gratifying to note that even the slight sharp rise at the start of the acetic acid titration curve is barely detectable; this feature can be brought out more clearly by assigning more small volume increments of the base, and suggesting that the indicator standards be prepared in 0.2 or 0.5-pH unit intervals rather than in intervals of integral values of pH (see Fig. 3 ) .
Figure 3. Clots doto: with 0.01 M NoOH.
"titration curve" of 25.00 rnl of 0.01 M HC8HaOr
In the figures shown, one notes a relative scarcity of points in the vertical region of the titration curves. This is caused by a combination of the nature of the very sharp change in pH around the equivalence point and the lack of buffering in the indicator standards when prepared as indicated above. The latter difficulty can be remedied by explaining the need for buffer solutions after the first experiment is completed, and incorporating buffer solutions in the indicator standards. If the
instructor prefers to discuss buffer solutions in regard to the titration curves, he can have the stockroom prepare the indicator standards ahead of time, using the buffer solutions of Clark and Lubs3 or commercially available solutions.4 The above experiments have been used by the author as described, first to introduce the topic of acid-base titrimetry, and then as an illustration of the properties of acids and bases; students are asked to elaborate on the latter aspect through preparation of a written report. Several improvements and variations on these experiments are immediately obvious. With a little ingenuity, one can alter the experiment on indicators to illustrate acid-base chemistry in non-aqueous solvents. Also, one can use the indicators to illustrate some problems in molecular structure, such as the matters of resonance and the effect of molecular structure on p K values. The application of the overhead projector to further instruction in the use and theory of indicators is apparel~t.~The indicator method of Stedman6 may also offer an avenue for effective modification. The use of newly available, low cost pH meters in this experiment would seem to defeat the whole purpose of providing familiarization with acidbase theory through the use of indicators. The author would like to acknowledge the help of the Photography Laboratory of the Kimberly-Clark Corporation in reproducing the figures shown in this paper. 'See taMe in KOLTROFF,I. M.,AND LAITENEN,It A,, "pH and Electro Titrations," 2nd ed., John Wiley & Sons, Inc., New York, 1941, p. 34. ' Obtainable from Hatmsnn-Leddon Ca., Philadelphia, Pa. SPIEGLER, K. S., ET AL., J. CHEM. EDUC.,39, 87 (1962). ' STEDMAN, D. F., J . CHEM.EDUC., 35, 456 (1958).
Magnetic Stirring Promotes Smooth Boiling Use of a seamless Teflon-covered magnetic stirring bar in conjunction with a magnetic stirrer effectively prevents violent bumping and has a tendency to deorease the foaming prevalent in so many reactions. A particular advantage of the Teflon bar is that vacuum distillations can be conducted without having to introduce extraneous gases. Distillations can be oonducted a t much lower pressures with consequent deorease in decomposition of thenndly sensitive campounda. Another (though minor) advantage is that ordinary still head8 and one-neck Basks o m be used. Stimng bars are effective when either oil baths or glass fabric heating mantles are used. They stand rough usage and are inert to almost all reagents and extremes of temperature. Magnetic stirring is also recommended in oontinuous liquid-liquid extractions-both in the extractor, t o insure better contact between the two liquids, and in the pot, to prevent bumping and discourage foaming. The bumping or foaming that may occur during a Soxhlet extraction, where the effectiveness of ordinary ebullators decreases as the extractina- solution becomes concentrated, can he prevented by magnetic stirring. Another advantage of the magnetic stirrer is that its effectiveness is not marred by a temporary increase in internal pregsure which farces liquid into capillaries and sometimes clogs them. In consequence, the temperature can fall without the danger that subsequent reheating will cause bumping.
JORDAN J. BLOOMFIELD UNIVERS~TY OF ARIZONA, TUCSON Volume 39, Number 7, July 1962
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