Quantitative in situ monitoring of citric acid phase transition in water using Raman spectroscopy

Citric acid exhibits two solid (anhydrous/monohydrate) phases in water. Below 34 °C, the more stable form being the monohydrate. This organic compound was selected as a model product to evaluate the continuous monitoring of solvent-mediated phase transition using in situ Raman spectroscopy. Specific calibration of the spectral data was developed at 15 °C for assessing in-line both the overall solid concentration in suspension (from 5% to 25%, w/w) and the composition of the solid phase (from 0% to 100% content of anhydrous). It is shown that, in addition to such key-measurements, supersaturation can also be computed in-line from the mass balance of the solute. In order to validate the technique, seeded phase transition experiments were performed at 15 °C and the kinetic process involved during the transition of anhydrous to monohydrate was successfully monitored. Both overall solid concentration and solid phase composition measurements were validated using off-line analytical methods (titration of solute concentration and DSC, respectively). The accuracy of the Raman measurements was found to be of the order of 5%.