WO2001057508A2

WO2001057508A2 - Measuring instrument suitable for measuring cations or anions, and membrane as part of the measuring instrument

Info

Publication number: WO2001057508A2
Application number: PCT/NL2001/000073
Authority: WO
Inventors: Alex Peter Van Loon; Michel Hendrikus Johannes Wissink; Cindy Huiskes
Original assignee: Hydrion B.V.
Priority date: 2000-02-01
Filing date: 2001-02-01
Publication date: 2001-08-09
Also published as: WO2001057508A3; AU2001237787A1; NL1014256C2

Abstract

The invention relates to a measuring instrument suitable for measuring cations or anions, comprising a measuring electrode contained in a housing, at least one wall of the housing being embodied as an ion-sensitive membrane made of a matrix material comprising an apolar ionoforic material. The matrix material is made of porous technical ceramics. The membrane is further provided with at least one substrate made of technical ceramics for feeding ionoforic material to the membrane. In addition, the invention relates to the membrane described as component of the measuring instrument according to the invention.

Description

Measuring instrument suitable for measuring cations or anions, and membrane as part of the measuring instrument

The present invention relates to a measuring instrument suitable for measuring cations or anions, comprising a measuring electrode contained in a housing, at least one wall of the housing being embodied as an ion- sensitive membrane made of a matrix material comprising an apolar ionoforic material.

Such a measuring instrument with such a membrane is used, for example, in ISEs (Ion-Selective Electrodes) . Ion-selective electrodes are used to determine with the aid of an external reference electrode the activity and indirectly also the concentration of ions in aqueous solutions. These sensors are of importance for pharmacologists, water purification experts, hydraulo- gists, ecologist, greenhouse gardeners, authorities on health matters, soil scientists, and for all specialists concerned with the ion composition of aqueous solutions. Ion-selective electrodes may be roughly subdivided into four types :

1. the ISE type that works with an oily liquid in which an ionoforic material is dissolved and wherein the oily liquid acts as membrane (liquid membrane type) ;

2. the ISE type with an ion exchanger incorporated in a PVC matrix, and the PVC and the incorporated plasticizer serving as membrane (PVC membrane type) ; 3. the ISE type wherein the ion-selective activity is obtained by using a crystal lattice in which the ion to be measured is present as very poorly soluble salt (solid state membrane type) ; and

4. the ISE type wherein the ion-selective activ- ity is obtained by means of a special kind of glass, which glass exhibits a pronounced preference for the absorption of a specific ion (glass membrane type) . Which type of sensor is preferred for measuring a particular ion, depends on the ion to be measured and on the field of application, and on the further composition of the water or the aqueous liquid in which measurement is to be carried out. A drawback of known ISEs and other measuring instrument such as ISFETs (Ion Sensitive Field Effect Transistors) is that they are fragile and easily damaged .

It is the object of the present invention to eliminate this drawback and to achieve other advantages that will be elucidated below. This object is achieved according to the invention by making the matrix material that forms the membrane of porous technical ceramics .

The use of technical ceramics in the matrix mate- rial greatly prolongs the life of the measuring instrument according to the invention; the matrix material used as ion-sensitive membrane in the measuring instrument according to the invention is porous, has a very high specific electrical resistance, is leakage proof and may, moreover, contain an extra supply of ionoforic material, with the result that the technical life of the membrane is longer than that of a known ISE membrane.

To this end the membrane is preferably provided with at least one substrate made of technical ceramics for feeding ionoforic material to the membrane. The substrate made of technical ceramics thus functions as a storage space for ionoforic material .

In addition, the ceramic membrane of the measuring instrument according to the invention is much less susceptible to mechanical damage, or deterioration due to the influence of chemicals when exposed to the external world than is the case with, for example, a membrane of the known ISE. If polluted, the ceramic membrane may if necessary be cleaned by, for example, polishing so that the entire surface becomes effective again. Mechanical damage of a known membrane results in a less consistent measuring signal and accelerated ageing of the sensor due to deterioration of the membrane. This aspect does not ap- ply to a ceramic membrane of the measuring instrument according to the invention.

