EP0705432A1

EP0705432A1 - Electrochemical sensor

Info

Publication number: EP0705432A1
Application number: EP94918441A
Authority: EP
Inventors: John Vincent The University of Newcastle DOBSON
Original assignee: University of Newcastle, The; Newcastle University of Upon Tyne
Current assignee: University of Newcastle, The; Newcastle University of Upon Tyne
Priority date: 1993-06-22
Filing date: 1994-06-17
Publication date: 1996-04-10
Also published as: WO1995000842A1; FI956179A0; FI956179A; AU6975894A; NO955153L; NO955153D0; CA2165827A1

Abstract

The invention concerns a device for use in determining the amount of contaminant or pollutant in a liquid sample. The device comprises a conditioning means for changing the chemical properties of the liquid sample so as to facilitate the determination or measurement of a preselected species, a reducing means for reducing the species to be detected and also a sensor for determining the amount of said species in said solution. The nature of the solution conditioning means, reducing means and sensor may be varied so as to allow for the determination of any preselect species. In addition, the invention provides for the maintenance of a sensor by the selective modification of a sensor's properties, which modification concerns the alteration of a sensor surface so as to maintain the integrity of same.

Description

ELECTROCHEMICAL SENSOR

The invention relates to a device for determining the presence of contaminants or pollutants in a sample and, particularly, but not exclusively the presence of nitrogenous substances or nitrogen oxygen combined in particular nitrates mode ammonia, ammonium and nitrites or phosphates, or azines or dimethylsulphide in a sample.

A device as aforedescribed can be used in a number of ways such as, for example, environmental sampling of water whereby the device is used to determine levels contaminant or pollutant in an aqueous phase.

The invention also relates to an apparatus for selectively modifying the surface of an electrode means so as to vary the functional characteristics of the surface and so modulate the activity of the sensor.

The apparatus as aforedescribed can be used in a variety of ways such as, for example, maintaining the integrity of an electrochemical sensor adapted to provide electrolytic measurements. In this example, the apparatus is used to modify the surface of an electrode so as to maintain its functional integrity and so ensure maintenance of sensitivity of a measuring device.

Nitrate, nitrite, ammonia ions are distributed at various levels throughout the eco-systems of the land, sea and other inland waters. Thus nitrate ions are found in the water we and animals drink, in the soil, and even within the organisms of the sea. Because of the deleterious effects or nitrate in living organisms and especially man, care and attention, when and where possible, is given to ensure minimum levels of nitrate and nitrite intake via food and drink.

Whilst natural phenomena such as geological strata, animal droppings, and plant decay are sources of nitrogenous species, it is well known that the present increasing measured levels of nitrate are attributed to agricultural activities and particularly to the use of chemical fertilizers. In addition nitrate contamination also results from domestic and industrial wastes.

Nitrogenous contamination is problematical because many nitrates and nitrites are very soluble in water, and are leached readily into the aqueous phase. Moreover, under the right conditions these contaminants can react with organics also present in the aqueous phase, for example, phenol to form potentially very dangerous and life threatening substances.

A number of agricultural and medicinal analytical methods have been created over the years to determine the levels of nitrate and nitrite contamination. Unfortunately, many of these methods are grouped in what are generally termed 'wet tests', and are only reliable when a number of time consuming preparative operations have been carried out under strict conditions. Moreover, some of these methods fail at concentrations of chloride found in seawater, and so are limited in their application and reliability. Further, some methods employ very poisonous reagents, for example the Griess Method uses cadmium, and cannot be used in an 'in— situ' measurement application. Accordingly, attempts to develop these

'wet' methods to allow for more applicability and reliability have not been successful. This is especially so where a rapid in-situ type of analysis is required.

In summary, previous colourimetric, ion selective electrodes (ISE), and present day electrochemical methods require considerable sample preparation and a strict regime of operation; both potentially prohibitive requirements which limit the use and flexibility of the methods. There are two conventional methods of analysis for the detection of the nitrate ion. One method, involves the reduction of the nitrate ion chemically to free ammonia and then distillation of the ammonia and determination of its concentration by titration or colourimetric means. The second method involves the use of 98% concentrated sulphuric acid and the addition of a molecule containing a phenolic group; the nitrate ion undergoes a chemical reaction to produce nitrophenol.

Nitrophenols are coloured variously when in solution and at different pH's. However, the intensity of colour provides the link to the original concentration of nitrate within the sample following subsequent colourimetric analysis. The nitration of phenolic groups has had some success when applied to analytical electrochemical methods where the concentration of nitrophenol is determined by a current peak height generated via its formation or destruction as a result of an electrochemical process.

However, both the above methods involve considerable sample preparation and obviously are not portable nor do they permit rapid analysis and so they are quite impractical for current requirements such as use in-situ.

The aim of the current invention therefore is to provide a device for rapidly and reliably determining in situ the presence of nitrogenous substances and especially the presence of nitrates in a sample.

In addition, the device of the invention also provides for the reliable determination, ideally in situ, of other contaminants or pollutants such as phosphates, azines or dimethylsulphide. However, it is not intended that the invention is limited to the aforementioned contaminants or pollutants, rather, as those skilled in the art will appreciate, the device of the invention can be modified, as hereinafter described, to reliably determine the existence of any preselected species.

