Sequential Iron Extraction

Introduction

The sequential iron extraction method was created by Eva Stueeken on 160620 following protocols in the UCR Lyons Lab. Their are three steps, performed sequentially on the same powder aliquot:

1. Sodium acetate (Fe in carbonate)
2. Sodium dithionate (Fe oxides, except magnetite)
3. Ammonium oxalate (magnetite)

Pyrite-bound iron is extracted separately with chrome-reduction and an independent powder aliquot. 

Adjust reagent quantities for smaller batches of samples

Step 1: sodium acetate 

Preparing the reagent:

• Weigh 82.03g of NaAcetate into a 1L glass beaker (using a weigh boat)
• Add ~950ml of DI-H2O
• Stir with a magnetic stir bar and monitor the pH with a probe
• Once all NaAcetate is dissolved, slowly add acetic acid (glacial) until the pH reaches 4.5 ± 0.1. This may require 50ml of acetic acid or more.
• ⇒  This makes a 1M NaAcetate solution, buffered with acetic acid. It can be stored for 2 weeks, but the pH may need to be readjusted.

Extraction procedure:

• Weigh 100-150mg of rock powder into a 15ml plastic centrifuge tube
• Pipette 10ml of the 1M NaAcetate solution into the tube
• Put the cap on, shake briefly (or vortex), then loosen the cap and let the sample sit for ~1h, such that CO2 gas from major carbonate dissolution can escape.
• After 1h or so, tighten the cap, then place the samples into the shaker table for 48h
• After 48h, centrifuge the samples.
• Pipette out 4ml into a Nalgene vial and discard the rest of the solution. The solid residue is now ready for Step 2.
• (For Fe analyses, prepare a diluted aliquot (e.g. 1:100) for the ICP-MS.)

 Step 2: sodium dithionate

Preparing the reagent:

• Weigh 58.82g of sodium citrate into a 1L glass beaker (using a weigh boat)
• Add 21ml of acetic acid and then fill up the beaker to 1L with DI-H2O
• Stir with a magnetic stir bar until the NaCitrate is completely dissolved. 
• ⇒ This makes the 0.2M NaCitrate/0.35M acetic acid buffer solution, which can be stored for up to 2 weeks.
• Take a fresh glass beaker and add 50g of sodium dithionate (= sodium hydrosulfite). 
• Fill up to the 1L mark with the buffer solution
• Stir with a magnetic stir bar while monitoring the pH
• Use ammonium hydroxide to bring the pH to 4.8 ± 0.1. If you exceed this pH, readjust with a few drops of acetic acid. 
• ⇒ This makes a 50g/L sodium dithionate solution buffered with 0.35M acetic acid and 0.2M sodium citrate to pH 4.8. This solution cannot be stored for more than 1 day.

Extraction procedure:

• Pipette 10ml of the sodium dithionate solution into the centrifuge tube with the solid residue from Step 1. 
• Shake well (vortex), then place on the shaker table for 2h. 
• After 2h, centrifuge, then pipette out 4ml of liquid into a Nalgene bottle. 
• Discard the rest of the solution. The solid residue inside the centrifuge tube is now ready for Step 3.  
• (For Fe analyses, prepare a diluted aliquot (e.g. 1:100) for the ICP-MS.)

Step 3: ammonium oxalate

Preparing the reagent:

• Weigh 28.42g of ammonium oxalate and 21.43g of oxalic acid into a 1L glass beaker
• Fill up to 1L with DI-H2O
• Stir with a magnetic stir bar until everything is dissolved
• Monitor the pH and adjust it to 3.2 ± 0.1 with ammonium hydroxide. If you exceed this pH, add a few grains of oxalic acid. 
• ⇒ This makes a 0.2M ammonium oxalate/0.17M oxalic acid solution, buffered with ammonium hydroxide to pH 3.2

Extraction procedure:

• Pipette 10ml of the ammonium oxalate solution to the solid residue from Step 2.
• Shake up (vortex), then place on the shaker table for 6h
• After 6h, centrifuge, then pipette out 4ml of liquid into a Nalgene bottle. 
• Discard the rest of the solution and discard the centrifuge tube with the solid residue
• (For Fe analyses, prepare a diluted aliquot (e.g. 1:100) for the ICP-MS.)

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