Investigation 16: Buffer Design

A buffer resists pH change because it holds a meaningful reserve of both a weak acid and its conjugate base. That reserve only exists in useful amounts when the pH you want is close to the acid's pKa, because pH = pKa + log(base/acid). When pH equals pKa, the ratio of base to acid is 1:1, which means roughly equal stockpiles of both forms are on hand to soak up whatever acid or base gets added. Pick an acid whose pKa is far from the target, and the equation forces a lopsided ratio, leaving almost nothing of one component to react with.

To build a buffer at pH 4.75, start by choosing an acid with a pKa near 4.75, acetic acid works well since its pKa is 4.76. Then use Henderson-Hasselbalch to find the acid-to-base ratio, plug in pH and pKa, solve log(base/acid) = pH - pKa, and convert back from the log. That ratio tells you how many moles of acetic acid versus acetate to dissolve together in the same solution. The actual concentrations you choose, as long as they keep that ratio, set the buffer capacity: a 1.0 M acid and 1.0 M base mix resists a much bigger addition of strong acid or base than a 0.01 M version of the same ratio, even though both sit at the identical pH.

This investigation ties directly into the error-analysis and experimental-design practices from this unit, since judging whether a buffer actually held its pH under titration means comparing measured values against the Henderson-Hasselbalch prediction and accounting for measurement uncertainty in volumes and concentrations.