Another advantage of the ceramic membrane according to the invention is that, due to the fact that the ionoforic material m the surface of the ceramic membrane has relatively much local freedom of movement in the matrix of pores enclosing it (as in a liquid membrane) , the measuring instrument reacts quickly to any change m ion composition m the solution in which it is placed. Known measuring instruments generally react slowly to a new measuring environment. With the measuring instrument according to the invention, low ion concentrations can be measured more easily than is the case with known membranes . Furthermore, due to the measuring instrument according to the invention being made of (porous) technical ceramics, it is able to easily withstand water pressures of up to a few bars. This makes it possible to measure at depths down to several tens of metres, without danger of the measuring instrument braking.

The measuring instrument according to the invention may optionally also be embodied such as to be refillable with ionoforic material.

Corresponding to a preferred embodiment according to the present invention, the porosity of the technical ceramics ranges from approximately 28-40%.

This provides an optimal balance between the strength of the membrane on the one hand, and the effectiveness on the other hand. To this end, the pores provided in the technical ceramics of the matrix material preferably have a uniform diameter ranging approximately from 50-1000 nm, still more preferably 80-100 nm. The pores m the technical ceramics of the substrate serving as storage space for extra ionoforic material preferably range from 100-1000 nm.

Corresponding to a preferred embodiment according to the invention, especially when a liquid ionoforic mate- rial is applied, the matrix material is provided with a coating of mesoporous technical ceramics.

This renders the ceramic membrane much less susceptible still to biological attacks by bacteria and fungi than it already was in comparison with a known membrane. The coating of the ceramic membrane is so hard and the pores in the coating are so small that microorganisms cannot grow through them. This property makes the measuring instrument according to the invention very effective for use in a biologically aggressive environment.

In connection with the life span and response speed of the measuring instrument according to the invention, it is desirable for the coating to be provided with pores that have a substantially uniform diameter ranging from approximately 2-7 nm, and for the porosity of the coating to be between 30 and 40%. Such a pore diameter suffices to allow ions to pass through the coating unimpeded, whereas the organic compounds pertaining to the ion exchanger, and indeed particularly the ionofore, are ef- fectively enclosed inside the ceramic membrane of the measuring instrument .

The thickness of the coating is preferably about 1-3 μm. With such a thickness the possibility of lacunae when applying the coating may be effectively avoided, and possible damage during use, or when cleaning the membrane, may be prevented. If the pores are required to have an even smaller diameter because ionoforic material of a lesser molecular size is used, it is generally possible to attain a pore diameter of minimally 0.3 nm by applying a thin layer of silica.

As already discussed earlier, it is preferred according to the present invention for the membrane to be provided with at least one substrate made of technical ceramics for feeding ionoforic material to the membrane, and for the technical ceramics of the substrate to have a substantially uniform porosity ranging from 100-1000 nm.

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the side of the membrane made of technical ceramics that is directed towards the housing, is provided with a coating to seal the pores, and a layer of adhesive is applied on the pore-sealing coating. The application of the pore-sealing coating prevents the ionoforic material in the technical ceramics from (very slightly) moving to the exterior anywhere save where the wall of the measuring instrument is embodied as membrane . In accordance with a further embodiment of the measuring instrument according to the invention, the sides of the membrane made of technical ceramic matrix material that are directed towards the inside of the housing, are provided with an ion-permeable plastic sealing layer, preferably PVC that may or may not comprise a softener and ionoforic material .

In accordance with a further embodiment of the measuring instrument according to the invention, the side of the membrane made of technical ceramics matrix material that is directed towards the outside of the housing, is provided with an ion-permeable plastic sealing layer.

In accordance with a favourable embodiment of the measuring instrument according to the present invention, at least part of the outside of the technical ceramics of the measuring instrument is provided with a silica coating.

In accordance with another favourable embodiment of the measuring instrument according to the present invention, at least the side of the membrane's matrix material directed towards the outside of the housing is hydrophobic .

The side of the membrane directed towards the outside of the housing may be made hydrophobic by means of coating it with an organic compound or a silicone com- pound .

The invention also relates to a membrane as described as component of the measuring instrument according to the invention. There are various ways of filling the matrix material with ionoforic material. Some examples are:

1. The ceramic matrix material is filled from the outward directed side with a PVC/ionoforic mixture, whereby said mixture fills the matrix material completely or partly.

