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 pKa. 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 pKa 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 pKa 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 2.87Ã10-4 mol mol-1 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.

中文翻译：

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EXPRESS：用于游离酸物质浓度监测的在线拉曼光谱和间接硬建模

我们提出了一种监测稀水溶液中离解羧酸物质浓度的方法。使用在线拉曼光谱结合间接硬建模 (IHM) 和多元曲线分辨率 (MCR) 对解离的酸物质进行量化。我们介绍了两种不同的基于滴定的硬模型 (HM) 校准程序，用于具有众所周知（方法 A）或未知（方法 B）酸解离常数 pKa 的水中的单个单质子或多元酸。在这两种方法中，只为每种酸物质准备了水中一种酸物质的光谱。这些光谱用于构建HMs。对于方法 A，HMs 使用计算的理想解离平衡进行校准。对于方法 B，我们通过将理想的酸解离平衡拟合到从光谱 HM 获得的酸峰面积来估计 pKa 值。HM 又是根据 MCR 数据构建的。因此，基于 IHM 的方法 B 与先验已知的 pKa 值无关，而是将它们作为校准程序的一部分提供。作为一个详细的例子，我们分析了水溶液中的衣康酸。对于所有酸性物质和水，在方法 A 和 B 的情况下，我们获得了低于 2.87×10-4 mol mol-1 的低 HM 误差。仅使用四个校准样品，IHM 产生的结果比偏最小二乘回归更准确。此外，我们将我们的方法应用于水中的甲酸、乙酸和柠檬酸，从而验证其作为过程分析技术的普遍性，用于定量监测含羧酸的过程。