pH curves

Question 1

What is the equivalence point?

Answer 1

Mid point of the inflection, enough titrant added to completely neutralise solution

Question 2

What can you do with a pH curve?

Answer 2

Question 3

What are the 4 types of acid base titrations?

Answer 3

• Strong acid + Strong base
• Strong acid + Weak base
• Weak acid + Strong base
• Weak acid + Weak base

Question 4

Notes about indicators

Answer 4

• Acid base indicator (weak acid dissociating to give an anion of a different colour)
• HIn (aq) + H2O (l) ⇌ H3O+ (aq) + In- (aq)

Hln and ln- are conjugate base pairs with different colours

If solution is acidic, equilibrium shifts left so more Hln is present so the colour of Hln will dominate

If alkaline, equilibrium shifts right so more ln- is present and ln- colour dominates

Endpoint of reaction is when balance between Hln and ln- is present, so Ka = [H+] [ln-] / [Hln] becomes Ka = [H+]

Taking negative logs of both sides, pKa = pH, so pKa of an indicator is the same as the pH of its endpoint

Question 5

How do we choose suitable indicators?

Answer 5

Indicators change colour over a narrow pH range centred around the kPa of the indicator.

Indicator is appropriate for the titration if pH range of indicator falls between rapid pH change of the titration.

Question 6

What indicator is suitable for a strong acid strong base titration?

Answer 6

• pH changes from 4 - 10 at end point
• Methyl Red and Phenolphthalein
• Methyl orange isn’t ideal but does indicate, so widely used

Question 7

What indicator is suitable for a strong acid weak base titration?

Answer 7

• pH changes from 4-7 at end point
• Methyl red
• Methyl orange can be used

Question 8

What indicator is suitable for a weak acid strong base titration?

Answer 8

• pH changes from 7-10
• Phenolphthalein

Question 9

What indicator is suitable for a weak acid weak base titration?

Answer 9

• No sudden pH change
• So no suitable indicators
• End point cannot be easily determined