PHYSICAL CHEMISTRY IN BRIEF

CHAP. 6: THERMODYNAMICS OF HOMOGENEOUS MIXTURES [CONTENTS] 161

6.5.3 Standard states

To determineμsti in relation (6.71), we need to choose thestandardorreference stateof
a given component of a given mixture. This choice is in principle arbitrary, but the following
standard states are usually chosen for practical reasons:

• Mixtures of gases
  For components in the gaseous phase, thestandard state of an ideal gas at standard
  pressure is used. In the case of this choice, μsti is the chemical potential of a pure
  substanceiin the state of an ideal gas at the temperature of the mixture and standard
  pressure, usuallypst= 101 325 Pa.


• Mixtures of liquids and solid substances
  For components in a liquid or solid phase, thestandard state of a pure substance at the
  temperature and pressure of the systemis used. μsti is the chemical potential of a pure
  substancei, which at the temperature and pressure of the mixture is in the same phase
  as the mixture.


• Diluted solutions
  For components present in mixtures in only a small amount (e.g. ions of salts in aqueous
  solutions or gases dissolved in liquids), one of the three following standard states may
  be chosen:
  
  
  • Standard state of a hypothetical pure substance at infinite dilution.
    μsti is the chemical potential of componentiobtained by linear extrapolation of the
    dependenceμi=f(lnxi) from the region of very low concentrations to the value at
    xi= 1 (i.e. hypothetically pure substance)
  
  
  • Standard state of a unit molar concentration.
    μsti is the chemical potential of componentiobtained by linear extrapolation of the
    dependenceμi=f(lnci) from the region of very low concentrations to the value at
    ci=cst= 1 mol dm−^3.
  
  
  • Standard state of unit molality.μsti is the chemical potential of componentiobtained
    by linear extrapolation of the dependenceμi=f(lnmi) from the region of very low
    concentrations to the value atmi=mst= 1 mol kg−^1.
    The same standard states as those used for the chemical potential are used for its derivatives
    with respect to temperature and pressure, i.e. entropy and volume [see (6.68) and (6.70)], and
    for other thermodynamic quantities, e.g. enthalpy or fugacity.