Triazine method: iron(II) ions react with a triazine derivative to form a violet complex. Iron(III) ions are reduced and thus also determined.

Natural waters as well as waste waters often contain some iron. It can be present as Fe(II) or Fe(III) ions if the pH value of the water is below 3 or if the water is free of oxygen. At higher pH values Fe(III) forms an insoluble oxyhydrate. Frequently the sample is in a transition state with finely dispersed iron oxides. In waste water and natural water containing humic acids iron is often present in the form of a complex salt.

The VISOCOLOR^® test kits only determine iron which is present in dissolved form as Fe²⁺or Fe³⁺ ions. Iron complexes are not covered unless they are decomposed by oxidation with nitric acid and sulphuric acid.