where formula_171 as its common logarithm is the activity coefficient, formula_172 is the integer charge of the ion (1 for H+, 2 for Mg2+ etc.), formula_173 is the ionic strength of the aqueous solution, and formula_174 is the size or effective diameter of the ion in angstrom. The effective hydrated radius of the ion, a is the radius of the ion and its closely bound water molecules. Large ions and less highly charged ions bind water less tightly and have smaller hydrated radii than smaller, more highly charged ions. Typical values are 3Å for ions such as H+,Cl−,CN−, and HCOO−. The effective diameter for the hydronium ion is 9Å. formula_175 and formula_176 are constants with values of respectively 0.5085 and 0.3281 at 25 °C in water .