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Unformatted text preview: nd pK3 = 6.4). Why do the pKa
values of the three ionizable groups differ from each other?
When the first proton dissociates from fully protonated citric acid (from either terminal COOH),
there is now a negative charge on the citrate(-1) molecule that makes it harder for the next H+ to
dissociate from either of the neighboring COOH groups on the same citrate molecule. At a pH
value where the second H+ has dissociated from citrate (from the other terminal COOH), there
are now two negative charges on the citrate(-2) molecule, which make it even harder for the last
H+ to dissociate from the last (the middle) COOH group on the same citrate molecule.
(c) At pH values above 7, citrate has a reasonably high affinity for heavy metal ions, e.g. Kd =
10-5-to-10-6 M for Fe2+, Co2+, Ni2+, Cu2+ and Zn2+. Why should citrate act like a metal-binding
molecule (a metal ion "chelator")?
Opposite charges attract. Citrate can act like a reasonably good metal cation chelator because,
above pH 7, its multiple negative charges can make strong and multiple (polyvalent)
electrostatic contacts with the permanent positive charges present in metal cations (i.e. the free
electrons on citrate are shared with / help fill the empty orbitals in the metal cations).
(d) In a solution containing a total concentration of 120 mM citrate, what citrate species are
present at pH 7.4 and what are their concentrations? (Hin...
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- Fall '08