6.2 Calculable Quantitation Methods
265
be ensured by using a solution of a (directly prepared) primary standard or a
secondary standard linked to a primary one via a preliminary titration. Box 6.9
describes two examples of titrimetric procedures that represent two different
situations as regards traceability. Figure 6.7 provides a schematic depiction of
the titrimetric process.
Box 6.9
The following two examples illustrate the more usual ways of establishing the traceability
chain in methods based on the ritrimetric technique.
Direct use of a primary standard
In the titrimetric determination of water hardness (viz. the (a 2 + and Mgl + concentrations) by
formation of metal chelates with EDTA (H4 Yl. a commercially available substance
(Na2H2Y ' 2 H 2 0) exists that can be used to prepare a titrant (R) solution of accurately known
concentration. An aliquot of sample is supplied with NH 4(I/NH 3 buffer at pH 10 and a metal -
lochromic indicator (neT). The burette is zeroed with the R solution and added dropwise over
the sample until the indicator changes colour.
Use of a secondary standard
When no primary standard for the intended purpose exists or those available are very expensive, one can use a secondary standard, of poorer purity, stability, etc., than a typical
primary standard (see Sect. 3.5.1). A solution of the secondary standard is prepared and an
aliquot titrated with a primary standard. Such is the case with the sodium hydroxide (NaOH)
solutions widely used in acid-base titrations. Solid NaOH cannot be a primary standard
because it is eaSily carbonated. An NaOH solution is made after washing the solid several
times that will contain an approximate reagent concentration [R ]appr.The solution is used to
zero the burette and is standardized with a solution of a primary standard (sodi um hydrogen
phthalate) prepared by weighing to the tenth of a milligram. A volume (practical mL) is thus
measured. The number of theoretical mL corresponding to the amount of acid standard is
calculated and the factor
theoretical m L
f= - -- -
practical m L
is obtained and multiplied by the approximate concentration of the secondary standard
solution in order to determine its actual concentration:
[NaOH] = [NaOHlappr . f
After this preliminary standardization, the sodium hydroxide solution can be used to titrate
acid analytes (e.g. to determine the acidity of commercially available vinegar).
The main titrimetric reaction (A + R ~ B), which can be an acid-base, complexformation, precipitation, redox, addition, condensation or substitution one,
must meet five essential requirements, namely:
(a) it should have a well-defined stoichiometry as this will be the basis for calculations;
265
be ensured by using a solution of a (directly prepared) primary standard or a
secondary standard linked to a primary one via a preliminary titration. Box 6.9
describes two examples of titrimetric procedures that represent two different
situations as regards traceability. Figure 6.7 provides a schematic depiction of
the titrimetric process.
Box 6.9
The following two examples illustrate the more usual ways of establishing the traceability
chain in methods based on the ritrimetric technique.
Direct use of a primary standard
In the titrimetric determination of water hardness (viz. the (a 2 + and Mgl + concentrations) by
formation of metal chelates with EDTA (H4 Yl. a commercially available substance
(Na2H2Y ' 2 H 2 0) exists that can be used to prepare a titrant (R) solution of accurately known
concentration. An aliquot of sample is supplied with NH 4(I/NH 3 buffer at pH 10 and a metal -
lochromic indicator (neT). The burette is zeroed with the R solution and added dropwise over
the sample until the indicator changes colour.
Use of a secondary standard
When no primary standard for the intended purpose exists or those available are very expensive, one can use a secondary standard, of poorer purity, stability, etc., than a typical
primary standard (see Sect. 3.5.1). A solution of the secondary standard is prepared and an
aliquot titrated with a primary standard. Such is the case with the sodium hydroxide (NaOH)
solutions widely used in acid-base titrations. Solid NaOH cannot be a primary standard
because it is eaSily carbonated. An NaOH solution is made after washing the solid several
times that will contain an approximate reagent concentration [R ]appr.The solution is used to
zero the burette and is standardized with a solution of a primary standard (sodi um hydrogen
phthalate) prepared by weighing to the tenth of a milligram. A volume (practical mL) is thus
measured. The number of theoretical mL corresponding to the amount of acid standard is
calculated and the factor
theoretical m L
f= - -- -
practical m L
is obtained and multiplied by the approximate concentration of the secondary standard
solution in order to determine its actual concentration:
[NaOH] = [NaOHlappr . f
After this preliminary standardization, the sodium hydroxide solution can be used to titrate
acid analytes (e.g. to determine the acidity of commercially available vinegar).
The main titrimetric reaction (A + R ~ B), which can be an acid-base, complexformation, precipitation, redox, addition, condensation or substitution one,
must meet five essential requirements, namely:
(a) it should have a well-defined stoichiometry as this will be the basis for calculations;