Vanadium (IV and V) speciation with a catechol derivative and tetrazolium salt

The possibility of application of 4-nitrocatechol (4-NC) - tetrazolium salt - organic solvent systems for liquid-liquid extraction-spectrophotometric vanadium (IV and V) speciation was studied. Six commercially available tetrazolium salts (2,3,5-triphenyltetrazolium chloride (TTC), 3-(2-naphtyl)-2,5-diphenyltetrazolium chloride (Tetrazolium Violet, TV); 3-(4,5-dimethyl-2- thiazol)-2,5-diphenyltetrazolium bromide (Thiazolyl Blue Tetrazolium, MTT), 2-(4-iodophenyl)- 3-(4-nitrophenyl)-5-phenyltetrazolium chloride (INT), 3,3’-(4,4’-biphenylene)bis(2,5- diphenyltetrazolium) chloride (Neotetrazolium chloride, NTC) and 3,3’-[3,3’-Dimetoxy(1,1’- biphenyl)-4,4’-diyl]bis(2,5-diphenyltetrazolium) chloride (Blue Tetrazolium chloride, BTC)) and three organic solvents (chloroform, 1,2-dichloroethane, 1-butanol) were tested in this study. The best results were obtained with MTT and chloroform. A direct, very simple, fast and sensitive procedure for vanadium (IV and V) speciation based on the spectral differences of the extracted ternary complexes of V(IV) and V(V) was developed. It was found that the optimization of the MTT to 4-NC ratio has a key role in the speciation results. At deficiency of 4-NC towards MTT the vanadium(IV) complex absorbed light at 390 and 570 nm, while the V(V) complex absorbed only at 390 nm. The corresponding molar absorptivities were calculated to be εV(IV)390 = 3.6×104 L mol-1 cm-1, εV(IV)570 = 1.7×104 L mol-1 cm-1 and εV(V)390 = 3.1×104 L mol-1 cm-1. The method was applied for vanadium (IV and V) speciation in industrial samples with low Fe and Al content (catalyst for oxidation of SO2) and good results were obtained in terms of precision and accuracy.