Unformatted text preview: t: Use the Henderson-Hasselbalch
At pH 7.4, only the citrate(-3) and citrate(-2) forms will exist at any appreciable concentration.
The Henderson-Hasselbach equation states, pH = pKa+ log [A-]/[HA]
In our specific case, pH = pKa + log [citrate(-3)] / [citrate(-2)].
If the concentration of citrate(-3) is X, then the concentration of citrate(-2) is the total citrate
concentration (C0) minus X.
Hence, pH = pKa + log (X/C0-X) or, in our case, 7.4 = 6.4 + log (X/C0-X).
Thus, 7.4 - 6.4 = 1 = log (X/C0-X); or, taking the antilog of both sides, 10 = X / 120 mM – X.
Simplifying, 11X = 1200 mM. Hence, X = 109 mM.
Thus, at pH 7.4, when the total citrate concentration is 120 mM, [citrate(-3)] = 109 mM and
[citrate(-2)] = 11 mM. In other words, most (~91%) of the citrate will be completely ionized, as
expected, given that pH 7.4 is above the pKa (6.4) of the least acidic COOH group. Question 4. Chirality. Which Alice is holding an amino acid that has the S stereochemistry? Is
that amino acid the D- or the L-isomer? Alice on the left is holding an amino acid with R stereochemistry and is a D-amino acid, whereas
it is the Alice on the right who is holding an amino acid with the S stereochemistry and is an Lamino acid (see p. 18 in your textbook)....
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- Fall '08
- pH, Citrate