In chemistry and manufacturing, electrolysis is a method of separating chemically bonded elements and compounds by passing an electric current through them.

History

• 1800 - William Nicholson and Johann Ritter decomposed water into hydrogen and oxygen.
• 1807 - Potassium was discovered by Humphry Davy

Overviews

Electrolysis involves the passage of an electric current through, in general, an ionic substance that is either molten or dissolved in a suitable solvent, resulting in chemical reactions at the electrodes. The positive electrode is called the anode, and the negative electrode is the cathode. To be useful for electrolysis, the electrodes need to be able to conduct electricity, and metal electrodes are generally used. Graphite electrodes and semiconductor electrodes are also used. An ionic compound, or a compound that reacts with the solvent to produce ions (such as an acid) is dissolved in an appropriate solvent, or an ionic compound is melted by heat. Then some free ions exist in the liquid. An electrical potential is applied between a pair of electrodes immersed in the liquid.

Each electrode attracts ions that are of the opposite charge. Therefore, positively-charged ions (called cations) move towards the electron-emitting (negative) cathode, whereas negatively-charged ions (termed anions) move toward the positive anode. The energy required to separate the ions, and cause them to gather at the respective electrodes, is provided by an electrical power supply. At the electrodes, electrons are absorbed or released by the ions, forming a collection of the desired element or compound.

Oxidation of ions or neutral molecules can take place at the anode, and the reduction of ions or neutral molecules at the cathode. For example, it is possible to oxidize ferrous ions to ferric ions at the anode:

$\backslash mathrm\{Fe^\{2+\}\_\{aq\}\; \backslash longrightarrow\; \backslash \; Fe^\{3+\}\_\{aq\}\; +\; \backslash \; e^-\; \}$ .

It is also possible to reduce ferricyanide ions to ferrocyanide ions at the cathode:

$\backslash mathrm\{Fe(CN)\_6^\{3-\}\; +\; \backslash \; e^-\; \backslash longrightarrow\; \backslash \; Fe(CN)\_6^\{4-\}\; \}$

Neutral molecules can also react at either electrode. For example: p-Benzoquinone can be reduced to hydroquinone at the cathode:

$\backslash mathrm\{+\; \backslash \; 2\; e^-\; +\; \backslash \; 2\; H^+\; \backslash longrightarrow\; \backslash \; \}$

In the last example, $H^\{+\}$ ions (hydrogen ions) also take part in the reaction, and are provided by an acid in the solution, or the solvent itself (water, methanol etc). Electrolysis reactions involving $H^\{+\}$ ions are fairly common in acidic solutions. In alkaline solutions, reactions involving $OH^-$ (hydroxide ions) are common.

The substances oxidised or reduced can also be the solvent (usually water) or the electrodes. It is possible to have electrolysis involving gases. For instance, fuel cells often use oxygen and hydrogen gases as reactants.