WO2006085761A1

WO2006085761A1 - A reference electrode, a membrane module for use therein, and a method for fabricating same

Info

Publication number: WO2006085761A1
Application number: PCT/NL2006/000071
Authority: WO
Inventors: Frans Mulckhuijse
Original assignee: HYDRION BV
Current assignee: HYDRION BV
Priority date: 2005-02-14
Filing date: 2006-02-13
Publication date: 2006-08-17
Anticipated expiration: 2007-08-14
Also published as: NL1028264C2

Abstract

The invention relates to a reference electrode (1) comprising a ceramic membrane module (6) , wherein the ceramic membrane module (6) is fabricated of partly interconnected ceramic particles forming a porous membrane, and wherein the particles have a substantially identical diameter of X nm, wherein X ranges from 40-450 nm, and wherein preferably 60% of the particles, preferably at least 75%, more preferably at least 90%, and still more preferably at least 99%, have a diameter in the range from 0.9 * X - 1.1 * X, more preferably from 0.95 * X - 1.05 * X, still more preferably from 0.99 * X - 1.01 * X.

Description

A reference electrode, a membrane module for use therein, and a method for fabricating same

The invention relates to a reference electrode, comprising a ceramic membrane module fabricated of partly interconnected ceramic particles forming a porous membrane, wherein a first surface of the ceramic membrane module is in contact with a fluid in the reference electrode and a second surface can be contacted with a fluid to be analysed.

The present invention further relates to a ceramic membrane module for use in a reference electrode . The invention finally relates to a method of fabricating a ceramic membrane module according to the invention .

Measuring electrodes are generally known in the art . From NL 1014256 , for example, an electrode according to the preamble is known. According to that publication, the membrane must at all times comprise a non-polar iono- phoric material , for which reason the outside of the membrane is sealed in order to form an impenetrable barrier for the ionophoric material . The obligatory presence of the ionophoric material and the seal provided for it on the membrane form a disadvantage for the electrode' s fabrication and use .

From DE 103 05 005 , a further electrode is known that works according to the " solid state" principle . For this purpose, a chamber of the electrode contains a solid salt . The solid salt is separated from the fluid to be analysed by means of a membrane . The salt dissolves gradually and exits via the membrane . The solubility rate is very variable and cannot be regulated. This electrode does not provide a reproducible method of measuring. So-called gel electrodes are also known, as described in US 5 071 537 and FR 02 10734. However, the principle applied here differs altogether from the principle applied in the present invention. The use of a ceramic membrane module in a reference electrode is already known in the art . To this end, according to the prior art, a non-porous body of a ceramic material is subj ected to lithographical or micromechanical treatments in order to provide channels through the ceramic membrane . Such a method is described in the international patent application under publication number WO 00/75648. The problem with the prior art membrane module is that the processes are difficult and costly. In order to create through-channels of a consistent shape and dimension, such that the porosity formed in this manner is consistent throughout the entire membrane module, the treatments must be executed with great precision. Moreover, this known membrane module is susceptible to clog- ging. Once a channel is obstructed it is not easily cleared. This seriously impairs the porosity. As a result, the reference electrode behaves unreliably, so that the known reference electrode needs to be calibrated regularly. It is the obj ect of the invention to provide an improved ceramic membrane module.

It is a particular obj ect of the invention to provide a ceramic membrane module that has a consistent porosity. It is a further obj ect of the invention to provide a ceramic membrane module that is only slightly susceptible to clogging.

It is also an obj ect of the invention to provide an improved ceramic membrane module that is inert and easy to clean.

An additional obj ect of the invention is to provide an improved reference electrode.

Finally, the obj ect of the invention is to provide an improved method for the fabrication of a membrane module .

In order to achieve at least one of the above- mentioned obj ectives , the invention provides a reference electrode according to the preamble, characterised in that the fluid in the reference electrode is a calibration fluid that can be fed through the membrane into the fluid to be analysed.

The flow rate at which the calibration fluid can be fed through the membrane into the fluid to be analysed is preferably

(a) adjustable, or

(b) predeterminable.

