Some reflections on concentration, activity, and electrode potential are made in order to explain the significance of apparently meaningless numbers such as extremely low solubility values calculated from certain solubility products. To this end, the nature of the electrode potential and the relation between activity and concentration is discussed. In particular, the effect of ion complexation on activity coefficients and on electrode potential is emphasized. Some practical examples are discussed for: a) the effect of gradually increasing the concentration of a ligand both on ionic strength and complex formation and b) the enormous difference between the activity of Hg2+ and the concentration of dissolved mercury in a HgS saturated solution.
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