Note: Descriptions are shown in the official language in which they were submitted.
2~8~7
95/00842 PCTIGB94/01319
ELECTROCHEMICAL SIENSOR
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 1or 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 b-e used in a variety of ways such
as, for example, maintaining the integrity ol 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 maintai
its functional integrity and so ensure maintenance of sensitivity of a
measuring device.
Nitrate, nitrite, ammonia ions are distributed at various levels throughout
tl1e 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
wo 95,00842 ~ 7 --2-- PCT/GB94/01319 ~
and where possible, is given to ensure minimum ievels 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 increasir,g 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
16 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
26 succes~rui. 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.
2 ~ 2 7
2 A
Devices and methods for measuring, for exarnple, nitrogen based ~species, ~such as ammonia,
may typically comprise, in their simplest forrn, electrodes. Such devices are so simple they
can only be used to measure species which are already conditioned, reduced and electrolytic.
A number of docl.m~nts, namely, JP 58 047 254 and US 4,166,775 and US 4,842,697 relate
to the detection of species using electrochemical methods in particular ion selective electrode
methods only and respectively: a method of quantifying ~mmmoni~c~l nitrogen in an
electrolyte; an electrochemic~l mo~ o~ g method for oxidizable gases; and, a method of
determining ~mm-~ni~ in a gas or liquid in an initially ~mmonil-m free electrolyte. None of
these documents indicate how a preselected sample species may be prepared, by cr)n~1itioning
1 0 and/or reduction prior to detection.
Other doc~.m.ont~ describe the conversion of a species into a detectable form, prior to transfer
to another piece of kit for determin~tic.n One such docunnent is US 3,616,273 in which
nitrogen is physically converted to gas prior to hydrogenation and subsequent d~L~ lion
of the ammonia by coulometric titration using an ion selective electrode technique. Another
1 5 such document is US 3,461,042 in which nitrogen is catalytically converted into ammonia,
followed by an ion selective electrode method involving electro~h~-mis~l titr~tion of the
~mmoni~ A furt-h-er document is JP 60 100760 wherein phosphor or sulphur are collvelled
to hydride for subsequent detection In ~d-lition DE 2 9~32 268 discloses reduction of
hydrogen cyanide to provide arnmonia, which is then detected.
Also JP 54 134693 discloses a method of de(~llllil.il~g inorganic nitrogen by conve~ion to
ammonia by passing the nitrogen through a column of base and reducing agent using a
hydrogen carrier at 50-300 C and subsequently determining t]he concentration of ~mmnni~ by
electroch~mic~l method and in particular an ion selective electrode method. None of these
documents disclose a device wherein samples can be rapidly and relatively det~rmined in-situ.
Indeed, the teaching of these documents particularly when viewed as a whole strongly
in(lic~tes that conversion to detectable form and detection are viewed in the art as distinct and
hMEi~DED SHEET
~,
separate problems, which problems are solved by distinct and sep~dte pieces of kit. This is
especially so since a nurnber of dirre.~ , occ~sion~lly hlter~ geable, physical and ch~mic~l
ways of conversion into a detect~ble species, are suggested in the art and a number of
methods of detection are also suggested. Many of these methods are based upon ISEs. Thus
5 the above methods involve mnlti~tep processes which steps are remote in time and geometry
and based on different specific principles.
A~.~ElID~ SHEET
95100842 ~ 2 7 PCT/GB94/01319
--3--
There are two conventional methods of analysis for the detection of the
nil;rate ion. One method, involves the reciuction 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
concenlr~lion of nitrophenol is determined by a current peak height
generated via its formation or destruction as a result of an electrochemical
1 5 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 subsPnces
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,
ral:her, 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 c:ommercially available tests
WO 9S/00842 ~ 8 2 7 PCT/GB94/OL~l9
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
6 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 c~lcul~te phosphate concentration.
Unfortunately these tests are unreliable bec~ se 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 dimethylsulphide 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 i, ILeyriL~ 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 bec~use, over time, a
2~ modification of the electrode surface occurs. Specifically, an electrodesurface 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 s~ f~;lory 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
~1~5~27
~jO 95/00842 PCT/GB94/01319
surface so as to increase the life effectivenes;s of a sensor embodying the
01ectrode.
According to the invention there is therefore provided a device for use in
cletermining 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
25 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.
2~ ~827
WO 95/00842 PCT/GB94/01319
--6--
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 preferabiy 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 eiectrode, for example platinum, and connected to
a DC power supply allowing the p~s~ge 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.
2 ~
~0 95/00842 PCT/GB94101319
--7--
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 elec:trode. 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 llow 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
SellSOr SO that the sample p~-~ses 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 l:he art will appreciate that at a low pH the Nl13 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
WO 95/00842 ~ 7 ~ ~ ~ 2 ~ PCT/GB94/OL~l9
--8--
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 skiiled 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
2~ 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.
2 ~
~0 95/00842 PCTIGB94/01319
_g_
l~he 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
5 lifetime of the electrodes is extended.
In a further aspect of the invention preferably l:he aforementioned device is
provided with an apparatus for modifying an ~lectrochemical suRace 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 ~ssessing 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 detectiion 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 suRace 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 suRace and
particularly on an electrode within a sensor is removed. Such an
apparatus is of particular importance when an elec-~rochemical 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;
WO 95/00842 2 ~ 2 ~ PCTtGB94/01319
-10-
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-proy,ar"nled 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 sllhst~ntially 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 di~d~"r"atic 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.
~) 9~/00842 PCT/GB94/OL319
-1 1 -
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 exampie, conditioner 1 will be
modified to provide the appropriate pH conditions of a species to b
measured. In some instances a conditioner 1 may not be used, that is to
say it may not be nece.ss~ry 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. Howeve!r, in other instances reducer
2 may be required.
Reducer 2 will comprise either a rechargable palladium wire sponge
housed in a flow through chamber as illustrated in Flgure 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 illu~ dted in Figure 3C. Each of these
hydrogen donating means will be described in greater detail hereinafter.
~educer 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 p~ss~ge of the volatile species to be measured, for example,
ammonia. On the upstream side of said chamber there are provided
wo 95,00842 2 ~ 2 7 --12-- PCTIGB94/01319~
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 inte, r~ce 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 flexibiiity of the device and enable sophistication of operation. The
function of each electronic unit will be described in greater detail
1 5 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 -1 2 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
~jO 95/00842 PCTIGB94101319
-13-
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.
~he difficulties of solvation of aqueous-soluble reagents are well known
and other methods or means may be available for controlled infusion of
concenlrdLed soluble reagent. Some of the methods include casting
r0agent 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
ar1d citric acid or indeed any acidic buffer. In a~ddition, 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
redl~ced 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
WO 95/00842 PCT/GB94/01319 ~
2 ~ -14-
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 s~ample 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 ele~,-l,uchemistry in which palladium
wire is made negative with respect to an inert electrode and connected to
a DC power supply allowing the p~s~ge 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.
~p 95/00842 2 ~ ~ ~ 8 ~ ~ PCT/GBg4/0l3lg
-15-
For phosphate measurement.
Complex of PO4, 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
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
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.
The working electrode 14 may be made from platinum, silver or gold and
be in the form of wires, rods or flags. The elecl:rode material may also be
deposited on membranes or other substrates. In the case if phosphate
del;ermination the working electrode may also in addition be a
molybdenum coating on the above substrates.
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 chloricle, or from mercuric oxide
or silver oxide with a filling of potassium hydroxide.. These reference
electrodes and other alternatives will be familiar to those skilled in the
field.
wo 95,00842 2 ~ 2 I PCT/GB94101319 ~
-16-
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
processe.s 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 neces.s~ry control of the sensor 4.
The potentiostat 5 and waveform generator 6 maintain the potential of the
working electrode 1 4A with respect to a reference electrode 1 6A 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
~) 9S/00842 ~ ~ 6 ~ ~ 2 7 PCT/GB94/OL~19
-17-
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 ~ccessible 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 inteRace for itsconversion 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 Jn the electronic circuitry at
control inteR~ce 10 to provide current for the r educer 2 by both direct and
periodic electrolysis of the sample flow to generate the necessary
hydrogen. A requirement in this circuit is th~t 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 inteRace (the in-situ generator 10) to have
sufficient power to overcome the electrical resistance of the sample
sl:ream, which could approach mega ohms in very pure water. The
charging of the reducer palladium sponge ~JIearly may be carried out
externally of the system.
WO 9S/00842 ~ PCT/GB94/01319
-18-
Such a circuit in operation is most effective when the sensor is required to
run continuousiy 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 c~p~hility 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
irlteyl ily 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.
2~82~
95/00842 PCT/GB94/OL319
-19-
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
arnount of current p~secl or generated as a signal and at a certain point
deactivates the electrochemical oxidation proc:ess. 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
coulombically 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 poten~ios~e t 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 or~anic 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
nil:rate, 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.
WO 95100842 PCT/GB94/01319--
?~ 7
--20--
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 1 x sized and shaped to
accommodate a plurality of disc structures as illustrated in Figure 6B. A
first end 2x of housing 1 x comprises a removable cap such as a screw cap
or a friction fit cap. Thus, manipulation of said cap results in ~cess 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 fiiter 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
2~ 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,
95/00~42 2 ~ 2 7 PCT/Gs94/~ s
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
6 those skilled in the art that where this arrange!ment 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, cmd especially the hydrogen
donating means and the solution conditioning means, can easily be
replaced as and when necess~ry.
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 t,vpe 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.
l he method with minor modifications and use of all or some of the
components of the system allows for the sirnultaneous measurement of
wo gs/00842 ~ 7 --22-- PCT/GI~94/01319 --
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.
nissolved Ammonia no components other than the ammonia sensor
of either type.
Ammonium iQn solution conditioner to make solution alkaline;
mixing coil;
ammonia sensor of either type.
Nitrite i~ 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);
~1 95t00842 ~ ~ ~ 5 ~ 2 7 PCT/GB94/01319
--23--
sensor employing bare metals;
operation electrochemicaily 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 necess~ry are, a PH conditioner
to create acidic conditions as hereindescribed;
and a complexing a.gent 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
- 25 determining the amount of a preselected contaminant or pollutant in a
sample.
WO 95/00842 PCT/GB94/01319 ~
2 7
--24--
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 altemative embodiment of the device of the invention
which is particularly favoured bec~use 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 s~ rdctory, 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.
~0 9~;/00842 ~16 ~ ~ 2 ~ PCT/GB94/01319
--25--
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 subse~lent 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 altemative metal for
example tracnine coating instead of silver.
In the second part of figure 7, part b, areas 5 are coated with an insulator,
~or example painting or spraying varnish PTr-E, 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 segm~snt. This segment could be
exposed through a hole in the instrument ciase 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
l:he 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
WO 95/00842 2 ~ 2 7 PCT/G~94/OL~l9--
-26-
different electrode assemblies for each diKerent 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 inFigure 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.
