seen to fall off drastically a t the lower flow rate, making the coulometric response invalid. This phenomenon was attributed primarily to increased sorption and spreading in the GC column, which could more than offset the advantage obtained due to increased efficiency. It follows that one can only reduce flow rates as long as GC response increases or remains constant. Figure 5 is a chromatogram obtained from an injection of 8 ml of ambient air in the New Brunswick, N.J., area. Identified peaks are CC13F, CH31, CH3-CC13, CC14, CHCl==CClZ, and CClFCC12. The identity of these peaks was not only established by retention data on different packings (SE30, Porapak Q), but also was confirmed by their efficiency of ionization. This was found to
Table 111. Effect of Flow R a t e on Detector Responses and Efficiencies for CC13Fand CHJ
Compound
CC13F CC13F CHJ CH~I
Flow rate, ml/min
Detector response, coulombs x 10’0
71.0 11.3 71.0 11.3
209.1 45.0 135.0 16.5
Ionization efficiency
0.73 1.00 (approx.) 0.60 1 . 0 0 (approx.)
be a highly reliable method of identification since it is extremely unlikely that two compounds will have the same retention times as well as equal ionization efficiencies. Table 11 shows the ambient concentrations of some of the compounds identified in Figure 5. Hitherto, it has been considered highly unlikely that a greater than coulometric response ( X z > XI) could be encountered in practice (2). In this laboratory, C C l F C H 2 , trans-CHCkCHCl, and CHzClz were found to yield greater than coulometric response as shown in Figure 6 . Figure 7 shows that for trans-CHCbCHC1 the response of the second detector is at least 265% greater than that of the first detector. Since such compounds are apparently rare, dual EC detectors in series can be used reliably to confirm their identification. Further, an increase in sensitivity is possible by using the second detector conventionally. Based on the mechanism for electron attachment proposed by Wentworth ( 4 ) , the greater than coulometric response is attributed to the products of ionization having greater electron affinities than the reactants. Received for review November 6 , 1973. Accepted March 7 , 1974. This project has been financed in part with Federal Funds from the Environmental Protection Agency under Grant No. EPA: 800833. Paper of the Journal Series, Agricultural Experimental Station, Cook College, Rutgers University-The State University of New Jersey, New Brunswick, N.J. 08903. ( 4 ) W. E Wentworth and Joe 75 (1968),
C. Steelhamrner, Advan. Chem. Ser.,
82,
Direct Current, Alternating Current, Pulse, and Anodic Stripping Voltammetric Methods with Glassy Carbon Electrodes in Hydrofluoric Acid A. M. Bond, T. A. O’Donnell, and R. J. Taylor’ Department of lnorganic Chemistry, University of Melbourne, Parkville 3052, Victoria, Australia
Polarography in hydrofluoric acid is severely restricted because of the necessity of constructing DME’s from nonsiliceous materials, among other reasons. The glassy carbon electrode (GCE) readily enables voltammetry to be undertaken in this solvent system. The considerable hydrogen overvoltage found on the GCE allows potentials almost as negative as those with mercury electrodes to be observed; in addition, the potential range in the positive direction is extended substantially. Electrodeposition of Ag, Hg, Cd, TI, Sn, and Pb on the GCE is studied in detail. A comparison of a large number of electroanalytical
techniques including dc, ac, and pulse voltammetry, indicates that the pulse methods offer substantial advantages over linear sweep methods. Pulse techniques extended the limit of detection for linear sweep voltammetry to 1 X for Ag(l) and Hg(ll) in 50% HF (5 X 10-4M) by at least an order of magnitude with excellent reproducibility. Anodic stripping voltammetry at the GCE was found to be sensitive but nonreproducible. However, in situ mercury plating anodic stripping voltammetry at the GCE provided excellent results.
