A buret (or burette) is a specialized piece of laboratory glassware used in analytical chemistry to dispense highly precise, variable amounts of liquid reagent. It is a long, calibrated tube that allows chemists to measure and deliver specific volumes of a solution with a high degree of control and accuracy. Its core purpose is to enable quantitative chemical reactions where the exact volume of one reactant needed to consume another must be carefully determined.
Defining the Buret and Its Structure
The buret is typically constructed from a long glass tube featuring a uniform internal diameter and an etched volumetric scale. Unlike most measuring vessels, the buret is calibrated with the zero mark at the top, and the volume markings increase downward toward the tip. A standard buret has a capacity of 50.00 milliliters (mL) with graduations often marked every 0.1 mL.
Controlling the flow of liquid is a valve called a stopcock, located at the tapered bottom end of the tube. This stopcock can be a glass or Polytetrafluoroethylene (PTFE) plug, which is rotated to regulate the release of the liquid, known as the titrant, from a narrow opening called the tip. The narrow tip facilitates the controlled, drop-by-drop delivery of the liquid required for precise reactions.
The Primary Function: Titration
The buret’s main application is in a quantitative chemical analysis technique called titration, which is used to determine the concentration of an unknown solution. In this process, a solution of precisely known concentration, called the titrant, is carefully added from the buret to a measured sample of the unknown solution, called the analyte. The buret allows the chemist to track the exact volume of the titrant added during the reaction.
Titration relies on observing a clear signal, usually a color change, that marks the completion of the chemical reaction between the titrant and the analyte. This observable signal is called the endpoint, which is designed to occur as close as possible to the equivalence point, the theoretical point where the reactants have completely neutralized each other. The buret permits the slow, incremental addition of the titrant to ensure the endpoint is not inadvertently passed. By measuring the volume required to reach this point, the concentration of the unknown analyte can be calculated.
Operating the Buret
Effective use of the buret begins with proper setup and preparation to ensure accurate volume delivery. The buret is first secured vertically to a stand using a specialized buret clamp. Before adding the titrant, the buret must be thoroughly cleaned and then rinsed two or three times with small portions of the titrant solution itself to prevent dilution or contamination by residual water.
The titrant is carefully poured into the buret, often using a funnel, until the liquid level is above the zero mark at the top of the scale. A critical step is purging the tip, which involves opening the stopcock briefly to allow liquid to flow out and completely fill the narrow space below the stopcock. This action removes any trapped air bubbles. Once the tip is clear of bubbles, the initial volume reading is recorded before the titration begins.
Ensuring Measurement Accuracy
The high precision of the buret stems from its fine calibrations and the technique used for reading the liquid level. Standard volumetric burets have a precision that allows readings to be recorded to the nearest 0.01 mL, significantly more accurate than glassware like graduated cylinders. To achieve this level of precision, the volume must be read from the bottom of the curved liquid surface, known as the meniscus.
It is necessary to position the eye exactly level with the meniscus when taking a reading to avoid parallax error, which is a distortion caused by viewing the liquid level from an angle. A small card with a dark line placed behind the buret can help make the meniscus more visible and easier to read accurately against the etched scale. The final volume is read using the same careful technique, and the difference between the initial and final volumes yields the exact volume of titrant delivered.