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:

• Weigh 30g of ZnAcetate into a 1L flask or beaker (using a weigh boat)
• Add 100ml of ammonium hydroxide
• Bring up to 1L with DI-H2O
• Stir with a magnetic stir bar until all solids are dissolved
• ⇒ This makes a 10% NH4OH – 3% ZnAcetate solution. It can be stored for a few weeks.

Silver nitrate trap solution:

• Weigh 30g of AgNO3 into a 1L flask or beaker (using a weigh boat)
• Add 100 mL of ammonium hydroxide (ammonia solution)
• Bring up to 1L with DI-H2O
• Stir with a magnetic stir bar until all solids are dissolved
• ⇒ This makes a 10% NH4OH – 3% AgNO3 solution. It can be stored for a few weeks.

Potassium iodate titrant solution:

• Weigh 10.700g of potassium iodate into a 500ml flask or beaker (using a weigh boat)
• Add 250ml of DI-H2O
• Add 50g of potassium iodide
• Bring up to 500ml with DI-H2O
• Stir with a magnetic stir bar until all solids are dissolved
• ⇒ This makes a 0.1M KIO3 solution with excess KI. The molarity of KIO3 needs to be known accurately. 

Chromium chloride solution for pyrite reduction:

• Weigh 266.45g of ClCl3∙6H2O (i.e. Cr3+) into a 1L glass beaker (using a weigh boat)
• Add 41.25ml of concentrated HCl (doesn’t need to be very accurate)
• Bring up to 1L with DI-H2O
• Stir with magnetic stir bar until all solids are dissolved. This may take an hour or more. 
• ⇒ This makes a 1M CrCl3 solution in 0.5M HCl, which can be stored for a few weeks. 
• Draw this solution under vacuum through a chromatography column packed with granular elemental Zn
• Alternatively, pour it into a flask with mossy zinc under a stream of N2
• Both of these steps reduce Cr3+ to Cr2+, while releasing H2
• ⇒ This makes a 1M CrCl2 solution in 0.5M HCl. This can only be stored for a few days.

Chrome reduction procedure

• Weigh the sample into the CR vessel (it may take 0.1-2 g of rock or more, depending on estimated pyrite content.)
• Add ~10ml of ethanol with a squirt bottle, using this to rinse off the sides of the vessel. The ethanol will help with reducing elemental sulfur during the reaction
• Optional: add some weak HCl (~1-2M) to dissolve carbonate, if rocks are carbonate-rich, which may later on generate excess amounts of CO2 and acidify the trap solution, thus preventing H2S from getting trapped. This step should not be done if acid-soluble sulfur species (pyrrhotite, sphalerite, galena, chalcopyrite etc.) are expected to be present, because they may be lost. In most cases, these are minor. 
• Attach the CR vessel to the extraction line and flush it with N2 gas
• In the meantime, set up the traps (Erlenmeyer flasks): fill each trap with either 30ml ZnAcetate solution or 30ml AgNO3 solution, attach to the line with bubble tube and let purge with N2 gas
• While vessels are purging, turn on the cooling water system (water pump)
• Turn on hotplates
• Take a 50-60ml syringe, suck up 35ml CrCl2 solution and 15ml concentrated HCl (i.e. they go together into the same syringe and mix before being added to the sample)
• Eject CrCl2 + HCl mixture from the syringe into the reaction vessel
• Let the sample react for 2h on the hotplate. N2 gas will carry produced H2S into the trap solution, where it re-precipitates as either Ag2S (brown to black) or ZnS (white)
• Product can be quantified by iodometric titration (ZnS) or careful weighing (Ag2S). For isotopes, Ag2S is recommended, because it is easier to handle

Iodometric titration with ZnS 

• Add 1ml of starch solution (excess) to each ZnS trap (Erlenmeyer flask with ZnS and residual ZnAcetate solution in it)
• Add a small magnetic stir bar
• Fill up the burette with 2.000ml of titrant and force it through one of the holes in the rubber stopper. The bubble tube stays in the other hole
• Using a small syringe, inject 10ml of 6M HCl through the bubble tube into the trap. This will dissolve the ZnS and generate H2S
• Try to keep the rubber stopper tight to minimize H2S escape
• While stirring with the magnetic stir bar (i.e. set flask onto a stir plate), slowly add titrant until the color of the solution changes to purple. Shake flask once and add a few more drops to make sure that the reaction has gone to completion

What happens in the titration?

• Excess KI in the titrant reacts with KIO3 to make I2 (the stoichiometry is 3 moles of I2 per 1 mole of IO3-, which becomes important in the calculation later): 

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

• Then I2 reacts with H2S to form elemental sulfur:

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

• o So with one mole of KIO3, one can convert 3 moles of H2S to S0.
• Then I2 + starch + I- = purple color. The color change happens, because once all H2S is consumed, I2 is present in excess, i.e. it is no longer consumed by H2S

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

• Weigh a filter paper (polycarbonate or other fine, smooth material), maybe with a weigh boat.
• Pour the solution through a vacuum filtration tower with the polycarbonate filter paper
• Wash the filtrate thoroughly by pouring DI-H2O after it
• Carefully remove the filter paper with the Ag2S on it and place it into a weigh boat (maybe the same as used before)
• Let the sample dry in an oven at <50 °C (otherwise weigh boat can deform or melt)
• Weigh the filter paper again with the Ag2S on it and subtract the empty mass to get the mass of Ag2S
• Convert to S and Fe using molar masses
• Transfer Ag2S into vial for storage for later S isotope analyses. 

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