BUFFERS

• Buffers consist of a weak acid and its conjugate base, resisting changes in pH when hydrogen ions or hydroxide ions are added
• In the graph of a weak acid like acetic acid, the midpoint represents the pKa, where 50% dissociation occurs
• At the midpoint, 0.5 equivalents of OH- have been added and half of the conjugate acid has dissociated, so [A-] equals [HA]
• As more OH- ions are added and move to the right on the curve, more of the conjugate acid molecules (HA) dissociate to generate H+ ions, resulting in only a small increase in pH
• Adding hydrogen ions to the buffer at its pKa (moving to the left of the midpoint) causes conjugate base molecules (A-) to combine with the added hydrogen ions to form HA, leading to almost no decrease in pH
• A buffer can only compensate for an influx or removal of hydrogen ions within approximately 1 pH unit of its pKa
• If the pH of a buffered solution changes from the pKa to 1 pH unit below the pKa, the ratio of [A-] to HA changes from 1:1 to 1:10
• More concentrated buffers are more effective because they contain a greater total number of buffer molecules per unit volume that can dissociate or recombine with hydrogen ions