1. Acids
Lesson 11
Indicators
Theory

2. Ishihara Test for Colour Blindness – if you can read all of the numbers you have good colour vision

3. Indicators
Indicators are organic chemicals with large complex formulas. They are weak acids where the acid and base forms have different colours.

4. Methyl Red

5. Acid Colour
Base Colour

6. 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
We use HInd to represent any indicator
HInd ⇄ H Ind-
HInd is
Transition Colour
Ind- is
Transition Point
= 5 ± 1
= 4 to 6
[HInd] = [Ind-]

7. For this lab you will make an Indicators Chart
Transition Point
Transition Colour
Colour Change as pH Increases
orange
Red to yellow
Mouat Red
4 - 6

8. There is one indicator in your lab that is diprotic and can change colour twice.
Fake blue
H2Ind ⇄ H HInd-
red
purple
blue
[H2Ind] = [HInd-]
HInd ⇄ H+ + Ind2-
blue
green
yellow
[HInd-] = [Ind2-]

9. For this lab you will make an Indicators Chart
Transition Point
Transition Colour
Colour Change as pH Increases
purple
Red to blue
Fake blue
1 - 3
orange
Red to yellow
Mouat Red
4 - 6
green
blue to yellow
Fake blue
7 - 9

10. HInd ⇄ H+ + Ind-
Red
Yellow
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

11. Look up Alizarin yellow on page 7 of your data package
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.

13. Look up Alizarin yellow on page 7 of your data package
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 = 2 =

14. 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 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] = [Ind-]
[HInd]
Ka = [H+]
Only at the transition point

15. Calculate the Ka for methyl orange.
Transition point pH = ( )/2 = 3.8
[H+] = 10-pH = = x 10-4 M
Ka = [H+] = 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 M
pH = -log[H+] = -Log[1.0 x 10-6]
= 6.00
The indicator is chlorophenol red because ( )/2 = 6.0

16. The formal lab report should contain the following in order
Title Page
Handout with purpose
Introduction
Data- coloured sheet
Analysis of data- indicator chart in order of increasing pH
Questions
Conclusion