Chemical Education Today
Letters Gravimetric Titrations Save Time, Expense, and Error by Using Weight Burets Two papers (1, 2) have shown the superiority of a cheap plastic squeeze bottle used as a weight buret over an expensive volumetric buret. These papers were published at a time, now past, when precise weighing was inconvenient. A weight buret is more accurate and more precise than a volumetric buret and requires less skill in titrimetry. A plastic squeeze bottle is cheaper and more robust than a glass volumetric buret. Students using squeeze-bottle burets for titrimetry obtained 0.1% accuracy (1), which is better than the 0.3% that good students can achieve volumetrically. Indeed, weight burets were used for specially accurate work long ago when their use was prohibitively inconvenient (3). Unlike volumetric titrimetry, training is not needed in proper drainage and cleanliness or in reading the meniscus or in the choice of Class A or B buret. The titrant is calibrated in mol/kg so the effect of temperature on w/v concentration is irrelevant. Concentrated titrants can be used without their traditional errors. Modern weighing technology would make automation straightforward. The published projects were done in academic labs. It is reasonable to suppose that industrial labs would also welcome the savings and convenience and increased reliability of squeeze-bottle burets. Improvements in technique are usually expensive, but this is cheap. It is time to resuscitate the weight buret. Literature Cited 1. Butler, Eliot A.; Swift, Ernest H. J. Chem. Educ. 1972, 49, 425–427. 2. Armstrong, Alfred R. Va. J. Sci. 1969, 20, 58–59. 3. Kolthoff, I. M.; Sandell, E. B.; Meehan, E. J.; Bruckenstein, S. Quantitative Chemical Analysis, 4th ed.; Macmillan: London, 1969, p 560.
In Support of Weight Titration Techniques I write in support of the recommendations of Stephen Hawkes on gravimetric titrations. Weight titration techniques are, in my opinion, by far the best way to bring “volumetric analysis” into the 20th (let alone 21st) century. Such techniques should have taken chemistry teaching by storm as soon as automatic balances good to 0.01 gram became commonplace. Not only are relatively inaccurate glass burets greatly outclassed, but so are pipets and volumetric flasks. Such glassware is expensive, hard to clean well, and difficult to use properly. Years ago I coined the term “molamity” to designate the concentration units of moles of solute per kilogram of solution, and my students routinely performed titrations with uncertainties of 1 part per thousand. My text, Chemical Equilibrium and Analysis (1), included a lab titled “High-precision Assay of Mohr’s Salt by Gravimetric Titration”. Students routinely achieved part per thousand results. W. B. Guenther published a good article titled “Supertitrations: High Precision Methods” (2). B. Kratochvil and C. Maitra published “Weight Titrations: Past and Present” (3), with many citations. A good technique is to use a squeeze bottle, as recommended by Hawkes, of the type that delivers through a channel molded into the side of the bottle. The top cap is replaced with a fine-tip medicine dropper so that the endpoint may be determined more precisely than a hard-to-control squeeze would permit. Squeezes are used to get close to the endpoint, and then the dropper is used to nail it down accurately. I like to have another medicine dropper in the receiving beaker, so that a small portion of the analyte can be kept in reserve to avoid over-running the endpoint. Gravimetric titrations should become the gold standard, and volumetric glassware should be seen in museums only. Literature Cited
Stephen J. Hawkes Department of Chemistry Oregon State University Corvallis, Oregon, 97331-4003
[email protected] 1. Ramette, Richard W. Chemical Equilibrium and Analysis; Addison-Wesley: Reading, MA, 1981. 2. Guenther, W. B. J. Chem. Educ. 1988, 65, 1087. 3. Kratochvil, B.; Maitra, C. American Laboratory 1983, 15, 22– 29. Richard W. Ramette Department of Chemistry, Retired Carleton College Northfield, MN 55057
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Vol. 81 No. 12 December 2004
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Journal of Chemical Education
1715
