Redox Potential and pH Measurements

One of the objectives of this experiment was to monitor in-situ redox and pH conditions in the EC. Knowledge of these two parameters is important in any interpretation of degradation reactions involving chloro-organic compounds in which electrons and protons are involved, as in reductive dechlorination, for example.

Commercial gel-filled pH probes were inserted into the soil column at 3 cm intervals, located between the anode and cathode (Figure 1). These were inserted through butyl rubber septa placed into holes drilled into the sides of the column. The pH probes were calibrated using standard buffer solutions before insertion into the soil column. Readings were taken daily for the duration of the experiment, by placing a reference electrode into a well at the top of the column, and connecting the reference and in-situ pH probes to an Orion Model EA 940 Expandable Ion Analyzer. All pH measurements were temperature compensated to 25 °C.

Amorphous black platinum Eh probes were inserted into the soil column at 1.5 cm intervals, located between the anode and the cathode (Figure 1). These probes were manufactured by us following the procedures outlined by Jackson (1958). The probes contain a 1 cm length of 1 mm diameter bright platinum wire set with epoxy inside a 6 cm long end-piece of a glass pipette tip. The inside end of the platinum wire is soldered to a copper wire lead. The tips of the bright platinum wire are polished to a flat mirror surface, washed in concentrated HC1 and rinsed in distilled water. Two such electrodes are then immersed into a platinizing solution consisting of platannic acid and lead acetate. The electrodes are then connected to two 1.5 V DC batteries in series, which results in the deposition of a dense black coating of amorphous platinum on the exposed ends of the bright platinum wire tips. The polarity ofthe DC current is reversed every 30 seconds until a satisfactory coating is achieved, usually after 3 to 5 minutes. In both soil and water systems these probes were well poised. Reproducible Eh measurements were routinely obtained using ASTM Standard Method C 1498 to measure the response of these black platinum electrodes.

It is not possible to make in-situ Eh-pH measurements while EO is in progress. The very small currents measured by the probes are severely interfered with by the voltage gradient in the EC. For this reason, the current was turned off during the daily Eh-pH measurements.

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