Assessment of Ionic Interferences to Nitrate and Potassium Analyses with Ion-Selective Electrodes

Ion-selective electrodes (ISEs) are simple tools used for rapid measurement of nitrate nitrogen (NO₃-N) and potassium (K) concentrations in plant sap. With the development of best management practices (BMPs), interest exists in using ISEs for soil leachate and soil and fertilizer solutions. Nitrate N and K concentrations in the 0 to 10,000 mg L^–1 ISE working range were measured in diluted solutions of common salts to assess ionic interference of calcium (Ca²⁺), ammonium (NH₄ ⁺), chloride (Cl^–), and sulfate (SO₄ ^2–). The effects of meter (replication) were unexpectedly significant in one out of three ranges for NO₃-N and K (P values of 0.50, 0.72, and 0.01 for NO₃-N and 0.99, 0.01, and 0.74 for K, for the 0–100, 100–1,000 and 1,000–10,000 mg L^–1 ranges, respectively). The responses of calculated NO₃-N and K concentrations to measured NO₃-N and K concentrations were linear, but slopes ranged from 0.85 to 1.54, from 0.24 to 2.72, and from 0.93 to 5.48 for NO₃-N and from 0.80 to 1.01, from 0.71 to 1.39, and from 0.93 to 2.21 for K for the 0–100, 100–1,000, and 1,000–10,000 mg L^–1 measuring ranges, respectively. All slopes were significantly different from zero, and several were significantly different from each other and the 1:1 line. Pairwise slope comparisons conducted with covariance analysis showed that SO₄ ^2– alone interfered with NO₃-N measurements at concentrations ranging from 34 to 171 mg L^–1, which was less than the manufacturer's information, and by its presence in combination with K⁺, NH₄ ⁺, Ca²⁺, and Cl^– within the medium and high concentration ranges. Potassium measurements were not subject to interference from the ions tested for all three concentration ranges. These results highlight the importance of using quality assurance / quality control (QA/QC) samples in the set of unknown samples to detect inacceptable departure from linearity in routine analysis. The increase in measurement variability from one range to the next showed the importance of keeping measurements within a single concentration range by using dilutions. Hence, ISEs may be used for field measurements of NO₃-N and K concentrations in soil leachate as well as soil and nutrient solutions and are therefore a practical BMP tool. However, ISEs should not be used as substitutes for the laboratory methods when official measurements are needed.