Chapter 2. Effects of Strong Bases on pH

Analyze the Data

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_feedback: When a weak acid is in aqueous solution of pH at or near its value of pK_a, 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 pK_a, 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 pK_a, 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 pK_a, 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.