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 am monium chloride soil extracts. It makes use of ethanol as a reaction accele rator. Full color development is achieved after a standing period of 2 hour s and the absorbance measurements were performed at 555 nm, against the bla nk. Under these conditions the interference of other elements usually found in the soil extracts were not important. The calibration graphs, in the ra nge from 0 to 1.0 mg L-1 Al+3, were fitted by 2nd order polynomial regressi ons. The protocol was tested with 10 representative soil samples from the S tate of Sao Paulo (Brazil), containing exchangeable aluminum varying from 3 0 to 200 mg Al kg(-1) of soil and variable amounts of iron. All samples wer e run in triplicate and the results averaged. The correlation among the pro posed procedure and the measurements obtained by ICP-AES were [Al,XO] = 0.9 6[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 KCI and NH4Cl extractions. For absorbance readings above 0.9 or below 0.1 it is advisable to repeat the determinations by taking sa mple aliquots of 1.00 or 10.00 mt, respectively. This analytical approach p roved to be appropriate for routine work.