Despite the fact that iron(1II) hydroxide is one of the most commonly used and efficient collectors of trace metals in sea water, its role remains ambiguous and unclarified to a substantial degree. rn brief discussions of its function, RILEY ANT) SICIRROW~ and JOYNEH et aZ.2 subscribed to the view which appears to be generally accepted, i.e., that iron(II1) hydroxide behaves in sea water as a negatively charged colloid capable of scavenging numerous cationic species. These include Al, As, Be, Co, Ga, Ge, the lanthanons, MO, Ni, Nb, Th, SC, Se, U, V, W and Yl. The collection is usually carried out by adding iron(III) chloride solution to the sea water. The negatively charged iron(TI1) hydroxide sol which is formed, collects on its surface the cations in the sea water. The result is a coagulated coprecipitate enriched with trace metals that can easily be separated from their aqueous environment by filtration. The species of interest in the precipitate is then determined by an appropriate method.
During the course of a study which necessitated the use of iron(lIr) hydroxide in sea water as a trace metal collector, it became clear that the commonly accepted colloidal role for iron(lI1) hydroxide was an over-simplified one and did not explain satisfactorily the wide variety of elements collected. Specifically, it did not appear to be in accord with the known chemistry of several of the elements which are believed to exist in aqueous solution over a wide PH range as anionic species and, consequently, under those conditions, are not capable of being collected by a negatively charged colloid.
This paper is concerned with a study of the behavior and role of. iron(II1) hydroxide in sea water under a variety of controlled conditions. Molybd&um was selected as the model trace element since it has been established3 that it exists in water as the molybdate anion (Mo042-) above the PH of 1.0. 1n addition, it was hoped, through the establishment of optimum conditions, to work out the details of a procedure for the determination of molybdenum in sea water, comparable to that recently reported by KIM AND ZEITLIN".
The general procedure adopted was to add known amounts of molybdenum to filtered sea-water samples. After addition of iron(III) chloride solution, the PH of the sea-water samples was adjusted to cover a range of 4.0 to 10.0. The coprecipitates were collected by filtration and the molybdenum present in each case determined by * Hawaii Institute of Geophysics Contribution No. 260.