Influence of Autocorrelation and Measurement Errors on Interpretation of Solubility Diagrams
Xu, Shihe | Harsh, James B.
The solubility method for determining solid-phase controls on solute activities in aqueous systems is subject to potential sources of misinterpretation. Measurement errors cause deviation of solid-phase solubilities from theoretical values, whereas autocorrelation between dependent and independent variables of a solubility diagram may lead to false interpretation of data points falling on a given solubility line. In this study, a Monte Carlo simulation was performed to determine the influence of measurement errors and autocorrelation on the interpretation of solubility data. For solid-phase control of Al activity at pH <6 for natural gibbsite, basaluminite, and jurbanite, deviation of apparent solubility from theoretical solubility lines of the solids due to measurement errors in pH, total inorganic monomeric Al, ionic strength, and SO₄ activity are usually small compared with the deviation expected when random numbers are used to generate a data set. As a result, solid-phase control was distinguishable from autocorrelation of random data except at pH 6 to 7. For a given metal oxide, autocorrelation interference is significant in a solubility study only when the OH/metal ratio of the predominant dissolved metal species is close to the OH/metal or O/Metal ratio of the solid phase. We concluded that the solubility method is a reliable and useful tool to study Al chemistry in acid soils if used with caution. Contribution from the College of Agriculture and Home Economics Research Center, Pullman, WA. Departmental Paper no. 9301-32, Project 0385.
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