Balances and Weighing

Weighing with various accuracy is required in laboratory practice. A technical chemical balance is used for weighing with an accuracy up to 0.01 g. A conventional analytical balance (Fig. 22a) can be used for weighing with an accuracy up to 0.000 2 g. 

Balances of both kinds can be equal-arm or single-pan ones. 

Balance Design. The principle underlying the design of technical chemical and analytical equal-arm balances is the same. A metal beam (equal-arm lever) is provided with three knife edges—two at its ends and one at its middle (Fig. 22b). 

The middle knife edge rests on a plate on the central column of the balance and is the point of support. In an analytical balance, the knife edges and plate are made from agate or synthetic sapphire. The stirrups carrying the balance pans rest on the side knife edges. The beam is provided with a long pointer that indicates the deviation of the beam from a horizontal position on a scale. When the beam is horizontal, the pointer is opposite zero of the scale. 

When a balance is not being used, all its moving parts should be lifted and the knife edges separated from their bearing surfaces by means of a device called an arrester. In this position of the balance, the beam cannot rock. This prevents excessive wear of the knife edges. 

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Figure 2.2 Photograph illustrating typical laboratory balances and weighing equipment.

Place a clean, dry weighing boat or appropriately sized sample tube onto a simple two-decimal-place balance (see p. 22) and zero (tare) the balance and weigh about 9.80 g of the chemical. 

Take a 100 mL plastic beaker, put it on a triple beam balance, and weigh into it 45 g of either your sample of ground commeal or flour or our sample. 

Solids.—The substance is heavier than water, insoluble in that liquid, and not in pmoder.—It is attached by a fine silk fibre or platinum wire to a hook arranged on one arm of the balance, and weighed. A beaker full of pure water is then so placed that the body is immersed in it (Fig. 1), and a second weighing made. By dividing the weight in air by the loss in water, the sp. gr. (water = 1.00) is obtained. Example ... 

Use a balance and weighing paper to measure 1.5 g of salicylic acid. Place the salicylic acid in a small test tube and add 3 ml of distilled water. Use a 10-mL graduated cylinder to measure the water. Then add 3 mL of methanol. Use a Beral pipette to add 3 drops of concentrated sulfuric acid to the test tube. WARNING Concentrated sulfuric acid can cause burns. Methanol fumes are explosive—keep away from open flame. Handle chemicals with care. 

On the other hand if we are clever we first put objects 1,2,3 5 on the balance and weigh them, then objects 2,3,4 6, then 3,4,5... 

Electronic balance and weighing paper Fumed alumina nanoparticles (Degussa)... 

Place the combustion boat on the balance and weigh an appropriate amount of the sample into the trench in the MgO powder. Enter the weight. 

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