Inline Raman Spectroscopy and Indirect Hard Modeling for Concentration Monitoring of Dissociated Acid Species

Abstract

We propose an approach for monitoring the concentration of dissociated carboxylic acid species in dilute aqueous solution. The dissociated acid species are quantified employing inline Raman spectroscopy in combination with Indirect Hard Modeling (IHM) and Multivariate Curve Resolution (MCR). We introduce two different titration-based Hard Model (HM) calibration procedures for a single mono- or polyprotic acid in water with well-known (method A) or unknown (method B) acid dissociation constants p<i>K</i><sub>a</sub>. In both methods, spectra of only one acid species in water are prepared for each acid species. These spectra are used for the construction of HMs. For method A, the HMs are calibrated with calculated ideal dissociation equilibria. For method B, we estimate p<i>K</i><sub>a</sub> values by fitting ideal acid dissociation equilibria to acid peak areas that are obtained from a spectral HM. The HM in turn is constructed on the basis of MCR data. Thus, method B on the basis of IHM is independent of a priori known p<i>K</i><sub>a</sub> values, but instead provides them as part of the calibration procedure. As a detailed example, we analyze itaconic acid in aqueous solution. For all acid species and water, we obtain low HM errors of less than 2.87×10<sup>-4</sup> mol mol<sup>-1</sup> in the cases of both method A and B. With only four calibration samples, IHM yields more accurate results than partial least squares regression. Furthermore, we apply our approach to formic, acetic, and citric acid in water, thereby verifying its generalizability as a process analytical technology for quantitative monitoring of processes containing carboxylic acids.

The Effect of Sulfate Electrolytes on the Liquid–Liquid Equilibrium of 2-MTHF/Water/5-HMF: Experimental Study and Thermodynamic Modeling

“…For the construction of pure component models (PCMs), complemental hard modeling (CHM) and IHM, the software PEAXACT ver. 5.3–5.4 (S-PACT, Aachen, Germany) is used. , Principles on IHM construction and calibration of dissociated acid species are presented by the authors in a previous article on the basis of Raman spectroscopy . In the present work, the concept is transferred to IR spectroscopy.…”

“…Thereby, the dissociation states of H 2 SO 4 , Li 2 SO 4 , and Na 2 SO 4 in the liquid–liquid system are analyzed. Motivated by prior work of our group, infrared (IR) spectroscopy is used to acquire multivariate data of the aqueous and organic phases. − Quantification of all species in both phases including dissociating sulfuric acid species is achieved by comprehensive Indirect Hard Modeling (IHM), transferring a recently published approach for Raman spectroscopy and dissociating carboxylic acids to IR spectroscopy and dissociating H 2 SO 4 . − Finally, we use the equation of state ePC-SAFT “revised” to model the system behavior in the measured temperature range.…”

On the use of Raman spectroscopy to characterize mass-produced graphene nanoplatelets

“…It has the advantages of relatively low cost, simple sample preparation, quick measurements, and automated analysis, offering clear benefits for quality control applications. It has been demonstrated in several application areas as an in-line process analysis and control method [9][10][11][12].…”

A Component Content Measurement Method Modified Using Indirect Hard Modeling for Polymer Blends Based on Raman Spectroscopy

“…In this way, the mixture spectrum is obtained by weighted superposition of pure component models whose parameters are simultaneously adapted to account for the nonlinear effects in the spectrum. At present, the IHM method has mainly been used for the determination of fuel gas compositions, 23 solute content, 24,25 monomer content, 26 and particle size. 27 Inspired by the previous research, this study would like to explore the possibility of combining a method modified using IHM method and Raman spectroscopy to realize the measurement of the component content of polymer blends.…”

“…In this way, the mixture spectrum is obtained by weighted superposition of pure component models whose parameters are simultaneously adapted to account for the nonlinear effects in the spectrum. At present, the IHM method has mainly been used for the determination of fuel gas compositions, 23 solute content, 24,25 monomer content, 26 and particle size. 27…”