Pipettes

PIPETTES (from French pipette—little tube), vessels for suctioning liquids or for measuring out a specific volume of liquids, as well as for transferring a measured liquid to another vessel (Fig. 1). The Mohr pipette has a single mark; graduated pipettes have divisions of ¹/₁₀ or ¹/₁₀₀ cm³. Eye pipettes and Pasteur pipettes serve for counting drops and have no divisions. Measuring liquids out of a pipette will be accurate if one adheres to the rule that when pouring out the liquid, the tip of the pipette must touch the wall of the vessel into which the liquid is being transferred at all times, and the pipette must be in a vertical position. After the complete outflow of the liquid, the pipette should be held in the same position for a few more seconds. The last drop from a Mohr pipette should not be blown out.

Pipettes with two lines (marks) are also used: one on the upper part of the tube and the other on the lower; the liquid should be released from the upper mark to the lower; the advantages of such pipettes are not substantial, and working with them is more difficult. For microanalysis needs, there are pipettes graduated in 0.001 cm³ (micropipettes). As with all instruments for volume determinations, when using a pipette it is necessary to take into account the temperature and specific gravity of the liquids: for example, when measuring an alcoholic solution with a specific gravity of 0.850, one must not forget that 5 cm³ of it is not 5 g, but only 4.25 g; conversely, in order to draw 5 g of such a solution with a pipette, one needs to measure 5.88 cm³ of it. Pipettes are widely used in bacteriological practice.

In cases where the measurement of exact volumes of liquids is required, such as in setting up serological reactions, in titrations of toxins, sera, and the like, graduated pipettes with a capacity predominantly from 1 cm³ to 10 cm³ with divisions from ¹/₁₀ to ¹/₁₀₀ cm³ are used.

Figure 1. Types of pipettes: 1 and 2—Mohr pipettes; 3 and 4—graduated pipettes; 5—eye pipette; 6—Pasteur pipette.

Figure 2. Preparation of Pasteur pipettes: 1—glass tube; 2—tube drawn into a capillary; 3—finished pipette; 4—pipette for sealing at both ends.

For seeding liquids onto nutrient media, for collecting liquid pathological and pathoanatomical material (taking blood, exudate, etc.), and in all other cases when it is required to transfer liquid from one vessel to another, Pasteur pipettes are used, manufactured from glass tubes 20–25 cm long and 5–7 mm in diameter (Fig. 2); the edges must be even and smooth. Small cotton plugs are placed in both ends of the tube and sterilized in this form in a Pasteur oven. After sterilization, the middle of the tube is introduced into the flame of a blowtorch and heated with continuous and rapid rotation. When the heated section of the glass becomes completely soft, the tube is quickly removed from the flame, its ends are stretched in opposite directions, and thus a capillary is drawn. The tube is held in a horizontal position so that the pipettes come out straight. The capillary is divided in half in a small flame; each end of the two pipettes thus obtained is sealed. Before use, the pipette should be passed through the burner flame several times and the end of the capillary broken off with sterile forceps. Pasteur pipettes can serve for preserving material taken in them; in this case, they are sealed at both ends. To facilitate sealing the end of the pipette opposite the capillary, when preparing a pipette intended for preserving material, one should heat the glass in the above-described manner and slightly stretch that part of the pipette which is adjacent to the cotton plug.