1. Acids and Bases
Titration curves

2. Acid – base titrations
The end point is reached when enough base has been added to react completely with the acid (or vice versa).
The pH at the end point is not necessarily 7.
For a strong acid – strong base titration, the end point has pH = 7
For strong acid – weak base titration the end point has pH < 7 why?
For a weak acid – strong base titration the end point has pH > 7 why?
A titration curve is a measurement of the recorded pH in a solution (on the y-axis) and volume of base added (on the x – axis) {or volume of acid added}.
The pH remains virtually constant while the base is added and then the pH changed dramatically as equivalence is approached. The pH reaches a plateau in the xs alkaline region (for base added to acid).
e.g. HCl + NH3 → NH Cl-
weak acid
e.g. CH3COOH + OH- → H2O + CH3COO-
weak base

3. Common indicators: methyl orange, methyl red, phenolphthalein
pH at end point = 7
Common indicators: methyl orange, methyl red, phenolphthalein
Note: starting pH = 1 (base added to acid) starting pH = 13 (acid added to base)
Vertical range pH = 3 – 11
Final pH = final pH = 1

4. Common indicators: methyl orange, methyl red
pH at end point = 5 – 6 why?
Common indicators: methyl orange, methyl red
Starting pH = 1 (base added to acid) starting pH = 11 (acid added to base)
Vertical range pH = 3-7
Final pH = 11 (base added to acid) final pH = 1 (acid added to base)
NH3 + HCl → NH Cl-
NH H2O  NH3 + H3O+ i.e. NH4+ undergoes hydrolysis to increase the [H3O+]

5. Common indicator - phenolphthalein
pH at end point = why?
Common indicator - phenolphthalein
Starting pH = 3 (base added to acid) starting pH = 13 (acid added to base)
Vertical range pH = 7-11
Final pH = 13 (base added to acid) final pH = 3 (acid added to base)
CH3COOH + OH- → CH3COO- + H2O
CH3COO- + H2O  CH3COOH + OH- i.e. the acetate ion undergoes hydrolysis to increase the [OH-]

6. Sketching titration curves
Decide whether the acid and base are strong or weak
Draw and label axes
Calculate the volume that has to be added to reach the equivalence point (the point at which the base reacts exactly with the measured amount of acid). Note, indicators are chosen so that the end point (the point at which the indicator changes colour) coincides with the equivalence point.
Estimate the pH
- at the start
- at the equivalence point
- at the end
Draw a smooth curve – make sure you know the range of the vertical section expected.

7. Explaining titration curves
Consider the addition of a 0.10 mol dm-3 NaOH solution from a (burette) to 20.0 cm3 of a 0.10 mol dm-3 solution of HCl. The pH at various point during the titration can be calculated:
before the addition of NaOH
after the addition of 10.0 cm3 NaOH
after the addition of 15.0 cm3 NaOH
after the addition of 18.0 cm3 NaOH
after the addition of 19.0 cm3 NaOH
after the addition of 19.5 cm3 NaOH
after the addition of 19.8 cm3 NaOH

8. after the addition of 20.0 cm3 NaOH
So, the addition of a small amount of strong base can change the pH dramatically around the equivalence point

9. Calculation of pKa from titration curves
In a titration between a strong base and a weak acid, the pH at the point when half the acid has been neutralised can be used to determine the pKa and hence Ka of the acid.
HA  H+ + A-
Ka = [H+][A-] and the value of Ka is constant
[HA]
At the halfway point:
[HA]halfway = [A-] halfway
ஃ Ka = [H+] halfway
and pKa = pHhalfway
See titration curve for weak acid – strong base

10. Indicators
Indicators are weak acids in which the acid molecule, HInd, is a different colour from its conjugate base, Ind-
HInd H2O  Ind H3O eq1
colour A colour B
In acid solution, the equilibrium will shift ______ producing more of the __________ ________, and therefore we see colour ___
If base is added, H3O+ + OH-  2H2O shifts to the ___________, thus the Ωm of eq 1 shifts _________ and we see colour ____

11. When does an indicator change colour?
For the reaction : HInd + H2O  Ind- + H3O+
Kind = [Ind-]eq[H3O+]eq
[HInd]eq
At the end point, [HInd] = [Ind-]
ஃ Kind = [H3O+]endpoint
pKind = pHendpoint
The noticeable colour change for an indicator usually occurs
over a range of 1 pH unit. E.g. methyl orange (pKind = 3.7)
is red below pH 2.7, gradually changes to orange at pH 3.7
and then to yellow at pH 4.7.

12. Choosing indicators Titration Vertical range Indicator pH range
Strong acid/strong base
3.5 – 10.5
Methyl red
Methyl orange
Phenolphthalein
Strong acid/weak base
3.0 – 6.5
Weak acid/strong base
6.5 – 10.5
Colours – Acid indicator base
red methyl orange yellow
colourless phenolphthalein magenta
red methyl red yellow
The choice of indicator depends on the pH at the equivalence point in the titration.
The point at which the indicator changes colour is the end point.
The pH range over which an indicator changes colour must lie completely within the vertical part of the titration curve.

13. Choosing indicators Choose an indicator for … these titrations
pH range
Methyl red
Methyl orange
Phenolphthalein
Bromothymol blue
Bromocresol green
Hydrochloric acid with ammonia solution
Ethanoic acid with ammonia solution
Nitric acid with potassium hydroxide