GB2503689A

GB2503689A - Phosphate Detection

Info

Publication number: GB2503689A
Application number: GB201211855A
Authority: GB
Inventors: Barry Jones
Original assignee: COMPACT INSTR Ltd
Current assignee: COMPACT INSTR Ltd
Priority date: 2012-07-04
Filing date: 2012-07-04
Publication date: 2014-01-08
Anticipated expiration: 2032-07-04
Also published as: GB201211855D0; GB2503689B

Abstract

A phosphate sensor 1 for sensing phosphate in solution 2 comprises a first electrode 3 with a cobalt surface immersed in the solution to generate an electrical potential, and a second electrode 4 electrically connected to the first electrode comprising a non-cobalt surface. Voltage source 11 is arranged to apply a voltage between the electrodes such that the electrical polarity of the first electrode is negative relative to that of the second electrode, thus maintaining a protection voltage to prevent cobalt oxidation. Reference electrode 5 is arranged for immersion in a reference solution to generate a reference electrical potential. Switch 12 is operable selectively to connect the reference electrode to the first electrode. An electrical potential difference between the first electrode and the reference electrode is detected when connected by the switch. A detection signal is output indicative of a detected phosphate concentration level according to the detected potential difference.

Description

Phosphate Detection The invention relates to the detection of phosphates and apparatus for phosphate detection. In particular, though not exclusively, the invention relates to detection of phosphates in solution.

Agricultural fertilizers commonly contain phosphates which may enter the water table through overuse or natural weathering processes. This can result in an overabundance of phosphate in natural bodies of water such as oceans, lakes and rivers. Similarly, there exists an association between algae and phosphate concentration levels in swimming pools. It has been suggested that problems of phosphates in oceans and groundwater has crossed over to swimming pools as well. This causes problems for swimming pool owners in the form of excessive, stubborn algal blooms and rapid chlorine consumption (additive to water) to try to combat it. Removal of phosphate is important to maintain good water quality. Once phosphates have been removed, it is important to maintain a low phosphate level. This will assist in cleaner water, improved chlorine performance and less algae. Phosphates enter swimming pool water from a variety of sources including dust and rain, runoff from lawns and gardens, chemicals added to the water, and leaves. Over time, this causes an increase in phosphate concentration.

Small amounts of algae attached to dust, leaves and the like are introduced into pools into pools continually. Nutrients like phosphate, together with sunlight permit algae to grow rapidly and become a threat to pool sanitisation due to a more rapid destruction of chlorine.

Chlorine is a reasonably effective but short-lived algaecide. Accurate long-term monitoring of phosphate levels in bodies of water is necessary to permit early warning of raised phosphate levels and the consequent raised levels of algae.

Some existing phosphate detectors employ a Cobalt sensing electrode and a detection mechanism based on the detected changes in electrical potential generated at the Cobalt electrode in response to oxidation reactions involving phosphates. In particular, when a Cobalt electrode first makes contact with water, a cobalt oxide film is formed on surface of the electrode. Subsequently, multiple reactions occur between Cobalt oxide and phosphates and a cobalt phosphate layer is formed.

3CoO + 2H2PO41+ 2H± 4 Co3(P04)2 + 3H20 3CoO + 2H2PO42+ H2O 3 Co3(P04)2 + 4Oft 3CoO + 2H2P043 + 3H20 4 Co3(PO4)2 + 6OH These reactions generate an electrical potential which, when measured relative to a constant reference potential is found to be proportional to the concentration of phosphate responsible for the reactions. In other words, the cobalt is found to exhibit a potentiometric-type response when exposed to phosphates in solution. However, the sensitivity of the Cobalt electrode falls rapidly over time as the reactions progress and Cobalt oxides build-up on the surface of the electrode reducing the reaction rate. The longer the Cobalt electrode is exposed to this reaction the less sensitive it becomes, until it losses all ability to act as a sensor. For this reason, such sensor ear only used for short periods and are not suitable for long-term monitoring in-situ.

The present invention aims to address these deficiencies.

In a first of its aspects, the invention may provide a phosphate sensor for sensing a phosphate in a solution comprising a first electrode with a Cobalt surface part arranged for immersion in the solution to generate an electrical potential, a second electrode electrically connected to the first electrode and comprising a surface part other than Cobalt arranged for immersion in the solution concurrently with the immersion of the first electrode, a voltage source for applying a voltage between the first and second electrode and a reference electrode for immersion in a reference solution to generate a reference electrical potential. Voltage terminals of the voltage source may be connected to (or arranged to be selectively connectable to) the first and second electrodes such that the negative voltage terminal is connected to the first electrode and the positive voltage terminal of the voltage source is connected to the second electrode. In this way, polarities of the electrical potentials applied to the first and second electrodes may be negative and positive respectively. The sensor may comprise a switch operable selectively to connect electrically the reference electrode to the first electrode. Preferably, a voltage sensor (e.g. a voltmeter) is arranged to sense an electrical potential difference between the electrical potentials of the first electrode and the reference electrode when electrically connected to the first electrode by the switch, and to generate an output value according to the sensed potential difference. The sensor may include a detector (e.g. a microprocessor) arranged to receive the output value and to generate a detection signal indicative of a detected phosphate concentration level according to the output value. The sensor may be an in-situ sensor arranged for long-term or permanent immersion in, and contact with, the solution being sensed.

