1. Starter In pairs discuss everything you remember about titrations:
How they are carried out.
What are indicators for?
You carry out a titration of a 25 cm3 acid solution (c= 0.5 moldm-3) with a 1 mol dm-3 solution of NaOH. Calculate the number of moles of acid after the addition of 10 cm3 of NaOH

2. Neutralisation
Interpret and sketch acid-base pH curves for strong and weak acid and bases.
Explain the choice of suitable indicator for acid-base titration

3. Simulation: http://users.skynet.be/eddy/titratie.swf
Strong acid/ strong base
Leave out

4. weak acid (CH3COOH) v. strong alkali (NaOH)
pH curves
Types There are four types of acid-base titration; each has a characteristic curve.
strong acid (HCl) v. strong base (NaOH)
weak acid (CH3COOH) v. strong alkali (NaOH)
strong acid (HCl) v. weak base (NH3)
weak acid (CH3COOH) v. weak base (NH3)
In the following examples, alkali (0.1M) is added to 25cm3 of acid (0.1M)
End points need not be “neutral‘ due to the phenomenon of salt hydrolysis

5. Equivalence point: the point in a titration at which the volume of one solution has reacted exactly with the volume of the second solution

6. strong acid (HCl) v. strong base (NaOH)

7. Equivalence point is the centre of the vertical section of the titration curve.

8. Titration curve for a strong acid–strong base titration

9. strong acid (HCl) v. strong base (NaOH)
pH 1 at the start due to 0.1M HCl (strong monoprotic acid)

10. strong acid (HCl) v. strong base (NaOH)
Very little pH change during the initial 20cm3
pH 1 at the start due to 0.1M HCl (strong monoprotic acid)

11. strong acid (HCl) v. strong base (NaOH)
Very sharp change in pH over the addition of less than half a drop of NaOH
Very little pH change during the initial 20cm3
pH 1 at the start due to 0.1M HCl (strong monoprotic acid)

12. strong acid (HCl) v. strong base (NaOH)
Curve levels off at pH 13 due to excess 0.1M NaOH
(a strong alkali)
Very sharp change in pH over the addition of less than half a drop of NaOH
Very little pH change during the initial 20cm3
pH 1 at the start due to 0.1M HCl (strong monoprotic acid)

13. strong acid (HCl) v. weak base (NH3)
Curve levels off at pH 10 due to excess 0.1M NH3
(a weak alkali)
Sharp change in pH over the addition of less than half a drop of NH3
Very little pH change during the initial 20cm3
pH 1 at the start due to 0.1M HCl

14. weak acid (CH3COOH) v. strong base (NaOH)
Curve levels off at pH 13 due to excess 0.1M NaOH
(a strong alkali)
Sharp change in pH over the addition of less than half a drop of NaOH
Steady pH change
pH 4 due to 0.1M CH3COOH (weak monoprotic acid)

15. weak acid (CH3COOH) v. weak base (NH3)
Curve levels off at pH 10 due to excess 0.1M NH3
(a weak alkali)
NO SHARP
CHANGE IN pH
Steady pH change
pH 4 due to 0.1M CH3COOH (weak monoprotic acid)
Types

16. 4.3 exercise 4

17. Indicator

18. Hin = H+ + In-

19. pH colour ranges for some common indicators19

20. COLOUR CHANGES OF SOME COMMON INDICATORS
Watch demo?
Why do you get a different result with a different indicator?
phenolthalein
Methyl orange
COLOUR CHANGES OF SOME COMMON INDICATORS pH 1 2 3 4 5 6 7 8 9 10 11 12 13 14
METHYL ORANGE
CHANGE
LITMUS
CHANGE
PHENOLPHTHALEIN
CHANGE

21. Indicator change colours around +- 1 unit of pH from their end point.
An indicator has to be chosen so that their end point is as close as the pH value of the titration’s equivalence point.

