The Burette: Master of Measurement
Anatomy of a Precision Instrument
To master the burette, you must first know its parts. Every component has a specific purpose in ensuring accurate liquid delivery. Imagine it as a specialized water gun for scientists, where every drop counts.
| Part Name | Description | Function |
|---|---|---|
| Graduated Tube | Long, cylindrical glass tube with fine markings. | Provides the scale for measuring liquid volume. Usually marked in milliliters (mL) and tenths of a mL. |
| Stopcock | A valve, often made of glass or PTFE[3], at the bottom of the tube. | Controls the flow of liquid. Turning the handle opens or closes the passage. |
| Tip | The narrow outlet below the stopcock. | Directs the liquid stream, allowing for drop-by-drop addition. |
| Burette Clamp | Metal clamp attached to a ring stand. | Holds the burette securely in a vertical, stable position. |
The Science of Reading the Meniscus
The most critical skill in using a burette is reading the volume correctly. Liquids in narrow glass tubes don't have a flat surface; they curve. This curve is called the meniscus. For water and most aqueous solutions, the meniscus is concave (curves upward). The rule is to always read the volume at the bottom of the meniscus, with your eye at the same level to avoid parallax error[4].
The volume of liquid dispensed is not simply the final reading. It is calculated by subtracting the initial reading from the final reading. The fundamental formula for any burette measurement is:
$V_{dispensed} = V_{final} - V_{initial}$
Where $V$ stands for volume. This simple subtraction is the heart of titration calculations.
A Step-by-Step Guide to Using a Burette
Proper procedure minimizes errors. Here is a standard sequence for setting up and using a burette in an experiment like a titration.
1. Rinsing & Filling: First, rinse the burette with a small amount of the solution you will be using (the titrant). This ensures any residual water does not dilute your solution. Then, use a funnel to fill the burette above the "0" mark. Carefully open the stopcock to let liquid fill the tip, removing any air bubbles.
2. Taking the Initial Reading: Slowly release liquid until the meniscus is exactly at or just below the 0.00 mL mark. This makes subtraction easier and reduces error. Record this precise initial volume.
3. The Titration Process: Place the flask containing the sample (analyte[5]) under the burette tip. While swirling the flask, open the stopcock to allow a steady stream of titrant. As you approach the expected endpoint (often signaled by a temporary color change), switch to adding drop by drop. Finally, add titrant half-drop by half-drop until the permanent color change (the endpoint) is reached.
4. Taking the Final Reading: Wait a few seconds for liquid on the walls to drain, then take your final meniscus reading. Now, apply the volume formula: $V_{dispensed} = V_{final} - V_{initial}$.
Putting It into Practice: The Vinegar Titration
Let's apply everything to a real-world example: determining the acidity of vinegar. Acidity is due to acetic acid ($CH_3COOH$). We can find its concentration by titrating it with a standard solution of sodium hydroxide ($NaOH$), a base. The burette will hold the $NaOH$ solution.
Scenario: You have a 0.1 M (molar) $NaOH$ solution in the burette. You measure 10.0 mL of vinegar into a flask and add an indicator[6] called phenolphthalein, which turns pink in base.
The Experiment: Your initial burette reading is 0.50 mL. You titrate until the vinegar solution turns a faint pink that lasts for 30 seconds. Your final burette reading is 42.60 mL.
Calculation:
First, find the volume of $NaOH$ used:
$V_{NaOH} = 42.60 \text{ mL} - 0.50 \text{ mL} = 42.10 \text{ mL}$.
Convert this to Liters: $42.10 \text{ mL} = 0.04210 \text{ L}$.
Using the formula for moles ($Moles = Molarity \times Volume$ in L):
$Moles_{NaOH} = 0.1 \text{ mol/L} \times 0.04210 \text{ L} = 0.004210 \text{ mol}$.
From the balanced chemical equation $CH_3COOH + NaOH \rightarrow CH_3COONa + H_2O$, we know the mole ratio is 1:1. So, moles of acetic acid = moles of $NaOH = 0.004210 \text{ mol}$.
This amount was in our 10.0 mL (0.0100 L) vinegar sample. Therefore, the concentration of acetic acid is:
$Concentration = \frac{0.004210 \text{ mol}}{0.0100 \text{ L}} = 0.421 \text{ M}$.
This precise calculation, enabled by the burette's accurate volume delivery, tells us the strength of the vinegar.
Burettes vs. Other Lab Tools
How does a burette compare to a graduated cylinder or a pipette? The key difference is precision and purpose.
| Tool | Best For | Precision | Key Feature |
|---|---|---|---|
| Burette | Dispensing variable, precisely measured volumes (e.g., Titration). | Very High (±0.05 mL) | Stopcock for controlled delivery; measures what is dispensed. |
| Graduated Cylinder | Measuring a fixed volume of liquid to be poured out. | Moderate (±1% of volume) | Easy to use; measures what it contains. |
| Volumetric Pipette | Transferring one exact, fixed volume (e.g., 25.00 mL). | Very High (±0.04 mL) | Single volume; "to deliver" (TD) design. |
Important Questions
A: Water molecules are attracted to glass more than to each other (adhesion > cohesion), pulling the water up the sides and creating a concave meniscus. For liquids like mercury, cohesion is stronger, creating a convex (domed) meniscus. For mercury, you would read the top of the meniscus. Always follow the specific rule for the liquid you are using.
A: The most common significant error is parallax error. This happens when your eye is not level with the meniscus, making the volume appear higher or lower than it actually is. To avoid it, always position yourself so the meniscus is at eye level. Another major error is not properly rinsing the burette with the solution to be used, which dilutes it and changes its concentration.
A: No. The stopcock material is key. A glass stopcock burette is suitable for most common solutions (acids, bases, salts). However, for strong alkaline solutions (like concentrated $NaOH$), a PTFE (Teflon) stopcock is necessary because strong bases can "freeze" a glass stopcock by reacting with it and fusing it shut.
Footnote
[1] Titration: A quantitative analytical technique where a solution of known concentration (titrant) is used to determine the concentration of an unknown solution (analyte). The point of complete reaction is called the endpoint.
[2] Stopcock: A valve that regulates the flow of liquid or gas from a pipe or container. In a burette, it is a tapered glass or PTFE plug that fits into a corresponding seat.
[3] PTFE: Polytetrafluoroethylene, a synthetic polymer known by the brand name Teflon. It is chemically inert and provides a smooth, non-sticking surface for stopcocks.
[4] Parallax Error: An error in reading an instrument because the observer's eye is not positioned perpendicularly to the scale and indicator (like the meniscus).
[5] Analyte: The substance whose concentration or composition is being determined in a chemical analysis. In a titration, it is the solution in the flask.
[6] Indicator: A substance that changes color in response to a chemical change (like pH). It is used to signal the endpoint of a titration.