In so far as phosphate is concerned, the measurement of phosphate ions in water is very important. However existing commercially available tests are not electrochemical, rather they are based upon the ability of the phosphate ion to be readily complexed with a number of VB and VIB early transitional elements and other elements in the periodic table such as Vanadium, Molybdenum etc. The complexes are formed when a number of sample preparative steps are are carried out. The resultant highly coloured complexes are then measured by eyed or by using primitive instrumental means so as to calculate phosphate concentration. Unfortunately these tests are unreliable because of limited sensitivity and specificity. Moreover, these tests only lend themselves to sampling and laboratory analysis techniques because continuous or in situ analysis is prohibitive because of the nature of the equipment that is needed. This form of analysis represents a significant set-back if a rapid, preferably on site, determination of contamination is required.

In so far as azines are concerned, because there is presently no way of determining their presence in the environment the device of the invention is an extremely valuable tool.

In so far as dimethyisulphide is concerned, it is well known that this algal product is a possible carcinogen and therefore its presence in the environment must be monitored.

A further aspect of the invention concerns the provision of a means for maintaining the integrity of a sensor and especially the surface of a sensor which is involved in electrochemical reactions. It is well known that electrochemical sensors often deteriorate in sensitivity over time and have to be periodically replaced. This is basically because, over time, a modification of the electrode surface occurs. Specifically, an electrode surface may be subject to oxidation as a result of use. In time, an oxide layer builds up to such an extent that the electrode is unable to function in a satisfactory manner and deteriorates eg sensitivity decreases . It is therefore desirable to provide a means for reversing this process and so restore the original electrode surface and minimize its replacement.

According to a further aim of the invention, there is therefore provided a means for modifying the surface of a sensor and particularly an electrode surface so as to increase the life effectiveness of a sensor embodying the electrode.

According to the invention there is therefore provided a device for use in determining the amount of contaminant or pollutant in a liquid sample comprising:

a solution conditioning means for changing the chemical properties of said liquid in a manner that enables the presence of a preselected species to be determined;

a reducing means for reducing said preselected species; and

a sensor for measuring the amount of said species so as to determine the amount of said contaminant or pollutant in the sample and further wherein said sensor is an electrochemical cell adapted to measure the level of said species.

Depending upon the nature of the species to be identified said reducing means and said sensor may comprise a single item or piece of equipment.

For example, in the instance where reduction is effected electrochemically using an electrode said reducing means and said sensor will comprise a single electrochemical cell and reduction and said determination will take place at a single electrode.

In the instance where the reducing means is separate from the sensor it, ideally, comprises a hydrogen donating means.

A device including a hydrogen donating means when used to measure, for example, nitrogen-based contamination, therefore ensures that there is sufficient hydrogen donating means to interact with compounds containing nitrogen so as to form ammonia and subsequently this ammonia is detected by an electrochemical sensor. The device operates rapidly, reliably and further, it is readily portable permitting rapid in-situ measurements to be made. It will be apparent from the above that a device in accordance with the invention is characterized by its adaption to use an electrochemical cell for measuring the presence of a pre-determined contaminant in a sample and particularly a water sample.

In the device for measuring, for example, an amount of nitrogen oxygen combined groups, the said hydrogen donating means according to the invention preferably comprises a hydrogen absorbing agent such as a hydrogen absorbing structure or lattice ideally made of a transition metal.

In a preferred embodiment of the invention said transition metal is palladium; alternatively, the transition element is zirconium.

A further advantage of this hydrogen donating means is that palladium charged with hydrogen, called palladium hydride, liberates the hydrogen that is contained in the lattice at a controlled and slow manner.

In an alternative embodiment said hydrogen donating means comprises palladium hydride which preferably is provided as a sponge of palladium wire which may be coiled so as to increase effective surface area of the hydrogen donating means.

In a yet further embodiment of the invention the hydrogen donating means, for the preparation of palladuim hydride, comprises an electrochemical means including conventional electrochemical circuitry so arranged as to provide for the generation of hydrogen when current flows via an external circuit. Preferably a solution of dilute sulphuric acid (0.1 molar) serves as the electrolyte and a palladium wire is made negative with respect to an inert electrode, for example platinum, and connected to a DC power supply allowing the passage of a few milliamps for a specific time. Preferably further still the arrangement comprises a flow through cell in which one electrode is made negative with respect to a second and hydrogen is liberated at the first. Examples of two appropriate electrodes are platinum and lead whereby the platinum is made negative with respect to the lead and hydrogen is liberated at the platinum electrode. This latter embodiment is an ideal in-situ method because it represents a flow through system and allows for periodic electrolysis of the sample solution flowing through the system.

Preferably a mixing means is provided between the hydrogen donating means and the sensor so as to provide for adequate mixing of the sample solution before measurements are taken.

Turning now to the sensor, ideally this would normally include a working, a reference and a counter electrode with sample fluid flow being in the direction from the working to the counter electrode. This arrangement is suitable for a flow through system.

In an alternative embodiment, the sensor comprises a plurality of electrodes which are isolated from the fluid flow by a semi-permeable membrane.

The advantage of this alternative embodiment is that the electrode components of the sensor exposed to the fluid flow are less likely to suffer from deterioration and are less effected by variation in flowrate of the sample.

