Controlling dissociation equilibrium enables selective desalting of alkaline electrosynthetic formate by Donnan dialysis

Aqueous alkaline electrosynthesis enables production of formic acid (HCOOH) or formate (HCOO − ) using renewable electricity under mild conditions, but coexisting of alkaline electrolyte and organic product hinders downstream separation. Although membrane‐based desalination technologies are efficient, these processes suffer from product loss from transmembrane organic migration. Here, we propose that controlling the dissociation equilibrium of HCOOH/ HCOO − enables selective desalination of alkaline electrosynthetic formate by cation‐exchange membrane‐based Donnan dialysis. By simply controlling the feed pH, HCOOH/HCOO − exists exclusively as negatively charged HCOO − , exhibiting strong Donnan repulsion from the membrane surface and a high diffusion barrier within the membrane. Furthermore, using industrial waste acid as the receiver, combined with a monovalent‐cation‐selective membrane, improves the economic viability of formate desalination. This strategy converts the alkaline formate stream into a neutral solution compatible with established downstream processes, providing an energy‐efficient, scalable separation route for alkaline electrosynthesis.