Aquamolal concentration

In any discussion of solution thermodynamics the choice of the standard is an iiq>ortant one and must be constantly kept in mind. Our standard state, the Conventional one, is the hypothetical solution of unit aquamolal concentration, taken to the Henry s law infinite dilution limit. (A one aquamolal solution is... 

H2O this is 1000 grams. Our choice of the aquamolal concentration scale Insures that all Isotope effects are conqiared at equivalent mole fractions.) The properties of the solution are known in a thermodynamlc sense when the standard and excess molal free energies, Pi and Ua, and and their appropriate tem erature and pressure derivatives, H and hS, H — — ... 

In this equation m refers to the aquamolal concentration, the s to the pure solvent effects, a Is the solvent activity and (j) the osmotic coefficient. Data for a number of different electrolyte solutions at one selected temperature are shown In Figure 6. The solutions all show a smaller VPIE than do the pure solvents. In the experiments AlnR Is measured as a function of temperature and concentration, and we have phenomenologically fit the data (58,59) using the extended Debye-Huckel theory. 

Concentrations of aqueous electrolyte solutions are conventionally expressed using the aquamolality scale (L = moles salt per 55.508 mol solvent (l,000g for H20)). Some typical solubilities (298.15K) are listed in Table 5.13. Almost all salts are less soluble in D20 than in H20. For those salts whose solubility increases with temperature, which is the ordinary behaviour, the isotope effects decrease with temperature. Writing the standard state partial molar free energy of pure solid salt as Pxsalt) and its standard state in solution as p, (HorD) we have on comparing the saturated solutions in H20 and D20,... 

A concentration scale for solutes in aqueous solutions, equal to moles of solute/55.51 mol water. It is frequently used in studies of solvent isotope effects. As pointed out by Schowen and Schowen the choice of standard states can change the sign for the free energy of transfer of a species from one solvent to another, even from HOH and DOD. The commonly used concentration scales are molarity, mole fraction, aquamolality, and molality. Free energies tend to be nearly the same on all but the molality scale, on which they are about 63 cal mol more positive at 298 K than on the first three scales. The interested reader should consult Table I of Schowen and Schowen ... 

It should be noted that there is sometimes an ambiguity in the concentration scales used because of the difference in densities between H2O and D2O the ratio K /K will have different values on the molarity and molality scales. It is probably most satisfactory to express all concentrations as moles per 55.51 moles of solvent this has been termed the aquamolality scale. Since H2O and D2O differ in molar volumes by only 0.36% the use of molarities will give virtually the same value for K /K. ... 

It is important here to make a correct choice of standard states or concentration units, just as in making a comparison of and K. The most logical choice is mole fractions, and for dilute solutions almost equivalent results are obtained by using aquamolalities (moles solute per 55.51 moles of solvent). 

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