Handling, Preparation and Storage of Standards

This section contains basic information on the handling, preparation and storage of standards, as well as basic calculations and nomenclature.

Handling

Observing the following recommendations will save considerable time, money, and frustration:

1. Never put solution transfer devices into the standard solution. This precaution avoids possible contamination from the pipette or transfer device.
2. Always pour an aliquot from the standard solution to a suitable container for the purpose of volumetric pipette solution transfer and do not add the aliquot removed back to the original standard solution container. This precaution is intended to avoid contamination of the stock standard solution.
3. Perform volumetric pipette solution transfer at room temperature. Aqueous standard solutions stored at 'lower' temperature will have a higher density. Weight solution transfers avoid this problem provided the density of the standard solution is known or the concentrations units are in wt./wt. rather than wt./volume.
4. Never use glass pipettes or transfer devices with standard solutions containing HF. Free HF attacks glass but it is sometimes considered safe to use glass when the HF is listed as trace and/or as a complex. However, many fluorinated compounds will attack glass just as readily as free HF.
5. Don't trust volumetric pipette standard solution transfer. Weigh the aliquot of the standard taken. This can be easily calculated provided the density of the standard solution is known. There are too many possible pipetting errors to risk a volumetric transfer without checking the accuracy by weighing the aliquot.
6. Uncap your stock standard solutions for the minimum time possible. This is to avoid transpiration concentration of the analytes as well as possible environmental contamination.
7. Replace your stock standard solutions on a regular basis. Regulatory agencies recommend or require at least annual replacement. Why is this precaution taken in view of the fact that the vast majority of inorganic standard solutions are chemically stable for years? This is due to the changing concentration of the standard through container transpiration and the possibility of an operator error through general usage (more info). A mistake may occur the first time you use the stock standard solution or it may never occur with the probability increasing with use and time. In addition, the transpiration concentration effect occurs whether the standard solution is opened / used or not and increases with use and increased vapor space (transpiration rate is proportional to the ratio of the circumference of the bottle opening to vapor space).

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