Patent Application
20140332398
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 measurement is important in the pharmaceutical industry, the food and beverage industry, the treatment and management of water and waste, chemical and biological research, hydrocarbon production and water supply monitoring. Nearly all water samples will have their pH tested at some stage during their handling as many chemical processes are dependent on pH.
One approach to pH measurements employs a solid-state probe utilising redox chemistries at the surface of an electrode. Some redox active compounds (sometimes referred to as redox active species) display a redox potential which is dependent on hydrogen ion concentration in the electrolyte. By monitoring this redox potential electrochemically, pH can be determined. Voltammetry has been used as a desirable and convenient electrochemical method for monitoring the oxidation and reduction of a redox active species and it is known to immobilise the redox active species on or in proximity to an electrode.
Prior literature in this field has included WO2005/066618 which disclosed a sensor in which two different pH sensitive molecular redox systems and a pH insensitive ferrocene reference were attached to the same substrate. One pH sensitive redox system was anthraquinone (AQ) and the second was either phenanthrenequinone (PAQ) or alternatively was N,N′-diphenyl-p-phenylenediamine (DPPD). WO2007/034131 disclosed a sensor with two redox systems incorporated into a copolymer. WO2010/001082 disclosed a sensor in which two different pH sensitive molecular redox systems were incorporated into a single small molecule which was immobilized on an electrode. WO2010/111531 described a pH metering device using a working electrode in which a material which is sensitive to hydrogen ions (the analyte) was chemically coupled to carbon and immobilised on the working electrode.
The pH sensitive redox systems in these disclosures have been compounds which undergo a 2-electron 2-proton redox reaction. In many instances the compounds have been quinones which undergo reversible redox conversion to and from hydroquinones.
It is known that phenolic compounds with a single hydroxy group can undergo electrochemical oxidation by a 1-electron 1-proton oxidation. It has been reported that the products of such oxidation are reactive and former polymers, so that the oxidation reaction is irreversible.
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. A pH sensor is often tested and calibrated using buffer solutions which have stable values of pH. The concentration of buffer in such a solution may be 0.1 molar or more. 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. Consequently 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.
This summary is provided to introduce a selection of concepts that are further described below. This summary is not intended to be used as an aid in limiting the scope of the subject matter claimed.
We have now found that a pH sensing electrode which is able to make measurements in unbuffered or weakly buffered aqueous solution can be made using certain substituted phenolic compounds. We now disclose here a method of making an electrode for the determination of pH, which comprises
The oxygen atom may be part of a group in which there is a double bond to the oxygen atom, such as a carbonyl, nitro or sulpho group. A carbonyl group may be part of an aldehyde, keto or ester group. The relationship between this oxygen atom and the phenolic hydroxyl can be depicted as a partial structure:
in which Y and the two carbon atoms connected to it are an aromatic ring with phenolic hydroxyl attached, and the oxygen atom joined to the ring through atom Z is able to participate in a hydrogen bond to the phenolic hydroxyl, as shown by a dotted line.
These phenolic compounds may be much less water-soluble than phenol itself and so may be applied to the conductive substrate by a process which deposits them onto the substrate. This may be application as a dispersion or solution in an organic solvent which is allowed to evaporate, leaving the phenolic compound immobilised on the surface of the substrate. Oxidation and polymerisation of the immobilised phenolic compound can then be brought about with the conductive substrate immersed in an electrolyte solution, which may be an aqueous solution and may be a buffer solution.
The conductive substrate may be metallic, for example a thin layer of platinum on an insulating substrate, or it may be a conductive form of carbon. Forms of carbon which have been used for electrodes, and which may be used as the substrate here, include glassy carbon, carbon fibres, carbon black, various forms of graphite, carbon paste, carbon epoxy and carbon nanotubes. The conductive substrate may for instance be a graphite electrode or a glassy carbon electrode.
Another aspect of the present disclosure provides an electrode for the determination of pH, comprising a conductive substrate bearing a water-insoluble, redox-active deposit which is a polymeric reaction product of the oxidation of a compound which has a phenolic hydroxy group attached to a carbon atom on an aromatic ring and also has an oxygen atom connected through one other atom to an adjacent carbon atom of the aromatic ring, such that said oxygen atom can form a hydrogen bond to the phenolic hydroxy group.
We have found that an electrode bearing the polymeric deposit resulting from oxidation and polymerisation of such a phenolic compound can be used for a measurement of pH of an aqueous liquid which contains little or no buffer. The potential at which the redox reaction gives maximum current flow is dependent on the pH of the liquid.
