VOLUMETRIC
SOLUTIONS
Normal
Solutions—Normal solutions are solutions that
contain 1 gram equivalent weight of the active
substance in each 1000 mL of solution; that is, an
amount equivalent to 1.0079 g of hydrogen or 7.9997
g of oxygen. Normal solutions and solutions bearing
a specific relationship to normal solutions, and
used in volumetric determinations, are designated as
follows: normal, 1 N; double-normal, 2 N;
half-normal, 0.5 N; tenth-normal, 0.1 N;
fiftieth-normal, 0.02 N; hundredth-normal, 0.01 N;
thousandth-normal, 0.001 N.
Molar
Solutions—Molar solutions are solutions that
contain, in 1000 mL, 1 gram-molecule of the reagent.
Thus, each liter of a molar solution of sulfuric
acid contains 98.07 g of H2SO4
and each liter of a molar solution of potassium
ferricyanide contains 329.25 g of K3Fe(CN)6.
Solutions containing, in 1000 mL, one-tenth of a
gram-molecule of the reagent are designated
“tenth-molar,” 0.1 M; and other molarities are
similarly indicated.
Empirical
Solutions—It is frequently difficult to
prepare standard solutions of a desired theoretical
normality, and this is not essential. A solution of
approximately the desired normality is prepared and
standardized by titration against a primary standard
solution. The normality factor so obtained is used
in all calculations where such empirical solutions
are employed. If desired, an empirically prepared
solution may be adjusted downward to a given
normality provided it is strong enough to permit
dilution.
All volumetric solutions,
whether made by direct solution or by dilution of a
stronger solution, must be thoroughly mixed by
shaking before standardization. As the strength of a
standard solution may change upon standing, the
factor should be redetermined frequently.
When solutions of a reagent
are used in several normalities, the details of the
preparation and standardization are usually given
for the normality most frequently required. Stronger
or weaker solutions are prepared and standardized in
the same general manner as described, using
proportionate amounts of the reagent. It is possible
in many instances to prepare lower normalities
accurately by making an exact dilution of a stronger
solution. Volumetric solutions prepared by dilution
should be restandardized either as directed for the
stronger solution or by comparison with another
volumetric solution having a known ratio to the
stronger solution.
Dilute solutions that are not
stable, as, for instance, potassium permanganate
0.01 N and more dilute sodium thiosulfate, are
preferably prepared by exactly diluting the higher
normality with thoroughly boiled and cooled water on
the same day they are required for use.
Blank
Determinations—Where it is directed that “any
necessary correction” be made by a blank
determination, the determination is to be conducted
with the use of the same quantities of the same
reagents treated in the same manner as the solution
or mixture containing the portion of the substance
under assay or test, but with the substance itself
omitted. Appropriate blank corrections are to be
made for all Pharmacopeial titrimetric assays (see
Titrimetry
 541 ).
All Pharmacopeial assays that
are volumetric in nature indicate the weight of the
substance being assayed to which each mL of the
primary volumetric solution is equivalent. In
general, these equivalents may be derived by simple
calculation from the data given under
Molecular Formulas and Weights,
in the Reference Tables.
Preparation and Methods of Standardization of Volumetric Solutions
The following directions
give only one method for standardization, but
other methods of standardization, capable of
yielding at least the same degree of accuracy,
may be used. The values obtained in the
standardization of volumetric solutions are
valid for all Pharmacopeial uses of these
solutions, regardless of the instrumental or
chemical indicators employed in the individual
monographs. Where the apparent normality or
molarity of a titrant depends upon the special
conditions of its use, the individual monograph
sets forth the directions for standardizing the
reagent in the specified context. For those
salts that usually are available as certified
primary standards, or that are available as
highly purified salts of primary standard
quality, it is permissible to prepare solutions
by accurately weighing a suitable quantity of
the salt and dissolving it to produce a specific
volume of solution of known concentration.
Acetic, hydrochloric, and sulfuric acids may be
standardized against a sodium hydroxide solution
that recently has been standardized against a
certified primary standard.
All volumetric solutions,
if practicable, are to be prepared,
standardized, and used at the standard
temperature of 25
 .
If a titration is carried out with the
volumetric solution at a markedly different
temperature, standardize the volumetric solution
used as the titrant at that different
temperature, or make a suitable temperature
correction.
|