Employing a Colour Developing Reagent

Some compounds naturally colored absorb visible light. Some compounds not colored e.g. Organic compounds with little or no conjugation absorb UV light.Some compounds do not absorb in the UV/VIS. If we want to use this technique we can employ a colour developing reagent. The coloring reagent binds to the unknown to form a colored complex.The intensity of the colour is directly proportional to the concentration of the unknown.
One example is Phosphate in water, this can be determined by adding ammonium moly date.This compound forms a blue complex with phosphate.The intensity of the colour is directly proportional to the concentration of the phosphate