Electrolysis of Brine

Submitted by ChemPRIME Staff on Thu, 12/16/2010 - 15:30

Three important chemicals, NaOH, Cl2, H2, can be obtained by electrolyzing an aqueousDescribing a solution in which the solvent is water. NaCl solutionA mixture of one or more substances dissolved in a solvent to give a homogeneous mixture. (brine). This forms the basis of the chlor-alkaliAny substance in aqueous solution that is bitter, irritating to the skin, and has a pH value greater than 7.0. industry. The diaphragm cell (also called a Hooker cell) in which the electrolysis is carried out is shown schematically in Fig. 1. At the cathodeThe electrode in an electrochemical cell where reduction occurs; the negatively charged electrode in a vacuum tube., water is reduced:

Figure 1 Diaphragm or Hooker cell for electrolysis I of brine (schematic). Since chloride ions are removed and hydroxide ions produced by the electrolysis, the electrolyte gradually changes from a solution of sodium chloride to a solution of sodium hydroxide.

2H2O + 2e → H2 + 2OH      (1a)

Chlorine is produced at the anodeThe electrode in an electrochemical cell where oxidation occurs. The positively charged electrode in a vacuum tube.:

2Cl → Cl2 + 2e      (1b)

Thus the overall reaction is

2H2O(l) + 2Cl(aq) → H2(g) + Cl2(g) + 2OH(aq)      (1)

Since the H2(g) and Cl2(g) might recombine explosively should they come in contact, the cathode must be entirely surrounded by a porous diaphragm of asbestos. Hence the name of this type of cell.

Both the H2(g) and Cl2(g) produced in Eq. (1) are dried, purified, and compressed into cylinders. Fresh brine is continually pumped into the cell, and the solution which is forced out contains about 10% NaOH together with a good deal of NaCl. [Remember that the spectator ions, Na+(aq), are not included in a net ionic equationThe equation for a reaction in which strong electrolytes are written as their separate component ions and in which species that are unchanged are omitted. such as Eq. (1).] H2O is allowed to evaporate from this solution until the concentrationA measure of the ratio of the quantity of a substance to the quantity of solvent, solution, or ore. Also, the process of making something more concentrated. of the solution reaches 50% NaOH, by which time most of the NaCl has crystallized out and can be recycled to the electrolysis. The NaOH is sold as a 50% solution or further dried to give crystals whose approximate formula is NaOH•H2O.

The considerable effort required to concentrate the NaOH solution obtained from diaphragm cells can be avoided by using mercury cells. The cathode in such a cell is mercury, and the cathode reaction is

Na+(aq) + e + xHg(l) → NaHgx(l)

The sodium metalAn element characterized by a glossy surface, high thermal and electrical conductivity, malleability, and ductility. produced in this reaction dissolves in the liquidA state of matter in which the atomic-scale particles remain close together but are able to change their positions so that the matter takes the shape of its container mercury, producing an amalgamAn alloy (metallic solution) containing mercury.. The liquid amalgam is then transferred to an- other part of the cell and reacted with water:

NaHgx(l) + 2H2O(l) → 2Na+(aq) + 2OH(aq) + H2(g) + xHg(l)

The 50% sodium hydroxide solution produced by this reaction contains no sodium chloride and can be sold directly, without being concentratedIncreased the concentration of a mixture or solution (verb). Having a large concentration (adjective). further. Up until 1970, however, chlor-alkali plants using mercury cells did not have adequate controls to prevent losses of mercury to the environment. About 100 to 200 g mercury was lost for each 1000 kg chlorine produced-apparently a small quantity until one realizes that 2 500 000 kg chlorine was produced by mercury cells every day during 1960 in the United States. Thus every 2 to 4 days 1000 kg mercury entered the environment, and by 1970 sizable quantities were being found in fish. Since 1970 adequate controls have been installed on mercury cells and most new alkali plants use diaphragm cells, but the very large quantities of mercury introduced into rivers and lakes prior to 1970 are expected to remain for a century or more.