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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

and pO

is produced. This

aqueous buffer solution is then used to determine the pCO

and pO

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.

pCO

and pO

and cNa