Abstract
A novel design of alkali chloride vapour-generating cell has been developed, which can serve as a calibration cell for quantitative ultraviolet absorption concentration measurements and meticulous spectral investigations of alkali compounds. The calibration cell was designed to provide alkali vapour of well-controlled concentrations and temperatures, and consisted of a sealed quartz cell measuring 0.4 m in length with a temperature-controlled reservoir containing solid alkali salt. The cell was placed in a furnace and the alkali vapours generated from the reservoir have direct access to the measuring chamber. Investigations of potassium chloride (KCl) were made on sublimated vapour at temperatures 650, 700, 750, 780, and 800 °C while the reservoir temperature was kept 50 °C lower to avoid condensation. The cell provides stable KCl vapour pressures, and the furnace provides a homogenous temperature profile along the cell. KCl vapour pressures are well characterised and conform the base for determination of the KCl concentration in the cell. The alkali chloride levels matched the concentration range of the absorption setup and indicated a previously employed calibration method to overestimate KCl concentrations. The KCl absorption cross sections for wavelengths λ=197.6 nm and λ=246.2 nm were calculated to be 3.4 × 10-17 and 2.9 × 10-17 cm2/molecule, respectively. The absorption cross section spectra did not show any structural differences with increasing temperature, which could indicate influence of dimers or significant changes of the population in the KCl vibrational states. The KCl absorption cross sections thus did not show any temperature dependence in the temperature region of 700-800 °C. Moreover, the applicability of the calibration cell for measurement of other alkali chlorides and hydroxides is discussed.
Highlights
Three different designs of high-temperature calibration cells for alkali compounds at atmospheric and lower pressures from the literature have been studied and are briefly described below
The salt reservoir temperature and subsequently the KCl vapour pressure in the calibration cell could be even better stabilized with an automatic temperature controller instead of manual control, as was the case for this experiment
The design and performance have been demonstrated for a calibration cell based on a cold finger, which can be employed to generate KCl vapour at stable well-defined concentration and temperature
Summary
Three different designs of high-temperature calibration cells for alkali compounds at atmospheric (open cell) and lower pressures (sealed cell) from the literature have been studied and are briefly described below. Vattenfall Research and Development AB has developed and patented an in situ alkali chloride monitor (IACM) based on UV absorption and designed a calibration cell for this instrument. This calibration cell operates at atmospheric pressure where solid. There is need for a calibration cell that generates vapours from alkali compounds at stable, well-defined concentration levels, and allows for optical measurements at well-controlled temperatures, which has motivated this research. The calibration cell presented in this paper is stable and robust, which facilitates handling and experimental reproducibility, which in turn result in accurate and reliable calibration data
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