Analyze the Data 2-2: Effects of Strong Bases on pH
The graph to the right illustrates the effect that the addition of a strong base such as sodium hydroxide has on the pH of an aqueous 0.1 M solution of an amino acid. Assume that prior to the addition of any OH−, the entire dissolved amino acid sample is in its fully protonated form. The addition of OH− causes the expected steep increase in solution pH until, between roughly 0.03 and 0.07 M NaOH, the solution pH remains almost constant at a pH of approximately 1.8. What causes the resistance to change in pH in this range? What are solutions that resist changes in pH called? What organic chemical group on the amino acid is most likely responsible for this phenomenon at pH 1.8? Additional base causes the pH to again increase rapidly until the base concentration reaches 0.15 M and 0.25 M, at which points the solution pH hovers around values of 6 and 9.3, respectively. What is the significance of these pH values? Which amino acid do you suspect is being titrated?
_feedback: When a weak acid is in aqueous solution of pH at or near its value of pKa, the weak acid will quickly establish an equilibrium with its conjugate base form and together the two will act to resist additional changes to the solution pH. Solutions in which weak acid/conjugate base pairs function to inhibit pH changes are called “buffers.” Buffers are at their most efficient when the concentrations of the weak acid and conjugate base forms are equal, as would be the case when precisely one half the amino acid concentration of sodium hydroxide has been added (0.05 M OH–), and at this point the solution pH should equal the pKa, of that weak acid group. At a low pH like 1.8, the buffering species on the amino acid must be a relatively strong type of weak acid like a carboxylic acid. Additional buffer points at pH 6 and 9.3 indicate that there are two additional chemical groups on the dissolved amino acid that can behave as buffers at the appropriate OH– concentrations. The pKa, of 9.3 likely corresponds to the amino group present on every amino acid. The ability of this amino acid to behave like a buffer at three different values of solution pH indicates that the amino acid side chain also has weak acid properties. Its apparent pKa, value of 6.0 identifies this amino acid as histidine due to the fact that its imidazole side chain functions as a buffer at this pH.
