indicators
DESCRIPTION
Indicators. This is the basis of indicators. Many chemicals, such as the pigments of pelargonia, have different colours depending on whether they are in the acid form or its conjugate base. Demonstration. 1) Pipette out 25ml of acid into a conical flask. - PowerPoint PPT PresentationTRANSCRIPT
Indicators
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.
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.
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.
Phenolphthalein
C
C
O
O
HO OH
pH > 10.0
C
C
O
O
-O OH
+ H+
pH < 8.2
Eg; Phenolphthalein
• Acid form – colourless, Conjugate base - pink
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
Methyl orangeMethyl orange
• Acid form – red, conjugate base – yellow.
Bromothymol blue
• Under acid conditions (LHS) bromothymol blue is yellow.
• Under basic conditions (RHS) it is blue.
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.
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
• 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
• 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
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.
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