The exposed surface part of the second electrode preferably comprises any electrically conductive material, examples include Steel, or Carbon, or optionally any other suitable conductor.

The first electrode preferably is arranged to act as a cathode and the second electrode is preferably arranged to act as an anode in conjunction with the cathode.

The phosphate sensor preferably includes a control unit arranged to control the switch unit to connect the reference electrode to the first electrode temporarily for operation of the voltage sensor in sensing the potential difference, and to maintain the first electrode electrically disconnected from the reference electrode otherwise.

The control unit may be arranged to periodically connect the reference electrode to the first electrode temporarily.

The phosphate sensor may comprise and electrode support member upon which the first and second electrodes are supported and at a surface of which the surface parts of the first and second electrodes are exposed.

The electrode support member may comprise electrically insulating material which electrically insulates the exposed surface parts of the first electrode from the exposed surface parts of the second electrode.

One of the first and second electrodes may be arranged at a surface of the electrode support member so as to surround the other of the first and second electrodes at that surface.

The invention may provide a phosphate monitoring apparatus arranged for monitoring phosphate concentration levels in a body of liquid, comprising a housing containing a phosphate sensor as described above wherein the housing includes one or more liquid access openings for admitting the liquid to contact the first and second electrodes simultaneously.

The detected concentration levels (or the fact of detection) may be conveyed to the user by any one or more of: (a) via an audible alarm on the monitoring apparatus; (b) via a visible alarm on the monitoring apparatus; (c) using a display screen showing the measured level with/without an alarm.

A sensing event may last for a period/duration of between about 10 sec. to about 20 sec. The control unit may be arranged to maintain the switch unit in a closed state for such a period to allow a measurement to be taken. The control unit may be arranged to periodically perform measurements over a preset period. For example, at intervals of 15 minutes for a period of about 12 months.

How often would you envisage making measurements in, say, a swimming pool -once a day automatically, once a week? There now follows an example of an implementation of the invention in one embodiment described with reference to the accompanying drawings of which: Figure 1 schematically illustrates a phosphate sensor; Figure 2 schematically illustrates an in-situ phosphate monitoring apparatus; Figure 3 schematically illustrates a relationship between an electrical potential difference detected by the phosphate sensor and a corresponding concentration of phosphate in a solution being monitored.

In the drawings like items are assigned like reference symbols for consistency.

Figure 1 schematically illustrates a phosphate sensor (1) according to an embodiment of the invention for sensing a phosphate concentration level within a body of water (2) such as a swimming pool, a pond or a river or lake. The sensor comprises a first electrode (3) composed of metallic Cobalt (Co) for immersion in the body of water being sensed, and a second electrode (4) composed of any conducting material (e.g. metallic stainless steel or Carbon, or other) also arranged for immersion in the body of water concurrently with the first electrode. The first and second electrodes are electrically connected to each other via electrical conduction lines (e.g. wires or strips, 9 and 10). A voltage supply and voltage measurement unit (11) is arranged to apply a small reverse voltage across the first and second electrodes (11), such that the negative terminal of the voltage supply is connected to the first electrode and the positive terminal of the voltage supply is connected to the second electrode.

As a consequence of the voltage applied between the first and second electrodes, the first electrode adopts a negative polarity (becomes a cathode) and the second electrode adopts a positive polarity (becomes an anode) when the two electrodes are concurrently immersed in the body of water. It has been found that this arrangement of polarities occurs when the active/immersed surface of the second electrode is formed from stainless steel or Carbon.

As a consequence of this polarity, the chemistries which result in the formation of cobalt-phosphate-oxide complexes on the first electrode and that are responsible for the build up of surface deposits are prohibited. The applied voltage is hereafter referred to as the "protection" voltage.

The consequence of this control of electrode polarities is that the build-up of Cobalt oxides at the surface of the first electrode is greatly inhibited and the sensitivity of the immersed Cobalt surface parts of the first electrode to the presence of phosphates in the water, is maintained or protected.

The phosphate sensor includes a reference electrode (5) electrically connected or connectable to the first electrode (3) via a switch unit (12). One switch contact of the switch unit is in electrical connection with the first electrode via an electrical conduction line (e.g. wire or strip, 9) and the other switch contact of the switch unit is electrically connected to the reference electrode via another electrical conduction line (e.g. wire or strip). The switch unit is controllable to open and close such that in the open state the first electrode is electrically isolated from the reference electrode, and in the closed state the first electrode in electrically connected to the reference electrode.

