Evaluation of Phosphate Ion-Selective Membranes for Real-time Soil Nutrient Sensing
A real-time soil nutrient sensor would allow the efficient collection of data with a fine spatial resolution, to accurately characterize within-field variability for site-specific nutrient application. Our goal was to evaluate the applicability of a phosphate membrane to the measurement of phosphate levels in soil extractants and to determine how previously developed nitrate and potassium membranes would be affected by the presence of phosphate. A type of PVC-based phosphate membrane containing an organotin compound, bis(p-chlorobenzyl)tin dichloride, was evaluated, along with the nitrate and potassium membranes, in pH 7 Tris buffer solution and Kelowna soil extractant for sensitivity and long-term stability. The phosphate membranes in the Tris buffer solution of pH 7 exhibited a response over a range of 10-5 to 10-1 mol/L phosphate concentrations with an average slope of -28.2 +1.5 mV per activity decade of dibasic phosphate. The response speed of tested electrodes containing phosphate, nitrate and potassium membranes was rapid, reaching an equilibrium response in less than 15 s. However, the phosphate membrane in the Kelowna solution of pH 8.5 was almost insensitive to different phosphate levels from 10-6 to 10-2 mol/L due to the presence of a high concentration of fluoride in the solution. In addition, the tin compound-based phosphate membranes had limited lifetimes of less than 14 days. It is not expected that the tested phosphate membranes could be used for phosphate detection in other soil extractants, such as Bray P1 and Mehlich III solutions, because they also contain high concentrations of fluoride.
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