A dropper, commonly encountered in a chemistry laboratory, is a fundamental piece of equipment used to manage and move small amounts of liquid. This tool facilitates the controlled addition or removal of reagents, solvents, or samples during a chemical process. Its main purpose is to achieve a precise physical transfer of liquid, not a precise measurement of its volume.
Nomenclature and Primary Role
While “dropper” is the common term, the scientific community refers to this instrument as a transfer pipette or a Pasteur pipette. This nomenclature clarifies its function as a tool for transferring small quantities of fluids. The design consists of a narrow tube, often with a fine tip, connected to a compressible rubber or plastic bulb that creates the necessary suction and pressure.
The primary function of the transfer pipette is the non-volumetric addition of substances. It is not used when the exact amount of liquid needs to be known. This makes it ideal for tasks such as adding an indicator solution drop-by-drop to a titration mixture or spotting a sample onto chromatography paper.
Common Types in the Laboratory
Laboratories utilize two primary versions of the dropper, distinguished mainly by their material and design: the glass Pasteur pipette and the disposable plastic transfer pipette. The glass Pasteur pipette features a long, narrow glass tube and requires a separate rubber bulb attached to the top end. This type is reusable, offers excellent chemical resistance, and can be sterilized for repeated use.
The disposable plastic transfer pipette is a single-piece unit, typically molded from polyethylene, incorporating the bulb directly into the body. These are favored for their safety, as they are shatterproof, and for convenience since they are discarded after a single use to prevent cross-contamination. While glass is better suited for volatile solvents that might react with plastic, the plastic version dominates in biological and general chemistry settings due to its low cost and disposability.
Technique for Safe and Effective Use
Proper technique begins before the pipette touches the liquid by first compressing the bulb to expel the air inside. The pipette tip is then submerged into the source liquid before the pressure on the bulb is slowly released to draw the fluid up into the tube. Drawing the liquid too quickly can cause air bubbles or cause the liquid to surge into the bulb, which can lead to contamination.
During the transfer step, the pipette should be held vertically to maintain a consistent column of liquid and minimize the effect of surface tension. When dispensing, the tip should be held against the inner wall of the receiving container, typically at a slight angle of 30 to 45 degrees. This allows the drops to run down the side of the vessel, ensuring the liquid is cleanly delivered and preventing a hanging drop on the tip.
Safety dictates that the tip of the dropper should never touch the source liquid container after the bulb has been squeezed. Furthermore, it should not be used to transfer between vessels without proper cleaning or disposal to avoid chemical cross-contamination. Always inspect the tip for cracks or chips, especially with glass versions, as these defects interfere with the formation of a uniform drop.
Accuracy and Limitations
A fundamental limitation of the standard dropper is its inherent lack of precision and accuracy for volumetric measurement. Unlike instruments such as volumetric pipettes or burettes, the transfer pipette is not calibrated to deliver a specific volume with high certainty. The volume of a single drop can vary significantly based on the pipette’s tip diameter, the liquid’s viscosity, and the user’s technique.
Therefore, these tools are unsuitable for quantitative analysis procedures, such as preparing standard solutions or performing accurate titrations, where even small volume errors can compromise the results. Droppers are best reserved for qualitative applications, including mixing, rough transfer of samples, or adding reagents when the instruction is simply to add a few drops. When an experiment requires high precision, a calibrated instrument with a distinct volume mark must be used instead.