A burette is a specialized piece of laboratory glassware designed for dispensing variable and precisely measured volumes of liquid. This long, slender tube is a high-precision volumetric instrument, distinguishing it from general measuring tools like beakers or graduated cylinders. Its design allows scientists to deliver a highly controlled amount of liquid, which is fundamental for accurate quantitative analysis in chemistry. Precision is achieved through its narrow diameter and fine graduation marks, enabling readings to a fraction of a milliliter.
Standard Burette Capacities
The capacity of a burette refers to the maximum volume of liquid it is designed to hold and dispense. The standard volumetric burette in most chemistry laboratories has a maximum capacity of 50 milliliters (mL). Other common sizes include 25 mL and 10 mL burettes, with smaller micro-burettes available for micro-scale work. The capacity size is selected based on the total volume expected to be used in an experiment.
The tube is marked with fine graduation lines that allow for granular volume measurements. A typical 50 mL burette is subdivided into increments of 0.1 mL. This level of subdivision enables a trained user to estimate the volume to an extra decimal place, allowing for a reading precision of plus or minus 0.05 mL.
The Primary Application of Burettes
The main reason burettes are engineered for precision is their primary application in titration. Titration is a chemical 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 and delivers the titrant solution in a controlled manner.
The process begins by filling the burette with the titrant and setting the initial volume reading. The titrant is then slowly added, drop by drop, into a flask containing the analyte. This controlled addition is made possible by a stopcock, a valve at the bottom of the burette that regulates the flow rate. The stopcock allows the flow to be stopped instantly or reduced to a trickle of single drops.
As the titrant is added, the reaction progresses until it reaches the endpoint. This endpoint is typically signaled by a sudden color change in the analyte solution, often due to an added chemical indicator. The total volume of titrant dispensed up to this point is the exact volume required for the reaction to complete.
The ability to precisely control the addition of titrant makes the burette irreplaceable for this analytical technique. Once the endpoint is reached, the final volume reading is recorded. The difference between the initial and final readings provides the exact volume of the known solution used, allowing the original concentration to be calculated.
Ensuring Measurement Accuracy
Achieving the high level of accuracy requires meticulous attention to technique and preparation. One fundamental step is correctly reading the liquid level by observing the meniscus. The meniscus is the curved surface formed by the liquid in the glass tube, and the reading must be taken at the very bottom of this curve for most solutions.
A common source of error is parallax, which occurs if the user’s eye is not level with the meniscus when reading. If the eye is positioned too high or too low, the line of sight intersects the volume scale inaccurately. To prevent this, the user must ensure their eye is perfectly aligned horizontally with the bottom of the meniscus.
Before dispensing liquid, both an initial and a final reading must be taken to determine the volume delivered. Although the initial volume is often adjusted to 0.00 mL for convenience, the volume dispensed is determined by the difference between the final and initial readings. This subtraction method isolates the exact volume consumed during the experiment.
Preparation of the glassware is important for maintaining precision. Before use, the burette must be clean, evidenced by the absence of water droplets clinging to the inner walls. It must also be conditioned by rinsing the interior two or three times with small portions of the titrant solution itself. This conditioning prevents residual water from diluting the titrant, which would compromise the measurement accuracy.