Mastering the use of a burette and pipette is a foundational skill in any chemistry laboratory. These precision measuring instruments are indispensable for quantitative chemical analysis, specifically for titrations and accurate volume transfers. Understanding the principles and proper handling of these tools is critical for obtaining reliable and reproducible experimental results.
Understanding the Burette and Pipette
Both the burette and pipette are designed to measure and deliver specific volumes of liquid with high accuracy. However, they serve different purposes and operate on different principles.
The Pipette: Precision Dispensing
A pipette (or pipet) is a laboratory tool used to transport a measured volume of liquid, often as a media dispenser. It comes in several types, but the most common are:
- Volumetric Pipette: Designed to deliver a single, fixed volume with the highest accuracy. It has a single graduation mark and a pear-shaped bulb.
- Mohr Pipette (Graduated Pipette): A straight tube with graduation marks, allowing for the measurement and delivery of variable volumes.
Pipettes are typically used to transfer a known volume of a sample or reagent into an Erlenmeyer flask or beaker for mixing or reaction.
The Burette: Controlled Dispensing
A burette is a long, graduated glass tube with a stopcock or valve at its lower end. It is used to dispense variable, but accurately known, amounts of a liquid. Burettes are most commonly associated with titration experiments, where a titrant of known concentration is slowly added to an analyte of unknown concentration to determine the equivalence point.
The Standard Titration Setup
The classic experiment utilizing both a burette and a pipette is a titration, specifically an acid-base titration. This experiment determines the concentration of an unknown acid or base.
Step-by-step procedure:
- Preparation and Cleaning: Rinse the burette with a small amount of the titrant (e.g., standard NaOH solution) and the pipette with the analyte (e.g., HCl solution). This prevents dilution of the reagents.
- Filling the Pipette: Using a pipette filler bulb, draw the analyte up slightly above the calibration mark. Slowly release the liquid until the bottom of the meniscus sits exactly on the mark.
- Transferring the Analyte: Drain the pipette's contents into a clean Erlenmeyer flask. Allow it to drain completely, but do not blow out the last drop (for volumetric pipettes).
- Adding an Indicator: Add a few drops of a suitable indicator to the flask. For an acid-base titration, phenolphthalein is commonly used, which is colorless in acid and pink in base.
- Filling the Burette: Fill the burette with the titrant solution. Record the initial volume reading at the bottom of the meniscus.
- Performing the Titration: Slowly open the stopcock to add titrant to the flask while swirling the flask continuously. As the endpoint approaches (indicated by a temporary pink color that disappears upon swirling), add the titrant drop by drop.
- Recording the Endpoint: The endpoint is reached when a permanent pale pink color persists for at least 30 seconds. Record the final volume reading on the burette.
Calculating Concentration from Titration Data
The data gathered from the burette readings and the known volumes/molarities are used to calculate the unknown concentration. The core principle is the stoichiometry of the reaction.
For a strong acid-strong base reaction, the relationship is:
Macid * Vacid = Mbase * Vbase
Where M is molarity and V is volume.
To ensure accuracy, the titration experiment should be repeated until concordant results are obtained (typically at least three trials where the volume readings agree within ±0.05 mL).
| Calculation Component | Description |
|---|---|
| Average Titrant Volume | The mean volume of titrant used across the concordant trials. |
| Calculated Molarity | The unknown concentration calculated using the average volume and the known concentration of the titrant. |
Critical Best Practices for Precision
Achieving accurate results requires meticulous technique. Ensure no air bubbles are present in the burette tip before starting, as they can lead to an overestimation of the volume dispensed. Always read the meniscus at eye level to avoid parallax error. Finally, proper cleaning of both the burette and pipette is paramount to prevent cross-contamination that could skew your results.
