Your instructor will explain how your specific balance is operated.
The primary goals are to keep all parts free from dust and corrosion, prevent contamination or any alteration in load (whether of sample or container), and steer clear of draft (air convection) errors. Before you weigh with any kind of analytical balance, you should get acquainted with a few general rules:
- Objects that are to be weighed should never be touched with your fingers. A clean piece of paper or tongs ought to be used.
- Perform the weighing at room temperature, thus preventing air convection currents.
- Chemicals must never be placed directly on the pan; instead, weigh them in a vessel (weighing bottle, weighing dish) or on powder paper. Any spilled chemicals should be brushed off at once with a soft brush.
- The balance case door must always be closed before you carry out the weighing. Air currents will make the balance become unstable.
Even though contemporary digital balances lack weights that users can adjust, corrosion can still lead to issues. Corrosive volatile substances (for instance, iodine or conc. HCl) must never be weighed in open containers on a balance.
Weighing of Solids
Solid chemical (nonmetal) materials are typically weighed and dried inside a weighing bottle. Weighing bottles feature standard tapered ground-glass joints, and hygroscopic samples (which absorb water from the air) can be weighed while the bottle is kept tightly capped. Replicate weighings can easily be performed by difference. The sample inside the weighing bottle is weighed, and afterward a portion is taken out (e.g., by tapping) and quantitatively moved to a vessel appropriate for dissolving the sample. The weighing bottle and sample are reweighed, and from the difference in weight, the sample's weight is calculated. The next sample is removed and the weighing is repeated to obtain its weight by difference, and so forth. This is demonstrated in the Laboratory Notebook example for the soda ash experiment.
It is clear that with this technique, an average of just one weighing per sample is needed, along with one extra weighing for the initial sample. Nevertheless, each weight corresponds to the difference between two weighings, meaning the overall experimental error results from the combined error of both weighings. Weighing by difference with the bottle capped must be employed if the sample is hygroscopic or otherwise cannot be exposed to the atmosphere prior to weighing. If atmospheric exposure produces no effects, then the bottles do not need to be capped.
For direct weighing, a weighing dish, weighing paper, or a weighing boat (all usually disposable) is used. The dish, paper, or boat is first weighed empty, and then again with the sample added. This calls for two weighings per sample. The weighed sample is transferred by tapping. Direct weighing is acceptable only when the sample is non-hygroscopic.
When performing highly precise weighings (for instance, to within a few tenths of a milligram or less), you need to be careful not to contaminate the weighing vessel with foreign material that may affect its weight. Special care should be taken not to get perspiration from the hands onto the vessel, since this can be quite significant. It is best to handle the vessel using a piece of paper. As an alternative, finger cots may be used. These are similar to just the fingertip portion of protective gloves. Solid samples must often be dried to a constant weight (e.g., ±0.5 mg for a 0.5 g sample). Highly insulating material, such as laboratory ware made from fluorocarbons, readily acquires static charge that affects weigh readings. Brushes with a built-in source of ionizing radiation (http://www.amstat.com/solutions/staticmaster.html) that help dissipate such charges are recommended for gently swiping such objects before weighing.
Weighing by difference is required for hygroscopic samples.
Weighing of Liquids
Liquids are typically weighed using the direct weighing method. The liquid is moved into a vessel that has been weighed beforehand (such as a weighing bottle), which is sealed with a cap to stop evaporation while being weighed, and then it is weighed. When a liquid sample is weighed by difference by pipetting out an aliquot from the weighing bottle, the inside of the pipet must be rinsed several times after the transfer is made. Care must be taken to avoid losing any sample from the pipet tip while transferring.
Types of Weighing: What Accuracy Do You Need?
Certain chemicals are never weighed on an analytical balance. Sodium hydroxide pellets, for instance, are so hygroscopic that they continuously take up moisture. The weight of a given quantity of sodium hydroxide cannot be reproduced (and its purity is unknown). To prepare a solution of known sodium hydroxide concentration, the sodium hydroxide is weighed on a rough balance and dissolved, and the solution is calibrated against a standard acid solution.
Sources of Error in Weighing
A number of potential sources of error have been noted, among them zero-point drift and buoyancy. Shifts in the surrounding temperature, or in the temperature of the object being weighed, are likely the most significant sources of error, producing a drift in the zero or rest point as a result of convection-driven air currents. Objects that are hot or cold need to be brought to ambient temperature prior to weighing. Hygroscopic samples can absorb moisture, especially in an atmosphere with high humidity. Contact between the sample and air, before and while weighing, must be kept to a minimum.
