Bismuth phosphate (BiPO₄)

Bismuth phosphate (BiPO₄) is an inorganic compound with the molecular formula BiPO₄ and a molecular weight of 303.95 g/mol. It is a white or off-white powder or monoclinic crystalline solid that does not melt upon heating. Its density is 6.32 g/mL at 25°C. Bismuth phosphate is slightly soluble in water and dilute acids but does not hydrolyze in boiling water. It is practically insoluble in alcohol and acetic acid but dissolves in concentrated nitric acid and hydrochloric acid. Below is a detailed introduction to bismuth phosphate:

Properties

Bismuth phosphate has a molecular formula of BiO4P and a molecular weight of 303.951741. Its CAS registry number is 10049-01-1. It exists as an odorless powder or monoclinic crystals. Its crystal structure can vary depending on the synthesis method, such as cubic sillenite-type or monoclinic forms. The lattice parameters of cubic sillenite-type bismuth phosphate are a = 10.1773(2) Å, while those of monoclinic bismuth phosphate are a = 6.4717(3) Å, b = 6.9385(3) Å, c = 6.7497(3) Å, and β = 103.696°. Its solubility product constant (pKsp) is 22.89. It is insoluble in water and alkali but soluble in hydrochloric acid.

Synthesis Methods

Solid-State Reaction Method: This is one of the most common synthesis methods. For example, Bi2O3 and K3PO4 can be dispersed in deionized water and then heated to 850°C for 2 hours to obtain BiPO₄. Another method involves reacting bismuth nitrate and ammonium dihydrogen phosphate. First, pure BiPO₄ is obtained by reacting bismuth nitrate and ammonium dihydrogen phosphate. Then, amorphous carbon-coated bismuth phosphate can be synthesized through a subsequent carbon-coating process.

Hydrothermal Synthesis Method: For instance, 97 mg (0.2 mmol) of Bi(NO3)3·5H2O and 26.3 mg (0.2 mmol) of (NH4)2HPO4 are mixed with 10 mL of deionized water and stirred in a 15 mL Teflon liner at room temperature until a homogeneous mixture is obtained. The Teflon liner is transferred into a stainless steel autoclave and heated at 220°C for 24 hours. The product is filtered and washed with deionized water, ethanol, and acetone, followed by drying in air at 80°C to obtain monoclinic BiPO₄. Cubic BiPO₄ can also be synthesized via hydrothermal methods. For example, Bi(NO3)3·5H2O and (NH4)2HPO4 are mixed with other additives and water, stirred, and subjected to hydrothermal treatment at 180°C for 24 hours. After cooling, the precipitate is filtered, washed, and dried to obtain cubic BiPO₄.

Solvothermal Synthesis Method: Ethylene glycol and glycerin are placed in a reaction vessel and mixed evenly. Tartaric acid is added and stirred until uniform. Then, bismuth nitrate pentahydrate is added and stirred. An equal molar amount of soluble phosphate is added, stirred, and transferred to a hydrothermal reactor for solvothermal reaction at 160°C for 24 hours. After centrifugation, washing, and vacuum drying at 60°C for 12 hours, bismuth phosphate photocatalysts with different morphologies are obtained.

Applications

Humidity Sensing: Bismuth phosphate exhibits promising humidity sensing properties. Cubic sillenite-type bismuth phosphate displays a capacitance change of up to four orders of magnitude over a relative humidity (RH) range of 11% to 95%, along with a linear adsorption/desorption relationship. Its humidity sensing behavior is related to the presence of a framework structure containing polarizable Bi³⁺ cations and moderate phosphate doping. Compared to monoclinic BiPO₄, cubic bismuth phosphate shows superior and more linear humidity sensing properties, faster response times, and better response behavior, making it a potential candidate for humidity sensors.

Optical and Magneto-Optical Fields: Bismuth phosphate nanocrystals synthesized in low-melting phosphate glass exhibit unique optical and magneto-optical properties. The optical transmission spectra of glass samples indicate that the red shift in the absorption edge is attributed to the presence of BiPO₄ nanoparticles. Faraday rotation tests on the glass nanocomposite show that the Verdet constant of BiPO₄ nanoparticles in phosphate glass is three times higher than that of BK-7 glass. Bismuth phosphate glass nanocomposites may have potential applications in magneto-optical devices.

Catalysis: Bismuth phosphate can serve as a catalyst in organic synthesis, such as the dehydration of alcohols to olefins, isomerization of olefins, nitration of aromatic hydrocarbons, polymerization of aldehydes, and other reactions. It is also a source of phosphates for solid-state exchange reactions to prepare metal phosphates like magnesium, calcium, and zinc.

Medical Field: Bismuth phosphate, along with its aluminate and subcarbonate forms, has been used to treat various gastrointestinal disorders. Additionally, bismuth compounds exhibit potent antimicrobial activity against Gram-positive and Gram-negative bacterial pathogens, Leishmania, and fungi. Recent studies have shown that bismuth compounds can act as antibiotic adjuvants to treat multidrug-resistant microbial infections, cancer, and viral infections, including SARS-CoV-2. However, due to its low bioavailability in physiological environments, many bismuth compounds with improved physicochemical properties and new administration modalities for bismuth release have been developed and biologically tested.

Nuclear Industry: Bismuth phosphate is used in the separation of plutonium from fission products.

Glass Manufacturing: It is used in the production of optical flint glass.

Safety Information

Bismuth phosphate is an irritant to the skin, respiratory system, and eyes. It is toxic if ingested. Long-term exposure may pose health risks. However, it is non-combustible and does not react violently with other chemical compounds. Its risk and safety statements indicate that it is classified as WGK Germany 3 and is listed in the TSCA inventory.

Market Outlook

Bismuth phosphate's unique properties make it widely used in fields such as chemicals, materials science, electronics, and medicine. With the development of industries like new energy and optoelectronics, the demand for high-purity bismuth phosphate is expected to grow. Research into its synthesis methods and applications will continue to advance, further expanding its market potential.