Acid Base Indicators

1

• Acid / Base Indicators

• acid / base indicators are weak acids and bases
  themselves.
• they change colour as the H3O changes in a
  solution.

eg
If HIn represents some indicator
Colour 1
Colour 2

• adding acid or base causes the equilm to shift.

• if acid is added to the above, the equilm shifts
  backwards and we see colour 1.

• if base is added, the equilm shifts forwards and
  we see colour 2.

2
eg
phenolphthalein
Colourless
Pink

• in acid, equilm is shifted left.

• in base, equilm is shifted right.

eg
bromthymol blue
Yellow
Blue

• in acid, this solution is yellow.

• in base it is blue.

• if equal amounts of HIn and In- are presentie
  In- HIn the solution will be green.

3

• we can write KA expressions for

KA

• the colour change of an indicator occurs at the
  transition point, that is when HIn In-

• at the transition point

KA H3O
KA

• thus, for any indicator, the transition point
  occurs when

H3O KA
or when
pH pKA
4
eg
Given KA 8.0 x 10-10 for phenolphthalein,
calculate the pH of the transition point.
KA H3O 8.0 x 10-10
pH 9.10
thus, phenolphthalein changes from colourless to
pink at pH 9.10 (slightly basic)
5
eg
at pH 6.00 bromthymol blue begins changing from
yellow to green. At pH 7.50 it just finishes
changing from green to blue. What is the KA for
bromthymol blue?
we assume the transition point is midway between
6.00 and 7.50.
Transition pH 6.75
pKA pH 6.75
KA antilog (-6.75)
1.8 x 10-7
Calculate the KA for
phenol red
alizarin yellow
methyl violet
5.0 x 10-8
8.9 x 10-12
1.6 x 10-1