Modeling of Salt Mixtures

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Author: Michael Steiger, Andreas Nicolai
English version by Christa Gerdwilker

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Salt mixtures – terminology

Salt mixtures can be either a mixture of different salts or solutions of them. Of particular interest are the properties of dissolved mixtures, i.e., salt solutions. Therefore, for this article Salt mixture is understood to be a solution of one or more salts.

Different types of salt mixtures can be defined, e.g., a solution of one cation and one anion is a binary solution, a solution of two cations and one anion or one cation and two anions forms a ternary solution, etc.

Definition of a salt(s) solution

A salt(s) solution is determined by the type and concentration of the ions it contains. A common concentration measurement is its molality (m), the ratio of the number of moles of salt to the mass of solute (m/kg), usually water. This is not to be confused with molarity (M) which is the ratio of the number of salt moles to the volume of water (m/liter). For very dilute solutions, both concentration values would be similar, but for concentrated solutions, the density of the salt solution is different to that of water.

For example, a binary 1 molal Na2SO4 solution consists of a mixture of Na = 2 m (mol/kg), SO4 = 1 m (mol/kg). In the case of a ternary solution, produced from 1 mol NaCl and 3 mol NaNO3 would result in the mixture Na = 4 m, Cl = 1 m, NO3 = 1 m.

Properties of salt mixtures

The physical, chemical and thermodynamic properties of salt mixtures can, in part, be measured directly, in part be identified indirectly and otherwise only be estimated or predicted through the use of modeling.

List of properties

The following listed properties are functions of temperature and composition (type of ions and their concentration).

  • Solution density
  • Water activity
  • Activity of ions
  • Osmotic potential
  • Thermal conductivity
  • Specific thermal capacity
  • Viscosity