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Title: Improved Scales for Metal Ion Softness and Toxicity

item Kinraide, Thomas

Submitted to: Environmental Toxicology and Chemistry
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 9/15/2008
Publication Date: 4/12/2009
Citation: Kinraide, T.B. 2009. Improved Scales for Metal Ion Softness and Toxicity. Environmental Toxicology and Chemistry. 28:525-533.

Interpretive Summary: Metals may be beneficial plant and animal nutrients, and they may be important toxicants, depending upon the identity and availability of the metal. Copper and zinc are good examples of metals that are essential in very small amounts but are toxic to both plants and animals in greater, but still small, amounts. Aluminum, on the other hand, is abundant but not essential for living things. It is a serious toxicant to plants in acidic soils, and it probably plays a role in Alzheimer’s disease. The present study has established a scale of metal ion reactivity based upon a property known as 'softness' (ions are charged, usually soluble, atoms). Softness refers to the ease with which the electron cloud of the atom can be deformed (thus the term softness). Metal ions that are soft are also very toxic, so the scale of softness also determines, in part, the scale of metal ion toxicity. These scales exceed in quality all previously published scales for softness and toxicity. These scales have both scientific and practical importance – the latter because it enables us to determine the identity and likely availability of metals as nutrients and toxicants and because it enables us to design purification systems and nutrient regimens for good health and safety.

Technical Abstract: Ten scales relating to chemical hardness or softness 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 logK values for metal ion binding to seven soft ligands. These ten scales were normalized and averaged. The resulting consensus scale for softness (S(Con)) appeared to be superior to any of the ten scales used in its construction based upon correlations among the scales. Other possible indicators of softness were examined including the standard electrode potential (E) and the bulk metal density (D), and E and D were also superior to most of the ten scales just mentioned. S(Con) may be computed from E, D, and the first ionization potential (I); R^2 = 0.867 for the equation S(Con) = aEI + bD. A consensus scale for toxicity (T(Con)) derived from studies with many different taxa correlated well (R^2 = 0.807) with S(Con) computed from the preceding equation, but incorporation of ion charge (Z) into the following equation, T(Con) = aS(Con) + bS(Con)Z + cZ, increased R2 to 0.923. Substitution of other softness scales for S(Con) into equations to predict T(Con) reduced the value of R^2. Thus S(Con) appears to be a superior scale for metal ion softness and toxicity, the latter being an interactive function of both softness and charge.