Potassium ferrocyanide trihyrate is a chemical compound with the formula C6H6FeK4N6O3. It is also known as potassium ferricyanide with the formula C6FeK4N6. It is used as a food additive in salt and salt substitutes. The compound has been studied for its electrochemical behavior and polymorphism.

Synthesis of Potassium ferrocyanide trihyrate

Weigh 5.68 grams of (NH4)4[Fe(CN)6] and 1.70g of KCl into a 100ml beaker, add 40ml of water to dissolve completely, referred to as sample A; Weigh 7.040 grams of manganese triacetate (III) Put it into a 250 ml flask, add 100 ml of concentrated phosphoric acid and stir until completely dissolved, the solution is transparent, referred to as sample B; Slowly add sample A dropwise into the beaker of sample B while stirring, and seal the mouth of the beaker with sealing glue. After standing at room temperature for 6 hours in the dark, the light blue crystals of Potassium ferrocyanide trihyrate with the molecular formula KMn 3+[Fe 2+(CN) 6] were obtained, and the crystals were washed repeatedly with high-purity water;The obtained crystals were vacuum-dried at 34°C for 24 hours to obtain dry crystals. The XRD pattern of the crystal powder is shown in Figure 1;In its crystalline powder XPS spectrum, the XPS spectrum of iron is Fe 2+2P spectrum as shown in Figure 2,The XPS spectrum of manganese is the spectrum of Mn 3+2P, as shown in Figure 3. The element ratio K:Mn:Fe measured by XPS is about 1:1:1; the element ratio K:Mn:Fe measured by ICP of the crystalline powder sample is 1: 1:1. Above data proves that the molecular structure of above-mentioned reaction gained compound is KMn3+[Fe2+(CN)6],It is called Potassium ferrocyanide trihyrate compound, and its crystal color is light yellow.[1]

Polymorphism of Potassium Ferrocyanide Trihydrate as Studied

The polymorphism of bulk powder samples of potassium ferrocyanide trihydrate (K4Fe(CN)6·3H2O, KFCT) has been studied using 1H, 13C, and 15N NMR spectroscopy in combination with X-ray diffraction. At room temperature, Potassium ferrocyanide trihyrate typically crystallizes in a monoclinic C2/c form, which converts irreversibly to a monoclinic Cc form upon cooling below ?25 °C. The structure of both of these forms has been determined using single-crystal X-ray diffraction. A less common metastable tetragonal I41/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 °C. Initial room-temperature 15N MAS NMR spectra and powder X-ray diffraction patterns of ground powder samples of Potassium ferrocyanide trihyrate prepared using a variety of crystallization methods suggested that only the C2/c form was obtained from a bulk crystallization. The 13C MAS NMR spectra consisted of six peaks with equal integrated areas, a result that is inconsistent with the 15N NMR spectra and known crystal structures. When the samples were not ground, the relative areas of the 13C NMR peaks were altered, indicating that the bulk samples in fact consisted of the two known forms of Potassium ferrocyanide trihyrate. Using the known temperature dependence of these two polymorphs, the 13C peaks corresponding to each of the C2/c and I41/a forms were assigned. The 13C 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 15N NMR spectra to the polymorphism of Potassium ferrocyanide trihyrate is surprising, and likely arises from a fortuitous overlap of the 15N NMR peaks of the two forms.[2]

In the solid state, the most commonly studied diamagnetic ferrocyanide compound is its hydrated potassium salt, K4Fe(CN)6·3H2O (herein referred to as Potassium ferrocyanide trihyrate). At ?25 °C, Potassium ferrocyanide trihyrate undergoes an interesting second-order phase transition from a paraelectric to a ferroelectric phase. The ferroelectric transition is believed to be initiated by changes in the dynamics of the water molecules, which undergo rapid reorientations about more than one axis at room temperature and become ordered when cooled below the Curie temperature.  The change in the water dynamics is accompanied by a phase change from monoclinic C2/c (a = 9.379 ?, b = 16.844 ?, c = 9.392 ?, β = 90.07°) above the transition temperature to monoclinic Cc (a = 9.353 ?, b = 16.755 ?, c = 9.359 ?, β = 90.08°) at low temperature. tudies of ferroelectricity in Potassium ferrocyanide trihyrate have been complicated by its tendency to form crystals that are twinned and as a result, some “single-crystal” experiments have been performed on what were actually twinned crystals. In practice, a bulk crystallization of Potassium ferrocyanide trihyrate at room temperature can produce a mixture containing the common monoclinic C2/c form, a less common tetragonal form (I41/a, a = 9.410 ?, c = 33.67 ?) whose structure has yet to be refined, and three forms that are composed of different types of twinned crystals. The tetragonal form is metastable and upon cooling below ?60 °C the individual crystals convert to the aforementioned monoclinic Cc form. When warmed back to room temperature, the same crystals convert to a twinned C2/c monoclinic form whose twin domains are related by a 90° rotation about the monoclinic b-axis.[3]

The relative intensities of peaks in the 13C NMR spectra of Potassium ferrocyanide trihyrate are also sensitive to grinding. After grinding using a mortar and pestle, the relative integrated areas of the six peaks are approximately equal. Variable temperature 13C NMR experiments performed on a ground sample of Potassium ferrocyanide trihyrate have demonstrated that additional peaks are present upon heating and such peaks arise from the formation of anhydrous KFC; no hydrates of KFC aside from the trihydrate are formed. In addition, powder X-ray diffraction experiments do not suggest the formation of an unknown polymorph of Potassium ferrocyanide trihyrate upon grinding but rather that grinding results in a near equal amount of the monoclinic and tetragonal polymorphs.

References

[1]WU XUEWEN& - WO2022/257736, 2022, A1

[2]Willans MJ, Wasylishen RE, McDonald R. Polymorphism of potassium ferrocyanide trihydrate as studied by solid-state multinuclear NMR spectroscopy and X-ray diffraction. Inorg Chem. 2009 May 18;48(10):4342-53.

[3]Toyoda, Hiroo; Niizeki, Nobukazu; Waku, Shigeru, Optical and structural anomalies in potassium ferrocyanide trihydrate crystals, Journal of the Physical Society of Japan (1960), 15, 1831-41CODEN: JUPSAU; ISSN:0031-9015.