ANALYTICAL CHEMISTRY, VOL. 46, NO. 8, JULY 1974
1063
Recently, reports of polarography at a dropping mercury electrode in concentrated hydrofluoric acid ( I ) and anhydrous hydrogen fluoride (2) have been published. In view of the experimental difficulties encountered in applying polarographic methodology to this extremely important solvent system, an alternative electrochemical method of voltammetry at a mercury pool electrode has also been investigated ( 3 ) . With mercury electrodes, the useable potential range for these solvents is limited by evolution of hydrogen in the negative region and by oxidation of mercury at positive potentials. Recently glassy carbon has been suggested as a solid electrode material (4, 5 ) providing a large hydrogen overvoltage (6). In addition, the electrode permits very anodic potentials to be studied and, therefore, it should allow species electroactive a t potentials considerably more positive than that for mercury oxidation to be studied. Furthermore, glassy carbon was expected to have the desirable property of being inert toward hydrofluoric acid and, as this solvent is also a remarkably good cleaning reagent, problems usually associated with surface contamination in aqueous media may be reduced. Finally, the use of glassy carbon also provides the possibility for the first time of using the highly sensitive analytical technique of anodic stripping voltammetry directly in hydrofluoric acid. The present paper therefore reports the use of voltammetry with a glassy carbon electrode (GCE) in concentrated hydrofluoric acid to extend the anodic potential range and to permit anodic stripping. Linear sweep dc and ac techniques as well as pulse voltammetry are reported and a critical comparison of each of these electroanalytical methods is made.
EXPERIMENTAL Reagent. Baker Analyzed (49.0%) and BDH Analar Grade Hydrofluoric Acid (48-49%) were used as both the solvent and supporting electrolyte. Silver(I), mercury(II), thallium(I), lead(II), and cadmium(I1) were added as their carbonates or nitrates. Tin(I1) was added as the fluoride and mercury(I1)as its nitrate. Electrodes. Glassy carbon electrodes were obtained from Tokai Electrode Manufacturing Co. Ltd., Tokyo, Japan. Typically, the glassy carbon was used as cylinders between 2 and 5 mm in diameter. The performance of these electrodes was critically dependent upon the method of preparation of the surface. The procedure adopted in this work was to polish the face t o be exposed to the solution, using 0-Yz micron diamond dust lapping compound. After this treatment, the face had a mirror-like surface, which was found essential to obtain reproducible results. Less vigorous polishing procedures gave rise to either poorly defined drawn out waves, or to no waves at all. Electrodes were sealed directly into shrinkable Teflon or polyethylene tubing or, with epoxy resin, into Kel-F tubing. The highly polished surface of the electrode was carefully kept free of any adhesive during the sealing procedure. A small amount of mercury was poured in the tube on top of the glassy carbon, and electrical contact was achieved by me,ans of a nichrome or platinum wire. After use, the electrodes were washed with distilled water, wiped with a tissue, and stored either in air or in 50% HF solution. Because the electrodes made by this process were not shielded, it was not expected that the results obtained should be exactly compatible with theories based on semi-infinite linear diffusion but they were used in this way for simplicity. Present address, Department of Physical Chemistry, University of Melbourne, Parkville 3052, Victoria, Australia. ( 1 ) A. M . Bond and T. A . O'Donnel1,Anal. Chem., 44, 590 (1972). (2) A . M . B o n d , T. A . O'Donnell, and A. B. Waugh, J . ElecfroanaL C h e m . , 39, 137 (1972). (3) A. M . Bond, T. A . O'Donnell, and R. J. Taylor, Anal. Chem.. 44, 464 (1972). (4) H . E. Zittel and F. J. Miller, Anal. Chem., 37, 200 (1965). (5) J. F. Alder, E. Fleet, a n d P. 0. Kane, J . Electroanal. Chem.. 30, 427 (1971). (6) H. Monien, H . Specker, and K . Z i n k e , Fresenius' 2. Anal. C h e m . . 226, 342 (1967).
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ANALYTICAL CHEMISTRY, VOL. 46, NO. 8, JULY 1974
All volumetric equipment (pipets and burets) was made of polyethylene. The bottles in which HF solutions were prepared and maintained were made from either polyethylene or Teflon. Dc voltammograms were recorded with either Metrohm Polarecord E261 or PAR Model 170 Electrochemistry System. All potentials are reported relative t o a saturated calomel reference electrode, and a three-electrode system, using tungsten or platinum wire auxiliary electrodes, was used. Phase-sensitive ac voltammetry was performed with the PAR Model 170 Electrochemistry System using an applied alternating potential of 10 mV peak to peak at frequencies listed in the tables and text. Non phase-sensitive ac polarography was carried out by coupling Metrohm ac Modulator E393 to the Polarecord. In this instance, an alternating potential of 10 mV rms at 50 Hz was used. Pulsed and derivative voltammograms were recorded with the PAR Model 170 Electrochemistry System using a 15-mseccurrent sampling time at the end of each 0.5-sec period of dc potential scan, using appropriate "Sample and Hold" circuitry. Concise details of these experimental procedures are given where appropriate in the text. All solutions were degassed with argon or nitrogen. No thermostating of equipment was attempted because of the poor heat transfer properties of the polymeric materials used to contain the working solutions. Temperatures given are ambient temperatures. All the voltammetric techniques except anodic stripping used potential scans in the negative direction, and the scans were not commenced until the system reached electrochemical equilibrium.