This may be accomplished, for example, by exerting a pres- sure on the calibration fluid with the aid of, for example, compressed gas or a pump, more specifically a micro- pump .

It is further preferred for the second surface to be at least partly covered by an upper layer that is im- permeable for both fluids, in order to reduce the exchanging surface area. If the surface is partly covered, the surface area permeable for the calibration fluid can be adjusted via the membrane to a desired value, so as to allow an accurate regulation of the flow rate. The invention further provides a ceramic membrane module for use in a reference electrode, characterised in that the particles have a substantially identical diameter of X nm, wherein X ranges from 40-450 nm, and wherein preferably 60% of the particles , preferably at least 75% , more preferably at least 90%, and still more preferably at least 99%, have a diameter in the range from 0.9 -X - 1.1 -X, more preferably from 0.95 -X - 1.05 -X, still more preferably from 0.99 -X - 1.01 -X.

The advantage of the ceramic membrane module ac- cording to the invention is that it is very easy to fabricate, while nonetheless the porosity can be determined accurately. Because the individual ceramic particles are partly interconnected, a three-dimensional porous structure is obtained. The chance of clogging is therefore very slight . The result of consistent porosity and insignificant susceptibility to clogging is a membrane module in a reference electrode that generates a more consistent po- tential , provides a more stable signal , and consequently provides a more precise measurement .

If a ceramic membrane module is preferred that irrespective of its thickness or its dimension has a con- sistent porosity throughout the entire module body, the particles have a substantially identical diameter of X nm, wherein X lies in the range from 40 - 450 nm and wherein preferably 99% of the particles have a diameter in the range from 0.9 -X - 1.1 -X, more preferably from 0.95 -X - 1.05 -X, still more preferably from 0.99 -X - 1.01 -X . If the ceramic particles have such a narrow particle size range, the porosity will be defined very precisely.

In order to accurately determine the porosity, the particles are stacked as close packing, preferably ac- cording to a cubic close packing or a hexagonal close packing, said particles being connected with each other on the contact surfaces . With a 100% ideal packing in accordance with the hexagonal close packing, which provides the closest packing, the porosity is approximately 26% . Pref- erably therefore, the voids content ranges from 26 to

32.5% by vol . , determined according to the mercury bath method.

In general , a ceramic material is inert with regard to most fluids . For the manufacture of a ceramic ma- terial , the usual materials known in the art may be used. Preferred is a starting compound or a combination of starting compounds capable of producing a ceramic material , preferably at least one of an oxide, a carbide, a nitride and a boride, such as one or several of zirconium oxide, aluminium oxide, silicon nitride, silicon carbide, titanium oxide and titanium carbide . All these substances are very wear-resistant and very resistant to low pH values . In addition, all these substances are biologically inert . However, other known starting compounds for the manufacture of ceramic materials can be used just as well . The size of the pores is to a large extent determined by the diameter of the ceramic particles . The diameter of the particles ranges preferably from 40-450 nm, more preferably from 100-400 run. If the diameter X of the ceramic particles is approximately 200 run, the pore size is shown to range from 50-70 run. Such an embodiment is preferred. Other preferable embodiments of the ceramic membrane module according to the invention will become apparent from the following description, as well as from the remaining subclaims .

Finally, the invention relates to a method for fabricating a ceramic membrane module according to the invention, comprising the regular stacking of ceramic particles and the subsequent sintering of the particles so as to obtain a coherent porous mass of ceramic particles that are superficially connected with each other . Such a ce- ramie membrane module is very robust and has a precisely defined pore size . As a consequence, the signal that can be obtained with this reference electrode is very accurate. The chance of the membrane module becoming obstructed is also very slight thanks to the fact that the porosity is oriented three-dimensionalIy, such that alternative feed-through passages for the reference fluid through the membrane module will be guaranteed.

The ceramic membrane module according to the invention is especially very suitable for incorporation into the reference electrode according to the invention .

