A buret is a specialized piece of laboratory glassware engineered for the precise dispensing of variable volumes of liquid in chemical analysis. It is a long, graduated tube used to deliver a highly controlled amount of one solution into another, primarily for quantitative experiments. This instrument is fundamental to volumetric analysis, where determining the concentration of an unknown substance depends on the accurate measurement of the volume of a known reagent added. The buret’s design provides greater accuracy than general-purpose containers like graduated cylinders.
Structure and Core Function
The typical buret consists of a long glass tube, or barrel, featuring fine volume markings along its length. These graduations are read in a descending manner, with the zero mark located at the top, opposite to most other volumetric glassware. At the bottom is a valve called a stopcock, which provides minute control over the liquid’s flow. The stopcock connects to a narrow dispensing tip, ensuring the liquid is delivered in a controlled stream or single drops.
The core function of the buret is its use in titration, a common analytical chemistry technique. During titration, a solution of known concentration, called the titrant, is placed inside the buret and slowly added to a second solution, the analyte, until a reaction endpoint is reached. The buret’s precise graduations permit measurement of the exact volume of titrant required to complete the reaction. This measurement allows for the calculation of the analyte’s concentration. Standard volumetric burets commonly hold a maximum volume of 50.00 mL, with readings estimated to the nearest hundredth of a milliliter.
Proper Usage Techniques
Preparation ensures the dispensed solution is not contaminated or diluted before the experiment begins. The glassware must first be cleaned, then rinsed two or three times with small portions of the titrant solution itself, a process known as conditioning. Conditioning prevents residual water droplets from diluting the titrant, which would change its concentration and introduce systematic error.
After conditioning, the buret is filled slightly above the zero mark, often using a small funnel, which should be removed immediately. Before taking the initial volume reading, the dispensing tip must be completely filled with liquid and contain no air bubbles, a process known as purging. Trapped air bubbles can later escape and be incorrectly measured as dispensed volume, leading to inaccurate results.
The flow of titrant is controlled by manipulating the stopcock, dispensing the liquid into the receiving flask. As the titration nears its endpoint—signaled by a color change from an indicator—the flow must be slowed to a drop-by-drop rate. This dropwise addition is necessary to avoid overshooting the reaction endpoint, which would invalidate the measurement.
Ensuring Measurement Accuracy
The buret is designed for high precision; Class A burets typically have a tolerance as low as +/- 0.05 mL for a 50 mL capacity. Achieving this accuracy depends heavily on the correct technique for reading the liquid level. The most common source of error is parallax, which occurs if the user’s eye is not level with the liquid’s surface when taking a reading.
To counteract parallax, the eye must be positioned exactly at the same horizontal level as the meniscus, the curved surface of the liquid. For most solutions, the reading is taken from the very bottom of this curve. Reading from above or below the meniscus will cause the volume to be incorrectly recorded, directly impacting the calculated dispensed volume.
The buret must be mounted perfectly vertically on a stand to ensure volume markings are correctly aligned relative to the liquid surface. After the final volume has been dispensed, a brief waiting period, typically about 30 seconds, is advised before taking the final measurement. This delay allows liquid clinging to the internal walls to drain down, preventing the measurement from being erroneously low. The true volume dispensed is calculated by subtracting the initial volume reading from the final volume reading.