2. The ceramic matrix material is first filled from the outside with a thin layer of ion-permeable plastic. Then the remainder of the ceramic matrix material is filled with a liquid ionoforic mixture, after which the inward-directed side is also provided with a thin ion- permeable plastic layer. These two thin plastic layers effectively shield the liquid ionoforic mixture against any water permeating from the electrolyte-containing solution as well as from the solution to be measured, while ion exchange with the aqueous phase remains possible. Said two thin plastic layers may consist, for example, of a PVC mixture comprising a softer and an ionoforic material .

3. The ceramic matrix material provided at its outside with a ceramic coating, for example, gamma- aluminium oxide, is completely filled with a liquid ionoforic mixture after which the inward-directed side of the matrix material is protected with a thin layer of ion- permeable plastic consisting of, for example, a PVC mix- ture provided with a softener and ionoforic material. If necessary, the surface of the ceramic coating is made hydrophobic.

The invention will now be elucidated with reference to the following description. Shown is in: - Figure 1, a cross-sectional view in perspective of a measuring instrument according to the invention;

- Figure 2, a cross-sectional view in perspective of an alternative embodiment of the measuring instrument according to the invention; - Figure 3, a cross-sectional view in perspective of an alternative embodiment of the measuring instrument according to the invention; - Figure 4, a cross-sectional view in perspective of an alternative embodiment of the measuring instrument according to the invention;

- Figure 5, a cross-sectional view in perspective of a further embodiment of the measuring instrument according to the invention; and

- Figure 6, a cross-sectional view in perspective of yet another embodiment of the measuring instrument according to the invention. Identical reference numbers refer to similar parts.

Figure 1 shows an ion-selective electrode (ISE) 1 comprising a measuring electrode 3 contained in a housing 2. The bottom side of the housing 2 is an ion-sensitive membrane 4 having a thickness of 0.5-2 mm and made of a layer of porous α-aluminium oxide 5 and a layer of α- aluminium oxide 6 with a filling comprised of a mixture of PVC and ionoforic material. Via an electrolyte-containing solution 7, the layer 6 makes electrical contact with the measuring electrode 3. As ionoforic material for the fill- ing of soft PVC it is possible to use a combination of potassium ionoforic valinomycin and tetraphenyl borate salts .

Figure 2 shows an alternative embodiment of the ISE of Figure 1. The layer of α-aluminium oxide 5 of the membrane 4 is filled with liquid ionoforic material 6a after the porous α-aluminium oxide was internally first made hydrophobic by a coating the internal surface with a thin layer of organic material.

Figure 3 shows an alternative embodiment of the ISE of Figure 1. The layer of α-aluminium oxide 5 of the membrane 4 is filled with liquid ionoforic material 6a and is covered with two thin layers 6b of PVC filled with ionoforic material. Prior to filling with ionoforic mixture, one layer of PVC 6b is applied to the outside of the porous α-aluminium oxide 5, and is absorbed as thin layer by the porous ceramics, while after filling with liquid ionoforic material, the other layer of PVC 6b is applied to the inside of the α-aluminium oxide 5. Figure 4 shows an alternative embodiment of the

ISE of Figure 1. The layer of α-aluminium oxide 5 is provided with a thin layer of mesoporous γ-aluminium oxide 8 having pores ranging from 2-7 nm, and being 1-3 μm thick. The ion- sensitive membrane 4 of the housing 2 is now filled with liquid ionoforic material 6a and is at the side directed inward sealed with a thin layer 6b of soft PVC filled with ionoforic material.

Figure 5, shows an alternative embodiment of the ISE of Figure 4. The layer of α-aluminium oxide 5 is thicker (2 to 5 mm) than in Figure 4, and is composed of a central portion 5b, and a portion 5a functioning as storage space for ionoforic material . In said thicker layer 5 a depression is provided so that the effective thickness of the central portion 5b of the membrane 4 is 0.5-1 mm. The portion 5a of the layer 5 included in the housing 2 made from, for example, plastic, functions as storage space for ionoforic material. The portion functioning as storage space 5a is preferably formed from α-aluminium θχ- ide having a larger grain size than that of the α- aluminium oxide of the central portion 5b of the layer 5. Underneath the layer 5 a further layer of γ-aluminium oxide 8 is provided.

Figures 6 shows an alternative embodiment of an ISE, the membrane 4 in this case being composed of a layer of α-aluminium oxide 5a serving as storage space on a measuring membrane of fine grain α-aluminium oxide 5b with a coating 8 of γ-aluminium oxide. The storage space 5a and the central portion 5b are sintered together. To the out- side of the central portion 5b a thin layer 9 of silica is applied. In this way the effective pore diameter of the membrane becomes less than 1 nm.

