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Wet Ashing Procedure for Determining P, K, Ca, Mg, Zn, Mn, Cu, Fe, S using HN03 AND HCL04

(This procedure is not recommended for ICP analysis.)

Reagents

  1. CONCENTRATED NITRIC ACID - HNO3
  2. 70% PERCHLORIC ACID - HCl04

Apparatus

  1. PERCHLORIC ACID FUME HOOD
  2. HOT PLATE*
  3. 150 mL BEAKERS
  4. WATCH GLASS COVERS
  5. 100 mL VOLUMETRIC FLASKS

Procedure

  1. Weigh 1.000 g sample into 150 mL beakers.
  2. Add 10 mL conc. HNO3.
  3. Place beakers on hot plate and cover with watch glass covers and heat at 4.5 until all organic matter is digested - clear yellow. Remove watch glasses and continue to heat to reduce volume to near dryness (do not bring to dryness).
  4. Remove beakers from hot plate and add 5 mL of 70% HCl04.
  5. Return to hot plate and digest at 5.5 until white fumes appear and solution is clear (silicates will be in bottom). If S is to be determined, allow to fume a minimum of 1 1/2 h.
  6. Remove beakers from hot plate. While still under the hood add about 10 mL of dH20 to beakers to cool samples and stop fuming.
  7. Quantitatively transfer sample into 100 mL volumetric flask with dH2O - cool to room temperature.
  8. Dilute to volume, stopper and shake flasks.
  9. Allow silicates to settle and transfer aliquot to test tubes. 
    *The setting 4.5 on the hot plate corresponds to about 100oC, the setting 5.5 corresponds to about 200-250oC.

References

  1. Ellis, R. J., Jr., J. J. Hanway, G. Holmgren, D. R. Kenney, and O. W. Bidwell. Sampling and Analysis of Soils, Plants, Waste Waters, and Sludge-Suggested Standardization and Methodology. Agricultural Exp. Station, Kansas State University, Manhattan, Res. Pub. 170. North Central Regional Pub. 230.
  2. Haynes, R. J. 1980. A Comparison of Two Modified Kjeldahl Digestion Techniques for Multi-Element Plant Analysis with Conventional Wet and Dry Ashing Methods. Comm. in Soil Sci. and Plant Anal. 11(5): 459-467.