Experiments have demonstrated the feasibility of detecting multiple species of toxic metal ions and other ions of interest dissolved in water, by means of voltammetry with electrically conductive artificial diamond electrodes. Diamond is attractive as an electrode material because it is highly chemically stable and exhibits the greatest useable range of potential (from +2 to –2 V in aqueous solution) than any known material.
The electrode material used in the experiments was a film of artificial diamond doped with boron to enhance electrical conductivity. The electrode film was grown on a molybdenum substrate by microwave plasma chemical vapor deposition from a gaseous mixture of methane and hydrogen in the presence of a disk of B2O3 as a dopant source. The diamond electrode was installed in an electrochemical cell along with an Ag/AgCl/NaCl reference electrode and a platinum-foil counter electrode. For each experiment, the cell was filled with a 0.5 M HNO3 containing the dissolved ions of interest and the experiment was performed at room temperature.
Initially, the diamond electrode was conditioned at +1.5 V vs the reference electrode for 30 seconds and the system was allowed to attain equilibrium. Then the voltammetric measurements were performed. The measurement techniques implemented by use of the electrodes included (1) differential pulse voltammetry (DPV) at a scan rate of 36.36 of mV/s, pulse height of 25 mV, pulse duration of 25 ms, scan increment of 2 mV, and step time of 55 ms and (2) square-wave voltammetry (SWV) at a pulse height of 25 mV, frequency of 60 Hz, and scan increment of 2 mV.
Each voltammetric procedure typically consists of the following events: The diamond electrode is first held at an anodic potential at which either oxygen or some other gas evolves (which gas depends on the specific solution). The potential applied to the electrode is scanned in the cathodic direction to reduce the metal ions present in the solution, and the resulting currents are measured. As the potential approaches the value at which the ions of each species are reduced, current begins to flow, eventually reaching a maximum. If multiple species are present in the solution, then several peaks are observed, either separately or superimposed on a rising baseline current as the potential is shifted cathodically. The current peaks thus obtained correspond to the complete reduction of each metal ion species from the solution. By determining the total charge or current corresponding to each peak and by comparing this charge or current with that obtained in a calibration solution, one can calculate the concentration of each species of ion in the solution.
The data from the experiments showed that cations of three species (Hg22+, Ag+, and Cu2+) can be detected in a single DPV or SWV scan over the range of potential from +0.799 to 0.1 V vs. a standard hydrogen electrode. This range is suitable for detecting the most toxic mercury ions in aqueous solutions.
It should eventually be possible to use diamond electrodes in place of conventional electrodes that contain mercury. It should also be possible to fabricate miniature diamond electrodes for electroanalytical detection of metal ions in ground water, plating solutions, and other aqueous solutions in hostile environments. The advantages of miniature diamond electrodes may include no need for stirring of electrolytes, low background currents, low double-layer capacitances, and no need to add supporting electrolytes or to cover electrochemical cells with neutral gases.
The conductive artificial diamond electrodes may be useful in analyzing Martian soil and sulfuric acid on Venus. Diamond is stable in sulfuric acid and can therefore be used to survey Venusian atmosphere and also to detect water.
This work was done by Rajeshuni Ramesham of Caltech for NASA's Jet Propulsion Laboratory. For further information, access the Technical Support Package (TSP) free on-line at www.nasatech.com/tsp under the Physical Sciences category. NPO-20726