Sulfate removal procedure
Based on Bodine and Fernalld (1973)
For sulfate removal, you'll be using a solution of 0.4N EDTA solution (this is a salt
available at the Barrows storeroom), buffered to a pH of 10-12 with NaOH pellets. Wear
gloves and safety glasses. To make the solution, weigh about 147 g (+/- 3g) into a 1 L
plastic bottle (I used a plastic funnel). Add filtered DI to make ~1 L of solution. In a
plastic weighing tray, weigh out ~3 g of NaOH pellets (also available from the storeroom,
but we have a lot), then use a plastic funnel to add to the 1 L bottle. Shake by hand
immediately since the NaOH starts dissolving and gets the bottom of the bottle pretty
warm. You can put the bottle on the shaking table while you get your first sample ready
to make sure it's completely dissolved. Check the pH using pH paper and dipping it in
droplets in the cap of the bottle.
1. Weigh some sample (40-100g depending on how much salt is in the soil and how
much salt-free soil you need) into a wide-mouth bottle that will fit in the centrifuge (I
used 250 ml).
2. Add EDTA solution, cap, then weigh the bottle. For your next sample, try to add
EDTA solution so that it weighs the same as the first. Keep track of your pairs so you
can load the centrifuge with the weight evenly distributed. It is easiest to work in batches
of 6, 12, or 18 so you have even sets for the centrifuge.
3. Load the samples on the shaking table and shake the samples at least 7 hours or
overnight.
4. Once the samples have been shaken, centrifuge at 3000 rpm for 1 hr. Decant most of
the solution into a large plastic beaker (I used the 4L one above the sink), but pour a little
(~5ml) into a glass beaker. Add a small squirt of acetone to the solution in the little
beaker-if it is cloudy then there is sulfate in the solution. If it does contain sulfate, then
repeat the process from step 2. If the solution remains clear, add water instead of EDTA
solution and repeat from step 3. Dispose of the decanted EDTA solution as described in
step 6.
5. Once the sample has been rinsed with water, decant the water then transfer the sample
into a pre-weighed, labeled beaker using a squirt bottle. Dry the sample in the oven
(50ºC) and weigh again. All done!
6. Wear gloves, safety glasses, and a lab coat. For ~600 ml of decanted EDTA solution,
dilute to at least 1:1 with tap water (do this in the fume hood) then add ~60 ml of 1N
HCl. Stir with a long glass stirring rod. This should make a solution with a pH of ~8
(test with pH paper) which can be disposed of by slowing pouring into the sink while
running the faucet to dilute further.