Stable free-radical complexing reagents in applications of electron spin resonance to the determination of metals

Abstract

Metals can be determined by electron spin resonance (e.s.r.) by using stable free radicals in which the molecules include complexing groups. Such reagents form complexes with metal ions which retain the properties of the free radicals producing the e.s.r. signal. The complexes formed can be separated from the excess of reagent and the metal concentrations measured from the signal intensity. This approach markedly expands the potential of e.s.r. as an analytical technique, because it is not limited to paramagnetic metals. The relatively high sensitivity of free radical determination by the e.s.r. method is an additional advantage. The properties of the spin-labelled β-diketone, 4-acetoacetyl-2,2,5,5-tetramethyl-3-imidazoline-1-oxyl, are described. Dissociation constants are reported for the enol form (p K a T = 6.56 ± 0.02) and for the protonated (at the imidazolyl nitrogen) form (p K a T = 1.91 ± 0.02), as well as partition constants for the hexane—water (log k D(C 6H 14) = 0.96 ± 0.02) and chloroform—water (log K D(CHCl 3) = 3.36 ± 0.04) systems. The reagent extracts Cu, Pb, Cd, Hg and Er readily; in the presence of caproic acid iron(III) is also extracted. Copper is extracted as a CuA 2, complex (log K D = 2.5 ± 0.5; log K ex = -2.80 ± 0.13; log β 2 = 14.3 ± 1.5). CuA 2 does not give an e.s.r. signal. Iron is extracted as a mixed complex with β-diketone and caproic acid. An extraction—e.s.r. method is reported for the determination of mercury with a detection limit of 2 × 10 -7 M.