It is particularly preferred for the membrane module to be fabricated such that it comprises two surfaces , which are disposed substantially parallel to and at a distance from each other . The flow of calibration fluid through the module can then be calculated quite simply. Finally, the invention relates to a method for the fabrication of a ceramic membrane module, comprising the step of covering the second surface with a material that is substantially impermeable to the calibration fluid and the fluid to be analysed, so as to form an upper layer thereon, and of subsequently removing a portion of the applied upper layer so as to form a further to be determined, freely accessible flow-through surface on the mem- brane, wherein the removal is realised by, for example, drilling, milling or the like . The method used for removing a portion of the upper layer preferably affords the possibility of previously determining which surface area is to be made accessible. By applying an upper layer and subsequently removing a portion thereof , a very small flow-through surface is created, thereby significantly restricting the flow-out of calibration fluid. As a result , contamination of the fluid to be analysed is negligible . Hereinafter the invention will be further elucidated by way of the figures .

Fig. 1 shows a schematic cross section of a reference electrode according to the invention.

Figures 2 , 3 and 4 show a detailed view of the membrane module of the reference electrode .

Fig. 5 is a representation of measuring results from a first test using a reference electrode according to the invention.

Fig . 6 shows a representation of measuring re- suits from a second test using a reference electrode according to the invention.

For example, the entire external surface of the membrane may be covered with a paste (optionally to cure) followed by drilling or milling a small hole into it . In this way a surface of a known diameter is obtained. Because the membrane has possibly been drilled, it is possible that the surface of the membrane at that hole is no longer exactly parallel to the other surface of the membrane . Nonetheless , a membrane fabricated in this manner is also considered to have two substantially parallel surfaces .

Identical reference numerals in the various figures have the same meaning. To make it easier to understand the invention, parts not necessary to the invention are not illustrated. Therefore the figures represent only a general picture of a reference electrode .

Fig. 1 shows a reference electrode 1 , which is comprised substantially of a housing 2 and a chamber 3 substantially enclosed by the housing 2. The chamber 3 contains a reference fluid 4. The chamber 3 also contains an electrode 5 , which is connected via a first end of the sensor 1 with an electrical connection 7 for conducting an electric signal . The second end of the sensor 1 is provided with and substantially closed off by a membrane module 6. Fig. 1 also shows a pump 8 for pumping a reference fluid from a storage vessel 9 into the chamber 3 of the reference electrode . Hereinafter the use of a reference electrode 1 as shown in Fig. 1 will be described in more detail .

A reference electrode 1 as shown in Fig. 1 , is immersed in a fluid to be analysed such that the part of the sensor 1 with the membrane module 6 is in the fluid. The pump 8 pumps a small amount of reference fluid from the storage vessel 9 into the chamber 3 , which chamber 3 was already completely filled with reference fluid 4 , such that a continuous stream of reference fluid 4 is pumped from the chamber 3 through the membrane module 6 into the fluid to be analysed. Said amount of reference fluid being pumped through the membrane module 6 is very small , such that the amount of contamination from the fluid to be analysed is undetectable . For the rest, the reference electrode 1 shown in Fig. 1 works exactly in the manner known in the art . The membrane module 6 should be in communication with the housing in such a way that all the outflowing fluid passes through the porous membrane module 6 and not along the membrane module 6.

The pump system 8 provides for a constant and ad- justable pressure with which the reference fluid is fed through the ceramic membrane . This makes it possible to produce a very consistent and regulated continuous outflow and thus to generate a very stable electrical signal . Instead of a pump, for example a micropump, the constant pressure may also be produced by means of a propellant, optionally contained in a balloon-like chamber, in the sensor . There are different ways of connecting the ceramic membrane module according to the present invention with the reference electrode . The Figures 2 , 3 and 4 show different detailed views of possible forms of connecting the ceramic membrane module with the housing 2 of the reference electrode 1. Fig. 2 shows a combination wherein one end of a substantially cylindrically shaped housing 2 of the reference electrode 1 is provided with a recess into which the ceramic membrane module 6 fits . The ceramic mem- brane module may, for example, be connected with the housing 2 by means of adhesion using, for example, glue or the like .

Fig . 3 shows an alternative embodiment, wherein the ceramic membrane module 6 is connected with an inner wall surface of the housing 2 along a circumferential surface 10. Here also , the connection may be provided, for example, by means of adhesion.