23. So the colour of the indicator changes over the pH range pKIn  1
HIn(aq) H+(aq) In¯(aq)
So the colour of the indicator changes over the pH range pKIn  1

24. Which indicator is the best?
The best indicator is C because it is closest to the mid point of the vertical
pH change, but B and D would work as well. A and E will not change
colour because they are outside the range of pH change

25. What happens when you titrate Na2CO3 with HCl?
Simulation
What happens when you titrate Na2CO3 with HCl?
What happens when you titrate H3PO4 with NaOH?
4.3 exercise 4

26. Choice of indicators for titrations
Teacher demo titration of NaOH/ acetic acid. Figure 3 in page 43.
2 titrations (1 with methyl orange, another with phenolphthalein).
4.3 exercise 4

27. Must change immediately in the required pH range
Acid-base indicators
Must have an easily observed colour change.
Must change immediately in the required pH range
over the addition of ‘half’ a drop of reagent.
To be useful, an indicator must
change over the “vertical” section of the curve where there is a large change in pH for the addition of a very small volume of alkali.
The indicator used depends on the pH changes around the end point - the indicator must change during the ‘vertical’ portion of the curve.
In the example, the only suitable indicator is PHENOLPHTHALEIN.
PHENOLPHTHALEIN
LITMUS
METHYL ORANGE
Extra slides from this one

28. COLOUR CHANGES OF SOME COMMON INDICATORS
Acid-base indicators
Must have an easily observed colour change.
Must change immediately in the required pH range
over the addition of ‘half’ a drop of reagent.
COLOUR CHANGES OF SOME COMMON INDICATORS pH 1 2 3 4 5 6 7 8 9 10 11 12 13 14
METHYL ORANGE
CHANGE
LITMUS
CHANGE
PHENOLPHTHALEIN
CHANGE

29. weak acid (CH3COOH) v. strong alkali (NaOH)
pH curves
Types There are four types of acid-base titration; each has a characteristic curve.
strong acid (HCl) v. strong base (NaOH)
weak acid (CH3COOH) v. strong alkali (NaOH)
strong acid (HCl) v. weak base (NH3)
weak acid (CH3COOH) v. weak base (NH3)
In the following examples, alkali (0.1M) is added to 25cm3 of acid (0.1M)
End points need not be “neutral‘ due to the phenomenon of salt hydrolysis

30. strong acid (HCl) v. strong base (NaOH)

31. strong acid (HCl) v. strong base (NaOH)
pH 1 at the start due to 0.1M HCl (strong monoprotic acid)

32. strong acid (HCl) v. strong base (NaOH)
Very little pH change during the initial 20cm3
pH 1 at the start due to 0.1M HCl (strong monoprotic acid)

33. strong acid (HCl) v. strong base (NaOH)
Very sharp change in pH over the addition of less than half a drop of NaOH
Very little pH change during the initial 20cm3
pH 1 at the start due to 0.1M HCl (strong monoprotic acid)

34. strong acid (HCl) v. strong base (NaOH)
Curve levels off at pH 13 due to excess 0.1M NaOH
(a strong alkali)
Very sharp change in pH over the addition of less than half a drop of NaOH
Very little pH change during the initial 20cm3
pH 1 at the start due to 0.1M HCl (strong monoprotic acid)

35. strong acid (HCl) v. strong base (NaOH)
PHENOLPHTHALEIN
LITMUS
METHYL ORANGE
Any of the indicators listed will be suitable - they all change in the ‘vertical’ portion

36. strong acid (HCl) v. weak base (NH3)
Curve levels off at pH 10 due to excess 0.1M NH3
(a weak alkali)
Sharp change in pH over the addition of less than half a drop of NH3
Very little pH change during the initial 20cm3
pH 1 at the start due to 0.1M HCl

37. strong acid (HCl) v. weak base (NH3)
PHENOLPHTHALEIN
LITMUS
METHYL ORANGE
Only methyl orange is suitable - it is the only one to change in the ‘vertical’ portion

38. weak acid (CH3COOH) v. strong base (NaOH)
Curve levels off at pH 13 due to excess 0.1M NaOH
(a strong alkali)
Sharp change in pH over the addition of less than half a drop of NaOH
Steady pH change
pH 4 due to 0.1M CH3COOH (weak monoprotic acid)

39. weak acid (CH3COOH) v. strong base (NaOH)
PHENOLPHTHALEIN
LITMUS
METHYL ORANGE
Only phenolphthalein is suitable - it is the only one to change in the ‘vertical’ portion

40. weak acid (CH3COOH) v. weak base (NH3)
Curve levels off at pH 10 due to excess 0.1M NH3
(a weak alkali)
NO SHARP
CHANGE IN pH
Steady pH change
pH 4 due to 0.1M CH3COOH (weak monoprotic acid)
Types

41. weak acid (CH3COOH) v. weak base (NH3)
PHENOLPHTHALEIN
LITMUS
METHYL ORANGE
NOTHING SUITABLE
There is no suitable indicator- none change in the ‘vertical’ portion.
The end point can be detected by plotting a curve using a pH meter.