21 buffer solutions

what is a buffer solution

a system that minimises pH changes when small amounts of an acid or a base are added

what do you need to prepare a buffer solution

a weak acid and its conjugate base

2 different methods for preparing a buffer

• a weak acid and a solution of one of its salts

• partial neutralisation of the weak acid

what happens when you add an acid to a buffer

• concentration of H+ increases

• the H+ ions react with the conjugate base, A-

• the equilibrium position shifts to the left, leaving most of the H+ ions

what happens when you add a weak acid to a molecule

• [OH-] increases

• the small concentration of H+ ions react with the OH- ions

• HA dissociates, moving the equilibrium position to the right to restore most of the H+ ions

equation for working out [H+] in buffers

how does the carbonic acid-hydrogencarbonate buffer system work to control blood pH

when an acid is added:

• concentration of H+ increases and the H+ ions react with the conjugate base HCO3-

• the equalibirum shifts to the left, removing most of the H+ ions

when OH- is added

• [OH-] increases and reacts with the H+

• H2CO3 dissociates, shifting equalibirum position to the right to restore most of the H+ ions

pH titration curve of adding an alkali to an aicd

pH titration curve for additions of an acid to a base

equivalence point

the volume of one solution that exactly reacts with another solution

indicator at the end point of a titration

• the indicator contains equal concentrations of HA and A-

• the colour will be in between the two extreme colours

what is an indicator

a weak acid that has a distinctly different colour from its conjugate base

what happens when a basic solution is added to an indicator

• OH- ions react with H+ in the indicator

• the weak acid, HA, dissociated, shifting the equilibrium position to the right

• this causes a colour change

what happens to the indicator if there is an initially basic solution and an acid is added

• H+ ions react with the conjugate base, A-

• the equilibrium position shifts to the left

How sensitive is the end point

most indicators change colour over a range of about 2pH units

how to choose an indicator

use pH titration curve graph and pick an indicator which changes colour in the range of the vertical line. No indicator is sutable for a weak acid and weak alkali