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Definition: The major species in which a cation is present, under a specified set of conditions. Although usually applied to cations, the term 'principal species' may also be applied to anions.
Examples: Zinc (II) may be present as Zn2+, Zn(NH3)2+, Zn(OH)2, etc. Equilibrium considerations may be used to determine which cation species is the principal species. For anions, at pH 8, the HCO3- ion is the principal species in the H2CO3—HCO3-—CO32- system.
