Iron test. Beakers showing the colour change in a test for the iron (III) ion. A solution containing iron (III) ions in water (forming a complex ion with the formula:[Fe(H2O)6]3+) is at left, and has an orange colour. The bottle contains potassium thiocyanate solution (colourless). When this is added to the iron (III) solution, a blood-red colour is formed (right). This is a result of the replacement of one of the water ligands (H2O) that is bonded to the iron ion (Fe) with a thiocyanate ion (SCN). The resulting complex ion has a formula of: [Fe(SCN)(H2O)5]2+