The reference electrode comprises a metallic Silver (Ag) electrode surface immersed within a quantity of Silver Chloride AgCI (6) contained within container (7). The reaction of the Silver reference electrode with the Silver Chloride within which it is immersed generates an electrical potential at the reference electrode which serves as a substantially constant reference electrical potential. It should be noted that this reference electrode system is merely an example, and one of a number of different alternative existing reference systems may be used such as would be readily appreciated by the skilled person. The container (7) of the reference electrode of Figure 1 is shown as not immersed in the body of water, however it may be so immersed if design preferences require.

The voltage supply and voltage measurement unit (11) has a voltage input terminal electrically connected to the electrical conduction line (9) connecting the first electrode to the switch unit and is operable to measure the electrical potential difference (voltage) between the first electrode and the reference electrode when the switch unit is in the closed state -whereby the reference electrode is electrically connected to the first electrode.

A detector unit (13) may be a microprocessor provided in the sensor and is connected to both the switch unit (12) and the voltmeter (11). The detector unit is arranged (e.g. programmed) to control the switch unit maintain an open state in the quiescent state such that the reference electrode is not connected electrically to the first electrode, and to periodically and momentarily control the switch unit to close to electrically connect the reference electrode and the first electrode. Concurrently, the detector unit (e.g. microprocessor) is arranged to receive a voltage reading output from the voltmeter representing the voltage across the first electrode and the reference electrode when the switch unit is in the closed state. The voltage reading represents a value of the potential difference between the electrical potentials of the first electrode and the reference electrode.

The detector unit (e.g. programmed microprocessor) is arranged to generate a detection signal indicative of a detected phosphate concentration level according to the voltage reading and to output the signal as an output which may be an electrical signal conveying information (e.g. the concentration level, or that the concentration level exceeds a threshold) or/and a visual/acoustic alarm signal. In particular, the detector unit is arranged to convert the received voltage reading into phosphate concentration reading according to a conversion relation of the form: Concentration = ax cxp(-b x V) moles Where a and b are constants and V is the value of the voltage reading (volts) from the voltmeter representing the electrical potential of the first electrode relative to the reference electrode when the switch unit is closed, momentarily. This relation is in accordance with the Nernst Equation as will be readily appreciated by the skilled person in this art, a schematic example of which is illustrated in Figure 3.

The conduction lines (e.g. wires) from the first, second and reference electrodes may, for example, be connected to a PCB (printed circuit board) embedded within the sensor. The PCB may contain an operational amplifier to amplify and buffer a high impedance voltage signal obtained from the electrodes, and a microprocessor chip arranged to control both the protection voltage supplied across the first and second electrodes, and the switch unit. The switch unit may comprise a Single Pole Single Throw (SPST) switch integrated circuit (IC). The microprocessor may also be arranged to process the buffered voltage generated across the first and reference electrode when the switch is closed, and to convert this voltage reading into a phosphate concentration reading according to a conversion relation in the form given above.

When the switch unit is momentarily closed, the protection voltage is removed and the first electrode is no longer polarised. Consequently, deposit-forming reactions may occur at the Cobalt surface of the first electrode during this time.

By only momentarily closing the switch unit to connect the first electrode to the reference electrode or sufficient time to permit the voltage reading to be generated for the sensor unit, the duration of this condition is minimised and so too is the extent of oxidation-type reactions at the Cobalt surface of the first electrode. Once a voltage reading is generated and received by the sensor unit, the sensor unit is arranged to control the switch unit to return to the open state thereby disconnecting the first electrode from the reference electrode and returning the polarity of the first electrode to its negative quiescent state by reapplying the protection voltage across the first and second electrodes. In this way, the build-up of Cobalt oxides at the exposed Cobalt surface of the first electrode is minimised and the sensitivity of the phosphate sensor is maintained and protected.

Figure 2 schematically illustrates a phosphate monitoring unit according to an embodiment of the invention, containing a phosphate sensor according to Figure 1. A tubular housing (20) possesses a fluid inlet opening (21) providing direct fluid communication to the bore of the housing which contains a sensing electrode unit (15), a reference electrode unit (18) and a control assembly (19) e.g. containing the microprocessor, op-amp and IC switch. In common with other potentiometric sensors, the reference electrode contains a liquid junction membrane which provides an electrical connection between the reference media and the sampling solution.

