Problem:The amino acid glycine has these pK_a's: 2.34 for COOH and 9.60 for NH₃⁺.

1. What is the ratio of [A⁻]/[HA] at pH 7.0? (For either acid)

2. Which form of glycine–X, Y, or Z–is most common at pH 7.0?
X has a charge = +1, Y has a charge = 0, and Z has a charge = −1.

Answer:

1. Using COOH = HA and COO⁻ = A⁻, 7.0 = 2.34 + log ratio, and the ratio = 4.57 x 10⁴ for [A⁻]/[HA].
     Using NH₃⁺ = HA and NH₂ = A⁻, 7.0 = 9.60 + log ratio, and the ratio = 2.51 x 10⁻³ for [A⁻]/[HA].

2. The most common form of glycine at pH 7 must be the form with COO⁻ and NH₃⁺, with a net charge = 0 (form Y).

top of page