In yet a further modified embodiment of the invention, said sample conditioning means is provided upstream of the reducing means and sensor so that the sample passes through a said conditioning means to ensure that the sample characteristics, for example pH, is modified to a predetermined value before entering the reducing means and sensor. This is particularly favoured where NH3 is to be measured. Those skilled in the art will appreciate that at a low pH the NH3 liberated will be present as NH4+ ion.

In the instance where nitrogen species are to be measured, or indeed any species, whose measurement is facillitated by an alkaline pH a suitable reagent to be included in the solution conditioner is trisodium phosphate which can be provided as a solid capsule or a powder and allows for slow dissolution with control of pH within the pH range of about 11 -12 or indeed higher. At this pH range, any ammonia generated is retained in solution mainly as free ammonia (NH3).

Preferably there is provided an electronic means for maintaining the potential of the working electrode with respect to the reference electrode contained in the sensor and measurement of the current generated by the oxidation of the species detected.

It will be clear to the worker skilled in the art that no further electronic interfaces need be specified but to refine the system a waveform generator an the electrode conditioner can be included.

A further refinement involves the incorporation of data retrieval means and PC display. A PC would be particularly advantageous because of the increase in sensitivity which it could provide by carrying out differential data post processing.

As previously mentioned, electrodes by their very nature when being used in oxidation and reduction (redox) reactions are subject to changes at the electrode surface. In addition, contaminants within samples can cause oxidation at the surface of the electrodes. This oxidation negates the function of the electrodes such that after a period of time the electrodes no longer function correctly. The affect of this oxidation is costly and requires careful monitoring so as to monitor the functioning capacity of the electrodes. The problem can be overcome by passing a reverse current through the electrode, the current having the exact same magnitude as that which has already passed through. The solution to this problem has not been suggested in the literature, there is therefore no prior art in this discipline to overcome the finite life of an electrode.

A further aim of the current invention therefore is to provide an apparatus for selectively modifying the surface of an electrode means so as to vary the functional characteristics of the surface and so modulate the activity of a sensor embodying the electrode means. The advantage of this aspect of the invention concerns the provision of an apparatus whereby the surface of an electrode, when used in redox reactions, can be modified providing, essentially, an infinite lifespan of use. The electrodes are periodically restored to a state whereby the lifetime of the electrodes is extended.

In a further aspect of the invention preferably the aforementioned device is provided with an apparatus for modifying an electrochemical surface of a sensor characterized in that it comprises:

a potentiostat means for generating a selected potential at said surface;

a current integrator for determining the total amount of current passing through the sensor; and

a logic unit for assessing the amount of said total current represented by the electrode reaction so as to calculate the amount of modifying current that modifies the functional characteristics of said surface whereby said potentiostat means is activated on detection of a pre-programmed modifying current value and provides a reverse potential at said surface, with respect to said modifying current, for a set time period until the current flowing through the surface substantially restores the original functional characteristics of said surface.

Thus, the potentiostat means provides a reverse potential such that a certain amount of build up of an oxide type layer on a surface and particularly on an electrode within a sensor is removed. Such an apparatus is of particular importance when an electrochemical sensor is required to run continuously for many days, or weeks.

According to a yet further aspect of the invention there is provided a method for modifying an electrochemical surface of a sensor, in preferably the aforementioned device, characterized in that it comprises:

generating a selected potential at said surface; determining the total amount of current passing through said sensor;

assessing the amount of said total current represented by an electrochemical reaction so as to calculate the amount of modifying current that modifies the functional characteristics of said surface;

generating a second selected potential at said surface in response to the detection of a pre-programmed modifying current value wherein said second selected potential is reversed with respect to the aforementioned selected potential, for a set period, until the current flowing through the said surface substantially restores the original functional characteristics of said surface.

An embodiment of the invention will now be described, by example only, with reference to and as illustrated in the accompanying Figures.

Figure 1 is a diagrammatic representation of the device in accordance with the invention;

Figure 2 shows diagrammatic representations of alternative sensor bodies;

Figure 3 shows diagrammatic representations of alternative reducing means and more particularly hydrogen donating means;

Figure 4 is a diagrammatic representation of a window sampler;

Figure 5 is a diagrammatic representation of the modifying apparatus of the invention;

Figure 6 is a diagrammatic representation of a simple form of the device in accordance with the invention; and

Figure 6B is a diagrammatic representation of the disc structures.

Figure 7 is a diagrammatic presentation of a disposable form of the device in accordance with the invention. Referring firstly to Figure 1 , there is illustrated a device in accordance with the invention for determining the concentration of a contaminant or pollutant in a sample. Basically, this device includes a solution conditioner 1 , upstream of a hydrogen donating means or reducer 2.

The nature of conditioner 1 will vary according to the nature of the preselected species to be measured. For example, conditioner 1 will be modified to provide the appropriate pH conditions of a species to be measured. In some instances a conditioner 1 may not be used, that is to say it may not be necessary to modify the characteristics of a liquid sample to make a particular measurement.

In the instance where production of ammonia is to be measured a solution conditioner is provided.

Similarly, reducer 2 will vary according to the nature of the preselected species to be measured. In some instances, reduction of a preselected species may take place electrochemically in sensor cell 4, where this occurs reducer 2 will not be required. However, in other instances reducer 2 may be required.

