WEIGHING BY DIFFERENCES (noun)

 

This phrase describes the technique that is used to accurately weigh a sample of solid matter, in conjuction with a top-loading balance and then an analytical balance.

A weigh boat is placed on a top-loading balance which is then "zeroed" so that the mass of the weigh boat reads 0.0 grams. The sample is then weighed out into the weigh boat to the approximate desired mass (say, 1.2 grams). The weigh boat and sample are then placed on the balance pan of an analytical balance and the combined mass of the weigh boat and the sample is recorded (say, 1.5633 grams). The solid matter is then dumped into a beaker, and the now empty weigh boat is weighed (say, 0.3216 grams).

This latter number is substracted from the first to yield the exact mass (1.2417 grams in our example) of the material that was dumped into the beaker to a high level of accuracy. This technique must be used since it is often the case that a few grains of the solid material being weighed out stick to the weigh boat.