This paper gives insight into chemical speciation. After a brief introduction, the methods available to study solution state of metal ions and the importance of speciation in biology are discussed through different examples. Speciation of toxic AlIII, beneficial CuII and CaII and potentially medicinal PdII and VOIV in various biofluids and tissues is discussed in order to describe the solution state of these metal ions in an organism. The relevance of ternary complex formation with endogenous biomolecules is emphasised. The importance of the biospeciation of AlIII ion in its transport and involvement in neurological disorders, of the actual chemical form of the insulin-mimetic VOIV complexes in its biological occurrence and CaII in mineralization/demineralization processes is discussed. The role of time, i.e., the kinetic aspects of biospeciation, is discussed in the case of the transport of AlIII and CuII and some PdII complexes (models of the anticancer PtII complexes) in blood serum.
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