A buret is a precision-calibrated glass tube equipped with a stopcock, designed for the highly accurate dispensing of variable liquid volumes. It is the standard instrument for volumetric analysis, primarily used in titration. Titration involves adding a solution of known concentration (the titrant) to a solution of unknown concentration until the chemical reaction is complete. This process allows for the exact determination of the unknown concentration. The buret’s fine graduations allow the dispensed volume to be measured with accuracy up to 0.05 mL.
Preparing the Equipment
Achieving precise titration results requires impeccably clean glassware, as residual chemicals or water can contaminate the titrant. The buret must first be thoroughly washed, typically using a laboratory detergent and a long-handled brush. Following the detergent wash, rinse it multiple times with tap water, and then perform a final three rinses with distilled or deionized water.
The standard test for cleanliness is observing the drainage: if water runs down the interior wall without leaving clinging droplets, the surface is clean. Once clean, securely mount the buret in a specialized clamp on a stand. Ensure it is perfectly vertical to prevent parallax errors during volume readings.
The stopcock must be checked for smooth operation and potential leaks before adding the titrant. For glass stopcocks, a thin layer of grease might be applied, but Teflon stopcocks require no lubrication. Place a waste beaker directly beneath the tip to catch initial rinses and any liquid dispensed during preparation.
Filling, Rinsing, and Zeroing the Buret
After cleaning, the buret must be rinsed with the actual titrant solution to prevent dilution from residual water droplets. Add a small volume (typically 3 to 5 mL) of the titrant, then tilt and rotate the buret horizontally so the solution coats the entire inner surface. Drain the solution through the stopcock into the waste beaker, repeating this process two or three times to ensure the concentration inside the buret is uniform.
Fill the buret using a funnel, pouring the titrant until the liquid level is slightly above the \(0.00 \text{ mL}\) mark. A crucial step for accuracy is the complete removal of any air bubbles trapped in the tip below the stopcock. If an air bubble escapes during titration, the volume it occupied will be incorrectly measured as dispensed liquid.
To remove the bubble, open the stopcock quickly and briefly to flush a stream of solution out of the tip into the waste beaker. Once the tip is full of liquid and free of bubbles, adjust the solution level. The bottom of the meniscus should rest either exactly on the \(0.00 \text{ mL}\) line or at a clearly readable value below it, which serves as the initial volume reading.
Executing the Titration and Reading the Volume
The titration process requires precise control over titrant delivery, managed by the stopcock. Manipulate the stopcock with one hand while continuously swirling the receiving flask containing the analyte and indicator with the other. This swirling ensures the titrant is immediately and completely mixed into the analyte solution, which is essential for observing the precise endpoint.
Initially, the titrant can be added relatively quickly. As the reaction proceeds and the endpoint nears, the addition rate must be significantly slowed. The approach to the endpoint is signaled by a temporary color change in the analyte solution that disappears upon swirling. At this stage, the titrant should be added dropwise, allowing each drop to mix completely before the next is added.
The endpoint is reached when the color change from the indicator persists after swirling, indicating the reaction is complete. To achieve maximum accuracy, employ a “half-drop” technique. Carefully turn the stopcock to form a small drop on the tip, touch the tip to the side of the flask, and wash the partial drop down with distilled water.
After the endpoint is reached, take the final volume reading with the eye positioned exactly level with the meniscus to eliminate parallax error. For most aqueous solutions, the reading is taken from the bottom of the concave meniscus curve. Using a piece of white paper or a specialized black card held behind the buret can sharpen the contrast, making the meniscus easier to align with the graduation marks. The final volume is estimated and recorded to the nearest \(0.02 \text{ mL}\) or \(0.05 \text{ mL}\). The volume of titrant dispensed is calculated by subtracting the initial reading from this final reading.
Maintaining Accuracy and Post-Use Care
Several factors can compromise titration accuracy.
Sources of Error
- Temperature changes, which affect solution density and volume.
- Contamination caused by dirty glassware.
- Parallax error, resulting from reading the meniscus from an angle other than eye level.
- A leaking stopcock or an overlooked air bubble in the tip, which causes the dispensed volume to be greater than the measured change.
Immediately after titration, the buret must be emptied and rinsed to prevent residue or etching on the glass surface. This is important when using strong basic solutions, which can react with the glass over time. Rinse the buret thoroughly multiple times, first with tap water and then with distilled water, ensuring the stopcock and tip are flushed clear.
For long-term storage, the cleaned buret should be completely dried, often by inverting it on a stand or in a dedicated rack. The stopcock should be left open or removed entirely to prevent it from seizing or becoming stuck. Proper post-use care ensures the buret maintains its precision and is ready for the next experiment.