Another aspect of this disclosure therefore provides a method of measuring the pH of an aqueous liquid wherein the concentration of buffer (if any) is not greater than 0.01 molar, comprising
Observing the redox reaction may be carried out by voltammetry which applies variable potential to the sensor electrode and determines the applied potential at a maximum current for redox reaction of the compound. More specifically, measuring pH may comprise applying a potential to the electrode in a sweep over a range sufficient to bring about at least one oxidation and/or reduction of the redox active deposit; measuring potential or potentials at the peak current for one or more said oxidation and/or reductions; and processing the measurements to give a determination of pH. If more than one potential is measured, the method may comprise averaging at least two potentials corresponding to peak currents and processing the average to determine the pH. Determination of pH from potential values may be done by comparing the potential with values observed in a calibration using buffer solutions of known pH.
For use with an unbuffered or weakly buffered liquid, the phenolic compounds may be devoid of any ionised or ionisable basic or acidic group other than phenolic hydroxyl, such as an amino, carboylic acid or possibly sulphonic acid group because this may disturb the pH in the vicinity of the electrode. A carbonyl group able to participate in hydrogen bonding as mentioned above may therefore be contained within an ester, aldehyde or ketone structure rather than in a carboxylic acid group. By way of illustration, instances of phenolic compounds which includes such structures are
Concentration of buffer is the total concentration of partially dissociated acid, base and/or salt which provides the stabilization of pH. The method and/or the use of a sensor may be carried out to measure the pH of an aqueous liquid which contains buffer at a concentration of at least 10−6 molar (0.001 mM) or possibly at least 5×10−6 molar (0.005 mM), or at least 10−5 molar or at least 10−4 molar. The concentration of buffer may perhaps be no more than 5×10−3 molar (5 mM) or even no more than 1 mM.
Because measurement can be made when buffer is at a low concentration, measurement can be performed on aqueous liquids where a small concentration of buffer may be present as a consequence of the origin of the liquid, for example measurement may be carried out on biological samples and natural products containing small concentrations of organic acids which are not fully ionized and provide some buffering of pH.
It is envisaged that some embodiments of the method may be carried out to measure the pH of aqueous liquid with a pH which is within two or three units of neutral. Thus the liquid may be mildly acidic from pH 4 or pH 5 up to pH 7 or mildly basic from pH 7 up to pH 9 or pH 10. The aqueous liquid may be liquid flowing within or sampled from equipment for processing the liquid and it may be a foodstuff or other material for human or animal consumption or an ingredient of such foodstuff or material. The aqueous liquid may possibly be one phase of a composition which is an emulsion, and it may be the continuous phase or a discontinuous phase of an emulsion.
Measurement of pH by the stated method can be carried out without measuring the buffer concentration. It is advantageous that the method can be employed when buffer concentration in the aqueous liquid is not known or is a parameter which cannot be controlled, without fear of an anomalous result because the concentration of buffer is low.
To carry out the determination pH, the sensor electrodes may be used as the working electrode of an electrochemical cell and maybe a component part of apparatus to determine pH. In a further aspect, the present disclosure provides apparatus to determine pH of water or other aqueous solution. Such apparatus may comprise:
an electrode for the determination of pH, comprising a conductive substrate bearing a water-insoluble, redox-active deposit which is the polymeric reaction product resulting from the oxidation of a compound which has a phenolic hydroxy group attached to a carbon atom on an aromatic ring and also has an oxygen atom connected through one other atom to an adjacent carbon atom of the aromatic ring, such that said oxygen atom can form a hydrogen bond to the phenolic hydroxy group;
means to apply potential to the electrode and observe current flow; and
a programmable computer connected and configured to receive current and/or voltage data from the electrode.
Such apparatus may be incorporated into equipment to process aqueous liquid, for instance process plant for water treatment, or to manufacture a pharmaceutical or a food product, and the computer which receives data from the sensor may be a computer which monitors or controls operation of that equipment. Thus this disclosure also provides equipment for processing water or other aqueous liquid, including:
a programmable computer operatively connected to control or monitor operation of the equipment,
an electrode for the determination of pH, comprising a conductive substrate bearing a water-insoluble, redox-active deposit which is a polymeric reaction product of the oxidation of a compound which has a phenolic hydroxy group attached to a carbon atom on an aromatic ring and also has an oxygen atom connected through one other atom to an adjacent carbon atom of the aromatic ring, such that said oxygen atom can form a hydrogen bond to the phenolic hydroxy group, and
means to apply potential to the electrode and observe current flow; wherein the computer is connected and configured to receive current and/or voltage data from the sensor.
Embodiments of apparatus may have a plurality of electrodes with the redox active deposit on one of the electrodes. An electrochemical sensor may also comprise a reference redox active compound, immobilized to the same or another electrode, where the oxidation and reduction of the reference redox active compound is substantially insensitive to pH.
