Technique for Controlling the Filling Rate of a Pipet In the recent wries ofsuagestiona tbr pipet fillcrr for student there has been no mention of the one step in the filling technique which enhnnees the effectivene~sof any oi the hulh adapters as wdl ns the standard red ruhber pipet bulb. If the contamer from which che student is to draw the sample h a . a flat hottwn whrch can he reached 11sthe trp of the prpet, merely resting the the vertical pipet on the bottom substantially slows the filling rate. Moderate downward pressure will nearly stop the flow even with the bulk completely depressed. The student is directed t o transfer a sufficient quantity of the solution t o be pipeted to a beaker. The wetted end of the pipet is inserted gently a short distance into the ordinary pipet hulh. To fill the pipet, the bottom ofthe pipet bulh and the stem of the pipet are grasped in the left hand and the hulk is depressed with the right hand. The pipet is lowered vertically into the solution in the beaker with the tip touching the bottom of the beaker. The tip is raised slightly from the bottom of the beaker as the pipet fills. Just before the enlarged portion of the transfer pipet is filled or as the level of the filling liquid nears the limit of the markings on a Mohr form pipet, the pipet is lowered until the tip touches the bottom of the beaker. A slight downward pressure on the pipet slows the filling rate to nearly astopand the bulk can be removed easily when the appropriate level is reached. By maintaining the slight downward pressure with the left hand, sufficient time is available to place the hulk on the desk before returning the right hand t o control the liquid level in the usual fashion. The advantage of this technique is that the pipet is already resting against the bottom of the beaker and any recoil of the pipet in the left hand as the right hand removes the hulk does not shatter the tip of the pipet or the solution container. Also.. the oioet is actuallv controlled hv both hands exceot for the bulb removal sten. One caution should be noted. Since the left hand is holding the piper at the tup of [he atem and many studenwpush the bulh to thesideor away from thernselvesto remove it insread of lifting it upand away from the sum, there isa trndenry ten "kirk" the beaker to thesidr or toward thestudmt. If the beaker is placed at n comfortable arms lenpthawny i n m the student instead ofon the edge of che hench, this prohlem cnn he allwinred. As with any pipeting technique, this procedure requires practice to acquire proficiency. Finally, rt will not eomplecely eliminate uver filling the piprt-but i r can greatl) reduce the frequency d that incident.
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Harris, W. E. and Kratochvil, B., J. CHEM. EDUC., 53,253 (1976). Hanson. R. H.. J. CHEM. EDUC.. 54.434 (1977). q ~ o t u m , k~..:lr...l. . ('HEM. E D I ~ ( ' . ; s ~ , < ; ~ ~ I S ~ X ~ . Hcklmg, G.(:...I. CHEM. EDIIC., 55,541 11931. '\'an Atta. H E., and Kambauk, D. 0.. J. C H E M EI)UC., 66,756 (1979). Miami University Oxford, OH 45056
474 I Journal of Chemical Education
Philip K.Welty
