The phosphine oxides Cyanex 921 and Cyanex 923 (Solvay, France) were used without further purification; they have the following compositions [37 (link)]: Cyanex 921 has tri-n-octylphosphine oxide as an active group, whereas Cyanex 923 is composed of four tryalkyl phosphine oxides. Besides their composition, the main difference between these two reagents is that Cyanex 921 is a solid whereas Cyanex 923 is a liquid, which removes any potential solubility problem when it is in the presence of organic diluents; in fact, Cyanex 923 has the potential to be used undiluted.
A quantity of 1 g/L chromium(VI) stock solution was prepared by dissolving potassium dichromate (Fluka, Switzerland) in distilled water, and working solutions were prepared from dilutions of the above. All the chemicals used in the experimentation, except the phosphine oxides and Solvesso 100 (aromatic diluent), were of AR grade. The solid support used in the present work was Millipore Durapore GVHP4700 (polyvinylidene fluoride) (75% porosity, 1.67 tortuosity, 12.5 × 10−3 cm thickness).
A quantity of 1 g/L chromium(VI) stock solution was prepared by dissolving potassium dichromate (Fluka, Switzerland) in distilled water, and working solutions were prepared from dilutions of the above. All the chemicals used in the experimentation, except the phosphine oxides and Solvesso 100 (aromatic diluent), were of AR grade. The solid support used in the present work was Millipore Durapore GVHP4700 (polyvinylidene fluoride) (75% porosity, 1.67 tortuosity, 12.5 × 10−3 cm thickness).