The original ferrozine method has been modified to sequentially determine t
he Fe(II)/Fe(III) speciation in small volumes of fresh and marine water sam
ples, at the submicromolar level. Spectrophotometric analyses of the Fe(II)
ferrozine complex are performed on a single aliquot before and after a redu
ction step with hydroxylamine. The procedure is calibrated using Fe(III) st
andards stable under normal conditions of analysis. It is shown also that t
he presence of high concentrations of dissolved NOM (natural organic matter
) do not create any significant artifacts. The method was used to measure F
e(II) and Fe(III) depth distribution in salt marsh pore waters and in a str
atified marine basin. (C) 2000 Elsevier Science Ltd. All rights reserved.