4.5 Metrological Traceability in Practice
49
to the mark with water. In the case of ‘outflow’ vessels—for example, graduated
pipettes—we weigh the mass of water after pouring it from a pipette. In practice, we
weigh the empty vessel, and then the one with a pipetted portion of the water. Measuring vessels can be calibrated independently in the laboratory by weighing water
contained in them. In this case, traceability refers to the certificate of calibration of
analytical balance and weights. Measuring vessels can be bought with a certificate
of calibration, which means that the process has been made by the producer and the
user buys a measuring vessel with an appropriate metrological service. Calibrating
the measuring vessel is always carried out by weight, which means that regardless
of whether we execute this process in the laboratory, use a balance or purchase a
service of calibration by a manufacturer of volumetric glassware, the measurement
is consistent with the SI unit—the kilogram.
In the case of the described procedure, measurement traceability must be ensured
only for the 100 mL volumetric flask, the one in which a sample solution is prepared.
Pipettes used for dispensing reagents (buffer solution and 1,10-phenanthroline) do
not need the ensured traceability since they are required only for the transfer of the
solutions of reagents, which are given in excess.
Figure 4.4 shows the volumetric flask with a volume of 500 mL (nominal capacity) with typical information provided by the manufacturer. According to the requirements of the German Committee for Standardization (DIN; Deutsches Institut für
Normung) glass measuring vessels should have the following markings:
– Producer (name and trademark)
– Nominal capacity (e.g., 500)
– Tolerance (e.g., ±0.25)
– The unit of measurement (e.g., mL)
– Calibration information (e.g., ‘inflow’ calibration)
– Applied temperature (the temperature of the water during calibration)
– Class of the vessels (class A is the highest quality class of measuring vessels).
Preparation of the series of calibration solutions
At this step, iron (II) nitrate of known purity is used, which is weighed and inserted
into the flask. To the flask, a portion of the solvent (e.g., 0.1 mol/L HNO 3 ) is added,
which permits the solubility of substances, and then the flask is filled up to the mark.
The process ensuring the traceability for weighing the reference substance is as
follows:
– We must ensure traceability of the reference weighing, since it is important to
know the exact weight of a reference substance and its molar mass;
– We must ensure traceability of the flask (weight calibration or buying a flask with
a calibration certificate from a competent manufacturer).
In order to prepare standard solutions, the iron (II) nitrate with a defined chemical
composition and sufficient stability, together with adequate and known purity is used.
It is rare that the purity of the reagents should be checked on-site in the laboratory. A
common practice is to purchase them from a producer of chemical reagents. In this
49
to the mark with water. In the case of ‘outflow’ vessels—for example, graduated
pipettes—we weigh the mass of water after pouring it from a pipette. In practice, we
weigh the empty vessel, and then the one with a pipetted portion of the water. Measuring vessels can be calibrated independently in the laboratory by weighing water
contained in them. In this case, traceability refers to the certificate of calibration of
analytical balance and weights. Measuring vessels can be bought with a certificate
of calibration, which means that the process has been made by the producer and the
user buys a measuring vessel with an appropriate metrological service. Calibrating
the measuring vessel is always carried out by weight, which means that regardless
of whether we execute this process in the laboratory, use a balance or purchase a
service of calibration by a manufacturer of volumetric glassware, the measurement
is consistent with the SI unit—the kilogram.
In the case of the described procedure, measurement traceability must be ensured
only for the 100 mL volumetric flask, the one in which a sample solution is prepared.
Pipettes used for dispensing reagents (buffer solution and 1,10-phenanthroline) do
not need the ensured traceability since they are required only for the transfer of the
solutions of reagents, which are given in excess.
Figure 4.4 shows the volumetric flask with a volume of 500 mL (nominal capacity) with typical information provided by the manufacturer. According to the requirements of the German Committee for Standardization (DIN; Deutsches Institut für
Normung) glass measuring vessels should have the following markings:
– Producer (name and trademark)
– Nominal capacity (e.g., 500)
– Tolerance (e.g., ±0.25)
– The unit of measurement (e.g., mL)
– Calibration information (e.g., ‘inflow’ calibration)
– Applied temperature (the temperature of the water during calibration)
– Class of the vessels (class A is the highest quality class of measuring vessels).
Preparation of the series of calibration solutions
At this step, iron (II) nitrate of known purity is used, which is weighed and inserted
into the flask. To the flask, a portion of the solvent (e.g., 0.1 mol/L HNO 3 ) is added,
which permits the solubility of substances, and then the flask is filled up to the mark.
The process ensuring the traceability for weighing the reference substance is as
follows:
– We must ensure traceability of the reference weighing, since it is important to
know the exact weight of a reference substance and its molar mass;
– We must ensure traceability of the flask (weight calibration or buying a flask with
a calibration certificate from a competent manufacturer).
In order to prepare standard solutions, the iron (II) nitrate with a defined chemical
composition and sufficient stability, together with adequate and known purity is used.
It is rare that the purity of the reagents should be checked on-site in the laboratory. A
common practice is to purchase them from a producer of chemical reagents. In this