Potentiometric pH probes remain the gold standard for the detection of pH but are not sufficiently sensitive to reliably detect ocean acidification at adequate frequency. Here, potentiometric probes are made dramatically more sensitive by placing a capacitive electronic component in series to the pH probe while imposing a constant potential over the measurement circuit. Each sample change now triggers a capacitive current transient that is easily identified between the two equilibrium states, and is integrated to reveal the accumulated charge. This affords dramatically higher precision than with traditional potentiometric probes. pH changes down to 0.001 pH units are easily distinguished in buffer and seawater samples, at a precision (standard deviation) of 28 μpH and 67 μpH, respectively, orders of magnitude better than what is possible with potentiometric pH probes.
We introduce here a general strategy to read out chronopotentiometric sensors by electrogenerated chemiluminescence (ECL). The potentials generated in chronopotentiometry in a sample compartment are used to control the ECL in a separate detection compartment. A three-electrode cell is used to monitor the concentration changes of the analyte, while luminol-H 2 O 2 system is responsible for ECL. The principle was shown to be feasible by theoretical simulations, indicating that the sampled times at a chosen potential, rather than traditional transition times, similarly give linear behavior between concentration and square root of sampled time. With the help of a voltage adapter, the experimental combination between chronopotentiometry and ECL was successfully implemented. As an initial proof of concept, the ferro/ferricyanide redox couple was investigated. The square root of time giving maximum light output changed linearly with ferrocyanide concentration in the range from 0.70 mM to 4.81 mM. The method was successfully applied to the visual detection of carbonate alkalinity from 0.06 mM to 0.62 mM using chronopotentiometry at an ionophore-based hydrogen ion-selective membrane electrode. The measurements of carbonate in real samples including river water and commercial mineral water were successfully demonstrated.
Since aquatic environments are highly heterogeneous and dynamic, there is the need in aquatic ecosystem monitoring to replace traditional approaches based on periodical sampling followed by laboratory analysis with new automated techniques that allow one to obtain monitoring data with high spatial and temporal resolution. We report here on a potentiometric sensing array based on polymeric membrane materials for the continuous monitoring of nutrients and chemical species relevant for the carbon cycle in freshwater ecosystems. The proposed setup operates autonomously, with measurement, calibration, fluidic control and acquisition triggers all integrated into a self-contained instrument. Experimental validation was performed on an automated monitoring platform on lake Greifensee (Switzerland) using potentiometric sensors selective for hydrogen ions, carbonate, calcium, nitrate and ammonium. Results from the field tests were compared with those obtained by traditional laboratory analysis. A linear correlation between calcium and nitrate activities measured with ISEs and relevant concentrations measured in the laboratory was found, with the slopes corresponding to apparent single ion activity coefficients γ(*)(Ca(2+)) = 0.55(SD = 0.1mM) and γ(*)(NO(3)(-)) = 0.75(SD = 4.7μm). Good correlation between pH values measured with ISE and CTD probes (SD = 0.2 pH) suggests adequate reliability of the methodology.
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