The Hydration Structure of Ni 2+ in Aqueous Solution by Neutron Diffraction Isotope Substitution: a Case Study on the Effects of Systematic Error

Abstract

The technique of neutron diffraction with isotope substitution has been used to probe the hydration structure around the Ni²⁺ ion in a concentrated aqueous solution of 3.87 m NiCl₂. The scattering from two heavy water solutions containing, respectively, nickel in natural abundance and enriched in the ⁶⁰Ni isotope, were measured under ambient conditions on the GLAD instrument at the Intense Pulsed Neutron Source, Argonne, using time-of-flight neutron diffraction. Because there have been several earlier neutron measurements, aqueous NiCl₂ is widely regarded as a reference system for studies of hydration in aqueous solution. An unexpectedly large difference in deuterium enrichment between our heavy water solutions was detected, initially from analysis of the scattering data. This imbalance, although on the face of it undesirable, has yielded a useful opportunity to gain insights into the effects of such a systematic error on the present results by comparing them to those from earlier measurements.