Manual of basic techniques for a health laboratory - libdoc.who.int

3. General laboratory procedures 75Fig. 3.49 Using a dropping pipetteFig. 3.50 Calibrating a dropping pipetteS: safety bulb.Micropipettes must be calibrated and maintained according to the instructions ofthe manufacturer.Calibrated dropping pipettesOrdinary calibrated dropping pipettes often deliver 20 drops per ml of distilledwater, thus 1 drop = 0.05ml. Hold the dropping pipette absolutely vertical to expelthe drops (Fig. 3.49).Calibration of dropping pipettes (Fig. 3.50)Using a volumetric pipette (see page 74), measure 1ml of water into a small tube.Draw the water into the dropping pipette to be calibrated. Count the number ofdrops delivered from the millilitre of water. Repeat the procedure three times tocheck the accuracy.PrecautionsPipetting by mouth is dangerous and should not be done. It can cause thefollowing:— infection— burns— poisoning— cuts.Always use a rubber safety bulb (see Fig. 3.50) with the pipette instead.3.4.2 Volumetric flasksVolumetric flasks are graduated to measure a certain volume when filled to thegraduation mark.