Acids/Bases
Lesson 11
Indicators
Theory
Ishihara Test for Colour Blindness – if you can read all
of the numbers you have good colour vision
Indicators
Indicators are weak organic acids or bases that display different colours for their
conjugate acid and base forms..
They are such complex molecules that their
names are usually abbreviated
HIn : an acid and an indicator
In- : a base and an indicator.
Methyl Red
Acid Colour
Base Colour
Explanation
Indicator: Methyl Red… Turns from red to yellow
0
2
4
HIn
HIn is
Transition range
5
6
⇄
H+
7
+
8
10
InIn- is
Transition Colour
=5±1
= 4 to 6
12
[HInd] = [Ind-]
14
Indicators Chart
Indicator
Transition range
Transition Colour
Colour Change as pH Increases
4-6
orange
Red to yellow
intense Red
0
2
4
5
6
7
8
10
12
14
HIn
⇄
H+
+
Red
InYellow
Le Chatelier’s Principle can be used to explain the colours of Red.
In an acid solution, [H+] is high and the equilibrium shifts left and turns red.
In a base solution, [H+] is low and it shifts right and turns yellow.
At pH = 5…the transition point
[HInd] = [Ind-]
Orange(mixture of red and yellow)
Look up Alizarin yellow.
It tells you that it changes from yellow to red as pH increases. This
means that the acid form is yellow and the base form is red.
To calculate the transition point, you have to average the two pH values
Transition Point
=
=
10.1
+
2
11.05
12.0
Alizarin Yellow
HIn
⇄
Acid form
yellow
H+
+
InBase form
red
Transition range 10.1 – 12.0
Transition point pH= 11.05 This means:
The solution is yellow when pH < 11.05
The solution is red when
pH > 11.05
Anything between
10.1 to 12.0 would be orange!
Alizarin Yellow
HIn
⇄
Acid form
yellow
H+
+
InBase form
red
At pH = 11.05, the transition point, the:
The solution looks orange, which is a blend of
Ka
=
[H+][Ind-]
[HInd]
Ka
=
[H+]
[HInd] = [Ind-]
yellow and red.
[HInd] = [Ind-]
Only at the transition point
Keep in mind…
Transition point, or end point, is the point where
the indicator is half way through its colour change.
Equivalence point or stoichiometric point, is the
point where mole ratio of the reaction is exactly
equal to the mole ratio required by the
stoichiometry of the reaction.
Keep in mind…
At the transition point of the indicator
Ka for the indicator = [ H+]
since [HIn] = [ In- ]
From this, we can figure out the solution’s pH
@ the transition point.
Ex: Calculate the Ka for methyl orange.
Transition point
[H+]
Ka
=
=
pH
10-pH
[H+] =
= (3.2 + 4.4)/2 = 3.8
=
10-3.8 =
2.0 x 10-4
1.58 x 10-4 M
Ex: An indicator has a Ka = 1.0 x 10-6, calculate the pH of
the solution at the transition point.
Ka
=
[H+]
=
1.0 x 10-6 M
pH = -log[H+]
=
=
-Log[1.0 x 10-6]
6.00
The indicator is chlorophenol red because (5.2 + 6.8)/2 = 6.0
TRY: answer the following questions:
A solution turns yellow when Orange IV is added and red when methyl orange is added.
A) what is the pH range for the solution?
B) what is pH transition ?
C) what is [H30+] of solution?
D) What is ka of indicator?
TRY: Alizarin Yellow changes from yellow to red at pH = 11.0
what colour is Alizarin Yellow in 1 x 10-5 M NaOH?
Universal Indicators
• It is an indicator solution which changes
colour several times over a range of pH
values.
– Colour changes are distinct enough to estimate
pH of a solution within + 0.5 pH
Summary of equations
At the transition point of the indicator
Ka for the indicator = [ H+]
since [HIn] = [ In- ]
pKa = pH
From Ind. Table, average pH range to get pH transition.
pH transition is a reference point for colours which means:
pH solutions > pH transition –basic form of indicator predominates(base color)
pH solutions < pH transition point – acidic form of indicator predominates(acid color)
pH solution in between the two pH numbers on your chart, means the color is a mixture
Practice Problems
Page 162
• 108, 110, 112, 114, 116, 117
For #116, consider Thymol Blue on Bottom.