Expression of solution composition

The composition of a solution, or melt, may be expressed in many different ways, e.g. mass per unit mass of solvent, mass per unit mass of solution, mass per unit volume of solvent, and so on. The mass unit may refer to the dissolved species itself or to a solvated form, e.g. a hydrate. 

For the expression of crystallization kinetics there is some theoretical justification for recording compositions on a molar basis, e.g. as kmolm (i.e. molL ), while mole fractions are most frequently used for thermodynamic calculations. Mass fractions are commonly used in the eonstruction of phase diagrams, although the use of mole fractions is recommended for the representation of reciprocal salt pair systems (section 4.7.2). 

For the purpose of expressing a mass balance on an item of process plant, there is considerable merit in expressing solution composition as mass of unsolvated solute per unit mass of solvent, particularly when temperature changes are expected. This avoids the need for further calculation to account for density changes. 

In view of the frequent need to make interconversions of composition units it is recommended that, whenever solution concentration measurements are made, the density of the solution at the relevant temperature is also measured and recorded (see section 3.9). 

Many of the above methods of solubility expression can lead to the use of the potentially misleading term percentage concentration . For instance, an expression such as a 10 per cent aqueous solution of sodium sulphate could be taken to mean, without further definition, any one of the following  

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Table 3.1 lists the interconversions between a number of the common expressions of solution composition. For convenience, the expressions are drafted in terms of aqueous systems, but the relationships are completely general if the terms unsolvated substance , solvate and solvent are substituted for anhydrous substance , hydrate , and water , respectively. 

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