Table 1.

Calculated ratios of [Fe(II)L]/[Fe(III)L] below which reduction of Fe(III)L to Fe(II)L by ${\text{O}}_2^{-*}$ is thermodynamically feasible for various Fe complexes in natural waters around pH 8.

Ligand (L)	Medium conditions	$\left[{\text{O}}_{\text{2}}^{-*}\right]$ (pM)	logKFe(II)L	logKFe(III)L	Maximum [Fe(II)L]/[Fe(III)L]
Desferrioxamine B (DFB)	pH 8.0, freshwater (I = 0.1 M)	1	7.0a	18.1a	3.7 × 10−14
		1000	7.0a	18.1a	3.7 × 10−11
	pH 8.0, seawater (I = 0.7 M)	1	4.05b	12.1c	4.2 × 10−11
		1000	4.05b	12.1c	4.2 × 10−8
Ethylenediaminetetraacetate (EDTA)	pH 8.0, freshwater (I = 2 mM)	1	9.24d	10.54e	2.4 × 10−4
		1000	9.24d	10.54e	0.24
	pH 8.0, seawater (I = 0.7 M)	1	5.37b	7.36f	4.8 × 10−5
		1000	5.37b	7.36f	4.8 × 10−2
Citrate	pH 8.2, seawater (I = 0.7 M)	1	5.40g	8.12h	9.0 × 10−6
		1000	5.40g	8.12h	9.0 × 10−3
Suwannee River fulvic acid (SRFA)	pH 8.1, seawater (I = 0.7 M)	1	7.5g	10.4g	5.9 × 10−6
		1000	7.5g	10.4g	5.9 × 10−3

^aCalculated at I = 0.1 M, ignoring formation of any other complexes using stability constants from Farkas et al. (2003).

^bCalculated as logK = p/(p − 1)[L]_T where [L]_T is total ligand concentration and p is proportion complexed as reported in Table 2 of Hudson et al. (1992).

^cMeasured value at pH 8.1 reported by Witter et al. (2000).

^dCalculated from complex formation and dissociation rate constants measured by Fujii et al. (2011).

^eCalculated from complex formation and dissociation rate constants measured by Fujii et al. (2010a).

^fMeasured value at pH 7.98 reported by Sunda and Huntsman (2003).

^gCalculated from complex formation and dissociation rate constants measured by Rose and Waite (2003b).

^hCalculated from complex formation and dissociation rate constants measured by Fujii et al. (2010a) for a citrate concentration of 100 μM (the apparent conditional stability constant varies with ligand concentration due to the existence of multiple complex species).