I digest the soil sample with HNo3 , perchloric and H2SO4 in the ratio 7:1:2 in a open beaker on hot plate at 900C but acid evaporate before digestion.
Perchlorate is a dangerous agens in the lab! We extract the samples with boiling aqua regia under reflux. Its an accepted method for (near) total concentration.
Di-Acid (HNO3-HClO4), Block Digester
Apparatus
Digestion tube
Block-digester
Atomic Absorption Spectrophotometer
Reagent
Nitric Acid (HNO3), concentrated
Perchloric Acid (HClO4), concentrated
Procedure
1. Weigh 0.5-1.0 g air-dry soil (0.15-mm) into a 300-mL calibrated digestion tube.
2. Add 3 mL concentrated HNO3 (in the fume hood), and swirl carefully, and then place tubes in
the rack.
3. Place the tubes rack in the block-digester, and then place a glass funnel in the neck of the tubes.
Slowly increase temperature setting to about 145 °
C for 1 hour.
4. Add 4 mL concentrated HClO4 and heat it to 240 o
C for further 1 hour.
5. Lift the tubes rack out of the block-digester, carefully place on a rack holder, and let tubes cool
to room temperature.
6. Filter through Whatman No. 42 filter paper and bring to 50-mL volume.
7. Each batch should contain at least one reagent blank (no soil).
8. Determine Fe, Mn, Cu, Zn, Cd, Ni, Pb, Co, and Cr by Atomic Absorption Spectrophotometer (reference ICARDA manual)
Perchlorate is a dangerous agens in the lab! We extract the samples with boiling aqua regia under reflux. Its an accepted method for (near) total concentration.
I totally agree with Dr Schnug on this. Although there are many acid digestion schemes, aqua regia has been widely recognized as suitable extract for determining the approximate levels of heavy metals (pseudototal analysis). More details about the aqua regia method you can find attached:
Article A comparison of wet oxidation methods for determination of t...
For any kind of samples including soils, the best method of digestion is acid digestion performed in a microwave digester, which is fast and accurate for detection of all metallic elements
A common accepted method of digestion of soil for metals is a nitric acid-hydrogen peroxide digest that can be done in either a digestion block or microwave digestor. Aqua regia is another procedure that is satisfactory for metals determination in soils.
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