A Question of Balance? Part 2: Putting Principles into Practice

A Question of Balance? Part 2: Putting Principles into Practice
Mar 01, 2006
Volume 19, Issue 3

In the first part of this article on analytical balances we spent some time looking at laboratory weighing from the perspective of the basic principles and common practices.1 Weighing is such a critical operation in all chromatography laboratories that we are amazed that there are so few references available outside analytical text books. For example, the European Pharmacopoeia (EP)2 doesn't have a monograph on weighing and it is also not mentioned in ISO 17025 for testing and calibration laboratories.3 In contrast, the United States Pharmacopeia (USP) does.4 In fact, it has two: General Chapters <1251> and <41>. Chapter <41> will be discussed later in this article but remember that compliance with this is a legal US requirement, whereas <1251> is only guidance, albeit very strong guidance! Ignore it at your peril.

The most surprising fact to us about USP <1251>, which involves weighing on an analytical balance, is that it needed to be written at all! However, anyone who graduated in analytical chemistry in the last 20 years or so probably wasn't trained in the same classical gravimetric and volumetric analysis disciplines as the present authors were (Well, at least Chris was! Bob spent most of his time dissecting rotting livers as far as we can tell). Chapter <1251> covers some of the aspects we discussed in Part 1 and also good weighing practices, which will be discussed later. This chapter omits to mention buoyancy and its impact on the accuracy of weighing. However, before we get on to that topic we need to look at selecting the correct analytical balance to meet our basic weighing accuracy needs.

The Right Balance for The Right Job

You would never weigh an analytical reference standard on an ordinary top pan balance that you use for weighing substances for preparing buffers and mobile phases, would you? Of course not, an analytical balance would always be used but what type? Take, for example, a common 4-place modern analytical balance that has a capacity of 160–200 g and a displayed resolution of 0.1 mg. OK, so what is the smallest sample weight that can be weighed on such a balance: 100 mg, 10 mg, 1 mg?

Like all good philosophical questions, it depends... The explanation comes from the USP General Chapter <41>, which specifies that the minimum weight (the minimum permissible load) for a balance is that which will have an error less than 0.10% of that weight. [Pedantically, this error is more correctly defined as the expanded relative uncertainty with a coverage factor of 3 — but like politicians we won't let the facts get in the way].

Those of you chromatographers who do not work in laboratories that use the USP may wonder why you should bother reading this section. However, the principles outlined in this general chapter of the USP are applicable in any laboratory that needs to weigh analytical reference material with any degree of accuracy. Read, absorb and apply as appropriate — it makes good scientific sense.

OK, back to the analytical science, the minimum weight, m min' , that can be weighed on an analytical balance, is simply found from Equation 1.

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