Simultaneous Spectrophotometric Measurement of Fe( I I ) and Fe( I I I ) in Atmospheric Water Slmo 0. Pehkonen, Ylgal Erel, and Mlchael R. Hoffmann"
Environmental Engineering Science, W. M. Keck Laboratories, California Institute of Technology, Pasadena, California 91 125
rn A new analytical method employing di-Ppyridyl ketone benzoylhydrazone (DPKBH) as a colorimetric chelating agent for the simultaneous determination of iron(I1) and iron(II1) in cloudwater has been developed. A spectrophotometric detection limit of 4 nM for both Fe(II1) and Fe(I1) with a linear response from 4 nM to 0.1 pM was established for samples extracted with CHC1,-H20. DPKBH chelation without CHC13 extraction showed a linear response from 0.1 to 30 pM. The molar extinction coefficients of the Fe(I1)-bis(DPKBH) (e2) and Fe(II1)bis(DPKBH) (e1) complexes are el and e2 = 3.6 X lo4 L mot1 cm-l at 370 nm and e2 = 1.1 X lo4 L mol-l cm-l a t 660 nm. Analytical interference studies on the possible changes in the oxidation state of iron with S(IV), oxalate, and other potential electron donors have also been carried out. This analytical method has been used to determine iron(I1)and iron(II1) simultaneously in cloudwater samples collected within the Los Angeles basin airshed. The concentration of Fe(I1) varied from 0.3 to 5 pM, and the concentration of Fe(II1) varied from 0.6 to 1.4 pM during several stratus cloud events. Introduction Iron is the fourth most abundant element in the earth's crust; it is present in a variety of rock and soil minerals as both Fe(I1) and Fe(II1) (1). Iron also plays a central role in the biosphere, serving as the active center of proteins responsible for O2 and electron transfer and of metalloenzymes such as oxidases, reductases, and dehydrases (2). The observed concentrations of total dissolved iron in natural water systems vary from 0.2 nM in midocean surface water (3)to 400 pM in polluted urban clouds (4). Determination of the oxidation state of iron in atmospheric water is important in light of recent studies reporting that iron may be the limiting nutrient for phytoplankton growth in the open ocean (5,6). Moreover, Fe(I1) is probably the preferred nutrient for phytoplankton (7). In addition, Fe(II1) serves as an effective catalyst for the autoxidation of SO2 to S042- in clouds (8, 9). In urban atmospheres, where S(IV) and oxidizable organic compounds are abundant, oxidation of S(1V) and organic compounds by Fe(II1) via direct electron transfer can take place. Laboratory experiments have shown that iron(II1) is an important oxidant of S(1V) (8-10) and organic pollutants (e.g., formaldehyde to formic acid) (11). Most investigations of iron in atmospheric water have been limited to the determination of its total dissolved concentration; very little is known about its oxidation state 0013-936X/92/0926-1731$03.00/0
(12,13). However, measurements of Fe(I1) in seawater (14) and in streamwater (15) have shown that it is present at significant levels in oxic environments. The determination of the oxidation state of iron in a variety of samples can be conventionally achieved by complexation with specific chelating agents followed by spectrophotometricmeasurement. Several chelating agents have been developed specifically for iron(II) determination. Tsuchiya and Iwanami (16) used 2-(diethylamino)-4hydroxy-5-nitroso-6-aminopyrimidine for the determination of iron(I1) in leaded brass, copper-nickel, and magnesium-aluminum alloys, while Zhang et al. (17) used 2- [ 2-(5-bromobenzothiazolyl)azo]-5-(dimethylamino)benzoic acid for the determination of iron(I1) in aluminum alloys. Wang et al. (18) utilized 4- [ (2-benzothiazoly1)azo]pyrocatechol for the determination of iron(I1) in water, and 2- [(6'-bromo-2'-benzothiazolyl)azo]-.5-carboxyphenyl was used by Sun et al. (19)for the determination of iron(II) in graphite. Ferrozine [3-(2-pyridyl)-5,6-diphenyl-1,2,4triazine-p,p'-disulfonic acid, monosodium salt monohydrate] as used originally by Stookey (20)and Carter (21) is the most common chelating agent used for Fe(I1) determination in water and blood serum samples. Several specific chelating agents for iron(II1) have also been developed, such as rneso-tetrakis(4-sulfopheny1)porphyrin used by Xu et ai. (22) for the simultaneous determination of iron(III), Cu(II), and Zn(II), while Vijayakumar et al. (23) used hydroxy-l-acetonaphthoneoxime for the determination of iron(II1) and, indirectly, of fluoride. Methods that involve the preconcentration of water samples followed by a spectrophotometric measurement using some of the above reagents have been used in the determination of very low iron concentrations in environmental samples. These methods include the pressure filtration through a silica gel immobilized 8-hydroxyquinoline cartridge followed by elution and complexation with ferrozine (24),the trapping of Fe(II1) with chelating resins that are analogous to naturally occurring siderophores (25), and flow injection analysis with chemiluminescence detection for the determination of low iron(I1) concentrations in seawater (26). While these methods can be used effectively in the case of low iron concentrations, they are, however, quite laborious and are limited to the determination of either Fe(II) or Fe(III). In addition, many of these techniques are unable to determine iron concentrations typically found in cloudwater in pristine environments, because of their relatively high detection limits (20-60 nM).
0 1992 American Chemical Society
Environ. Scl. Technol., Vol. 26, No. 9, 1992
1731
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Figure 2. AbsorDtion soectrum of FdIIkDPKBH in CHCI, at DH 5. . . 1-im cell, with [Fe(II)]'= 25 pM.
Figure 1. Coordination complex of Fe-DPKBH.
Di-2-pyridyl ketone benzoylhydrazone (DPKBH) was selected in this study as a potential chelating agent for iron, because it is known to complex both iron(I1) and iron(II1) (27,28). In addition to iron, DPKBH will also complex other metal ions such as cobalt, copper, nickel, and zinc (27). However, the concentrations of these metals in most atmospheric water systems are several orden of magnitude lower than Fe and thus are unlikely to cause interferences (29, 30). We investigated the use of DPKBH both with and without a preconcentration procedure in order to cover a wide range of Fe concentrations. In order to test our procedures with atmospheric water samples, cloudwater samples from the Los Angeles basin were collected and iron(I1) and labile iron(II1) [iron(III) that is not complexed strongly (stability constant
