A burette is the core glass instrument in titration analysis, used to accurately measure the volume of standard solution consumed during titration. Correctly operating the burette and effectively removing air bubbles from the tip are key to ensuring accurate titration results. This article details burette types, debubbling methods, titration techniques, and precautions.
Removing air bubbles from the burette tip is the most critical and error-prone step. Bubbles cause the reading to be too high, making the titration result too high. The standard debubbling operation is: tilt the burette about 45°, quickly open the stopcock to let liquid rush out of the tip and carry the bubble out.
1. Burette Types and Selection
1.1 Classification by Stopcock Type
- Glass Stopcock Burette: The stopcock is glass and integrated with the barrel. Suitable for non-alkaline solutions (alkaline solutions corrode glass, causing it to stick).
- PTFE (Teflon) Stopcock Burette: The stopcock is PTFE, resistant to acid and alkali corrosion. Suitable for all types of solutions, including alkaline solutions.
- Pinch-cock Burette (alkaline burette): No glass stopcock; liquid flow is controlled by a rubber tube and glass bead. Suitable for alkaline solutions.
1.2 Classification by Scale Direction
| Type | Scale Direction | Zero Position | Use Case |
|---|---|---|---|
| Standard Burette | 0 at top, full at bottom | Upper end (0.00 mL) | Acid-base titration, redox titration |
| Reverse-scale Burette | 0 at bottom, full at top | Lower end (0.00 mL) | Special titration (e.g. Karl Fischer) |
1.3 Common Specifications
Common burette specifications in the laboratory: 10mL, 25mL, 50mL. The 50mL burette is most commonly used, with a minimum division value of 0.1mL, readable to 0.01mL.
2. How to Remove Air Bubbles from Burette Tip (Critical Step!)
If there are air bubbles in the burette tip, the bubbles will be carried out by the liquid during titration, making the reading too high and the titration result too high. Therefore, you must check and remove bubbles after filling the burette.
Fill the standard solution into the burette, open the stopcock, let the liquid flow out, and lower the liquid level to near the zero mark. At this point, observe whether there are bubbles in the tip.
Hold the upper end of the burette with one hand, tilt the burette about 45° so that the tip points upward (note: not downward!).
With the other hand, quickly open the stopcock to let the liquid rush out of the tip rapidly. The impact of the liquid will carry the air bubbles out of the tip. Repeat 2~3 times until there are no bubbles in the tip.
After debubbling, vertically clamp the burette on the burette stand, adjust the liquid level to the zero mark (0.00 mL), wait 10~15 seconds before reading (to let the liquid in the tip completely drain out).
1. For alkaline burettes (pinch-cock type), when debubbling, bend the rubber tube upward, then squeeze the area near the glass bead to let liquid rush out of the tip.
2. If bubbles reappear repeatedly, the burette tip may be dirty or damaged. Clean or replace the burette.
3. After debubbling, check again for bubbles in the tip (observe against light).
3. Titration Techniques (Improve Accuracy)
Technique 1: Left hand operates stopcock, right hand swirls the flask
The standard operation is: left hand controls the burette stopcock (or squeezes the rubber tube), right hand swirls the conical flask. This allows precise control of droplet speed while fully mixing the solution.
Technique 2: Fast dropping in early titration stage
In the early stage of titration (far from the endpoint), you can drop faster (3~4 drops per second) to save time. But near the endpoint, you must add drop by drop or half-drop at a time.
Technique 3: Rinse the flask wall with wash bottle before endpoint
When the solution color starts to change but is not yet stable (approaching the endpoint), use distilled water (wash bottle) to rinse the inner wall of the conical flask to wash down splashed liquid on the wall and ensure the reaction is complete.
Technique 4: Endpoint judgment should be "light color and persistent"
Taking phenolphthalein indicator as an example: at the endpoint, the solution changes from colorless to light pink, and does not fade within 30 seconds. If the pink color is too deep, it means over-titration; repeat the experiment.
Technique 5: Eye level with liquid surface when reading
After titration ends, keep the burette vertical, eye level with the liquid surface, read the scale corresponding to the lowest point of the meniscus. The reading should be estimated to 0.01mL.
4. Burette Usage Precautions
- Burette must be calibrated before use: New or long-unused burettes should be sent to a measuring agency for calibration to ensure volume accuracy.
- Rinse with the titrant before filling: Rinse 2~3 times with a small amount of standard solution to avoid water in the tube diluting the solution.
- Alkaline solutions cannot use glass stopcock burettes: Alkali corrodes glass, causing the stopcock to stick. Use PTFE stopcock or pinch-cock burettes.
- Titration speed should not be too fast: Too fast can cause local over-titration, making the endpoint appear early and the result low.
- Burette must be vertically clamped: Tilting causes reading errors and may prevent liquid from fully draining.
- Wait before reading after titration: After titration ends, liquid may still be hanging at the tip or draining slowly from the inner wall. Wait 10~15 seconds before reading.
- Avoid damaging the burette tip: A damaged tip causes unstable liquid flow and affects precision. Handle carefully and replace promptly.
5. Frequently Asked Questions (FAQ)
• Phenolphthalein: Colorless → light pink (and does not fade within 30 seconds)
• Methyl orange: Red → yellow
• Starch indicator (iodometry): Blue appears and does not fade within 30 seconds
The endpoint color should be light and persistent; too deep means over-titration.