WO2017133952A1

WO2017133952A1 - Device and method for electrochemically sensing the ph of a liquid

Info

Publication number: WO2017133952A1
Application number: PCT/EP2017/051576
Authority: WO
Inventors: Shandy ZHU
Original assignee: Koninklijke Philips N.V.
Priority date: 2016-02-04
Filing date: 2017-01-26
Publication date: 2017-08-10

Abstract

The invention presents a device for electrochemically sensing the pH of a liquid using electrolysis, the device comprising: a working electrode; a counter electrode; a current detector arranged to detect a current flowing between the working electrode and the counter electrode;a voltage detector arranged to detect a voltage between the working electrode and the counter electrode; a controller arranged to apply an electrical test sequence to the working electrode and the counter electrode, and to determine the pH of the solution liquid using the detected current and detected voltage during the electrical test sequence. Further, a method for electrochemically sensing the pH of a liquid is presented.

Description

Device and method for electrochemically sensing the pH of a liquid

FIELD OF THE INVENTION

The present invention relates to devices and methods for electrochemically sensing the pH of a liquid.

BACKGROUND OF THE INVENTION

Water with different pH values can have different uses in various applications, e.g. domestic applications. For example weak acid (e.g. pH 5.5 - 6.5) water is suitable for skin washing, since it can bring the skin pH into weak acid and accelerate the skin barrier recovery. Strong acid (e.g. pH < 5.5) water has a proven of capability of anti-bacteria.

Weak alkaline (e.g. pH 7.5 - 10) water may be regarded as being suitable for drinking and good for health, while strong alkaline (e.g. pH > 10) water is known to increase the solubility of organic compounds, which can for example remove the pesticides from vegetable leaves.

Moreover, water pH significantly affects the cooking and brewing process and water with a correct pH can bring benefits of better nutrition and taste. However, the above benefits can only be correctly delivered when water pH is in the desired range. Therefore, a pH sensor may be required in order to determine the pH of the water or other liquid.

SUMMARY OF THE INVENTION

It is an object of the invention to provide an improved device and method for electrochemically sensing the pH of a liquid.

According to an aspect of the present invention, there is provided a device for electrochemically sensing the pH of a liquid, the device comprising: a working electrode; a counter electrode; a voltage detector arranged to detect a voltage between the working electrode and the counter electrode; a controller arranged to apply an electrical test sequence to the working electrode and the counter electrode, and to determine the pH of the liquid using the detected voltage during the electrical test sequence. The voltage detector may detect the voltage directly, or may detect the voltage indirectly. The electrical test sequence may include be sufficient to cause oxidation and/or reduction of water in the liquid. The liquid could be any analyte, such as food or other home analyte.

The electrodes of such a device are both solid state and there is no need for a reference solution. Hence, such devices are more robust and do not need special storage conditions than some conventional devices. Such devices also do not leak and can be easier to maintain.

By using electrolysis, the pH of the liquid can be determined in a convenient way. Hence, by detecting water pH by using all-solid-state electrodes, with a water electrolysis-based concept, compared with existing methods, such embodiments of the invention can completely eliminate the usage of reference electrode and inner reference solution. It presents advantages as being solution-free and maintenance-free, being simple in structure, being easy to minimization and being potentially portable, and being safe and low cost.

In some embodiments, the electrical test sequence is a current signal sequence.

In some embodiments, the electrical test sequence is a four step squared current potentiometry sequence.

In some embodiments, the counter electrode is a capacitive counter electrode.

In some embodiments, the counter electrode is any one of a carbon electrode, a metal oxide electrode or a polymer capacitor electrode.

In some embodiments, the working electrode is an inert material capable of triggering a water electrolysis reaction.

In some embodiments, the working electrode is any one of a platinum electrode, a silver electrode, a silver chloride electrode, a ruthenium electrode, or an iridium electrode.

