The Quantitative Assay of Minerals for Fe²⁺ and Fe³⁺ Using 1,10‐Phenanthroline: I. Sources of Variability

Abstract

Factors affecting the reliability of results using 1,10‐phenanthroline (phen) in the quantitative assay of minerals for Fe2+ and total Fe were studied. The greatest source of variability was the photochemical reduction of ferric‐phen species during Fe2+ analyses. It is believed that oxo‐bridged dinuclear complexes of Fe2+ and phen are formed in the mineral digests, which then are susceptible to photochemical reduction by wavelengths < 400–500 nm. The rate of photoreduction of a solution obtained by digesting oxidized nontronite was sufficient to increase the predicted Fe2+ content of the clay from about 0 to 2.45% by simply leaving the sample for 1 hour in normal fluorescent light in the laboratory; and for partially reduced nontronite, from 3.25 to 4.68%. The rates of increase for the two clays were 0.0024 and 0.0014 absorbance units/min, respectively. Interferences due to this phenomenon were eliminated when samples were kept in the dark or under subdued red light. Other factors that adversely influence the accuracy and precision of the standard curve included (i) the amount of 48% HF used to digest the sample; (ii) addition of chemical reducing agents such as hydroxylamine hydrochloride and hydroquinone; (iii) the order in which phen is added relative to other reagents; (iv) pH; and (v) the length of time allowed for color development if phen is added only to the final dilution.

Role of Structural Hydrogen in the Reduction and Reoxidation of Iron in Nontronite

“…The reversibility of the reduction of structural Fe 3+ has been reported by others (Stucki and Roth, 1976;Stucki and Anderson, 1981;Stucki et aL, 1984b), although Rozenson and Heller-Kallai (1976) suggested that the structural changes occurring on reduction are reversible only if A1-OH-Fe associations are involved, but not if the associations are Fe-OHFe. The above results suggest that the structural changes which occur during reduction are reversible even ifFe-OH-Fe associations are involved.…”

Simulated atmospheric processing of iron oxyhydroxide minerals at low pH: Roles of particle size and acid anion in iron dissolution

“…Ferrous iron was measured colorimetrically with 1,10-phenanthroline, which forms a complex with Fe(II) that absorbs light at 510 nm (37). For Fe(II) analysis, 200 μL of a 5 mM 1,10-phenanthroline solution and 200 μL of an ammonium acetate buffer were added to 1 mL of sample.…”

Effects of Reduction and Reoxidation of Structural Iron on the Surface Charge and Dissolution of Dioctahedral Smectites

“…Each clay was Na+-saturated using a 1 N solution of NaCI, fractionated to <2/zm, dialyzed and freezedried. The unit-ceil formulae reported in Table 1 were calculated using the same weight percentages for Si, A1, and Mg that were obtained in earlier studies (Low, 1980;Stucki et al, 1976); but the values for exchangeable Na +, Fe 2+, and total Fe were revised according to the results obtained from improved methods for Fe 2+ and total Fe analysis (Stucki and Anderson, 1981;Stucki, 1981).…”