Sensing NO₃-N and K Ions in Hydroponic Solution Using Ion-Selective Membranes

Rapid on-site sensing of nitrate-nitrogen and potassium ions in hydroponic solution would increase the efficiency of nutrient use for greenhouse crops cultivated in closed hydroponic systems while reducing the potential for environmental pollution in water and soil. Ion-selective electrodes (ISEs) are a promising approach because of their small size, rapid response, and the ability to directly measure the analyte. The capabilities of the ISEs for sensing nitrate and potassium in hydroponic solution can be affected by the presence of other ions such as calcium, magnesium, sulfate, sodium, and chloride in the solution itself. This study was conducted to investigate the applicability of two ISEs consisting of TDDA-NPOE and valinomycin-DOS PVC membranes for quantitative determinations of NO₃-N and K in hydroponic solution. Nine hydroponic solutions were prepared by diluting highly concentrated paprika hydroponic solution to provide a concentration range of 3 to 400 mg/L for NO₃-N and K. Two of the calibration curves relating membrane response and nutrient concentration provided coefficients of determination (R²) greater than 0.98 and standard errors of calibration (SEC) of less than 3.79 mV. The use of the direct potentiometry method, in conjunction with an one-point EMF compensation technique, was feasible for measuring NO₃-N and K in paprika hydroponic solution due to almost 1:1 relationships and high coefficients of determination (R² greater than 0.97) between the levels of NO₃-N and K obtained with the ion-selective electrodes and standard instruments. However, even though there were strong linear relationships (R² greater than 0.94) between the NO₃-N and K concentrations determined by the Gran's plot-based multiple standard addition method and by standard instruments, hydroponic NO₃-N concentrations measured with the ISEs, on average, were about 10% higher than those obtained with the automated analyzer whereas the K ISE predicted about 59% lower K than did the ICP spectrometer, probably due to no compensation for a difference between actual and expected concentrations of standard solutions directly prepared.