Computer Interfacing

Teacher Information: The Effect of Alcohol on Biological Membranes

Suggestions and Tips

• The light source for the red-pigmented solution is the blue LED (470 nm).
• Isopropyl alcohol may be used in place of 1-propanol, if necessary. If rubbing alcohol is used, remember that it is already diluted to 70% or so. The actual dilution is usually listed on the jar. You will need to recalculate the volumes used for this alcohol.
• Use 95% ethanol and absolute methanol.
• Beets will stain hands and clothing. Lab aprons and gloves should be worn to prevent staining.
• Obtain large, red beets from a local store. Students will cut these into small 1 cm X 1 cm X 0.5 cm pieces. To save class time, you may want to cut a 1 cm X 1 cm rectangle for each group. They can slice 0.5-cm sections from this. The end pieces should be discarded, however, as they will dry out and behave differently than freshly cut surfaces.
• The cuvette must have at least 1.5 mL of solution in order to get a valid absorbance reading. If students fill the cuvette as described in the procedure, they should easily be in this range.
• Since there is some variation in the amount of light absorbed by the cuvette if it is rotated 180°, you should use a water-proof marker to make a reference mark on the top edge of one of the clear sides of all cuvettes. Students are reminded in the procedure to align this mark with the white reference mark to the right of the cuvette slot on the colorimeter.
• The use of a single cuvette in the procedure is to eliminate errors introduced by slight variations in the absorbance of different plastic cuvettes. If one cuvette is used throughout the experiment by a student group, this variable is eliminated.
• The use of cotton swabs is recommend to dry a cuvette after water or a solution has been emptied from it. The cotton swab will remove any remaining droplets in the cuvette. After completely drying the cuvette using both ends of the cotton swab, it is no longer necessary to rinse with the new solution.
• Satisfactory results can be obtained if a different cuvette is used for each solution of the experiment. For best results, the cuvettes for one student lab team should be matched. Each cuvette in a matched set absorbs light (when empty) at approximately the same level. To match a set of cuvettes, first calibrate the colorimeter using the method described in Appendix A. Use a clean, dry cuvette for the 100% calibration instead of a distilled water blank. Put a reference mark on one of the clear sides of the cuvette so it is always oriented the same way in the cuvette slot. Place each cuvette in the batch into the colorimeter and record percentage transmittance values for each. When you are finished, group the cuvettes according to similar %T values. Each of these groups represents a set of matched cuvettes.
• This experiment gives you a good opportunity to discuss the relationship between percentage transmittance and absorbance. At the end of the experiment, students can press the mouse on the absorbance label of the vertical axis to access the pop-up menu showing the columns. If they choose the column "T" (percentage transmittance), they will change the graph to a plot of percent transmittance vs. concentration. You can also discuss the mathematical relationship between absorbance and percent transmittance, as represented by either of these formulas:

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