That makes sure that the flow of liquid is always from left to right - preventing any of the sodium hydroxide solution formed finding its way back to where chlorine is being produced.Ĭhlorine is produced at the titanium anode according to the equation: Notice that there is a higher level of liquid on the anode side. The solution can seep through it from the anode compartment into the cathode side. The diaphragm is made of a porous mixture of asbestos and polymers. If you are interested, you can find pictures and descriptions of real electrodes by doing a Google search on diaphragm cell and looking for manufacturers' websites. For example, in a real diaphragm cell, the electrodes are not a single block of metal. Note: This is a simplification of a real cell.

The diaphragm and membrane cells are designed so that all the products are kept separate. Therefore, if you are trying to manufacture chlorine and sodium hydroxide rather than bleach, you have to keep the chlorine and sodium hydroxide apart as well. However, chlorine also reacts with sodium hydroxide solution to produce a mixture of sodium chloride and sodium chlorate(I) - also known as sodium hypochlorite. Obviously, the two gases need to be kept apart. If chlorine comes into contact with hydrogen, it produces a mixture which will explode violently on exposure to sunlight or heat. The need to keep all the products separate In other words, sodium hydroxide solution is being formed around the cathode. The net effect of this is that there is a build up of sodium ions and these newly-produced hydroxide ions around the cathode. So this reaction happens:Īs the hydrogen ions are converted into hydrogen gas, the water equilibrium tips to the right to replace them. It is much easier for a hydrogen ion to pick up an electron than for a sodium ion. Sodium ions and hydrogen ions (from the water) are attracted to the negative cathode. The chlorine has to be purified to remove this oxygen. The chlorine is, however, contaminated with small amounts of oxygen because of a reaction involving hydroxide ions giving up electrons as well. Two chloride ions each give up an electron to the anode, and the atoms produced combine to give chlorine gas. The major reaction at the anode is therefore: It is actually easier to liberate hydroxide ions (to give oxygen) than chloride ions (to give chlorine), but there are far, far more chloride ions arriving at the anode than hydroxide ions. The negative ions, chloride and hydroxide, get attracted towards the positively charged anode. The simplification is fine for this topic.Īt any one time, the concentration of hydrogen ions or hydroxide ions will be very small - the position of equilibrium lies well to the left-hand side. That would need two water molecules on the left-hand side of the equilibrium. The H + (aq) ions could be better shown as hydroxonium ions, H 3O +. Note: Strictly speaking, this is a simplification. The hydrogen and hydroxide ions come from the equilibrium: The chemistry of the electrolysis process In fact, the electrolysis of sodium chloride solution can be used to make three useful substances - chlorine, sodium hydroxide and hydrogen. Read the background chemistry section below, and then concentrate on the cell you need to know about.Ĭhlorine is manufactured by electrolysing sodium chloride solution.

For UK A level (or equivalent) purposes, you will only need to know details of one of these cells. Both cells rely on the same underlying chemistry, but differ in detail. This page describes the manufacture of chlorine by the electrolysis of sodium chloride solution using a diaphragm cell and a membrane cell. MANUFACTURING CHLORINE USING A DIAPHRAGM AND A MEMBRANE CELL The diaphragm and membrane cells for the manufacture of chlorine

Cathode anode diagram