DIN method: N,N-Dimethyl-1,4-phenylene diamine reacts with hydrogen sulphide to form a compound which changes to leucomethylene blue. This compound is oxidised to methylene blue using iron(III) ions (Reaction basis according to DIN 38405-D26/27).

Sulphide is measured in an acidic medium; vigorous mixing can cause evaporation of gaseous hydrogen sulphide which escapes analysis.