Polarography

A polarographer

The polarography (a special case of voltammetry ) is an electrochemical process for the qualitative and quantitative analysis of chemical elements and compounds , especially ions and molecules in a solution . While stationary electrodes are used in voltammetry, mercury drop electrodes are used in polarography . It was developed in 1922 by Jaroslav Heyrovský and is based on measuring the electrolysis current on a mercury drop electrode. With the help of polarography it is possible to electrolytically deposit even base metals because of the large overvoltage of hydrogen on mercury at strongly negative potentials and to measure the current flowing in the process. This represents the analytical signal.

construction

Scheme of a polarograph

The mercury drop electrode consists of a mercury reservoir and a capillary from which the mercury drops fall into a solution to be examined. It is used as a working electrode (also measuring electrode) in polarography and is an ideally polarizable electrode, i. This means that an electrical potential can be impressed on it without a charge passing over the electrode-solution phase boundary, provided there are no depolarizers in the solution. Depolarizers are oxidizable or reducible substances. If this is the case, however, the charge passes through , the substance depolarizes the working electrode, and a current flows .

In a simple two-electrode arrangement, the counter electrode also takes on the function of the reference electrode . A three-electrode arrangement is more favorable, in which the electrolysis current flows via a counter electrode made of noble metal or carbon , while the reference electrode remains de-energized. As a reference electrode, an electrode of the second type , e.g. B. a calomel electrode or a silver-silver chloride electrode . The advantages are the longer service life of the reference electrode and less interference with the applied potential due to overvoltage effects on the counter electrode.

Measurement and concentration determination

During the measurement, a voltage that changes linearly over time is specified and the resulting current is recorded. When a substance in the solution causes a permeation reaction, there is an increase in current, i. that is, a step appears in the current-voltage curve. The position of the potential halfway up this level (half-level potential) is characteristic of every chemical species, which enables a qualitative analysis. The height of the step (i.e. the current) is given by the diffusion limit current, which occurs when the diffusion of the analyte from the interior of the solution to the electrode surface is the rate-determining reaction step . This enables quantitative analysis, since the diffusion limit current is related to the concentration of the analyte via the Ilkovič equation (a numerical equation ):

${\displaystyle {\overline {I_{\mathrm {g} }}}=607\,n\,D^{\frac {1}{2}}\,u^{\frac {2}{3}}\,t^{\frac {1}{6}}\,c_{\mathrm {L} }}$

${\displaystyle {\overline {I_{\mathrm {g} }}}}$ : temporal mean of the current that is limited by diffusion (mean diffusion limit current) in microampere (µA)

n : number of charges

D : diffusion coefficient in centimeters per second (cm ² s ⁻¹ )

u : mass flow of the outflowing mercury in milligrams per second (mg / s)

t  : lifetime of a drop in seconds (s)

c _L : Concentration of the analyte (depolarizer) inside the solution in mol per liter (mol / l)

Temperature in ° C	Density of mercury in g / cm³	Value of the factor
19.2	13.54783	606.502
20.0	13.54587	606,561
22.0	13.54096	606.707
25.0	13.53360	606,927
26.0	13.53115	607,000
32.8	13.51451	607.499

The equation was first derived from Dionýz Ilkovič . The factor K, also called Ilkovič constant, of around 607 results from solving the diffusion equation for the growing drop and averaging over the drop time. The exact theoretical value is given by the following expression:

${\displaystyle {\frac {K}{\mathrm {\tfrac {C\cdot cm^{3}}{g\cdot mol}} }}={\frac {F{\frac {6}{7}}{\sqrt {\frac {7}{3}}}{\sqrt[{3}]{36}}}{100\pi ^{\frac {1}{6}}\rho ^{\frac {2}{3}}}}={\frac {4\cdot 3^{\frac {7}{2}}\cdot F}{25\cdot {\sqrt {7}}\cdot \pi ^{\frac {1}{6}}\cdot \rho ^{\frac {2}{3}}}}\approx 2{,}3369{\frac {F}{\rho ^{\frac {2}{3}}}}}$

ρ is the density of the mercury, F the Faraday constant . The example values ​​in the table on the right show that the theoretical numerical value 607 should apply between 19.2 ° C and 32.8 ° C. The half-wave potential and the diffusion limit current are the characteristic values ​​for the type and quantity of the depolarizer (analyte) in the selected supporting electrolyte. The applicability of polarography is limited by some factors, such as: B. the occurrence of a capacitive current, which leads to an interference signal that raises the detection limit. In addition, drop peaks and so-called polarographic maxima occur (when the current rises above the limit current for various reasons).

