Green Vitriol
Sulphuric acid was probably discovered by alchemists in the thirteenth century.
In the middle ages it was made by distilling iron(II) sulphate crystals:
2FeSO4.7H2O -------> Fe2O3 + H2SO4 + SO2 + 13H2O
Since iron(II) sulphate was known as green vitriol, the acid formed was called oil of vitriol.
Lead Chamber process
In the eighteenth century sulphuric acid was made by burning a mixture of sulphur, air and potassium nitrate (nitre) over steam. The sulphur burns in air to form sulphur dioxide, then oxides of nitrogen acted as a catalyst to convert sulphur dioxide to sulphur trioxide, which then reacted with the steam to make sulphuric acid which condensed on the walls of the container.
In 1740 the first large-scale sulphuric acid plant was built, using the nitre method. At first large glass containers were used, but in 1746 a new plant was built using lead-lined chambers. The process became known as the lead chamber process. The concentration of acid produced by this method was about 70% sulphuric acid and it was sold as chamber acid.
Chamber acid could be concentrated to 78% by evaporating off the water, but if more concentrated acid is required a different method of manufacture must be used.
The contact process
In 1831 Peregrine Phillips discovered that finely divided platinum would catalyse the oxidation of sulphur dioxide to sulphur trioxide, and that at temperatures between 400oC and 500oC conversion was almost 100%. Above this temperature the percentage of sulphur trioxide formed decreases, below this temperature the rate of reaction is too slow to be economic. The final acid produced was much more concentrated than that produced in the lead chamber process.
When using platinum, special care must be taken to remove impurities in the sulphur and air, since the catalyst can be easily deactivated or ‘poisoned’ by trace impurities. These problems were somewhat reduced when a new catalyst was discovered - vanadium pentoxide. Since the hot gases come into contact with the catalyst bed the process became known as the contact process.
In the contact process sulphur is first purified, burnt in air to form sulphur dioxide and the mixture of sulphur dioxide and air passed over the catalyst beds. There the sulphur dioxide is oxidised to sulphur trioxide. Finally the sulphur trioxide is dissolved in 98% sulphuric acid solution (98% sulphuric acid is still 2% water). As the concentration of acid rises, water is slowly added to keep the concentration at 98%.