An Inexpensive Solution to Automated Redox Measurements.

Measurement of redox potential has many benefits in field studies as small changes in dissolved oxygen content can be measured as larger changes in electrical potential.  Difficulties arise with field measurements due to the length of time required for measurements to stabilize, which often limits the number of redox probes installed. Automated redox measurements could solve this problem, but the often negative voltage outputs from the platinum electrode (relative to the calomel electrode) limit the types of data loggers available. This study documents the design and testing of a relatively low cost (< $35, batteries included) voltage-adjusting system to allow the automation of measurements taken with platinum-tipped electrodes and recorded on a Hobo® datalogger. Standard electrical components (LDO voltage regulators, operational amplifiers, 110 kOhm resistors, and 100 pF capacitors), perf-boards, and enclosures were purchased from local suppliers (Newark Electronics and Radio Shack). The LDO voltage regulator allows a constant voltage output (3.3 V) while maintaining a very low quiescent current (1.6 µA). Components were chosen for an operating temperature range as low as -40^o C). The voltage adjuster will convert ‑600 and +600 mV inputs from the platinum electrode (relative to the calomel electrode) to outputs of 0.25 and 2.45 V, respectively for the data logger. This range makes the new automated probe system functional over a very wide range of possible field Eh values.