Reducer 2 will comprise either a rechargabie palladium wire sponge housed in a flow through chamber as illustrated in Figure 3A; a palladium wire sponge connected to circuitry for the purpose of hydrogen charging as shown in Figure 3B; or a conventional electrochemical hydrogen donating in-situ means as illustrated in Figure 3C. Each of these hydrogen donating means will be described in greater detail hereinafter.

Reducer 2 is connected to a mixing coil 3 which is in fluid connection with a sensor 4.

Sensor 4 comprises either a series of electrodes housed within a flow through chamber as illustrated in Figure 2A; or a partitioned chamber as illustrated in Figure 2B, including a membrane or filter to enable the selective passage of the volatile species to be measured, for example, ammonia. On the upstream side of said chamber there are provided working electrodes for detecting the presence of ammonia. In this latter arrangement or partitioned chamber the electrodes are shielded from contact with the sample fluid which flows in a circuitous fashion on one side of said membrane or filter.

The electronic control means comprises a number of units including a potentiostat 5, waveform generator 6, electronic conditioner 7, data retrieval means 8, PC display 9, control interface 10 and alternative control interface 11.

It will be understood that the device will function when sensor 4 is connected to potentiostat 5, which potentiostat 5 controls the working electrode potential of sensor 4. However, when desired additional electronic members 6, 7, 8, 9, 10 and 11 will be provided so as to increase the flexibility of the device and enable sophistication of operation. The function of each electronic unit will be described in greater detail hereinafter.

In use, a sample to be analysed, and which may contain the species of interest such as nitrate ions is drawn into the system via a solution conditioner 1. This chamber contains a reagent, for example trisodium phosphate, to ensure that the sample on exit from the solution conditioner 1 , is alkaline and has a pH of about 11-12 or above. The reagent trisodium phosphate occurs as a solid capsule or powder and allows for slow dissolution over a period of time to provide a pH within the required range. The specific range is selected so as to ensure that the ammonia generated is retained in solution as free ammonia (NH3). However, it is within the scope of this specification to measure the presence of the ammonium ion.

The solution conditioner 1 in its simplest form would be a tube containing a reagent, for example, trisodium phosphate, in the form of compressed pellets. The pellets are placed end to end and would dissolve successfully provided the pellet lengths are covered and only the ends are exposed.

The pellets would need to fit snugly within the tube. A further preferred form could be a reagent cylinder of appropriate dimensions, fitting snugly into conditioner 1 , the cylinder having small access holes for the liquid aqueous sample to penetrate to allow for controlled solvation of the reagent.

Ideally, trisodium phosphate would be used to provide an alkaline pH but other suitable reagents could be used, for example potassium hydroxide or sodium carbonate.

The difficulties of solvation of aqueous-soluble reagents are well known and other methods or means may be available for controlled infusion of concentrated soluble reagent. Some of the methods include casting reagent on the inner walls of a tube or tubes, leaching of reagent from a matrix of saturated absorbent which is replaced automatically, or controlled infusion of concentrated soluble reagent. These methods are well known to those in the field.

In the instance where the device is to be used to measure a phosphorous species the conditioner 1 includes a reagent that will lower the pH (to about pH2 -4) of a liquid sample. Such a reagent may be citric acid, tartaric acid, lactic acid, ascorbic acid or a combination of sodium citrate and citric acid or indeed any acidic buffer. In addition, a complexing agent will also be provided by conditioner 1 , such an agent may be Ammonium molybdate, Quinoline molybdate, Quinoline Vanadate, 1 , 2, 4 amino naptho sulphonic acid or a combination of Quinoline molybdate and Quinoline Vanadate. Other examples of complexing agents may be found in the literature.

On exit from the solution conditioner 1 , the sample, now at a predetermined pH, for example, in the instance where a nitrogen species is to be measured pH 11-12, passes through a second chamber, a reducing chamber 2. It is at this stage that the species or nitrate ion is reduced by hydrogen. We describe methods resulting in electron donation but it is not intended that the invention should be limited to these methods. A first method involves the use of a previously hydrogen charged palladium sponge (see below), existing as palladium hydride (Figure 3A). The overwhelming advantage of this system is that the palladium wire is re-chargeable resulting in a quick and efficient provision of hydrogen. Alternatively, hydrogen can be generated using a flow through cell (Figure 3C) which has appropriate electrodes to permit the frequent and periodic electrolysis of the sample solution. In this instance, two electrodes are sufficient, for example platinum and lead, the platinum is made negative with respect to the lead, hydrogen is then liberated at the platinum electrode.

The palladium sponge is charged by electrochemistry in which palladium wire is made negative with respect to an inert electrode and connected to a DC power supply allowing the passage of a few milliamps of current for a specified time. In this instance, a solution of dilute sulphuric acid, for example 0.1 molar, serves as the electrolyte. This is shown in Figure 3B.

On exit from the reducing chamber 2, the sample passes through a mixing means, for example a tube in the form of a coil 3 in order to allow adequate mixing and equilibration of the reduction process. Finally, the sample is passed into a sensor 4 adapted to identify the presence of a preselected species such as NH3; created as a result of the above interaction of the sample with the hydrogen donator. Within sensor 4 an electrochemical process occurs whereby the sample, in the form of ammonia, is oxidized. The magnitude of signal produced by the sensor 4 is determined by the concentration of ammonia which in turn is dependent on the concentration of nitrate in the sample.

