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Sample Preparation for Determining Ions in Dark Colored Sodic Soil Extracts

USDA-ARS, Soil and Water Management Research Unit, Route 1, 3793 N. 3600 E., Kimberly, ID 83341

* Corresponding author.

ABSTRACT

Saturation paste extracts of sodic soils (pH >8.5 and electrical conductivity <4.0 dS m^–1) usually contain dark colored, suspended organic matter that interferes with colorimetric, turbidimetric, potentiometric, ion chromatographic (IC) and to a lesser extent, atomic absorption spectrophotometry and flame emission procedures. Bicarbonate also interferes with formate analysis by IC in those extracts. This study was conducted to develop a simple pretreatment method for removing those interferences without introducing new interferences. Fifteen mL extract samples were titrated to pH 8.4, then to 4.7 and finally to a pH range of 3.0 to 3.5 with standardized H₂SO₄ or HCl. The first two end points determine CO^2-₃ and HCO^-₃ concentrations. The third pH adjustment removed the HCO^-₃ interference from the formate analysis and allowed for organic matter coagulation. The acid choice depended on whether Cl^- or SO^2-₄ was to be measured later. To remove the organic matter, the extracts were contrifuged and forced through a 0.2-µm nylon filter following 0.08 M AlCl₃ or 0.04 M Al₂(SO₄)₃ treatments. Formate and acetate concentrations were determined by IC. The Cl^- concentrations were determined by colorimetric, and IC procedures. The SO^2-₄ concentrations were measured turbidimetrically and by IC. The extracts were analyzed for Ca²⁺ and Mg²⁺ by atomic adsorption spectrophotometry, and Na⁺ and K⁺ by flame emission. Titrating the extracts as described provided CO^2-₃ and HCO^-₃ data, removed the HCO^-₃ interference from the formate analysis, and allowed Al³⁺ to coagulate the suspended organic matter, which was then removed by centrifugation and filtration. This pretreatment did not interfere with any of the analytical methods tested, except for cation determination by IC and anion determination by IC when methyl orange was used as a pH end point indicator.

Contribution from USDA-ARS.

Received for publication January 7, 1988.

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