Methods

A polarogram recorded with tactile polarography. The reference electrode was a saturated calomel electrode (SCE). The graphic evaluation of a polarographic step with half-step potential E _1/2 and step height I is also shown

These problems, as well as the requirement for higher resolution and accuracy, have resulted in several improved polarography methods:

• Rapid polarography : the drop is knocked off mechanically
• Touch polarography : the current is measured just before the drop falls
• Derivative polarography: instead of the step curve, the 1st derivative of this curve is used
• Differential direct current polarography : Use of two synchronously dripping mercury drip electrodes
• Cathode ray polarography : the entire potential area in question is traversed during a single drop life
• AC polarography : a low-frequency AC voltage is superimposed on the applied DC voltage
• Pulse polarography : a rectangular voltage pulse is applied at the end of the drop's life
  • Normal pulse polarography: A voltage pulse is applied that increases from drop to drop. In between, the voltage is zero. The current is registered at the end of each voltage pulse. Step signals are obtained.
  • Differential pulse polarography: a voltage ramp that increases linearly over time is used and a constant voltage pulse is added at the end of each drop's life. The current is registered before the beginning and the end of each voltage pulse. The difference between the two gives the current measured value. This gives peak-shaped signals.
• Kalousek polarography : positive square-wave pulses are superimposed on a direct voltage that becomes more negative or square-wave pulses that become more negative are superimposed on a constant direct voltage
• Inverse voltammetry : the substance to be determined is cathodically deposited on the hanging mercury drop electrode ( HMDE ) and redissolved by running the potential in a positive direction, with a current peak then occurring. It is not actually polarography because the mercury does not drip during the measurement. Instead, other electrode materials such as precious metals , carbon or, more recently, bismuth are also used in inverse voltammetry .

These methods can in part be further subdivided.

Status

Polarograph

Polarography is basically suitable for precise analysis in a small concentration range of a large number of inorganic and organic substances. Because of the large negative potential range of mercury, a cathodic (reductive) conversion takes place predominantly. The heyday of polarography extended from the 1930s to the 1980s. It was the first widely used instrumental analysis method. In the form of atomic spectrometry (element analysis ) and chromatography (organic analysis), significant alternative methods have emerged in the past decades, which are characterized in particular by a larger range of determinable analytes.

Advantages and disadvantages

The high achievable accuracy ( precision approx. 1%), low investment costs and the possibility of element species analysis are advantageous . In its modification as differential pulse polarography and inverse voltammetry, polarography has very good detection strength for many analytes (in some cases the best of all instrumental methods, e.g. ppq range for platinum metals). The measuring range can be more than 6 orders of magnitude . Polarography can provide valuable information for the elucidation of redox reaction mechanisms in aqueous and non-aqueous solutions. The constantly renewing and almost ideally smooth electrode surface of the mercury drop is particularly advantageous.

Disadvantages are the potential for interference from surface-active substances, the often low selectivity and the handling of mercury. Although the latter is completely recycled, the use of the polarograph is limited to the chemical laboratory. Although a large number of substances can be determined and most of the disturbances can be avoided, the execution of the analyzes requires special knowledge and experience.

Polarography is still of great importance in special areas of activity:

Sample media with a high salt load

Galvanic baths, sea water samples and sample solutions from melt decomposition contain high concentrations of alkali metal salts. These cannot be easily removed. Higher salt concentrations interfere with many instrumental analysis methods such as atomic spectroscopy. The disturbing influence of this sample matrix can only be reduced by diluting it. However, this reduces the detection strength of the entire analysis method. In polarography, these salts serve as the base electrolyte and do not interfere further.

Occasional or special examinations

Compared to other instrumental methods, polarography is associated with only low investment costs. It offers possibilities for laboratory automation, such as sample changers. Several manufacturers also offer modern computer-controlled polarographs today (2008). It can therefore be worthwhile to purchase a polarograph instead of an atomic spectrometer if there is only a small number of samples. The same applies to a chromatography device if only a few and always the same organic analytes are routinely determined (quality control).

See also

• Clark electrode
• Concentration polarization

Individual evidence

1. ^ Günter Henze: Polarography and Voltammetry. Basics and analytical practice. Springer, Berlin et al. 2001, ISBN 3-540-41394-4 , p. 1 f. ( limited preview in Google Book search).
2. ↑ Ralf Martens-Menzel: Physical chemistry in analytics. An introduction to the basics with application examples . Springer, Berlin et al. 2010, ISBN 978-3-8348-9781-7 , p. 169 , doi : 10.1007 / 978-3-8348-9781-7 ( limited preview in Google book search).
3. ↑ Hans Peter Latscha, Gerald Walter Linti, Helmut Alfons Klein: Analytical Chemistry. Basic chemistry knowledge III . 4th, completely revised edition. Springer, Berlin et al. 2004, ISBN 3-642-18493-6 , p. 354 , doi : 10.1007 / 978-3-642-18493-2 .
4. ^ Klaus J. Vetter: Electrochemical Kinetics. Theoretical and Experimental Aspects . Academic Press, New York et al. 1967, p. 223 .
5. ^ Georg Schwedt: Analytical Chemistry. Basics, methods and practice. Thieme, Stuttgart et al. 1995, ISBN 3-13-100661-7 , p. 158.
6. ↑ Karl Cammann (Ed.): Instrumental analytical chemistry. Procedures, applications and quality assurance. Spectrum - Akademischer Verlag, Heidelberg et al. 2001, ISBN 3-8274-0057-0 , pp. 7-50 f.

Web links

Commons : Polarography  - collection of images, videos and audio files