COMPARISON OF THE LINEAR VANDENBERG RUZIC TRANSFORMATION AND A NONLINEAR FIT OF THE LANGMUIR ISOTHERM APPLIED TO CU SPECIATION DATA IN THE ESTUARINE ENVIRONMENT

Copper complexation measurements in estuarine organic-rich samples wit h differential pulse anodic stripping voltammetry (DPASV) and differen tial pulse cathodic stripping voltammetry (DPCSV) show a relatively la rge scatter in the data, due to a high organic matter content. Two met hods for the estimation of the complexation parameters according to th e Langmuir isotherm are compared: the linear transformation of Van den Berg/Ruzic and the non-linear method according to Wilkinson. For DPAS V measurements in estuarine samples the values of the conditional stab ility constant (K'), estimated with the linearization method are syste matically lower than those estimated with the Wilkinson method. The Va n den Berg/Ruzic linearization is more susceptible to outliers or data points that do not follow the model of the Langmuir isotherm. For DPC SV measurements, where the data followed the Langmuir isotherm more cl osely, this difference could not be demonstrated. The non-linear metho d is to be preferred because this method is more suited to the error s tructure of the data, which is of constant absolute magnitude. Moreove r it offers the advantage that the standard error of the estimated par ameters can be calculated consistently.