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Inorganic and Applied Chemistry
general an indicator may be represented as ”HIn”. Colour indicators exhibit one colour with the proton and
another colour without the proton. A well-known example is phenolphthalein which is opaque in its HIn
form while the colour changes to violet in it’s In- form. We will briefly look into how such a molecule may
function as indicator in order to achieve knowledge of what is going on during titration.
We look at a representative hypothetical color indicator HIn which is a weak acid with a Ka value of 1.0 · 10-
(^8) M. HIn is in equilibrium with:
HIn(aq) H+(aq) + In-(aq)
The indicator is red in its HIn form while the colour is blue when on the In- form. Correspondingly, the
equilibrium reaction the following equilibrium fraction is achieved:
HIn
In
H
K
HIn
K H In a
a
Assume that a few drops of indicator is added t a solution where pH = 2.0. From above we get that the
relation between In- and HIn is:
1000000
1
10
- 0108
(^)
M
M
H
K
pH
a
Acids and bases
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