Oxoanions defined

The enthalpies of formation of aqueous ions may be estimated in the manner described, but they are all dependent on the assumption of the reference zero that the enthalpy of formation of the hydrated proton is zero. In order to study the effects of the interactions between water and ions, it is helpful to estimate values for the enthalpies of hydration of individual ions, and to compare the results with ionic radii and ionic charges. The standard molar enthalpy of hydration of an ion is defined as the enthalpy change occurring when one mole of the gaseous ion at 100 kPa (1 bar) pressure is hydrated and forms a standard 1 mol dm-3 aqueous solution, i.e. the enthalpy changes for the reactions Mr + (g) — M + (aq) for cations, X (g) — Xr-(aq) for monatomic anions, and XOj (g) —< XO (aq) for oxoanions. M represents an atom of an electropositive element, e.g. Cs or Ca, and X represents an atom of an electronegative element, e.g. Cl or S. 

Various generic names have been used for oxoacids and oxoanions. Because there are many of them, it is difficult to define the terms unambiguously and consistently. But the following statements may be helpful. 

Define acids, bases, oxoacids, oxoanions, and hydrates. 

To model the sorption of oxoanions onto the solid surface with PHREEQC2, it is necessary to define the reactive surface by a set of surface complexation constants and parameters describing the availability of surface sites. 

Many salts crystallized from aqueous solutions are not water-free but take the form of well-defined hydrates. Other solid phases contain water associated in changing amounts. A classic case is water coordinated onto oxoanions, for example,... 

---