8.1: Osmosis and Dialysis Lab Procedure

Learning Objectives

• To introduce the concept of solute concentration and osmosis and dialysis.
• Using different solute concentrations observe the process of osmosis.
• Use dialysis to separate out glucose and chloride ions from a mixture of chloride ions, glucose, and starch.

 

Background

 Health care professionals work with a variety of solutions, the most common type probably being intravenous (IV) solutions. A solution is a homogeneous mixture of one or more solutes dissolved in a solvent. In a solution, the substance that is present in the greater amount is called the solvent, while the substance (or substances) that is present in the smaller amount is called the solute. Although most common solutions are liquids, and the most common solvent is water, solutions can be made from solvents and solutes that are liquids, solids or gases. Body fluids are examples of liquid solutions while the air we breathe is an example of a solution containing gaseous components.

 

Diffusion

Diffusion is the process in which a substance moves from an area of high concentration to an area of lower concentration. It is important for membranes to be semi-permeable. This difference in concentration is referred to as a concentration gradient. This movement does not require any external energy, but uses the free energy intrinsic to the system.

Figure \(\PageIndex{1}\): Concentrated dye diffuses along the concentration gradient until reaching equilibrium (no net movement).

If membranes were universally permeable, they would not legitimately serve their purpose as membranes; certain substances need to be kept out of a cell, and others kept in.

If membranes were not at all permeable, there would be no interface between the cell and its environment–effectively starving the cell. Membranes, being selectively permeable, allow in nutrients and other necessary substances, and also provide for the purging of cell waste.

 

Osmosis

Osmosis is a special case of diffusion. Instead of observing the net change in solute, osmosis follows the net movement of solvent across a semipermeable membrane. Since a semi-permeable membrane permits specific things to pass through, some solutes are partitioned.

Figure \(\PageIndex{2}\): A semi-permeable membrane allows the solvent to pass but not this red salt molecule. The water moves along the concentration gradient (of water). This movement of water causes an osmotic pressure.

A cell lacking a cell wall is affected greatly by the tonicity of the environment. In a hypertonic solution where the concentration of dissolved solute is high, water will be drawn out of the cell. In a hypotonic solution where the concentration of dissolved solute is lower than the interior of the cell, the cell will be under great osmotic pressure from the environmental water moving in and can rupture.

Figure \(\PageIndex{3}\)

Plants have rigid cell walls composed of cellulose. These cell walls permit for maintenance of cellular integrity when the external environment is hypotonic (less dissolved substances). In this situation, the water moves into the cell. Without the cell wall, the cell would burst open from the excessive water pressure entering the cell. This state of swelling is referred to as turgid, resulting from turgor pressure.

Figure \(\PageIndex{4}\): Cell walls of a plant retain the shape of the cell despite the state of external tonicity.

When the exterior environment is hypertonic, (greater amount of dissolved substances), the reverse condition occurs whereby the cellular fluid exiting the cell reduces the size of the cytoplasm. This condition is referred to as plasmolysis.

 

Dialysis

Dialysis is the separation of colloids from dissolved ions or molecules of small dimensions, or crystalloid, in a solution. A colloid is any substance that is made of particles that are of an extremely small size: larger than atoms but generally have the size of 10-7 cm ranging to 10-3 cm. A crystalloid is a substance that has some or all of the properties of a crystal or a substance that forms a true solution and diffuses through a membrane by dialysis. Dialysis is a process that is like osmosis. Dialysis is possible because of the unequal rates of diffusion through a semipermeable membrane.

Figure \(\PageIndex{5}\): Graphic showing the diffusion of solutes across a membrane during dialysis. Image used with permission from wikipedia (Potcherboy)

This experiment will introduce you to the concept of solute concentration, osmosis and dialysis. You will be given solutions with different solute concentrations which you will use to observe the process of osmosis. Finally, dialysis is used to separate out glucose and chloride ions from a mixture of chloride ions, glucose, and starch.

