What happens when potassium iodine reacts with starch?
The iodine/ potassium iodide solution is used as an indicator for starch (And conversely, starch is used as an indicator for free iodine in iodometric titrations) A deep blue colour develops when starch is mixed into a solution of Potassium iodide/iodine – which nermally has a yellow/brown colour. Amylose in the starch is responsible for the formation of a deep blue colour in the presence of iodine. The iodine molecule slips inside of the amylose coil. The reaction seems to be a physical one, where the free iodine is physically held in the amylose chain, forming a localised concentration if iodine, which is clearly visible. A chemical complex is not formed. Iodine – KI Reagent: Iodine is not very soluble in water, therefore the iodine reagent is made by dissolving iodine in water in the presence of potassium iodide. This makes a linear triiodide ion complex with is soluble. The triiodide ion ion slips into the coil of the starch causing an intense blue-black colour. KI ↔ K+ + I- I2 + I
