IMPROVEMENTS TO THE QUANTITATIVE ASSAY OF NONREFRACTORY MINERALS FOR FE(II) AND TOTAL FE USING 1,10-PHENANTHROLINE

A method using 1,10-phenanthroline (phen) to quantify Fe(II) and total Fe in nonrefractory minerals was modified to improve the accuracy and precision and to eliminate the inconvenience of performing much of th e analysis under darkroom conditions. Reagents were combined to minimi ze solution-handling errors, volumes of the reagent additions were det ermined gravimetrically and the acid-matrix solution was preheated to near-boiling before sample contact. The darkness requirement, which st ems from the photoreduction of Fe(III) to Fe(II) in the presence of ph en, was eliminated by the use of opaque amber-colored hi,oh-density-po lyethylene bottles during the digestion step and for storage of the di gestate and subsequent dilutions before Fe(II) analysis. Reduction of Fe(III) for total-Fe analysis was accomplished either by exposure to l ight from a Hg-vapor lamp or by reaction with hydroxylamine, NH2OH. Al though the minimum periods required for adequate reduction ranged from 1.5 to 4 h, the optimum reduction periods were between 6 and 10 h. Wh en standard samples containing Fe(II) and MnCl2 were digested and anal yzed for total-Fe using the light treatment (with incidental heating t o 35-45 degrees C), significant decreases and in some instances, oscil lations, in absorptivity were obtained. Similar experiments with NH2OH or with CrCl3 showed no effect. The absorptivity of most digestates s tored in opaque bottles was stable for at least 2 weeks, although dige states with Mn concentrations above 3 mu g mL(-1) showed proportional decreases in absorptivity. Analysis of 8 geochemical reference materia ls by the modified method (using NH2OH) yielded excellent agreement wi th published values and a mean relative standard deviation of 0.6%. To tal-Fe results obtained using the light treatment, however, were gener ally lower (similar to 2% relative) than the NH2OH values, although th is difference decreased with longer irradiation periods. Use of NH2OH was deemed preferable because it was simpler, faster, minimized interf erences from Mn and eliminated the need for specialized apparatus. Las tly, MICA Fe was shown to be unreliable as a primary reference materia l for Fe(II) determinations.