3.1.17 buffer composition (HL) and pH

chem whatarethemechanismsofchemicalchange

Reactivity 3.1.17 - the pH of a buffer solution depends on both:

• the or of its acid or base
• the ratio of the concentration of acid or base to the concentration of the conjugate base or acid

see 3.1.10 and 3.1.11 acid and base dissociation constants (HL)

different buffers for different values

considering an acidic buffer made of the generic weak acid and its salt :

two approximations can be made based on the interpretation of these reactions:

1. the dissociation of the weak acid is so small that it can be considered to be negligible

2. the salt is considered to be fully dissociated into its ions

following these approximations:

for basic buffer solutions, the equivalent equations are:

these expressions show that the of a buffer solution can be determined directly from:

• the or values of its component acid or base

• the ratio of initial concentrations of acid and salt used to prepare the buffer

• an equimolar solution can be prepared by a neutralisation reaction where half of the starting acid or base is converted into salt

  • alternatively, the buffer can be prepared directly by mixing the acid or base with an appropriate amount of its salt

• the mixture will contain the unreacted acid or base and its salt in equimolar amounts.

following the equations above, equimolar buffer solutions are at the of their values

factors that can influence buffers

dilution

• and are equilibrium constants and so are not changed
• the ratio of acid or base concentration (as both components will be decreased by the same amount)

diluting a buffer alters the amount of acid or base it can absorb without significant changes in - the buffering capacity

this depends on the molar concentrations of its components, so it decreases as the concentrations are decreased by dilution

temperature

• as temperature affects the value of and , it affects the of the buffer
• a constant temperature should be maintained in all work involving buffers
  • ie calibration of meters

challenge questions

10. contrast and explain how the of each of the following types of solution responds to dilution:
    a) strong acid
    increases

    b) weak acid
    increases

    c) buffer
    stays the same as its depends on , which changes with temperature, and the ratio of concentrations of the salt to the concentration of the acid or base stays the same as both are decreased by the same amount