General Operation and Protocol
Place the balance on a clean level table away from drafts and v
ibrations. The balance room must be near a constant temperature, i. e. within 5 o C. Plug in the balance to 110 VAC, turn the balance switch on, and allow it to warm up for 15 minutes. Never put chemicals directly on the balance pan. Remember opening and closing laboratory doors will create a draft. No one may use any balance without first learning about the nature of an analytical balance from a qualified instructor.
A. Tarring (Zeroing, i. e. no need to subtract two numbers)
Never use the maximum tarring capabilities of a balance. The balances can tare up to their total weight capacity; however, the tare should be 5 grams or less. Use a piece of paper to weigh chemicals or a small-aluminum pan. Never expect accurate weights for a 0.0100 gram sample in a 200 gram beaker. If the last person to use the balance did not re-tare the balance, with out a load, the reading may be negative. Avoid electrostatic charge when making weighings.
1. Place an empty container or weighing paper on the pan in the center of the weighing pan. How much does the tare weigh? Do not proceed to the next step if the tare container has a weight greater than 40 grams.
NOTE: For larger weights use a triple beam balance, accuracy of 0.05 gram, see below.
Select a container which allows easy transfer of the weighted sample for the next step in the procedure.
2. Press the Zero key to tare the empty container on the balance. Depress the zero key once and read 0.000 grams. Repeat depressing the zero key, if necessary.
B. Weighing a Sample using tarring.
3. Place the same tarred sample container in the center of the balance pan and add the sample to the container.
4. The balance is stable when the display is constant; it shows the weight of the sample.
5. Read and record the mass of the sample in a notebook and re-zero.
6. Remove the sample and its container from the balance pan and re-zero the balance.
7. Record your name and date in the balance log book.
Note: Here are some general rules on the use of an electronic balance and a few tips to follow to insure proper care whenever you are using an electronic balance:
* Do not bump or place objects on the bench after zeroing the balance.
* Weigh powders on paper or light weight pans or small containers. Handle objects with tongs, tweezers, gloves, or paper to prevent fingerprints dirt or grease.
* Let hot objects cool.
* Hygroscopic materials will absorb water during weighing. Similarly, the weight of solution will change as it evaporates.
* When making repetitive weighings always use the same procedure.
* DO NOT leave spilled chemicals on or around the balance! Report any spill so it may be cleaned up in a properly.
