Acid-washing soils to remove carbonates

  1. This procedure should be done in a fume hood with safety glasses and gloves.
  2. Soil samples should be oven dry, homogenized, and finely ground.
  3. Prepare a 1N HCl solution in a 500 ml volumetric flask. You will need about 10-15ml of this per soil sample
  4. Add deionized water to a 500ml volumetric flask to 5 cm below the junction of the lower bulb and the neck (allowing room for the acid to be added).
  5. Using a graduated cylinder, or a glass pipette, slowly add 42ml of 36-38% by weight HCl to the water.
  6. Cover the top of the flask and gently swirl the solution until well-mixed.
  7. Add deionized water up to the 500ml mark on the neck.
  8. Swirl again
  9. Label the flask with a sharpie, cover the top, and wrap with Parafilm.
  10. Now mix 250ml of diluted solution (0.5N HCl). This will be the primary acid wash.
  11. Add 100ml of deionized water to a 250ml volumetric flask.
  12. Add 125ml of the 1N HCl to the flask.
  13. Fill the flask to the measurement line (250ml) with deionized water.
  14. Mix as above.
  15. Label the flask with a sharpie, cover the top, and wrap with Parafilm.
  16. Label a clean 50ml Erlenmeyer flask with a soil sample id number, and add about 2ml of that soil to the flask. This is roughly 1cm depth in a 2.5x5cm tall sample vial.
  17. Add 15ml of the weaker 0.5N HCl solution. It should bubble as CO~2~ is produced (if carbonates are present).
  18. Let sit in the fume hood for 8 hours, or overnight. Bubbling should stop.
  19. Use pH paper to test the pH of the solution. If it is acidic (below 6.5) then there was sufficient acid to neutralize dissolve the carbonates in the soil. If not, add 5ml of the stronger 1N HCl solution and wait until the reaction is complete.
  20. Filter the soil solution through a clean ceramic funnel with a #30 glass filter paper (5.5cm diameter), smooth side up, into a 1-2L filter flask (Erlenmeyer with a sidearm). The sidearm can be attached to a water aspirator or a vacuum to aid filtration.
  21. Rinse 5 times with ~10ml of deionized water each time. You an estimate, but be sure the soil is well rinsed.
  22. Label a clean petri dish with the correct soil sample id number (this label can be pulled from the Erlenmeyer flask above) and place the filter paper with the rinsed soil in it.
  23. Dry all the soil samples in a drying oven at 70°C overnight, then transfer to a new labelled storage vial.
  24. Samples may need to be ground again before loading for EA-IRMS analysis.