3.5 Analytical Chemical Standards
113
13.5.1 Types
Analytical chemical standards can be classified into two, mutually nonexclusive, categories according to (a) intrinsic properties or (b) nature and
purpose (see Fig. 3.8).
Box 3.8
Determining the acidity of commercially available vinegar entails using a "standard" solution
of sodium hydroxide (i. e. one of accurately known concentration, which will be one basis for
traceability). Solid sodium hydroxide does not meet the purity and stabil ity requirements for
use as a primary standard. Therefore, an experimental traceability link must be established
between the secondary standard (NaOH) and a primary standard of acid character (e.g.
potassium hydrogen phthalate, which meets the previous requirements).
A solution about 0.1 Min NaOH is thus prepared. With due care, the solution is used to fill
a conventional or automatic burette. In another operation, an amount of 0.8642 g of potassium hydrogen phthalate is weighed on a balance and dissolved in water. This latter solution
is supplied with phenolphthalein as ind icator and titrated; in the titration, which is equivalent
to standard izing the titrant, a volume of 40.15 mL (practical volume) of the NaOH solution is
used. A simple stoichiometric calculation reveals that the amount of primary standard initially weighed should have consumed 42.15 mL of an exactly 0.0001 M solution of NaOH
(theoretical volume) . The titrimetric factor, defined as the ratio
theoretical volume 42.15
f =
= - = 1.0550
practical volume
40.15
is the number by which the approximate NaOH concentration (0.1 M) will have to be multiplied in order to determine the actual titrand concentration. Once the NaOH solution is
standardized, it can be used to titrate the vinegar samples - always exerCising good care in
handling the solutions.
Based on intrinsic properties, analytical standards can be of two types, namely:
(a) Primary analytical standards, which are stable, homogeneous substances of
well-known properties (e.g. purity, species concentrations) that can be
directly related to chemical standards (e. g. atomic weights) via a traceability
chain provided the specific uncertainty involved in deriving the characteristic quantity is considered [e. g. a purity of (99.5 ± 0.2) % J.
(b) Secondary analytical standards, also known as "working standards", are
substances that lack the properties required for direct use as analytical
standards (e.g. stability, purity, homogeneity) but have to used because an
appropriate primary standard for the intended purpose either does not exist
or is unavailable or expensive. Secondary standards must be traced to a
primary standard through testing (e. g. by standardizing a titrant solution as
described in Box 3.8).
113
13.5.1 Types
Analytical chemical standards can be classified into two, mutually nonexclusive, categories according to (a) intrinsic properties or (b) nature and
purpose (see Fig. 3.8).
Box 3.8
Determining the acidity of commercially available vinegar entails using a "standard" solution
of sodium hydroxide (i. e. one of accurately known concentration, which will be one basis for
traceability). Solid sodium hydroxide does not meet the purity and stabil ity requirements for
use as a primary standard. Therefore, an experimental traceability link must be established
between the secondary standard (NaOH) and a primary standard of acid character (e.g.
potassium hydrogen phthalate, which meets the previous requirements).
A solution about 0.1 Min NaOH is thus prepared. With due care, the solution is used to fill
a conventional or automatic burette. In another operation, an amount of 0.8642 g of potassium hydrogen phthalate is weighed on a balance and dissolved in water. This latter solution
is supplied with phenolphthalein as ind icator and titrated; in the titration, which is equivalent
to standard izing the titrant, a volume of 40.15 mL (practical volume) of the NaOH solution is
used. A simple stoichiometric calculation reveals that the amount of primary standard initially weighed should have consumed 42.15 mL of an exactly 0.0001 M solution of NaOH
(theoretical volume) . The titrimetric factor, defined as the ratio
theoretical volume 42.15
f =
= - = 1.0550
practical volume
40.15
is the number by which the approximate NaOH concentration (0.1 M) will have to be multiplied in order to determine the actual titrand concentration. Once the NaOH solution is
standardized, it can be used to titrate the vinegar samples - always exerCising good care in
handling the solutions.
Based on intrinsic properties, analytical standards can be of two types, namely:
(a) Primary analytical standards, which are stable, homogeneous substances of
well-known properties (e.g. purity, species concentrations) that can be
directly related to chemical standards (e. g. atomic weights) via a traceability
chain provided the specific uncertainty involved in deriving the characteristic quantity is considered [e. g. a purity of (99.5 ± 0.2) % J.
(b) Secondary analytical standards, also known as "working standards", are
substances that lack the properties required for direct use as analytical
standards (e.g. stability, purity, homogeneity) but have to used because an
appropriate primary standard for the intended purpose either does not exist
or is unavailable or expensive. Secondary standards must be traced to a
primary standard through testing (e. g. by standardizing a titrant solution as
described in Box 3.8).