Coulometry

Coulometry is a method of electrochemistry.

Coulometry is a method to determine the quantitative amount of an oxidizable substance or reducible compound. It was developed by the Hungarian László and Zoltán Somogyi Szebellédy in 1938. It was not until 1950, found the method wider application. Coulometry based on the measurement of the electric charge and the quantity of electricity, which is converted to a working electrode. The coulometry is the Electrogravimetry very similar, but in this case the oxidized or reduced substances are not deposited on the electrode but remain in solution and the full implementation can be (eg Manga Nation) appears only through an indirect indicator. According to Faraday's law, the electric charge is proportional to the amount of unreacted material. With complete electrochemical conversion of the substance to be determined ( the analyte ) and lack of electrochemical side reactions can be calculated by means of the Faraday constant amount of analyte. Since at the counter electrode must also run an electrochemical reaction in order to close the circuit, it must be ensured that the reaction products can not reach the area of the working electrode. This can (eg halogen with silver counter electrode as a sparingly soluble silver halide) done by a diaphragm or by chemical bonding. The coulometry finds application, for example in the determination of the water content according to Karl Fischer in the trace range, or in the quantification of adsorbable organic halogens ( AOX) in water samples.

• 2.1 Indication methods
• 2.2 Example

Potentiostatic coulometry

In potentiostatic variant coulometry the electrode potential is held constant with the aid of a potentiostat. This potential control is very advantageous to eliminate side reactions. The disadvantage is that the current drops sharply because of the steadily declining analyte concentration. This can take a long time to complete the experiment. The end of the reaction is assumed when the current drop reaches 99.9 percent. The requested amount of electricity can be calculated from the integral of the current over time. For very small electrode surface, high concentrations and large volumes, the analysis can take days to complete. For this reason, coulometry in a pronounced degree is a trace analytical method.

This analysis usually takes an analog integrating circuit ( integrator circuit ), or a computer program.

The calculation of the deposited or converted mass results from the following relations: Faraday's law: Amount of substance: Used and solved for the mass m of the following applies: Here, M is the molar mass in g / mol, Q is the charge in coulombs determined experimentally, z is the charge number which corresponds to the reduction and oxidation of the change of the oxidation number, and F is the Faraday constant, C / mol.

Examples

Reduction of metal ions to metal mercury or platinum electrodes:

Change in the oxidation state ( valence state ) on platinum electrodes:

Oxidative precipitation of halides on silver electrodes:

Galvanostatic coulometry

In the galvanostatic coulometry variant of the electrolysis current is kept constant by a galvanostat. This consists in the simplest case of a battery, a resistance of several kilo-ohms and a potentiometer connected in series with the electrochemical cell. The kilo-ohm resistor limits the electric current because it has by far the highest resistance in the circuit. Of advantage are the simple device technology and the rapid implementation. The disadvantage is that the electrode potential is changed during the reaction and thus side reactions by other measures (eg cleaning steps in the sample preparation) must be excluded. The end of the reaction must be indicated by an indication method ( for example, by pH measurement). This method can therefore also be regarded as a " titration with electrons ".

Since the current is kept constant, the following relationship applies to the unreacted charge:

Indication methods

The methods below can be used for end point indication. It should be noted that often the quality of the method is dependent on the analyte and the background matrix, such as pH buffers, etc.

PH indicator: The pH indicator is at pH = 7 at best; the further one travels to pH = 0 or pH = 14, the worse the indication. At very low analyte concentration, the change in pH is not large enough to detect a change in pH, since water also has buffering properties.

Conductometric Indication: For reactions which do not extend between pH = 6 and pH = 8, the conductivity of protons or hydroxide ions is too great. Further conductive salt is always added in excess in order to prevent migration of the analytes in the electric field. Therefore, the conductivity of the solution is generally high. At low analyte concentrations, the conductivity is not significantly changed. Thus, an indication is difficult to impossible, as long as the analyte concentration is low.

Photometric Indication: For very low initial concentration of the extinction of most analytes is too small to perceive a significant change in absorbance. Due to the high concentration of electrolyte and auxiliary reagent, matrix-dependent faults can occur.

Biamperometre indication by means of an indicator electrode: It creates an indicator electrode in a very small potential or a very small current and is an auxiliary reagent in the solution, which will be implemented, instead of the analyte. Because one must allow for a reaction at the cathode and at the anode for the unimpeded flow of current and the dissolved salt is only present in an oxidation state ( oxidized or reduced ), only a small leakage current flows. Once you start with coulometry, the auxiliary reagent is implemented, which then converts the analyte and responds back. The important for the oxidation / reduction species is converted back, the current flow is therefore very small. After the conversion of the analyte, the oxidised and the reduced form is present in the solution by the reaction. As a result, the indicator electrode (which usually consists of two Pt - pins ) of the current to flow, now that the electrochemical processes can run at the anode and at the cathode. Considering a defined current to the potential drops, puts you at a fixed potential, the current increases after the full implementation of the analyte. Since the galvanostatic coulometry a potential increase with time occurs, an auxiliary reagent must be used in excess anyway, which can be negligible, the potential increase. This auxiliary reagent can then be easily used for endpoint determination. Fortunately, the voltage drop and the current increase in the solution is not dependent on the analyte concentration, but of the indicator electrode surface ( keep as small as possible ) and the concentration of the auxiliary reagent.

Example

It should be determined cerium (IV ) ions. Here, these ions are reduced in the determination:

The actual reduction takes place in that an excess of added auxiliary agent (for example, an iron ( III) salt ) is reduced in the cathodic electrolysis, and then the reduced form from the oxidation provides the electrons:

Until all the cerium (IV ) ions, are reduced, the concentration of the iron ( III) ions is constant, and a constant current flows. The end point of the coulometric determination is reached when the current decreases.

Chronocoulometry

When chronocoulometry is basically a chronoamperometry, ie there is a potential jump experiment is carried out and followed the change of the electrolysis current with high temporal resolution ( microseconds). However, it is integrated over time to obtain the converted electrical charge. The goal is to determine the surface befindlicher at the working electrode materials. This will be implemented in the shortest possible time, while in the electrolyte solutes only have to diffuse to the electrode surface. The electric current, which is caused by the latter process, according to the fall Cottrellgleichung AB. Therefore, a distinction can be calculated between the implementation of dissolved and deposited materials.

There are two different type of Coulometers:

• Coulometer as devices with which the analytical method coulometry is performed. For example, there are such coulometric water determination at trace level or determination of CO2 in gases.

• Coulometer for the determination of electrical values ​​in a DC circuit, namely, the total charge or a constant current. In the 19th century and the first half of the 20th century, these devices, which had been invented by Michael Faraday, have been widely used in science and technology. Coulometer were called in the 19th century voltameter, details such as the description of the different types, see there. This coulometer not perform quantitative analysis, as described elsewhere in this Article, and no longer belong to the analytical coulometry.