Electrochemical sensor

Abstract

A voltammetric pH sensor, especially for characterising wellbore fluids, comprises a plurality of electrodes with a redox active organic compound attached to an electrode and having at least one functional group convertible electrochemically between reduced and oxidized forms with transfer of at least one proton between the compound and surrounding aqueous phase, wherein the compound has at least one substituent group which promotes hydrogen bonding at a said functional group and thereby increases the reaction rate of proton transfer. The substituent group may form an internal hydrogen bond with a redox-convertible group or may enhance polarity to promote electrostatic interaction with water molecules and reduce activation energy. Typical examples include alizarin or 1,2-dihydroxy-anthraquinone (RH=72-48-0), quinizarin or 1,4-dihydroxy-anthraquinone (RN=81-64-1), 2-acetoxy-benzoquinone (RN=1125-55-9), chloranil or 2,3,4,5-tetrachloro-benzoquinone (RN=118-75-2) and 1,4-diamino-2,3-dichloro-anthraquinone (RN=81-42-5) deposited on a glassy carbon electrode. In this way, anomalous measurements at low ionic strength and low concentrations of pH buffering species can be overcome.

Description

FIELD OF THE INVENTION[0001]Embodiments of this invention relate to electrochemical sensors for detecting and monitoring analytes, in particular for determining pH. Fields in which the invention may be utilised include, although are not restricted to, the analysis of water at the Earth's surface and also the analysis of a subterranean fluids which may be in an aquifer or downhole in a hydrocarbon reservoir.BACKGROUND OF THE INVENTION[0002]There are numerous circumstances in which it is desirable to detect, measure or monitor a constituent of a fluid. One of the commonest requirements is to determine hydrogen ion concentration (generally expressed on the logarithmic pH scale) in aqueous fluids which may for example be a water supply, a composition in the course of production or an effluent. The determination of the pH of a solution is one of the most common analytical measurements and can be regarded as the most critical parameter in water chemistry. Merely by way of example, pH meas...

AI Technical Summary

Benefits of technology

[0011]In a first aspect this invention provides an electrochemical sensor comprising a redox active compound, which may be an organic compound, convertible electrochemically between reduced and oxidized forms with transfer of at least one proton (i.e a hydrogen atom in ionised form) between the compound and surrounding aqueous phase, wherein the compound has at least one substituent group which increases the reaction rate of proton transfer. Such a substituent group may reduce the activation energy for transfer of a proton and / or may be a substituent group which promotes hydrogen bonding at the functional group.

[0013]One possibility for a substituent group to promote hydrogen bonding is a group containing oxygen or nitrogen and positioned to participate in hydrogen bonding with a water molecule also able to hydrogen bond to the redox active functional group. More specifically, this substituent may be a hydroxyl group. Another possibility is a group containing a hetero atom (which is neither carbon nor hydrogen) which increases the polarity of the molecule and electrostatic interaction with water molecules in the electrolyte. Such a group may be an electron withdrawing group positioned to be able to withdraw electrons from the redox active functional group. Without limitation as to mode of action, we believe that these possibilities promote hydrogen bonding between the redox active functional group and an adjacent water molecule and this is believed to reduce the activation energy for proton transfer and so increase the reaction rate of proton transfer.

[0015]Aromatic compounds which have two groups convertible between a reduced hydroxyl form and an oxidised keto form by a two electron, two proton reaction have been previously been found to be particularly suitable as pH sensitive redox active species: anthraquinone is a common example. The present invention may use a compound of this character, with structure and substituent groups which promotes hydrogen bonding at the hydroxyl / keto functional groups and thereby increases the reaction rate of proton transfer. Thus, in another aspect this invention provides an electrochemical pH sensor comprising a redox active organic compound which is immobilized to an electrode and which comprises at least two fused aromatic rings with oxygen or nitrogen-containing substituents, convertible between reduced and oxidised forms, on adjacent fused aromatic rings in positions allowing formation of an internal hydrogen bond between the reduced form of one said substituent and the oxidised form of another said substituent.

Problems solved by technology

In case of samples taken downhole, this change may also happen due to degassing of a sample (seal failure), mineral precipitation in a sampling bottle, and chemical reaction with the sampling chamber.

An issue with electrochemical sensors (particularly those involving detection mechanisms involving proton transfer) is the ability to make electrochemical measurements without a buffer and / or similar species that can facilitate proton transfer reactions.

Measurements can be particularly difficult, and error prone, in low ionic strength media, without pH buffering species and / or other species facilitating proton transfers.

Measuring the pH of rainwater, and natural waters with very low mineralization, is noted as being particularly difficult.

It has been discovered that electrochemical sensors utilising an immobilized redox compound can give good results when used in a buffered aqueous solution, and yet fail to do so when used in an unbuffered solution.

Batchelor-McAuley et al “Voltammetric Responses of Surface-Bound and Solution-Phase Anthraquinone Moieties in the Presence of Unbuffered Aqueous Media” J. Phys. Chem. C vol 115 pages 714-718 (2011) attribute this phenomenon to depletion of H+ ion concentration in the vicinity of the electrode resulting in a significant local change in pH adjacent to the electrode and thus an erroneous determination of pH within the bulk solution.

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