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Extraction with hot water is the most widely used procedure to determine boron (B) in soils for the diagnosis of the nutrient availability for plants. However, this procedure is tedious for routine conditions and requires some special precautions. An alternative extraction procedure was developed, consisting in the extraction of B with a 1.25 g/L solution of barium chloride (BaCl 2 ) or with water heated in a domestic microwave oven. Boron was determined either by inductively coupled argon plasma emission spectrometry (ICP-AES) and by spectrophotometry with azomethine-H. Good correlations were found between the extraction by boiling water under reflux and the extraction by the microwave heating for 13 Brazilian soils, and the contents of B obtained by spectrophotometry did not differ significantly from those obtained by ICP-AES. Considering the calculated standard deviations, it can be concluded that microwave heating followed by ICP-AES determination is an adequate procedure for the determination of hot water extractable B in soils.

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Aflow injection analysis (FIA) procedure for the speciation of Cr(lll) and Cr(VI) using the 1,5-diphenyIcarbazide (DPC) method is presented. As Cr(lll) does not interfere in the Cr(VI) -DPC reaction, both Cr(VI) and total chromium [after the on-line oxidation of Cr(lll) by Ce(lV)] are sequentially determined. Cr(lll) is obtained by difference. Under the experimental conditions described, the calibration graphs are linear up to 2 pg ml-1 of Cr(VI) and 4 pg ml-1 of Cr(ll1). The detection limits found were 18 ng ml-1 for Cr(VI) and 55 ng ml-1 for Cr(lll), at a signal to noise ratio of 3. The common interfering elements in the Cr(VI) -DPC reaction were investigated under dynamic FIA conditions. The FIA method was also compared with the conventional spectrophotometric procedure.

An on-line procedure for the oxidation of Cr(lll) to Cr(VI) was developed. The extent of oxidation was followed using the 1,5-diphenyIcarbazide (DPC) spectrophotometric method accommodated in a flow injection analysis (FIA) system. Cerium(tV) sulphate and potassium peroxydisulphate were tested as oxidising agents. Despite the high oxidation power of the peroxydisulphate ion in acidic solution, the use of Ce(lV) is more convenient under dynamic conditions. The best experimental conditions were obtained a t 45°C using a 0.025% rnlVCe(lV) solution in 0.07 M H2S04 with the Cr(lll) solutions containing up to 3.0 x 10-2 M H2S04.