Passage I (Questions 1-7)

    Buffers are aqueous solutions of weak acids and their conjugate base. The pH of solution is dictated by the Henderson-Hasselbalch equation: pH = pKa + log base/acid. This means that a buffer solution should be mixed in a manner where the pKa of the acid is close to the pH desired. The following chart lists the Ka and pKa values for some common monoprotic weak acids:

Acid Ka value pKa value
 H2NCONH3+ 6.6 x 10-1 0.18
 HF 6.8 x 10-4 3.17
 HCNO 3.5 x 10-4 3.49
 HCO2H 1.7 x 10-4 3.78
 H3CCOCO2H 1.4 x 10-4 3.89
 C6H5CO2H 6.5 x 10-5 4.19
 C6H5NH3+ 2.3 x 10-5 4.64
 H3CCO2H 1.8 x 10-5 4.74
 C5H5NH+ 7.1 x 10-6 5.16
 HClO 3.5 x 10-8 7.49
 B(OH)3 5.9 x 10-10 9.22
 NH4+ 5.6 x 10-10 9.26
 HCN 4.9 x 10-10 9.32
 H3CNH3+ 2.2 x 10-11 10.66
 (H3C)2NH2+ 1.9 x 10-11 10.77


    Polyprotic acids can also be used in making buffers. A difficulty that arises with polyprotic acids involves the two or more pKa values. For any given polyprotic acid, the conjugate pair will buffer at the respective pKa for the acid of the conjugate pair. For instance, carbonate/bicarbonate will buffer at a pH around 10.8 because pKa2 of carbonic acid is 10.81. Carbonic acid/bicarbonate will buffer at a pH around 6.4 because pKa1 of carbonic acid is 6.37. The following table lists the Ka values for some common polyprotic acids:

Acid Ka1 value Ka2 value Ka3 value
 H2C2O4 5.6 x 10-2 5.1 x 10-5
 H2SO3 1.3 x 10-2 6.3 x 10-8
 H3PO4 6.9 x 10-3 6.2 x 10-8 4.8 x 10-13
 H2CO3 4.3 x 10-7 1.5 x 10-11
 H2S 8.9 x 10-8 3.8 x 10-13


    As a point of interest, carbonates and phosphates are believed to be the major contributors to buffering in the blood. For years it was believed that carbonate played the major role, but recent research indicates that phosphate may play a more significant role than carbonate in the overall buffering.