In some embodiments, the device further comprises a probe portion, wherein the working electrode and the counter electrode are formed at an end portion of the probe portion for contact with the liquid.

In some embodiments, the controller is arranged to determine the pH of the solution by calculating (p_Pi_us, where φ_Ρι_η5 = φ⁺ + φ^~; wherein (p⁺is the measured electrode potential when a positive current i₊ flowing through the solution triggers water oxidation, and φ^~ is the measured electrode potential when a negative current Lflowing through the solution triggers water reduction.

In some embodiments, φ⁺ = φ£₂/Η20 + Δ £^"— φ_κ+ wherein φ£₂/Η20 is ^me potential required for water oxidation, ΔΕ is the electrode polarization, and φ_κ+ is the electrode potential of the counter electrode under i₊ . In some embodiments: Ψ02/Η20 = Ψθ2/Η2θ + ^ (^^) = φ₀ ⁺ ₂' _/Η20 - 0.0S91pH + 0.0148 lg[02] wherein φ o₂' /H2o is ^me potential required for water oxidation when the partial pressure of oxygen P02 is 101325 Pa and [H+]=l mol/L, R is the universal gas constant; T is the absolute temperature; n is the number of moles of electrons transferred in the cell reaction or half-reaction, and F is the Faraday constant.

In some embodiments:

φ^~ = Φ_Η+/_Η2 - ΔΕ - ψ_κ_ .

wherein φ_#+_/Η2 *^{s me} potential required for water reduction, ΔΕ is the electrode polarization, and φ_κ_ is the electrode potential of counter electrode under i_ .

In some embodiments: PH+/H2 = PH+/H2 + ^ 1η( ) = φ-'_+/Η2- Ο-Ο591ρΗ - Ο.Ο296^[Η2]

wherein φ_#'+_/Η2 *^{s me} potential required for water reduction when the partial pressure of hydrogen PH2 is 101325 Pa and [H+]=l mol/L, which is defined as 0 V.

In some embodiments, the device further comprises a power supply arranged to apply voltage across the working electrode and the counter electrode.

In some embodiments, the device further comprises a current detector arranged to detect a current flowing between the working electrode and the counter electrode, wherein the controller is arranged to determine the pH of the solution using the detected current and detected voltage during the electrical test sequence.

According to an aspect of the present invention, there is provided a method for electrochemically sensing the pH of a solution, the method comprising: applying an electrical test sequence to a working electrode and a counter electrode in contact with the liquid;

detecting the voltage between the working electrode and the counter electrode during the electrical test sequence; determining the pH of the liquid using the detected voltage during the electrical test sequence.

According to an aspect of the present invention, there is provided a device for electrochemically sensing the pH of a liquid, the device comprising: a working electrode; a counter electrode; a current detector arranged to detect a current between the working electrode and the counter electrode; a controller arranged to apply an electrical test sequence to the working electrode and the counter electrode, and to determine the pH of the liquid using the detected current during the electrical test sequence. BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention will now be described, by way of example only, with reference to the accompanying drawings, in which:

FIG. 1 A shows an example pH electrode according to one type;

FIG. IB shows an example pH electrode according to another type;

FIG. 2 is an illustration of a device for electrochemically sensing the pH of a liquid according to an embodiment of the invention;

FIG. 3 is an illustration of a device for electrochemically sensing the pH of a liquid according to another embodiment of the invention;

FIG. 4A is an illustration of a patterned current signal, and FIG. 4B shows potential signal detected using the patterned current signal of FIG. 4A;

FIG. 5 shows pH detection according to an example;

FIG. 6 shows pius as a function of pH;

FIG. 7 is an illustration of a device for electrochemically sensing the pH of a liquid according to another embodiment of the invention; and

FIG.8 is a flow chart according to an embodiment of the invention.

DETAILED DESCRIPTION OF THE EMBODIMENTS

The majority of the conventional commercial available pH electrodes are based on the concept of an H+ sensitive membrane. For example, a widely used pH electrode type is based on glass membrane (see FIG. 1 A). Others are based on polymer membranes (see FIG. IB), which contains H-ionophores.

