Transcript Lesson 13.

Acids
Lesson 13
Indicators
Theory
Indicators
Indicators are organic chemicals with large complex formulas.
They are weak acids where the acid and base forms have different
colours.
Methyl 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.
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+
+
Ind-
Acid form
Base form
yellow
red
The solution is yellow when the pH < 11.05
The solution is red when the pH > 11.05
At the transition point pH = 11.05, the yellow [HInd] = [Ind-] red and
the solution looks orange, which is a blend of yellow and red.
Ka
=
[H+][Ind-]
[HInd]
Alizarin Yellow
⇄
HInd
H+
+
Ind-
Acid form
Base form
yellow
red
The solution is yellow when the pH < 11.05
The solution is red when the pH > 11.05
At the transition point pH = 11.05, the yellow [HInd] = [Ind-] red and
the solution looks orange, which is a blend of yellow and red.
Ka
=
[H+][Ind-]
[HInd]
Ka
=
[H+][Ind-]
[HInd]
Alizarin Yellow
⇄
HInd
H+
+
Ind-
Acid form
Base form
yellow
red
The solution is yellow when the pH < 11.05
The solution is red when the pH > 11.05
At the transition point pH = 11.05, the yellow [HInd] = [Ind-] red and
the solution looks orange, which is a blend of yellow and red.
Ka
=
[H+][Ind-]
[HInd]
Ka
=
[H+][Ind-]
[HInd]
=
[H+]
Only at the trans point
Calculate the Ka for methyl orange.
Transition point
[H+]
=
10-pH
Ka
=
[H+]
pH
= (3.2 + 4.4)/2 = 3.8
=
=
10-3.8 =
1.58 x 10-4 M
2 x 10-4
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
There are a few indicators that are diprotic and can change colours
twice. You have one of those in this lab and it is called thymol blue.
H2Ind
⇄
H+
+
HInd-
HInd-
⇄
H+
+
Ind2-
8
10
pH
0
2
4
6
7
12
14
There are a few indicators that are diprotic and can change colours
twice. You have one of those in this lab and it is called thymol blue.
H2Ind
Red
⇄
H+
+
HInd-
HInd-
⇄
H+
+
Ind2-
8
10
pH
0
2
4
6
7
12
14
There are a few indicators that are diprotic and can change colours
twice. You have one of those in this lab and it is called thymol blue.
H2Ind
Red
⇄
H+
+
HIndblue
HInd-
⇄
H+
+
Ind2-
8
10
Blue
pH
0
2
4
6
7
12
14
There are a few indicators that are diprotic and can change colours
twice. You have one of those in this lab and it is called thymol blue.
H2Ind
Red
⇄
H+
+
HIndblue
HInd-
⇄
H+
+
Ind2-
Blue
Yellow
pH
0
2
4
6
7
8
10
12
14
There are a few indicators that are diprotic and can change colours
twice. You have one of those in this lab and it is called thymol blue.
H2Ind
Red
⇄
HInd-
⇄
H+
+
purple
[H2Ind] = [HInd-]
H+
+
Blue
HIndblue
Ind2Yellow
pH
0
2
4
6
7
8
10
12
14
There are a few indicators that are diprotic and can change colours
twice. You have one of those in this lab and it is called thymol blue.
H2Ind
Red
⇄
HInd-
⇄
H+
+
purple
[H2Ind] = [HInd-]
Blue
H+
+
green
HIndblue
Ind2Yellow
[HInd-] = [Ind2-]
pH
0
2
4
6
7
8
10
12
14