Additions of concentrated nitric acid, hydrochloric acid, and hydrogen peroxide digest soils and dissolve metals that could potentially become environmentally available. The final extract is diluted and analyzed using ICP-OES to determine metal content. EPA method 3050b is followed. A standard is analyzed with every batch of unknowns. Depending on the characteristics of the sample, this procedure can be expected to take between two and four hours to complete.

Principle

Soils are digested in nitric acid, hydrochloric acid, and hydrogen peroxide. The reagents will digest all organic matter and dissolve almost all of the metals that might become available in the environment. 

Equipment

  • Beakers (50 or 100 ml)
  • Watch glass
  • Glass stir rods
  • Adjustable pipettes
  • Hot plate
  • Filter paper
  • Funnels
  • Fume hood
  • 100 ml volumetric flasks

Reagents

  • Concentrated nitric acid
  • Concentrated hydrochloric acid
  • 30% H2O2
  • DI water

Procedure

  1. Soil samples should be dried at 105°C and ground and sieved to 2mm.
  2. Weigh approximately 1g of dried soil into the beaker. Record the exact weight.
  3. Add 10 ml of 1:1 HNO3, mix, and cover. Heat for 15 minutes without boiling.
  4. Allow the sample to cool, add 5 ml of concentrated HNO3, cover, and heat for 30 minutes.
  5. Continue to add HNO3 in 5 ml increments with 15 minutes of heating until no more brown fumes are produced.
  6. Heat uncovered without boiling until the volume has reduced to approximately 5 ml.
  7. Allow to cool.
  8. Add 2 ml of water and 3 ml of 30% H2O2. Heat until effervescence subsides.
  9. Repeatedly add H2O2 in 1 ml aliquots with a few minutes of heating until no more effervescence is produced, up to a maximum of 10 ml.
  10. Cover the sample and heat until the volume is reduced to approximately 5 ml.
  11. Allow to cool.
  12. Add 10 ml of HCl and cover.
  13. Heat for 15 minutes.
  14. Filter, collecting the digestate in a 100 ml volumetric flask.
  15. Dilute to volume using DI water.

 

Quality control

  • Use beakers large enough that sample isn’t lost due to violent effervescence.
  • Use beakers small enough that 5 ml completely covers the bottom.
  • Do not use the same stir rod in different beakers.