Full Paper A New Calibration Free pH-Probe for In Situ Measurements of Soil pH Heike Kahlert,* a Tim Steinhardt, a Jürgen Behnert, b Fritz Scholz a a Universität Greifswald, Institut für Chemie und Biochemie, D-17489 Greifswald, Soldmannstr. 23, Germany *e-mail: hkahlert@uni-greifswald.de b DPST Behnert GmbH, Friedhofstr. 26, D-27576 Bremerhaven, Germany Received: January 21, 2004 Final version: March 15, 2004 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 ( 4 mVor 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 2 , NO  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. Keywords: pH-probe, pH measurement, Soil pH, In situ measurements 1. Introduction All chemical, biological and physical processes of soil generation and especially the availability and storing capacity of plant nutrients are controlled by the soil pH. In numerous studies of plant growing [1 – 2], distribution and growing of soil bacteria [3, 4] and of forest ecosystems [5 – 6] the soil pH plays an important role. The soil pH is one of the most common and important parameter for evaluating the quality of a soil. Most of the nutrients and trace elements are best soluble and bio-available at pH values between 5.5 and 6.5. Until now there is no single and general accepted method for measuring the pH of soils or sediments [8, 9]. Several studies have shown that the measured pH values depend strongly on the analytical conditions such as soil to solution ratio, the way and time of suspending the soil (stirring or shaking), the way of pH measurement (suspen- sion effect) and the nature and concentration of the salt solution [10, 11]. The most commonly accepted procedure is to collect a certain amount of soil sample, dry and sieve it in laboratory, suspend the dried material in water or salt solutions (KCl, CaCl 2 ) and measure the pH with a standard glass electrode [10]. This procedure is time consuming and some expertise in drying soil material and calibrating the pH meters is mandatory. Beside this, there exist also methods for in situ measurements of soil pH [7], i.e., with special test kits or pH meters with glass electrodes, ISFETs or metal/metal oxide electrodes. All these electrodes have the disadvantage that they need individual calibration. Further, glass electrodes are fragile and therefore their applicability is limited for in situ measurements in soils. ISFETs are not stable over longer periods. These are some of the reasons why, until now, in situ measurement do not play an important role. It has been shown in earlier studies [12], that composite electrodes based on quinhydrone are useful for calibration free measurements in solutions with pH values up to 8.5. These electrodes have the same properties as a conventional quinhydrone electrode but additionally they can be used in emulsions and they can be produced as very robust electro- des. The theory of the potential dependence of the quinhydrone electrode is described elsewhere [13]. Here it may suffice to mention that the potential of the quinhydrone composite electrodes follows the Nernst equation and calibration is not necessary for fundamental reasons pro- vided that a stable reference electrode is used. The robust- ness, the calibration free function and the fact that in general the soil pH is between 3.0 and 9.0 made composite electro- des based on quinhydrone interesting for the construction of a pH-probe which can be used for in situ measurements. The aim was to develop a sensor which can be directly drilled into the soil or the sediment to allow in situ pH determinations of soils without any preliminary sample preparation and electrode calibration. 2058 Electroanalysis 2004, 16, No. 24 # 2004 WILEY-VCH Verlag GmbH&Co. KGaA, Weinheim DOI: 10.1002/elan.200403059