Question

A solution of 0.146 M cysteine is titrated with 0.0487 M HNO3. The pKa values for cysteine are 1.70, 8.36, and 10.74, corresponding to the carboxylic acid group, thiol group, and amino group, respectively. Calculate the pH at the equivalence point.

Answer 1

Cysteine titration would have three equivalence points

pKb1 = 3.26

pKb2 = 5.64

pKb3 = 12.3

Ist equivalence point,

pH = 1/2(pKb1 + pKb2)

     = 1/2(3.26 + 5.64)

     = 4.45

IInd equivalence point,

pH = 1/2(pKb2 + pKb3)

     = 1/2(5.64 + 12.3)

     = 8.97

IIIrd equivalence point,

Since the volume of cysteine solution is not mentioned, we cannot calculate the final volume and thus the IIIrd equivalence point.