Constraint on the chemical potentials of hydrogen and proton in recombination L. L. Sales, F. C. Carvalho and H. T. C. M. Souza Eur. Phys. J. C, 83 6 (2023) 466 Published online: 04 June 2023 DOI: 10.1140/epjc/s10052-023-11671-z