The method of X-ray diffraction with isomorphic substitution has been extended to aqueous solutions containing complex ions. In a study of isomorphism between NO3-(aq), BrO3-(aq) and ClO3-(aq), it is found that the NO3- and BrO3- ions are isomorphic up to the level of the first-order difference indicating a similarity in their anion-hydration structure. Based on the isomorphism of these two ions, and because the water molecules are only weakly perturbed from their structure in pure water, a second-order difference calculation was made and the ion-ion pair correlation function of these two ions obtained. The results show that the most probable ion-ion separation is between 5 AA and 11 AA and the distance of closest approach is 4.8 AA. NO3- and BrO3- are found, however, not to be isomorphic with ClO3-.
Burke, CAE., Adya, AK., & Neilson, GW. (1991). A structural study of the aqua-anions NO3-, BrO3- and ClO3- by difference methods of X-ray diffraction. Journal of Physics Condensed Matter, 3, 837 - 850. https://doi.org/10.1088/0953-8984/3/7/008