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18.6 - Indicators
18.6.1: Describe qualitatively how an acidbase indicator works. Use HIn H+ + In- or similar.
Indicators are weak acids (or bases) which have a different colour in the ionic and molecular forms.
When a weak acid is in solution its molecular form predominates and this colour is seen. However, as the equilibrium involves hydrogen ions then it is pH sensitive. Addition of hydrogen ions will force the equilibrium in the direction of the molecular form (left hand side) whereas when base is added to the equilibrium it moves in the direction of the ionic form (the conjugate base) and this colour is seen.
CH3COOH CH3COO- + H+
Hence, in this equilibrium, addition of hydrogen ions will push the equilibrium in the direction of the CH3COOH molecules, whereas addiiton of base will react with the hydrogen ions from the equilibrium and pull the equiklibrium to the right hand side. In the case of ethanoic acid there will be no colour change so it could not be used as an indicator.
We generally represent the weak acids that are indicators by the formula HIn, meaning that this is the molecular form of the acid that is going to lose the hydrogen ion. In this case the ionic form will be In-. For the indicator to be useful the colour of HIn must be different from In-.
If we use phenolphthalein as an example then the molecular form of phenolphthalein (HIn) has no colour. Hence in acidic solution the phenolphthalein is colourless.
HIn H+ + In-
low pH (high concentration of H+ ions) pushes equilibrium to the left
If the phenolphthalein is in basic solution then the excess OH- ions react with the H+ ions from the equilibrium pulling it to the right hand side and the colour of the IN- ion is seen, which is red in the case of phenolphthalein.
HIn H+ + In-
high pH (high concentration of OH- ions) pulls equilibrium to the right
18.6.2: State and explain how the pH range of an acidbase indicator relates to its pKa value.
pKa of indicators
As indicators have a different coulour whn in the molecular form to when in the ionic form it seems logical to assume that the point of changing colour will be when there is 50% of both forms present i.e. the equilibrium HIn H+ + In- lies exactly in the middle.
If we apply the equilibrium law to this situation then:
And when the equilibrium lies exactly in the centre then:
[In-] = [HIn]
In which case we can cancel them out from the equation to give:
Ka = H+
pKa = pH
The consequence of this answer is that the indicator will change colour when the pH is the same value as its pKa value. Hence, indicators have different regions of operation. As the change in pH is usually large at the equivalence point this means that provided the pH change takes place through the pKa of the indicator then it can be used for a titration.
18.6.3: Determine an appropriate indicator for a titration, given the equivalence point of the titration and Ka (or pKa) values for possible indicators.
Choice of indicator
From section 18.6.2 it becomes apparent that the indicator must be chosen to suit the titration being performed.
Titrations involving strong acids and bases can be performed using any indicator as the pH change at the equivalence point is large (from 3 to 11 or vice versa) - most indicators operate within this range.
Titrations involving strong acids and weak bases have an equivalence point in the acidic region of the pH scale. The choice of indicator will depend on the actual expected pH at the equivalence point. It makes sense to select an indicator with a pKa right in the middle of the pH change at the equivalence point. In practice, there are few indicators in common use. For strong acids and weak bases methyl orange is the indicator of choice.
for titrations involving weak bases and strong acids use methyl orange
For weaK acids and strong bases use phenolphthalein
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