A third alternative embodiment is shown in Fig . 4 , wherein the ceramic membrane module 6 is initially in- serted into a recess in the end of the housing 2 , substantially in a manner identical to that shown in Fig.2 , but wherein the membrane module 6 is positioned by a screw- detachable coupling member 12 , connected by means of thread 11 with the housing 2. An alternative manner of fabricating the ceramic membrane module entails the moulding or extrusion of ceramic particles . In that case the particles will probably not be placed in accordance with by approximation ideal close packing, for example, a cubic close packing or a hexagonal close packing. However, in cases where such a precise definition of the pore size is not necessary, a method of this kind provides an inexpensive alternative .

Especially if a precise definition of the pore size, and thus a by approximation ideal stacking of the particles is desired, the ceramic particles are preferably packed by means of sedimentation and are subsequently sintered. From the detailed view in Fig. 2 may be inferred that the membrane module 6 is provided with an upper layer 13. In this way at least part of the surface of the membrane module is protected against contact with the fluid to be analysed. The upper layer is not permeable to the two fluids . A small opening 14 in the upper layer 13 provides a contact surface between the membrane module and the fluid to be analysed, so as to make analysis possible . The upper layer to be applied on the membrane module may be provided with this opening beforehand, but it is also possible to apply an upper layer on the membrane module and to provide an opening afterwards . The opening in the upper layer may be provided approximately in the middle of the membrane module, but a more lateral position is also possible . It is also possible to provide several openings in the upper layer. It is then necessary to ensure that reference fluid 4 is able to flow from the chamber 3 through the membrane module 6 into the fluid to be analysed. Due to the reduction of the flow-through surface of the membrane module, and hence of the exchanging surface, it is possible to perform a more accurate measurement than with a membrane module that is not covered. The size of the opening is preferably smaller than 5 mm² , more preferably smaller than 2 mm² , and still more preferably smaller than 1 mm² , and even still more preferably smaller than 0.2 mm². This also allows a substantial reduction of the flow rate of the reference fluid to the fluid to be analysed, so that there is less effect on the fluid to be analysed. An additional advantage is that the reference electrode itself may be very robust while providing a very accurate measurement .

A table below shows an overview regarding a number of ceramic particles . AKP stands for a registered trademark of the Sumitomo Company (Japan) . The diameter of the particles mentioned relates to at least 99% of the total of particles present . All these particles are found in the range mentioned in the next column. The last column gives the pore size to be obtained with ideal packing.

Fig . 5 shows a test result obtained with a reference electrode according to the present invention, wherein measurement commences at point A; at point B fresh refer- ence fluid was fed into the storage vessel 9 , the composition differing somewhat from the original reference fluid. The signal measured from point B therefore differs somewhat from the signal measured before . However, as can be clearly seen, during the 25 days ' duration of the test virtually no variation was observed in the measured signal .

The test conditions were as follows : the ceramic material consisted of particles of ceramic aluminium oxide, type AKP 50 (obtainable from the Sumitomo company in Japan) , the ceramic particles having a diameter of 200 nm (99%) and a pore size of 50-70 nm. The thickness of the membrane module was 1.5 mm. The flow-through surface of the membrane was 35 mm². The overpressure generated by the micropump 8 was 2-4 bars . The resulting flow rate was 0.5 to 4 microlitres per minute . The reference fluid was comprised of a liquid having a 0.1 molar KCl and 0.1 molar KNO₃ concentration.

Fig. 6 shows a test result of a reference electrode according to the present invention . It can be clearly seen that during the period of 12 days of testing no variation in the measured signal was observed.

The test conditions were as follows : the ceramic material consisted of particles of ceramic aluminium ox- ide, type AKP 50 (obtainable from the Sumitomo company in Japan) . The ceramic particles had a diameter of 200 nm ( 99%) and a pore size of 50-70 nm. The thickness of the membrane module was 1.5 mm. The flow-through surface of the membrane was in this case reduced to less than 0.1 mm². The overpressure generated by the micropump 8 varied from 1 to 4 bars . The flow rate thus obtained was 3 microlitre per hour .

The reference fluid was a fluid of a 3 molar KCl concentration.

