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Observe osmosis with a thistle tube osmometer. This device consists of a hollow bell attached to a long tube. The tube is filled with molasses or sugar solution, and the large opening of the tube is covered with dialysis tubing and secured with a rubber band. The thistle tube osmometer is placed in a beaker of water and clamped to a ring stand (see picture below). The level of the molasses or sugar solution is indicated with a wax mark from a marker.
Examine the setup throughout the lab period. You may find that the liquid in the thistle tube eventually stops rising. This occurs when the gravitational pressure equals the force exerted by the process of osmosis. This amount of force required to balance or equilibrate osmosis is the osmotic pressure.
In class you will see 4 osmometer setups (similar but slightly different than the figures above). The solutions in the dialysis bags (the 'bell' of the osmometer) that are attached to the graduated pipettes contain a Dextrose solution mixed with a red indicator dye so as to identify the level of the dextrose in the pipette more readily. The first osmometer labeled "100%" contains a saturated solution of Dextrose and is set as the standard. The second osmometer contains a solution that is diluted to 50% of the concentration of the standard and labeled "50%". The third osmometer is labeled "5%" and contains a solution that has a concentration of Dextrose that is only 5% as concentrated as the standard. The fourth osmometer contains distilled water and is labeled "0%".
Diffusion and osmosis can be demonstrated simultaneously in one setup. Obtain a 15 cm section of dialysis tubing that has been soaked in distilled water. Tie or fold and clip one end of the tubing to form a leakproof bag. Half fill the bag with 15ml of a 1% protein (albumin) solution and 15ml of a 10% NaCl solution (The resulting percent solutions change when they are combined... see diagram below). Also add a 3 ml sample of the same solution into each of four test tubes labeled "Inside Start."
Now tie the bag closed with a leakproof seal. Wash the bag with distilled water.
Place the bag in a 250 ml beaker containing 40ml of a 10% starch solution and 160ml of a 40% glucose solution (again the resulting percentages change because of the combination of the solutions). Place 3 ml samples of the fluid from the beaker in each of four test tubes labeled "Outside Start." The starting conditions are summariezed in the figure below. This experiment will run for approximately 1 hours. Go on to the other experiments while this experiment runs in the background.
3378, Guru Nanak Marg, Near Pul Chameli, Ambala Cantt,
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