Mix the solution well using one of the techniques described below. Students often will not mix well enough to cause the liquid in the neck of the flask to mix with the bulk solution. Thus, the concentration of the solution in the neck of the flask is much lower than expected and the concentration of the bulk solution is much higher than expected. Mixing can be accomplished manually or mechanically once the solution is diluted to the calibration mark.
Manually: Stopper the flask with a glass or plastic stopper and shake the solution holding the flask with both hands. Never shake a volumetric flask while only holding the neck of the flask. Do not use your hand or finger to stopper the solution.
Mechanically: A magnetic stirring bar can be added to a volumetric flask that has been filled to the mark. Carefully place the stirring bar in the flask by tilting the flask and sliding the bar down the neck of the flask. Dropping the stirring bar down the neck of an untilted flask may break the bottom of the flask. Then, stir the solution until the solution is completely homogeneous. The solution in the neck of the flask must be the same concentration as that in the rest of the flask.
