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Total vapor pressure of solutions can be determined by a method using a Baratron gauge with the 0.001 Torr accuracy. The concentration of sample solution is controllable by adding the amounts of solvents from the gas-handling manifold. We can determine xA-dependence of the total pressure in binary A-B system. The partial pressure of A, pA, can be obtained by Boissonnas equation. Then the excess chemical potential of A and concentration fluctuation of the solution is estimated by pA.

Figure. Schematic diagram of apparatus for vapor pressure measurement. 1: Box which keeps the temperature constant at 40 ^{o}C; 2: temporary gas tank (500 mL); 3: temporary gas tank (5 L); 4: Baratron® differential pressure transducer (connected to digital multi-meter); 5 and 6: solvent cell; 7: water bath at 25 ^{o}C; 8: sample cell; 9: vacuum system (a rotary pump and a diffusion pump).