Hydration number, Stokes-Robinson

The different hydration numbers can have important effects on the solution behaviour of ions. For example, the sodium ion in ionic crystals has a mean radius of 0 095 nm, whereas the potassium ion has a mean radius of 0133 nm. In aqueous solution, these relative sizes are reversed, since the three water molecules clustered around the Na ion give it a radius of 0-24 nm, while the two water molecules around give it a radius of only 017 nm (Moore, 1972). The presence of ions dissolved in water alters the translational freedom of certain molecules and has the effect of considerably modifying both the properties and structure of water in these solutions (Robinson Stokes, 1955). 

The method of calculating these values was proposed by R. A. Robinson and R. H. Stokes. The Gibbs energy G of a system containing 1 mole of uni-univalent electrolyte ( t = 1, total number of moles of ions being 2) and n0 moles of solvent is expressed for solvated ions (dashed quantities) on one hand and for non-solvated ions (without dash) on the other it is assumed that jU0 = jUo and that the hydration number h (number of moles of the solvent bound to 1 mole of the electrolyte) is independent of concentration ... 

An extended form of the Debye-Huckel equation is the hydration one of Robinson and Stokes (11). It contains two adjustable parameters, ap and h, where h is rilated to the hydration number. It can be fitted to y for several electrolytes for concentrations in excess of 1 m. Their equation has the valuable feature of describing not only the salting-in but also the salting-out part of the y+ versus m curve. It should be noted, however, that the... 

Stokes and Robinson achieved remarkable success with a one-parameter equation, the single parameter being a hydration number. For a salt of the type AB or AB2, they wrote the equation... 

The Stokes-Robinson model was later modified by its creators for use in more concentrated solutions by accounting for dehydration equilibria (12). The original model was also modified by Nesbitt (12), who made the hydration number a decreasing function of the ionic strength. Other workers too numerous to mention here have also attempted to do something with hydration theory. 

The added empirical quantity bl was replaced by Stokes and Robinson (1948) by an expression in the hydration number of the electrolyte hE = v+ h+ +v-h-, with the stoichiometric coefficients v+ + = ve, according to ... 

Nesbitt (1982) pointed out that using hydration numbers which are independent of solution concentration is unreasonable. For example, at about 4 molal MgCl2, and using the Stokes and Robinson hydration number (A) of 13.7, virtually all the water in solution would reside in hydration shells, leaving none as solvent. Nesbitt (1982) suggested a minor variation in the hydration scheme, in which water is partitioned between solvent and hydration shells, such that... 

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