Comparing burets with different capacities, a 50-mL buret having a typical drop volume of 0.05 mL has a predicted optimum RSD of about 0.05 mL/50 mL or 0.1%, and a 2-mL microburet about 0.01 mL/2 mL or 0.5% RSD, a factor of five larger. The RSDs for the 0.1 M KHP/0.1 M NaOH titrations were 0.5% and 1%, respectively, calculated from the reported 95% confidence intervals. Spreadsheet simulations of the 0.1 M KHP–0.1 M NaOH titrations shed some light on the discrepancy between the predicted and reported RSDs, showing that the volume intervals for a phenolphthalein indicator color change are ~0.1 mL for the 50mL buret (about two times larger than the 0.05 mL volume resolution) and ~0.01 mL for the 2-mL buret (about equal to the 0.01 mL volume resolution). There are two contrasting ways to view the comparison. On one hand, it shows an advantage of microscale titrations which have sharper endpoints (much larger endpoint slopes, ΔpH/ΔV), as noted anecdotally by Singh et al. (2) and described quantitatively in this Journal by Rossi et al. (3). On the other hand, the comparison is biased in favor of the microburet because the conditions selected result in a titration that is not optimized for a 50-mL buret, the volume interval of the color change (and therefore the uncertainty in the volume measurement) being greater than the volume resolution.

Literature Cited

1. Singh, M. M.; McGowan, C.; Szafran, Z.; Pike, R. M. J. Chem. Educ. 1998, 75, 371.
2. Singh, M. M.; McGowan, C. B.; Szafran, Z.; Pike, R. M. J. Chem. Educ. 2000, 77, 625.
3. Rossi, M. V.; Suárez Iha, M. E. V.; Neves, E. A. J. Chem. Educ. 1990, 67, 65.

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