The concentration osmometer

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Abstract
A “concentration osmometer” which is easy to operate and very simply constructed is described. The principle of the measurement is based on the comparison of the unknown solution to a standard (in this case, a solution of polyethylene glycol Carbowax 6000 in water) which has a known calibrated value of the activity of water. During the measurement, the concentration of the two solutions changes as a result of free flow of water across the dividing semipermeable membrane, while the pressure in both solutions remains equal. By measurement of the amount and the direction of flow, the concentration of the standard which would be in equilibrium with the unknown solution can be interpolated. Examples are given of experiments performed with solutions of ionized PMA. The apparatus described has been employed in a concentration range of activities 10−1 to 10−4 M.