Weighing is the most quantitative operation in the laboratory. A weighing error of a few milligrams can render an entire experiment useless, making proper balance technique essential.

Balance Types

• Top-loading balances: readability of 0.01–0.001 g. Used for general weighing where high precision is not critical. Capacity is typically 200–2000 g.
• Analytical balances: readability of 0.0001 g (0.1 mg). Used for preparing standards, analytical samples, and precise formulations. They have a draft shield and require a stable, vibration-free surface.
• Microbalances: readability of 0.001–0.0001 mg (1 µg). Used for very small samples, typically in a temperature-controlled room on an anti-vibration table.

Factors Affecting Accuracy

• Air currents: open doors, ventilation, and breathing near the balance cause fluctuations. Close the draft shield before taking a reading.
• Temperature: allow samples and containers to equilibrate to room temperature before weighing. Warm objects create convection currents.
• Static charge: dry samples or plastic containers can accumulate static charge, causing erratic readings. Use antistatic devices or ionizers.
• Moisture: hygroscopic samples absorb water from the air. Weigh them quickly or use a desiccator.
• Vibration: place balances on a heavy stone bench, away from doors and equipment.

Weighing Techniques

There are two main methods for weighing:

Direct weighing: place a container on the balance, tare it, add the sample until the desired mass is reached. Suitable for non-hygroscopic solids and liquids.

Weighing by difference: weigh the sample plus container, transfer the desired amount, and reweigh the container. The difference is the mass transferred. This method does not require taring and accounts for sample left in the container. It is preferred for hygroscopic materials.

Best Practices

• Level the balance before use using the adjustable feet and bubble level.
• Calibrate daily with the internal calibration weight or certified external weights.
• Use tweezers or gloves to handle weights and containers — fingerprints add mass.
• Record the mass to the full readability of the balance.
• Never weigh chemicals directly on the balance pan — always use a weighing boat, paper, or container.
• Clean spills immediately using a brush or damp wipe (never compressed air, which blows particles into the mechanism).