Potentiometric Electrodes in Ocean Monitoring: Past, Present, and Future

It was not until the early 1970’s that glass electrodes would be routinely deployed on CTD’s and that a moving vessel could obtain oceanographically meaningful horizontal pH profiles by simply pumping seawater into a cell on deck containing an electrode array. pH electrodes in seawater streams improved with continued use. This effect was attributed to the fact that the glass membrane slowly dissolved in the flowing seawater stream, thereby preventing fouling. This effect was also observed for the pressed crystal Cu²⁺ ISE. In summary, ISE’s that are to provide continuously accurate ocean measurements need to be both physically robust and ablative to minimize fouling. Four types of ISE membranes qualify for continuous field use: membranes based on silicate glasses, membranes based on chalcogenide glasses, polycrystalline membranes typically based on pressed mixtures of metal sulfides and PVC-based membranes doped with ionophores for selectivity, and available for a wide variety of ionic and even non-ionic species. The PVC membrane ISE have been made sufficiently robust to changes in temperature and pressure that they have been used to make measurements on the surface of Mars. The last 20 years have seen the development of the “electronic tongue” or ET. The ET is a multisensory system that consists of an array of cross-sensitive chemical sensors (typically potentiometric, but without sharp selectivity) and appropriate chemometric tools for data processing to extract information about analytes in a sample. ET produces a “spectrum-like” response that can be analyzed to yield both quantitative and qualitative information not only of the specific analytes but also of the organic matrix that binds to ions in seawater. This information may lead to an enhanced understanding of microbial/chemical activity from surface to deep sea vents. Electrodes “bundled” into very small packages, and the recent progress in microelectronics and microfluidics indicate that intelligent sensor packages suitably mounted on ship’s hulls, ROV’s and other probes, will contribute much to our future understanding of the marine world.