In the Classroom edited by
JCE DigiDemos: Tested Demonstrations
Ed Vitz Kutztown University Kutztown, PA 19530
A Demonstration of Sample Segregation submitted by:
Mark D. Fritz Department of Chemical Technology, University of Cincinnati, Cincinnati, OH 45206;
[email protected] checked by:
Stephen B. Brumbach Department of Chemistry, Western Wyoming Community College, Rock Springs, WY 82902-0428 JudithAnn R. Hartman Department of Chemistry, United States Naval Academy, Annapolis, MD 21402-5026
Success in chemical analysis depends on starting with a good sample (1). A good sample accurately represents the bulk material from which it is taken. The approach of teaching sampling through statistics is well documented in this Journal (2–9). The demonstration described here illustrates an avenue that is less frequently considered: that of sample segregation. For mixtures of solid particles, it is important for students to recognize that a good sample can be turned into a bad sample by simply handling it. The result can be a nonrepresentative, segregated sample. Sample segregation is easy to demonstrate using colored beads, dry beans and rice, or other inexpensive materials. The result is a simple, visuallycompelling demonstration. Handling a mixture of solid particles can lead to segregation if the particles are free flowing and differ in size and shape (10, 11). Pouring or stirring such a mixture creates a velocity gradient in the mixture, which will separate particles by size. The mechanism typically involves the movement of the smaller particles through the matrix of larger ones. The result is a nonuniform mixture. Sample segregation can occur when handling a variety of materials. Pharmaceuticals, for example, are often mixed as powders before being pressed into tablets. A drug’s active ingredient may be a fine powder mixed with a filler of larger particles. Stirring such a formulation can cause the small particles to sift through the large particles, so that the active drug ends up at the bottom of the batch. Chemists who sample and analyze this batch must be wary of sample segregation. They employ a variety of handling and sampling techniques (12–14), such as coning and quartering, and riffling, to prevent or correct for sample segregation.1 Other materials where sample segregation can influence sampling and analysis include soils in environmental analysis, ores in mining and steel making, coal in power generation, powdered detergents in consumer products manufacturing, and grains in agriculture. Materials • Mixture of plastic beads, 4-mm diameter red beads and 2.5-mm diameter white beads, enough to fill a 300mL beaker. The beads are available from craft stores. An alternative to beads is a mixture of dry red kidney beans and dry white rice. Because of the larger particle size, at least 450 g (16 ounces) of the bean and
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rice mixture is needed for an effective demonstration. These are available at grocery stores. •
Rod, stick, or pen for stirring the mixture.
• Two beakers for holding, pouring, and viewing the mixture. For the bead mixture, tall 300-mL beakers work well. For the beans and rice, 800-mL or larger beakers are needed.
The mixture used in this demonstration has two components with particles that are big enough to see easily and have different sizes. The two components of the mixture must have different colors to make the segregation graphic. Also, the two types of particles must differ in diameter by a ratio of at least 1.3:1 because the larger the ratio of particle sizes, the greater the tendency for particles to segregate (10). You may be able to think of other readily-available materials that meet these criteria and would also provide an effective demonstration. Procedure This procedure demonstrates how the simple actions of stirring or pouring can segregate a mixture and create a nonuniform sample. The bead mixture is described, but the procedure for the beans and rice is the same.
Stirring Show students a uniform mixture of beads in a glass beaker.2 Ask the students to consider stirring the mixture as a way to help in collecting a representative sample. Let a student stir the beaker for a minute with a stirring rod or a pen. Ask the student to stir in a circular motion and to reach well into the beaker. The beads will separate, with large red beads accumulating in a layer at the top of the mixture. The smaller white beads will accumulate and be visible at the bottom (Figure 1). Show the students the surprising result: stirring a mixture can “unmix” it! Pouring Show students a uniform mixture of beads in a glass beaker.2 Pour the mixture into a second beaker. You can also ask a student to do the pouring. The beads will separate, especially if you pour the mixture down one side of the receiving beaker. It helps to hold the receiving beaker at a slight angle.
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In the Classroom
Figure 1. Sample segregation caused by stirring. Large red beads rise to the top of the mixture.
Figure 2. Sample segregation caused by pouring. Large red beads accumulate on one side of the mixture.
Typically, one side of the beaker will be enriched in large red beads, and the other side will be enriched in small white beads (Figure 2). Show students the beads in the second beaker, turning the beaker or passing it around so that students can see the separated pools of red and white beads.
side and odd-numbered ones to the other side. Each coning and quartering or riffling operation cuts the mixture in half without segregation and may be repeated to collect a sample of any desired size (see refs 12–14 ). 2. Preparing the uniform bead mixture to start the demonstration takes practice. Some mixing techniques will actually separate the mixture! One approach that works well is to cover the beaker, hold it on its side, and alternately shake and swirl it.
Discussion After doing these demonstrations I ask students what would happen if a sub-sample was scooped for analysis from the top of the segregated mixture. Then I extend the discussion to sampling and analyzing mixtures such as pharmaceutical powders, soils, ores, coal, detergents, and grains. These materials are frequently stirred, poured, or otherwise handled in ways that can segregate them before sampling. Many students assume that stirring a mixture before sampling it will mix it and help produce a good sample. They are surprised to learn that stirring can segregate it! They are also surprised to see that pouring, a common and seemingly benign action, can produce a segregated sample. This demonstration has proven useful for illustrating the importance of sample handling for students studying analytical chemistry and environmental chemistry. It can be followed by a discussion of coning and quartering, riffling, and other sample handling techniques, as well as statistical treatments of sampling. Notes 1. Coning and quartering involves pouring a mixture into a cone-shaped pile and then cutting the flattened pile into quarters. Two opposite quarters are rejected, and the remaining two quarters are retained. Riffling involves feeding a mixture into a row of small chutes arranged so that even-numbered chutes deliver to one
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Literature Cited 1. Herrington, B. L. J. Chem. Educ. 1937, 14, 544. 2. Kratochvil, B.; Reid, R. S.; Harris, W. E. J. Chem. Educ. 1980, 57, 518–520. 3. Kratochvil, B.; Reid, R. S. J. Chem. Educ. 1985, 62, 252. 4. Bauer, C. F. J. Chem. Educ. 1985, 62, 253. 5. Cohen, R. D. J. Chem. Educ. 1991, 68, 902–903. 6. Cohen, R. D. J. Chem. Educ. 1992, 69, 200–203. 7. Guy, R. D.; Ramaley, L.; Wentzell, P. D. J. Chem. Educ. 1998, 75, 1028–1033. 8. Vitt, J. E.; Engstrom, R. C. J. Chem. Educ. 1999, 76, 99– 100. 9. Ross, M. R.; Bacon, D. W.; Wolsey, W. C. J. Chem. Educ. 2000, 77, 1015–1016. 10. Carson, J. W.; Royal, T. A.; Goodwill, D. J. Bulk Solids Handling 1986, 6, 139–144. 11. Principles of Powder Technology; Rhodes, M., Ed.; Wiley: Chichester, England, 1990; pp 76–79. 12. Dunn, J. G.; Phillips, D. N.; van Bronswijk, W. J. Chem. Educ. 1997, 74, 1188–1190. 13. Willey, J. D.; Avery, G. B., Jr.; Manock, J. J.; Skrabal, S. A.; Stehman, C. F. J. Chem. Educ. 1999, 76, 1693–1694. 14. Pierce, W. C.; Haenisch, E. L. Quantitative Analysis, 3rd ed.; Wiley: New York, 1948; pp 60–62.
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