An improved analytical protocol for the routine spectrophotometric determination of exchangeable aluminum in soil extracts with xylenol orange.

The proposed analytical protocol is applicable to potassium chloride and ammonium chloride soil extracts. It makes use of ethanol as a reaction accelerator. Full color development is achieved after a standing period of 2 hours and the absorbance measurements were performed at 555 nm, against the blank. Under these conditions the interference of other elements usually found in the soil extracts were not important. The calibration graphs, in the range from 0 to 1.0 mg L -1 Al +3 , were fitted by 2nd order polynomial regressions. The protocol was tested with 10 representative soil samples from the State of Sao Paulo (Brazil), containing exchangeable aluminum varying from 30 to 200 mg Al kg -1 of soil and variable amounts of iron. All samples were run in triplicate and the results averaged. The correlation among the proposed procedure and the measurements obtained by ICP-AES were [Al,XO] = 0.96[Al,ICP] - 7.38 with r 2 =0.9950 and [Al,XO] = 1.06[Al,ICP] - 6.43 with r 2 =0.9970, for the KCl and NH 4 Cl extractions. For absorbance readings above 0.9 or below 0.1 it is advisable to repeat the determinations by taking sample aliquots of 1.00 or 10.00 mL, respectively. This analytical approach proved to be appropriate for routine work.