Tributyl phosphate
Tributyl phosphate (TBP), also known as tri-n-butyl phosphate, is an organophosphorus compound with the molecular formula C₁₂H₂₇O₄P.[1] It is a colorless, odorless liquid that serves primarily as a polar aprotic solvent and extractant in industrial applications.[1] TBP's most prominent use is in the PUREX process for nuclear fuel reprocessing, where it selectively extracts uranium(VI) and plutonium(IV) from nitric acid solutions, facilitating the separation of fissile materials from spent nuclear fuel.[2] Beyond nuclear chemistry, it functions as a plasticizer for cellulose esters and resins, a component in aircraft hydraulic fluids and brake fluids, and a flame retardant in polymers.[3] Its solvency properties also find application in extracting rare-earth metals and as a defoaming agent.[1] The compound exhibits moderate toxicity, classified as harmful if swallowed (Acute Toxicity Category 4), a skin irritant, and suspected of carcinogenicity based on animal studies.[4] Inhalation or ingestion can lead to nervous system effects, while environmental release poses risks to aquatic life due to its persistence and bioaccumulation potential.[1] Despite these hazards, TBP remains essential in specialized industrial processes owing to its chemical stability and extraction efficiency under acidic conditions.[5]Physical and Chemical Properties
Molecular Structure and Characteristics
Tributyl phosphate (TBP), with the molecular formula C12H27O4P, is a trialkyl phosphate ester derived from phosphoric acid, consisting of a central phosphorus(V) atom bonded to one oxo group (P=O) and three n-butoxy groups via P-O-C linkages.[1][6] This tetrahedral organophosphorus structure features ester bonds that link the polar phosphoryl moiety to the hydrophobic butyl chains (CH3(CH2)3-), conferring amphiphilic properties essential for its function as a neutral solvating ligand.[1] The polarity of TBP arises primarily from the electron-withdrawing P=O bond and the oxygen lone pairs in the P-O-C framework, enabling coordination to Lewis acidic metal cations such as uranyl (UO22+) through donor-acceptor interactions.[7] These structural elements underpin its solvency for polar metal complexes, allowing selective extraction from aqueous phases into organic media while the alkyl chains promote solubility in nonpolar diluents like dodecane or kerosene.[7] Relative to triethyl phosphate, which bears shorter ethyl chains and exhibits higher hydrophilicity, TBP displays increased lipophilicity that enhances organic-phase partitioning and extraction distribution ratios for actinides, though it may form third phases under high metal loading.[8] In comparison, trioctyl phosphate with longer octyl chains offers even greater lipophilicity and potentially stronger extraction for certain metals but at the cost of higher viscosity, which can impede kinetics in industrial processes.[9] This intermediate chain length in TBP optimizes the balance between solvation power and phase compatibility for extractant applications.[9]Physical Properties
Tributyl phosphate (TBP) is a colorless to pale yellow, odorless liquid at ambient temperatures, with a molecular weight of 266.32 g/mol.[1][10] Its low volatility is indicated by a vapor pressure of approximately 0.15 Pa at 25 °C and a relative vapor density of 9.2.[11] Key physical constants include a melting point of −80 °C and a boiling point of 289 °C at standard pressure.[12] Density ranges from 0.972 to 0.979 g/cm³ at 20 °C, while dynamic viscosity is approximately 3.4 mPa·s at 20 °C, influencing its flow characteristics in industrial handling and storage.[6][13]| Property | Value | Conditions |
|---|---|---|
| Density | 0.977 g/cm³ | 20 °C |
| Viscosity | 3.5 mPa·s | 25 °C |
| Water solubility | 0.28–0.4 g/L | 25 °C |