Pasteur pipette calibration

Modem Erlenmeyer flasks and beakers have approximate volume calibrations fused into the glass, but these are very approximate. Somewhat more accurate volumetric measurements are made in the 10-mL graduated cylinders. For volumes less than about 4 mL, use a graduated pipette. Never apply suction to a pipette by mouth. The pipette can be fitted with a small rubber bulb. A Pasteur pipette can be converted into a calibrated pipette with the addition of a plastic syringe body [see Fig. 11(d)] or you can calibrate it at 0.5, 1.0, and 1.5 ml and put three file scratches on the tube this eliminates the need to use a syringe with this Pasteur pipette in the future. Also see the Pasteur pipette calibration marks in the back of this book. You should find among your equipment a 1-mL pipette, calibrated in hundredths of a milliliter [Fig. 11(a)]. Determine whether it is designed to... 

For the determination of nitrate, use a 1 dram vial with a polyethylene stopper (Kimble No. 60975-L) as a reaction vessel. Introduce a 0.20ml aliquot of aqueous sample into the vial, followed by 1.0ml of thiophen free benzene. Catalyse the reaction by addition of 1.0ml of concentrated sulphuric acid. Shake the vial for lOmin. Remove the benzene layer immediately from the reaction vial with a Pasteur pipette, place it in a separate vial and analyse by gas chromatography with electron capture detection for the nitrobenzene concentration generated. Treat standard solutions of potassium nitrate in the same manner to generate a standard calibration plot relating nitrobenzene concentration to peak height. If higher precision is desired (approximately 4% relative standard deviation), add 2,5-dimethylnitrobenzene to the benzene prior to reaction. 

Make up to the calibration mark with solvent - add the last few drops from a Pasteur pipette until the bottom of the meniscus is level with the calibration mark. 

A convenient way of dispensing liquids when a great deal of accuracy is not required is to use a disposable pipette, or Pasteur pipette. Two sizes of Pasteur pipettes are shown in Figure 8. Even though accurate calibration may not be required when these pipettes are used, it is nevertheless handy to have some idea of the volume contained in the pipette. A crude calibration is, therefore, recommended. 

Additional Pasteur pipettes can be calibrated easily by holding them next to the pipette calibrated in Laboratory Exercise 2 and scoring a new mark on each pipette at the same level as the mark placed on the calibrated pipette. We recommend that several Pasteur pipettes be calibrated at one time for use in future experiments. 

Collect all the distillate in a 15-mL screw-cap centrifuge tube. Using a calibrated Pasteur pipette (see Experiment 1), add 2.0 mL of methylene chloride (dichlo-romethane) to extract the distillate. Cap the tube securely and shake it vigorously with frequent venting. Allow the layers to separate. Using a Pasteur pipette, transfer fhe lower mefhylene chloride layer to a clean, dry, 5-mL conical vial. Repeat this extraction procedure two more times with fresh 1.0-mL portions of methylene chloride and combine all the methylene chloride extracts in the same 5-mL conical vial that you used for the first extraction. If there are drops of water in the vial, it will be necessary to transfer the methylene chloride solution with a dry Pasteur pipette to another dry conical vial. 

Before running the column, assemble the following glassware and liquids. Obtain five dry test tubes (16 X 100-mm), and number them 1 through 5. Prepare two dry Pasteur pipettes with bulbs attached. Calibrate one of them to deliver a volume of about 0.25 mL. Place 10.0 mL hexane, 6.0 mL 70% hexane-30% acetone solution (by volume), 6.0 mL acetone, and 6.0 mL 80% acetone-20% methanol (by volume) into four separate containers. Clearly label each container. 

Transfer about 10 mL of anhydrous diethyl ether into a dry 25-mL Erlenmeyer flask and stopper the flask. Use the flask to store your dry ether during the course of this experiment. During the experiment, remove the ether from this flask with a dry calibrated Pasteur pipette. 

A Pasteur pipette may be supplied by your instructor for dropwise addition of a particular reagent to a reaction mixture. For example, concentrated sulfuric acid is often dispensed in this way. When sulfuric acid is transferred, take care to avoid getting the acid into the rubber or latex dropper bulb. It is best to avoid the rubber dropper bulb entirely by using one-piece transfer pipettes made entirely of polyethylene. These plastic pipettes are available in 1- or 2-mL sizes. They come from the manufacturers with approximate calibration marks stamped on them (Figure 55B). 

In some cases, however, your instructor may have available a calibrated pipette for transferring nonlimiting reagents that may damage an automatic pipette. For example, a calibrated Pasteur pipette may be used with concentrated acids. 

---