Final answer:
Being within 1 unit of pKa indicates the solution is a buffer with similar concentrations of the acid and base forms. A pH 1 unit above pKa reflects a ten-fold increase in the base-to-acid ratio, and 1 unit below indicates a ten-fold decrease. This pH range is ideal for buffer effectiveness.
Step-by-step explanation:
When the pH is within 1 unit of the pKa, this indicates that the concentration of the protonated form of the compound (the acid, HA) is similar to the concentration of the deprotonated form (the base, A−).
This is characteristic of a solution that is acting as a buffer, which maintains a stable pH when small amounts of acid or base are added.
The Henderson-Hasselbalch equation, which relates pH, pKa, and the ratio of [A−]/[HA], explains that when the pH is equal to the pKa, the concentrations are equal, and pH changes of +1 or -1 reflect a ten-fold increase or decrease, respectively, in the ratio of the base to the acid concentration.
The logarithmic nature of the pH scale means that a change in one pH unit reflects a ten-fold change in the concentration of hydrogen ions (H+).
Therefore, when the pH is within 1 unit above or below the pKa, the solution is in the buffer region, and significant amounts of both the protonated and deprotonated forms of the buffer are present.
This is crucial for the buffer's ability to resist changes in pH. For example, if the pH is pKa +1, the ratio [base]/[acid] is increased by 10, and if the pH is pKa -1, the ratio is decreased to 0.1. Thus, the buffer is most effective when the pH is close to its pKa value.