RESULTS AND, DISCUSSION One of the desirable features of the glassy carbon electrode is the possibility of being able to undertake trace analysis and electrochemical studies a t considerably more positive potentials than DME polarography or voltammetry at stationary mercury electrodes will permit. Glassy carbon should enable the direct determination of mercury itself and silver for example, neither of which can be readily determined with a dropping mercury electrode in hydrofluoric acid. Silver(1). Linear Sweep dc Voltammetry. In linear sweep voltammetry under conditions of semi-infinite linear diffusion, relationships of the type
i,
=
kn' ' A C ( D U )''
(1)
apply to reversible electrode processes (7) where i, = peak current, k = constant, A = electrode area, n = number of electrons, u = scan rate of dc potential, C = concentration, and D = diffusion coefficient. After the charging current is subtracted from the instantaneous current, a linear relationship is usually found for the calibration curve of i, us. C, with most types of electrode processes. The limit of concentration detection is therefore reached when the charging or background current masks the faradaic current. For silver(I),the electrode process being considered is
Ag(1)
+e
A4g(0)
(2)
where metallic silver is deposited on the glassy carbon. Figure 1 shows a plot of i, us. concentration for the above electrode process with linear sweep voltammetry. For concentrations greater than 2 x 10-4M, the calibration curve was found to be linear. Below this level to the detection limit of 5 x lO-5M, curvature became evident, probably because of the inability to precisely subtract the charging current. The reproducibility of i, was found to be excellent. On the same solution, using a scan rate of 1 volt per min, and a silveriI) concentration of 5 x IO-SM, 20 consecutive scans gave a reproducibility of &1.3%; while on 10 individually prepared solutions, this figure was &2.2%. Repro(7) R
s. Nicholson and
I . Shain, Ana/ Chem 36, 706 (1964)
30-
I
*L
b
40 u A-
J.
I 67 da Volt
0'5
04
63
62
O7
vs 5 C E
O6 Volt
O4 vs
O3
S C E
@*
/x
0
20 [ Ag
,
,
40
60
Figure 2. ( a ) . Dependence of E, and i, on scan rate for the A g ( l ) s Ag(0) electrode process. [Ag(l)] = 5 X 10T3M,T = 15.6 "C.v = ( l ) , 5 mV/sec, (2),10 mV/sec, ( 3 ) , 20 mV/sec, (4), 50 mV/sec. ( b ) . Phase-selective ac voltammogram. Alternating potential = 10 mV, p-p. Frequency = 100 Hz. v = 50 mV/sec. Same conditions as Figure 2 ( a )
lI)1 y x IO'
Figure 1. Plot of ip vs. C for the electrode process Ag ( I ) Ag(0) in 50% HF. v = 1 volt per minute. T = 14.8 "C
+e
G
ducible measurements of the peak potential, E,, to within *5 mV or better are also readily accessible for this metal deposition reaction. Figure 2a shows that both E , and i, are dependent on the scan rate of dc potential. Table I shows quantitatively the scan rate dependence of these parameters. E , becomes more negative and i,/u1/2 decreases with increasing scan rate. E , also shows considerable dependence on concentration, becoming more positive with increasing concentration. The deposition of a metallic silver film onto the glassy carbon surface implies that an exact theoretical description of the electrode process would be most difficult as the activity of the silver deposit must vary during the recording of a single voltammogram (8). The expected dependence of E , on silver ion activity, ~ . 4 ~ +assuming , constant unit activity of the deposited silver is given by the expression
RT 0.0218 E o f 2.3log(a,,-) - ___ (3) nF n a t constant u (8). Experimentally, E , varied with CAg+in the expected direction, and a plot of E , US. log C.AgT was
E,
=
