Concentration element

Copper concentration element

A concentration element (also a concentration cell ) is a galvanic cell in which a usable electrical voltage is created due to chemical reactions similar to a battery .

The special thing about such a concentration chain is that it consists of two half-cells, which are constructed in the same way and only differ in the electrolyte concentration. The two electrodes are made of the same material and the electrolytes contain the same types of ions. The chemical reaction in one half-cell is therefore the reverse of the reaction in the other when current flows. However, since the two half-cells have the tendency to equalize the concentrations , a measurable voltage is created.

Today, the concentration chain is primarily used for demonstration purposes, since it can be used to qualitatively show and quantitatively check the concentration dependency of the electrode potential or the chemical potential . This enables them to be used to derive the Nernst equation .

Example: metal dissolution and deposition

Often concentration cells are used in which metal electrodes are immersed in solutions of a salt of the same metal, e.g. B. Zinc electrodes in zinc salt solutions, iron electrodes in iron chloride solutions or copper electrodes in copper sulfate solution. Then the metal will go into solution in one cell, and it will be deposited again in the other. Since the concentrations equalize when the current flows, the metal in the half-cell with the diluted electrolyte will dissolve and deposit in the one with the more concentrated one.

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

On the side with the more dilute solution, the metal of the electrode is oxidized to metal ions, releasing electrons; it passes over into the diluted solution as ions and increases the concentration in the half-cell. Due to the release of electrons, this is the negative electrode, the negative pole. Since oxidation takes place here, it is by definition the anode . As with batteries, the negative pole is the anode. The released electrons migrate to the other electrode via an electrical conductor. There they reduce the metal ions present in the concentrated solution to elemental metal, which attaches to the electrode. Because of the reduction , this is the cathode . Since the metal ions bring a positive charge with them when they attach, this is the positive pole. The concentration of this half-cell decreases. The concentration balance between the two half-cells takes place via a salt bridge . This salt bridge is designed in such a way that it does not allow electrons or positively charged metal ions (cations) to pass through. The only particles that are let through are the negatively charged anions that are present in the saline solution. The concentrations of the two half-cells thus approach each other until there is no longer any difference in concentration. This completes the circuit.

Calculating the voltage

The Nernst equation can be used to calculate the potential difference of the galvanic cell . Since the standard potential (see electrochemical series ) of the two half-cells is the same, the Nernst equation can be simplified to the following term:

${\displaystyle U={\frac {R\cdot T}{z\cdot F}}\cdot \ln \left({\frac {c_{\mathrm {A} }}{c_{\mathrm {D} }}}\right)}$, with . ${\displaystyle c_{\mathrm {A} }>c_{\mathrm {D} }}$

Designations:

• ${\displaystyle z}$: Number of transition electrons per oxidation / reduction
• ${\displaystyle c_{\mathrm {A} }}$: Electrolyte concentration in the acceptor half-cell (cathode)
• ${\displaystyle c_{\mathrm {D} }}$: Electrolyte concentration in the donor half-cell (anode)
• ${\displaystyle R}$: Universal gas constant | Universal or molar gas constant, R = 8.31447 J mol ⁻¹ K ⁻¹ = 8.31447 CV mol ⁻¹ K ⁻¹
• ${\displaystyle T}$: absolute temperature (= temperature in Kelvin )

At temperatures between 21.7 ° C and 26.7 ° C, this results in the equation

${\displaystyle U={\frac {0{,}059\ \mathrm {V} }{z}}\cdot \log \left({\frac {c_{\mathrm {A} }}{c_{\mathrm {D} }}}\right)}$, with ; ${\displaystyle c_{\mathrm {A} }>c_{\mathrm {D} }}$

further numerical values ​​are given in the article electrode slope.

If = the half-cells are the same. There is no potential difference between the electrodes and the voltage that can be measured between the half-cells is 0 V. ${\displaystyle c_{\mathrm {A} }}$ ${\displaystyle c_{\mathrm {D} }}$

Web links

• Concentration element ( Memento from July 18, 2013 in the Internet Archive ) (PDF file; 97 kB)
• Chemistry experiment ( Memento from February 11, 2013 in the web archive archive.today )
• Concentration dependence (PDF file; 192 kB)