In the Laboratory edited by
The Microscale Laboratory
Arden P. Zipp SUNY-Cortland Cortland, NY 13045
A Comparative Study of Microscale and Standard Burets Mono M. Singh,* Cynthia B. McGowan, Zvi Szafran, and Ronald M. Pike National Microscale Chemistry Center and Chemistry Department, Merrimack College, No. Andover, MA 01845; *
[email protected] In analytical chemistry, many titrations are performed change from wine-red to purple-blue is sharper and easier to routinely. We have developed an improved microscale buret, detect than in macroscale titrations. In Mohr titrations (3b; whose construction and use were described previously in this personal communication with Norman Clark, Christer Journal (1). In this paper, the use of a microburet in general Gruvberg, and Felix Schultze), the end point was determined and analytical chemistry will be illustrated by results from by the formation of red silver chromate. At the macroscale acid–base, oxidation–reduction, precipitation, complexometric, level, the relatively large amount of silver chloride precipitate and pH titrations. The data obtained by students using the formed masked the initial color formation of the silver chromicroburets is compared with data obtained using Beral pipets mate. In contrast, the end point was readily visible at the and as standard 50- and 10-mL burets. Microscale titrations microscale level. performed using less than 2 mL of titrant proved to be as accurate as macroscale techniques. We emphasize several verBenefits of Microburets satile features of using a microscale buret: cost effectiveness, Cost Effectiveness time savings, and accessibility for the physically challenged. Upper-class students performed all titrations according A set of two microburets (with 0.001-mL tolerance) costs to procedures described in the literature (2–4 ). The HCl less than $20. This is substantially less expensive than a 10solution (0.0974 M, Aldrich) was used as a prestandardized mL buret ($80) or a 50-mL buret ($60). Furthermore, since solution, and the sodium hydroxide solution (0.0983 M, two microburets are utilized, calibrated pipets (5 and 25 mL) Aldrich) was given as an unknown. The base solution was costing approximately $10 each are eliminated. Additional always delivered from a buret (macro or micro) or a Beral savings are realized for both chemicals and chemical waste pipet (5). In microscale titrations, a second microburet was disposal. The chemical cost for microscale Mohr titration for used to transfer fixed volumes of HCl solution. In macro 100 students (performing at least three titrations using 2– titrations, known volumes of HCl solution were transferred 8-mL samples) was $45 and 3 L of waste solution was prousing a fixed-volume pipet. The duplicate set of titrations duced, whereas the 50-mL scale had a chemical cost of $575 (using the same solutions) was also performed using 10-mL and generated 20–30 L of waste solution. Moreover, in burets and preweighed and calibrated Beral pipets. An average microscale titration, the use of K2CrO4 indicator is drastiof 5–8 trials were made for each titration. Table 1 summarizes cally reduced. the data from these experiments. Time Savings Titrations were performed using 50-mL class A burets (macro titration) and microburets (micro titration). To comThe average time needed to set up a standard 50-mL pare the 50-mL buret with the microburet, the NaOH was buret and to perform a titration using 10–25 mL of solution standardized with a standard potassium hydrogen phosphate as the analyte is about 25 minutes. Using a microburet, the solution (0.0974 M). With each buret, four trials were made. same titration can be performed within about 3–5 minutes. The average molarity determined by the microburet was Moreover, the prelaboratory preparation time is drastically 0.09175 with a 95% confidence level of ± 0.0016. With the reduced. In comparison to time needed for a regular buret, 50-mL buret, the average molarity was determined to be the dismantling, rinsing, and storing of microscale burets is 0.09212 with a 95% confidence level of ± 0.0008. From these data, one can observe the following Table 1. Acid–Base Titrations trends. The NaOH molarities determined using the Technique for Amount of HCl, Base RSD Error microburet and the 10-mL buret, within relative deSD NaOH Delivery 0.0974 M Molarity (%) (%) viation, were identical. The Beral pipets and spot Microburet 2.000 mLa 0.09813 0.00038 0.41 0.20 plates gave much poorer results for the standardiza0.62 1.1 1.000 mLa 0.09720 0.00057 tion. The data from 50-mL buret are comparable 1.000–1.232 mLb 0.09722 0.00101 1.0 1.1 with those obtained using 2.00-mL burets. 0.09781 0.00102 1.0 0.51 Buret, 10 mL 2.000 mLa The microburets were also successfully used in 5.000 mLa 0.09940 0.00044 0.40 1.1 other types of titrations such as potentiometric, com≈ 0.5 g c Weighed Beral pipet 0.09053 0.0033 3.6 7.9 plexometric, redox, and Mohr’s precipitation titration 0.00329 3.9 16 Calibrated Beral pipet 15 drops 0.0826 (2– 4 ). During a 2.5-h laboratory period, students 0.00837 11 19 20 drops 0.07921 were able to complete a potentiometric titration (2). 4.3 16 25 drops 0.08285 0.00353 In complexometric (using Na 4EDTA) titraaDelivered from a fixed-volume pipet. bDelivered from a microburet. cAverage tions, the indicator (Eriochrome Black T) color mass. JChemEd.chem.wisc.edu • Vol. 77 No. 5 May 2000 • Journal of Chemical Education
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not a time-consuming process. Further, the storage of microburets does not pose any problem in terms of space or dust and oil accumulation.
Easy Accessibility Another significant aspect of microscale burets is their easy accessibility to physically handicapped students. Students in wheelchairs, for example, were able to perform microscale titrations with ease, on a lowered laboratory benchtop. This was not possible with the much taller standard burets. This was also advantageous to shorter students, who were able to perform the titrations without climbing on stools or laboratory benches (with opportunities for accidents). Conclusions The use of microburets has proved to be a very versatile and analytically accurate alternative to using standard burets. Considering the ease of performance, cost effectiveness, time saving, and analytical rigor described above, the environmentally friendly microburet is of great value to the instructional laboratory. Acknowledgments We thank Norman Clark, Pentucket High School, MA, and Christer Gruvberg and Felix Schultze, Kattegatt-
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gymnasiet, Halmstad, Sweden, for providing us with the data obtained by their students on microscale precipitation titrations using AgNO3. We thank Lori Murphy for the acid–base titrations. The funding from TURI, Commonwealth of Massachusetts, is also acknowledged. Literature Cited 1. Singh, M. M.; Szafran, Z.; Pike, R. M. J. Chem. Educ. 1991, 68, A125. Singh, M. M.; McGowan, C.; Szafran, Z.; Pike, R. M. J. Chem. Educ. 1998, 75, 371. 2. Singh, M. M.; Pike, R. M.; Szafran, Z. Microscale and Selected Macroscale Experiments for General and Advanced General Chemistry: An Innovative Approach. Wiley: New York, 1995. 3. (a) Szafran, Z.; Pike, R. M.; Singh, M. M. Microscale Chemistry for High Schools: Volume 1; Kendall/Hunt: Dubuque, IA, 1996. (b) Szafran, Z.; Pike, R. M.; Singh, M. M. Microscale Chemistry for High Schools: Volume II; Kendall/Hunt: Dubuque, IA, 1996. 4. Szafran, Z.; Pike, R. M.; Foster, J. C. Microscale General Chemistry Laboratory with Selected Macroscale Experiments; Wiley: New York, 1993. 5. Slater, A.; Rayner-Canham, G. Microscale Chemistry Laboratory Manual; Addison Wesley: Reading, MA, 1994; pp 73– 77.
Journal of Chemical Education • Vol. 77 No. 5 May 2000 • JChemEd.chem.wisc.edu
