A buret is a specialized piece of volumetric glassware designed for the accurate dispensing of a specific volume of liquid. Its primary application is in titration, a precise laboratory method used to determine the unknown concentration of a substance by reacting it with a solution of known concentration. The validity of the final analytical result depends entirely on the accuracy with which the titrant solution is measured and delivered. Therefore, meticulous preparation of the buret is a non-negotiable step to ensure the integrity and reliability of the quantitative analysis.
Initial Cleaning and Inspection
Before chemical preparation can begin, the buret must be physically clean, as residual dirt or grease can severely compromise volume measurements. The initial step involves washing the glassware with a mild laboratory detergent and tap water to remove contaminants. Following the detergent wash, the buret must be rinsed multiple times with tap water to eliminate all soap residue.
The cleanliness of the inner glass surface is assessed by observing how water drains down the walls after rinsing with distilled or deionized water. If the water beads up or forms droplets rather than sheeting down in a uniform film, it indicates the presence of hydrophobic contaminants like grease, a phenomenon known as “water breaks.” If water breaks are observed, the cleaning process must be repeated until the inner surface maintains a continuous, unbroken film of water.
The mechanical function of the stopcock valve, which controls the flow of liquid, requires close inspection. The valve should turn smoothly without excessive force or sticking, allowing for fine control over the drop rate. The stopcock must also be tested for leaks by filling the buret with water and observing the tip over a period of a few minutes. A leaking stopcock will lead to an inaccurate volume measurement and must be addressed before proceeding.
Conditioning the Buret
Once the buret is physically clean and leak-free, it must be conditioned with the specific titrant solution intended for use in the experiment. This chemical preparation prevents the dilution of the titrant that would occur if the buret walls were still coated with rinse water. Any remaining water film would mix with the concentrated titrant, effectively lowering the actual concentration of the solution delivered.
Conditioning is performed by introducing a small volume, typically between five and ten milliliters, of the standardized titrant into the clean buret. The stopcock is closed, and the buret is carefully removed from its stand and held nearly horizontal. The technician slowly rotates the buret, ensuring that the titrant solution makes contact with and coats every square millimeter of the inner glass surface.
This rotational coating process allows the titrant to saturate the walls and displace the last traces of water, ensuring that the concentration of the solution delivered in the titration is accurate. After coating the walls, the small volume of titrant is drained completely through the tip of the buret into a waste beaker, effectively flushing the tip and stopcock assembly. This entire conditioning procedure is typically repeated two or three times to guarantee that the internal surface is fully equilibrated with the concentration of the titrant solution.
Proper conditioning is a necessity for achieving the high degree of precision expected from volumetric analysis. The act of rinsing with the titrant ensures that the delivered volume corresponds precisely to the volume of the known-concentration solution, maintaining the integrity of the subsequent calculations. By eliminating the variable of water-induced dilution, the results obtained from the titration can be considered reliable for determining the unknown concentration.
Loading and Zeroing the Titrant
With the buret thoroughly conditioned, the final steps involve loading the titrant and setting the initial volume measurement to zero. The titrant solution is introduced into the buret using a clean, dry funnel, which helps prevent spillage and ensures the liquid flows smoothly down the glass walls. The solution should be filled well above the zero milliliter mark to provide sufficient volume for the subsequent adjustment.
The funnel must be removed immediately after filling, as residual drops clinging to the funnel’s stem could fall into the buret later, artificially increasing the volume. Before setting the final volume, the buret tip must be checked for the presence of air bubbles lodged below the stopcock valve. Trapped air must be expelled by rapidly opening the stopcock to allow a short burst of liquid to flow through, as these bubbles would displace liquid and lead to an erroneously high volume reading.
Once the air is removed, the solution level is adjusted precisely to the zero mark. This is achieved by slowly opening the stopcock and dispensing the excess liquid into a waste container. The bottom of the curved liquid surface, known as the meniscus, must be exactly tangent to the 0.00 mL calibration line. Reading the meniscus at eye level prevents parallax error, ensuring the volume reading is accurate.
The buret is now prepared, containing a known-concentration solution ready for accurate and precise dispensing. The final volume reading is recorded as the starting point, and the titration can commence with the assurance that the volume delivered will accurately reflect the chemical reaction.