Chemistry, Third edition

Diluting standard solutions

A dilute standard solution can also be made by carefully diluting a more concentrated

standard solution. In this instance, a known volume of the concentrated standard

solution is delivered into a standard volumetric flask, and the solution diluted to the

graduation line with deionized water. Pipettesare designed to deliver specific

volumes of liquid. They can contain various volumes (from 0.50 to 200 cm^3 ) and the

volume of the pipette is marked on its bulb or shown by graduation marks on the side

of the pipette (Fig. 9.2).

STANDARD SOLUTIONS 139

Fig. 9.2Different types of

pipette.

Standard solutions are made up in special flasks, called volumetric flasks, which

have been calibrated to contain a definite volume of liquid. A graduation line is

marked on the neck of the flask. When the flask contains solution to this level it con-

tains the volume of solution marked on the front of the flask. Some volumetric flasks

are shown in Exercise 9C.

BOX 9.2

Making a standard solution

A standard solution of, for example, sodium carbonate, is made by weighing out the required

mass of sodium carbonate using a very sensitive balance called an analytical balance. The

mass is generally measured to the nearest 0.0001 g. The solid is then completelydissolved in

deionized water in a beaker and the resulting solution transferred to a volumetric flask. The

beaker is rinsed with deionized water a few times and the washings also transferred to the

flask, so that allthe sodium carbonate solution is transferred. The flask is then carefully topped

up to the graduation mark on the flask with deionized water, so that the bottom of the solution

meniscus is ‘sitting on’ the graduation line when viewed at eye level. Finally, the flask is

shaken well, so that the concentration of the solution it contains is homogeneous.