Complexation, Association or Dissociation:

Some salts have a tendency to form complexes whose colours are different from those of the simple compounds. For example, the colour of cobalt chloride changes from pink to blue due to the following complex formation.

2 CoCl₂   ↔   Co[CoCl₄]

pink                            blue

The degree of complex creation increases along with increase in concentration, therefore, Beer's law does not hold at high concentrations. Same discrepancies are found while the absorbing solute dissociates or associates within solution since the nature of the species in solution depends on the concentration.