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Soil Screening Method for Arsenic

 

(Digestion Method)

Scope and Application: (Digestion Method)
1. This method is applicable to the determination of Inorganic Arsenic in soil.
2. The method is applicable in the range from 5 to 500 mg As/kg soil

Sample Handling and Preparation:
3. Dry soil for at least 1 hour at 60°C or until completely dry.
4. Grind the dried soil into a fine powder using a coffee grinder or mortar and pestle and mix until sample is pulverized. (a Coffee Grinder works well)

Interferences:
5. Test can eliminate up to 2 ppm of Hydrogen Sulfide.
6. Iron concentrations above 9000 mg Fe/kg in soil will give low Arsenic results.
7. Lead concentrations above 1500 mg Pb/kg in soil will give low Arsenic results. (Note: Lead levels of 5000 ppm or greater are considered as Superfund Contamination.) The lead poisons the zinc reaction and suppresses the generation of Hydrogen and Arsine gas. To minimize lead interference, 0.2 g of Potassium Iodide (KI) should be added in the digestion procedure.

Equipment/Apparatus Needed:
8. Heating Block (Hach® COD Reactor Model 45600 or Equivalent)
9. Borosilicate screw cap style glass test tube (16 x 125 mm, Pyrex # 99449-16x or 99449-16xx or Equivalent) with Teflon lined screw cap (Pyrex # 9998-15 or Equivalent)
10. Transfer Pipette
11. Thermometer
12. 50 mL or 100 mL Volumetric Flask

Reagents Needed:
13. 50% (v/v) Hydrochloric Acid [HCl]

14. Distilled Water (or Arsenic-free Tap Water)

Safety Considerations:
15. Use a well-ventilated fume hood when handling Hydrochloric Acid (concentrated or 50%).

16. Wear Personal Protective Equipment (Gloves, Safety Glasses/Goggles, Lab Coat or Apron) when handling Hydrochloric Acid.

Digestion Procedure:
17. Weigh 0.5 g of the dried soil and transfer to a glass test tube.

18. Pipette 4.5 mL of 50% (v/v) HCl into the test tube, secure the screw cap tightly on the test tube and shake upright for 5 seconds. (To minimize particles clinging to the upper walls of test tube and cap, it is recommended not to invert the test tube.)
19. Place test tube in heating block for 1 hour at 95°C. Mix sample at least twice during digestion by carefully shaking test tube upright. 20. After digesting the soil for 1 hour, remove the test tube from the heating block and allow to cool.
21. Cautiously open test tube containing Digested Soil Sample and point cap away from eyes and body. Transfer the cooled digest to a 50 mL volumetric flask. Wash the test tube several times with Distilled or Arsenic-free water and add wash water to the flask without exceeding 50 mL volume. Fill to 50 mL volume with distilled water.

Test Procedure:
22. Transfer 10 mL of the 50 mL diluted digest to the Reaction Bottle supplied in the Arsenic Quick™ Kit (Part # 481396). Fill the Reaction Bottle to the upper marked line with Arsenic-free tap water or Distilled water.
23. Follow the standard test procedure for the Arsenic Quick™ Kit starting with Step 3 on page 3.

Calculation:
24. Multiply test result by 1000 (Example: 50 μg/L becomes 50 mg/kg)

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