Amperometry

The amperometry is an electrochemical method for the quantitative determination of chemical substances. In amperometric titration , the electrochemically generated current flow is used as evidence of the completeness of a conversion.

General

The amperometric method is characterized by the measurement of an electrolysis current on a working electrode while a time-constant electrochemical potential is applied. Amperometry is thus derived from voltammetry , in which the electrolysis voltage changes over time. For a titration , either the existing solution (Titrand) or the titrated solution (titrator) must contain substances that can be oxidized or reduced at the electrodes. The current intensity is measured as a function of the amount of solution added. If the addition is ended during the titration or if a reactant is completely used up, the electrolysis current rises or falls again to the starting potential (depending on the redox system).

The measured electrolysis current is directly proportional to the concentration of the converted substance. This allows unknown concentrations to be determined with the aid of a calibration function . Commonly used materials for working electrodes are: platinum, gold, carbon, mercury and silver. With the Clark electrode , dissolved oxygen is reduced at a constant potential. This principle of oxygen determination is widely used in industry and environmental analysis. The working electrodes of amperometric sensors can be coated with a layer that reacts selectively with the substance to be analyzed. Amperometric glucose sensors that are modified with the enzyme glucose oxidase are very widespread in medical diagnostics . In its function as a biocatalyst , this enzyme converts the analyte grape sugar (glucose) into gluconic acid and hydrogen peroxide . This consumes oxygen. The increase in the hydrogen peroxide concentration or the decrease in the oxygen concentration is actually registered amperometrically, depending on the electrolysis potential selected.

Chronoamperometry

As the name suggests ( ancient Greek χρόνος chrónos "time" and μέτρον métron "measure, scale"), the time dependence of the current is measured and evaluated in chronoamperometry. With this relaxation method, the changing electrolysis current is registered after a potential jump. Before this, a potential is applied to the working electrode at which the analyte is not yet converted. When the potential changes suddenly to a new value that is constant over time, the oxidation or reduction of the analyte begins and an electrochemical current begins to flow. This current has its maximum value immediately after the potential jump and then drops. The course over time is described by the Cottrell equation (published by Frederick Gardner Cottrell in 1903).

${\ displaystyle I = {\ frac {zFA {\ sqrt {D}}} {\ sqrt {\ pi t}}} c}$

Herein mean:

I - electrolytic current

z - number of electrons transferred

F - Faraday's constant (96,485.3 As / mol)

D - diffusion constant (among other things depending on the viscosity of the solution and the size of the diffusing particles)

A - electrode surface

t - time

c - initial concentration of the reacted substance

The product is constant for the investigated substance in a certain period of time during the measurement and is dependent on the initial concentration c , the diffusion constant D and the number of transferred electrons z (change in the oxidation state of the substance). Therefore, the Cottrell equation can be used to calculate the initial concentration or the change in the oxidation state or the diffusion constant. ${\ displaystyle I \ cdot {\ sqrt {t}}}$

Biamperometry

In this simplified variant of the amperometry, two identical working electrodes are used, each made of a platinum wire, for example. Between the two electrodes there is a small resistance (e.g. 10 ohms), between the anode electrode and the positive pole there is a large resistance (e.g. 4 kilo ohms), with battery voltages of 0.5 to 1 V the current is applied measured at the anode. An electrolysis current can only flow if both electrodes are subject to conversion. This occurs when both components are electroactive species and the electrical voltage between the two electrodes is sufficiently high (usually 10–100 mV). This method is used for dead-stop titration.

Dead-stop titration is used, for example, to detect small traces of water by means of Karl Fischer titration. Sulfur dioxide reacts with iodine and water to form iodide and sulfuric acid. The inactive iodide is still present before the end point. As soon as there are free iodine molecules, the current increases.

literature

Ullmann's Encyclopedia of Technical Chemistry, 4th edition, keyword: electrochemical analysis methods.
Georg Schwedt, Analytical Chemistry, Wiley-VCH, 2nd edition 2008, ISBN 978-3-527-31206-1
Karl Cammann (Ed.), Instrumental Analytical Chemistry, Spectrum Academic Publishing House, Heidelberg - Berlin, 2001.

Individual evidence

↑ Frederick Gardner Cottrell : The residual current in galvanic polarization, viewed as a diffusion problem . In: Wilhelm Ostwald, Jacobus Henricus van 't Hoff (ed.): Journal for physical chemistry . 42U, no. 4 . Wilhelm Engelmann / De Gruyter, October 1903, ISSN 2196-7156 , p. 385–43 , doi : 10.1515 / zpch-1903-4229 ( online in the Internet Archive ).
↑ Bernd Speiser: Electroanalytical Methods I: Electrode Reactions and Chronoamperometry . In: Chemistry in Our Time . tape 15 , no. 1 , February 1981, ISSN 1521-3781 , pp. 21-26 , doi : 10.1002 / ciuz.19810150105 ( wiley.com ).

[1] Frederick Gardner Cottrell : The residual current in galvanic polarization, viewed as a diffusion problem . In: Wilhelm Ostwald, Jacobus Henricus van 't Hoff (ed.): Journal for physical chemistry . 42U, no. 4 . Wilhelm Engelmann / De Gruyter, October 1903, ISSN 2196-7156 , p. 385–43 , doi : 10.1515 / zpch-1903-4229 ( online in the Internet Archive ).

[2] Bernd Speiser: Electroanalytical Methods I: Electrode Reactions and Chronoamperometry . In: Chemistry in Our Time . tape 15 , no. 1 , February 1981, ISSN 1521-3781 , pp. 21-26 , doi : 10.1002 / ciuz.19810150105 ( wiley.com ).