Wednesday 28 October 2015

Techniques on Weighing

Analytical balanceThere are many different types of balances or scales available to measure the mass of a sample. The selection of the balance depends on the mass of the object or sample and the precision needed for the measurement. A top-loading balance is used to determine the approximate mass of the sample needed. In this course, we will be mostly using electronic analytical balances (Figure 1). These balances are easy to operate and are capable of measuring to 0.0001 g. Models of these balances vary in the labs. Consult your instructor and operating manual specific to the model of the balance.

There are two methods of weighing:

1. Weighing by difference – This is a technique which is used when it is important to know the precise amount of the sample that has been transferred
to a reaction mixture.

2. Weighing by taring – This technique is used when the mass of the empty container is not important. The empty container is ‘zeroed’ or tared on the

These two methods will be described in detail below.

The Balance Room

Balances are sensitive to drafts, changes in temperature, or the vibrations caused by moving people. The balances are stored in a separate room to minimize these variables and are placed on concrete tables.

Balances are very expensive and are sensitive to attack by corrosive chemicals. Do not take liquid into the balance room. When possible, chemicals should be added to the weighing container outside of the balance chamber. During the weighing process, the weighing container should be placed on a clean surface, such as a kimwipe, so that the bottom of the container does not pick up any dust. It is important that you clean up all chemical spills. If in doubt consult your instructor.

Weigh boats (Figure 2) are disposable containers used for weighing. They are made of polypropylene plastic and are inexpensive. A used weigh boat should be discarded in the waste container. All chemicals and spatulas that are used should be returned to their proper places. Depending on the experiment, other types of weighing container could be a porcelain crucible, an aluminum plate or a small beaker
balance pan
The balance room must be kept tidy. Materials taken into the balance room include datasheets, pen and the sample to be weighed. Enter mass measurements directly on the datasheet with your pen. Before you leave your balance, make sure:

1. The balance and the area around it is clean. Spills inside the balance should be brushed off using the brush on top of the balance. Spills on the concrete table should be cleaned using Kimwipes.

2. Close all the doors of the balance.

3. Turn off the balance.

Weighing by difference

1. Pre-weigh an approximate quantity of the sample into a weigh boat using a toploading balance.

2. Record all the masses for each step directly on the datasheet in pen.

3. Turn on the balance by pressing on the control bar. After a few seconds, the display will read 0.0000.

4. Open the sliding glass door (on the side) and place the sample/weigh boat on the balance pan. Close the sliding glass door and wait until the reading is stable. Record the value.

5. Transfer the sample into a beaker.

6. Weigh the emptied weigh boat on the analytical balance. Do not brush off any sample particles from the emptied weigh boat. Record the mass of the emptied weigh boat.

7. The difference between the two weighings is the mass of the sample transferred into the beaker.

Weighing by taring

1. Place a weigh boat on the balance pan. Close the doors and wait for the reading to stabilize. Press briefly on the control bar or the tare button and the display changes to 0.0000 g. The weight of the weighing boat is now tared.

2. Remove the weigh boat from the balance and set it on a piece of kimwipe. With a spatula, carefully add the sample to the weigh boat. Place the weigh boat back on the balance pan. Close the doors and wait for the reading to stabilize. Record the mass of the sample.

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