Acids and bases

• An indicator is a molecular substance that changes colour as the pH of a solution changes
• An indicator is an equilibrium system of a weak acid and its conjugate base.
• Methyl red is a common indicator that has an acid structure that is a bright red colour. It is represented by HIn (In for indicator; the H indicates it is an acid). The conjugate base formed by the loss of the proton is a yellow colour. It is represented by In-
• When methyl red is added to an acidic solution with higher [H3O+], the equilibrium shifts to the left. The hydronium ions react with the In– to form HIn. This results in an increase in [Hin]. Hence, methyl red is red in an acid.
• When methyl red is added to an acidic solution with higher [H3O+], the equilibrium shifts to the left. The hydronium ions react with the In– to form HIn. This results in an increase in [Hin]. Hence, methyl red is red in an acid.

  
• When methyl red is added to a basic solution with higher [OH- ], the hydroxide ions react with the H3O+ and decrease [H3O+]. The equilibrium shifts to the right. This results in an increase in [In- ]. Hence, methyl red is yellow in a base

  
• A buffer is a solution that resists a change in its pH when a small amount of strong acids or bases is added
• Buffers are solutions that contain a conjugate acid– base pairing where either the acid or base is weak.
• Increasing [H3O+ ], the equilibrium of the system is disrupted.

  The ethanoate ion (CH3COO- ) will be consumed by reacting with the added acid (H3O+).

  Equilibrium shifts to the left.

  Since much of the added H3O+ ion is removed, there is a minimal increase in the overall [H3O+].

  There is a minimal decrease in the pH or increase in the acidity of the buffer solution.
• Increasing [OH-]

  The ethanoic acid (CH3COOH) will be consumed by reacting with the added OH- , decreasing [CH3COOH], the equilibrium of the system is disrupted.

  Equilibrium shifts to the left.

  Since much of the added OH- ion is removed, there is a minimal increase in the overall [OH- ].

  There is a minimal increase in the pH or basicity of the buffer solution.
• The extent to which a buffer solution can absorb the extra acid or base added, is called the buffer capacity.
• The buffer capacity depends on two factors: • High concentration of weak acid/base • Equal concentration
• The buffer capacity depends on two factors: • High concentration of weak acid/base • Equal concentration
• Buffer solutions
• A buffer solution resists changes in pH when small amounts of an acid or alkali are added to it.