It will be apparent to those skilled in the art that where, as in the case with nitrogen species, the nature of the electrochemical measurement is oxidation, then the provision of a separate reducer 2 is required. Where the nature of the electrochemical measurement is reduction a separate reducer 2 is not required,.

For nitrate measurement.

NO3 - is first reduced to NH3 which is then oxidised electrochemically. It is the current from the oxidation process which is measured. For phosphate measurement.

Complex of P04, molybate is formed which can either be:

1. Reduced and the reduction current is measured.

2. Reduced and then the complex is oxidised and it is the current from 5 the oxidation process which is measured.

In either case it is likely we are considering the effects of valency changes of the molybate atom, possibly penta and hexavalent.

Consider first the sensor illustrated in Figure 2A. In this instance, the o sensor body may contain "bare" electrodes as a working 14, a reference

16 and a counter electrode 15. In this case, the electrode materials are exposed directly to the sample. An alternative sensor body 12 is illustrated in Figure 2B in which a semi-permeable membrane 13 provides a barrier between the "bare" electrodes 14, 15 and 16 and the sample.

5 The working electrode 14 may be made from platinum, silver or gold and be in the form of wires, rods or flags. The electrode material may also be deposited on membranes or other substrates. In the case if phosphate determination the working electrode may also in addition be a molybdenum coating on the above substrates.

0 Counter electrodes 15 may be of similar metals to the working electrodes

14 or may be made from lead or any other easily electrochemically oxidisable or reducible material that is not too soluble.

Reference electrodes 16 may be made from silver, silver chloride or calomel, with a filling of either potassium chloride, or from mercuric oxide 5 or silver oxide with a filling of potassium hydroxide.. These reference electrodes and other alternatives will be familiar to those skilled in the field. All the electrodes are mounted in suitable carriers to permit their sealing into the main body 12.

Considering sensor body 2B in more detail. This arrangement is most suited to a gas sensor such as an ammonia gas sensor. It could comprises a sealed unit, generally referenced 17, in which the sample containing ammonia would enter the unit via port 18 and exit via port 19. The working 14a, reference 16a and counter 15a electrodes are held behind a thin semi-permeable membrane 13 through which gas can permeate and thus react at an inner electrode surface. This form of construction is exploited in dissolved gas-liquid sample sensors. The inventor has several patents protecting this latter type of sensor, the numbers of these patents are GB 2 066 965 and GB 1 585 070. In this type of sensor body shown in Figure 2B, the working electrode metal may be sputtered onto the membrane in such a manner as to contact with the internal filling of the sensor. The fillings of the sensor can be selected from those already known, for example, potassium hydroxide and potassium chloride mixtures. This sensor body, 2B, is favoured because it prolongs the life expectancy of the sensor mainly because the electrodes are not exposed to liquid contaminants.

As shown in Figure 1 there are ideally three electronic units incorporated in the device, a potentiostat 5, a waveform generator 6 and a conditioner 7. These units are used to control all the various electrochemical processes taking place in the device. It is however of note that the device could be operated simply by the provision of the potentiostat but where greater flexibility, long term use and sophistication is required, all three units are deployed. The potentiostat 5, waveform generator 6 and conditioner 7 are all for providing the necessary control of the sensor 4. The potentiostat 5 and waveform generator 6 maintain the potential of the working electrode 14A with respect to a reference electrode 16A contained in the sensor.

Each of the three electronic units employ well-known circuits and are not of a special nature. Thus the specifications are not given here but the units are adapted to provide for a range of voltages such as between -1.8 volts to +1.0 volts, with respect to the reference electrode 16, 16A. The current output may be within the range microamps to milliamps. The waveform generator 6 provides for linear sweeps of potential of 1 to 300 millivolts per second between the limits stated. Desirably, the waveform should be capable of being held at any potential if required. Moreover, square wave pulses of magnitude between 0 to 100 millivolts of 1 to 100 hertz are a useful feature. Many of these capabilities are accessible in modern circuits.

In the preferred embodiment shown in Figure 1 , a data retrieval means 8 and PC display 9 are also provided. The current signal is passed from the potentiostat 5 to a data retrieval unit 8 containing electronic interface for its conversion to digital RS232 type output for direct input to a PC 9 for display. Clearly, data retrieval 8 and PC display 9 are preferred refinements of the overall process. A simple current measuring system coupled in series with the sensor 4, for example current measuring resistor or current to voltage converter, allows recording on a digital voltmeter or pen recorder. However, should a PC be in place, considerable advantages in terms of increases in sensitivity can be made by carrying out differential data post processing.

As is shown in Figure 1 , provision is made in the electronic circuitry at control interface 10 to provide current for the reducer 2 by both direct and periodic electrolysis of the sample flow to generate the necessary hydrogen. A requirement in this circuit is that the current is not passed whilst the sensor 4 is providing information, ie is in an ON/OFF operation. Alternatively, control interface 11 provides a similar current and may be included to allow for the periodic charging by hydrogen of the palladium wire sponge. Both circuits would be simple trigger controlled constant current circuits. For the control interface (the in-situ generator 10) to have sufficient power to overcome the electrical resistance of the sample stream, which could approach mega ohms in very pure water. The charging of the reducer palladium sponge clearly may be carried out externally of the system. Such a circuit in operation is most effective when the sensor is required to run continuously for many days, weeks unattended provided that sufficient sample conditioner was available. Clearly, in the long term, deployment of the in-situ control interface (hydrogen generator reducer/generator 10) is the most effective.

