Slice-Selective NMR for Quantitative Analysis of Liquid–Liquid Biphasic Systems: A Tool for Organic Chemists

Biphasic systems are ubiquitous across modern chemistry, yet direct and quantitative analysis of solute distribution across phases remains experimentally nontrivial. While many chemists rely on qualitative intuition to reason about partitioning, confirming these assumptions typically requires time-consuming sample manipulation. We demonstrate that slice-selective NMR spectroscopy─a straightforward technique available on modern NMR instruments─can be used to directly measure spatially resolved concentrations. This allows users to quantify how species partition between immiscible phases without disrupting the equilibrium, offering a powerful and broadly accessible tool for reaction monitoring, mechanistic study, and process troubleshooting. We demonstrate the application of this technique to measure logP values, monitor phase-transfer catalysis, study biphasic reaction kinetics, and quantify the influence of counterion identity and ionic strength on phase distribution. In each example, slice-selective NMR enables an understanding of underlying chemical behavior that would otherwise be difficult to capture. We aim to highlight this technique not as a niche spectroscopic curiosity but as a general-purpose tool that can be readily adopted by synthetic and process chemists. This work serves as both a tutorial introduction and a practical demonstration of how spatially selective NMR can demystify the behavior of biphasic systems and bring quantitative rigor to problems often approached qualitatively.