As shown in FIG. 1A, there is a pH electrode 10 with an inner reference electrode 11, an inner reference solution 12 and a glass membrane 13. As shown in FIG. IB, there is a pH electrode 20 with an inner reference electrode 21, an inner reference solution 22 and a pH sensitive membrane 23.

In such pH electrodes, an ion sensitive membrane is the key component for sensing water pH. When the outer surface contacts with an unknown solution, the inner surface is in contact with the inner reference solution 12, 22. The inner reference electrode 11, 21 is required to form an electric circuit with constant contact potential.

Though electrodes of the type shown in FIG. 1 A and IB are widely used in laboratory settings, they can suffer from various problems for the domestic use. For example, both types of electrode may suffer from a leakage problem, because the inner reference solution 12, 22 may leak out during use. They may also suffer from maintenance problems, because the electrodes should be kept in a special chemical solution when the device is not being used, typically 3M KC1 solution, which is also used typically as inner reference solution 12, 22.

In addition, there may be other problems such as the need for frequent calibration, relatively high cost, fragile and difficult for transportation, and so on.

To avoid the use of an inner reference solution for a pH electrode, one method is to directly coat a layer of ion selective membrane on the conductive substrate. However, for an undetermined e-to-ion process, the electrode potential is unpredictable and will be changing during the use, which results in poor reproducibility.

To avoid the use of inner reference solution, another method for a pH electrode is to use metal oxides, whose electrode potential is determined by water pH.

However, there are a number of problems associated with this approach. For example, the metal oxide may react with H⁺ in acid solution and release toxic ions. Also, this approach may be associated with low reversibility, because the pH response curve may be changed after long time contact with strong acid/alkaline water. Furthermore, such approaches may suffer strong interference from redox species.

FIG. 2 shows a device 100 for electrochemically sensing the pH of a liquid according to an embodiment of the invention. The device 100 comprises a working electrode 110, a counter electrode 120, a voltage detector 130, and a controller 140.

The voltage detector 130 is arranged to detect a voltage between the working electrode 1 10 and the counter electrode 120. In use, the working electrode 110 and the counter electrode 120 may be placed in contact with a liquid (not shown). The voltage detector 130 may detect the voltage directly, or may detect the voltage indirectly. The liquid could be any analyte, such as food or other home analyte.

The controller 140 is arranged to apply an electrical test sequence to the working electrode 110 and the counter electrode 120. The controller 140 may determine the pH of the liquid using the detected voltage during the electrical test sequence. The electrical test sequence may cause electrolysis of the liquid (e.g. be sufficient to cause water oxidation/reduction). Hence, the controller 140 can control the supply of current to the electrodes and analyse the results provided by the voltage detector 130. By performing data analysis, the controller 140 can determine the pH of the liquid.

The electrical test sequence may be a current sequence or a voltage sequence. For example, as discussed below, the electrical test sequence may be a four step squared current potentiometry sequence. The electrical test sequence may be other types of potentiometry sequence.

In this embodiment, the controller 140 is arranged to determine the pH of the solution by calculating (p_Pi_us, where (p_Pi_us = φ⁺ + φ^~. Here, φ⁺ is the measured electrode potential (i.e. the potential difference between the working electrode 110 and the counter electrode 120) when a positive current i₊ flowing through the liquid triggers water oxidation, and φ^~ is the measured electrode potential when a negative current i_ flowing through the liquid triggers water reduction.

In this embodiment, the device 100 may be connected to an external power supply. The external power supply may the current for the electrical test sequence. The controller 140 may also be powered by the external power supply. In other embodiments, the device 100 may comprise the power supply.

The power supply (internal or external) may be a programmed power supply that can provide required potentials. The controller may collect the information from the voltage detector control the power supply and carry out data analysis.

