Polarography

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 operating in the voltammetry with stationary electrodes, are used in the polarography dropping mercury electrodes. It was developed in 1922 by Jaroslav Heyrovský and based on measuring the electrolysis current at a dropping mercury electrode. By means of polarography, it is possible to electrodeposit and base metals due to the high overvoltage to hydrogen of mercury at very negative potentials and to measure the current flowing thereby. This represents the analytic signal

Construction

The dropping mercury electrode consists of a mercury reservoir and a capillary of the mercury drop in a solution to be examined fall. It is used as a working electrode (also measuring electrode ) used in the polarography and is ideal polarizable electrode, i.e., can be their impart an electric potential without causing a charge transfer across the electrode -solution phase boundary, provided in the solution are not depolarizers. Depolarizers are oxidizable or reducible substances. If this is the case, but, it is to charge transfer, the substance depolarizes the working electrode, and a current flows.

In a simple two -electrode assembly, the counter electrode also takes on the function of the reference electrode. Conveniently a three-electrode arrangement, wherein the electrolysis current through a counter electrode made ​​of noble metal or carbon, while passing through the reference electrode is de-energized. As a reference electrode is usually used an electrode of the second kind, such as a calomel or a silver -silver chloride electrode. The advantages lie in the longer durability of the reference electrode and less disturbance of the applied potential by overvoltage effects at the counter electrode.

Measurement and determination of concentration

In the measurement, a linear time -varying voltage is specified and registers the resulting current. If a substance in the solution causes a charge-transfer reaction, there is an increase in current, that is, in the current-voltage curve occurs one level. The location of the potential at half the height of this step ( half-wave potential ) is characteristic of each chemical species, thus a qualitative analysis is possible. The height of the step (that is the power ) is given by the diffusion current, which then sets, when the diffusion of the analyte from the inside of the solution to the electrode surface is the rate-determining step reaction. This results in the possibility for quantitative analysis because of the diffusion limiting current to the concentration of the analyte ( an integer value equation) connected via the Ilkovič equation:

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

ρ is the density of mercury, F is the Faraday constant. On the example values ​​in the table opposite can be seen that the theoretical value 607 should apply between 19.2 ° C and 32.8 ° C. The half-wave potential and the diffusion current are the characteristic values ​​for the type and amount of the depolarizer ( analyte ) in a supporting electrolyte selected. The applicability of the polarography is limited by several factors, such as the occurrence of a capacitive current, resulting in a spurious signal, which up converts the detection limit. Also, come drop tines and so-called polarographic maxima ( when the current for various reasons rises above the limiting current ) on.

Methods

These problems as well as the requirement of higher resolution and accuracy has led to various enhanced Polarografiemethoden:

• Rapid- polarography: the drop is mechanically cut off
• Derivativpolarografie: instead of stepping curve is used the first derivative of this curve
• Difference Gleichstrompolarografie: using two synchronized dropping mercury electrode drip
• Kathodenstrahlpolarografie: all eligible potential area is traversed during a single drop of life
• Wechselstrompolarografie: the applied DC voltage is a low frequency alternating voltage is superimposed
• Pulspolarografie: at the end of the drop life, a rectangular voltage pulse is applied Normalpulspolarografie: It is applied a voltage pulse, which increases from drop to drop. Between the voltage is zero. At the end of a voltage pulse, the current is recorded. This gives step-like signals.
• Differenzpulspolarografie: it is driven a temporary ramp voltage ramp and added a constant voltage pulse at the end of every drop of life. The current is registered before the beginning and before the end of each voltage pulse. The difference between the two gives the actual measured value. This gives peak -shaped signals.

These methods may be partially subdivided.

Status

The polarography is in principle suitable for detailed analysis in a small range of concentrations of many inorganic and organic substances. Because of the large negative potential range of mercury it takes place predominantly a cathodic (reductive ) implementation. The heyday of polarography ranged from the 30s to the 80s of the 20th century. She was the first widely applied instrumental method of analysis. In the form of atomic spectrometry ( elemental analysis ) and chromatography (organic analysis) arose in the last decades significant alternative methods, in particular, identifiable by a wider range analytes distinguished.

Pros and Cons

Advantageously, the high achievable accuracy ( precision 1% ), low investment costs and the possibility of element species analysis. In their modification as Differenzpulspolarografie inverse voltammetry and polarography which has for many analytes a very good detection sensitivity (in some cases the best of all instrumental methods, such as ppq - range PGMs ). The measuring range can comprise more than 6 orders of magnitude. For the examination of Redoxreaktionsmechanismen in aqueous and non-aqueous solutions, the polarography can provide valuable information. Of particular advantage is the constantly renewing and almost ideally smooth electrode surface of the mercury drop.

Disadvantages are the Störmöglichkeiten by surface-active substances, which are often low selectivity and the use of mercury. The latter is indeed completely recycled, restricts the use of the polarograph but on the chemical laboratory. Although many substances determined and most of the failures can be avoided is to perform the analyzes but preceded by special knowledge and experience.

The polarography has today a great importance in specific areas of responsibility:

Galvanic baths, seawater samples and sample solutions from melting outcrops contain high concentrations of alkali metal salts. These can not be easily removed. Higher salt concentrations interfere with many instrumental analytical methods such as atomic spectroscopy. One can reduce the disturbing influence of this sample matrix only by dilution. However, this reduces the proof strength of the entire analytical procedure. In polarography, these salts are used as supporting electrolyte and do not disturb on.

Occasional or special studies

Compared with other instrumental methods, the polarography is associated with only low investment costs. It offers possibilities of laboratory automation, such as sample changer. Several manufacturers offer these days (2008 ) to modern computer-controlled polarograph. Therefore, it may be worthwhile to purchase a polarograph with only a small sample volume instead of an atomic spectrometer. The same applies to a Chromatografiegerät if routinely only a few and always the same organic analytes are to be determined ( quality control).