48
4 Metrological Traceability
– Transfer quantitatively weighed sub-samples to volumetric flasks (100 mL), pour
20 mL of acetate buffer with an appropriate pH and leave for 20 min, stirring the
contents of each flask several times during this time;
– Add the solution of ascorbic acid, to reduce Fe(III) to Fe(II). After completion of
the reduction process, add 5 mL of 1,10-phenanthroline and wait another 15 min,
so as to allow the formation of a colored complex;
– Fill the flask to the mark.
Before measurements, calibrate the UV–Vis spectrophotometer, using a series of standard
solutions containing increasing concentrations of iron, prepared from a stock solution of iron
nitrate of known purity. Colorful solutions of the iron complex with 1,10-phenanthroline
can be prepared in the same way as the procedure followed in the preparation of sample
solutions. Simultaneously, also prepare a blank solution, to which buffer, reducing agent,
and a complexing reagent are added. The absorbance of successive standard solutions must
be measured. Then, the absorbance of the sample solution of leaves can be measured and
the content of iron in solution calculated.
The procedure described above is a relatively straightforward example of laboratory practice. Nevertheless, it allows the most important items related to ensure
traceability to be highlighted. In practice, it is worth first distinguishing the physical quantities, for which the establishment of traceability is known. In the abovedescribed example, weighing occurs repeatedly and always allows reference to the
traceability to a unit of mass, the kilogram.
The reference to the unit of mass is valid for the following stage of the analytical
procedure:
– Precise weighing of the three portions of about 1 g each.
In this case, the metrological status of analytical balance and weights is important,
and the records in the calibration certificate allow the measurement traceability to
the kilogram to be found, as well as the accompanying uncertainty.
WARNING! At the first measurement of sample weight, before the drying, there is
no need to show traceability, since this step is indicative of estimated weight (about
5 g), providing the possibility of further weighing of at least 1 g per sample (the
procedure requires three sub-samples). Also, the drying conditions (temperature,
time) do not need in this case to ensure the traceability of thermometer and clock
since the assessment of the correctness of the drying process does not result from
the process at a specific temperature (in which case it would be necessary to ensure
the consistency of measurement temperature). In this case, the important difference
is in mass between successive drying stages.
The use of measuring vessels, flask and pipette
In the case of measuring vessels, it is worth always finding the traceability to the SI
unit of mass—the kilogram. Although in practice we use the volume of a flask or
pipette, the calibration of measuring vessel is accomplished by weighing the liquid
contained in a vessel, taking into account the appropriate correction of temperature
for expansion of water and glass. In the case of ‘inflow’ vessels—for example, volumetric flasks—we weigh the empty container, and subsequently, the vessel is filled
4 Metrological Traceability
– Transfer quantitatively weighed sub-samples to volumetric flasks (100 mL), pour
20 mL of acetate buffer with an appropriate pH and leave for 20 min, stirring the
contents of each flask several times during this time;
– Add the solution of ascorbic acid, to reduce Fe(III) to Fe(II). After completion of
the reduction process, add 5 mL of 1,10-phenanthroline and wait another 15 min,
so as to allow the formation of a colored complex;
– Fill the flask to the mark.
Before measurements, calibrate the UV–Vis spectrophotometer, using a series of standard
solutions containing increasing concentrations of iron, prepared from a stock solution of iron
nitrate of known purity. Colorful solutions of the iron complex with 1,10-phenanthroline
can be prepared in the same way as the procedure followed in the preparation of sample
solutions. Simultaneously, also prepare a blank solution, to which buffer, reducing agent,
and a complexing reagent are added. The absorbance of successive standard solutions must
be measured. Then, the absorbance of the sample solution of leaves can be measured and
the content of iron in solution calculated.
The procedure described above is a relatively straightforward example of laboratory practice. Nevertheless, it allows the most important items related to ensure
traceability to be highlighted. In practice, it is worth first distinguishing the physical quantities, for which the establishment of traceability is known. In the abovedescribed example, weighing occurs repeatedly and always allows reference to the
traceability to a unit of mass, the kilogram.
The reference to the unit of mass is valid for the following stage of the analytical
procedure:
– Precise weighing of the three portions of about 1 g each.
In this case, the metrological status of analytical balance and weights is important,
and the records in the calibration certificate allow the measurement traceability to
the kilogram to be found, as well as the accompanying uncertainty.
WARNING! At the first measurement of sample weight, before the drying, there is
no need to show traceability, since this step is indicative of estimated weight (about
5 g), providing the possibility of further weighing of at least 1 g per sample (the
procedure requires three sub-samples). Also, the drying conditions (temperature,
time) do not need in this case to ensure the traceability of thermometer and clock
since the assessment of the correctness of the drying process does not result from
the process at a specific temperature (in which case it would be necessary to ensure
the consistency of measurement temperature). In this case, the important difference
is in mass between successive drying stages.
The use of measuring vessels, flask and pipette
In the case of measuring vessels, it is worth always finding the traceability to the SI
unit of mass—the kilogram. Although in practice we use the volume of a flask or
pipette, the calibration of measuring vessel is accomplished by weighing the liquid
contained in a vessel, taking into account the appropriate correction of temperature
for expansion of water and glass. In the case of ‘inflow’ vessels—for example, volumetric flasks—we weigh the empty container, and subsequently, the vessel is filled