25. Extraction and spectroscopic methods can be used to determine the dissociation
constant (Ka) of a monoprotic organic acid (HA) and its distribution coefficient (Kd)
between the organic phase and the aqueous phase. When extracting organic acids, it is
assumed that only HA is soluble in the organic layer, and its concentration is [HA]org；
while HA and A-
exist in the aqueous layer, and their concentrations are [HA]aq and
[A-
]aq, respectively. The distribution coefficient is defined as Kd = [HA]org/[HA]aq. In an extraction experiment, take 10-mL of an aqueous solution containing 3.5
mM HA at pH 5.0； extract it with 15-mL of an organic solvent. Separate the mixed
solution into aqueous and organic layers. Take 1.0 mL of the aqueous layer； put it into
a 1.0-cm (path length) cuvet； measure the absorbances at 400 nm and 450 nm； the
observed absorbances are 0.30 and 0.62, respectively. Given the following information:
molar absorptivity ε (in M-1
.cm-1
): ε400(HA) = 2000, ε400(A-
) = 250, ε450(HA) = 200, ε450(A-
) = 1500. Answer the following questions.
(A) Find [HA]aq and [A-
]aq in the aqueous layer. 4%
(B) Find [HA]org in the organic layer. 2%
(C) Find the acid dissociation constant Ka. 2%
(D) Find the distribution coefficient Kd. 2%
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