Standard hydrogen electrode

The term hydrogen electrode is generally used for electrodes, is where hydrogen gas H2 developed or consumed. The hydrogen electrode is an important tool for measurements in electrochemistry and physical chemistry: it is one of the most important reference electrodes, that is, it is used to determine the potential of the other electrode by a simple voltage measurement. The potential is the most important parameter for describing the electrochemical state of an electrode.

Particularly important are the normal (NHE ) and the standard hydrogen electrode (SHE ), since its potential is defined as the zero standard potential scale. The potential of electrodes is hydrogen by the reaction:

Described.

Standard hydrogen electrode ( SHE)

The standard hydrogen electrode ( Standard Hydrogen Electrode Sheet shortly SHE) is normalized to an ion activity of 1 mol / L and a hydrogen pressure of 1013 hPa at each temperature. You will initially be used as any reference electrode for the purpose of providing a well-defined potential. In addition, it is for the determination of standard potentials particularly suitable, since its potential is defined as the zero of the potential scale. The absolute electrode potential of the hydrogen electrode is at 298.15 K according to IUPAC recommendation 4.44 ± 0.02 V.

In contrast to the standard hydrogen electrode is the standard hydrogen electrode works (NHE = Normal Hydrogen Electrode ) with the ion concentration of the hydrogen under otherwise identical conditions.

Design and function

The potential of the hydrogen electrode is based on the following reaction:

The technical problem at a hydrogen electrode is that hydrogen can accept as a gas electrode neither form, nor is electrically conductive. Therefore, we used an auxiliary electrode in the form of a neutral noble metal ( platinum). This is flushed with hydrogen gas adsorbed on the platinum surface, and this covered with a very thin layer of hydrogen atoms. Thus one obtains a " hydrocarbon block " consisting of platinum inside - the hydrogen electrode.

The standard hydrogen electrode, which defines the zero point of the standard potential scale, therefore, consists of a platinized (ie electrolytically plated with platinum black ) platinum foil, immersed in an acid solution having an ionic activity of the protons of. The platinum electrode is washed by hydrogen gas at a pressure of 1013 hPa and a temperature of 298.15 K. The hydrogen gas adsorbed on the partially platinum, forms an equilibrium with the hydrogen ions of the acid and so defines the electrochemical potential.

The electric potential, which is formed on this electrode, the standard potential, which is defined as being identical to zero.

Meaning and handling

Since standard potentials are measured relative to the standard hydrogen electrode, this electrode has a great importance for Physical Chemistry. Its management is very laborious: First, it requires the handling of hydrogen gas, the hydrogen-oxygen formation has to be avoided by closed vessels. An oxygen-free atmosphere in the electrolytic vessel is also necessary, since oxygen may distort the potential of the electrode. On the other hand, a platinum electrode is susceptible to contamination, as no traces of grease may be present, for example, fingerprints; to ensure a high catalytic activity, they should be re- Platinum. For accurate measurements, also the pressure control is required. For these reasons, hydrogen electrodes for routine tests are rarely used, as well as rare for research, except these were aimed to determine standard potentials. Much more often calomel or silver -silver chloride electrode (Ag / AgCl ) were used where no gaseous reactants occur, so that virtually no pressure dependence occurs. Today mainly Ag / AgCl electrodes are used to prevent the toxic calomel. They are also commercially measured readily available in many different variants in contrast to hydrogen electrodes.

Normal hydrogen electrode ( NHE)

In the experiment, the standard conditions of SHE can not be set. These include the activity of the proton and the gas pressure of 1013 mbar. Therefore, it is recommended to use at atmospheric conditions as the electrolyte, a 1 mol / l hydrochloric acid and hydrogen gas. In this case one speaks of the normal hydrogen electrode. The deviations from the SHE are minimal.

Reversible hydrogen electrode (RHE )

→ Main article: Reversible hydrogen electrode

For some electrochemical studies also hydrogen electrodes are used, which are operated at ambient pressure; the hydrogen electrode for water electrolysis often operate at overpressure. For investigations in acids or alkalis is also sometimes used a hydrogen electrode in the same solution as the measured electrode; it is not separated by a salt bridge. In this way it can be achieved that does not change the measured potential with the pH.

In the electrolysis of water occurs surges: The cell voltage required is due to kinetic inhibition higher than the equilibrium voltage. The over-voltage increases as the current density at the electrodes. The measurement of equilibrium potentials is therefore possible without power.