When using an analytical balance, there are three common weighing techniques: direct weighing, fixed mass weighing, and differential weighing. Choosing the right method depends on the nature of the sample and the specific requirements of the analysis. Direct weighing is typically used when a precise mass of a sample is needed, such as in routine analyses where simplicity and accuracy are essential. Fixed mass weighing is suitable when multiple samples need to be weighed for parallel testing, ensuring consistency across measurements. Differential weighing, also known as subtraction weighing, is ideal for hygroscopic, volatile, or corrosive substances, as it minimizes exposure and ensures accurate measurement by subtracting the initial weight from the final.
The analytical performance of a balance includes sensitivity, unequal arm effect, and indication variability. Sensitivity refers to how responsive the balance is to small changes in mass, often measured by the number of divisions the pointer moves per unit mass. Unequal arms occur when the two sides of the balance beam are not perfectly equal in length, which can affect accuracy. Indication variability refers to the repeatability of the balance’s readings when the same mass is measured multiple times without altering the setup.
To ensure proper maintenance and use of an analytical balance:
1. Place the balance in a dedicated, temperature-controlled room that avoids direct sunlight, drafts, and moisture.
2. Position it on a stable, vibration-free platform, ideally anchored to the wall or a concrete base.
3. Before each use, check that the balance is level and functioning properly, and adjust the zero point if necessary.
4. Ensure the sample mass does not exceed the balance’s capacity. Handle the balance with care to avoid damaging the knife edges.
5. Add or remove samples and weights only after the balance is closed. Always close the door before reading the result.
6. Allow the sample to reach room temperature before weighing, especially for hygroscopic, volatile, or corrosive materials. Weigh them in sealed containers to prevent contamination or loss.
7. Use tongs or tweezers to handle weights, never your hands. Return weights to their designated positions after use.
8. Only use the same balance and set of weights throughout a single experiment to maintain consistency.
9. After weighing, record the mass, check the balance components, and secure the balance cover.
10. Keep the balance clean and dry. Regularly replace desiccants, dust the surfaces, and perform routine checks to maintain accuracy.
Common issues and solutions include:
1. **Lifting ear misalignment**: Adjust the lifting ear using pliers and reposition the small pillar if it's off-center.
2. **Tray height issues**: Remove the tray, adjust its position, and secure it properly.
3. **Pointer instability**: Loosen the front screw and adjust the pillar height until the pointer stabilizes.
4. **Balance blockage**: Check for friction between the inner and outer dampers and adjust their positions carefully.
5. **Index disk malfunction**: Tighten the index plate screw, check for oil or loose parts, and adjust the eccentric if needed.
6. **Photoelectric system problems**: Adjust the light source alignment, focus the magnifying glass, and correct mirror angles to ensure clear readings.
Proper handling and regular maintenance are crucial for maintaining the precision and longevity of an analytical balance.
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