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

• Published in: Electroanalysis (New York, N.Y.) Vol. 16; no. 24; pp. 2058 - 2064
• Main Authors: Kahlert, Heike, Steinhardt, Tim, Behnert, Jürgen, Scholz, Fritz
• Format: Journal Article
• Language: English
• Published: Weinheim WILEY-VCH Verlag 01.12.2004; WILEY‐VCH Verlag
• Subjects: In situ measurements; pH-probe; pH measurement; Soil pH
• ISSN: 1040-0397; 1521-4109
• DOI: 10.1002/elan.200403059

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 (±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 S2−, NO$\rm{ {_{3}^{-}}}$ and Fe3+ 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.