reasonably linear with a slope of about 73 mV a t 13 "C, compared with an expected theoretical value of 56 mV. Linear Sweep ac Voltammetry. Alternating current techniques often provide considerable improvement on dc methods in terms of limit of detection and wave shape for reversible electrode processes (9),uncomplicated by kinetic and other factors. Figure 2b shows a phase sensitive ac voltammogram for Ag(1). A characteristic of all ac techniques used in this work was that poorly defined broad ac waves resulted. All the characteristics of the ac electrode process are indicative of the electrodeposition of silver being irreversible and exhibiting considerable complexity, probably with respect to the glassy carbon surface and the activity of metallic silver. From the analytical point of view, the ac method appears entirely without value for the electrode processes examined. Pulse Voltammetric Methods. The dc voltammetric method is severely limited to concentrations above about 10-4M because of the high charging current. Pulse voltammetry, where the potential is applied in a series of pulses, rather than using a continuous dc ramp, provides discrimination against charging current and, in principle, could be expected to be substantially superior to dc volt(8) T. Berzins and P. J . Delahay, J. Amer. Chem. SOC.,75, 555 (1953). (9) W. L. Underkofler and I , Shah. Anal. Chem.. 37, 218 (1965).
Table I. Dependence of E , a n d i, on Scan R a t e for the Ag(1) =Ag(O) Electrode Process in 50% HF at t h e GCE= scan rate, u
ip/v1i2,
(mV/=)
E,, volt us SCE
5 10 20 50
0.505 0.485 0.465 0,440
pA-sec'i2 ip,
PA
23 30 37 52
mV-li2
1.03 0.95 0.83 0.74
[Ag(I)] = 5 X lO-3M, T = 15.6 'C.
ammetry for this and other reasons (IO, 11). However, no pulse voltammetry a t the GCE appears to have been reported. Several pulse techniques are available. Figure 3a shows a normal pulse voltammogram of Ag(1) in 50% HF. It can be seen that this curve is particularly well defined and most suitable for analytical purposes. It has the typical shape of a dc polarogram obtained a t a dropping mercury electrode, and a plot of the limiting current us. concentration was found to be linear down to 5 x 10-6M. The reproducibility a t the 10-4M level was f l . 6 % . The limit of detection was about 1 x 10-6M, and the technique is substantially superior to linear sweep dc voltammetry. The improvement offered by the pulse technique can probably be attributed to the discrimination against charging current and the simplification of the electrode process. With the pulse method, the potential is periodically returned to an initial value where the redissolution or oxidation of silver occurs. Thus, whenever the pulsed potential is applied, the electrode process essentially occurs a t a renewed, clean GCE surface, and complications introduced by the deposition of silver as in linear sweep voltammetry are obviated. Substantial advantage of pulse techniques has been observed a t other stationary electrodes (IO,11). In view of the excellent nature of the normal pulse polarogram, the derivative pulse method, in which the difference in current between two consecutive pulses is measured as a function of dc potential, would be expected to provide a most convenient form of readout. Figure 3b shows this to be the case. Another pulse technique, differential pulse voltammetry, in which a pulse of fixed height is applied periodically, in addition to the dc potential is often useful for trace analysis. Figure 3c shows a differential pulse voltammogram of a 10-4M silver solution. The presence of silver is readily de(10) €. P. Parry and R. A. Osteryoung, Anal. Chem., 36, 1366 (1964) (11) K. 8.Oldharn and E. P. Parry, Anal. Chem., 38, 867 (1966).
