Polymorphism of Potassium Ferrocyanide Trihydrate as Studied by Solid-State Multinuclear NMR Spectroscopy and X-ray Diffraction

Abstract

The polymorphism of bulk powder samples of potassium ferrocyanide trihydrate (K(4)Fe(CN)(6).3H(2)O, KFCT) has been studied using (1)H, (13)C, and (15)N NMR spectroscopy in combination with X-ray diffraction. At room temperature, KFCT typically crystallizes in a monoclinic C2/c form, which converts irreversibly to a monoclinic Cc form upon cooling below -25 degrees C. The structure of both of these forms has been determined using single-crystal X-ray diffraction. A less common metastable tetragonal I4(1)/a form is also known to exist at room-temperature. This tetragonal form also converts to the monoclinic Cc form upon cooling, although this phase transition is irreversible and occurs at -60 degrees C. Initial room-temperature (15)N MAS NMR spectra and powder X-ray diffraction patterns of ground powder samples of KFCT prepared using a variety of crystallization methods suggested that only the C2/c form was obtained from a bulk crystallization. The (13)C MAS NMR spectra consisted of six peaks with equal integrated areas, a result that is inconsistent with the (15)N NMR spectra and known crystal structures. When the samples were not ground, the relative areas of the (13)C NMR peaks were altered, indicating that the bulk samples in fact consisted of the two known forms of KFCT. Using the known temperature dependence of these two polymorphs, the (13)C peaks corresponding to each of the C2/c and I4(1)/a forms were assigned. The (13)C NMR spectra and powder X-ray diffraction results demonstrate that upon grinding, a near 50-50 mixture of the two forms is always produced, rather than a new form entirely. The insensitivity of the (15)N NMR spectra to the polymorphism of KFCT is surprising, and likely arises from a fortuitous overlap of the (15)N NMR peaks of the two forms.

Local Structure in Americium and Californium Hexacyanoferrates – Comparison with Their Lanthanide Analogues

“…The presence of two peaks can be explained by the fact that K 4 [Fe II (CN) 6 ]·3H 2 O can crystallize in two crystallographic structures with a slight difference in CN bond lengths. [30] The spectrum of Am III /Fe II exhibits a band at 2069 cm -1 . In comparison with /Fe II , the peak position is shifted to higher energies.…”

Effect of Nanoscale Confinement on the Crystallization of Potassium Ferrocyanide

“…This is also supported by in situ observation of their crystallization by optical microscopy, which revealed that the diagonal edge of the octagonal crystals often grew faster than the other edges, transforming the shape into a rectangular one (Figure 2 Table S1 and Figure S3). The SAED data demonstrated that the rectangular and octagonal crystals both correspond to the monoclinic form of KFCT 42 (Figures 6a, 6b and 6c and 6d)., while the hexagonal-shaped crystals are tetragonal (Figure 6e and 6f). 52 Effects of Humidity on Crystals Precipitated in Bulk.…”

“…KFCT has a metastable tetragonal form and a thermodynamically stable monoclinic polymorph, and also exhibits a number of twinned crystals. 42,43 Potassium ferrocyanide is used in the food industry to prevent the caking of table salt and to precipitate heavy metals from wine, 44 and has applications as a catalyst, an ion exchanger, photosensitizer and a supramolecular magnetic material. 45,46 KFCT also exhibits a number of unusual solid-state transitions, which have elicited considerable interest from a fundamental point of view.…”

“…54,55 More recent studies have revealed the complexity of the KFCT system, and have shown that it comprises a number of polymorphs which are very close in free energy. 42,43,47,49,51 The five different forms include a monoclinic and a tetragonal polymorph and three different twinned crystals.…”

Investigation of electrochemical behavior of potassium ferricyanide/ferrocyanide redox probes on screen printed carbon electrode through cyclic voltammetry and electrochemical impedance spectroscopy

“…Both redox probes are water soluble and fluorescent. Potassium ferricyanide and potassium ferrocyanide are often used as a tool in physiological experiments [16][17][18]. K3Fe(CN)6…”