Evaluation of diffusion for inorganic-nitrogen analysis of natural water and wastewater

Public concern that agricultural use of N fertilizers may have adverse effects on environmental quality and human health has led to a growing need for reliable data concerning the concentration of inorganic N in ground and surface water. A study was conducted to compare the accuracy and precision of simple Mason-jar diffusion methods for quantitative determination of NH₄/⁺ and NO₃/^- in a wide variety of water and wastewater, relative to colorimetry, ion-selective potentiometry, and steam distillation. Good agreement among methods was generally obtained with standard solutions prepared using deionized water; however, substantial differences often were observed with natural and anthropogenic samples, because of either Cl^- interference in measurements with the NO₃/^- electrode or CO₃/^2- interference in distillation. Analytical accuracy also was evaluated by measuring recovery of N added as (NH₄)₂SO₄ or KNO₃ (6 mg N L^-1). With most of the samples studied, quantitative recovery (97-103%) was not achieved by potentiometry or distillation. Quantitative recoveries usually were achieved by a manual Berthelot method for colorimetric determination of NH₄/⁺, whereas recovery was often incomplete when NO₃/^- analyses were performed with an automated flow-injection system using Cd²⁺ reduction. Regardless of the sample matrix, diffusion was always accurate in measuring recovery of NH₄/⁺ or NO₃/^-.