and hydrogen is evolved about the cathode.
The chlorine ions on being discharged at the anode in similar manner may either be given off as chlorine gas, or may attack the water, as represented in the equation
2. Electrolysis of water. The reason for the addition of sulphuric acid to water in the preparation of oxygen and hydrogen by electrolysis will now be clear. Water itself is not an electrolyte to an appreciable extent; that is, it does not form enough ions to carry a current. Sulphuric acid dissolved in water is an electrolyte, and dissociates into the ions 2 H+ and SO4—. In the process of electrolysis of the solution, the hydrogen ions travel to the cathode, and on being discharged escape as hydrogen gas. The SO4 ions, when discharged at the anode, act upon water, setting free oxygen and once more forming sulphuric acid:
SO4 + H2O = H2SO4 + O.
The sulphuric acid can again dissociate and the process repeat itself as long as any water is left. Hence the hydrogen and oxygen set free in the electrolysis of water really come directly from the acid but indirectly from the water.
3. Electrolysis of sodium sulphate. In a similar way, sodium sulphate (Na2SO4), when in solution, gives the ions 2 Na+ and SO4—. On being discharged, the sodium atoms decompose water about the cathode, as in the case of sodium chloride, while the SO4 ions when discharged at the anode decompose the water, as represented in the equation
SO4 + H2O = H2SO4 + O
Fig. 33