| Source:
School of Earth & Atmospheric Sciences, Georgia Institute of Technology,
Atlanta, GA, U.S.A. |
| Reference:
J. D. Ritchie and E. M. Perdue, Geochim. Cosmochim. Acta, 67,
85-96 (2003). |
| Ten-ml
solutions containing 0.36 ± 0.01 g L-1 (on a dry ash-free basis)
of an IHSS sample in 0.1 M NaCl were titrated from their initial pH values
of 3.0-3.3 to final pH values of 10.5-10.7 in 25-35 minutes, using 6-8
mL increments of carbonate-free 0.100 M NaOH.
All titrations were conducted under a N2 atmosphere in a thermostated
water bath at 25.00 ± 0.02 Celsius. At least three titrations were obtained
for each sample. The stock solutions of soil, peat, leonardite, and Summit
Hill humic acids also contained 0.002 M NaOH, which was needed to effectively
disperse/dissolve those samples, and an equivalent quantity of HCl that
was added just before a titration was started. |
| Carboxyl
is the charge density (meq/g C) at pH 8.0; Phenolic is two times
the change in charge density (meq/g C) between pH 8.0 and pH 10.0. |
| The overall charge density (in meq/g
C) of a humic substance increases systematically with pH (or with
the concentration of H+). The equation on the right,
known historically as a modified Henderson-Hasselbalch equation
for two classes of binding sites, was used to fit titration data
and obtain the set of model fitting parameters in this |
 |
| table.
The fitting parameters of the model were obtained by a nonlinear least-squares
fit of the model to aggregated sets of replicate titration data.
Q1 and Q2 are the maximum charge densities
of the two classes of binding sites, Log K1 and
Log K2 are the mean log K values for proton binding by the
two classes of sites, and n1 and n2 are
empirical parameters that control the width (in log K) of a class of proton
binding sites. N is the number of fitted titration data points, and
RMSE is the root mean-square error for fitting this model to the data
(see Reference). This model, also known in more recent literature as a
Langmuir-Freundlich equation, is the primary building block of the NICA class
of models.
|
| An *
means that a sample is no longer available, and nd means that an
item was not determined. |