Indicators. Many chemicals, such as the pigments of pelargonia, have different colours depending on...
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![Page 1: Indicators. Many chemicals, such as the pigments of pelargonia, have different colours depending on whether they are in the acid form or its conjugate.](https://reader036.fdocuments.us/reader036/viewer/2022081519/56649f395503460f94c5643e/html5/thumbnails/1.jpg)
Indicators
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Many chemicals, such as the pigments of pelargonia, have different colours depending on whether they are in the acid form or its conjugate base.
This is the basis of indicators.
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Demonstration• 1) Pipette out 25ml of acid into a
conical flask.• 2) Fill a burette with the base.• 3) Add a few drops of universal
indicator.• 4) Titrate until the colour change
indicates pH 7 has been achieved. Note the volume.
• 5) Repeat using phenolphthalein.• 6) Repeat using methyl orange.
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Indicators as weak acids.
Indicators are weak acids.
HIn H⇌ + + In-Acid
Conjugate base
In acid the eqm is to the LHS
HIn ⇌ H+ + In-
Solution has the colour of the undissociated acid form.
In alkali equm is to the RHS.
HIn ⇌ H+ + In-
Solution has the colour of the conjugate base.
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Phenolphthalein
C
C
O
O
HO OH
pH > 10.0
C
C
O
O
-O OH
+ H+
pH < 8.2
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Eg; Phenolphthalein
• Acid form – colourless, Conjugate base - pink
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Methyl orange
NN
+
CH3
CH3
O- Na
+
O
O
SN
H
+ H+NN
CH3
CH3
N S
O
O
O- Na
+
pH < 3.2
pH > 4.4
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Methyl orangeMethyl orange
• Acid form – red, conjugate base – yellow.
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Bromothymol blue
• Under acid conditions (LHS) bromothymol blue is yellow.
• Under basic conditions (RHS) it is blue.
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End points• At the end point;
• [In-] = [HIn]• So the colour will be half way between the
two.• The end point should be sharp, such that the
addition of a small amount of acid or base causes a complete colour change.
• NB The contribution of the indicator to the pH of a solution is minute as only a few drops are used.
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pH and endpoint
• HIn ⇌ H+ + In-
• Kin = [H+] [In-] / [HIn]• pKin = pH + log ([HIn] / [In-]) • pH = pKin - log ([HIn] / [In-])• At the end point [HIn]= [In-]• pH = pKin – log1• log 1 = 0• So pH = pKin
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• It is assumed that the “acid” colour is totally
obscured by the “base” colour when [In-] = 10 x [HIn]
• Substituting;
• pH = pKin - log ([HIn] / [In-])
• pH = pKin - log([HIn] / 10[HIn])
• log 0.1 = -1
• pH = pKin – (-1)
• pH = pKin + 1
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• Similarly the “base” colour will be obscured when;
• [HIn] = 10 [In-] • Subsituting;• pH = pKin - log ([HIn] / [In-])• pH = pKin - log(10[In-] / [In-]) • log 10 = 1• pH = pKin – (+1)• pH = pKin - 1
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Effective range of an indicator
• Generally the effective range of an indicator is;
• pKin +/- 1
• ie 2 pH units.• For a titration the end point of the indicator
must correspond with the equivalence point. • The point where neutralisation has taken
place and the reaction has finished.
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Indicator Acid colour
Base colour
pKinpH range
Methyl orange
red yellow 3.71 3.2 – 4.4
Methyl red yellow red 5.1 4.8 - 6
Bromo thymol blue
yellow blue 7 6 – 7.6
Phenol phthalein
colourless pink 9.4 8.2 - 10