es025751a_si_001.pdf (186.86 kB)
Download fileModeling Complexometric Titrations of Natural Water Samples
journal contribution
posted on 2003-03-18, 00:00 authored by Robert J. M. Hudson, Eden L. Rue, Kenneth W. BrulandComplexometric titrations are the primary source of metal
speciation data for aquatic systems, yet their interpretation
in waters containing humic and fulvic acids remains
problematic. In particular, the accuracy of inferred ambient
free metal ion concentrations and parameters quantifying
metal complexation by natural ligands has been challenged
because of the difficulties inherent in calibrating common
analytical methods and in modeling the diverse array of
ligands present. This work tests and applies a new method
of modeling titration data that combines calibration of
analytical sensitivity (S) and estimation of concentrations
and stability constants for discrete natural ligand classes
([Li]T and Ki) into a single step using nonlinear regression
and a new analytical solution to the one-metal/two-ligand equilibrium problem. When applied to jointly model
data from multiple titrations conducted at different
analytical windows, it yields accurate estimates of S, [Li]T,
Ki, and [Cu2+] plus Monte Carlo-based estimates of the
uncertainty in [Cu2+]. Jointly modeling titration data at low-
and high-analytical windows leads to an efficient
adaptation of the recently proposed “overload” approach
to calibrating ACSV/CLE measurements. Application of
the method to published data sets yields model results with
greater accuracy and precision than originally obtained.
The discrete ligand-class model is also re-parametrized,
using humic and fulvic acids, L1 class (K1 = 1013 M-1),
and strong ligands (LS) with KS ≫ K1 as “natural components”.
This approach suggests that Cu complexation in NW
Mediterranean Sea water can be well represented as
0.8 ± 0.3/0.2 mg humic equiv/L, 13 ± 1 nM L1, and 2.5 ±
0.1 nM LS with [Cu]T = 3 nM. In coastal seawater from
Narragansett Bay, RI, Cu speciation can be modeled as
0.6 ± 0.1 mg humic equiv/L and 22 ± 1 nM L1 or ∼12 nM
L1 and ∼9 nM LS, with [Cu]T = 13 nM. In both waters,
the large excess (∼10 nM) of high-affinity, Cu-binding ligands
over [Cu]T results in low equilibrium [Cu2+] of 10-14.5±0.2
M and 10-13.3±0.4 M, respectively.
History
Usage metrics
Categories
Keywords
humicligandmodeling Complexometric TitrationsRIdata sets yields model resultsmetal ion concentrationsL 1 classNW Mediterranean Sea waterNatural Water Samples Complexometric titrationsmodeling titration datametal speciation datafulvic acidsmethodparameters quantifying metal complexationnMK iCu10 13 MACSV