1. pH CURVES & INDICATORS Richard Grime – Ripon Grammar School
How pH indicators work
methyl orange
phenolphthalein
Choosing an appropriate indicator
pH curves
strong acid strong base
strong acid weak base
weak acid strong base
weak acid weak base
SUMMARY
diprotic acid
H2C2O4 / NaOH
diprotic base
Na2CO3 / HCl

2. INDICATORS HIn  H+ + In- colour 1 colour 2
Indicators are weak acids which have a different colour to their conjugate base
HIn  H+ + In-
colour colour 2
low pH: equilibrium pushed left = colour 1
high pH: equilibrium pushed right = colour 2

3. Methyl orange
pH < 3.2 
pH > 4.4

4. acid
end-point
alkali

5. Phenolphthalein 
pH < 8.2
pH > 10.0

6. phenolphthalein
acid
alkali

7. INDICATORS For an indicator to work in a titration:
The pH range of the indicator’s colour change must be within the range of the pH change at the end-point.

8. INDICATORS Indicator colour at low pH pH range of colour change
high pH
methyl orange
red
3.2 – 4.4
orange
phenolphthalein
colourless
8.2 – 10.0
purple

9. Strong acid – Strong base
pH at equivalence
= 7

10. Strong acid – Strong base

11. Strong acid – Strong base

12. Strong acid – Weak base
pH at equivalence
< 7

13. Strong acid – Weak base

14. Strong acid – Weak base

15. Weak acid – Strong base
pH at equivalence
> 7

16. Weak acid – Strong base

17. Weak acid – Strong base

18. depends on relative strength of acid and base
Weak acid – Weak base
pH at equivalence
depends on relative strength of acid and base

19. Weak acid – Weak base

20. Weak acid – Weak base

21. SUMMARY

22. SUMMARY

23. SUMMARY

24. H2C2O4 v NaOH
H2C2O4 + OH- ↓
HC2O4- + H2O
HC2O4- + OH- ↓
C2O42- + H2O

25. H2C2O4 v NaOH
H2C2O4 + OH- ↓
HC2O4- + H2O
HC2O4- + OH- ↓
C2O42- + H2O

26. H2C2O4 v NaOH
H2C2O4 + OH- ↓
HC2O4- + H2O
HC2O4- + OH- ↓
C2O42- + H2O

27. Na2CO3 v HCl
CO32- + H+ ↓
HCO3-
HCO3- + H+ ↓
H2O + CO2

28. Na2CO3 v HCl
CO32- + H+ ↓
HCO3-
HCO3- + H+ ↓
H2O + CO2

29. Na2CO3 v HCl
CO32- + H+ ↓
HCO3-
HCO3- + H+ ↓
H2O + CO2