To avoid the risk of explosion from using perchloric acid, the following method uses a mixture of nitric and hydrochloric acids (aqua regia). This extractant has been satisfactorily used by ADAS for over 10 years. Overnight soaking in cold aqua regia reduces frothing on heating. The digestion period has been extended to 3 h, which also makes it suitable for plant material; if there is excess organic matter in the sample, slightly more aqua regia is added at the start (Bob Llewelyn, ADAS Wolverhampton, 2001, personal communication). Note: TCA recommends extraction method BS EN 13650 with determination by ICP or ISO 11047. Details of behrotest® workstations for aqua regia digestion are available at the website:
http://www.behr-labor.com/ehtemel/prods/01404a.html Some mercury may be lost by the procedure given below, but excellent results have been reported for the self-contained LECO AMA254 advanced mercury analyser; details from LECO Corporation are at: http://www.leco.com/organic/ama/ama254.htm
• Digest acid - Safety note: Wear PPE and use a fume cupboard when handling concentrated acids which emit fumes. Carefully add 390 ml HCl (approximately 36% m/m HCl) to 360 ml of water and stir with a glass rod to mix. Slowly add 250 ml HNO3 (approximately 70% m/m HNO3) and stir slowly to mix, avoiding splashes. It is recommended that extra pure, spectroscopy (AAS/ICP) grade acids are used for trace element analyses.
• Nitric acid-KCl reagent, 8.75% m/m HNO3 - carefully add 125 ml HNO3 (approximately 70% m/m HNO3) to 40 ml of KCl solution (see below) and dilute with water to 1 l and mix.
• Potassium chloride solution, 5% m/v - dissolve 5 g of KCl in water and make up to 100 ml.
Procedure (extraction). Weigh 1.2 g air-dry soil, sieved to ^2 mm in a stainless steel sieve and then ground in a pestle and mortar, into a borosilicate boiling tube that has a graduation at 60 ml. Add 15 ml of the digest acid and carefully swirl to wet the sample thoroughly. Place a small glass funnel into the neck of the tube (to permit refluxing during the heating stage) and allow to stand overnight. Place the tube in a thermostatically controlled aluminium heating block and raise the temperature to 50°C and maintain this temperature for 30 min. Raise the temperature to 120°C, and digest for 3 h. Allow to cool, remove the funnel carefully to avoid acid drips, and make up to the 60 ml mark with the nitric acid-KCl reagent. Filter through a Whatman 541 paper, discarding the first few millilitres, and retain for analysis. Carry out a blank determination omitting the soil.
• Standard solutions - make up standard solutions, diluting with nitric acid-KCl reagent, to cover the following concentration ranges in pg element ml-1: Cd, 0-0.2; Cu, 0-2; Pb, 0-2; Ni, 0-0.5 and Zn, 0-1.5. • Releasing agent - dissolve 2.68 g lanthanum chloride heptahydrate (LaCl3.7H2O) in water and dilute to 100 ml.
Procedure (determination). Pipette 20 ml of each standard, extract and the blank solution into 50 ml beakers and add 1 ml releasing agent, then swirl to mix. Determine using AAS or ICP. If necessary, dilute a fresh sample extract or standard solution with the appropriate amount of nitric acid-KCl reagent to bring the readings on scale, then add releasing agent as above. The most sensitive resonance lines for AAS are (nm): Cd, 228.8; Cu, 324.8; Ni, 232.0; Pb, 217.0 and Zn, 213.9.
Calculation. If the measured concentration of trace element was y pg ml-1, this becomes y x 10-3 mg ml-1. A sample of 1.2 g soil was extracted into 60 ml solution, therefore 1 kg is equivalent to 103(60/1.2) ml. Therefore the measured concentration is equivalent to yx 10-3 x 103(60/1.2) mg kg-1, which becomes 50y mg kg-1 air-dry soil.
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