31-07-2012, 09:57 AM
Introduction to Electroanalytical Chemistry
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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)
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
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
Fundamentals
Electrical charge, q, is measured in coulombs ©. The charge associated with chemical species is related to the number of moles through the Faraday constant, F=96,485.3 (~96,500) C/mole.
Electrical current, I, is measured in Amperes (A). Current is the amount of charge that passes in a unit time interval (seconds).
Ohm's law relates current to potential (E) through the resistance ® of a circuit by E=IR. The potential is measured in Volts (V) and the resistance in Ohms ().
More About pH Measurements
The surface of the glass is hydrated, which allows exchange of hydronium ions for the cation in the glass (sodium or lithium).
There are four interface regions, the external solution and hydrated glass, hydrated glass and dry glass on the outside, dry glass and hydrated glass on the inside, and hydrated glass and the internal solution.
If the glass is uniform, the two hydrated glass/dry glass interfaces should be identical and should have the same junction potential.
Since the glass interface junction potentials then cancel each other, the junction potential is then the difference between the internal and external solutions.
[attachment=29607]
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)
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
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
Fundamentals
Electrical charge, q, is measured in coulombs ©. The charge associated with chemical species is related to the number of moles through the Faraday constant, F=96,485.3 (~96,500) C/mole.
Electrical current, I, is measured in Amperes (A). Current is the amount of charge that passes in a unit time interval (seconds).
Ohm's law relates current to potential (E) through the resistance ® of a circuit by E=IR. The potential is measured in Volts (V) and the resistance in Ohms ().
More About pH Measurements
The surface of the glass is hydrated, which allows exchange of hydronium ions for the cation in the glass (sodium or lithium).
There are four interface regions, the external solution and hydrated glass, hydrated glass and dry glass on the outside, dry glass and hydrated glass on the inside, and hydrated glass and the internal solution.
If the glass is uniform, the two hydrated glass/dry glass interfaces should be identical and should have the same junction potential.
Since the glass interface junction potentials then cancel each other, the junction potential is then the difference between the internal and external solutions.