Norman S. Radin
Northwestern University Medical School Chicago, Illinois
Rapid Dispensing of Liquids for Large Laboratory Classes
W h e n solutions or liquid reagents are made available to students in stock bottles, there is a marked tendency for the students to spill some of the liquids and take somewhat more than is needed. In the case of corrosive liquids, the spillage becomes an even greater nuisance. The spillage and wastage can be reduced greatly by offering the liquids in burets or pipets of the automatically filling type, but such dispensing is slow and many of these fragile glass dispensers are required for large laboratory classes. We have found that this problem can be handled conveniently and a t reasonable cost by the use of automatic syringes. These devices, which are available from most laboratory supply houses, are ordinarily used for highly repetitive dispensing problems, such as loading test tubes with media for bacterial assays, loading vials with pharmaceutical solutions, or adding reagents for analysis of many samples. They are essentially small pumps which deliver a constant volume with each stroke. The automatic syringes may be obtained in motor-driven types and hand-operated types. Some of the basic elements of the automatic syringe are shown in the accompanying figure. A hypodermic syringe ( A ) serves as the pump. The metal part (B), which attaches to the tip of the syringe, contains two valves. The inlet valve, on the side, leads by a flexible tube (C) to the stock bottle containing the solution to bedispensed. The exit valve, on the bottom; leads by a wide bore needle (with a blunt end) (D) to the student's test tube or flask. The syringe is held in a metal device which contains a spring and a plunger. When the student presses down on the plunger, the glass syringe plunger is forced down and liquid is expelled into the container. Release of the plunger allows the spring to return the glass plunger and retill the syringe from the stock bottle. A screw adjustment (E) in the metal holder controls the amount of liquid dispensed with each stroke. We have been using two commercially available automatic syringes, the Adams Aupette' and the Cornwall continuous p i p e t t ~ r . ~ In the case of the former, we recommend only the 5-ml size; the l-ml size is too fragile for student use and the 10-ml size seems to have a spring that is too stiff for girls. In the case of the latter device, we recommend the 2-ml and the 5-ml sizes. The metal valves make the apparatus unsuitable for use with acidic solutions. Two days' exposure to 0.2 N NaOH is not detrimental. Use with organic solvents rewuires ~olvethvleneor Teflon tubing. Clay-Adams Co., No. A-2700. 'Becton, Dickinson and Co., No. 1251 (2 ml) and No. 1261 (5 ml). Furnished with cannula No. 1250 NR.
When strong acids are to be dispensed, we replace the metal valves with Teflon valve^.^ These valves have delivery tips which do not require needles. We use polyethylene tubing4 between the inlet valve 2nd stock bottle. Because of a difference in dimensions, only the Aupette syringe holder can be used with the Teflon valve; it is necessary to replace the Aupette syringe by an ordinary 5-ml hypodermic syringe with glass tip. Certain techniques in the use of these syringes are helpful. First, i t is usually necessary to keep the level of liquid in the stock bottle below the tip of the automatic syringe to avoid siphoning. Second, it is neces-
F i g ~ r e1. Avtomotic syrnge IAdom. A~petvel. A, hypodermic ryrnge; B, outomal'c valve LO;?;C, Rex'ble tdbing ono linker; D, connula. F, rolrme control. lPhotograpn by corrtery of Scienl~RcPradurt~.Evonrton. Il 'no, 1
sary to prime the pumping system each time a new solution is started. Third, it is useful to hang a card over each automatic syringe advisiug the student as to the number of "squirts" to take and which container to use. The syringes tend to trap a small air bubble (from dissolved air), and the valves do not operate precisely the same way with each stroke, so the reproducibility of delivery volumes is not as good as that of a volumetric pipet. For this reason, the syringes are not used to dispense exact portions of standards or unknowns for quantitative analysis. While we have used this technique only in the biochemistry laboratory, it may be found useful in other student laboratories. Although the technique cannot be economically extended to the dispensing of large volumes, one can sometimes use concentrated solutions and have the students dilute them at their tables. a
Tri-R Instruments, 14413 Jmmiea Ave., Jamaim 35, N. Y. Co.
'Clay-Adrtms
Volume 36, Number 1 1 , November 1959
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579
