Question

      Return to the issue of finding the equivalence point. Assume you are using a colorimetric indicator (one which changes color at the equivalence point). One way in which such an indicator (In) can work is that it undergoes a reaction with substance T itself, and that reaction leads to a product that has a different color than the unreacted indicator. In other words, a reaction like this occurs:

                                    In (color A) + T à Product (color B)

Since the Indicator is typically added to the analyzed sample (in the flask) before the titration is begun, Indicator is present all the time. In choosing and using indicators then, there are strategies one can employ to make sure that problems are minimized so as not to affect measurement accuracy. Make of list of your suggestions.   Hint: Think about equilibrium and concentrations.

Answer 1

1.choose an indicator whose pH range falls within the pH change of the reaction. For example, in the titration of a strong acid with a strong base, the pH quickly changes from 3 to 11. The indicator phenolphthalein, whose range spans from pH 8

1a. pH paper contain several indicators and combine their pH ranges to offer an entended range of pH sensitivity.these paper display a unique colour for each pH unit and come with their own colour chart.

2.Suppose an indicator which is a weak acid, with the formula HIn. At equilibrium, the following chemical equation is established.

HIn(aq)	+	H2O(l)		In-(aq)	+	H3O+(aq)
acid				base
color A				color B

The acid and its conjugate base have different colors. At low pH, the concentration of H₃O⁺is high and so the equilibrium position lies to the left. The equilibrium solution has the color A. At high pH, the concentration of H₃O⁺is low and so the equilibrium position thus lies to the right and the equilibrium solution has color B.

Phenolphthalein is an example of an indicator which establishes this type of equilibrium in aqueous solution:

colorless (acid)	magenta (base)