![]() One obtains PB by the addition of Fe(III) salts to a solution of 4. The story of "Turnbull's Blue" (TB) illustrates the complications and pitfalls associated with the characterization of a composition obtained by rapid precipitation. The variability of PB's composition is attributable to its low solubility, which leads to its rapid precipitation vs. Again, the composition is notoriously variable due to the presence of lattice defects, allowing it to be hydrated to various degrees as water molecules are incorporated into the structure to occupy four cation vacancies. The Fe(III) centers, which are high spin, are surrounded on average by 4.5 N centers and 1.5 O centers, the latter from water. The Fe(II) centers, which are low spin, are surrounded by six carbon ligands. These workers concluded that the framework consists of Fe(II)-CN-Fe(III) linkages, with Fe(II)-C distances of 1.92 Å and Fe(III)-N distances of 2.03 Å. By growing crystals slowly from 10M HCl, Ludi obtained crystals wherein the defects were ordered. Since X-ray diffraction cannot distinguish C from N, the locations of these lighter elements is deduced by spectroscopic means as well as distances from the Fe centers. Parallel studies were conducted on related materials such as Mn 3 2 and Co 3 2 (i.e., Co 5(CN) 12). The assignment of the structure and the formula resulted from decades of study using IR spectroscopy, Moessbauer spectroscopy, and X-ray and neutron crystallography. The chemical formula of PB is Fe 7(CN) 18(H 2O) x where 14 ≤ x ≤ 16. The precise identification of PB was complicated by three factors: (i) PB is extremely insoluble but also tends to form colloids, (ii) traditional syntheses tend to afford impure compositions, and (iii) even pure PB is structurally complex, defying routine crystallographic analysis. Japanese painters and woodblock print artists likewise did not have access to a long-lasting blue pigment until they began to import Prussian blue from Europe, though cobalt blue had been used extensively by Chinese artists in blue and white porcelains for centuries prior.ĭespite being one of the oldest known synthetic compounds, the composition of PB was uncertain until recently. European painters previously used a number of pigments such as indigo and smalt which tended to fade, and the extremely expensive ultramarine. The pigment is significant as the first stable and lightfast blue to be widely used. Due to a contaminated source of potash, he obtained the blue instead. The chemist and paint maker Heinrich Diesbach had intended to prepare a red lake pigment. ![]() Prussian Blue was the first modern dye to be synthesized and was the result of an accident.
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