Synthesis and Quantitation of <i>m</i>,<i>p</i>-ESBSA: An Impurity in Brilliant Blue FCF and Fast Green FCF

3-[<i>N</i>-Ethyl-<i>N</i>-(4-sulfophenyl)amino]methylbenzenesulfonic acid (<i>m</i>,<i>p</i>-ESBSA) is a common impurity in Brilliant Blue FCF (Food Blue No.1: <b>B1</b>) and Fast Green FCF (Food Green No.3: <b>G3</b>). For the quality control of <b>B1</b> and <b>G3</b>, quantitative analysis of <i>m</i>,<i>p</i>-ESBSA is required. However, no analytical-grade <i>m</i>,<i>p</i>-ESBSA suitable as a quantitative standard is readily available. In this study, we established an efficient synthesis that provides <i>m</i>,<i>p</i>-ESBSA with a quality that can be used for quantitative analysis. The synthetic route to <i>m</i>,<i>p</i>-ESBSA involves inexpensive starting materials, and the product was structurally characterized by detailed NMR analysis. In addition, the suitability of the synthesized samples for quantitative analysis was confirmed by ¹H-qNMR analysis and quantitation of food dye products. The results showed that the purity of the synthesized <i>m</i>,<i>p</i>-ESBSA was adequate for use as an analytical standard in quantitative analyses.