162
4 The Measurement Process in Chemistry
Inorganic analytes present in a matrix of organic (e.g. metal traces in petroleum) or biological nature (e.g. iron in serum) require destruction of organic
matter - an alternative to sample dissolution. This operation involves the
oxidation of organic matter (viz. that of carbon, hydrogen, nitrogen and sulphur
to CO2, H20, NxOy and S02, respectively) by using appropriate reagents in
different possible ways. So-called "dry procedures" heat the sample at
550-600°C in an appropriate container (a crucible); the oxidant is obviously
atmospheric oxygen. This procedure has the disadvantage that it results in
partial or even complete losses of volatile elements such as arsenic or mercury.
"Wet procedures" heat the sample with an oxidizing acid (HN03, HCI04) or an
acid mixture in a appropriate vessel (see Box 4.7). There are alternative, special
procedures for the same purpose such as those based on the Sh6ninger oxygen
flask or the Kjeldahl process for the determination of organic and ammonia
nitrogen, respectively.
__ Box4.7
The determination of metal traces in organic or biological samples by wet methods entails
the use of an oxidizing acid (e. g. HNOJ, HCI04) in the presence or absence of an additional
oxidant (e.g. H 2 0 2 ) to decompose the sample into a residue of metal oxides readily dissolved
in acids or a concentrated solution of the oxides.This treatment can be performed in a variety
of ways.
Those samples that contain abundant moisture (e. g. biological materials) are best freezedried and then subjected to high vacuum in order to remove the water and break the matrix
structure (e. g. any macromolecules in it). The reSUlting residue can be readily dissolved in an
organic acid under mild conditions.
Alternatively, organic and biological samples can be attacked with an oxidizing acid in a
flask furnished with a condenser, with heating for an interval of half an hour to several hours.
This procedure is also slow and hazardous (it involves the use of sizeable volumes of toxic,
polluting solvents).
In order to expedite the process and reduce the volume of oxidizing acid needed, samples
can be digested in an air-tight enclosure known as a "pressurized reactor'~ This is usually a
Teflon container lined with the same polymer material that is accommodated inside a steel
cylinder which is opened and closed via threaded parts. Once the sample and acids have been
inserted, the reactor is shut and heated. The combined effect of temperature and the pressure
inside the vessel is thus exploited.
Microwave ovens have recently proved highly efficient for digestion inside pressurized
reactors. In fact, the effect of concentrated microwave energy is a more drastic attack in a
shorter time. The reactors must be made ofTeflon or quartz and the steel housing removed to
facilitate irradiation of the sample and avoid reflection of waves.
Solid samples that cannot be dissolved in an acid require disaggregation, usually by fusion with a solid alkali (e.g. Na2C03), alkali-oxidant (e.g. Na202),
alkali-reductant (e.g. KeN + C) or acid (KHS04 ),depending on the nature of the
sample matrix, in a platinum or nickel crucible. Once cool, the molten mass is
dissolved in an appropriate solvent.
4 The Measurement Process in Chemistry
Inorganic analytes present in a matrix of organic (e.g. metal traces in petroleum) or biological nature (e.g. iron in serum) require destruction of organic
matter - an alternative to sample dissolution. This operation involves the
oxidation of organic matter (viz. that of carbon, hydrogen, nitrogen and sulphur
to CO2, H20, NxOy and S02, respectively) by using appropriate reagents in
different possible ways. So-called "dry procedures" heat the sample at
550-600°C in an appropriate container (a crucible); the oxidant is obviously
atmospheric oxygen. This procedure has the disadvantage that it results in
partial or even complete losses of volatile elements such as arsenic or mercury.
"Wet procedures" heat the sample with an oxidizing acid (HN03, HCI04) or an
acid mixture in a appropriate vessel (see Box 4.7). There are alternative, special
procedures for the same purpose such as those based on the Sh6ninger oxygen
flask or the Kjeldahl process for the determination of organic and ammonia
nitrogen, respectively.
__ Box4.7
The determination of metal traces in organic or biological samples by wet methods entails
the use of an oxidizing acid (e. g. HNOJ, HCI04) in the presence or absence of an additional
oxidant (e.g. H 2 0 2 ) to decompose the sample into a residue of metal oxides readily dissolved
in acids or a concentrated solution of the oxides.This treatment can be performed in a variety
of ways.
Those samples that contain abundant moisture (e. g. biological materials) are best freezedried and then subjected to high vacuum in order to remove the water and break the matrix
structure (e. g. any macromolecules in it). The reSUlting residue can be readily dissolved in an
organic acid under mild conditions.
Alternatively, organic and biological samples can be attacked with an oxidizing acid in a
flask furnished with a condenser, with heating for an interval of half an hour to several hours.
This procedure is also slow and hazardous (it involves the use of sizeable volumes of toxic,
polluting solvents).
In order to expedite the process and reduce the volume of oxidizing acid needed, samples
can be digested in an air-tight enclosure known as a "pressurized reactor'~ This is usually a
Teflon container lined with the same polymer material that is accommodated inside a steel
cylinder which is opened and closed via threaded parts. Once the sample and acids have been
inserted, the reactor is shut and heated. The combined effect of temperature and the pressure
inside the vessel is thus exploited.
Microwave ovens have recently proved highly efficient for digestion inside pressurized
reactors. In fact, the effect of concentrated microwave energy is a more drastic attack in a
shorter time. The reactors must be made ofTeflon or quartz and the steel housing removed to
facilitate irradiation of the sample and avoid reflection of waves.
Solid samples that cannot be dissolved in an acid require disaggregation, usually by fusion with a solid alkali (e.g. Na2C03), alkali-oxidant (e.g. Na202),
alkali-reductant (e.g. KeN + C) or acid (KHS04 ),depending on the nature of the
sample matrix, in a platinum or nickel crucible. Once cool, the molten mass is
dissolved in an appropriate solvent.