Sub-topic #2: Weighing Techniques

• Direct Weighing

  • For non-hygroscopic samples
• Direct Weighing

  • A clean and dry container is placed on the balance.
• Tare – sets the balance to zero with the container or

  any other object not part of the sample.
• Tare Weight – mass of the empty vessel or weight of

  the container or object excluded from the

  measurement of the sample.
• Most balances allow the tare to be automatically

  adjusted to read a mass of zero.
• The sample is then transferred to the container and the new mass is measured.
• Weighing by Difference

  • For hygroscopic samples
• Hygroscopic – samples that absorb moisture or water from the environment (e.g. methanol, glycerine,

  ethanol, silica gel)
• Weighing by Difference

  • Hygroscopic samples are weighed indirectly
• Weighing by Difference

  • The hygroscopic sample is placed in a covered weighing bottle and the combined mass of the sample and container is determined.
• Weighing by Difference

  • A portion of the sample is removed, then the weighing bottle with the remaining sample is reweighed.
• Weighing by Difference

  • The difference between the two masses = mass of the transferred sample