Demonstration of the traffic light reaction. Initially the beaker contains a solution of glucose, a sugar, to which a small amount of the indicator indigo carmine is added. This forms a deep blue solution below pH 11.4. The solution is then made more alkaline (higher pH) by the addition of sodium hydroxide, turning it green. The indicator has three colours relating to three oxidation states. Straight after mixing, it has reacted with oxygen in the air to produce its green oxidised form. Over time, the glucose reduces this first to the red intermediate form, and eventually to the yellow fully-reduced form. Note that at the end of the clip, the layers nearest the air are more oxidised. This reaction is reversible, and shaking the solution oxidises it again.