ANALYTICAL C H E M I S T R Y , VOL. 46, NO. 8, J U L Y 1974
1065
b
C a
Volt
Volt v s S C E
Volt v s S C E
vs S C E
Figure 3. (a). Normal pulse voltammogram of a 5 X 10-4M A g ( l ) solution in 50% H F at the GCE. T = 15.6 "C. Initial potential = 0.800 volt vs. SCE. v = 10 mV/sec. ( b ) .Derivative pulse voltammogram of a 5 X 10-4M solution of A g ( l ) in 50% H F . T = 15.6 OC. Initial potential = 0.800 volt vs. SCE. v = 10 mV/sec. (c).Differential pulse voltammogram of a 1 X 10-4M solution of A g ( l ) in 50% HF. T = 15.6 "C.Pulse amplitude = 25 mV. v = 5 mV/sec
yL
Volt v s S C E
03
04 05
0 6 0 7
OB
vdt vs S C E
Figure 4. Dc stripping curves at the GCE for a 5 X 10-5M A g ( l ) solution in 50% H F after a 3-minute electrolysis. T = 13.6 "C. ( a ) Metrohm instrumentation, electrolysis potential 0.2 volt vs. SCE. v = 1 volt/minute. ( b ) PAR instrumentation, electrolysis potential 0.0 volt vs. SCE. v = 20 mV/sec
-' D u n 1
OB
ds d4
07 06 Vdt
-1 . 0'3
02
1s S C E
06
05
04 0 3 02 volt v5 S C E
Q
Figure 5. Linear sweep dc, ac, and pulse voltammograms of the Hg(ll) F? Hg(0) electrode process. T = 13.8 "C. [Hg(ll)] = 5 X 10-3M. (a) Linear sweep dc voltammogram. v = 50 mV/ sec. ( b ) Linear sweep ac voltammogram. Alternating potential = 10 mV p-p, frequency = 100 Hz, v = 10 mV/sec. Phase
sensitive detection. (c) Normal pulse polarogram. Initial potential = 1.000 volt vs. SCE. v = 20 mV/sec
tected, but the readout form is not as convenient or well defined as the normal or derivative pulse polarogram, and these latter techniques are considered to be superior for the determination of silver. Derivative Linear Sweep dc Voltammetry. As an alternative to normal linear sweep voltammetry, the derivative readout form can be used. With silver(1) a t the GCE, the derivative readout is well defined and gives clearly defined peaks. However, little advantage over other techniques was found. Stripping or Inverse Voltammetry. Since the Ag(1) F! Ag(0) electrode process in H F has been shown to lead to electrodeposition of a metallic silver film on the GCE, the very sensitive electroanalytical technique of anodic stripping should be possible (6, 12, 13). In the present work, a controlled potential of 0.0 or 0.2 volt us. SCE was used with a 3-minute electrolysis. Stirring with a Teflon-coated magnetic bar was used for the first 2 minutes. Figure 4 shows two stripping curves for silver at the 5 x lO-5M level. These curves are exceedingly well defined, and for qualitative analysis the stripping method was found to be useful down to at least lO-?M.
Unfortunately, however, the reproducibility was exceedingly poor and at 5 x lO-5M only *15% reproducibility was obtained. Similar findings were noted with ac and pulse stripping methods. Presumably the silver does not deposit reproducibly on the GCE under the conditions of this work. Undoubtedly, lower levels of silver would be detected with longer electrolysis times coupled with rotationof the electrode (14, 15). Mercury(I1). Linear Sweep dc, ac, and Pulse Methods. The determination of mercury is of considerable importance and, in the HF solvent system, mercury gives an extremely well defined dc wave at the GCE, with characteristics very similar to silver. The overall electrode process being considered is
( 1 2 ) M. Kopanicaand F. Vydra,d. Electroanal. Chem., 31, 175 (1971). (13) E. Ternrnerman and F. Verbeek, Anal. Chim. Acta, 58, 263 (1972).
(14) M. Stulikova and F. Vydra, J. Electroanal Chem., 42, 127 (1973). (15) T. M. Florence, J. Eiectroanal. Chem., 27, 273 (1970).
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ANALYTICAL CHEMISTRY, VOL. 46, NO. 8, J U L Y 1974
Hg(I1)
+ 2e
e Hg(0)
(4)
Figures 5a, b, and c show that a well-defined dc linear sweep voltammogram, an ill-defined ac voltammogram, and an excellent pulse voltammogram are obtained a t the GCE in 50% HF. Other characteristics such as limit of detection, reproducibility, etc., are essentially the same as for silver.
Voltammetry at the GCE at Negative Potentials.
