A burette is a specialized piece of laboratory glassware designed for the highly accurate dispensing of liquid volumes in chemical analysis. This long, narrow tube allows chemists to measure and deliver specific amounts of a solution with a level of precision unmatched by general laboratory equipment like beakers or flasks. Its function is centered on quantitative analysis, where the exact volume of a reactant is a necessary factor for calculating the concentration of an unknown substance. The burette serves as a foundational instrument for any experiment that requires the controlled and measurable addition of a liquid reagent.
Physical Structure and Components
The burette’s design is optimized for its high-precision function, consisting primarily of a long, straight glass tube with a uniform inner diameter. Volume markings, or graduations, are etched along the length of the tube, typically allowing for readings with a precision of 0.1 milliliters or even finer increments. The zero mark is located at the top, and the volume measurements increase as the liquid level moves down the scale.
The flow of liquid is managed by a small valve called a stopcock, situated at the bottom of the tube just above the tapered tip. The stopcock can be precisely turned to control the rate of liquid addition, allowing for the delivery of a steady stream or even single, isolated drops. This controlled dispensing is the physical mechanism that enables the burette to achieve its high degree of accuracy in volume measurement.
Essential Role in Titration
The primary application for the burette in a chemistry laboratory is in the technique known as titration. Titration is an analytical method used to determine the unknown concentration of a substance (the analyte) by reacting it with a solution of known concentration (the titrant). The burette holds the titrant and allows for its gradual, measurable addition to the analyte solution until the reaction reaches completion.
The procedure relies on the precise determination of the volume of titrant required to completely react with the analyte. Because the calculation of the analyte’s concentration depends directly on this volume, the burette’s ability to dispense and measure liquid accurately is crucial. Without the controlled delivery and fine volume scale provided by the burette, the results of the concentration analysis would be unreliable.
Practical Steps for Accurate Measurement
Achieving the burette’s inherent precision requires careful preparation and technique. Before use, the burette must be cleaned with deionized water and then “conditioned” by rinsing it with a small amount of the solution it will dispense. This conditioning prevents dilution of the titrant. Once secured vertically in a stand, the burette is filled, ensuring the liquid level is above the zero mark and that no air bubbles are trapped in the tip below the stopcock.
The most demanding aspect of using a burette is accurately reading the liquid level, which involves observing the meniscus. The meniscus is the curved surface the liquid forms due to surface tension, and for most aqueous solutions, it curves downward (concave). To avoid parallax error, the chemist must position their eye exactly level with the liquid and read the volume from the bottom of this curved surface.
The volume of liquid delivered is determined by taking the difference between the initial volume reading and the final volume reading after the titration is complete. Readings are typically recorded to two decimal places, estimating the final digit between the 0.1 milliliter markings. This meticulous process ensures the dispensed volume is measured with the highest degree of accuracy.