Integrated coulometric sensor-actuator devices

Electrochemical Control of pH in Nanoliter Volumes

The electrochemical management of the proton concentration in miniaturized dimensions opens the way to control and parallelize multistep chemical reactions, but still it faces many challenges linked to the efficient proton generation and control of their diffusion. Here we present a device operated electrochemically that demonstrates the control of the pH in a cell of ∼140 nL. The device comprises a microfluidic reactor integrated with a pneumatic mechanism that allows the exchange of reagents and the isolation of protons to decrease the effect of their diffusion. We monitored the pH with a fluorescence marker and calculated the final value from the redox currents. We demonstrate a large pH amplitude control from neutral pH values beyond the fluorescence marker range at pH 5. On the basis of the calculations from the Faradaic currents, the minimum pH reached should undergo pH ∼ 0.9. The pH contrast between neutral and acid pH cells can be maintained during periods longer than 15 min with an appropriate design of a diffusion barrier.

Feasibility of Using an Electrolysis Cell for Quantification of the Electrolytic Products of Water from Gravimetric Measurement

A gravimetric method for the quantitative assessment of the products of electrolysis of water is presented. In this approach, the electrolysis cell was directly powered by 9 V batteries. Prior to electrolysis, a known amount of potassium hydrogen phthalate (KHP) was added to the cathode compartment, and an excess amount of KHCO₃ was added to the anode compartment electrolyte. During electrolysis, cathode and anode compartments produced OH^-(aq) and H⁺(aq) ions, respectively. Electrolytically produced OH^-(aq) neutralized the KHP, and the completion of this neutralization was detected by a visual indicator color change. Electrolytically produced H⁺(aq) reacted with HCO₃^-(aq) liberating CO₂(g) from the anode compartment. Concurrent liberation of H₂(g) and O₂(g) at the cathode and anode, respectively, resulted in a decrease in the mass of the cell. Mass of the electrolysis cell was monitored. Liberation of CO₂(g) resulted in a pronounced effect of a decrease in mass. Experimentally determined decrease in mass (53.7 g/Faraday) agreed with that predicted from Faraday's laws of electrolysis (53.0 g/Faraday). The efficacy of the cell was tested to quantify the acid content in household vinegar samples. Accurate results were obtained for vinegar analysis with a precision better than 5% in most cases. The cell offers the advantages of coulometric method and additionally simplifies the circuitry by eliminating the use of a constant current power source or a coulometer.

Microelectrode generator-collector systems for electrolytic titration: theoretical and practical considerations

Electochemical generator-collector systems, where one electrode is used to generate a reagent, have a potentially large field of application in sensing and measurement. We present a new theoretical description for coplanar microelectrode disc-disc systems where the collector is passive (such as a potentiometric sensor) and the generator is operating at constant flux. This solution is then used to develop a leading order solution for such a system where the reagent reacts reversibly in solution, such as in acid-base titration, where a hydrogen ion flux is generated by electrolysis of water. The principal novel result of the theory is that such devices are constrained by a maximum reagent flux. The hydrogen ion concentration at the collector will only reflect the buffer capacity of the bulk solution if this constraint is met. Both mathematical solutions are evaluated with several microfabricated devices and reasonable agreement with theory is demonstrated.

Solid State Sensor for Simultaneous Measurement of Total Alkalinity and pH of Seawater

A novel design is demonstrated for a solid state, reagent-less sensor capable of rapid and simultaneous measurement of pH and Total Alkalinity (A_T) using ion sensitive field effect transistor (ISFET) technology to provide a simplified means of characterization of the aqueous carbon dioxide system through measurement of two "master variables": pH and A_T. ISFET-based pH sensors that achieve 0.001 precision are widely used in various oceanographic applications. A modified ISFET is demonstrated to perform a nanoliter-scale acid-base titration of A_T in under 40 s. This method of measuring A_T, a Coulometric Diffusion Titration, involves electrolytic generation of titrant, H⁺, through the electrolysis of water on the surface of the chip via a microfabricated electrode eliminating the requirement of external reagents. Characterization has been performed in seawater as well as titrating individual components (i.e., OH^-, HCO₃^-, CO₃^2-, B(OH)₄^-, PO₄^3-) of seawater A_T. The seawater measurements are consistent with the design in reaching the benchmark goal of 0.5% precision in A_T over the range of seawater A_T of ∼2200-2500 μmol kg^-1 which demonstrates great potential for autonomous sensing.

No more conventional reference electrode: transition time for determining chloride ion concentration

Ion selective electrodes (ISE) are used extensively for the potentiometric determination of ion concentrations in electrolytes. However, the inherent drift in these measurements and the requirement of a stable reference electrode restrict the feasibility of this method for long-term in-situ applications. This work presents a chronopotentiometric approach to minimize drift and avoid the use of a conventional reference electrode for measuring chloride ion concentration. An anodic current pulse is applied to a Ag/AgCl working electrode which initiates a faradaic reaction that depletes the chloride ions near the electrode surface. The rate of change in potential at the Ag/AgCl electrode, due to chloride ion depletion, reaches an inflection point once the chloride ions deplete completely near the electrode surface. The moment of the inflection point, also known as the transition time, is a function of the chloride ion concentration and is described by the Sand equation. It is shown that the square root of the transition time is linearly proportional to the chloride ion concentration. Drift in the response over two weeks is negligible: 59 μM/day when measuring 1 mM of Cl(-) ions using a 10 A m(-2) current pulse. The transition time at a specific ion concentration can be tuned by the applied current pulse, e.g., in a solution containing 5 mM chloride ions, the transition times with current pulses of 10 and 20 A m(-2) are 1.56 and 0.25s, respectively. The moment of inflection determines the response, and thus is independent of the absolute potential of reference electrode. Therefore, any metal wire can act as a pseudo-reference electrode, enabling this approach for long-term and integrated-sensor applications such as measurement inside concrete structures.