Kidney Dialysis

Note to Teachers: This lab is an inquiry-based lab. The idea is for students to come up with their own procedure. They must decide what will be held constant, what will be varied, and how to make their measurements. The notion is to have students learn science by doing science the way scientists do it. Students may either write narrative answers to the questions as posed in the objective section (complete with graphs) or write a full lab report. I expect that it will require anywhere from 1 to 3 full hours of classroom time, depending on the level of complexity you require from the students. Students may extend their investigations at home in order to get a better grade. If you try it, please get in touch (my email address is on the home page) and let me know how it worked for you.


Looking at a model of a process can help you to understand the process. Kidneys use the process of diffusion across a membrane to filter out unwanted products of metabolism and excess water. Separation of molecules using membranes is called dialysis and dialysis is something you can do in the lab. The idea in this lab is to explore dialysis to get a basic idea of how it works and then to use that knowledge to do some further research.

You have already become somewhat familiar with dialysis from reading an article in ChemMatters (4/2001). In this lab you will perform a few demonstrations of the key concept important in dialysis: diffusion. To make it real science you will also do some research into the following questions:
Top: a bag containing a solution of two kinds of molecules is placed into water. The bag has pores only big enough to allow the small molecules to pass.
Bottom: the small molecules have equal concentrations on both sides.
  1. Using a plastic bag as a dialysis membrane determine which substances can move through the bag: cornstarch, tincture of iodine (I2 molecules in liquid solution), water.
  2. By reasoning from which molecules can move through the membrane you will determine the relative size of the pores in the membrane as compared to the size of the cornstarch, I2, and H2O molecules.
  3. How does the concentration of cornstarch affect the process of diffusion across the membrane?
  4. How does the concentration of I2 affect the process of diffusion across the membrane?
  5. How does temperature affect the process of diffusion?



Diffusion is a molecular-level process in which molecules pass through a porous membrane. The membrane has holes big enough to let some molecules pass through but small enough that others can’t pass. Molecules that can fit through will travel through the membrane in the direction that takes them from higher concentration to lower concentration. When the concentration of the molecule is the same on both sides of a membrane then diffusion happens at the same rate in both directions. Water molecules generally can pass in both directions through a membrane but this is not always the case.

Diffusion can be used to separate molecules of different sizes. See the illustration at right. An analogy might be a screen door: dust and dirt can pass through but bugs are kept out.

A definition, in case you need it: The concentration of a chemical is just a measure of how much of it there is in a volume of liquid or gas. A practical example: an 8-oz. glass of water with one spoonful of sugar tastes a lot less sweet than the same glass of water with 5 spoonfuls of sugar.

Cornstarch belongs to a group of biological molecules called polysaccharides. Starches in general are large molecules made up of tens to hundreds or even thousands of individual sugar molecules. These large molecules usually have twisted and convoluted structures with a lot of pockets in them. Tincture of iodine contains I2 molecules and is a light brown color. When you add tincture of iodine to a starch solution it forms a complex with the starch by sitting in the many pockets in the molecule. The complex has a characteristic dark blue color. Because this complex forms it is possible to use iodine to determine whether any starch is present.

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The procedure for this lab is mostly up to you. After your teacher gives you an introduction to the lab it is your decision about how to proceed. Some things to keep in mind:

Tips and Pointers

Here are a few things you need to know how to do in order to be successful in answering the objective questions.


Before you start any dialysis it would be wise to take a look at what color the iodine solution is (see below for how to make it. Also, you should take a look at the range of colors produced when you add cornstarch to an iodine solution. Make yourself familiar with what happens when iodine and cornstarch meet.

How to Make the Cornstarch Solution
  1. Make a cornstarch paste by mixing 3 g of cornstarch with 20 mL of water in a 250 mL beaker.
  2. Stir the slurry so that it is mixed completely.
  3. Measure about 180 mL of hot tap water into another beaker.
  4. Pour the hot water into the beaker with the cornstarch paste while mixing.

This is your standard cornstarch solution. When trying to find out how concentration affects diffusion you should use a measured amount more or less than the 3 g of cornstarch used here.

How to Make the Iodine Solution
  1. Measure about 150 mL of water into a 250 mL beaker.
  2. Measure 3 mL of tincture of iodine using the graduated cylinder
  3. Add the tincture of iodine to the water and stir.

This is your standard iondine solution. Place baggies containing cornstarch solution into this bath to start the dialysis process. It can be re-used for more than one trial so do not throw it out!

How to Do Dialysis

To do dialysis for any of your experiments pour some cornstarch solution into a baggie, seal the bag and then let the bag rest in the iodine solution. Be cautious and do not let any cornstarch solution get on the outside of the bag. When taking observations of the diffusion process let the bag stay in the iodine solution but take it out every two or three minutes to look at it. It may take as long as 15 minutes for the process to be complete.


For a 3 write down what you did in the lab describing why you did what you did. Answer the questions in the objective section using your data and graphs. Use complete sentences and your best writing style.
For a 4 do what it take to get a 3 but in a formal lab report.
For a 5 do what it takes to earn a 4 and additionally propose and carry out an Above & Beyond activity.

Possible Above & Beyond Activities:
  1. Research diffusion and osmosis. Define them and explain how they are different in a few well-written paragraphs. Give examples.
  2. What is reverse osmosis? Find out and write about it in a few well-written paragraphs with examples.
  3. Do an experiment at home to find out whether salt or sugar can diffuse through the material of a plastic sandwich/snack bag. It is important not to use freezer or storage bags. Report your results as part of your lab write-up.

This page owes much of its inspiration to an article (unfortunately, it is not available online) from the student-oriented magazine ChemMatters published by the American Chemical Society (ACS). Specifically the April 2001 issue of ChemMatters.
Last updated: May 13, 2007 Home