In a laboratory, a dropper is a simple tool for transferring small amounts of liquid one drop at a time. It is made of a narrow tube, usually glass or plastic, with a rubber or elastomer bulb at the top. Squeezing and releasing the bulb draws liquid into the tube and then expels it as drops. Droppers are used for non-critical tasks such as adding indicator solution, placing a drop of reagent or transferring a small sample.<\/p>\n
A laboratory dropper, also called a Pasteur pipette or a dropping pipette, has a tapered tip and a wider upper end that fits a rubber teat. Glass versions are common because glass resists many solvents and can be cleaned or sterilised. Disposable plastic droppers are widely used where cross-contamination must be avoided. Some bottles for indicators and reagents also come with a dropper fitted to the closure, so that the liquid can be dispensed without a separate tool.<\/p>\n
A dropper is not a measuring instrument. The size of a drop depends on the liquid, the temperature, the shape of the tip and the angle and speed at which the bulb is squeezed. As a rough guide, many aqueous liquids give about 20 drops per ml, but this figure varies widely and should not be used for quantitative work. When you need a defined volume, use a volumetric pipette, a graduated pipette, a burette or an adjustable micropipette instead.<\/p>\n