pH-electrode for soils

In short: quarterly to half-yearly.

Explanation: In routine laboratory operation with pH combination electrodes, it is common practice to check these pH electrodes again daily using buffer solutions and to recalibrate them if necessary. Such a daily recalibration of pH electrodes installed in the soil is neither feasible nor reasonable. This applies to both field and laboratory experiments in vessels or soil columns.
A calibration is always connected with the de-installation of the pH electrode and the subsequent re-installation. The re-installation is not without disturbances at the contact surface electrode/substrate, so that a frequently repeated calibration must worsen the measuring result over time. This also applies if you measure over a longer period of time (e.g. months) and then have to reckon with an actual shift in the calibration values over this period of time caused by electrode drift.

However, a reasonable interval for recalibration of our pH electrodes can only be estimated from previous tests and experiences with our electrodes. Laboratory tests by Thiele-Bruhn et al. (2015) showed that the measured values of the pH electrodes remained stable down to a few mV or a few hundredths of a pH unit even after several months of continuous use. Experience over a longer period than Thiele-Bruhn et al. (2015) is available so far from three practical applications, each covering a period of between 14 and 37 months (as of June 2017).

The ecoTech pH electrodes permanently installed over these periods and tested for their calibration values at intervals of several months showed deviations of max. 0.2 to 0.3 pH units in buffer solutions for pH 4, 7 and 10. In all cases, a downward trend of the displayed pH values was observed. In order to find a compromise between inappropriate disturbance of the experimental set-up through deinstallation and reinstallation on the one hand and the best possible calibration values on the other, we currently (as of June 2017) recommend recalibration at quarterly to half-yearly intervals. According to our experience so far, deviations in the range of approx. 0.1 pH units are likely to occur. Significantly larger deviations indicate either pending maintenance of the reference system or a technical fault, which may be localised in the reference electrode, the pH electrode or in the measuring system.

Sources:
Thiele‐Bruhn, Sören, Stefan Wessel‐Bothe, and Marc‐Oliver Aust. ""Time‐resolved in‐situ pH measurement in differently treated, saturated and unsaturated soils."" Journal of Plant Nutrition and Soil Science 178.3 (2015): 425-432.

As far as we know, there is no time limit. We have been producing our redox electrodes in close cooperation with Prof. Dr. Tim Mansfeldt, University of Cologne, since 2004. During this time, we have not become aware of any case in which the continuous use of these electrodes has resulted in a problem. In a project by Professor Mansfeldt himself, eighteen of these electrodes have been in continuous use since March 2010, some of them almost all year round in groundwater areas with intermittent overtopping of > 150 cm water column. All sensors are still working perfectly today (as of 09-2020).

After intensive consultation with Prof. Mansfeldt, University of Cologne, we jointly decided against further developing our redox electrodes in this direction. Although the construction of a multiple probes would not be a problem from a purely technical point of view, in our experience the quality of the measured values would suffer. The main reason for this is the risk of insufficient contact between the platinum surface and the surrounding soil material.

Explanation: The geometry of our single electrode is carefully designed so that the installation involves minimal soil disturbance and the platinum pin, which is only 1 mm thick, is pierced into the soil. This ensures intensive contact between Pt and the undisturbed soil material. In the case of a tubular probe with several measuring levels, this can hardly be guaranteed under normal installation conditions. The following circumstances may result in this:

1. a borehole must be cut out for a tubular probe. In most cases, the walls of such boreholes are not exactly straight; this can cause the platinum surfaces to have insufficient contact with the ground in individual areas, and oxygen to reach deeper areas along gaps and then oxidise areas such as reducing areas. This is usually not visible when the probe is inserted but can have serious effects on the measured values. 2.

2 In order to avoid the contact problem mentioned under 1, it is in our opinion also not advisable to choose a significantly smaller borehole in order to force contact between the probe surface and the borehole wall. An unavoidable consequence of the force applied when pushing a probe into a borehole that is too small would be the forced dragging of soil material along the borehole wall and thus potentially a smearing of zones with different redox potentials.

3. improving the contact of the probe and borehole wall by slurrying is also not advisable because the redox reading is generated directly at the electrode/soil contact surface. However, the redox potential of artificially introduced disturbed material may be very different from that of the autochthonous substrate.

4. even with a perfectly placed tube probe, changing contact pressure with the soil substrate must be expected over time. This can be caused by temperature differences and the differing expansion coefficients of the probe and the surrounding soil, or by different moisture conditions with resulting swelling and shrinking processes.

To have any possibility at all to collect data continuously at one and the same measuring point, even over years!

Explanation: The main cause of ageing of measuring systems for ORP and pH is the ageing of the reference electrode. By separating the measuring electrode from the reference electrode, it is possible to maintain and, if necessary, replace the latter, while the measuring electrode does not have to be manipulated. This is of particular importance because the measured values of pH and especially of redox often stabilise only after a longer period of equilibration. However, maintenance that would require the removal and re-installation of the Pt and pH electrodes would result in recurring shifts in the measured values, and the continuity of years-long time series would be considerably disturbed by this.

The service life of a reference electrode can be considerably extended by regular maintenance, so that measurements can be taken continuously with the same equipment for years. A particular advantage is that the actual measuring electrodes for redox and pH do not have to be dismantled for cleaning and refreshing the reference electrodes, because they remain permanently at the same measuring point.

Our modules are designed for the simultaneous measurement of redox and pH values. The electrodes can then be clamped to the measuring module as desired. Each module can record up to seven measured values and is thus also suitable for small vessel experiments. For larger numbers of electrodes, several measuring modules can be connected in series. The use of a reference electrode is particularly advantageous when redox and pH electrodes are used simultaneously in the same measuring system. Thus, a single reference electrode can be combined with any number of redox and pH electrodes and kept fresh for several years.