A more realistic approach to speciation using the IUPAC Stability Constants Database

The lUPAC Stability Constants Database (SC-Database), designed to contain all significant published constants, provides the most complete and accessible route to stability constants in the literature. Collection of new constants is becoming less significant since most are now measured to confirm a mechanism or to identify bonding centers, not to provide data for general use. AS a result, constants are often measured less rigorously and are of lower accuracy. What are required now are critical evaluations of existing data, coupled with accurate study of a number of important and superficially simpler systems (e.g., complexes with some inorganic ligands). Calculation of species distribution curves is a major use of stability constants. Ways of reflecting possible errors in the calculated curves are now required. Historically, curves have been drawn as sharp lines, but these could only result from using the exact stability constants with an accurate model for the system. Two techniques for demonstrating the effect of errors are outlined. Constants are dependent on ionic strength and temperature changes. Specific ion interaction theory (SIT) is the most general method of compensating for ionic strength changes up to about 5 molal. Software to correct constants for ionic strength changes, prepared under recent IUPAC projects, is described.