pH and pKa Main Concept: The pH is an important characteristic of aqueous solutions that can be controlled with buffers. Comparing pH to pKa allows one to determine the protonation state of a molecule with a labile proton.

pH and pKa Buffer Solutions

HC2H3O2 C2H3O2- - A buffer solution contains a large concentration of both members in a conjugate acid-base pair - conjugate acid reacts with added base and conjugate base reacts with added acid - pH of buffer is related to pKa and concentration ratio of acid and base forms - buffer capacity is related to absolute concentrations of acid and base forms

- relationships can be used to reason issues such as ratio of acid to base forms in a given buffer, impacts on buffer capacity for added acid or base, and choice of an appropriate conjugate acid-base pair for a desired buffer pH (including polyprotic acids)

- If [A-]/[HA] starts as 1, only when ratio changes by factor of 10 does a 1 pH unit change occur  adding small amounts of either acid or base does not change ratio much, so pH changes are much smaller for buffers than unbuffered solutions - Weak acids and conjugate bases make good buffers; Strong acids and bases do not

- takes much more base to change pH of a weak acid solution because of large reservoir of undissociated weak acid

- comparing pH of a solution to pKa of any acid in solution, concentration ratio between acid and base forms of that acid (the protonation state) can be determined - For example, if pH < pKa, acid form has a higher concentration than the base form - If pH > pKa, base form has a higher concentration than the acid form - Applications of this include use of acid-base indicators, protonation state of protein side chains (including acids or proteins with multiple labile protons), and pH required for acid-catalyzed reactions