In order that the device may function in a pre-programmed manner and thus take periodic samples at pre-determined time intervals, a window sampler is provided and is illustrated in Figure 4. The sampler includes a potentiostat 5, a sample and hold comparator 20, ideally an output to analogue devices 21 or a comparator and display means for recording and displaying information, a time circuit 22, a waveform generator 23, and a power supply for all units 21 -23.

The circuit has the capability of providing measurement of sensor signals over a particular range whilst the sensor 4 is undergoing continuous linear or stepped potential sweeping. The advantage of such a circuit is that conditioning of the sensor electrode is maintained for example by continuous cleaning and to some extent the activation of working electrode surface in the "bare" metal type of sensor as illustrated in Figure 2A and a reactivation of working electrode in the alternative sensor, Figure 2B. The sample and hold parts of the circuit carry the measured current signal between each sweep and update so that a continuous readout is provided.

In the instance where the device is to be used for prolonged periods of time, it is advantageous to monitor, and indeed control, the functional integrity of the sensing electrodes, to this end a working electrode modifier or reactivator, as illustrated in Figure 5, is provided.

This modifier or reactivator includes the circuitry illustrated below dotted line A. It will be understood that this circuitry may be integrated into any electrochemical sensing device so as to maintain the integrity of a sensing electrode.

Basically the circuitry includes a current integrator 24, logic means 25, potentiostat 26 and ideally a timer 27. It will be apparent to those skilled in the art that the modifier shown in Figure 5 can be used advantageously with the sensor shown in Figure 2A which will be exposed to the sample and therefore to contaminants contained therein.

The modifier of Figure 5 enables the correct catalytic activity to be maintained on the surface of the working electrode 14 -14A. An oxide type layer forms on the metal surface of the working electrode 14 14A with continuous operation, the working electrode itself is oxidised and is passivated. The oxide layer ultimately grows thicker in proportion to the amount of current passed or generated as a signal and at a certain point deactivates the electrochemical oxidation process. In the circuit shown in Figure 5, the current integrator 24 measures the total amount of current flowing through the sensing electrode, circuit logic means 25 then determines what fraction of the total current is responsible for modifying the functional characteristics of the electrode, thus there is determined couiombically the amount of current passed and the amount of oxidation which occurs over an interval of time. At a pre-determined level of oxidation, the potentiostat 26 is activated so that a reverse potential is applied by the second potentiostat 26 and the functional effects are reversed for a pre-determined time interval until the integrity of the electrode is restored or deposited oxide layer removed.

It is to be realised that the components of the embodiment described above are used for nitrate detection, but other species, for example ammonia, ammonium ion and nitrogenous organic species, for example nitrophenol, may also be detected. Moreover, by replacing the sensor 4 with a sensor for the detection of other types of product from other reaction, for example, nitrite giving an azo-dye could be detected. For example, in the instance when aqueous dissolved organic molecules, or nitrate, are to be detected after reduction or helping to produce another product species, the reduced molecule or product could be detected at the

"bare" metal type sensor described in Figure 2A. Further, as previously mentioned with appropriate modification of the solution conditioner 1 and sensor 4, other species such as phosphorous species, azines and DMS can also be measured.

Figure 6 represents an alternative embodiment of the invention which is ideally suited for quick and easy use in-situ. The alternative embodiment comprises a hollow cylindrical housing 1x sized and shaped to accommodate a plurality of disc structures as illustrated in Figure 6B. A first end 2x of housing 1x comprises a removable cap such as a screw cap or a friction fit cap. Thus, manipulation of said cap results in access being gained to the inside of housing 1 x.

Positioned within housing 1x are a plurality of discs arranged in a pre¬ determined manner so as to function in accordance with the invention. At a first inner most end of said housing 1x there is provided a first counter electrode 3x. Positioned adjacent electrode 3x is a reference electrode 4x; adjacent electrode 4x there is positioned a working electrode 5x. It will therefore be apparent that electrodes 3x, 4x, and 5x are positioned in substantially conventional manner. Adjacent electrode 5x there is provided a disc, which ideally is made of filter paper, onto which there has been impregnated palladium hydride powder. This disc represents the hydrogen donating means.

In the instance where the device is to be used to measure the presence and amount of ammonia within a solution, this hydrogen donating means may be omitted.

In addition, in the instance where the device is to be used to measure a species by a reduction reaction at said electrodes this hydrogen donating means may be omitted.

Adjacent this latter disc is a disc made of filter paper and impregnated with a conditioning agent such as trisodium phosphate, or indeed any other appropriate agent/complex as previously mentioned, Optionally, there may be provided within the device a structure representing a capillary rise to facilitate diffusion of a sample through the device. Alternatively, a number of layers of filter paper may be positioned in between each of said above described layers and it will be apparent to those skilled in the art that where this arrangement is provided diffusion of the sample will arise thus ensuring that a sample passes sequentially and progressively through the housing and so through the disc structures.

Conventional circuitry is located towards the inner most end of the housing so as to ensure that detection of a species such as ammonia is represented by an electrical signal.

