Chemical Education Today
The author replies:
rather than the given + → HCl Note also that this discussion leaves out the complication of the oxygen molecule, which is clearly present in the reaction mixture. H•
Cl•
Literature Cited 1. Egolf, L. M.; Keiser, J. T. J. Chem. Educ. 1993, 70, A208. 2. Laidler, K. J. Chemical Kinetics, 3rd ed.; Harper and Row: New York, 1987; pp 295–300. 3. Miller, J. C.; Gordon, R. J. J. Chem. Phys. 1981, 75, 5305–5310. 4. National Institute of Standards and Technology at http:// fluid.nist.gov/cgi- bin/CKMechReact (accessed May 1998). Richard Schwenz and Lynn Geiger Department of Chemistry and Biochemistry University of Northern Colorado Greeley, CO 80639
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Precision and Accuracy in Measurements I enjoyed the article by Richard Treptow entitled, “Precision and Accuracy in Measurements: A Tale of Four Graduated Cylinders” (J. Chem. Educ. 1998, 75, 992–995). I found the “concept charts” (Figures 4–5) helpful and intend to use them in the spectrochemistry training seminars I teach for users of our analytical instrumentation. However, there is a problem with the graduated cylinder experiment as it is really about the resolution of a measuring instrument and how this can affect the precision. The poor precision of the cylinders graduated in 1 mL divisions is clearly worse than those with 0.1 mL graduations, as illustrated by the measurements given in Table 1. But this is a matter of the resolution of the respective measuring instruments. It is possible that this distinction was omitted for pedagogical purposes. Although the two are related, there is a very real difference between precision and resolution and this should be noted. I must thank the author for pointing out that poor resolution can also result in poor precision, a relation that escaped me in a recent note on the terminology of measurement (Phys. Teach. 1997, 35, 15–17). Perhaps due to a concentration on instrumental analysis, I noted only the opposite effect: Consider the five measurements, 15.1, 15.2, 15.1, 15.0, and 15.2. If the instrument could not resolve the decimal, then the measured values would all be 15, with zero standard deviation and perfect precision. There are many aspects of the measurement process beyond just accuracy and precision (both short and long term). These include resolution, sensitivity, response time, range, and input impedance. There are interesting and often curious inter-relationships between all of them. Volker Thomsen Spectro Analytical Instruments, Inc. 160 Authority Dr. Fitchburg, MA 01420
I thank Volker Thomsen for his thoughtful response to my article. Had I known of his publication in The Physics Teacher I would surely have cited it. Here are a few comments concerning his letter. My article acknowledges that the two cylinders graduated in 1-mL divisions suffer from poor resolution when it states that they are readable only to the tenths place. I defined error to be anything that causes a measurement to differ from the true value. By this definition, an error results from the combined effects of poor instrument resolution and poor skill of the person using the measuring device. I choose not to separate the effects of resolution and skill since they are intimately linked. By keeping them together, I never find myself in the predicament of viewing a set of scattered measurements, such as 15.1, 15.2, 15.1, 15.0 and 15.2, and having to conclude that they have zero standard deviation and perfect precision. Richard S. Treptow Department of Chemistry and Physics Chicago State University Chicago, IL 60628-1598
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Corrections The review of Statistical Mechanics for Chemists appearing on page 1217 of the October 1998 issue should have listed the reviewer as: John Gunn Department of Chemistry Université du Montréal Montreal, PQ, Canada
❖❖❖ The review of Molecular Mechanics across Chemistry appearing on page 31 of the January 1999 issue should have listed the first author as Anthony K. Rappé. ❖❖❖ The review of The Art of Molecular Dynamics Simulation appearing on page 171 of the February 1999 issue should have listed the reviewer as: Stephen P. Molnar Foundation for Chemistry Upper Arlington, OH 43212-1112
JChemEd.chem.wisc.edu • Vol. 76 No. 4 April 1999 • Journal of Chemical Education
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