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Indicators

Key Concepts

Examples of Indicators:

25oC

Acid

neutral

Base

 
[H+]
(mol/L)
100 10-1 10-2 10-3 10-4 10-5 10-6 10-7 10-8 10-9 10-10 10-11 10-12 10-13 10-14  
pH 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 pH range
Universal
indicator
red red orange-
red
orange pale
orange
orange-
yellow
pale
yellow
green-
yellow
green dark-
green
blue blue blue blue blue  
cyanidin
(red cabbage water)
red red red cerise purple blue blue blue aqua-
marine
emerald-
green
lime lime yellow yellow yellow  
blue litmus indicator red red red red red red red blue blue blue blue blue blue blue blue 5.0 - 8.0
red litmus indicator red red red red red red red red blue blue blue blue blue blue blue 5.0 - 8.0
phenol-
phthalein indicator
colour
-less
colour
-less
colour
-less
colour
-less
colour
-less
colour
-less
colour
-less
colour
-less
pink pink pink pink pink pink pink 8.3 - 10.0
thymol blue indicator yellow yellow yellow yellow yellow yellow yellow yellow yellow blue blue blue blue blue blue 8.0 - 9.6
phenol red indicator yellow yellow yellow yellow yellow yellow yellow yellow red red red red red red red 6.8 - 8.4
bromo-
thymol blue indicator
yellow yellow yellow yellow yellow yellow yellow blue blue blue blue blue blue blue blue 6.2 - 7.6
methyl red indicator pink pink pink pink pink pink yellow yellow yellow yellow yellow yellow yellow yellow yellow 4.4 - 6.0
bromo-
cresol green indicator
yellow yellow yellow yellow yellow pale blue-
green
blue-
green
blue-
green
blue-
green
blue-
green
blue-
green
blue-
green
blue-
green
blue-
green
blue-
green
3.8 - 5.4
methyl orange indicator red red red red yellow yellow yellow yellow yellow yellow yellow yellow yellow yellow yellow 3.1 - 4.4
bromo-
phenol blue
yellow yellow yellow yellow blue blue blue blue blue blue blue blue blue blue blue 3.0 - 4.6
cresol red red red red yellow yellow yellow yellow yellow yellow yellow yellow yellow yellow yellow yellow 0.2 - 1.8
pH 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 pH range

Choosing an appropriate indicator for a titration:

  • an appropriate indicator will change colour at the equivalence point of the titration.

    Litmus is not used in titrations because the pH range over which it changes colour is too great.

    Universal indicator which is actually a mixture of several indicators displays a variety of colours over a wide pH range so it can be used to determine an approximate pH of a solution but is not used for titrations.

  • Determine what species are present at the equivalence point & deduce the pH at the equivalence point

    pH of salts formed from
    reactions of acids & bases
    (25oC)
    Strong Base Weak Base
    Strong Acid pH = 7 pH < 7
    Weak Acid pH > 7 pH = 7

  • Use the table of indicators to choose an indicator which changes colour over a pH range that includes the equivalence point

Examples:

Strong Acid - Strong Base titration

HCl(aq) + NaOH(aq) → NaCl(aq) + H2O(l)
At equivalence the only species present will be NaCl(aq) & H2O(l)
The solution of a salt of a strong acid and a strong base will have a pH=7
NaCl(aq) will have a pH=7
A suitable indicator would be bromothymol blue (pH range 6.2 - 7.6) or phenol red (pH range 6.8 - 8.4)

Strong Acid - Weak Base titration

HCl(aq) + NH3(aq) → NH4Cl(aq)
NH4Cl is the salt of a strong acid & a weak base, so a solution of NH4Cl will have a pH < 7 (NH4+ is a weak acid)
A suitable indicator would be methyl orange (pH range 3.1 - 4.4) or methyl red (pH range 4.4 - 6.0)

Weak Acid - Strong Base titration

CH3COOH(aq) + NaOH(aq) → CH3COONa(aq) + H2O(l)
CH3COONa is the salt of a weak acid & a strong base, so a solution of CH3COONa will have a pH > 7 (CH3COO- is a weak base)
A suitable indicator would be phenolphthalein (pH range 8.3 - 10.0) or thymol blue (pH 8.0 - 9.6)

Dissociation constants for indicators:

let HIn be the acid form of the indicator and In- the base form and KIn the dissociation constant for the indicator

HIn + H2O In- + H3O+

KIn = [In-][H3O+]
[HIn]
KIn
[H3O+]
= [In-]
[HIn]

The colour of the indicator at any pH is determined by the ratio [In-]:[HIn]

As pH increases, [H3O+] decreases, [In-]:[HIn] increases, so the indicator has increasingly more of the colour of the base form and less of the acid form

It is important to use as little indicator as possible during a titration since the indicator itself reacts with the titration's reactants


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