-06
-04 Volt
VI
-08 SCE -05
-06
6 l
Volt vs AqlAqCl
Figure 6. Comparison of a pulse voltammogram at a GCE and a Teflon DME polarogram for a 1 X 10-4M solution of cadmiurn(ll) in 50% HF. T = 16 'C. (a) Normal pulse voltammogram. Initial potential = -0.4 volt vs. SCE. v = 10 mV/sec. ( b ) Teflon DME polarogram
Glassy carbon is a most versatile and useful electrode material because, in addition to providing a greatly extended positive potential region, it, like mercury, has a considerable hydrogen overvoltage. Hydrofluoric acid is a moderately acidic medium and previous work has shown that with mercury electrodes ( I , 3), the negative potential region is limited at about -0.9 to -1.0 volt us. SCE by hydrogen evolution. Evolution of hydrogen does not occur until about -0.8 volt us. SCE at the GCE and the useable cathodic range is almost as great as with mercury electrodes. In principle, therefore, it should be possible to compare polarography with a Teflon DME with voltammetry a t the GCE. The reduction of Sn(II), Cd(II), Tl(I), and Pb(II), which have been studied previously with a Teflon DME ( I ) , were therefore investigated this time simultaneously with both types of electrodes. At the dropping mercury electrode, these species are reduced reversibly or almost reversibly. However, with glassy carbon, nonreversibility
i
b
I
'1
0.4 u
T i,
C
Ab
!
1I
' 1
'
3uA
)
I I I
1
\
) 1
,I
,
-09 - 0 8 -07 -06 - 0 5 Volt v s S C E
d1 -1
,
,'
,
,
,
- 0 9 -06 -07 -06 - 0 5 - 0 4 - 0 3 Volt v s s c E
-09 - 0 8
-07 -06 - 0 5 -04 vs S C E
Volt
Figure 7. Anodic stripping voltammograms of cadmium in 50% H F at an in situ mercury plated GCE. Deposition potential = -0.9 V vs. SCE. Deposition time = 3 min. [ C d ( l l ) ] = 2 X 1 0 - 5 M . [ H g ( N 0 3 ) 2 ] = 5 X lOV4M. v = 10 mV/sec. T = 16 "C. ( a ) Dc linear sweep, ( b ) derivative linear sweep, (C)
normal pulse
At a scan rate of 20 mV/sec, E , was 0.45 volt us. SCE and became less positive with increasing scan rate. As is characteristic of many aqueous solvent systems, mercury and silver have similar E , values and the simultaneous determination of both mercury and silver is not possible. The conclusion that ,pulse voltammetry is the best of the electroanalytical techniques used, was strongly confirmed with mercury. Anodic Stripping or Inverse Voltammetry. The anodic stripping of mercury from glassy carbon was studied in considerable detail, using several voltammetric techniques. Controlled potential electrolysis for 3 min was carried out at 0 volt us. SCE. As for silver, extremely sharp well-defined curves are obtained, but the reproducibility was only f12% at the 10-5M level. The E , value for the oxidation of mercury is also considerably more positive than the E , value for reduction of mercury(I1) and it is concluded that, as for silver, the electrode process possesses a degree of irreversibility. Ac and pulse anodic stripping curves also provided a reasonably sensitive qualitative method for detection of mercury, but the reproducibility was essentially the same as the dc method. Presumably, rotation of the electrode, and methods of the kind employed in (14) are required for the best results for the stripping of mercury from a GCE.
was found in all cases. Lead(II), for example, was found to be not reduced within the available potential range, nor was thallium(1). Tin(I1) gave behavior analogous to mercury and silver, as did cadmium. Figures 6a and b provide a comparison of a normal pulse voltammogram obtained a t a GCE and a polarogram recorded with a Teflon DME on a 1 x 10-4M solution of Cd(I1) in 50% HF. Because deposition of the metals onto mercury surfaces to form an amalgam does appear to proceed reversibly for these systems, polarography is a superior analytical technique, provided of course that the fabrication of a Teflon DME can be undertaken. Recently, Florence (15-17) has shown that anodic stripping voltammetry with a rotating glassy carbon electrode, using mercury plating in situ is a convenient and highly sensitive technique for the determination of many metals. In 50% HF, mercury has been shown to be deposited onto glassy carbon, and polarographic data suggest reversible or quasi-reversible electrode processes occur a t mercury electrodes for Pb, Cd, Sn, and T1. Hence, the in situ anodic stripping voltammetry should be applicable to the HF solvent system. Work with wax-impregnated graphite (16) T. M. Florence, J. ElectroanaL Chem., 26, 293 (1970). (17) T. M. Florence, J. €/ectroana/. Chem., 35, 237 (1972).
