III. OSMOTIC POTENTIAL DETERMINATION The Osmette™ osmometer measures the osmolarity of a solution by determining the freezing point depression of a sample. A small amount of liquid in an Eppendorff centrifuge tube is cooled rapidly in the instrument while its temperature is recorded. When the temperature drops well below freezing (supercooling) a small rod in the sample is vibrated to allow ice crystals to form. The temperature of the liquid stabilizes at the freezing point. The freezing point measured is electronically converted to osmolarity which appears as a series of digits on the instrument’s readout. The conversion factor used by the instrument is 0.54 mol °C-1. We could have determined this manually by similar manipulation in an ice bath and observation of an ASTM 52C BUTA BPR thermometer (range: -10 to +5 C) The Wescor™ vapor pressure osmometer measures the osmolarity of a solution by determining the vapor pressure depression by thermocouple hygrometry. The sample chamber is cooled rapidly to a temperature below the dew point of the sample. As water condenses on the thermocouple, the temperature of the thermocouple stabilizes at the dew point. The depression of this dew point is used to determine the osmolality of the sample. A sample of 1 millimole kg-1 would have a depression of 0.00031 °C. The instrument must be very accurate in measuring temperature! 1. Chop an undamaged part of the remainder of the potato, grind it to a pulp in a mortar, and squeeze out a small volume of sap into an Eppendorff centrifuge tube. If necessary this can be capped and frozen for thawing and later use. Do not add any water to this liquid at any time! 2. The liquid should be centrifuged for 2 minutes in the microcentrifuge. 3. As directed by Dr. Koning, use the micropipette to transfer the required volume into the osmometer. Record the resulting osmolarity in the laboratory record. 4. Use this information to perform the calculations requested in the laboratory record.