Ionic Association in Ammonium Fe(II) Sulfate and Ammonium Fe(III) Sulfate Aqueous Solutions by Ultrasonic Relaxation Spectroscopy

In this work, an ultrasonic relaxation spectroscopic study of aqueous ammonium Fe(II) sulfate, aqueous ammonium Fe(III) sulfate and the corresponding ternary system has been undertaken. A variety of acoustic parameters including relaxation frequency, relaxation amplitude and speed of sound were determined as a function of solution concentration. In addition, the adiabatic compressibility and the molar volume change during the ionic association in ammonium Fe(II) sulfate and ammonium Fe(III) sulfate aqueous solutions were also estimated from the acoustic data. This approach facilitated a comprehensive characterization of the three systems across different concentrations. In the two binary systems, the presence of an ion association mechanism was identified involving the divalent and trivalent iron ions, with the sulfate anions, respectively. Furthermore, in the ternary system, an internal sphere oxidation–reduction mechanism occurred between the divalent and trivalent iron ions. All ions within each solution play an active role in shaping the structure of water molecules, owing to the prevailing kosmotropic characteristics specific to each solution. The results are examined within the context of the current phenomenological understanding in the field.