Chemical Education Today
Letters Precision in Microscale Titration The authors reply: We would like to make the following comments in response to Julian Roberts. First, Roberts’s letter refers to RSD values. We calculated the RSD values by following standard methods described in analytical chemistry textbooks, using the titration data from students’ hands-on work in the laboratory. We do not know if the method of calculation of RSD as suggested by Roberts is applicable in the manipulation of laboratory titration data. Further, we have found that in our original paper, “A Comparative Study of Microscale and Standard Burets” ( J. Chem. Educ. 2000, 77, 625–626), there was a typographical error in reporting the tolerance; it should read 0.01 instead of 0.001. If this corrected tolerance is used in calculating the RSD as suggested by Roberts, then it would fall in the range ~0.6 instead of 0.06. If we assume that Roberts’s approach is valid, then this range is comparable to the value (~0.4) mentioned in our paper. Second, we did state that students could read the delivered volume in a 2-mL buret to four significant figures after the titration is done. This is correct. Roberts says that we do not have the volume resolution to deliver an aliquot to four significant figures. This is also correct. However, we never
claimed we did. Our students are asked to read the initial and final volumes and the third decimal point reading is the estimated one. If the volume delivered is more than 1 mL, this leads to four-significant-figure reading. The accuracy to which one can arrive at the endpoint is affected by the volume resolution but not the precision to which we can measure the quantity of delivered titrant. Third, when we compared the performance of 50-mL and 2-mL burets in instructional laboratories we did so by taking into account the results obtained by students, cost, ease, time requirement, and waste reduction. Fourth, concerning Roberts’s statement that the comparison of microtitration to macrotiration is biased, we want to point out that the conditions under which we do the microscale titrations are similar to actual conditions used in a teaching laboratory. Generally, neither macroscale nor microscale titrations are optimized for volume resolution relative to volume interval. Thus, we are not biasing the experiment—it is the way these experiments are typically done that does this. Mono M. Singh, Cynthia B. McGowan, and Zvi Szafran National Microscale Chemistry Center and Chemistry Dept. Merrimack College North Andover, MA 01845
JChemEd.chem.wisc.edu • Vol. 79 No. 8 August 2002 • Journal of Chemical Education
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