From abstract: "Bis(di-n-hexylphosphinyl)methane, HDPM, [(C6H13)2P(O)]2CH2, has been studied as an extractant for a variety of metals. HDPM was evaluated as an extractant for uranium(VI) and compared with tri-n-octylphosphine oxide, TOPO, (C8H13)3PO. In nonpolar solvents, HDPM forms a polymeric-like substance with compounds of uranium(VI). Viscosity measurements indicate that the molecular weight of this polymeric-like substance is about 100 times greater than the corresponding complex with TOPO. Polymer formation occurs only when nonpolar solvents are used as diluents for HDPM and is easily avoided by using polar solvents such as 1,2-aichlorobenzene. HDPM forms 1:1 and 2:1 complexes with uranium(VI) nitrate. Equilibrium constants for these complexes as well as that for the 2:1 TOPO complex were calculated and it was shown that the over-all constant is at least ten times larger for the HDPM complex than for the TOPO complex. The effect of concentration of various mineral acids, extractant concentration, temperature, and diluents on the extraction of uranium are discussed."