For the record should be noted that the flow rate may be further reduced by using a ceramic material wherein the ceramic particles used for the fabrication of a ceramic membrane module have a smaller diameter . For exam- pie, AKP 70 or a similar starting composition may be used.

The embodiments shown in the figures and expounded in the description are only schematic representations of a number of feasible embodiments . The invention is not limited to the embodiments shown in the figures . For example, the upper layer shown in Fig. 2 may also be applied with the other embodiments in the other figures . According to a particular preference the pressure, which as described above and shown in Fig . 1 is supplied by a pump 8 , may also be generated by a compressed gas . The gas may be stored in a gas storage container and may be connected with the chamber 3 via a pipe and a control valve . Optionally, the gas storage container may be connected with the storage vessel 9 via a control valve, in which case the pump 8 may be dispensed with . Instead of the pump 8 , the control valve may then be positioned between the vessel 9 and the chamber 3. All the variations on this theme lie within the scope of a person skilled in the art .

The invention is limited by the appended claims only.

Claims

1. A reference electrode, comprising a ceramic membrane module fabricated of partly interconnected ceramic particles forming a porous membrane, wherein a first surface of the ceramic membrane module is in contact with a fluid in the reference electrode and a second surface can be contacted with a fluid to be analysed, characterised in that the fluid in the reference electrode is a calibration fluid that can be fed through the membrane into the fluid to be analysed.

2. A reference electrode in accordance with claim

1 , characterised in that the flow rate at which the calibration fluid can be fed through the membrane into the fluid to be analysed is

(a) adjustable, or (b) predeterminable .

3. A reference electrode in accordance with claim 1 or 2 , characterised in that the second surface is at least partly covered by an upper layer that is impermeable for both fluids , in order to reduce the exchanging surface area .

4. A ceramic membrane module for use in a reference electrode in accordance with one of the claims 1 to

3 , characterised in that the particles have a substantially identical diameter of X nm, wherein X ranges from 40-450 nm, and wherein preferably 60% of the particles , preferably at least 75%, more preferably at least 90% , and still more preferably at least 99% , has a diameter in the range from 0.9 ^«X - 1.1 «X, more preferably from 0.95 -X - 1.05 -X, still more preferably from 0.99 -X - 1.01 -X.

5. A ceramic membrane module according to claim

4 , characterised in that the particles are stacked as close packing, preferably according to a cubic close packing or a hexagonal close packing, more preferably according to a hexagonal close packing, said particles being connected with each other on the contact surfaces thereof .

6. A ceramic membrane module according to claim 4 , characterised in that the voids content ranges from 26 to 32.5% by vol . , determined according to the mercury bath method.

7. A ceramic membrane module according to claim

4 , characterised in that the ceramic particles comprise at least one starting compound suitable for the manufacture of a ceramic material , for example an oxide, a nitride, a carbide, or a boride, such as one or several of aluminium oxide, zirconium oxide, titanium oxide, titanium carbide, silicon carbide and silicon nitride .

8. A ceramic membrane module according to claim 4 , characterised in that X ranges from 100-400 nm.

9. A ceramic membrane module according to claim 4 , characterised in that the same is fabricated such that it comprises two surfaces , disposed substantially parallel to and at a distance from each other .

10. A method of fabricating a ceramic membrane module according to one of the claims 4-9 for use in a reference electrode, comprising the regular stacking of ceramic particles and the sintering of the particles so as to obtain a coherent porous mass .

11. A method of fabricating a ceramic membrane module according to one of the claims 4-9 for use in a reference electrode according to one or several of the claims 1-3 , comprising the regular stacking of ceramic particles and the sintering of the particles so as to obtain a coherent porous mass .

12. A method of fabricating a ceramic membrane module according to one of the claims 4-9 for use in a reference electrode according to one or several of the claims 1-3 , comprising the step of covering the second surface with a material that is substantially impermeable to the calibration fluid and the fluid to be analysed, so as to form an upper layer thereon, and of subsequently removing a portion of the applied upper layer so as to form a further to be determined, freely accessible flow-through surface on the membrane, wherein the removal is realised by, for example, drilling, milling or the like .