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Titrations are covered in detail in the 'Quantitative analysis' topic of the Analytical chemistry course. When we carry out a titration we need a signal to tell us that the end point has been reached. The signal is often an indicator changing colour but in some titrations the signal is the formation of a precipitate.

The concentration of chloride ions in solution can be determined by adding silver nitrate solution in the presence of potassium chromate(VI) in a 'Mohr' titration. The end point is signalled by the formation of a blood red precipitate of silver chromate(VI). This type of titration is particularly useful in finding out about the concentration of sodium chloride in the environment.

To understand why this precipiate is produced at the end point of the titration we need to explore the solubility equilibria of silver chloride and silver chromate(VI). When we add silver nitrate solution to the chloride ion solution there will be Ag⁺, Cl^– and CrO₄^– ions in solution. A precipitate of Ag₂CrO₄ will only be formed when the ionic product exceeds its solubility product.