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Balances and Glassware for Solution Preparation

Author(s): David R. Caprette, PhD

Open Beam Balances

We frequently use balances to measure quantities of solid substances. Sometimes, we use them to measure quantities of liquids. For simplicity of discussion, the terms "mass" and "weight" will be used interchangeably, although they are not equivalent terms.

The choice of a balance depends on the quantity to be weighed and the accuracy needed. The design may trade capacity for accuracy, or vice versa, because we don't need the same accuracy when weighing a large quantity as we need with a small quantity. For example, the error introduced by an inaccuracy of 0.1 gram is 10% when weighing 1 gram, while it is only 0.01% when weighing one kilogram.

With any balance, the weight of the container must be subtracted from the total weight so that a desired mass (net weight) is correctly determined. To get a balance to display only net weight, one first "tares" the instrument by placing weighing paper or a weigh boat or other container on the pan and setting the display to read zero.

A typical open beam or hanging pan balance is accurate to the nearest 0.1 gram. Such a balance is quite suitable for quantities above 100 grams. In fact, the error introduced when weighing 10 grams is only 1%, which is well within reasonable limits for biological buffers.

The pointer of a calibrated trip balance is straight up when there is nothing on either pan, indicating that the pans are in balance. Sliders on one or more beams are used to add weight to one side of the balance (usually the left side), causing one pan to drop. When an equal weight is placed on the opposite pan, the two pans are again balanced. 

One way to use a trip balance is to weigh a container, pre-set the beams to read the weight of the container plus a desired net weight, and then add material to the container until the pointer indicates balance. A second procedure is to place identical containers on each pan so that the desired net weight can be pre-set using the beams and material added to the balance point.

The mass of an unknown can be determined by placing it on the pan and moving the weights until a balance point is reached (the weight of the container must be subtracted, of course).

A two pan open beam balance is especially convenient for balancing centrifuge tubes. In a centrifuge, the rotor must be balanced by placing tubes of equal weight opposite each other. An unbalanced rotor can fly off its mount, shatter, or both, thus damaging the centrifuge and possibly causing serious injuries to personnel.