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The Magnesium Problem in Nitrate Determination by Steam Distillation¹

ABSTRACT

Steam distillation of NH₃ is commonly used to quantify the amount of mineral N in soil and plant materials. A problem with NO₃^- recovery by this method was encountered in an NH₃ volatilization study. When NH₄NO₃ solutions were collected in acid and analyzed for NH₄⁺-N and NO₃^--N, incomplete recovery of NO₃^- occurred. As the volume or normality of H₂SO₄ added to the distilling solution increased, more of the MgO used to raise the pH of the distilling solution was solubilized, which caused NO₃^- recovery in the first 25ml of distillate to decrease. Only 80% NO₃^--N recovery was obtained with 20 ppm of Mg²⁺ in the distilling solution. Soils with 3 meq exchangeable Mg²⁺/100 g depressed NO₃^--N recovery by 15% using the standard distillation procedure. An interaction between Mg²⁺ and Devarda's alloy evidently inhibits NO₃^- reduction.

¹ Contribution of the Nebraska Agric. Exp. Stn., Lincoln, Nbr., as Paper no. 5972 Journal Series.

² Former Graduate Student, Research Soil Chemist, Soils and Fert. Res. Branch, TVA, and Professors, respectively, Univ. of Nebraska, Lincoln, NE 68583.

Received for publication April 4, 1980. Accepted for publication July 9, 1980.