A Low-Cost, Repeating, Precision Pipetter for Student Use In 1973 Blaekman and Sanchez1 described a modification of the "Cornwall pipetter" in which disposable plastic tips were attached to the syringe with a short length of Tygon tubing. This has proven to be a most useful and relatively inexpensive laboratory tool. One problem was that the volume that could he handled was limited hy the size of the plastic tip, and a subsequent communication (Roberts et 01, 1914)2recommended nesting of the plastic tips to permit use with larger volumes. This arrangement is not particularly suitahle for student use, and this note recommends an alternate modification to handle s wide range of volumes. The Cornwall pipetter is ideal for use in teaching laboratories where i t can provide safe, rapid, precision pipetting of most liquids, with safety being a major factor. However, the easily broken and expensive to replace precision glass syringes supplied with the metal pipetting holder do not stand up well to student use. I t was found that disposable plastic syringes could be modified to fit the metal holder. The lip a t the top of the barrel of the plastic syringe is trimmed with a pen knife until it fits into the metal retaining ring of the holder. This modification takes about two minutes. Other parts fit normally. into the 10 ml-holder. A 3-ml ~ l a s t i cswinee fits into the 2-ml holder. a 10-ml swinee , " into the 5-ml holder. and a 20-ml swinee They can he usid tipipet up to 2.5. I , and 12 k l of liquid, respecrively. For applira'tionr whpre strong n r ~ d s obases r are used. it has heen found that diapmahle glass syringescan alsu be modified LO fit the holders. In placeofthe kniie, one uscsaxamng dewee and a pair of pliers to reduce the size of the rim on the harrel. The markings on plastic disposable syringes are not too accurate, so it is recommended that they be set by weighing the volume of water dispensed. A quicker method of setting the syringe is to attach a volumetric (or graduated) pipet to the "Luer" tip using suitable pieces of Tvmn tubing. The volumetric o i ~ eist then immersed to its calibration line in a cvlinder of water, andthe holder is adjusteduniiiwater rekhes the line. ~mz&ioneliminates the error which would occur deeause air in the plastic syringe is under tension when supporting the weight of water in a full pipet. Once set the plastic syringe has remarkable reproducibility.
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R e ~ u l t sof Using the Apparatus t o Weigh Various Sampler of Water
Nomindi w u m e
(mi)
.
.
Mean weight (pl" Standard deviation Relative average deviation
10
(%I
10.089 0.013 0.12
7.5 7.506 0.006 0.08
5
4.979 0.003 0.06
2 2.010 0.008 0.40
1.5 1.502 0.002 0.12
1.0
1.000 0.004 0.38
0.5 0.506 0.002 0.37
aTen sampler per trial.
The manufacturer has indicated that the metal pipetting holder is svsilatde a s a separate item. This holder, with the disposable syringes, makes an excellent instrument rugged e n o u ~ hfor use in the teaching iahoratory uith the precision re. quired for research purposes. This work wasan offshoot of aproject supported by grant # 996-150 from the Research Grants Committee of Wilfrid Laurier University.
' Blackman, D., and Sanchez, G. R., J. CHEM. EDUC., 50,851 (1913).
Roberts, C. A.,Bowman, G. C., and Blackman, D., J. CHEM. EDUC., 51,694 (1914). Wilfrid Laurier University Waterloo, Ontario, Canada N2L 3C5 2
David R. Peirson
Volume 54, Number 3, March 1977 1 173
