Fe in sea water

Mokiyevskaya, V.V., 1962. Methods of determining Fe in sea water and interstitial water. Trudy Inst. Okeanologii Akad. Nauk S.S.S.R. (Trans. Inst. Oceanology Acad. Sci. U.S.S.R.), 54 115-124 (in Russian). 

Flameless atomic absorption spectroscopy using the heated graphite furnace is a sensitive method for analyzing environ-mental samples for trace metals. High salt concentrations cause interference problems that are not totally correctable by optimizing furnace conditions and/or using background correctors. We determined that samples with identical ratios of major cations have trace metal absorbances directly related to their Na and trace metal concentrations. Equations and curves based on the Na concentration, similar to standard addition curves, can be calculated to overcome the trace element interference problem. Concentrations of Pb, Cd, Cu, and Fe in sea water can be simply (ind accurately determined from the Na concentration, the sample absorbance vs. a pure standard, and the appropriate curve. 

The average rates of corrosion of Fe-36Ni alloy exposed to alternate immersion in sea-water are appreciably greater than those that occur when the alloy is exposed to marine atmospheres. Although the rates of corrosion are significantly below those observed for mild steel (Table 3.32) the superiority over mild steel in not so great with respect to pitting attack. 

Nickel-iron alloys fully immersed in sea-water may suffer localised corrosion which can be severe under conditions where oxygen is constantly renewed at the surface and the formation of protective corrosion products is hindered, e.g. in fully-aerated flowing sea-water. In quieter, less oxygenated conditions, average corrosion rates of Fe-36Ni are low and well below those for mild steel, as exemplified in the data given in Table 3.33 . However the resistance to localised attack is not improved to the same extent. 

Table 3.33 Resistance of Fe-36Ni alloy and mild steel to corrosion in sea-water-...

Example 3-1 Voltammogram (a) was obtained for adsorptive stripping measurements of Fe(III) in sea water. Voltammograms (b) and (c) show successive standard additions of 4 ppb Fe(III). Find the concentration of Fe(III) in the sample. 

Outside the zone of mixing the Fe content is negligible—10-20, more often 3-6 mg/m. Skopintsev and Popova (1960) point out that the Fe content in sea water is not more than 50-60 jug/1 according to the data of various authors. 

The forms of occurrence of iron in sea waters were investigated in detail by Mokiyevskaya (1962). The total amount of Fe is the sum of dissolved, colloidal, and suspended iron of organic and inorganic origin ... 

Iron of inorganic dissolved compounds (bicarbonates, sulfates, chlorides, fluosilicates, etc.) may enter into the dissolved form of iron of inorganic origin (Fcj"" ), but their existence is governed by an acid environment with a pH not higher than 3. As a rule the pH in sea water is close to 8 ( 0.5). Under these conditions iron compounds are easily hydrolyzed and converted into hydroxides, which form colloidal solutions in sea water. In appropriate conditions colloidal hydroxide condenses to clots of gel and converts to the suspended state. Therefore there are practically no ionic forms of iron (Fe "" proper). As early as 1937 Cooper (1937) concluded, on the basis of the solubility product and activity of ferrous and ferric iron ions and FeOH ions, that until equilibrium is reached sea water may contain about 10 jiig/1 of iron ions in true solution at pH = 8.5 the amount of ionic Fe in ferric form is still less—10 which corresponds to the extremely... 

Colloidal iron consists mainly of colloidal solutions of hydroxides and phosphates of Fe (Fe, " 7 ) and colloidal solutions of the enumerated organic compounds (Fe , ). Direct determination of colloidal iron in sea waters is difficult and usually dissolved iron means the sum of Fe j + Fe. , . 

Manganese and iron hydroxides are extremely insoluble in sea-water. Maximum concentrations of free ions in sea-water are 0.887 x 10 pg Fe/1 and 1.09 x 10 pg Mn/1, the excess manganese and iron building colloidal hydroxides which sooner or later precipitate. 

Cobalt is mostly incorporated into the iron phase of manganese nodules. However, theoretically, it would suit much better in the manganese phase which has experimentally been confirmed by Giovanoli and Brutsch There is a theory, not yet proved by experiments, staling that Co is oxidized to Co in sea-water. This process should be catalyzed by Fe(OH)3, The final product of oxidation, Co(OH)3, forms a solid solution with Fe(OH)3 The theory does not include a discussion about the possibilities of diadochic incorporation of cobalt after cristalliza-tion of iron hydroxide. 

A procedure was developed for the determination of total and labile Cu and Fe in river surface water. It involved simultaneous solvent extraction of the metals as diethyldithio-carbamates (ddc) and tfac complexes. The complexes were extracted by isobutyl methyl ketone (ibmk) and the solution subjected to flame atomic absorption spectrometry. Variables such as pH, metal complex concentration, reaction time, ibmk volume and extraction time were optimized. Prior to the solvent extraction a microwave-assisted peroxydisulfate oxidation was used to break down the metallorganic matter complexes in the river surface waters . Trifluoroacetylacetone was used as a chelation agent for the extraction and quantitative determination of total Cr in sea water. The chelation reaction was conducted in a single aqueous phase medium. Both headspace and liquid phase extractions were studied and various detection techniques, such as capillary GC-ECD, EI-MS (electron-impact MS) and ICP-MS, were tested and compared. The LOD was 11-15 ngL Cr for all the systems examined. The method provided accurate results with EI-MS and ICP-MS, while significant bias was experienced with ECD °. ... 

Trace metals in sea water are present at very low concentrations (normally lower than 10 A/) thus, the study of metal distribution and speciation poses problems that have been overcome only in the last few decades. A prerequisite for determination of trace metals is the collection and processing of uncontaminated samples for the element of interest. Improvements in analytical procedures have led to a decrease by some orders of magnitude in the reported concentration of some elements, especially so for ubiquitous and contamination-prone elements, such as Fe or Pb. These changes lead to the concept of oceanographic consistency as a criterion for accepting trace chemical measurements (83). 

Sharma and Millero (1988) determined the corresponding second-order rate constants k 0 = 2.1 104 and iCi=8.7 102 A/-1s-1 in sea water. The di and ii iehlorocomplexes were not sufficiently reactive to produce detectable rate constants. Thus the chloride ion, which stabilizes the soft reactant Cu(I) inhibits the oxygenation, whereas OH, which stabilizes the product Fe(III), accelerates i lie rale of Fe(II) oxidation. The reaction of Cu(I) with 02 represents an interesting test case because the reverse reaction has been measured by pulse tadiolysis. We may therefore apply the principle of microscopic reversibility to the electron-transfer step ... 

Due to the high insolubility of the most stable oxidation state of iron in sea water, Fe(III), the concentration of iron in the ocean as a whole has remained relatively unaltered despite the appreciable man-made inputs11,12. In estuarine and coastal waters, especially near large cities, increases have been noted13. The iron found in sea water is mainly in the form of suspended material, the concentration of which varies with depth14. For iron concentrations in sea waters near the surface, we estimate a present day value in excess of the value quoted in Table 1 of 5 jug/1. 

The reaction of dissolved Fe with oxygen is known to be fast and its rate was determined in sea-water by Millero et al. (1987) ... 

The stability field of Fe in seawater is best illustrated by the use of an Eh, pH diagram (Fig. 11.4). Solid Fe(OH)3 is shown as the metastable form of iron at the conditions prevalent in sea-water (Eh +0.4 V, pH 8). Actually, akageneite (P-FeOOH) is the more stable form found in deep-sea manganese nodules (see Sect. 11.4.8) but its free energy of formation has not been determined. 

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