Iodine (I – or I3 –) acts as a mordant and as a trapping agent. A mordant is a substance that increases the affinity of the cell wall for a stain by binding to the primary stain, thus forming an insoluble complex which gets trapped in the cell wall. In the Gram stain reaction, the crystal violet and iodine form an insoluble complex (CV-I) which serves to turn the smear a dark purple color. At this stage, all cells will turn purple