Inexpensive and Accurate Micropipets and Pvlcnometers a
F . L. HAHN Instituto Quimico Agricola Nacional, Guatemala City, Guatemala
(Translated by Rdph E. ~i.sper,University of Cincinnati) stem becomes narrower. Consequently, it is best to have the mark in a very fine capillary constriction. (2) Under this condition, it will not even be necessary to place a mark on the pipet and then subjectively to use this as a point of reference, because the liquid will automatically adjust itself to a height that is fixed by the capillary forces. Both of these predictions have been verified experimentally; the reproducibility of the volumes delivered by pipets filled in this fashion has been excellent. If the chemist makes these pipets himself he will find it difficult to produce instruments of a particular desired capacity. (A commercial instrument maker probably would be able to solve this problem.) However, in the majority of microchemical exercises, i t makes no difference whether the pipet discharges, for instance 0.2 ml. or 0.193ml., provided that the volume is known and constant. The writer uses two types of these pipets. The first (Figure I), used for ordinary measurements, is provided with two marks. The second variety (Figure 2) has one mark and a longer discharge tip. This latter type is used if one liquid must be blown into another, for s instance when preparing a series of dilutions. They are also used to introduce the last reagent when running catalyzed time reactions. The instant a t which the operator hears the air bubbles pass through the C solution is taken as the starting time. The pipet is filled by sucking the solution above the FIGURE 3 mark A and then holding the instrument a t an angle, with the tip resting against the wall of the container. The liquid runs out until its level reaches the mark and automatically stays a t that height. The pipet is then held horizontal and the outside is wiped off with filter paper. The contents are then allowed to flow out into the reaction vessel until the liquid level drops to the lower mark (I),or the pipet is emptied by blowing (2). The reproducibility of the volumes discharged was HERE is no lack of automatic pipets for the tested by making 10 trials and using the deviations measurement of very small volumes of liquids. from the average value to calculate the mean error of However, none of these is so accurate or so simply constructed as those described in this paper. They are the individual trials. The result was *0.25 mg. = easily fashioned from a piece of ordinary glass tubing. 0.00025 ml. Accordingly, 0.25 ml. can be measured Their use can facilitate one of the tedious operations within 0.1 per cent, and 0.025 ml. within 1 per cent. This same principle may be used in the construction of microchemistry, namely, the measurement of samples of pyknometers. In this case it is easily possible to and reagents by means of fine transfer pipets. This new design was based mainly on two ideas. repeat the standardization or filling several times and (1) A slight variation in the height to which the pipet to take the mean of all the trials. The design of the is filled has less effect on the volume delivered as the pyknometers is shown in Figure 3.
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The liquid is drawn above the constriction A and the excess is then allowed to flow out, with the tip C in contact with the liquid in the original container. The liquid in the pyknometer will then stop in the constriction. The pyknometer is then held horizontal and the tip is wiped with bibulous paper. The apparatus is then turned upside down and suspended, at B, on the balance. If the constriction a t A and the capillary are too wide the liquid will not stay in the constriction when the pyknometer is inverted. In that event the pyknometer is laid flat on the balance pan. In both cases, the liquid will retreat a little from the tip C into the capillary, and is thus adequately prevented from evaporating.' When repeat-
ing the measurement i t is sufficient again to suck the liquid a little above A and to let the excess run out until the liquid remains stationary. The mean error of the single trials with a l-ml. pyknometer was t0.16 mg. = 0.00016 ml. The average error of 10 fillings was +=0.05mg. = 0.00005 ml. These results signify that the accuracy attained here in the measurement of a volume not exceeding one milliliter is such that the temperature must be determined within 0.2'C., since otherwise the inaccuracy due to temperature variation will exceed the inherent instrumental error.
Cf.HAHN,Mikrochemica Acla, 3, 7 (1938)