 

Experimental Procedure

Safety

WEAR YOUR SAFETY GOGGLES

Materials: 3 dialysis tubing, three 250 mL beakers, pipette, distilled water, thread, 10% (m/v) sodium chloride solution, and 30% sodium chloride solution.

 

Part A. Osmosis

1. You will be given three pieces of dialysis tubing of equal length (8-10 cm). Soak them in a beaker with water for about 10 min to hydrate them. Tie a knot at the end of each bag and fill half with 10% (m/v) sodium chloride solution using a pipette. Gently squeeze out any air above the liquid and close bag with another knot. Rinse bags with tap water, pat dry using a paper towel to remove any liquid from the outside of the bag and weigh the bag. Record the weight of each bag. DO NOT MIX UP BAGS!
2. Label three labeled beakers and fill with (a) 50 mL of distilled/deionized water, (b) 50 mL of 10 % (m/v) sodium chloride, and (c) 50 mL of 30% sodium chloride. Be sure to keep track of the bags placed in each solution by labeling the beakers. Allow the bags to remain undisturbed in each solution for 30 min. Gently swirl the beaker every 5 minutes. At the end of the 30 minutes quickly remove each bag, rinse the bag well with distilled water, gently pat the bag completely dry to remove any liquid from the outside the bag and reweigh the bag. Record your results for each bag.

 

Part B. Identification Tests for Chloride, Glucose and Starch

Materials: 10% NaCl solution, AgNO3 solution, 10% glucose solution, 1% starch solution, Benedict’s reagent, DI water, test tubes.

1. Chloride test: (wear gloves for this part of the experiment!) Place 5-6 drops of 10% NaCl in a test tube and 5-6 drops of deionized (DI) water in another test tube. The test tube with water will be the control or comparison sample. Test for Cl - by adding 1 drop of AgNO3 to each. Record your observations. Compare the results for the NaCl solution with the results of the water sample.
2. Glucose test: Place 1.0 mL of 10% glucose solution in a test tube. Place 1.0 mL of DI water into another test tube. Add 1.0 mL of Benedict’s reagent to the glucose and water. Heat both tubes in a warm water bath for 5 minutes. Record your observations. Remove tube from the water, label them and keep on table for comparison reasons. Note: Each time you carry out the glucose test, the tube containing Benedict’s reagent must be heated in a boiling water bath.
3. Starch test: Place 1.0 mL of 1% starch solution in one test tube and 1.0 mL of DI water in another test tube. Add 1 drop of iodine reagent to each. Record your observations. Compare the results for the starch sample with the results for the water sample.

 

Part C. Dialysis

Materials: 10% NaCl solution, AgNO3 solution, 10% glucose solution, Benedict’s reagent, dialysis tubing, thread, 100 mL beakers, distilled water.

In a test tube combine approx. 1 mL of 10% NaCl, 1 mL of 10% glucose, and 1 mL of 1% starch solution. Soak a dialysis tube in a 150 mL beaker with distilled water for about 10 min to hydrate it. Tie a knot at one end of a 4-5 cm long piece of dialysis tubing. Fill the dialysis bag with the mixture, and tie a knot at the other end, rinse bag with water to remove any of the mixture that may be on the outside of the bag, pat dry and place in a beaker with about 30 mL of distilled water, making sure bag is submerged.

1. Immediately remove 3 mL of the water in the beaker (surrounding the dialysis bag). Divide the sample into 3 test tubes (about 1.0 mL each). Repeat each of the three identification tests that you did in part B on these three samples (one test per sample). A substance is present in the sample if a test for that substance gives the same results as in the identification tests in part B. Record the result as positive (+). If the substance is absent (there is no chemical reaction), record the test as negative (-).
2. Repeat the procedures in part B after 15 minutes of placing the bag in the beaker.
3. Repeat the procedures in part B after 30 minutes of placing the bag in the beaker.
4. After the 30 minutes test, carefully open the bag and remove the contents and perform the three identification tests for chloride, glucose and starch.