voltammetry
mass transport
methods that measure current as a function of applied voltage
Working Electrode
Solution is well mixed ( e.g., by stir bar or flowing stream)
- i.e., monitoring Faradaic current from an electrochemical reaction
Stagnant solution (Nernst Diffusion Layer)
Nernst Diffusion Layer – viscous viscous drag causes a thin film of solution to Bulk Solution become stagnant at the electrode (well mixed) surface = 10-2-10-3 cm depending on stirring and solution viscosity - mass transport only by diffusion in film
Working Working Electrode – electrode at which the reaction of analytical interest occurs . Electrode is small (microelectrode, mm2) to very small (ultramicroelectrode, µm 2)
analyte experiences “resistance to mass transfer” ( i.e., diffusion-limited transport) across the Nernst film
Diffusion current
linear sweep voltammetry
current observed when the rate of electrode reaction is limited by the rate of diffusion diffusion of the analyte to the electrode surface
n = number of electrodes in half reaction = Faraday’s constant F = A = A = surface area of electrode DA = diffusion coefficient of analyte ! = thickness of Nernst diffusion film cA = analyte concentration in bulk solution
The characteristics of the linear sweep voltammogram depend on a number of factors: *The rate of the electron transfer reaction(s) *The chemical reactivity of the electroactive species *The voltage scan rate
cyclic voltammetry
atomic microscopy and CV
varying the applied potential at a working electrode in both forward and reverse directions (at some scan rate) while monitoring the current
rarely used for quantitative determinations widely used for the study of redox processes for understanding reaction intermediates for obtaining stability of reaction products
relative to the onset of bulk Bi deposition which is 190 ± 5 mV vs SHE in 0.1 M HC10 4. CHEN CH, KEPLER KD, GEWIRTH AA, et al. ELECTRODEPOSITED BISMUTH MONOLAYERS ON AU(111) ELECTRODES - COMPARISON OF SURFACE X-RAY-SCATTERING, SCANNINGTUNNELING-MICROSCOPY, AND ATOMIC-FORCE MICROSCOPY LATTICE STRUCTURES JOURNAL OF PHYSICAL CHEMISTRY 97 (28): 7290-7294 JUL 15 1993
stripping voltammetry Stripping analysis is an analytical technique that involves (i) pre concentration of a metal phase onto a solid electrode surface or into Hg (liquid) at negative potentials and (ii) selective oxidation of each metal phase species during an anodic potential sweep.
Experimental Details (1) Preconcentration step (2) Stripping step Edep V , l a i t n e t o P
! (1)
tdep (2) Eclean
M1n+
pre concentration occurs via adsorption or electrochemical reaction stripping can be either cathodic (reduction) or anodic (oxidation)
Working electrode: Electrode at which the reaction of
Time, s
interest occurs. (Reduction or Oxidation) Electrode materials: Hg, Hg-film, gold, carbon
E1o’
A , t n e r r u C
E2o’ M2n+
Potential, V
Parameters of interest Peak potential Peak current Peak charge Peak width
The metals strip off near their respective E 0’.
Deposition Step Hg: spherical droplets
Stripping Step
Thin-film of spherical Hg
Mn+ + ne-
!
M(Hg)
1) The stripping peak current ( and charge) is proportional to the concentration of the metal in or on the electrode, and thereforeto its concentration in the sample solution 2) Peak potentials serve to identify the metal ions in the sample
Application of Faradays law enables the concentration of the metal in the amalgam, (C M), to be calculated:
Thin films of Hg: Peak current (Ip) is given by,
where A = Area, l = thickness,
Ip = k CA = n2F2 !1/2 A l CA
CA = concentration, ! = scan rate
CM = iL td / nF VHg
7.5 µm
Control Parameters ! Film morphology and architecture
!
Deposition potential
!
Deposition time
2.7 RT
iL: limiting current for the deposition of the metal
35.0
F: Faraday
30.0
Cd(II)
Variation of peak currents vs. deposition time
Deposition times
VHg: Volume of the Mercury electrode td: Deposition time
Incase of a metallic film deposited on a inert solid electrode, the amount, in moles, of metal deposited on the surface (M) is: M = iL td / nF
) A 25.0 µ ( t 20.0 n e 15.0 r r u 10.0 C
30s 60s 90s 120s 150s 180s
5.0 0.0 -5.0 -1000
-900
-800
-700
-600
-500
Applications of ASV Advantages of Stripping Voltammetry 1) Linear Dynamic Range: 4-5 orders of magnitude 2) Limits of Detection: (S/N = 3), high pptto low ppb range 3) Response precision: < 2-3 % is common 4) Sensitive to oxidation states 5) Low cost for instrumentation and running 6) Allows Standard addition: no matrix effects 7) Compact
1) Environmental Analysis: Water, soil samples etc. 2) Clinical Analysis: Blood, Urine samples 3) Food Analysis 4) Gasoline and Oil analysis 5) Pharmaceutical analysis
Major Interferences in ASV 1) Overlapping stripping peaks caused by similarity in oxidation potential 2) Presence of surface-active organic compounds that adsorb on the Hg surface and inhibit metal deposition 3) Formation of intermetallic compounds (e.g., Cu-Zn) which affect peak size and position
Source: http://www.icsu-scope.org/downloadpubs/scope51/images/fig13.1.gif