Acids
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 organic chemicals with large complex formulas. They are weak acids
where the acid and base forms have different colours.
Methyl Red
Acid Colour
Base Colour
Experiment
One drop of buffer of each pH is placed on an acetate
Indicator: Mouat Red
One drop of the each indicator is added to each buffer
0
2
4
5
6
7
8
We use HInd to represent any indicator
HInd
Hind is
Transition Point
⇄
H+
+
10
IndInd- is
Transition Colour
=5±1
= 4 to 6
12
[HInd] = [Ind-]
14
For this lab you will make an Indicators Chart
Indicator
Transition Point
Transition Colour
Colour Change as pH Increases
4-6
orange
Red to yellow
Mouat Red
0
2
4
5
6
7
8
10
12
14
There is one indicator in your lab that is diprotic and can change colour twice.
Fake blue
H2Ind
⇄
H+
HInd-
+
purple
red
blue
[H2Ind] = [HInd-]
HInd-
⇄
blue
H+
Ind2-
+
green
yellow
[HInd-] = [Ind2-]
0
2
4
6
7
8
10
12
14
For this lab you will make an Indicators Chart
Indicator
Transition Point
Transition Colour
Fake blue
1-3
purple
Red to blue
Mouat Red
4-6
orange
Red to yellow
Fake blue
7-9
green
blue to yellow
0
2
4
6
7
8
Colour Change as pH Increases
10
12
14
HInd
⇄
H+
+
Red
IndYellow
Le Chatelier’s Principle can be used to explain the colours of Mouat Red.
In acid [H+] is high and it shifts left and turns red.
In base [H+] is low and it shifts right and turns yellow.
At pH = 5
[HInd] = [Ind-]
orange
Look up Alizarin yellow on page 7 of your data package. 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.
Look up Alizarin yellow on page 7 of your data package. 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.
Transition Point
=
=
10.1
+
2
11.05
12.0
Alizarin Yellow
HInd
⇄
H+
+
IndAcid form
Base form
yellow
red
The solution is yellow when the pH < 11.05
The solution is red when the pH > 11.05
At pH = 11.05, the transition point, the:
[HInd] = [Ind-]
The solution looks orange, which is a blend of
yellow and red.
Ka
=
[H+][Ind-]
[HInd]
Ka
=
[H+]
[HInd] = [Ind-]
Only at the transition point
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 x 10-4
1.58 x 10-4 M
An indicator has a Ka = 1.0 x 10-6, calculate the pH of 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
The formal lab report should contain the following in order
1.
2.
3.
4.
5.
6.
7.
Title Page
Handout with purpose
Introduction
Data- coloured sheet
Analysis of data- indicator chart in order of increasing pH
Questions
Conclusion