Volumetric glassware is used when it is necessary to prepare solutions of a precisely known concentration. Volumetric flasks are manufactured to a high accuracy and are labelled with their volume and a precision. Different grades of volumetric glassware are available depending on how accurate the solution needs to be for a particular purpose. Examples of where accurate solutions may be required include:

• Preparation of standard solutions for calibration curves

• Preparation of solutions of a known concentration for use when titrating against an unknown

• In the preparation of samples for analysis

In order to prepare a volumetric solution, it is necessary to first determine the volume and concentration required. This varies depending on the purpose of the solution and the volumetric glassware available to you. In many cases you will only require a small volume to analyse, however it is worth considering the compromise between making up very small amounts of solution which minimises waste, and the increase in errors with weighing and volumes when preparing very small amounts of solution. For some very dilute solutions it may be easier (and more accurate) to make up a more concentrated solution initially and then dilute a portion of it to give a solution at the desired concentration.

Notes on preparing stock solutions

Volumetric glassware is manufactured to a high degree of precision. As a result, the glassware needs to be treated well to avoid affecting its accurate volume. Sources of heat can easily distort the glassware and as a result volumetric glassware should be protected from extremes of heat. This includes not placing volumetric glassware in ovens and avoiding washing up the glassware in very hot water along with the notes above regarding exotherms during dissolution.

The term accurately-approximately is often used when preparing volumetric solutions. This indicated that is important the exact concentration of the solution is known, but it does not need to be precisely the concentration indicated to prepare. For instance, if you were asked to prepare a 0.100 M solution and prepared a 0.098 M solution, this would be perfectly acceptable but you would use the precise concentration in any subsequent calculations.

When preparing solutions it is not necessary that volumetric flasks are perfectly dry prior to use. It is often easier to rinse the flask with a small volume of the solvent that will be used to make up the next solution in the flask than it is to dry the insides of the flask.

Micropipettes offer a convenient alternative to graduated pipettes and their use is commonplace, especially in microbiology. They can often be utilised effectively in chemistry laboratories, however consideration needs to be given to their reactivity with any reagents which may be handled using them. Many organic chemicals will react with the plastic tips used with micropipettes which may affect the accuracy of dispensed volumes or could pose risk with undesired reactions. As a result, micropipettes are generally restricted to use with dilute aqueous solutions.

Sometimes it is necessary to prepare solutions which contain multiple compounds. For these solutions, the amounts required of each component would be calculated separately and combined in a single volumetric flask which would be brought to the mark. It is especially important to consider the compatibilities of any chemicals combined in solution, especially prior to being diluted.