Method 711a Preparation of plant sample solution by dry combustion

Reagents.

• Hydrochloric acid, approximately 36% m/m HCl.

• Hydrochloric acid, approximately 6 M - mix equal volumes of hydrochloric acid, approximately 36% m/m HCl and water.

Procedure. Transfer 2 g dried sample, ground to 1 mm, into a silica basin, place in a cool muffle furnace, and increase the temperature to 500°C, which is maintained overnight. Note: if the sample is not oven-dried, take a separate sample for moisture determination. The ash should be whitish-grey. (If some carbon remains, cool the basin, moisten with water, dry at 102°C, then reheat at 500°C.) Cover with a watch glass and slightly displace to allow the addition of 10 ml of approximately 6 M HCl, avoiding loss of solution by effervescence. Remove and rinse the watch glass into the basin. Place the basin on a water-bath and evaporate the solution to dryness. When dry, continue heating for 1 h either on the water bath or in an oven at 102°C. Moisten the residue with 2 ml of hydrochloric acid, approximately 36% m/m HCl, cover the basin with a watch glass and gently boil for 2 min. Add approximately 10 ml of water and again boil. Remove the watch glass and rinse into the basin. Transfer the contents of the basin quantitatively into a 50-ml volumetric flask and dilute to the mark with water and mix. Filter through a 90mm Whatman No. 541 filter paper, rejecting the first few millilitres, and retain the remainder for analysis in a polythene sample tube. Perform a blank extraction. Suitable determinations include Ca, K, Mg, Mn, P and Na.

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