The sensing electrode unit comprises a cylindrical member attached to and extending from the control assembly. The sensing electrode unit is formed from an electrically insulating material (17) such as plastic in the centre of a terminal flat end of which is embedded a Cobalt electrode (the first electrode) such that a circular/disk electrode end surface of Cobalt is exposed at the terminal end of the cylindrical member. The conduction line (9) which electrically connects the first electrode to the switch unit (12) passes axially along and within the cylindrical member to the control assembly which contains the switch unit. An annular second electrode of stainless steel or Carbon (4) is also embedded within the flat terminal end of the cylindrical member to expose a circular electrode ring surface circumscribing the exposed first electrode surface centred within the ring and being electrically separated therefrom by the insulating material (17) of the sensing electrode unit. The conduction line (10) which electrically connects the first electrode to the second electrode passes along and within the cylindrical member to the control assembly. The reference electrode and reference media (e.g. vessel of Silver Chloride) is housed within a reference electrode unit (18) attached to the control assembly.

The switch unit (12), voltmeter (11) and detector unit (13) (e,g, collectively comprising the PCB containing the microprocessor, op-amp and switch) are all housed within the control assembly (19) and are electrically connected, and operable, as described above with reference to Figure 1.

The phosphate monitoring unit may be located within a body of water, such as a swimming pool, a pond or a lake/river, either temporarily or permanently by placing the sensor unit within the water to be monitored such that the exposed electrode surfaces of the first and second electrodes (3, 4) are immerse simultaneously in water which enters the sensor unit via the fluid inlet opening (21).

It will be appreciated that the above embodiments are intended as illustrative and non-limiting examples to aid understanding, and that modifications, variants and adaptations such as would be readily apparent to the skilled person are intended to be encompassed by the invention as defined by, for example, the appended claims.

Claims

CLAIMS: 1. A phosphate sensor for sensing a phosphate in a solution comprising: a first electrode with a Cobalt surface part arranged for immersion in the solution to generate an electrical potential; a second electrode electrically connected to the first electrode and comprising a surface part other than Cobalt arranged for immersion in the solution concurrently with said immersion of the first electrode; a voltage source arranged to apply a voltage between the first electrode and the second electrode such that the electrical polarity of the first electrode is negative relative to the electrical polarity of the second electrode; a reference electrode for immersion in a reference solution to generate a reference electrical potential; a switch operable selectively to connect electrically the reference electrode to the first electrode; a voltage sensor arranged to sense an electrical potential difference between the electrical potentials of the first electrode and the reference electrode when electrically connected to the first electrode by said switch, and to generate an output value according to the sensed potential difference; a detector unit arranged to receive said output value and to generate a detection signal indicative of a detected phosphate concentration level according to the output value.
2. A phosphate sensor according to any preceding claim in which the exposed surface part of the second electrode comprises an electrically conductive material.
3. A phosphate sensor according to any preceding claim in which the first electrode is a cathode and the second electrode is an anode.
4. A phosphate sensor according to any preceding claim including a control unit arranged to control said switch unit to conned the reference electrode to the first electrode temporarily for operation of said voltage sensor in sensing said potential difference, and to maintain the first electrode electrically disconnected from the reference electrode otherwise.
5. A phosphate sensor according to claim 4 in which the control unit is arranged to periodically connect the reference electrode to the first electrode temporarily.
6. A phosphate sensor according to any preceding claim comprising and electrode support member upon which said first and second electrodes are supported and at a surface of which said surface parts of said first and second electrodes are exposed.
7. A phosphate sensor according to claim 6 in which said electrode support member comprises electrically insulating material and said first electrode is separated from said second electrode by said insulating material.
8. A phosphate sensor according to any of claims 6 to 7 in which one of said first and second electrodes is arranged at said surface of the electrode support member so as to surround the other of said first and second electrodes.
9. A phosphate monitoring apparatus arranged for monitoring of phosphate concentration levels in a body of liquid, comprising a housing containing a phosphate sensor according to any preceding claim wherein the housing includes one or more liquid access openings for admitting said liquid to contact said first and second electrodes simultaneously.
10. A method for sensing a phosphate in a solution comprising: immersing a first electrode with a Cobalt surface part in the solution to generate an electrical potential; immersing a second electrode in the solution concurrently with said immersion of the first electrode being electrically connected to the first electrode and comprising a surface part other than Cobalt; applying a voltage between the first electrode and the second electrode such that the electrical polarity of the first electrode is negative relative to the electrical polarity of the second electrode; providing a reference electrode immersed in a reference solution to generate a reference electrical potential; selectively to connecting electrically the reference electrode to the first electrode; with a voltage sensor sensing an electrical potential difference between the electrical potentials of the first electrode and the reference electrode when electrically connected to the first electrode, and generating an output value according to the sensed potential difference; with a detector unit receiving said output value and generating a detection signal indicative of a detected phosphate concentration level according to the output value.
11. A phosphate sensor substantially as described in any one embodiment hereinbefore with reference to the accompanying drawings.
12. A method of monitoring phosphate substantially as described in relation to any embodiment herein, with reference to the accompanying drawings.