“…The liquid model considers two cations, Na + and K + , and two anions, SO 2À 4 and S 2 O 2À 7 . In reality, the liquid might also contain the anions S 3 O 2À 10 [6,18], O 2À and O 2À 2 [31][32][33], or dissolved molecular SO 3 [7], but it is expected that these species occur in very low concentrations in the (Na,K) 2 (SO 4 ,S 2 O 7 ) system, and therefore do not affect the chemical equilibrium to any great extent. The Gibbs free energy of the quadruplets is defined as follows: For the unary quadruplet containing two cations A and two anions X, the Gibbs free energy is defined as [30]:…”