Is Iron Sulfide Soluble?

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Iron sulfide is an insoluble chemical substance with the formula FeS. It is found naturally as the mineral pyrrhotite, troilite and in igneous rocks such as syenites and hematites. It is used as a pigment in hair dyes and ceramics and in glasses and bottles. It is also used in lubricants and anodes, to treat exhaust gases, and as an alloying agent to improve the machine ability of carbon, alloy and stainless steel castings. It is also known to have enzyme-like properties, which suggest a potential for use in biomedical applications.

Pyrophoric iron sulfide may form in vessels, storage tanks and pipelines that have been exposed to asphalt, aromatic tars, sour crude oil, and high-sulfur fuel oils. When the pyrophoric material comes into contact with oxygen, it will react to produce iron oxide, or rust, and sulfur dioxide (SO2). The resulting reaction is extremely exothermic and can cause fires and explosions. To prevent this, pyrophoric iron sulfide is usually kept wet or filled with an inert gas such as nitrogen to prevent the oxidizing reaction from occurring.

Whether or not iron sulfide is soluble depends on its physical and chemical characteristics, the temperature and pressure of the solvent, and the pH of the solution. The solubility of a substance is defined as the saturation concentration (Sc)—the number of moles of the solute that will dissolve in a given amount of solvent at a particular temperature and pressure. Sc values are determined by using X-ray diffraction techniques to measure the structure of the solute and the solvent. The Sc value for a solute will decrease as the concentration of the solute increases, but it will never reach zero.

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