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Balances and Glassware for Solution Preparation

Author(s): David R. Caprette, PhD

Top Loading and Analytical Balances

Most modern balances are electronic, with digital displays. A top loading electronic balance is probably the most frequently used weighing device in a typical laboratory. The capacity of such a balance is similar to that of an open beam balance, namely 200 grams or more, and a typical accuracy is plus or minus 0.01 grams. Electronic balances are convenient because it is necessary only to place a container on the pan, tare it to zero, and then add material to the container until the desired mass is displayed. There is no "balancing act" involved. Unfortunately, a typical electronic balance costs well over $1,000.

Electronic balances should be placed on stable surfaces and away from strong drafts, since air currents and vibrations affect the stability and accuracy of the readings. They also are designed with adjustable feet so they can be leveled.

When accuracy of better than 0.01 grams is desired and/or when quantities of less than a gram are measured, an analytical balance is preferred. The pan, which is placed upon or suspended from a sensitive weighing apparatus, is protected from drafts by a housing with doors through which materials can be introduced or removed. The sequence of operations is to tare the instrument with a piece of weighing paper on the pan with doors closed, introduce material through an open door, close the door to get a final precise weight, and then turn off the balance to remove the material.

Analytical balances cost perhaps twice as much as top loading electronic balances, depending on the level of accuracy and on the capacity of the balance. Because these devices are so expensive they should be kept scrupulously clean. Use a brush to remove any trace amounts of chemicals that may remain on a pan or the floor of an analytical balance, for example. Stainless steel pans should be washed frequently and rinsed with high quality water. Many chemicals are corrosive, and even non-corrosive materials tend to absorb water, promoting corrosion.