1. Problem 2-2, Benjamin (2002). Gibbs energy change and equilibrium constants for a series of reactions.
2. Problem 3-7, Benjamin (2002). Hypochlorous acid additions.
3. Over the years, researchers have strived
to quantify the acid-base behavior of natural organic matter. A recent
effort by Ritchie and Perdue (2003; "Proton-binding study of standard and
reference fulvic acids, humic acids, and natural organic matter," Geochimica
et Cosmochimica Acta 67, 85-96) compared the acid-base properties of
fourteen isolated fractions of natural organic matter available from the
International Humic Substances Society.
(a) Ritchie and Perdue quantified the amount of carboxyl
functional groups in the organic matter as the charge density at pH 8.0
(defined as Q1) and the amount of the phenolic functional
groups as twice the change in charge density between pH 8.0 and 10.0 (defined as
Q2). First, what do they mean by "carboxyl" and
"phenolic" (use the values of K1 and K2 in
Table 1 in your explanation)? Second, why are the amounts of carboxyl and
phenolic charge density defined for the pH ranges of <8.0 and 8.0 to 10.0,
respectively?
(b) Ritchie and Perdue considered the amount of organic
matter in the ionic strength of the titration solutions. Explain in your
own words how they did this. They concluded that the concentration of
organic matter was a negligible contributor to ionic strength. For the
standard Suwannee River fulvic acid sample, the first one in Table 1, calculate
the maximum value of the concentration of the "ith organic anion,
S[orgi-],
and compare it to the concentration of sodium in the titrations solutions.
(c) Use the titration data presented in Figure 1 to show the
features of the titration curve (the carboxyl and phenolic ranges, the pK values
in Table 1). Do the values of Q1 and Q2
derived from the graph match the data presented in Table 1? Do the acidity
constants pK1 and pK2 correspond to any
features of the graph as you would expect for the titration of an organic acid?
Why or why not?
Last updated on September 09, 2007 at 04:06 PM by Joe Ryan