An advantage of this embodiment of the invention is that, as mentioned, it is easy to use in that it can be simply placed in contact with the sample for quick and easy analysis. Further, the disc, and especially the hydrogen donating means and the solution conditioning means, can easily be replaced as and when necessary.

For economy of operation, the following components of the electrochemical sensor are recommended for short term use, that is up to 8 hours, a solution conditioner 1 ; a reducer of the type 3A (palladium sponge wire); a sensor body as described in Figure 2A (flow through "bare" metal type sensor); a window sampling (Figure 4) or at least a constant potentiostatic control at a suitable fixed potential for ammonia oxidation. In this arrangement, the calibration checks would be made.

For long term use, for example days and weeks or continuous use, the following components are recommended, a solution conditioner as appropriate, this could effectively be in concentrated liquid form provided that space was available for an infuser; a detector preferably if possible as described in Figure 2B (with a semi-permeable membrane); a continuous but periodic hydrogen generator in-situ (10); and use of the circuits as illustrated in Figures 4 and 5.

The method with minor modifications and use of all or some of the components of the system allows for the simultaneous measurement of not only nitrate but also free ammonia, ammonium ion, and nitrite ion and indeed other preselected species as aforein described.

Thus for:

Nitrate components necessary are, solution conditioner to create alkaline conditions, for example Trisodium phosphate or other;

a reducer hydrogen charged palladium wire sponge or electrochemical generator of hydrogen;

mixing coil;

ammonia sensor - bare metal or membrane type.

Dissolved Ammonia no components other than the ammonia sensor of either type.

Ammonium ion solution conditioner to make solution alkaline;

mixing coil;

ammonia sensor of either type.

Nitrite ion solution conditioner, for example, sodium acetate mixed with alpha-naphthylamine and sulphanilic acid in a form for easy dissolution into the sample stream;

mixing coil immersed in a cooling bath (crushed ice) or use of micro electronic coolers (Joule Thompson); sensor employing bare metals;

operation electrochemically via potentiostat etc, of the three electrode type sensor to detect the subsequent azo-dye.

It is inferred that for nitrate, ammonia and ammonium ion the electronic equipment stated would be operated to optimize working electrode conditions and for efficient ammonia oxidation.

Figure 5 gives a schematic only of a possible flow through layout if all four nitrogen-combined species were being required to be simultaneously detected. Other arrangements obviously are possible, for example, where only one sensor is employed and channelled from different sample sources, nitrate, ammonia, ammonium ion, nitrite, nitrophenol etc.

Thus for:

Posphate Components necessary are, a PH conditioner to create acidic conditions as hereindescribed; and a complexing agent as hereindescribed.

Optional a reducer as hereindescribed

(please see page 15)

Mixing coil

Phosphate sensor - bare metal only.

Those skilled in the art will appreciate that measurement by the apparatus of the invention of other species is undertaken by making modifications to the said apparatus of the sort hereindescribed so as to ensure that the appropriate chemical and electrochemical conditions are provided for determining the amount of a preselected contaminant or pollutant in a sample. determining the amount of a preselected contaminant or pollutant in a sample.

The invention therefore provides means for rapid, reliable in-situ sampling and also a means for prolonging the lifespan of such means and indeed many types of amperometric sensor detecting species other than those indicated here. For example carbon monoxide sensors.

Figure 7 shows an alternative embodiment of the device of the invention which is particularly favoured because of its disposable characteristics.

In part A of Figure 7 there is shown a device including electrodes numbered as 1 counter, 2 reference, 3 detector working and 4 hydrogen generating working electrodes. Further, the device of Figure 7 comprises a potentiostat which is switched in and out to provide a controlled output signal proportional to the nature of the species to be measured, for example proportional to the nitrate concentration. Thus, initially, current is passed through either (a)two separate and additional electrodes and, other than the three electrodes in the sensor arrangement. One of these two additional electrodes generates hydrogen, or (b) use of a setting, one of a number, arranged by a switch which causes hydrogen to be generated at one of two working electrodes in the sensor. The other working electrode is only used for detection, of generated or otherwise present species such as ammonia. The counter and reference electrode of the sensor could be used to complete the circuit when the first working electrode is being used to generate hydrogen, or, (c) such a potential is applied to this sensor electrode assembly that hydrogen is generated at the counter, or less satisfactory, at the working electrode. For (a), (b), (c), this first stage mode of measurement would be for a few seconds only. The idea being that time is allowed for the hydrogen generated to diffuse and reduce the NO 3-ion to form free ammonia. At a second state the detector mode is switched in so that for (a) stage 1 is off, or, (b) the second working electrode, ie. the sensor working electrode is switched in, and the first switched out, or, (c) a second potential is switched in via the potentistat so enabling electrochemical oxidation of ammonia to be detected by the sensor electrode assembly. The technical preference probably for stage 1 , variation (b) will be chosen.

The assembly illustrated in Figure 7 could be manufactured as follows. A substrate is masked to allow for laying down silver on various tracks either by say PCB, photo-etch board, with subsequent silver plating on exposed copper in the required areas, or evaporation of silver onto a suitable thin sheet film or an insulator. With ease different variations could be carried out for example, making electrode 3 with a coating of alternative metal for example tracnine coating instead of silver.

