Concentration cell

A condensing element (also concentration cell ) is a galvanic cell in which a battery similar to a useable electric voltage generated due to chemical reactions.

The special feature of such a concentration range is that it consists of two half-cells, which are of identical construction, and differ only in the electrolyte concentration. Thus, the two electrodes consist of the same material, and the electrolyte containing the same types of ions. The chemical reaction in one half-cell is therefore at the reversal of the current flow in the other reaction. However, since the two half-cells will tend to equalize the concentrations, yet results in a measurable voltage.

The concentration of chain now serves mainly for demonstration purposes, since one can qualitatively show the concentration dependence of the electrode potential and the chemical potential and verify quantitatively with her. This way you can use them to derive the Nernst equation.

Example: metal dissolution and deposition

Often concentration cell are used in which metal electrodes are immersed in solutions of a salt of the same metal, such as zinc salt zinc electrodes in solutions of iron electrodes in solutions of ferric chloride or copper electrodes in copper sulphate solution. Then, in one cell, the metal will go into solution in the other it is redeposited. Since equalize the concentrations during the current flow, the metal itself is in the half-cell to solve the dilute electrolytes and those deposited with the more concentrated.

If you connect the cell short, so that current can flow and can the reactions take place, the following will take place:

On the side with the more dilute solution of the metal of the electrode is oxidized to metal ions with release of electrons; it is as an ion into the dilute solution and increases the concentration of the half-cell. Due to the electron charge that is the negative electrode, the negative pole. Since a oxidation takes place, it is by definition the anode. As with the batteries so the negative here; the anode. The freed electrons travel through an electrical conductor to the other electrode. There they reduce the present metal ions in the concentrated solution to elemental metal which accumulates on the electrode. This is due to the reduction in the cathode. Since the metal ions bring a positive charge, if they attach themselves, this is the positive terminal. The concentration of these half cell decreases. Of the concentration equalization between the two half-cells is carried out via a salt bridge. This salt bridge is designed so that it lets through as possible neither electrons nor the positively charged metal ions (cations). The only particles that are allowed to pass the negatively charged anions present in the salt solution. The concentrations of the two half-cells thus approach each other until there is no concentration difference exists. In this way the electric circuit is closed.

Calculating the voltage

For the calculation of the potential difference of the galvanic cell can use the Nernst equation. Since the standard potential (see Electrochemical Series ) of the two half-cells is the same, the Nernst equation can be simplified to the following expression:

Designations:

• Z: Number of transfers electrons per oxidation / reduction
• Electrolyte concentration in the Akzeptorhalbzelle (cathode)
• Electrolyte concentration in the Donatorhalbzelle (anode)

In the case = no voltage between the half- cells is to measure more.