Cation log z/r < +0.48
Cation electronegativity < 1.2HydrogenHydrogenOxygenCationCation log z/r > +1.08
Cation electronegativity > 1.9OxygenCationCation log z/r between +0.48 and +1.08
Cation electronegativity between 1.2 and 1.9OxygenCationHydrogen(a)(b)(c)Fig. 5.1Relationship between cation properties and force of repulsion between a cation and
the hydrogen ions in a water molecule. The logz/rdata should be compared to Fig. 5.2.
Electronegativity is explained in Box 4.2. After Raiswell et al. (1980). (a) Large, low-charge,
electropositive ions (e.g. K+) are surrounded by water molecules such that the centre of the
negative charge on the oxygen is aligned toward the cation. (b) Smaller, more highly charged,
electronegative cations (e.g. Fe^3 +) interact more strongly with the water molecule forming a
bond to the oxygen and displacing one H+to form hydroxide. (c) Small, highly charged,
strongly electronegative cations (e.g. P^5 +) react to displace H+ions from the water, forming an
oxyanion.