1. A guide for A level students KNOCKHARDY PUBLISHING
INDICATORS & pH CURVES
A guide for A level students
KNOCKHARDY PUBLISHING

2. Indicators www.argonet.co.uk/users/hoptonj/sci.htm INTRODUCTION
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3. 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
Titration curve; phosphoric acid
Check list

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¯).

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

6. 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

7. 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

8. 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
LITMUS
METHYL ORANGE

9. 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 (0.1M) is added to 25cm3 of acid (0.1M)
End points need not be “neutral‘ due to the phenomenon of salt hydrolysis

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

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

12. 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)

13. 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)

14. 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)

15. 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

16. 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

17. 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

18. 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)

19. 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

20. 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

21. 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.

22. 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

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
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.
17.50cm3
35.00cm3

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

25. 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.

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

27. 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.

28. 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
pH of H3PO = 1.5

29. 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
pH of NaH2PO4 = 4.4
pH of H3PO = 1.5

30. 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
pH of Na2HPO4 = 9.6
pH of NaH2PO4 = 4.4
pH of H3PO = 1.5

31. 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
pH of Na3PO = 12
pH of Na2HPO4 = 9.6
pH of NaH2PO4 = 4.4
pH of H3PO = 1.5

32. 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 phosphoric acid
Explain the shape of the titration curve for acid -sodium carbonate reactions
CAN YOU DO ALL OF THESE? YES NO

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35. © 2004 JONATHAN HOPTON & KNOCKHARDY PUBLISHING
INDICATORS & pH CURVES
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© 2004 JONATHAN HOPTON & KNOCKHARDY PUBLISHING