Preparing capillary tubes primarily involves sealing one end using heat, typically from a Bunsen burner, to create a closed container for various laboratory analyses, such as melting point determination.
Capillary tubes are slender, thin-walled glass tubes with a very small internal diameter, widely used in scientific laboratories for tasks requiring precise handling of minute liquid or solid samples. Their preparation often involves sealing one end to contain the sample securely. This guide focuses on the most common preparation method: sealing one end with heat, and then preparing them for specific applications like melting point analysis.
Sealing One End of a Capillary Tube
Sealing one end of a capillary tube is a fundamental step for many applications, creating a closed bottom to hold a sample. This is typically done using a heat source.
Method 1: Using a Bunsen Burner or Microburner
This is the most common and effective method for sealing glass capillary tubes in a laboratory setting.
Materials Needed:
- Capillary tubes: Unsealed, open at both ends.
- Bunsen burner or microburner: A controlled flame source.
- Heat-resistant mat or stand: For safety.
- Safety goggles: Essential for eye protection.
- Gloves (optional): To keep tubes clean and avoid fingerprints.
Step-by-Step Sealing Process:
- Prepare Your Workspace: Ensure your work area is clear, and the Bunsen burner is connected to a gas supply and ready for use. Put on your safety goggles.
- Light the Burner: Adjust the Bunsen burner to produce a soft, blue, non-luminous flame. A smaller, hotter flame from a microburner can also be effective.
- Position the Capillary Tube: Hold the capillary tube securely, near one end, with your thumb and forefinger or a pair of tweezers. Ensure the end you intend to seal is free from obstructions and easily accessible.
- Heat the End: Gently roll the very tip of the open end of the capillary tube in the hottest part of the flame. Continuously roll the tube between your fingers to ensure even heating and prevent the glass from becoming brittle or misshapen in one spot.
- Observe for Melting and Collapse: As the glass heats, it will soften. Keep rolling it around in the flame until the end begins to visibly glow red-hot and the opening collapses and seals completely. You will notice the tip melting and forming a rounded, closed bottom.
- Remove from Flame and Cool: Once the end is completely sealed and has collapsed into a rounded shape, carefully remove the tube from the flame. Allow it to cool completely on a heat-resistant surface before handling further. Avoid touching the sealed end while it's hot, as it can cause burns or distort the seal.
Tips for Success:
- Even Heating: Constant rotation of the tube is crucial for a smooth, strong seal.
- Avoid Overheating: Excessive heating can cause the glass to thin out too much or form a large, irregular bulb.
- Practice: If you are new to this, practice with a few disposable tubes until you achieve a consistent, well-sealed end.
- Inspect the Seal: After cooling, visually inspect the sealed end to ensure there are no tiny pinholes or cracks that could allow your sample to escape.
Filling Capillary Tubes for Analysis
Once a capillary tube has one end sealed, it's ready to be filled with a sample for specific applications. A common application is melting point determination.
For Melting Point Determination
Melting point capillary tubes are typically filled with a finely ground solid sample.
Materials Needed:
- Sealed capillary tubes: Prepared as described above.
- Solid sample: Finely powdered and dry.
- Watch glass or clean surface: To hold the sample.
- Small spatula or micro-spatula: For handling the sample.
- Long glass tube (e.g., stirring rod) or a smooth, hard surface: For compacting the sample.
Step-by-Step Filling Process:
- Prepare the Sample: Ensure your solid sample is completely dry and finely powdered. If necessary, use a mortar and pestle to grind it into a fine powder. This ensures good contact and even heating during analysis.
- Collect Sample: Place a small amount of the powdered sample onto a clean watch glass or hard surface. Dip the open end of the sealed capillary tube into the sample. The powder will adhere to the inside walls by capillary action.
- Transfer and Tap: Invert the capillary tube so the sealed end is down. Gently tap the top of the tube against the benchtop, or carefully flick it with your finger. This helps the sample fall to the bottom (sealed end) of the tube.
- Compact the Sample: For accurate melting point readings, the sample needs to be tightly packed at the bottom of the tube.
- Method A (Tapping): Drop the capillary tube, sealed end first, through a long glass tube (like a stirring rod) held vertically, onto a hard surface (e.g., a counter or floor). The impact will compact the sample. Repeat this a few times until the sample is tightly packed.
- Method B (Flicking): Hold the tube vertically with the sealed end down and gently flick the side of the tube with your finger. This also helps in compacting.
- Achieve Correct Height: Aim for a sample height of approximately 2-3 mm at the bottom of the tube. Too much sample can lead to inaccurate or broad melting ranges. Add more sample and compact if needed, or remove excess if there's too much.
- Store (if not using immediately): Store filled tubes in a clean, dry container to prevent contamination or moisture absorption until ready for use.
Important Considerations for Melting Point Samples:
- Purity: Use pure, dry samples for accurate results. Impurities can depress and broaden the melting range.
- Packing: Consistent and tight packing is crucial for reproducible results. Loose packing can lead to uneven heating and a wider observed melting range.
- Sample Height: Keep the sample height consistent across different tubes for comparison.
Other Considerations
Cleaning Capillary Tubes
While most laboratory capillary tubes are disposable, some specialized tubes (e.g., for chromatography) might be reusable. Cleaning typically involves rinsing with appropriate solvents, followed by distilled water, and then drying in an oven. However, for applications like melting point, disposable tubes are standard and recommended to avoid cross-contamination.
Cutting Capillary Tubes
Pre-manufactured capillary tubes come in standard lengths. If a shorter length is required, they can be carefully cut using a specialized glass cutter or by scoring with a file and then snapping, followed by fire-polishing the cut edge if it needs to be smooth and safe. However, for most basic preparations, buying the correct length is more practical.
| Preparation Step | Purpose | Key Technique | Safety Notes |
|---|---|---|---|
| Sealing One End | Create a closed bottom for sample containment | Rolling in Bunsen burner flame until red-hot | Wear safety goggles; allow to cool slowly |
| Filling with Sample | Introduce material for analysis (e.g., solid) | Tapping or dropping to collect sample | Ensure sample is dry and finely powdered |
| Compacting Sample | Ensure dense, uniform sample for accuracy | Tapping through a long tube onto a hard surface | Avoid aggressive handling that could break tube |
Preparing capillary tubes correctly is essential for obtaining accurate and reliable results in various laboratory procedures. By carefully following these steps, you can ensure your samples are properly contained and prepared for analysis.
