Diffraction Studies on Concentrated Aqueous Hydrochloric Acid Solutions

Abstract

AbstractX‐ray diffraction and neutron diffraction experiments with H/D isotopically substituted aqueous 21 mol% hydrochloric acid solutions were carried out in order to obtain detailed features concerning the intermolecular hydrogen‐bonded structure in highly concentrated aqueous acidic solutions. Structure parameters, namely, intermolecular distance, root‐mean‐square amplitude, and coordination number, for the nearest neighbor H3O+···H2O interaction were determined from the least‐squares‐fitting analysis of the observed X‐ray interference term. These were, respectively, r(H3O+···H2O) = 2.45(1) Å; l(H3O+···H2O) = 0.11(1) Å; and n(H3O+···H2O) = 1.8(1). The intermolecular nearest neighbor distances, r(H···H) = 2.02(5) Å and r(O···H) = 1.69(2) Å, were determined from the least‐squares fit to partial structure factors, aij (Q), derived from the observed neutron intermolecular interference terms for sample solutions with different H/D isotopic ratios. The present values of intermolecular distances are significantly shorter than those for pure liquid water, implying that extremely strong hydrogen bonds exist in concentrated aqueous acidic solutions.