ABL90 FLEX reference manual Appendix – Traceability to the primary standards at Radiometer
II-3
Traceability
The primary pH standards are traceable to the definitive method for pH. The
definitive method is based on a Hydrogen Electrode System. The primary pH
standards are obtained from the Danish primary laboratory for Electrochemistry
(DPLEC) at the Danish Institute of Fundamental Metrology (DFM). This primary
laboratory is accredited by Danish Accreditation (DANAK accreditation no. 255)
Certification is done in accordance with the method recommended by the
International Union of Pure and Applied Chemistry (IUPAC). The Hydrogen
Electrode System of DLPEC is validated by comparison with Standard Reference
Materials (SRMs) produced by the National Institute of Standards and
Technology (NIST). The primary standards are therefore also traceable to NIST.
The IUPAC-recommended method is described in Ref. 1.
The NIST SRMs used are: 186I/II, 185, 187, 191-192.
Using the primary pH standards, the secondary pH standards are certified in the
Metrology Section. These are normally of the same composition as the primary
buffers, tapped into 2-mL glass ampoules and heat sterilized. The secondary
buffers are stored at 5 °C. Measurements of the secondary buffers are done
using a glass electrode with a saturated calomel reference electrode and a liquid
junction of saturated KCl. The liquid junction is a vertical, cylindrical and open
liquid junction. Measurement of a secondary buffer is done using a primary
buffer together with a certified secondary buffer as standards for making a 2-
point calibration of the glass electrode arrangement.
The primary gases used are Standard Reference Materials (SRMs) produced by
NIST. The NIST SRMs used are: 1674b, 2625a, 2658a and 2659a. The NIST
SRM gases are used to validate primary gravimetric working gas standards,
certified by Scott Medical, Air Liquide or Air Products. The primary gravimetric
working gas standards are validated using a computer-controlled gas
chromatography system, introducing the NIST SRM gases as samples and
comparing the obtained results with the certified values.
The primary gravimetric working gas standards are used as standards in the gas
chromatography system, so that the composition of secondary working gas
standards can be determined.
By using the secondary working gas standards in a tonometer together with an
aqueous buffer solution, a solution with a known pCO
2
and pO
2
is produced. This
aqueous buffer solution is then used to determine the pCO
2
and pO
2
of
secondary working standards. These secondary working standards are aqueous
buffer solutions kept in 2-mL ampoules.
The primary working standards used are gravimetric standards produced from
KCl and NaCl Suprapur, produced by Merck. These primary working standards
are validated using Standard Reference Materials (SRMs) produced by NIST, so
that traceability to NIST is achieved. The NIST SRMs used are: 919a (NaCl) and
999 (KCl). Validation of the primary working standards is done using a flame
photometer together with the NIST SRMs.
The flame photometer method of validating the primary working standards is
described in Ref. 2.
The primary working standards are used to determine the sodium and
potassium concentrations of the secondary working standards. The
concentrations of the secondary working standards are measured using a flame
photometer. The determination of the sodium and potassium concentrations of
fluorocarbon-based secondary standards is done using ion-selective K and ion-
selective Na electrodes on the ABL735 analyzer. The determination takes place
using the primary working standards.
pH
pCO
2
and pO
2
cK
+
and cNa
+
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