Description of a van't Hoff apparatus
The Van't Hoff equilibrium box is a chamber containing a mixture of reactants and products at equilibrium.  It is connected to an array of cylinders, each containing one of the pure reactants or products.  The chamber itself is operated at constant total pressure and temperature.  The cylinders are each connected to the equilibrium box through a semipermeable membrane that only passes that species into the chamber.  Pure species are introduced in stoichiometric proportions to the chamber through these membranes at the same partial pressures as in the reaction mixture, and pure products are removed in stoichiometric proportions at the same partial pressures as in the reaction mixture.  So the operation of the equilibrium box is reversible, and the delta G in going from pure reactants in to pure products out is zero.