Electrodes may be positioned in the equipment to be exposed to liquid flowing within the equipment, or taken from it as a sample, possibly by automated sampling under control of the computer. A programmable computer may monitor the proper operation of equipment and give a readout to a human operator, or the computer may itself control operation of the equipment.
An electrode was prepared using the phenolic compound salicylaldehyde which has the structure
Powdered salicylaldehyde was dissolved in dichloromethane at a concentration of 1 mg/ml. A 10 microliter (10 μL) aliquot of this solution was spread onto the surface of a glassy carbon electrode and allowed to dry. The electrode was then used as the working electrode of an electrochemical cell in which the electrolyte was pH 4 buffer. Square wave voltammetry (Frequency=25 Hz, Step Potential=2 mV, Amplitude=0.02V) was carried out to assess the electrochemical response.
The above electrode preparation procedure was repeated with a number of variations:
The results of square wave voltammetry in pH 4 buffer solution are shown in
An electrode prepared as above using salicylaldehyde was used as the working electrode of an electrochemical cell which was also provided with a silver/silver chloride reference electrode and a stainless steel counter electrode. The electrolyte in the cell was buffered electrolyte having pH increased in steps from pH2 to pH10. Square wave voltammetry was carried out at each pH. The potential at which oxidative current reached a peak progressively shifted to lower values as pH was increased.
This experiment was repeated using stirred buffer solutions as electrolyte. The results are included in
Similar results were obtained with electrodes prepared using 2-hydroxybenzylalcohol, 2-hydroxypropiophenone and 2-nitrophenol, showing that the deposits obtained from electrochemical oxidation of all of these phenolic compounds were redox active and sensitive to pH of the electrolyte.
An electrode prepared as above using salicylaldehyde was again used as the working electrode of an electrochemical cell, and square wave voltammetry was carried out with three buffer solutions having pH 4, 7 and 9 as electrolyte. The voltammetric responses are shown as dashed lines in
Analogous experiments were carried out using electrodes prepared as above using salicylic acid, 2-hydroxypropiophenone, 2-hydroxybenzylalcohol and 2-nitrophenol. The resulting data is summarised in the following table.
The data in the table shows that salicylaldehyde and 2-hydroxypropiophenone provide electrodes suitable for measuring pH of an unbuffered or weakly buffered solution. The electrode prepared using hydroxybenzylalcohol was not so accurate, attributed to a weaker hydrogen bond between the two hydroxyl groups. The electrode prepared using salicylic acid gave an inaccurate result, suggesting that the carboxylic acid functionality contained within the molecule was controlling the pH of the unbuffered electrolyte within the diffusion layer of the electrode.
In some embodiments, a redox active deposit, as disclosed here, which is sensitive to the analyte concentration/pH may be used jointly with a redox active compound which is substantially insensitive to the concentration of analyte/pH. This species which is independent of analyte concentration may then function as a reference and the potential of the sensitive compound may be determined relative to the potential of the compound which is insensitive to the concentration of analyte/pH. Possible reference molecules, insensitive to hydrogen ion concentration are K5Mo(CN)8 and molecules containing ferrocene such as potassium t-butylferrocene sulfonate.
The redox active deposit, as disclosed here, may be formed on part of the area of a conductive substrate and a reference redox active compound which is substantially insensitive to the concentration of analyte/pH may be immobilized on another part of the same substrate to form an electrode with both redox systems or it may be immobilized on another electrode. The two electrodes may then be connected together so that only a single voltammetric sweep is required.
An electrode as disclosed herein could be incorporated into a wide variety of tools and equipment. Possibilities include use in tools which are located permanently downhole, use in tools which are conveyed downhole, for instance at the head of coiled tubing or by drillpipe or on a wireline, use in underground, undersea or surface pipeline equipment to monitor liquid flowing in the pipeline, and use in a wide variety of process plant at the Earth's surface, including use in water treatment.
Measuring apparatus may comprise both a sensor and a control unit providing both electrical power and measurement. A control unit such as 62 may comprise apparatus such as a power supply, voltage supply, or potentiostat for applying an electrical potential to the working electrode 32 and also a detector, such as a voltmeter, a potentiometer, ammeter, resistometer or a circuit for measuring voltage and/or current and converting to a digital output, for measuring a potential between the working electrode 32 and the counter electrode 36 and/or the reference electrode 34 or 35 and for measuring a current flowing between the working electrode 32 and the counter electrode 36 (where the current flow will change as a result of the oxidation/reduction of a redox species). The control unit may in particular be a potentiostat. Suitable potentiostats are available from Eco Chemie BV, Utrecht, Netherlands.
