De aldehyden in de monsteroplossing en de formaldehyde in de vergelijkingsoplossing reageren met Schiff's reagens onder vorming van roodgekleurde complexen, waarvan de kleurintensiteit vergeleken wordt.
The aldehydes in the test solution, and the formaldehyde in a comparison solution, react with Schiff's reagent to produce red coloured complexes, the intensities of which are compared.