For a 12.7 mm diameter pellet, approximately 200-250 mg of KBr powder is typically required to produce a disc around 2 mm thick. This amount ensures proper pellet formation and transparency for spectroscopic analysis. The quantity scales with the pellet's surface area, meaning larger diameters would proportionally require more material. The sample powder mixed with KBr usually ranges from 1-2 mg for this pellet size, maintaining an optimal ratio for accurate measurements.
Key Points Explained:
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Standard KBr Quantity for 12.7 mm Pellets
- 200-250 mg is the recommended range for pure KBr powder to form a pellet of this diameter.
- This amount yields a disc thickness of ~2 mm, which balances structural integrity and light transmission for FTIR spectroscopy.
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Sample-to-KBr Ratio
- Only 1-2 mg of sample powder is needed when mixed with KBr for analysis.
- The low sample concentration ensures the pellet remains transparent, avoiding signal saturation in infrared measurements.
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Scaling with Pellet Area
- The required KBr mass scales with the square of the pellet's radius/diameter.
- Example: A 25.4 mm (double the diameter) pellet would need ~800-1000 mg of KBr, as the area increases fourfold.
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Practical Considerations for Purchasers
- Consistency: Pre-weighed KBr aliquots (e.g., 250 mg) can streamline workflow for high-throughput labs.
- Material Quality: Opt for high-purity, dry KBr to minimize spectral interference from moisture or impurities.
- Pellet Thickness Control: Adjust KBr quantity slightly if thicker or thinner pellets are needed for specific applications.
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Why This Matters in Spectroscopy
- Proper KBr quantity ensures uniform pellet density, critical for reproducible absorbance measurements.
- Overloading KBr can lead to brittle pellets, while insufficient amounts may cause uneven surfaces or poor transparency.
For labs regularly preparing 12.7 mm pellets, bulk purchasing KBr in 100 g or 500 g bottles is cost-effective, while sample powders may be acquired in smaller quantities due to minimal usage per test.
Summary Table:
| Key Parameter | Value for 12.7 mm Pellet |
|---|---|
| KBr Powder Needed | 200-250 mg |
| Pellet Thickness | ~2 mm |
| Sample Powder Required | 1-2 mg |
| Scaling Factor | ∝ (Diameter)² |
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