A control unit 62 which is a potentiostat may sweep a voltage difference across the electrodes and carry out voltammetry so that, for example, linear sweep voltammetry, cyclic voltammetry, or square wave voltammetry may be used to obtain measurements of the analyte using the electrochemical sensor. The control unit 62 may include signal processing electronics.
A control unit 62 may be connected to a computer 63 which receives current and/or voltage data from the sensor. This data may be the raw data of applied voltage and the current flowing at that voltage, or may be processed data which is the voltage at peak current. A control unit 62, such as a potentiostat may itself be controlled by a programmable computer 63 giving a command to start a voltage sweep and possibly the computer will command parameters of the sweep such as its range of applied voltage and the rate of change of applied voltage.
A schematic of a microsensor 50 incorporating such a surface is shown in
An application of an embodiment of the present invention may be in the monitoring of underground bodies of water for the purposes of resource management. From monitoring wells drilled into the aquifers, one or more sensors may be deployed on a cable from the surface—either for short duration (as part of a logging operation) or longer term (as part of a monitoring application).
The deployment of such a pH sensor within producing wells on a cable may provide information on produced water quality. Also, the pH sensor may be deployed in injection wells, e.g. when water is injected into an aquifer for later retrieval, where pH may be used to monitor the quality of the water being injected or retrieved.
Before completion of a well, the modular dynamics tester is lowered into the well on the wireline 812. After reaching a target depth, i.e., the layer 842 of the formation which is to be sampled, the hydraulic arms 834 are extended to engage the sample probe tip 836 with the formation. The o-ring 840 at the base of the sample probe 836 forms a seal between the side of the wellbore 844 and the formation 842 into which the probe 836 is inserted and prevents the sample probe 136 from acquiring fluid directly from the borehole 814.
Once the sample probe 836 is inserted into the formation 842, an electrical signal is passed down the wireline 812 from the surface so as to start the pump 832 and the sensor systems 816 and 830 to begin sampling of a sample of fluid from the formation 842. The electrochemical sensor 816 can then measure the pH of the formation effluent.
While the preceding uses of the electrochemical sensor are in the hydrocarbon and water industries, embodiments of the present invention may provide an electrochemical sensor for measuring pH in a wide range of industries, including food processing, pharmaceutical, medical, water management and treatment, biochemistry, research laboratories and/or the like.
Electrodes may be made by a process which utilizes screen-printing onto a substrate. Stencil designs may delineate the components of the electrode. Constituents of the electrode may possibly be sequentially deposited onto the electrode. By way of example, carbon/graphite may be deposited onto an insulating substrate, which may comprise a plastic, polyester and/or the like. The carbon/graphite will provide a conducting substrate area. A reference electrode, such as silver/silver-chloride may then be deposited as a paste onto the electrode. The phenolic compound may be applied to the area printed with carbon/graphite and then electrochemically oxidized and polymerized.
A polymer coating on top of an electrode may prevent diffusion of a redox species from the working electrode, but still allow for interactions between an analyte and one or more of the redox species disposed on the working electrode.
This electrode 111 could be used in combination with a hand-held potentiostat, for instance to measure pH of a sample in a beaker 127 as shown in
A polymer coating 110 may serve to prevent leaching, diffusion and/or the like of the redox species 114, 123 into the surrounding fluid. This may be important where it is not desirable to contaminate the fluid, for example the fluid may be water in a water treatment process, a batch of a pharmaceutical process, a food substance or the like. In other aspects, the electrochemical sensor/working electrode may be subject to human contact in use and it may be desirable to prevent such contact with the redox species. Alternatively or in addition, the application of the polymer coating 110 to the working electrode 111 may serve to anchor the redox species 114, 123 to the working electrode 111. As such, methods of fabrication of the working electrode may be used wherein the redox species are not chemically coupled to the working electrode 111. At the same time, the polymer coating 110 should allow the fluid 125 to permeate, diffuse or otherwise come into contact with the redox species 114 and 123 on the working electrode 111. Merely by way of example the polymer coating 110 may comprise a polysulphone polymer or a polystyrene polymer. Other polymers may be used provided the polymers do not interfere with the operation of the sensor. Methods to deposit the polymer coating 110 in a generally uniform layer over the working electrode 111 include spin coating onto the working electrode 111, dip coating onto the working electrode 111, and application using solvent evaporation onto the working electrode 111.
It will be appreciated that the example embodiments described in detail above can be modified and varied within the scope of the concepts which they exemplify. Features referred to above or shown in individual embodiments above may be used together in any combination as well as those which have been shown and described specifically. Accordingly, all such modifications are intended to be included within the scope of this disclosure as defined in the following claims.
