HOW LONG SHOULD A PIPET DRAIN? B. I. LYON
AND
.-,
L. J. BIRCHER
Vanderbilt University, Nashville, Tennessee
AN EXAMINATION of the directions for the use of the pipet as given in leading textbooks on quantitative analysis shows that there is a possibilitywf confusion in the minds of students as to how long the pipet should drain. It is acknowledged that under certain conditions it makes no difference how long the pipet is allowed to drain and that under other conditions the error may be insignificant, hut even then it is unfortunate from a pedagogical standpoint that conflicting statements are made by the several authors. Under existing conditions of instruction and practice it is conceivable that these conflicting instructions may lead to inaccurate work. In this study twenty-three textbooks giving instructions in volumetric procedure were examined. Of these books, five state specifically that in emptying the pipet after continuous flow has ceased, the pipet should not be allowed to drain, but that the tip should be touched immediately to the surface of the liquid or the vessel to remove the last drop to complete delivery. Five books, although not stating specifically that the pipet should be allowed to drain, refer to the Bureau of
Standards specifications. Four hooks state that after continuous flow has ceased, the pipet should drain fifteen seconds; one book states twenty seconds; one, thirty seconds; one, a minute; and two state that it should drain, but fail to say how long. Two books do not say to drain but do say to hold until it is "empty," and two others apparently give no instructions for emptying the pipet. The procedure recommended by the United States Bureau of Standards states that "After continuous, unrestricted outflow ceases, the tip should be touched with the wet surface of the receiving vessel to complete the emptying." To render this less subject to variable interpretation some authors insert the word "immediately" after the word "touched" in their directions. Since some pipets, apparently of the better quality, drip rapidly for a second or two after continuous flow ceases, it would seem best to say: after the continuous outflow and the first rapid dripping cease, touch the tip of the pipet immedietely to the wet surface of the receiving vessel to complete the emptying.
