Activity coefficient
Activity coefficient of a liquid or solid mixture:
The activity coefficient of a substance A is denoted by the symbol fA.
In a liquid or solid mixture containing the molar fractions c A, c B ..., of substances A and B, fA is a dimensionless quantity defined by:
fA = lA / lA* cA,
wherel A is the absolute activity of A in the mixture and l A* is the absolute activity of pure substance A at the same temperature and the same pressure.
fA can also be obtained by starting from the chemical potential equation:
RT ln(c A fA) = m A - m A*
where m A is the chemical potential of A in the mixture and m A* is the chemical potential of pure substance A at the same temperature and the same pressure. It results from this definition that the activity coefficient fA of a substance A tends towards 1 when its molar fraction c A tends towards 1.
Activity coefficient in solution, e.g. an aqueous solution:
The activity coefficient g A of a substance A in a solution containing the molalities mA, mB ..., of A, B, ... in a solvent S, is a dimensionless quantity defined by:
g A = (lA / mA )/ (lA / mA )µ [we have not defined the multiplying coefficient alpha]
where lA it is the absolute activity of A in the solution, the denominator corresponds to the limiting value for an infinite dilution mA ---> 0, the same temperature and the same pressure.
g A can also be obtained starting from the chemical potential by:
RT ln( mAg A) = mA - (mA - RT ln mA)µ
where mA is the chemical potential of A in the solution. It results from this definition that the activity coefficient g A of a substance A in solution A tends towards 1 when its molality tends towards 0.