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The underlying principle of Raman spectroscopy is based on the interaction between light and matter, specifically molecules' inelastic scattering of photons. When a monochromatic beam of light, typically from a laser source, interacts with a sample, most scattered light has the same frequency as the incident light. This is known as Rayleigh scattering.
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Inline Raman Spectroscopy and Indirect Hard Modeling for Concentration Monitoring of Dissociated Acid Species.

Alexander Echtermeyer1, Caroline Marks1, Alexander Mitsos1,2,3

  • 1Process Systems Engineering (AVT.SVT), 9165RWTH Aachen University, Aachen, Germany.

Applied Spectroscopy
|October 27, 2020
PubMed
Summary

This study introduces a new method using Raman spectroscopy and indirect hard modeling (IHM) to accurately measure carboxylic acid concentrations in water. The approach is versatile and can determine acid dissociation constants, offering a robust tool for chemical process monitoring.

Keywords:
ALSIHMMCRProcess analyticsRaman spectroscopyalternating least squarescarboxylic aciddissociationindirect hard modelingitaconic acidmultivariate curve modelingpKa estimation

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Area of Science:

  • Analytical Chemistry
  • Spectroscopy
  • Chemical Engineering

Background:

  • Accurate quantification of dissociated carboxylic acid species in aqueous solutions is crucial for monitoring chemical processes.
  • Traditional methods may require a priori knowledge of acid dissociation constants (pKa) or extensive calibration sets.

Purpose of the Study:

  • To develop and validate an inline Raman spectroscopy approach combined with indirect hard modeling (IHM) for quantifying carboxylic acids in dilute aqueous solutions.
  • To introduce two calibration methods (A and B) for IHM, one requiring known pKa values and the other estimating them.
  • To demonstrate the generalizability of the method for various carboxylic acids.

Main Methods:

  • Inline Raman spectroscopy coupled with indirect hard modeling (IHM) and multivariate curve resolution (MCR).
  • Development of two titration-based hard model (HM) calibration procedures (Method A: known pKa; Method B: unknown pKa).
  • Preparation of spectra for individual acid species and water for HM construction.

Main Results:

  • Achieved low hard model errors (< 2.87 × 10⁻⁴ mol mol⁻¹) for all analyzed acid species and water using both methods.
  • Method B (IHM) accurately estimated pKa values, making it independent of prior knowledge.
  • Demonstrated superior accuracy compared to partial least squares regression with only four calibration samples.
  • Successfully applied the approach to itaconic, formic, acetic, and citric acids, confirming its broad applicability.

Conclusions:

  • The proposed IHM approach provides accurate and generalizable quantitative monitoring of carboxylic acids in aqueous solutions.
  • The method's ability to determine pKa values enhances its utility for process analytical technology (PAT).
  • This technique offers a powerful alternative for real-time analysis in chemical manufacturing and research.