Acids and Base Equilibria

Buffer Solutions resist chang in pH. A typical solution is made up by mixing a weak acid with its conjugate base or a weak base with its conjugate acid. The weak bases tends to neutralize any extra acid added to the buffer, and the weak acid tends to neutralize any extra based added. However, with enough acid is base the capacity of the buffer can always be overwhelmed.

HA(aq) + H₂O(l) <==> H₃O⁺(aq) + A^-(aq)

As long there is an appreciable amounts of the weak acid and its conjugate base, the hydronium ion concentration is

The concentration ratio, not the concentration governs the hydronium ion concentration. If more acid or base is added the ratio changes slightly and keeps the buffer at near its buffered pH.

If either of the [HA] or [A^-] goes to zero, the ratio goes to zero or infinity, and the equation describing the buffer loses its meaning. Buffer action no longer occurs. Both acid and conjugate base must be present for buffer action.

If we consider a buffer when a weak base B is placed into solution with with its conjugate acid,

B(aq) + H₂O(l) <==> HB⁺(aq) + OH^-(aq)

we can calcuate the buffered contration of hydroxide ion,

Strong Acid-Base titration is a procedure for determining the amount of an acid or base in solution. During a titration, acid and base are mixed in a controlled fashion. The progress is follwed by measuring the pH of solution as a function of titrant (whats added). See Figures 6-9 and 6-10 for typical examples.

H₃O⁺(aq) + OH^-(aq) <==> 2 H₂O(l)

starting point

₃

⁺

As the reaction approaches equivalence, some base has been added. Assuming that any bases that has been added neutralizes the same amount (mols) of acid, the concentration of H₃O⁺(aq) is the remaining of moles acid divided by the total volume.

At the equivalence point, the amount of base and acid are exactly equal. The only concentration of H₃O⁺(aq) comes from the autoionization of water. The pH is then 7.0.

Beyong the euaivalence point more base has been added then is neccessary to neutralize all the acid. The excess strong base would give rise to OH^-(aq)

Titration of a weak acid with a strong base has the same four regions. However there is an equilibrium associated with such a problem. See problem 6-45.

Weak Acid/Strong Base Titration

starting point

₃

⁺

Between the starting point and the equivalence point we have a buffer solution. The titrant (strong base) has neutralize some of the weak acid and formed it conjugate base. We can calcualte the equilibrium concentration using the equations that describe buffer solutions (see above).

At the equivalence point, moles of acid equals moles of base. All the weak acid has been converted to its conjugate base. The conjugate base wants H⁺ and extracts it from water producing some weak acid and hydroxide ion. We can calculate the equilibrium concentrations, knowing K_b.

After the equivalene point, moles of base overwhelms the buffer and the excess added hydroxide ions dominate.

Polyprotic Acids contain two or more H atoms that can dissociate. For example H₂SO₄ is a diprotic acid and H₃PO₄ is a triprotic acid. Polyprotic acids donate hydrogen ion in stages with the first stage dominating the latter stages.

_a1

At high pH, most of the concentration of a polyprotic acid will be in the form of the most negative conjugate base. At low pH, the more acidic (higher Ka) form of the poluprotic acid will exist.