A New Calibration Free pH‐Probe for In Situ Measurements of Soil pH

Abstract

A new pH-probe was developed for in situ determination of soil pH. It consists of a stainless steel tube with a plastic inset containing the indicator electrode, the reference electrode and a temperature sensor at the end of the tube. The indicator electrode is a quinhydrone composite electrode that does not need to be calibrated, because it acquires almost the theoretical predicted potential and has a constant formal potential and slope under all fabrication conditions. The pH-probe has a low standard deviation (AE 4 mV or 0.07 pH units). The response time is short (5 s). To characterize its function the soil pH-probe was used to analyse pond and arable soil samples. The results were compared with those obtained with a conventional combined glass electrode. To evaluate the results, measurements were performed (i) in natural wet soil samples (in situ conditions), (ii) after drying and moistening the soil samples (moistened samples) and (iii) after drying the soil samples and mixing with bidistilled water (soil solutions; generally accepted method in laboratories). The minimum water content required to obtain stable potentials in soil samples was 10%. The influence of S 2À , NO À 3 and Fe 3þ as naturally available reducing and oxidising agents on the potential response of the pH-probe was investigated. All the obtained results demonstrate that the developed pH-probe is a powerful tool to measure the pH of a soil sample under in situ conditions without a calibration step.

Instrument-Free Control of the Standard Potential of Potentiometric Solid-Contact Ion-Selective Electrodes by Short-Circuiting with a Conventional Reference Electrode

“…17 Ion selective membranes 9,12 and solid electrolyte sensors 18 in the coulometric measurement mode as well as stopped-flow anodic stripping coulometry on thin gold electrodes 19 have been suggested as means of achieving calibration-free operation. A calibration-free method to measure soil pH directly on site 20 and the interesting approach of using an internal standard iontopohoresis device for calibration-free glucose measurements have been reported. 21 Low-cost and disposability have been addressed for calibration-free optical pH sensors, 8 planar potentiometric pH sensor chips based on nanoporous platinum, 17 lead selective sensors for environmental monitoring 14 and for all plastic Cu 2+ -, 22 Ca 2+ -and Pb 2+ -selective sensors, 23 all of which (except the optical one) contain also a reference electrode in the solid state.…”

“…Ion selective membranes , and solid electrolyte sensors in the coulometric measurement mode as well as stopped-flow anodic stripping coulometry on thin gold electrodes have been suggested as means of achieving calibration-free operation. A calibration-free method to measure soil pH directly on site and the interesting approach of using an internal standard iontopohoresis device for calibration-free glucose measurements have been reported …”

The effect of soil moisture content and soil texture on fast in situ pH measurements with two types of robust ion-selective electrodes

“…Previous studies have investigated the effect of varying SMC on the pH measurement on different soils (Schaller and Fischer, 1981;Adamchuk et al, 1999;Kahlert et al, 2004;Oliviera et al, 2018;de Souza Silva and Molin, 2018;Patil et al, 2019). However, most of the studies have been carried out with glass electrodes and detailed evaluation of antimony electrode performance at low moisture regimes in different soil textures is lacking.…”

Differential Sensor for PH Monitoring of Environmental Objects

“…Measuring of pH value is realized in different laboratories, both stationary and mobile, including a those working in the field conditions [6][7][8][9]. Potentiometric analysis methods are effective for the control of the environment.…”