Weigh each sample 20.0 g used 2-digit balance.
2. Grind soil sample and dried solid sample was weight 10 g with 2-digit balance into a 250-ml beaker.
3. 0.5 ml of DI water and 10.00 ml HNO3 was add into a sample and a few boiling chips into the slurry samples and cover with a watch glass.
4. The sample was heat at 100 C on a hot plate for 2 hours in hood.
5. 3 ml of 30% H2O2 was drop-wise into a boiled sample and heat the sample on the hot plate for 1 hours in hood
6. Dilute to the mark with DI water and mix well
7. Pipette the sample 20.00 mL from 200-mL volumetric flask into a 25-mL volumetric flask
8. Added 0.50 mL of hydroquinone solution and 0.75 mL of o-phenanthroline solution into each volumetric flask
9. Dilute to the mark with sodium acetate buffer and mix well
10. Allow the standard solutions and blank solution to stand at l0 minutes
11. Used Vis-Spectrophotometer measure the absorption spectrum of the highest concentration of the iron solution by measuring the absorbance 510 nm
12. Setting Vis-Spectrophotometer by used blank solution
13. Then measure the absorbance of each solution 3 replicates
14. Record the absorbance of each solution