A New Micro-osmometer

Abstract

This apparatus measures the rate of distillation at room temperature between the unknown solution and a known standard solution under carefully defined conditions. It is calibrated using solutions of known osmotic pressure. It is there fore an empirical vapour-pressure osmometer. The sample (o-o6 ml.) is placed in the capsule C (Fig. i) and the standard solution contained in the pipette P, where it is held at a fixed position due to surface tension at the jet. The upper meniscus is located on the eye-piece scale of the microscope M. The capillary tube above the meniscus and the space inside the screw plunger S are air filled. By turning the screw S the standard solution can be extruded to hang as a drop from the tip of the jet. After a measured time (20 min.) the drop is again drawn into the pipette to the fixed position at which it comes to rest automatically, and the change in volume, due to condensation or evaporation, measured on the microscope scale. If the sample has a surface tension not much lower than that of water, the osmometer can be used in the reverse way, the sample being drawn into the pipette and the standard in the capsule. Used in this way only 0 001 ml. of the sample is needed. A separate calibration curve is required for each standard (Fig. 2) and each standard has a range of about 9 atm. ( A = 0 75o C.). The standard deviation of single determinations is ±0 06 atm. ( A = 0 005° C.). This is uniform over the range investigated (0-28 atm. or i = 2-3° C.). Satisfactory comparisons between the osmometer and the cryoscopic method have been obtained with samples of plant sap (Table I). The osmometer has the advantage of needing only a small volume of solution which can be viscous, opaque, and need not be free from cell debris, &c. It operates at physiological temperatures and needs no very stringent temperature control. It is simple to construct and manipulate and occupies less than 1 hour per deter mination of which only 15 minutes is taken up in operation.