Mercury Frit Electrode, a N e w Indicator Electrode for Peak Voltammetry SIR: The theory and analytical import of voltammetry with linearly varying potential at stationary electrodes has been discussed in an authoritative paper by Nicholson and Shain (6). Voltammograms so obtained exhibit peaks at potentials related to t’he relevant polarographic half-wave potentials. Single sweep voltammetry has been employed utilizing the mercury pool, hanging drop, as well as various solid indicator electrodes. With application of sufficiently fast scan rates ( = 1 volt/sec., oscilloscopic polarography), peak polarograms are also obtained at the dropping mercury electrode. Rapid scan rates, however, possess the attendant liability of large capacitance current.s. Slow scan rates at stationary electrodes minimize these, but under these conditions most conventional solid electrodes are subject to convective effects, which distort the current-voltage curves. In the extreme case, the expected peak voltammograms may degenerate into sigmoid-shaped curves reminiscent of classical polarographic waves ( 5 ) . Prevalence of ideal peak voltammograms is contingent upon the absence of mising of the solution by natural and/or forced convection. Consequently, peak voltammograms are a characteristic of diffusion electrodes where molecular or ionic diffusion is the mode of transport of the electroreactive species from the bulk of the solution to the electrode -surface. .I\ novel indicator electrode has been de:leloped which yields peak voltammograms at the slow potential scan rates (10-3-10-2 volt/sec.) normally employed in d.c. polarography. This mas accc,mplished through immobilization of the electrolyte solution within the pores of a borosilicate glass frit in contact with an overlying mercury head of 7-10 cm. This mercury frit electrode (MFE)and the complementary electrolysis cell are illustrated in Figure 1. ‘The glass matrix employed for restraining convection consisted of a cylindrical fritted glass disk of coarse porosity (Pyres S o . 39570-1OC made of glass So. 7740 hy Corning Glass Works, Corning, K‘, Y . )which had the following characteristics ( 2 , 3): diameter, 8.5 mm.; t,hickness, 2 mm.; pore diameter, 40 to 60 microns; porosity (fraction of 1398
ANALYTICAL CHEMISTRY
TUNGSTEN LEAD WIRE /
ELECTROLYTE SOLUTION
-
Pt EL
POROUS SEPTUM
LUCITE
GLASS ELBO LEADING TO
:EpR,E,N,c,E, ~_-_ _ I ,
/
TAPERED JOINT
SUGORTING ELECTROLYTE
Figure 1 . Mercury frit electrode (MFE) with machined Lucite electrolysis cell
penetrable void pore space referred to the total cylindrical volume between the external frit boundaries), e = 0.28
*
0.01.
Conceptually the LIFE can be visualized as a membrane electrode (1) in which the thickness of the membrane (frit) exceeds the thickness of the diffusion layer a t all times and where the pore diameter is far greater than molecular dimensionh. Typical .voltammograms obtained at the LIFE for the Kmujt controlled electrode reactions: T1+ e Hg e Tl(Hg) (1)
+ + Pb+l + 2e + Hg Cd+2 + 2e + Hg
Pb(Hg)
(2)
;j Cd(Hg) (3) are shown in Figure 2. As a first approximation, a modified form of the Randles-Sevcik Equation is applicable to the MFE, viz. : i, = k-4.4,,s(U/q)”2n3’51’2C (4)
where i, dencres t!,e peak current expressed in amperes and IC = 0.1463 53’2.R-:’2 T-1 2 The symbol 8 de-
notes the faraday constant (96,500 coulombs). The product Acre represents a measure of the effective area (Le., the area open to diffusion) of the MFE. A,, denotes the circular cross section of the frit disk (expressed in sq. cm.) and c is the porosity defined earlier. D is the diffusion coefficient of the electroreactive species expressed in sq. cm./sec. The symbol q denotes a dimensionless tortuosity factor which corrects for the hindrance to diffusion resulting from bent (and/or obstructed) pores within the frit. For an ideal matrix, composed solely of parallel cylindrical pores oriented normal to the electrode surface, q = 1 and Dq-’ = D. The other notations in Equation 4 have their usual significance: n refers to the number of Faradays per mole involved in the electrode reaction, v is the potential scan rate in volts per second and C denotes the bulk concentration of electroreactive species, expressed in moles per liter. Equation 4 represents a straight-forward adaptation of the general theory presented by Nicholson and Shain (6): It is an analog of
Equation 25 in their paper. Equation 4 differs from the corresponding Nicholson-Shain expression for a conventional planar electrode by the factor ~ q - ' / ~ which is accounted for by the geometry of the MFE. The significance of the parameters E and q has been satisfactorily elucidated in recent studies on porous electrodes, of the type employed in fuel cells. Reference (2) may be consulted for a bibliography of the relevant literature and for a rigorous derivation of Equation 4. Whenever a specified electroreactive species is involved a t a given M F E kA,,t(D/q)
=
const.
(5)
and Equation 4 can be formulated as:
i,, = const. V ~ / ~ C (6) The proportionality relationships implicit in Equation 6 have been thoroughly verified. Data obtained in a large (>100) number of experiments have substantiated that Potential of MFE,volts - 2 s (;)u==conBt,
=
const.'
when: 5 X lo-'
Figure 2.
