Preparing solutions of accurately known concentration from solids

To make up a volumetric solution, calculate the amount of solid required (the solute) and weigh accurately, approximately the correct amount required, usually into a weighing boat or appropriately sized beaker. It is not necessary to weigh precisely the calculated amount of solute, but it is important to know exactly how much material is going into the solution. The solid should be transferred from the weighing vessel into a conical flask of similar size to the volumetric flask being used. The weighing vessel should then be washed into the conical flask with the solvent which will be used to dissolve the compound. It is important to consider the material the weighing vessel is made from. Plastic disposable weighing boats are not suitable for use with many organic solvents as the boat will dissolve, but are fine for use with water as the solvent. Washing of the residual compound from the weighing boat is best accomplished by angling the weighing boat above the conical flask and using a Pasteur pipette to wash any remaining solid from the boat into the conical flask below. Beakers would be washed out by rinsing the edges with solvent using a Pasteur pipette and pouring the solution  If it is impractical to wash the solid from the weighing boat, then the alternative is to reweigh the weighing boat after tipping the compound into the conical flask and calculating the exact mass that has been added to the conical flask. It some cases it may be appropriate to weigh directly into the conical flask and remove the need for a weighing boat. However, weighing into narrow necked flasks is generally discouraged as this often results in spilling compounds, especially where larger quantities are being weighed out.

Once your compound has been transferred to the conical flask, add roughly a third of the required solvent volume and dissolve the compound. Depending on the solubility this may require use of a magnetic flea and stirrer hotplate or for small amounts use of a sonicator in order to agitate the mixtures into dissolving.

Once dissolved, the solution should be transferred to the volumetric flask using a glass funnel by carefully pouring the solution through the funnel and into the volumetric flask. The washings in the conical flask should be transferred to the volumetric flask by adding small volumes of solvent to the conical flask and swirling to dissolve any residues, then carefully transferring these to the volumetric flask. Once all washings have been transferred, the funnel should also be rinsed into the volumetric flask using a Pasteur pipette and small volumes of solvent. The volumetric flask can then be carefully filled until the meniscus reaches the graduation mark. This is best achieved using a Pasteur pipette to ensure that you don’t overshoot the graduated mark on the volumetric flask. For most liquids, the bottom of the meniscus should be resting on the graduated mark when viewed at eye level. Stopper the flask and invert the solution several times to ensure that the solution is mixed thoroughly.

Ensure that you calculate the exact concentration of your solution and use this concentration in any required calculations. Once the solution has been made up accurately it can be transferred to other containers for storage or use as required. Ensure you label your solution.

The above procedure will need to be modified when dissolving compounds which are exothermic or endothermic when dissolved in solvent. A common example would be sodium hydroxide pellets which generate a fair amount of heat when dissolving. In order to prepare a solution for these sorts of compounds, it is necessary to add the solid in small portions into the solvent in order to allow any change in temperature to be controlled.