In the second part of figure 7, part b, areas 5 are coated with an insulator, for example painting or spraying varnish PTFE, plastic etc and by use of masks. Thereby the area exposed would comprise of the electrodes mentioned and at 8 bare ends for allowing electrical connections to electrodes.

A pre-prepared segment of a porous material for exampleVyon, filter paper etc impregnated and containing the necessary conditioning agent, for example trisodium phosphate for nitrate is then placed over the exposed nitrode area. This segment may be fixed in place by adhesing the corners or left loose as illustrated by reference 9.

The electrode assembly is now ready for use and may be inserted into a suitable design of instrument whereby in the arrangement electrical connections to the electronic interface within. The connection being held in place via c clip etc. A drop sample is then added through a well in the instrument body and onto the porous segment. This segment could be exposed through a hole in the instrument case to form small wells. The instrument is then switched on and the previously mentioned stages 1 & 2 proceed for measurement.

Clearly for simultaneous measurement of nitrate, ammonia, ammonium, ion and nitrite several electrode assemblies could be accommodated on the same unit by miniaturisation or by exposing the several different surface areas at the disposable electrode assemblies by the use of a movable cover or slide on the instrument case. Alternatively, the different electrode assemblies for each different determination. Moreover it is possible with the current sequence of determinations to provide different assemblies for determining different species.

It will be apparent to those skilled in the art that the assembly illustrated in Figure 7 may be modified, in accordance with the information presented herein and/or common general knowledge to provide an assembly adapted to measure a preselected contaminant or pollutant.

The invention therefore concerns the device for use in determining, in situ, a preselected ion species, and ideally the device is a simple and more preferably disposable device for said determination.

Claims

1. A device for use in determining the amount of contaminant or pollutant in a liquid sample comprising: a solution conditioning means for changing the chemical properties of said liquid in a manner that enables the presence of a preselected species to be determined; a reducing means for reducing said preselected species; and a sensor for measuring the amount of said species so as to determine the amount of said contaminant or pollutant in said liquid; and further wherein said sensor is a electrochemical cell adapted to measure the amount of said species.

2. A device according to Claim 1 wherein said reducing means and sensor comprise separate units.

3. A device according to Claims 1 & 2 wherein said reducing means comprises a hydrogen donating means.

4. A device according to Claim 3 wherein said hydrogen donating means comprises a hydrogen adsorbing agent.

5. A device according to Claim 4 wherein said agent is a hydrogen adsorbing structure or lattice.

6. A device according to Claim 4 wherein said structural lattice is made of a transition metal.

7. A device according to Claim 6 wherein said metal is palladium.

8. A device according to Claim 6 wherein said metal is zirconium.

9. A device according to Claim 7 wherein said palladium is in the form of a wire, sheet, gauze or mesh.

10. A device according to Claim 9 wherein said wire is coiled.

11. A device according to Claim 3 wherein said hydrogen donating means comprises an electrochemical means including electrochemical circuitry so arranged as to provide for the generation of hydrogen when current flows via an external circuit.

12. A device according to Claim 11 wherein a solution of dilute sulphuric acid serves as an electrolyte.

13. A device according to Claims 11 or 12 wherein Palladium serves as an electrode.

14. A device according to Claim 11 wherein Platinum and lead are used as electrodes.

15. A device according to Claim 14 wherein the apparatus is adapted for the flow through of a sample.

16. A device according to Claims 2 - 15 wherein a mixing means is provided downstream of the reducing means or hydrogen donating means and upstream of the sensor.

17. A device according to any preceding claim wherein said sensor includes a working electrode, and a reference/counter electrode and the apparatus is adapted such that sample liquid flows in the direction from the working electrode to the counter electrode.

18. A device according to any preceding claim wherein a semi-permeable membrane is provided so as to isolate the sensor, and particularly the sensor electrodes, from the sample stream.

19. A device according to any preceding claim wherein said solution conditioning means is provided upstream of the reducing means and sensor.

20. A device according to any preceding claim wherein electronic means for maintaining the potential of the working electrode with respect to the reference electrode is provided and a measurement of the current generated by an electrochemical reaction is detected.

21. A device according to any preceding claim wherein there is further provided wave from generator.

22. A device according to any preceding claim wherein there is further provided an electrode conditioner.

23. A device according to any preceding claim wherein there is further provided an apparatus for modifying an electrochemical surface of said sensor characterised in that the apparatus comprises: a potentiostat means for generating a selected potential at said surface; a current integrator for determining the total amount of current passing through the sensor; a logic unit for assessing the amount of said total current represented by the electrode reaction so as to calculate the amount of modifying current that modifies the functional characteristics of said surface whereby said potentiostat means is activated on detection of a pre-programmed modifying current value and provides a reverse potential at said surface, with respect to said modifying current, for a set time period until the current flowing through the surface substantially restores the original functional characteristics of said surface.

24. A method for modifying an electrochemical surface of a sensor according to any preceding claim, characterised in that it comprises: generating a selected potential at said surface; determining the total amount of current passing through said sensor; assessing the amount of said total current represented by the electrochemical reaction so as to calculate the amount of modifying current that modifies the functional characteristics of said surface; generating a second selected potential at said surface in response to the detection of a pre-programmed modifying current value; wherein said second selected potential is reversed with respect to the aforementioned selected potential, for a set period, until the current flowing through the said surface substantially restores the original functional characteristics of the surface.