Some opinions of an innocent bystander regarding the Hofmeister series

• Hofmeister's “water affinity” of ions is quantified from colligative properties.
• Hydration (Hydrodesmic) numbers are the number of waters strongly bound to ions.
• Mole fractions of water must pertain only to the amount not strongly bound to ions.
• Extents of complete dissociation and of ion pair formation are quantified.
• Logic vs. misconceptions: properly treated, water solutions behave “ideally”.

Hofmeister's “water absorbing effect” of ions is quantified. A clear definition is provided for the term “hydration number” as the average number of water molecules bound to the solute more strongly (by at least 13.3 kcal mol− 1) than are bound to other waters. Such hydration numbers are reported for various ions of electrolytes along with the extent of ion pair formation of each salt. Experimental measurements of various colligative properties demonstrate that the hydration numbers and extents of ion pair formation remain constant over large ranges of concentration and that solutions behave “ideally” up to the point where about 50% of the total water is strongly bound to the solute and is not available to act as solvent for additional solute.

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