A buffer is a solution that resists a change in pH when acid or base is added and contains significant quantities of a weak acid and its conjugate base or a weak base and its conjugate acid. Adding small quantities of acids or bases shifts the equilibrium, reducing the number of protons or hydroxide ions in solution. A buffer can be made by adding a weak acid to a salt of its conjugate base, providing the metal cation will not itself affect pH.A buffer of acetic acid (CH3COOH), for example, could be made by mixing acetic acid with potassium acetate (CH3COOK). In aqueous solution, potassium acetate completely dissociates into potassium ions (K+) and acetate ions (CH3COO–). Potassium ions (K+) do not play a role in the buffer because the pH is affected only by the acid and its conjugate base. The acetic acid molecules can donate a proton to the added base, and the acetate ions (CH3COO–) can absorb protons (H+) from any acid that is added.
If the ratio is close to 1, the pH will be close to the pKa of the weak acid. Also, consider that and the fact that the logarithm of an exponential is approximately equal to the exponent. The exponent of Ka therefore gives a rough indication of the value of pH. As more A– is added, the ratio becomes greater, and as HA is added, the ratio becomes smaller. Keeping the ratio within an order of magnitude in either direction therefore keeps the pH range within about ±1.Knowing the pH of a buffer system is important when deciding which buffer to use for a specific reaction. The best choice of buffer is one that will maintain the optimal pH for the reaction taking place. Buffer pH can be estimated by assuming that the initial concentrations of acid and conjugate base will be the same, making the Henderson-Hasselbalch equation:
What is the pH of this solution? Will this be an effective buffer for maintaining pH around 5.0?
A buffer solution has a finite number of acid and conjugate base particles in it. It therefore can absorb a finite number of strong acid or base ions. The buffer capacity is the number of moles of strong acid, protons (H+), or strong base, hydroxide ions (OH–), that can be added to one liter of solution before the pH changes by 1 in either direction.A liter of 1 M buffer, for example, contains one mole of HA and one mole of A–.
The third column shows the concentration of ascorbic acid, which starts as 0.030 moles. Then 0.0050 moles of HCl is added, which completely dissociates, adding an additional 0.0050 moles of H+ to the solution. The H+ ions will be absorbed by the C6H7O6– ions, making the [HC6H7O6] in the final solution 0.035 moles.
The equilibrium amount of H3O+ will be zero in this case because it is entirely absorbed by the conjugate base ascorbate (C6H7O6–). The first column shows the change in concentration of the conjugate base, which decreases as the HCl is added and is used up in producing weak acid molecules.