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First HCl optical gas standard worldwide

News on Environment and Climate

Optical spectrometer for SI traceability of hydrogen chloride measuring instruments

PTBnews 3.2022
Especially interesting for

operators of biogas/biomethane digesters

operators of industrial combustion/incineration/power plants

operators of clean room centers

reference and analytical laboratories

Hydrogen chloride gas (HCl) occurs as an impurity in biogas/biomethane, stack emissions from combustion processes and in the air of clean rooms and is harmful to health. In particular for legally prescribed measurements, SI traceability has been a problem to this day. The first spectroscopic HCl primary standard worldwide has now been developed and metrologically validated at PTB.

During fermentation in a biogas digester, acids are formed. If chlorides make their way into the process (e.g., coming from waste products from the food industry), corrosive hydrochloric acid (i.e., the aqueous solution of the HCl gas) may form. HCl may also be formed by the combustion of lignite, coal, domestic waste or biomass (wood/straw). Even in the air of clean room centers, the smallest traces of HCl have to be measured to prevent quality fluctuations of the semiconductors produced.

Calibration gases used to implement the traceability of HCl measuring instruments to the SI are problematic: These gas mixtures lack stability, and they are mostly only available as HCl-N2 mixtures leading to instrument systematic calibration errors in gas matrices other than nitrogen.

The new optical gas standard (OGS) of PTB eliminates these shortcomings. In the future, it will be possible to use the OGS for HCl quality control measurements and for calibration services on HCl sensors. The OGS does not require pre-calibration with a reference gas. Instead, it relies on SI-traceable HCl spectral data as well as on gas pressure and temperature measurements. In this way, it is possible to directly determine absolute HCl gas species concentrations that are traceable to the SI.

PTB’s HCl OGS is a special optical spectrometer based on direct tunable diode laser absorption spectroscopy (dTDLAS). A diode laser in the mid-infrared, an infrared semiconductor detector and one of PTB’s own data evaluation procedures form the basis for the OGS.

The HCl OGS was validated for the first time by comparing it to a new primary gravimetric HCl gas standard of the Korean metrology institute (KRISS) within the scope of a bilateral comparison. At an HCl concentration of 100 μmol/mol, the comparability between PTB and KRISS was 0.3 μmol/mol (i.e., 0.3 % relatively). The OGS is currently being used within the scope of a new global metrological comparison at 30 μmol/mol HCl in N2. In laboratory studies conducted at PTB, it was shown that by using other laser wavelengths, high-resolution measurements in air and in biomethane can be realized. It is therefore expected that it will be possible to offer metrological calibration services in the range from 200 μmol/mol to 10 μmol/mol. Moreover, the HCl OGS approach is suited both to field calibration and direct measurements of HCl.

Since stable, SI-traceable gravimetric HCl gas mixtures are not routinely commercially available in gas cylinders (or dynamic HCl generation systems), this new standard can, in the medium term, help industry reduce the operating costs for HCl measurements and increase the accuracy of these measurements.


Javis Nwaboh
Department 3.4
Analytical Chemistry of the Gas Phase
Phone: +49 531 592-3156
Opens local program for sending emailjavis.nwaboh(at)ptb.de

Scientific publication

J. Nwaboh, Z. Qu, O. Werhahn, V. Ebert: EURAMET 1498 Final Report – Hydrogen Chloride in Nitrogen (2022)