Electrochemistry

three principal sources for the analytical signal potential, current, charge a wide variety of experimental designs are possible

The simplest division is between bulk methods, which measure properties of the whole solution (Conductometric methods) Interfacial methods, in which the signal is a function of phenomena occurring at the interface between an electrode and the solution in contact with the electrode.

Interfacial Electrochemical Methods

Controlling and Measuring Current and Potential Electrochemical measurements are made in an electrochemical cell, indicator electrode The electrode whose potential is a function of the analyte’s concentration (also known as the working electrode). counter electrode The second electrode in a two-electrode cell that completes the circuit. reference electrode An electrode whose potential remains constant and against which other potentials can be measured.

there are only three basic experimental designs: (1) measuring the potential under static conditions of no current flow; (2) measuring the potential while controlling the current; and (3) measuring the current while controlling the potential.

Ohm’s law The statement that the current moving through a circuit is proportional to the applied potential and inversely proportional to the circuit’s resistance (E = iR).

potentiometer A device for measuring the potential of an electrochemical cell without drawing a current or altering the cell’s composition.

Manual potentiostat potentiostat A device used to control the potential in an electrochemical cell.

Galvanostat galvanostat A device used to control the current in an electrochemical cell.

Electrochemical cell

Ag(s) + Cl–(aq) t AgCl(s) + e–

The cathodic reaction (the right-hand cell) is the reduction of Fe3+ to Fe2+ Fe3+(aq) + e– t Fe2+(aq) The overall cell reaction, therefore, is Ag(s) + Fe3+(aq) + Cl–(aq) t AgCl(s) + Fe2+(aq) The electrochemical cell’s shorthand notation is Ag(s) | HCl (aq, M), AgCl (sat’d) || FeCl2 (aq, M), FeCl3 (aq, M) | Pt Note that the Pt cathode is an inert electrode that carries electrons to the reduction half-reaction. The electrode itself does not undergo oxidation or reduction.

Potential and ConcentrationÑThe Nernst Equation The potential of a potentiometric electrochemical cell is given as Ecell = Ec – Ea

Liquid junction liquid junction potential A potential that develops at the interface between two ionic solutions that differ in composition, because of a difference in the mobilities of the ions (Elj).

Electrodes SHE

Calomel Electrode(SCE)

Silver/silver chloride electrode

Metallic indicator electrode

First kind

Second kind

Membrane Electrode

Glass ion selective electrodes

Voltammetric measurements hanging mercury drop electrode An electrode in which a drop of Hg is suspended from a capillary tube. dropping mercury electrode An electrode in which successive drops of Hg form at the end of a capillary tube as a result of gravity, with each drop providing a fresh electrode surface. static mercury drop electrode An electrode in which successive drops of Hg form at the end of a capillary tube as the result of a mechanical plunger, with each drop providing a fresh electrode surface. amalgam A metallic solution of mercury with another metal.

Hg electrodes

Current in voltammetry faradaic current Any current in an electrochemical cell due to an oxidation or reduction reaction. cathodic current A faradaic current due to a reduction reaction. anodic current A faradaic current due to an oxidation reaction.

Mass Transport migration, convection. Diffusion diffusion The movement of material in response to a concentration gradient. diffusion layer The layer of solution adjacent to the electrode in which diffusion is the only means of mass transport.

Diffusion layer

Three common shape of voltammogram

Voltammetric Techniques Polarography Potential excitation signal by the Ilikovic equation (ilim)max = 706nD 1/2 m 2/3 t 1/6 C A (ilim)avg = 607nD1/2m2/3t1/6CA

Voltammetric Techniques Hydrodynamic Voltammetry –In hydrodynamic voltammetry current is measured as a function of the potential applied to a solid working electrode. Stripping Voltammetry –which is composed of three related techniques: –anodic, cathodic, and adsorptive stripping voltammetry.

Stripping

Normal pulse voltametry

Differential pulse

Staircase polarography

Suare wave polarography

Cyclic voltammetry i E E time

Application Clinical Samples Differential pulse polarography and stripping voltammetry have been used to determine the concentration of trace metals in a variety of matrices, including blood, urine, and tissue samples. The determination of lead in blood is of considerable interest due to concerns about lead poisoning.

Besides environmental and clinical samples, differential pulse polarography and stripping voltammetry have been used for the analysis of trace metals in other samples, including food, steels and other alloys, gasoline, gunpowder residues, and pharmaceuticals. Voltammetry is also an important tool for the quantitative analysis of organics, particularly in the pharmaceutical industry, in which it is used to determine the concentration of drugs and vitamins in formulations.

Evaluation Scale of Operation Voltammetry is routinely used to analyze samples at the parts-per-million level and, in some cases, can be used to detect analytes at the parts-per-billion or parts-per-trillion level. Accuracy The accuracy of a voltammetric analysis often is limited by the ability to correct for residual currents, parts-per- million level, accuracies of ±1–3%

Evaluation Precision Precision is generally limited by the uncertainty in measuring the limiting or peak current. Under most experimental conditions, precisions of ±1– 3%. One exception is the analysis of ultratrace analytes in complex matrices by stripping voltammetry,(precisions as poor as ±25% Sensitivity In many voltammetric experiments, sensitivity can be improved by adjusting the experimental conditions Selectivity Selectivity in voltammetry is determined by the difference between half-wave potentials or peak potentials, with minimum differences of ±0.2–0.3 V required for a linear potential scan, and ±0.04–0.05 V for differential pulse voltammetry.

evaluation Time, Cost, and Equipment Commercial instrumentation for voltammetry ranges from less than $1000 for simple instruments to as much as $20,000 for more sophisticated instruments. In general, less expensive instrumentation is limited to linear potential scans, and the more expensive instruments allow for more complex potential-excitation signals using potential pulses. Except for stripping voltammetry, which uses long deposition times, voltammetric analyses are relatively rapid.