Electrolysis of acidified water
Earlier in this chapter we described the electrolysis of acidified water in a Hofmann voltameter. In this case, only the water is decomposed. The small quantity of acid added simply provides a vast number of extra ions to make the solution more highly
conducting. Water acidified with sulphuric acid contains hydrogen ions, H +, and hydroxyl ions, OH -, produced by dissociation of the water, together with hydrogen ions, H +, and sulphate ions, S04 z-, from the sulphuric acid. When the current is passing H + ions migrate towards the cathode. Here they receive electrons from it and become neutral hydrogen atoms. The H atoms then combine in pairs to form molecules of hydrogen, Hz, which are liberated in the form of gas bubbles. An ion which has received an electron from the electrode in this manner is said to be discharged.
The action at the cathode may be summarized by the following equations, in which e represents an electron:While the H + ions are moving towards the cathode, the OH – and S04 z- ions move towards the anode. Here only the OH – ions lose electrons to the anode and are discharged. The S042 – ions remain in solution. This is called the preferential discharge of ions. When thus liberated, pairs of OH groups combine to form water, H20, and oxygen atoms, O. Finally, the oxygen atoms combine in pairs to form oxygen molecules, O2, which are set free in the form of gas bubbles. The action at the anode may therefore be symbolized as follows: