Most of us who work in laboratories use an analytical balance as part of our daily routine. In fact, it’s such a familiar object that we rarely give it much thought, unless there’s a problem with it. The UK’s Laboratory of the Government Chemist has just released a video into the public domain covering the essentials of analytical balance use. The video was recorded to support training under the Valid Analytical Measurement (VAM) programme in 1999 and provides an authoritative guide on correct use of the balance.
For quantitative analytical work, it is generally acceptable to weigh to a nominal accuracy of 0.1 % or better, which means that you need at least four significant figures on the balance read-out. So, for example, if the required sample mass is 10 g, a balance displaying at least two decimal places (a 2-place balance) would be needed. For a mass of 10 mg (0.01 g), though, you’d need to use a 5-place balance.
One of the most important considerations, especially for 4-place and 5-place balances, is where to put them. They should be located in an area that is free of draughts, vibration, temperature fluctuations, strong magnetic fields (e.g. not close to NMR spectrometers) and electrical interference. Make sure that the surface on which they stand is level and stable, and don’t site balances in an area of the laboratory that is dusty or where volatile corrosive substances are in use.
Keep it Clean
Residues of solid or liquid samples on a balance pan are a common source of weighing error. Mop up liquid spills with a clean tissue and brush spilt powder off the top of the balance with a fine-haired brush. Remember to clean underneath the pan of analytical balances as well.
Choice of Container
Containers used for weighing samples should be clean, dry, at ambient temperature, and designed to make it easy to transfer the sample to the vessel required for the next stage of analysis (e.g. a volumetric flask). Don’t use a container that is larger than necessary for the size of sample. For example, if you need to make 1 g of sample up to 500 mL, weigh the sample into a smaller vessel and then transfer it to the flask. Don’t weigh directly into the volumetric flask. For volatile liquids or hygroscopic solids, use a stoppered container to minimise changes in sample mass. Use gloves when weighing mg quantities of sample to prevent oils from your fingers adding to the weight of the container.
Choice of Spatula
It sounds obvious; use a small spatula for weighing small amounts, and a large spatula for large amounts. For accurate weighing of very small amounts, it’s impossible to exercise fine control using a large spatula, but the right tool (a microspatula, shown on the left of the photograph) may not be available in your laboratory. If you don’t have one, you can save yourself a lot of time and frustration by buying a pack; they’re not expensive!
One of the commonest problems when using a balance, particularly the more sensitive types, is an unstable reading. Drift can be caused by an open balance door, a container or sample that is hotter or cooler than the balance, loss or gain of moisture/solvent from the sample, a balance that isn’t levelled, the build-up of static charge (see below) or vibrations from adjacent equipment.
Weighing errors can occur when samples become electrostatically charged. This problem is particularly noticeable for micronized drugs and other fine powders of non-conducting materials, and will become more pronounced under conditions of low humidity. The attractive or repulsive forces between the sample and balance components caused by the build-up of static charge result in a drift in the balance reading. Some analytical balances are equipped with an accessory that ionizes the air inside the balance compartment to dissipate the charge on the sample.