Page 135 - Vogel's TEXTBOOK OF QUANTITATIVE CHEMICAL ANALYSIS

PREPARATION AND STORACE OF STANDARD SOLUTIONS 3.28

apparatus may be programmed to reproduce automatically a given number of
cycles), until the required degree of purification has been achieved.

3.28 PREPARATION AND STORAGE OF STANDARD SOLUTIONS
In any analytical laboratory it is essential to maintain stocks of solutions of
various reagents: some of these will be of accurately known concentration (i.e.
standard solutions) and correct storage of such solutions is imperative.
Solutions may be classified as:
1. reagent solutions which are of approximate concentration;
2. standard solutions which have a known concentration of some chemical;
3. standard reference solutions which have a known concentration of a primary
standard substance (Section 10.6);
4. standard titrimetric solutions which have a known concentration (determined
either by weighing or by standardisation) of a substance other than a primary
standard.
The IUPAC Commission on Analytical Nomenclature refers to (3) and (4)
respectively as Primary Standard Solutions and Secondary Standard Solutions.
For reagent solutions as defined above (i.e. 1) it is usually sufficient to weigh
out approximately the amount of material required, using a watchglass or a
plastic weighing container, and then to add this to the required volume of
solvent which has been measured with a measuring cylinder.
To prepare a standard solution the following procedure is followed. A
short-stemmed funnel is inserted into the neck of a graduated flask of the
appropriate size. A suitable amount of the chemical is placed in a weighing
bottle which is weighed, and then the required amount of substance is transferred
from the weighing bottle to the funnel, taking care that no particles are lost.
After the weighing bottle has been re-weighed, the substance in the funnel is
washed down with a Stream of the liquid. The funnel is thoroughly washed,
inside and out, and then removed from the flask; the contents of the flask are
dissolved, if necessary, by shaking or swirling the liquid, and then made up to
the mark: for the final adjustment of volume, a dropping tube drawn out to
form a very fine jet is employed.
If a watch glass is employed for weighing out the sample, the contents are
transferred as completely as possible to the funnel, and then a wash bottle is
used to remove the last traces of the substance from the watch glass. If the
weighing scoop (Fig. 3.8; Section 3.25) is used, then of course a funnel is not
needed provided that the flask is of such a size that the end of the scoop is an
easy fit in the neck.
If the substance is not readily soluble in water, it is advisable to add the
material from the weighing bottle or the watchglass to a beaker, followed by
distilled water; the beaker and its contents are then heated gently with stirring
until the solid has dissolved. After allowing the resulting concentrated solution
to cool a little, it is transferred through the short-stemmed funnel to the
graduated flask, the beaker is rinsed thoroughly with several portions of distilled
water, adding these washings to the flask, and then finally the solution is made
up to the mark: it may be necessary to allow the flask to stand for a while
before making the final adjustment to the mark to ensure that the solution is
at room temperature. Under no circumstances may the graduated flask be heated.