Advertisement

Improved scales for metal ion softness and toxicity

Authors

  • Thomas B. Kinraide

    Corresponding author
    1. Agricultural Research Service, U.S. Department of Agriculture, Beaver, West Virginia, 25813–9423
    • Agricultural Research Service, U.S. Department of Agriculture, Beaver, West Virginia, 25813–9423
    Search for more papers by this author

Abstract

Ten scales relating to chemical hardness or softness of metal ions were compiled. These included eight published scales such as those of Pearson, Ahrland, Klopman, and Misono. Another scale consisted of the logs of the solubility products of metal sulfides, and yet another was a consensus scale constructed from —log K values for metal ion binding to seven soft ligands. These 10 scales were normalized and averaged. The resulting consensus scale for softness (σCon) appeared to be superior to any of the 10 scales used in its construction based on correlations among the scales. Other possible indicators of softness were examined, including the standard electrode potential (E0) and the bulk metal density (ρMetal), both of which were also superior to most of the 10 scales just mentioned. Vales for σCon may be computed from E0, ρMetal, and the first ionization potential (IP), R2 = 0.867, for the equation σCon = aE0IP + bρMetal. A consensus scale for toxicity (TCon) derived from studies with many different taxa correlated well (R2 = 0.807) with σCon computed from the preceding equation, but incorporation of ion charge (Z) into the following equation, TCon = aσCon + bσConZ + cZ, increased R2 to 0.923. Substitution of other softness scales for σCon into equations to predict TCon reduced the value of R2. Thus, σCon appears to be a superior scale for metal ion softness and toxicity, the latter being an interactive function of both softness and charge.

Ancillary