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Standard Operating Procedures

Chromium Reduction

Introduction

Chromium reduction for pyrite extraction (S isotopes, sequential Fe) was originally written by Eva Stueeken on 160620 following the protocols in the UCR Lyons Lab. This method extracts pyrite-bound sulfur from rocks, which can be converted to pyrite-bound iron, assuming FeS2 stoichiometry. 

Adjust reagent quantities for smaller batches of samples.

Preparing chemicals

Zinc acetate trap solution:

Silver nitrate trap solution:

Potassium iodate titrant solution:

Chromium chloride solution for pyrite reduction:

Chrome reduction procedure

Iodometric titration with ZnS 

What happens in the titration?

IO3- + 5I- + 6H+ = 3I2 + 3H2O

H2S + I2 = S0 + 2H+ + 2I-

Conversion formula to calculate pyrite-sulfur content:

[ml titrant used] / [1000 ml/L] = L of titrant used

[L of titrant used] * [0.1 M KIO3] = moles of KIO3 used (need exact molarity of titrant) 

[moles of KIO3 used] * [3] = moles of H2S converted to S0 = moles of pyrite-S in sample

[moles of S] * [32.07 g S/mol S] = grams of pyrite-S in sample

[grams of S] / [mass of rock powder] * [100] = % pyrite-S in sample

Formula to calculate pyrite content:

Molar mass of pyrite FeS2 = (55.85 g Fe/mol Fe) + 2* (32.07 g S/mol S) = 119.99 g/mol

Fraction of sulfur = (2*32.07)/(119.99) = 0.5345

Iron to sulfur ratio = 55.85/(2*32.07) = 0.8708

⇒ Pyrite content of sample in % = [% pyrite-S] / 0.5345

⇒ Pyrite-bound Fe content of sample in % = [% pyrite-S] * 0.8708

Extraction of Ag2S for weighing and sulfur isotopes