In laboratory preparation of nitric acid, the mixture of potassium nitrate and concentrated sulphuric acid should not be heated above 200°C because at a higher temperature, potassium sulphate is formed. Potassium sulphate is deposited as a hard, solid mass in the retort, which is difficult to remove. Moreover, most of the nitric acid decomposes at a higher temperature to form reddish brown fumes of nitrogen dioxide and the acid acquire a yellow colouration.
