Pressure-induced pH changes in aqueous solutions – On-line measurement and semi-empirical modelling approach

The main aim of this work is to investigate the effect of CO2 and N2O under pressure in aqueous solution – both on-line and in real-time. In particular, the paper presents the kinetics of pH behaviour of either slightly buffered minimal medium (MM) or non-buffered solution (Ringer solution) during high-pressure treatment at 5, 30 and 60 bar at 25 °C.An innovative spectrophotometric approach has been developed: an optical fiber conducted the inlet signal of a light source into the high-pressure reactor, while another fiber conducted the outlet signal to a spectrometer, thus, obtaining measurements on-line and real-time.CO2 treatment resulted in a pH decrease down to 3.0 in Ringer solution with the equilibrium being reached after 800 s, while the slightly buffered MM allows only for a decrease down to 4.5 with equilibrium being reached after slightly more than 40 s at 60 bar.N2O treatment, as expected, did not lead to any acidification of the treated medium at any conditions tested.Furthermore, a mathematical model describing both the solubilization kinetics of CO2 and in aqueous solution and the decrease of extracellular pH has been implemented. This model may easily be applied to different reactor configurations. The theoretical results and the goodness of the fit has been verified using the experimental results showing a good accuracy of the complex phenomena with a simplification that is of vital relevance for technical processes.

Graphical abstractFigure optionsDownload full-size imageDownload as PowerPoint slideResearch highlights▶ CO2 and N2O solubilisation kinetics under pressure in aqueous solution. ▶ On-line and in real-time innovative spectrophotometric measurement method of the pH. ▶ Validated mathematical modelling of both the solubilisation kinetics of CO2 in aqueous solution and the decrease of extracellular pH. ▶ Best fitting of the measured pH drop time profile, yielding the mass transfer coefficient realized in the experimental conditions. ▶ Possibility to extend the proposed general methodology to different experimental configurations.