A spectroelectrochemical instrument has been developed for measuring the total organic carbon (TOC) content of an aqueous solution. Measurements of TOC are frequently performed in environmental, clinical, and industrial settings. Until now, techniques for performing such measurements have included, variously, the use of hazardous reagents, ultraviolet light, or ovens, to promote reactions in which the carbon contents are oxidized.
The instrument now being developed is intended to be a safer, more economical means of oxidizing organic carbon and determining the TOC levels of aqueous solutions and for providing a low power/mass unit for use in planetary missions.
The proposed instrument exploits an electrochemical-oxidation principle that has also been investigated as the basis of a method of disposing of toxic organic industrial wastes. The method has found limited use, largely because the most common electrode materials (non-diamond carbon-based materials and such metals as platinum, silver, gold, mercury, and nickel) eventually become fouled or oxidized when operated at the high anodic potentials (between 2 and 2.5 V versus a standard hydrogen electrode) needed for efficient oxidation of organic compounds. An effort to overcome this limitation has led to consideration of electrodes consisting of substrate materials (e.g. silicon or titanium) coated with diamond that has been heavily doped with boron to promote p-type semiconductivity to a nearly metallic level. Such boron-doped diamond (BDD) electrodes have been found to be robust, capable of withstanding high anodic potentials, and resistant to self-oxidation.
This new TOC instrument (see figure) includes a BDD electrode, a counter electrode, and a reference electrode in a cell containing an aqueous solution to be tested. A positive potential of about 2.5 V versus the reference electrode is applied to the BDD electrode to cause the organic material in the solution to become oxidized, thereby producing H2O and CO2. A headspace above the solution traps escaping CO2. The concentration of the CO2 can be measured by a miniature infrared absorption spectrometer comprised of a tunable diode laser and an associated wavelength-selective detector subsystem. The TOC of the solution is proportional to the concentration of CO2 in the headspace.
This work was done by Sam Kounaves at Tufts University for Goddard Space Flight Center. GSC-14814-1
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