Advantages of Electroanalytical Methods

Advantages of Electroanalytical Methods

• Matched against a wide range of spectroscopic and chromatographic techniques, the techniques of electroanalytical chemistry find an important role for several reasons:

• Electroanalytical methods are often specific for a particular oxidation state of an element
• Electrochemical instrumentation is relatively inexpensive and can be miniaturized
• Electroanalytical methods provide information about activities (rather than concentration)

History of Electroanalytical Methods

• Michael Faraday: the law of electrolysis

• “…the amount of a substance deposited from an electrolyte by the action of a current is proportional to the chemical equivalent weight of the substance.”

• Walter Nernst: the Nernst equation (Nobel Prize 1920)

• Jaroslav Heyrovsky: the invention of polarography: (Nobel Prize 1959)

Main Branches of Electroanalytical Chemistry

• Key to measured quantity: I = current, E = potential, R = resistance, G = conductance, Q = quantity of charge, t = time, vol = volume of a standard solution, m = mass of an electrodispensed species

Main Branches of Electroanalytical Chemistry

• Potentiometry: measure the potential of electrochemical cells without drawing substantial current

• Examples: pH measurements, ion-selective electrodes, titrations (e.g. KF endpoint determination)

• Coulometry: measures the electricity required to drive an electrolytic oxidation/reduction to completion

• Examples: titrations (KF titrant generation), “chloridometers” (AgCl)

• Voltammetry: measures current as a function of applied potential under conditions that keep a working electrode polarized

• Examples: cyclic voltammetry, many biosensors

Electrochemical Cells

Electrochemical Cells

• Galvanic cell: a cell that produces electrical energy

• Electrolytic cell: a cell that consumes electrical energy

• Chemically-reversible cell: a cell in which reversing the direction of the current reverses the reactions at the two electrodes

Conduction in an Electrochemical Cell

• Electrons serve as carriers (e.g. moving from Zn through the conductor to the Cu)

• In the solution, electricity involves the movement of cations and anions

• In the salt bridge both chloride and potassium ions move

• At the electrode surface: an oxidation or a reduction occurs

• Cathode: the electrode at which reduction occurs
• Anode: the electrode at which oxidation occurs

Faradaic and Non-Faradaic Currents