In this embodiment, the pH value may be detected by controlling and measure the voltage (current) signal during electrolysis. This approach can provide a pH measure with little or no H₂ gas being generated.

Other indirect pH calculation methods may be used by, for example, calculating (p_Phts from some other ways rather than directly detect it. Hence, in some embodiments, the pH value is calculated using (p_Pi_us , and (p_Pi_us could either be detected or calculated from other parameters.

In other embodiments, the device 100 may further comprise a current detector arranged to detect a current flowing between the working electrode and the counter electrode, and the controller 140 may be arranged to determine the pH of the liquid using the detected current and detected voltage during the electrical test sequence

Compared to conventional devices, the device 100 is uses solid state working and counter electrodes. Hence, here is no need for a reference solution. Therefore, such a device is more robust and does not need special storage conditions. The device also does not leak because it contains no liquid and is easier to maintain because it does not require calibration.

FIG. 3 shows a device 200 for electrochemically sensing the pH of a liquid according to another embodiment of the invention. The device 200 comprises a working electrode 210, a counter electrode 220, a voltage detector 230, a current detector 235, a controller 240, and a power supply 250.

The voltage detector 230 is arranged to detect a voltage between the working electrode 210 and the counter electrode 220, and the current detector 235 is arranged to detect a current flowing between the working electrode 210 and the counter electrode 220. In other embodiments, the current detector 235 need not be present. In other embodiments still, the current detector 235 may be present and the voltage detector 230 need not be present.

In use, the working electrode 210 and the counter electrode 220 may be placed in contact with a liquid (not shown) whose pH is to be measured.The liquid could be any analyte, such as food or other home analyte.

The controller 240 is arranged to control the power supply 250 to supply an electrical test sequence to the working electrode 210 and the counter electrode 220 when they are in contact with the liquid. As discussed in more detail below, the controller 240 is arranged to determine the pH of the liquid using the detected voltage during the electrical test sequence. The controller 140 can control the supply of current to the electrodes and analyse the results provided by the voltage detector 230 and current detector 235. By performing data analysis, the controller 240 can determine the pH of the liquid.

The detection in this embodiment is based on water electrolysis. According to the theory of electrochemistry, the potential required for water oxidation and reduction depends on the pH condition.

In this embodiment, the working electrode 210 is an inert material capable of triggering a water electrolysis reaction. In this embodiment, the working electrode 210 is a platinum electrode. However, in other embodiments, other materials such as silver, silver chloride, ruthenium, iridium, etc. could be used.

In this embodiment, the counter electrode 220 is a capacitive counter electrode. In this embodiment, the counter electrode 220 is a carbon electrode. However, in other embodiments, a metal oxide, polymer capacitor, or other capacitive materials may be used as the counter electrode.

Potential required for water oxidation:

Anode: 2H₂0- 4e 0₂ + 4H Equation 1 :

ΨΟ₂/Η₂Ο = ΨΟ₂/Η₂Ο +^ ^R ^S ⁼ Ψ¾/Η₂Ο - 0-0591PH + 0.0148 lg[0₂] In Equation 1 ,

is ^me potential required for water oxidation and

Ψ02/Η20 is ^me potential required for water oxidation under standard conditions (P02 =latm and [H+] =1 mol/L), which is equal to 1.229 V; R is the universal gas constant (8.314 J-K-l mo 1-1); T is the absolute temperature; n is the number of moles of electrons transferred in the cell reaction or half-reaction; F is the Faraday constant (9.648 x l04 C -mol-1).

Potential required for water reduction:

Cathode: 2H⁺ + 2e ^ H₂

Equation 2: PH^~+/H₂ = PH^~+/H₂ + ^ 1η(^) = φ-'_+/Η2- 0.0591ρ - 0.0296^[Η₂]

In Equation 2, φ_#+_/Η2 *^{s me} potential required for water reduction and φ_#'+_/Λ is the potential required for water reduction under standard conditions (PH₂ =l atm and [H+] = 1 mol/L), which is defined as 0 V.

It will be appreciated that φ J₂/H2o ^and Φ//+/Η2 ^cannot be simply detected by electrochemical methods. For a two-electrode system, when a positive current i₊ flowing through triggers the water oxidation, the electrode potential measured is according to Equation 3.

Equation 3 :

φ⁺ = Ψο2/Η2θ + ^{Δ E} - <PR+ In Equation 3, ΔΕ is the electrode polarization (which may be considered a constant in this equation), which is unpredictable during the detections. φ_κ+ is the electrode potential of counter electrode under i₊.

When a negative current i_ flowing through the liquid triggers water reduction, the detected electrode potential is according to Equation 4.

Equation 4:

In Equation 4, φ_κ_ is the electrode potential of counter electrode under i_.

When employing a capacitive counter electrode, e.g. carbon electrode, only symmetrical charging and discharging process occurs. If i₊ =— i_, then φ_κ+=φ_κ_=φ_κ. Therefore, to remove the interference of ΔΕ, p_Pi_usis used in this embodiment to detect water pH.

Equation 5 :

⁽ppius = φ⁺ + φ^~ = Ψ02/Η20 + ΨΉ+/Η2 - ^{2 (}PR

= Ψ02/Η20 + ΨΉ+/Η2 - °-¹¹⁸ P^H + 0.0148Z#[O2] - 0.0296Z#[H2] - 2φ_κ It will be appreciated that φ⁺ + φ^~ is independent of ΔΕ. When the change of

0.01481g[O2]— 0.02961g[H2] is ignorable, Equation 5 can be simplified as Equation 6.

Equation 6:

pius ⁼ Constant- 0.118 pH .

This method is more sensitive and the slope, which is 118 mV/pH, is around twice of that of traditional pH electrode.

In this embodiment, Four Step Squared Current Potentiometry (SCP-4S) is used for the detection. The patterned current signal (FIG. 4A), which is generated by the power supply 250 and is described by parameters of il, i2, tl and t2, is used for the detection.

FIG. 4A shows the patterned current signal used for SCP-4S detection, and FIG. 4B shows potential signal detected. SCP-4S is a branch of step potentiometry and is a known technique in electrochemical analysis. It will be appreciated that four step signals are widely used in signal analysis field for noise filters.

In the SCP-4S scheme of this embodiment, ii is the charging current to quickly fully charge the electrode, and i₂ is the detection current. The potential collected between the working electrode and counter electrode is shown as FIG. 4B. φ⁺ and φ^~ are defined as the potential at the end of each period. When the potential reaches φ⁺ and φ^~, stable and continuous water oxidation/reduction takes place at the working electrode surface. After several cycles of detection, (p_Pius is calculated for pH evaluation.

Whereas conventional pH electrodes employ a fragile reference electrode, which contains an inner reference solution, the device of FIG. 3 comprises all- so lid- state electrodes and works in a proactive detection mode, during which a current flowing through is required to trigger the water electrolysis reaction.

In an example, the optimized detection parameters are ii =800 μΑ, i₂ =50 uA, ti=t₂=0.5 s. The potential at the end of each pulse is collected as φ⁺ and φ^~ and is used for pH evaluation. The experiment conditions used for the pH detection is shown below in Table 1, and are shown in relation to FIG. 5 and FIG. 6.

Table 1 :

FIG. 5 shows pH detection in 100 mM phosphate-buffered saline (PBS) with pH values of 4.43, 5.38, 5.88. 7.01, 8.04 and 8.92, under the optimized condition as listed in Table 1. FIG. 6 shows (p_Pi_us as a function of pH. The results are an average of three independent detections. As shown in FIG. 6, (p_Pi_us turns out linear in relation to the water pH. Hence, (p_Pi_us can be used for pH detection.

The SCP-4S scheme is used in this embodiment to help filter noise signal and improve detection accuracy. [However, square wave potentiometry, cyclic voltammetry and even linear scan voltammetry are also applicable may be used, potentially with varied detection accuracy.

FIG. 7 shows a device 300 for electrochemically sensing the pH of a liquid according to another embodiment of the invention. The device 300 comprises a working electrode 310, a counter electrode 320, a voltage detector 330, a controller 340, and a power supply 350.

In this embodiment, the working electrode 310 is a platinum electrode, and the counter electrode 320 is a carbon electrode. In this embodiment the working electrode 310 and the counter electrode 320 are formed in a probe portion 360 of the device. The voltage detector 330, controller 340, and the power supply 350 (which in this embodiment is a battery) are formed in a handle portion 370 of the device 300.

The device 300 may operate in the way descried above in relation to FIG. 2. Hence, the power supply 350 (i.e. a battery) may power the controller 340, which applies the test sequence to the electrodes. Such a device provides a convenient hand held pH sensor. Such a sensor may be suited to domestic or other environments

The device 300 may or may not be provide with an outer sheath (not shown) to protect the electrodes 310, 320, as the electrodes 310, 320 are robust and safe for direct contraction with food or other home analyte.

In other embodiments, the device could be configured in other ways. For example, the device could be rod shaped with the electrodes 310 and 320 at one end of the rod. Alternatively, the device could be assembled with other appliance such as spoon, cup and etc.

FIG. 8 is a flow chart according to an embodiment of the invention. This method may be used with the device for electrochemically sensing the pH of a liquid according to an embodiment of the invention according to any of the discussed embodiments.

At step S10, an electrical test sequence to the working electrode and the counter electrode in contact with the liquid.

At step SI 1, the voltage between the working electrode and the counter electrode during the electrical test sequence is detected.

As discussed, embodiments of the invention may provide a device for electrochemically sensing the pH of a liquid using electrolysis, the device comprising: a working electrode; a counter electrode; a current detector arranged to detect a current flowing between the working electrode and the counter electrode; a voltage detector arranged to detect a voltage between the working electrode and the counter electrode; a controller arranged to apply an electrical test sequence to the working electrode and the counter electrode, and to determine the pH of the solution liquid using the detected current and detected voltage during the electrical test sequence. The voltage detector may detect the voltage directly, or may detect the voltage indirectly. The electrical test sequence may cause electrolysis of the liquid.

Embodiments of the invention may provide an all-solid-state two-electrode pH sensor. Such a pH sensor follows the water electrolysis mechanism and comprises a solid- state working electrode, a solid-state counter electrode, a programmed power supply, a current detector, a voltage detector and a controller. Four Step Squared Current

Potentiometry (SCP-4S) methods may be used to detect the potentials required for water oxidation and reduction, according to which the liquid pH is calculated.

It will be appreciated that the hardware used by embodiments of the invention can take a number of different forms. For example, all the components of the controller could be provided by a single component, or different components of the system could be provided on separate devices. More generally, it will be appreciated that embodiments of the invention can provide a system that comprises one device or several devices in communication.

It will be appreciated that the term "comprising" does not exclude other elements or steps and that the indefinite article "a" or "an" does not exclude a plurality. A single processor may fulfil the functions of several items recited in the claims. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measures cannot be used to an advantage. Any reference signs in the claims should not be construed as limiting the scope of the claims.

Although claims have been formulated in this application to particular combinations of features, it should be understood that the scope of the disclosure of the present invention also includes any novel features or any novel combinations of features disclosed herein either explicitly or implicitly or any generalisation thereof, whether or not it relates to the same invention as presently claimed in any claim and whether or not it mitigates any or all of the same technical problems as does the parent invention. The applicants hereby give notice that new claims may be formulated to such features and/or combinations of features during the prosecution of the present application or of any further application derived therefrom.

Claims

CLAIMS:

1. A device (100, 200, 300) for electrochemically sensing the pH of a liquid, the device (100, 200) comprising:

a working electrode (110, 210, 310);

a counter electrode (120, 220, 320);

a voltage detector (130, 230, 330) arranged to detect a voltage between the working electrode (110, 210, 310) and the counter electrode (120, 220, 320);

a controller (140, 240, 340) arranged to apply an electrical test sequence to the working electrode (110, 210) and the counter electrode (120, 220, 320), and to determine the pH of the liquid using the detected voltage during the electrical test sequence.

2. The device according to claim 1, wherein the electrical test sequence is a current signal sequence.

3. The device according to claim 2, wherein the electrical test sequence is a four step squared current potentiometry sequence.

4. The device according to any preceding claim, wherein the counter electrode (120, 220, 320) is a capacitive counter electrode.

5. The device according to any preceding claim, wherein the working electrode (110, 210, 310) is an inert material capable of triggering a water electrolysis reaction.

6. The device according to claim 5, wherein the working electrode (110, 210, 310) is any one of a platinum electrode, a silver electrode, a silver chloride electrode, a rutheniumelectrode, or an iridium electrode.

7. The device according to any preceding claim, further comprising a probe portion (370), wherein the working electrode (310) and the counter electrode (320) are formed at an end portion of the probe portion (370) for contact with the liquid.

8. The device according to any preceding claim, wherein the controller (140, 240, 340) is arranged to determine the pH of the solution by calculating ⁽p_Pi_us, whereppius = φ⁺ + φ^~;

wherein (p⁺is the measured electrode potential when a positive current i₊ flowing through the liquid triggers water oxidation, and φ^~ is the measured electrode potential when a negative current i_ flowing through the liquid triggers water reduction.

9. The device according to claim 8, wherein:

wherein φ O₂/H₂O is ^me potential required for water oxidation, ΔΕ is the electrode polarization, and φ_κ+ is the electrode potential of the counter electrode (120, 220, 320) under i₊.

10. The device according to claim 10, wherein:

ΨΟ₂/Η₂Ο = <Po₂/H₂o + ^ 1^η( ¾^) = φζ_/Η2θ- 0.0591ρΗ + 0.0148 lg[0₂]

wherein φ O₂' /H₂O is ^me potential required for water oxidation when the partial pressure of oxygen P02 is 101325 Pa and [H⁺]=l mol/L, R is the universal gas constant; T is the absolute temperature; n is the number of moles of electrons transferred in the cell reaction or half-reaction, and F is the Faraday constant.

11. The device according to any one of claims 8 to 10, wherein:

wherein φ_#+_/Η2 is the potential required for water reduction, ΔΕ is the electrode polarization, and(p_R_ is the electrode potential of counter electrode (120, 220, 320) under i_.

12. The device according to claim 11, wherein:

Η_+/Η₂ = <PH H₂ ⁺ S^ln(S¾

- 0-0591PH - 0.0296Z# [H₂] wherein φ^'+_/Η2 is the potential required for water reduction when the partial pressure of hydrogen PH2 is 101325 Pa and [H⁺]=l mol/L, which is defined as 0 V.

13. The device according to any preceding claim, further comprising a power supply arranged to apply voltage across the working electrode (110, 210, 310) and the counter electrode (120, 220, 320).

14. The device according to any preceding claim, further comprising a current detector (235) arranged to detect a current flowing between the working electrode (110, 210, 310) and the counter electrode (120, 220, 320), and wherein the controller (140, 240, 340) is arranged to determine the pH of the liquid using the detected current and detected voltage during the electrical test sequence.

15. A method for electrochemically sensing the pH of a liquid, the method comprising:

applying an electrical test sequence to a working electrode (110, 210, 310) and a counter electrode (120, 220, 320) in contact with the liquid;

detecting the voltage between the working electrode (110, 210, 310) and the counter electrode (120, 220, 320) during the electrical test sequence;

determining the pH of the liquid using the detected voltage during the electrical test sequence.