A burette is a precision volumetric glass instrument designed for the highly accurate dispensing of liquid reagents in a laboratory setting. Its primary application is in titration, a quantitative chemical analysis method used to determine the concentration of an unknown substance. Unlike glassware that measures a fixed volume, the burette allows for the delivery of variable amounts of liquid, known as the titrant, with exceptional control. It is engineered to measure the exact volume delivered with a typical precision of 0.01 to 0.05 milliliters, making it indispensable for reliable analytical chemistry procedures.
The Physical Structure and Design
The burette is a long, narrow glass tube calibrated for precise volume measurement. Its elongated design ensures that a small change in liquid height corresponds to a minimal change in volume delivered, enhancing accuracy. The tube is typically made of borosilicate glass, chosen for its chemical inertness and low thermal expansion, which keeps the calibration consistent.
The tube features a finely etched graduated scale where the zero mark is located at the top, unlike measuring cylinders. Volume markings increase as they descend toward the tip, reflecting the volume dispensed. The volume delivered is calculated by taking the difference between the initial and final liquid level readings.
At the lower end, the burette tapers into a finely bored glass or plastic tip that restricts the liquid flow rate. This narrow opening allows the titrant to be delivered in small, controllable drops, enabling the analyst to approach the reaction endpoint precisely. The bore size often allows for a delivery rate of approximately 0.05 milliliters per drop.
Liquid flow is regulated by a valve known as a stopcock, positioned just above the tip. Modern burettes often utilize a Polytetrafluoroethylene (PTFE) stopcock, which requires no lubrication and offers excellent chemical resistance. Older or specialized glass stopcocks, however, require a thin layer of grease to create a seal and prevent leakage, ensuring the measured volume is exactly the volume delivered.
Essential Preparation Steps
Before use, the burette must undergo preparation to ensure the delivered volume is accurate and the reagent concentration is unchanged. The first step involves thorough cleaning to remove residual chemicals or contaminants. This is typically done using detergent and distilled water, ensuring the inner glass surface drains uniformly without leaving droplets.
Following the wash, a conditioning rinse is performed using the actual titrant solution. A small volume is added, rotated to coat the inner surface, and drained through the tip. This process removes any remaining water clinging to the glass, preventing dilution of the titrant and avoiding systematic error.
The cleaned burette is securely mounted vertically to a retort stand using a clamp. The titrant is poured using a funnel to fill the instrument above the zero mark, taking care to avoid splashing or introducing air bubbles. The funnel is then removed to prevent drops from falling into the solution during the procedure.
Before zeroing the volume, any air bubbles trapped in the tip below the stopcock must be removed. These bubbles displace liquid, leading to an inaccurate reading of the dispensed volume. The bubble is usually dislodged by rapidly turning the stopcock to allow a quick burst of liquid flow, ensuring the tip is completely filled.
Finally, the liquid level is set to the zero mark, or a clearly readable initial volume, by slowly draining the excess titrant. The reading is taken by bringing the eye level parallel to the bottom of the meniscus (the lowest point of the curved liquid surface). This alignment prevents parallax error, ensuring the initial volume is recorded accurately before the titration begins.
Operating the Burette for Precise Volume Delivery
Once prepared and filled, volume delivery begins by carefully manipulating the stopcock to introduce the titrant into the receiving flask containing the analyte. Initially, the stopcock can be opened for a steady stream of titrant to quickly advance the reaction. As the reaction progresses and approaches the expected volume, the flow rate must be drastically reduced to ensure precision.
The receiving flask must be continuously swirled to ensure complete mixing of the titrant with the analyte solution. This constant agitation prevents localized concentration gradients that could lead to a premature endpoint indication. The addition rate is slowed to a dropwise pace, sometimes even half-drops, as the color change begins to linger longer before disappearing.
The endpoint is reached when a single drop of titrant causes a permanent change in the color of the analyte solution. This visual cue, often mediated by an added chemical indicator, signals that the exact stoichiometric amount of titrant has been added. The care taken with the final drops directly determines the accuracy of the analytical procedure.
As soon as the endpoint is achieved, the stopcock is closed to halt the flow of liquid, and the final volume reading must be taken without delay. The eye is positioned level with the bottom of the meniscus to avoid parallax error. This final reading is typically recorded to two decimal places, reflecting the high precision of the glassware.
The total volume of titrant delivered is calculated by subtracting the initial volume reading from the final volume reading. For example, if the initial reading was 0.00 milliliters and the final reading was 24.85 milliliters, the dispensed volume is 24.85 milliliters. This calculated volume is then used in stoichiometric calculations to determine the unknown concentration of the analyte.
To ensure reliability, the titration procedure is usually repeated several times, often in triplicate, to obtain concordant volumes. Concordant results agree within a narrow range (typically 0.05 to 0.10 milliliters), confirming that the precise volume required for the reaction has been consistently measured.