1. Indicators

2. 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.

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

4. 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+]

5. 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+]

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

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

8. 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
9.1-14
Methyl orange
Red
Yellow
Bromothymol blue
Blue
Phenolphthalein
Colourless
Pink
Combined colour
ORANGE
YELLOW
GREEN
PURPLE

9. 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.

10. End of Presentation