Sources of Uncertainty in Weighing

Balances of interest in this book are often classified with respect to their resolution, i.e., the smallest mass difference that can be reliably measured. Generally, the better the resolution, the lower the maximum mass that can be reliably weighed. Thus, the term analytical balance is generally applied to a device that can measure up to a few hundred grams with an uncertainty of 0.1 mg a semi-microbalance can weigh up to a few tens of grams with an uncertainty of 0.01 mg a microbalance is limited to a few grams with an uncertainty of 0.001 mg and an ultramicrobalance can handle only very small loads but can provide uncertainties of as httle as 0.1 p.g. 

A critical factor that must be considered before weighing an analytical standard is whether or not the balance is 

To achieve good performance in weighing with modem balances, precautions must be taken to avoid a series 

An extreme case of problems related to humidity arises when the analytical standard to be weighed is extremely hygroscopic, to an extent that it is never possible to completely remove all the water from an otherwise chemically pure standard. A real example of this problem arose (Burton 2005) in the course of developing analytical standard solutions for saxitoxin and other shellfish toxins (Section 11.2.2). It was impossible to weigh these 

Of course the preceding thought-experiment, in which calibration of the balance is conducted in a vacuum, is an unrealistic example of the buoyancy effect. Consider now a realistic case where the calibration of the balance and the weighing of the nnknown object are both conducted at atmospheric pressure now the variation arises because the densities (and thus the volumes) of the standard mass and the unknown are different. The buoyancy force on the steel standard is stiU the same (equivalent to the gravitational force on 1.5 x 10 g), but in this case this buoyancy force is accounted for in the calibration procedure. Assume that the object to be weighed is a powdered or liquid chemical with density 1 g.cm and also of true mass 10.0000g, so its volume is lO.Ocm the mass of air that it displaces is thus 12 x 10 g. In other words, the upward (buoyancy) force on the sample is greater than that on the steel standard used to calibrate the balance by an amount equivalent to a mass of (12 — 1.5 = 10.5) X 10 g, and the indicated mass on 

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