An Interfacial Label-Free Electrochemical Approach to in-Situ Soil pH Monitoring

Abstract

Soil is a critical entity of Earth’s ecosystem. Healthy soil supports a landscape that is more resilient to the impacts of drought, flood, or fire. It also helps regulate the Earth’s climate. Real time assessment of soil health is imperative to improving crop yield as well to sustain an ever growing human population. Soil health encompasses moisture content, phosphorus, pH, nitrate, and other parameters. One of the most important parameters is soil pH which affects nutrients availability to plants and nutrients leaching in the soil. Current soil pH assessment techniques require destructive sampling of the soil where a core of soil is collected then treated at a lab before measuring the pH. This method measures pH at time points and doesn’t provide a continuous real time monitoring of pH. This work utilizes commercial screen printed electrodes with a pH sensitive coating for low cost continuous in-situ soil pH measurement. The coating is made up from alizarin an anthraquinone compound that undergoes a redox reaction in the presence of hydrogen ions and nafion an ion permeable membrane. Square wave voltammetry (SWV) is utilized since the output current signal shows the redox of alizarin while rejecting the capacitive current that’s inherent in soil. This redox peak current potential is dependent on the concentration of hydrogen ions which is gives a direct measure of pH. Due to the sensitivity of SWV, it is possible to measure unbuffered soil ph (without CaCl2 and KCl to improve the electrolyte ionic strength) accurately. The nafion layer protects the alizarin from dissolving in water present in soil as well as promoting the diffusion of hydrogen ions into the coating thus improving sensitivity. The alizarin has been used previously in screen printed electrodes however, this is the first work to our knowledge to utilize it to measure soil pH in-situ over a period of time. The sensor output was first calibrated in different unbuffered soil textures against the gold standard glass electrode. The calibrated response of the sensor in unbuffered sandy loam soil slurry is shown in Fig. 1. Then the sensor accuracy was validated by testing different soil samples using the sensor and later mixing it with deionized water and measuring the pH using the glass electrode. The sensor’s stability was evaluated by taking a measurement every hour for 48 hours without removing it from the soil sample. The temporal study showed a coefficient of variation less than 1% and an average reading of within 0.3 pH of the actual pH of the soil sample.Fig. 1 Calibrated response of proposed sensor in sandy loam soil slurries with 40% water Figure 1