Determination of the Osmotic Pressure of Biological Fluids

Abstract

IN 1938 Blegen and Rehberg¹ described a method for determining the osmotic pressure of biological fluids based on the measurement of the initial rate of outflow of water from one solution through a semipermeable membrane into another solution of somewhat higher osmotic concentration. As semipermeable membranes they used small collodion bags impregnated with copper-ferrocyanide². The collodion bag is alternately filled with the unknown fluid and with a solution of sodium chloride of known strength, and is connected to a piece of capillary tubing (0·2–0·5 mm. internal bore) which is bent at right angles so as to form a longer, horizontal arm of about 15 cm. length carrying a millimetre scale. When the bag containing the unknown fluid or the sodium chloride solution is immersed in a sodium chloride solution of higher concentration, the meniscus begins to move, and the rate of movement, which is taken to be proportional to the concentration difference, is determined at intervals.