1. INDICATORS & TITRATION CURVES

2. Indicators CONTENTS Acid-base indicators - theory
Titration curves - introduction
Titration curve; strong acid - strong base
Titration curve; strong acid - weak base
Titration curve; weak acid - strong base
Titration curve; weak acid - weak base
Titration curve; acid - carbonate
Check list

3. Acid-base indicators
General Many indicators are weak acids and partially dissociate in aqueous solution
HIn(aq) H+(aq) In¯(aq)
The un-ionised form (HIn) is a different colour to the anionic form (In¯).

4. Acid-base indicators
General Many indicators are weak acids and partially dissociate in aqueous solution
HIn(aq) H+(aq) In¯(aq)
The un-ionised form (HIn) is a different colour to the anionic form (In¯).
Apply Le Chatelier’s Principle to predict any colour change
In acid - increase of [H+]
- equilibrium moves to the left to give red undissociated form
In alkali - increase of [OH¯]
- OH¯ ions remove H+ ions to form water; H+(aq) OH¯(aq) H2O(l)
- equilibrium will move to the right to produce a blue colour

5. Acid-base indicators
General Many indicators are weak acids and partially dissociate in aqueous solution
HIn(aq) H+(aq) In¯(aq)
The un-ionised form (HIn) is a different colour to the anionic form (In¯).
Apply Le Chatelier’s Principle to predict any colour change
In acid - increase of [H+]
- equilibrium moves to the left to give red undissociated form
In alkali - increase of [OH¯]
- OH¯ ions remove H+ ions to form water; H+(aq) OH¯(aq) H2O(l)
- equilibrium will move to the right to produce a blue colour
Summary In acidic solution
In alkaline solution

6. 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
PHENOLPHTHALEIN
METHYL RED
METHYL ORANGE 1 2 3 4 5 6 7 8 9 10 11 12 13 14
CHANGE pH
BROMOTHYMOL BLUE
Ka (HIn) = [H3O+][In-]
[HIn]

7. Acid-base indicators Must change immediately in the required pH range
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
BROMOTHYMOL BLUE
METHYL ORANGE mL

8. strong acid (HCl) v. strong base (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 (0.1mol L-1)
is added to 25 mLof acid (0.1mol L-1)
End points may not be “neutral‘ due to the phenomenon of salt hydrolysis

9. strong acid (HCl) v. strong base (NaOH)mL

10. strong acid (HCl) v. strong base (NaOH)
pH 1 at the start due to 0.1mol -1 HCl (strong monoprotic acid) mL

11. strong acid (HCl) v. strong base (NaOH)
Very little pH change during the initial 20mL
pH 1 at the start due to 0.1mol L-1 HCl (strong monoprotic acid) mL

12. 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 20mL
pH 1 at the start due to 0.1mol L-1 HCl (strong monoprotic acid) mL

13. strong acid (HCl) v. strong base (NaOH)
Curve levels off at pH 13 due to excess 0.1mol L-1 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 20mL
pH 1 at the start due to 0.1mol L-1 HCl (strong monoprotic acid) mL

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

15. strong acid (HCl) v. weak base (NH3)
Curve levels off at pH 10 due to excess 0.1mol L-1 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 20mL
pH 1 at the start due to 0.1mol L-1 HCl mL

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

17. weak acid (CH3COOH) v. strong base (NaOH)
Curve levels off at pH 13 due to excess 0.1mol L-1 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 0.1mol L-1 CH3COOH (weak monoprotic acid) mL

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

19. weak acid (CH3COOH) v. weak base (NH3)
Curve levels off at pH 10 due to excess 0.1mol L-1 NH3
(a weak alkali)
NO SHARP
CHANGE IN pH
Steady pH change
pH 4 due to 0.1mol L-1 CH3COOH (weak monoprotic acid) mL
Types

20. weak acid (CH3COOH) v. weak base (NH3)
PHENOLPHTHALEIN
LITMUS
METHYL ORANGE
NOTHING SUITABLE mL
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.

21. Other pH curves - acid v. carbonate
Sodium carbonate reacts with hydrochloric acid in two steps...
Step Na2CO3 + HCl ——> NaHCO3 + NaCl
Step NaHCO3 + HCl ——> NaCl + H2O + CO2
Overall Na2CO HCl ——> 2NaCl + H2O + CO2

22. Other pH curves - acid v. carbonate There are two sharp pH changes
Sodium carbonate reacts with hydrochloric acid in two steps...
Step Na2CO3 + HCl ——> NaHCO3 + NaCl
Step NaHCO3 + HCl ——> NaCl + H2O + CO2
Overall Na2CO HCl ——> 2NaCl + H2O + CO2
There are two sharp pH changes
The second addition of HCl is exactly the same as the first because the number of moles of HCl which react with the NaHCO3 is the same as that reacting with the Na2CO3.
17.50mL
35.00mL mL

23. Other pH curves - acid v. carbonate There are two sharp pH changes
Sodium carbonate reacts with hydrochloric acid in two steps...
Step Na2CO3 + HCl ——> NaHCO3 + NaCl
Step NaHCO3 + HCl ——> NaCl + H2O + CO2
Overall Na2CO HCl ——> 2NaCl + H2O + CO2
There are two sharp pH changes
First rapid pH change around pH = 8.5 due to the formation of NaHCO3 .
Can be detected using phenolphthalein mL

24. Other pH curves - acid v. carbonate There are two sharp pH changes
Sodium carbonate reacts with hydrochloric acid in two steps...
Step Na2CO3 + HCl ——> NaHCO3 + NaCl
Step NaHCO3 + HCl ——> NaCl + H2O + CO2
Overall Na2CO HCl ——> 2NaCl + H2O + CO2
There are two sharp pH changes
First rapid pH change around pH = 8.5 due to the formation of NaHCO3 .
Can be detected using phenolphthalein
Second rapid pH change around pH = 4 due to the formation of acidic CO2 .
Can be detected using methyl orange. mL

25. Other pH curves - polyprotic acids (H3PO4)
Phosphoric acid is triprotic; it reacts with sodium hydroxide in three steps...
Step 1 H3PO NaOH ——> NaH2PO H2O
Step 2 NaH2PO4 + NaOH ——> Na2HPO H2O
Step 3 Na2HPO4 + NaOH ——> Na3PO H2O

26. Other pH curves - polyprotic acids (H3PO4)
Phosphoric acid is triprotic; it reacts with sodium hydroxide in three steps...
Step 1 H3PO NaOH ——> NaH2PO H2O
Step 2 NaH2PO4 + NaOH ——> Na2HPO H2O
Step 3 Na2HPO4 + NaOH ——> Na3PO H2O
There are three sharp pH changes
Each successive addition of NaOH is the same as equal number of moles are involved.

27. What should you be able to do?
REVISION CHECK
What should you be able to do?
Recall the definition of a weak acid
Understand why indicators can be made from weak acids
Understand why indicators must change colour quickly over a small pH range
Recall and explain the shape of titration curves involving acids and bases
Explain why particular indicators are used for certain titrations
Explain the shape of the titration curve for acid -sodium carbonate reactions (extension)
CAN YOU DO ALL OF THESE? YES NO

28. You need to go over the relevant topic(s) again
Click on the button to
return to the menu

29. Try some past paper questions
WELL DONE!
Try some past paper questions

30. INDICATORS & TITRATION CURVES
THE END