Anlaytical Scales and Laboratory Compounding

By: Robert Thomson

We are further going to constrain our discussion to recipes that call for the mixing of solids only (even though they may be used to create solutions at a later point in the process). Recipes used in laboratories usually call for the ingredients to be measured by weight, and usually specify the weight in the metric system (g, Kg, mg, microg), and only rarely specify solids by volume. If our recipes are going to specify ingredients by weight we must consider the types of scales and balances we are likely to encounter in the lab. These are the Laboratory Balance (which is indeed a scale in its present form); which measures a. to one milligram in scales with ranges of several hundred grams (generally thought of as providing one part in one hundred thousand for accuracy), b. to 10 milligrams in scales with ranges of several Kilograms, and c. to 100 milligrams in scales with ranges of several 10’s of Kilograms. Next comes the Analytical Balance (also a scale in its present form); which measures to 0.1 milligram in scales with ranges of hundreds of grams (generally thought of as one part in one million for accuracy). Next is the Semi-Microbalance; which measures to 0.01 milligrams (or 10. micrograms) in scales with ranges to tens of grams. Lastly, the microbalance; which measures to 1. micro-gram in scales with ranges to several grams. The last two categories of scales have special problems because of their very high sensitivities. Things such as the weight of the displaced air (which brings barometric pressure, temperature, and humidity into consideration), the weight of dust that may fall on the weigh pan, and other factors not considered in weighing heavier objects. The recipes which are compounded in the laboratory are frequently experimental and require very small quantities of very expensive and rare substances. These are prototypical mixtures that may be replicated in large quantities at a later date, depending upon the results achieved by the tiny laboratory quantity. It is vital, therefore, that great care be taken to qualify the compounding process. The scale used must be calibrated regularly and documented as such. A print out of the compounding process, delivered by the scale, which includes the date and time, is very valuable. There are standards for these processes and they provide excellent guides. The use of a computer, desktop or lap-top, can be invaluable in capturing the required details. There is surprisingly little in the way of automated dispensing aids to help with the chore of weighing out the small amounts of ingredients. It seems that the variety of substances combined with variety of packaging poses a serious

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