A buret is a specialized piece of laboratory glassware engineered for the highly accurate measurement and dispensing of liquid volumes. It typically consists of a long, calibrated glass tube with a valve at the bottom, allowing chemists to add small, controlled amounts of liquid to a reaction mixture. Its primary function is to provide a variable but highly measurable volume, making it a standard tool in quantitative chemical analysis where exact concentrations must be determined.
Anatomy and Precision Features
The buret’s design centers on its long, narrow glass tube, which is precisely graduated, often marked in increments of 0.10 milliliters (mL) for standard 50 mL burets. This narrow bore is intentional; it ensures that a small vertical distance on the scale corresponds to a very small change in dispensed volume, maximizing the precision of the reading. The volume markings read from zero at the top to the maximum volume at the bottom, indicating the volume dispensed rather than the volume contained.
Control over the liquid flow is maintained by a stopcock, a specialized valve located near the bottom of the tube. Modern burets often feature a PTFE (Teflon) plug stopcock that does not require lubrication and resists chemical etching. By manipulating this valve, the operator can regulate the flow from a steady stream to a single, minuscule drop, which is then delivered through the fine glass tip. This combination allows the buret to achieve a precision typically around \(pm 0.02\) to \(pm 0.05\) mL, significantly surpassing the accuracy obtainable with common measuring pipettes or graduated cylinders.
The Core Process: Titration
The most common application for the buret is in a procedure known as titration, a volumetric technique used to determine the unknown concentration of a substance. Titration works by slowly adding a solution of known concentration, called the titrant, to a solution of unknown concentration, known as the analyte. The two solutions are mixed until the reaction between them is complete.
The reaction completion, or the equivalence point, is typically signaled by a sudden color change in the analyte solution, often achieved by adding a chemical indicator. The exact point where the indicator changes color is called the endpoint, which is intended to closely match the theoretical equivalence point.
The calculation of the unknown concentration relies on the exact volume of titrant required to reach this endpoint. Even a difference of \(0.05\) mL in the dispensed volume can introduce a noticeable percentage error into the final calculated concentration.
The operator must use the stopcock to add the titrant drop-wise as they approach the expected endpoint, allowing the reaction to fully complete before the next drop is added. The volume recorded from the buret represents the minimum amount needed to neutralize or react completely with the analyte, providing the most accurate possible concentration value.
Essential Steps for Accurate Measurement
Achieving the buret’s maximum inherent accuracy requires following a specific set of preparatory steps, beginning with a process known as conditioning. Before the first measurement, the buret must be rinsed three times with small portions of the titrant solution itself, rotating the tube to coat the entire interior surface. This step removes any residual water or contaminants that might dilute the titrant, ensuring the concentration remains uniform throughout the experiment.
Once conditioned, the buret is filled above the zero mark, and the stopcock is briefly opened to allow the liquid to flow rapidly through the tip to eliminate any air bubbles trapped below the stopcock. A bubble released during the titration would lead to an inaccurate volume reading. The initial volume is then adjusted precisely to the zero mark or another convenient starting volume.
The reading of the liquid level must be performed meticulously to account for the meniscus, which is the curved surface formed by the liquid in the narrow tube. Because most aqueous solutions “wet” the glass, the liquid surface curves downward; the volume is always read by aligning the bottom of this curve with the graduation mark at eye level. Reading from above or below the meniscus introduces parallax error, which compromises the measurement.
During the titration, the stopcock is operated with one hand to control the flow rate, while the other hand gently swirls the receiving flask to ensure thorough mixing. As the color change approaches, the operator slows the addition to a single drop at a time, sometimes using a wash bottle to rinse a hanging partial drop into the solution, confirming the exact volume needed to reach the endpoint.
Cleaning and Storage
The buret should be cleaned immediately after use to prevent etching of the glass or staining of the interior walls. Cleaning typically involves rinsing with deionized water, and occasionally with a mild detergent solution, ensuring no residue remains to affect future measurements.
For storage, the buret should be clamped vertically. If it uses a glass stopcock, the plug should be removed or loosened to prevent seizing. The fine glass tip is the most fragile part and must be protected from accidental impact to avoid chipping, which would compromise the precision of drop formation.