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Pure sulphuric acid and formaldehyde yields the reaction in addition to an oxidation-reduction reaction and also an avoidance of excess formaldehyde. This oxidizing agent can be sodium amalgam to reduce oxalic acid while the reducing agent is nitric acid which oxidizes an alcohol. Reduction of the oxalic acid in done in a sulphuric acid solution while avoiding a temperature raise inside the cathode. Once the reduction has ended, the oxalic acid is diminished and the freed glyoxylic acid produces the indol molecules, which gives the red/purple color.