Chemical Education Today
Letters Beer’s Law Revoked? In the recent article “Using Calculator-Based Laboratory Technology to Conduct Undergraduate Chemical Research” (J. Chem. Educ. 2001, 78, 694–696) I was quite surprised to see the percent transmission versus concentration plot yield a straight line (Fig. 1). I thought absorbance versus concentration gave the straight lines. I shall immediately pour all my Lambert beer down the drain and go on the wagon. Roy W. Clark Middle Tennessee State University Murfreesboro, TN 371323
[email protected] The author replies: No need to pour your Lambert beer down the drain! The original buckminsterfullerene data collected for Figure 1 are as follows: 0.05 mg/mL, 79% T 0.10 mg/mL, 70% T 0.20 mg/mL, 60% T 0.36 mg/mL, 39% T 0.42 mg/mL, 31% T Indeed, a plot of %T versus concentration gives the expected curve when plotted carefully. The curve fit for Figure 1 was drawn by a program that assumed a linear function. Conversion of the above %T values to absorbance values using the equation: A = 2 – log %T gives the following data for the above five samples: 0.05 mg/mL, 0.102A 0.10 mg/mL, 0.155A 0.20 mg/mL, 0.222A 0.36 mg/mL, 0.409A 0.42 mg/mL, 0.509A A plot of absorbance versus concentration gives a linear curve as expected (slope = 1.070 ± 0.0758, R 2 = .9926). Since the CBL data were obtained with a colorimeter probe that measures percent T directly, we wanted to keep the data in the original form. Thank you for pointing out the graphing error. Yet another reason why (in the days when such instruments only measured percent T ) we taught students to convert %T values to A values before plotting in Beer’s law experiments. Not even the mighty buckminsterfullerene molecule is able to exhibit chemistry that revokes Beer’s law. Maureen Kendrick Murphy Department of Chemistry & Physics Huntingdon College Montgomery, AL 36106
[email protected] JChemEd.chem.wisc.edu • Vol. 78 No. 12 December 2001 • Journal of Chemical Education
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