ANALYTICAL CHEMISTRY, VOL. 46, NO. 8, J U L Y 1974
1067
ac
-06 - 0 5 - 0 4 - 0 3 - 0 2 Volt
VI
SCE
Figure 8. Ac and dc anodic stripping voltammogram of lead in 50% HF at an in situ mercury plated glassy carbon electrode. [ P b ( l l ) ] = 2 X 10-5M. [ H g ( N 0 3 ) 2 ] = 10-3M. Scan rate = 10 mV/sec. T = 15.4 "C. For ac curve, alternating potential = 10 mV p-p, frequency = 100 Hz. Phase sensitive detection employed electrodes (mercury plated) (18) suggests pulse and other methods could offer substantial advantages over dc methods used by Florence. Figure 7a shows a cadmium anodic stripping curve for 2 x lO-5M cadmium(I1) and 5 x 10-4M mercury(I1) solutions in 50% H F using in situ mercury plating. A deposition potential of -0.9 volt us. SCE was applied for 3 min with a 2-min stirring period to simultaneously deposit both cadmium and mercury. The presence of the mercury leads to the observation of an extremely sharp well-defined stripping peak and behavior which is close to reversible is indicated; 1 x lO-7M cadmium was readily detected and the half height width of (30 2) mV reveals that excellent resolution is achieved. The reproducibility a t the 10-6M level was *4% and considerably better than in the absence of the mercury film. The in situ mercury plating technique is therefore found to be substantially superior to direct electrodeposition onto the GCE. Figure 7b shows a derivative cadmium anodic stripping curve for the same solution as in Figure 7a. For reversible electrode processes observed with the in situ mercury plating, either the positive or negative branches of the derivative stripping voltammogram or the peak to peak height can be used for determination of cadmium. Similar results were obtained with the other metals.
*
(18) J. B. Flato,AnaL Chem., 4 4 ( l l ) , 75A (1972).
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ANALYTICAL C H E M I S T R Y , VOL. 46, NO. 8, J U L Y 1974
Figure 7c shows a pulse stripping voltammetric curve for cadmium on the GCE plated in situ with mercury. The readout is very similar to the linear sweep dc curve. A derivative pulse stripping voltammogram is similar in shape to the derivative dc curve shown in Figure 7 b . As was found with the linear sweep reduction voltammetry, the pulse methods were also more sensitive than dc techniques in the anodic stripping work. An improvement in the limit of detection is suggested with the pulse techniques, because a t the limit of detection of the dc method of about 10-7M, the pulse anodic stripping curves were still extremely well-defined. Figure 8 shows dc and ac linear sweep stripping curves for lead(1I) in 50% H F using the mercury coated GCE. Lead(I1) is not electroactive in the useable potential range on a GCE, but the mercury film enables a reversible stripping curve to be obtained. Although the ac curve is well-defined, it does not offer any apparent advantages over the dc stripping curves.
CONCLUSIONS The GCE enables voltammetry to be readily undertaken in concentrated hydrofluoric acid. The high hydrogen overvoltage means that potentials almost as negative as those with mercury electrodes can be studied but, in addition, positive potentials can be observed over a much wider range. Electrodeposition of metals onto the GCE in 50% H F does not appear to be reversible for Ag, Hg, Cd, T1, Sn, or Pb, although the last four species are reduced reversibly or quasi-reversibly on mercury electrodes. Pulse voltammetry at the GCE appears to offer substantial advantages over dc and ac methods for the systems studied. Anodic stripping voltammetry using GCE plated in. situ with mercury enables derivative and pulse stripping techniques to be used advantageously in concentrated HF. ACKNOWLEDGMENT The authors gratefully acknowledge the assistance of A. B. Waugh in construction of the GCE's, the Geology Department of this University for use of equipment and materials associated with the fabrication of the GCE's, and T. M. Florence for helpful discussions. Received for review July 11, 1973. Accepted November 20, 1973. We would like to thank ARGC for financing the purchase of much of the instrumentation. One of us (RJT) is grateful to the Australian Commonwealth Department of Education for a Commonwealth Post-Graduate Research Award.
