A sensor designed to determine the amount and concentration of analyte in a sample having a volume of less than about 1 μL. The sensor has a working electrode coated with a non-leachable redox mediator. The redox mediator acts as an electron transfer agent between the analyte and the electrode. In addition, a second electron transfer agent, such as an enzyme, can be added to facilitate the electrooxidation or electroreduction of the analyte. The redox mediator is typically a redox compound bound to a polymer. The preferred redox mediators are air-oxidizable. The amount of analyte can be determined by coulometry. One particular coulometric technique includes the measurement of the current between the working electrode and a counter or reference electrode at two or more times. The charge passed by this current to or from the analyte is correlated with the amount of analyte in the sample. Other electrochemical detection methods, such as amperometric, voltammetric, and potentiometric techniques, can also be used. The invention can be used to determine the concentration of a biomolecule, such as glucose or lactate, in a biological fluid, such as blood or serum. An enzyme capable of catalyzing the electrooxidation or electroreduction of the biomolecule is provided as a second electron transfer agent.

Small volume in vitro analyte sensor

The dissolution kinetics of major sedimentary carbonate minerals

A surface complexation model of the carbonate mineral-aqueous solution interface

A sensor utilizing a non-leachable or diffusible redox mediator is described. The sensor includes a sample chamber to hold a sample in electrolytic contact with a working electrode, and in at least some instances, the sensor also contains a non-leachable or a diffusible second electron transfer agent. The sensor and/or the methods used produce a sensor signal in response to the analyte that can be distinguished from a background signal caused by the mediator. The invention can be used to determine the concentration of a biomolecule, such as glucose or lactate, in a biological fluid, such as blood or serum, using techniques such as coulometry, amperometry, and potentiometry. An enzyme capable of catalyzing the electrooxidation or electroreduction of the biomolecule is typically provided as a second electron transfer agent.

Method of manufacturing small volume in vitro analyte sensors

Experimental determination of porosity and permeability changes induced by injection of CO2 into carbonate rocks

Channel and tubular electrodes

The requirements of any experimental investigation of a reaction, such as dissolution or precipitation, at the solid-liquid interface are delineated, and it is shown that no previously adopted experimental method successfully meets all of these criteria. A new strategy for studying such reactions is therefore proposed, and described specifically for the dissolution kinetics of calcium carbonate in acidic aqueous solution. This technique involves locating a calcite crystal in part of one wall of a rectangular duct, through which reactant flows under laminar conditions, and positioning either an amperometric or potentiometric detector electrode immediately adjacent to and downstream of the crystal. The rate of the reaction at the crystal surface is then followed by measuring the concentration of the reactant or products reaching the electrode as a function of solution flow rate. In this way both transport (to the crystal and detection system) and the surface topography of the crystal are defined and controllable, and, most importantly, the concentrations interrogated by the detector electrode are those pertaining at the crystal surface and not the bulk values, which are those probed with other (previous) methods. A mechanistic model for the dissolution reaction at pH H + + CaCO 3 → C a 2 + + HCO 3 − , is developed, and it is recognized that half-, first- and second-order reactions in surface H + constitute possible candidate heterogeneous processes. The model also takes the following homogeneous (solution) reactions into account: HCO 3 − + H + ⇌ H 2 C O 3 ⟶ k H 2 C O 3 C O 2 + H 2 O , K H 2 C O 3 = a H + a HCO 3 − / a H 2 C O 3 The backward implicit finite difference method is used to provide theory for the proposed flow cell experiment. Specifically this relates the rate of an n th order process at the crystal surface to the transport-limited current at an amperometric detector electrode, as a function of solution flow rate and cell-crystal-electrode geometry. The computational method selected allows both the full parabolic velocity profile and any value of n to be considered. The validity of the numerical approach is checked by invoking the Leveque approximation (linearizing the parabolic velocity profile) and solving the problem analytically for n = 0 and 1. The numerical method is extended so that both parallel heterogeneous and homogeneous reactions are covered, specifically so that theory describing the mechanistic model can be derived as related to the flow cell experiment. The design and construction of the necessary experimental apparatus is described, along with experiments devised to prove the validity of the experimental approach. The results of flow cell dissolution experiments carried out with 0.25–1.0 mM HCl solutions are presented, and it is shown for the first time that the reaction between H + and calcite is not transport-controlled. Assuming k H 2 C O 3 = 20 s − 1 and K H 2 C O 3 = 3.06 × 1 0 − 4 M , a best fit to the data is obtained for a first-order heterogeneous process with a rate constant of (0.043 ± 0.015) cm s -1 .

The dissolution of calcite in aqueous solution at pH < 4: kinetics and mechanism

SUMMARY. 1. A new experimental method for the study of kinetics and mechanism of reactions at the solid-liquid interface has shown that the dissolution of calcite in acidic waters is, under conditions of high mass transport, controlled by the first order heterogeneous reaction of H+ at the interface and not by diffusion as previously thought.



2. The implications of this for lake liming strategies (aimed at countering the effects of ‘acid rain’) are significant in that under typical liming conditions, the rate of calcite dissolution will be surface controlled and consequently appreciably slower than previously considered.

The dissolution of calcite in acid waters: mass transport versus surface control

A technique is described that allows an assessment of the various candidate rate laws that have been proposed to predict the dissolution kinetics of calcite under high pH conditions. A combination of theoretical modelling and experimentation allows us to choose the following rate law as that which best fits the observed data: rate ( mol c m − 2 s − 1 ) = k − k ′ [ C a 2 + ] s [ CO 3 2 − ] s ′ , where k ′ = k / K sp and K sp is the solubility product of calcium carbonate. The modelling developed differs from previous studies in that it deals in terms of surface concentrations of reactants, [ Ca 2 + ] s and [ CO 3 2 − ] , as opposed to those present in bulk solution.

The Dissolution of Calcite at pH > 7: Kinetics and Mechanism

A general computational strategy is presented for the calculation of the chronoamperometric responses arising from potential-step experiments at rotating disc electrodes. The method is applicable to a wide range of electrode reaction mechanisms and theoretical results are given for single- and double-potential-step experiments for ECE, DISP1, DISP2, EC' and CE reactions. For the last, the treatment is extended to cover the case where reactants have grossly unequal diffusion coefficients. Steady-state behaviour is also deduced. The extent to which the various mechanistic pathways can be distinguished is identified and the necessary experiments defined.

Richard G. Compton

Papers

Channel and tubular electrodes

The dissolution of calcite in aqueous solution at pH < 4: kinetics and mechanism

The dissolution of calcite in acid waters: mass transport versus surface control

The Dissolution of Calcite at pH > 7: Kinetics and Mechanism

Rotating Disc Electrodes: The Theory of Chronoamperometry and Its Use in Mechanistic Investigations