Indicators

What is an Indicator? An indicator is a weak organic acid or base with different colours for its conjugate acid and base forms. Ex: HIn + H2O < In- + H3O+ yellow red When put in an acid, equilibrium shifts to the yellow form. When put into a base, equilibrium shifts to the red form.

The Ka Expression for an Indicator For indicators, their Ka :

An Indicator’s End Point or Transition Point If an indicator is in an acid solution and base is added, the point at which the colour change is halfway through is called its end point or transition point. At this point the [In-] and [HIn] are equal and thus cancel from the equilibrium expression. At this point the Ka = [H3O+]

An Indicator’s End Point or Transition Point At the transition point the pH will be equal to the pKa. At this point the Ka = [H3O+]

Determining the End Point or Transition Point Alizarin Yellow R changes from yellow to red in the pH range from 10.1 to 12.0. To calculate the transition point, find the halfway pH by averaging: (10.1+12.0)/2 = 11.05 = 11

Universal Indicators A universal indicator is an indicator solution which changes colour several times over a range of pH values.

Making a Universal Indicator By mixing methyl orange (pKa = 3.8) with bromothymol blue (pKa = 6.8) with phenolphthalein (pKa = 9.1), a universal indicator is formed which changes colours over a wide pH range. pH range 0-3.8 3.8-6.8 6.8-9.1 9.1-14 Methyl orange Red Yellow Bromothymol blue Blue Phenolphthalein Colourless Pink Combined colour ORANGE YELLOW GREEN PURPLE

Universal Indicator Paper Impregnating paper with a universal indicator solution produces a paper that can be dipped into solutions with a wide range of pH to determine their various pH readings.

End of Presentation