3.5 Analytical Chemical Standards
115
a physical standard ranking (see Box 3.5). As a rule, physical standards are
either RMs issued by manufacturers or, less often, CRMs.
(b) Pure (and mixed) substances used as standards, which are chemical
materials consisting of one or more substances the identity and (high)
purity of which is assured by a manufacturer or, less frequently, a certifying
body. The certified properties can be physical (e. g. the boiling point of
phenol), physico-chemical (e. g. the UV absorption spectrum for a holmium
filter or the IR absorption spectrum for polystyrene film) or chemical (e.g.
the redox equivalent of KI03) . This type of standard can be used to calibrate
an instrument, an apparatus or a relative analytical method, to determine
an analyte (by titrimetry) or to experimentally standardize secondary
standards. The purity of some standards [e. g. polycyclic aromatic hydrocarbons (PAHs) and their mixtures, dioxins] is certified by a national or
international organization engaged on chemical metrology.
__ Box 3.9
Calibration of a UV-visible molecular absorption spectrophotometer
A photometer can be calibrated via two parameters, namely: the signal (transmittance or
absorbance) it provides and the wavelength it uses, which rep resent the quantitative and
qualitative concepts, respectively, discussed in the text. Also, a photometer can be calibrated
for a specific (MP by cali brati ng its response to a standard of the measurand (viz. establishing
the absorbance-analyte concentration relation).
A wide va riety of available measurement standards exists for the qualitative and
quantitative ca libration of photometers; such standards can be of two types depending on
their state of aggregation (solids and solutions) and purpose (absorbance and wavelength
standards).
Absorbance standards are used for the quantitative calibration of spectrophotometers
by comparing the absorbance A or molar absorptivity E [derived from Beer's law (A = £Ie))
experimentally obtained with the standard and its certified (CRM) or stated value (RM).
Specific liquid absorbance standards can be prepared by weighing appropriate amounts
of pure or mixed substances and dissolving them in a given volume of a solution of controlled
pH. Alternatively, they can be purchased as RMs in sealed vials.
One typical example of solid standard of this type is potassium dichromate, which is
available as both an RM and a CRM (NIST); the latter has a certified E value and comes with
comprehensive documentation regarding preparation of its solutions and usage to make
absorbance measurements. A solution of coba lt and nickel in 0.1 M HCl04 that is sold in sealed
ampoules as a CRM with certified absorbance values at four different wavelengths (302,395,
512 and 678 nm) is a typical dissolved standard.
Wavelength standards are used to calibrate monochromaticity in order to ensure that the
wavelength set and that actually employed by a spectrophotometer will coincide. There are
various types of standards for this purpose. Glow discharge lamps emit at highly precise
wavelengths (e.g. the low-pressure mercury lamp at 253.65 and 184.96 nm); as a result,
spectra recorded under their light facilitate comparison of reference and actual wavelengths.
Holmium oxide and neodymium-praseodymium glasses, commonly known as holmium and
didymium filters, are also used for this purpose.
115
a physical standard ranking (see Box 3.5). As a rule, physical standards are
either RMs issued by manufacturers or, less often, CRMs.
(b) Pure (and mixed) substances used as standards, which are chemical
materials consisting of one or more substances the identity and (high)
purity of which is assured by a manufacturer or, less frequently, a certifying
body. The certified properties can be physical (e. g. the boiling point of
phenol), physico-chemical (e. g. the UV absorption spectrum for a holmium
filter or the IR absorption spectrum for polystyrene film) or chemical (e.g.
the redox equivalent of KI03) . This type of standard can be used to calibrate
an instrument, an apparatus or a relative analytical method, to determine
an analyte (by titrimetry) or to experimentally standardize secondary
standards. The purity of some standards [e. g. polycyclic aromatic hydrocarbons (PAHs) and their mixtures, dioxins] is certified by a national or
international organization engaged on chemical metrology.
__ Box 3.9
Calibration of a UV-visible molecular absorption spectrophotometer
A photometer can be calibrated via two parameters, namely: the signal (transmittance or
absorbance) it provides and the wavelength it uses, which rep resent the quantitative and
qualitative concepts, respectively, discussed in the text. Also, a photometer can be calibrated
for a specific (MP by cali brati ng its response to a standard of the measurand (viz. establishing
the absorbance-analyte concentration relation).
A wide va riety of available measurement standards exists for the qualitative and
quantitative ca libration of photometers; such standards can be of two types depending on
their state of aggregation (solids and solutions) and purpose (absorbance and wavelength
standards).
Absorbance standards are used for the quantitative calibration of spectrophotometers
by comparing the absorbance A or molar absorptivity E [derived from Beer's law (A = £Ie))
experimentally obtained with the standard and its certified (CRM) or stated value (RM).
Specific liquid absorbance standards can be prepared by weighing appropriate amounts
of pure or mixed substances and dissolving them in a given volume of a solution of controlled
pH. Alternatively, they can be purchased as RMs in sealed vials.
One typical example of solid standard of this type is potassium dichromate, which is
available as both an RM and a CRM (NIST); the latter has a certified E value and comes with
comprehensive documentation regarding preparation of its solutions and usage to make
absorbance measurements. A solution of coba lt and nickel in 0.1 M HCl04 that is sold in sealed
ampoules as a CRM with certified absorbance values at four different wavelengths (302,395,
512 and 678 nm) is a typical dissolved standard.
Wavelength standards are used to calibrate monochromaticity in order to ensure that the
wavelength set and that actually employed by a spectrophotometer will coincide. There are
various types of standards for this purpose. Glow discharge lamps emit at highly precise
wavelengths (e.g. the low-pressure mercury lamp at 253.65 and 184.96 nm); as a result,
spectra recorded under their light facilitate comparison of reference and actual wavelengths.
Holmium oxide and neodymium-praseodymium glasses, commonly known as holmium and
didymium filters, are also used for this purpose.