Complementary Quantitation of Inorganic Counterions and Organic Components Using ICP OES and qNMR for Organo-Sulfonic...

Organic salts are widely used as chemical reagents, pharmaceuticals, and food-related materials. However, the quantitative relationship between organic ions and their inorganic counterions is often assumed based on the reagents used during manufacturing rather than determined experimentally, leading to ambiguity in salt property interpretation and complicating quality control. In this study, we establish a complementary organic-inorganic quantification strategy which enables whole sample quantification; the strategy combines the quantitative ¹H NMR (qNMR) analysis of organic sulfonic acids with inductively coupled plasma optical emission spectrometry (ICP OES) for the direct determination of inorganic countercationsLi⁺, Na⁺, and K⁺using a standard-addition method with minimal sample preparation. The validity of this approach was confirmed using benzenesulfonic acid salts, achieving accurate countercation recovery (98.4-101.5%) with low deviation (<2.9%). Similarly, the content of the prepared potassium benzenesulfonate (1-K) was determined as 99.0% based solely on qNMR (Mol_NMR × MW_salt), whereas the value determined by the complementary quantification strategy (Mol_NMR × MW_organic ion + Mol_ICP × MW_counterion) was 93.3%, indicating that this approach helps avoid misestimation of the salt purity. The method was further applied to a synthesized disulfonic acid, 3-[N-ethyl-N-(2-sulfophenyl)-amino]-methylbenzenesulfonic acid (m,o-ESBSA), a potential tar dye impurity, which exhibited problematic ¹H NMR signal broadening after silica-gel purification, hindering qualitative and quantitative analysis. Complementary organic-inorganic quantification revealed that the broadened ¹H NMR spectrum of m,o-ESBSA depends on the presence of equimolar countercations (Li⁺, Na⁺, and K⁺) introduced by silica-gel purification or ion-exchange treatment, providing correlation data that are useful for the countercation design of the analytical reagent. This strategy provides a robust and widely applicable framework that enables the determination of the actual molar ratios of whole organic salt samples, thereby clarifying composition ratios that were previously ambiguous.