Feasibility of rapid detection of soil nitrate-nitrogen content using Superfloc127 in ion-selective electrode

Abstract：Sample pretreatment is one of the key steps for soil nutrient analysis. It greatly affects the detection efficiency of Ion-selective electrode (ISE) based soil nitrate-nitrogen (NO3--N) detection. Aiming at shortening the standing time, the polymer flocculant Superfloc127, one of conventional non-ionic polyacrylamide chemicals, was reported to be applied in sample pretreatment of soil detections based on the ultraviolet-visible spectrophotometry (UV-VIS). In this study, the feasibility of employing Surperfloc127 into the soil NO3--N rapid detection by using ISE was discussed. First, the theoretical analysis of the dynamic electrode process was conducted under the stationary-state conditions, which described the assumed electrochemical equilibrium of the ionic exchange, ionic flux and ionic co-extraction under zero current condition in artificial nitrate solution. A phase boundary potential model was built to prove the ignorable influences produced by Superfloc127 on the solvent polymeric ISE with the inner filling solution. The calculated results of the phase boundary potential model showed that less degradation of the linear detection ranges (LDR) was observed when Superfloc127 was added into the nitrate sample. Theoretically, Superfloc127 was demonstrated acceptable influences on the performance of nitrate ISE. And then, soil samples was prepared to further verify the applicability of Superfloc127. Soil testing was carried out by the self-developed PPy (NO3--N)-membrane nitrate solid-state ISE (PPy-NSISE) and the commercial PVC-membrane nitrate ISE (PVC-NISE). The previous developed soil macronutrient automatic detecting stand was integrated for the measurements throughout the whole study. The effects of Superfloc127 on the performances of PPy-NSISE and PVC-NISE were evaluated by comparing the response sensitivities in standard nitrate solutions. The concentrations of superfloc127 were artificially controlled to 0, 25, 50 and 75 mg/L. 10 soils covering 4 different types were selected. Each soil was divided into three sets. Soil set1 was measured with ultraviolet spectrophotometry. Soil set2 was pretreated without Superfloc127 and analyzed by the ISEs. Soil set3 was pretreated with Superfloc127 and analyzed with the ISEs. Results showed that the standing time, which represented the duration from the ending instant of the shaking process to the instant of obtaining transparent soil extracts, was enormously shortened from 1.5h to 10s by the addition of Superfloc127. The PPy-NSISE and the PVC-NISE produced good sensitivities with the response slopes of -51.4 and -52.1 mV/decade. Neglectable influence made by Superfloc127 was observed on the changes of sensitivities of PPy-NSISE and PVC-NISE. The determination coefficient (R2) of linear regression between ISEs based soil NO3--N measurements and the UV-VIS results were 0.69 and 0.31 for PPy-NSISE and PVC-NISE, respectively. After continuous determination of 12h, the absolute response slopes of the PPy-NSISE and the PVC-NISE were reduced to 35.1 and 25.4 mV/decade. Moreover, the ISE performance couldn’t be recovered even by the re-activation process. In summary, Superfloc127 could greatly improve the efficiency of soil pretreatment. But it will fasten the ISE deteriorating promptly. Therefore, it was not recommended to use Superfloc127 in ISE based soil NO3--N detection. Researches should be carried out to clarify the mechanism of Superfloc127. The attempts of enhance the pretreatment efficiency of ISE based soil detection were greatly encouraged.