The present invention will now be elucidated with reference to an example.

Example 1

A measuring instrument according to the invention, of the type represented in Figure 1 above, having a matrix made of porous technical ceramics and filled with a mixture of an ionoforic material, was compared with a sodium-glass electrode of the make Radiometer (type ISE 21Na) and with a sodium electrode having only a PVC membrane. While taking measurements and also afterwards, the surface of the ceramic membrane of the measuring instrument according to the invention was scrubbed with a small nylon brush in order to test the wear resistance of the surface. After scrubbing for three hours, no signs of wear or change in measuring sensitivity were observed in the ceramic sensor according to the invention, while the other two electrodes exhibited obvious damage after similar brushing treatment.

Claims

1. A measuring instrument suitable for measuring cations or anions, comprising a measuring electrode contained in a housing, at least one wall of the housing being embodied as an ion-sensitive membrane made of a ma- trix material comprising an apolar ionoforic material, characterized in that the matrix material is made of porous technical ceramics.

2. A measuring instrument according to claim 1, characterized in that the porosity of the technical ceram- ics ranges from approximately 28-40%.

3. A measuring instrument according to claim 1 of 2, characterized in that the technical ceramics of the membrane is provided with pores having a substantially uniform diameter ranging from approximately 50-1000 nm.

4. A measuring instrument according to one of the preceding claims, characterized in that the matrix material is provided with a coating of mesoporous technical ceramics .

5. A measuring instrument according to claim 4, characterized in that the coating is provided with pores that have a substantially uniform diameter ranging from 2- 7 nm.

6. A measuring instrument according to claim 4 or 5, characterized in that the thickness of the coating is approximately 1-3 μm.

7. A measuring instrument according to one of the claims 4-6, characterized in that the porosity of the coating is between 30 and 40%.

8. A measuring instrument according to one of the preceding claims, characterized in that the membrane is provided with at least one substrate made of technical ceramics for feeding ionoforic material to the membrane.

9. A measuring instrument according to claim 8, characterized in that the technical ceramics of the sub- strate has a substantially uniform porosity ranging from 100-1000 nm.

10. A measuring instrument according to one of the preceding claims, characterized in that the matrix material is formed of α-aluminium oxide (α-Al₂o₃) .

11. A measuring instrument according to one of the claims 4-10, characterized in that the coating is formed of γ-aluminium oxide (γ-Al₂o₃) .

12. A measuring instrument according to one of the claims 10 or 11, characterized in that the matrix material is formed from α-aluminium oxide in powder form having a substantially uniform grain size of between 180 nm and 3500 nm, which is compacted and subsequently sintered for several hours at a temperature between 1050 and 1300°C and then - if necessary - polished prior to applying the coating to the smooth side.

13. A measuring instrument according to claim

12, characterized in that the coating is applied by at least one dip-coating for the application of boehmite (A100H) in colloidal form, followed by calcination at a temperature between 400 and 800°C.

14. A measuring instrument according to claim

12 or 13, characterized in that at least the sides of the membrane made of technical ceramics that is directed towards the housing, is provided with a coating to seal the pores .

15. A measuring instrument according to claim

14, characterized in that a layer of adhesive is applied on the pore-sealing coating.

16. A measuring instrument according to one of the preceding claims, characterized in that the side of the membrane made of technical ceramic matrix material that is directed towards the inside of the housing, is provided with an ion-permeable plastic sealing layer.

17. A measuring instrument according to one of the preceding claims, characterized in that the side of the membrane made of technical ceramic matrix material that is directed towards the outside of the housing, is provided with an ion-permeable plastic sealing layer.

18. A measuring instrument according to claim 16 or 17, characterized in that the plastic sealing layer is PVC.

19. A measuring instrument according to claim 18, characterized in that the plastic sealing layer comprises ionoforic material.

20. A measuring instrument according to one of the preceding claims, characterized in that at least part of the outside of the technical ceramics of the measuring instrument is provided with a silica coating.

21. A measuring instrument according to one of the preceding claims, characterized in that at least the side of the membrane's matrix material directed towards the outside of the housing is hydrophobic.

22. A membrane as described as component of the measuring instrument according to one of the preceding claims .