HALF-CELL FOR MEASURING A pH VALUE, METHOD FOR PRODUCING A HALF-CELL, AND POTENTIOMETRIC SENSOR

Abstract

A half-cell for measuring a pH value of a measuring medium is disclosed including a tube-shaped carrier element and a pH-sensitive glass membrane connected to an end section of the carrier element. At least the end section of the carrier element includes a zirconia-containing and/or alumina-containing ceramic. A method for manufacturing a half-cell for pH value measurement and a potentiometric sensor are further disclosed.

Claims

1. A half-cell for measuring a pH value of a measuring medium, the half-cell comprising: a tube-shaped carrier element having an end section; and a pH-sensitive glass membrane connected to the end section of the carrier element, wherein at least the end section of the carrier element includes a zirconia-containing and/or alumina-containing ceramic.

2. The half-cell of claim 1, wherein the ceramic has a content of zirconia and/or alumina of at least 80% by weight.

3. The half-cell of claim 1, wherein the ceramic has a content of zirconia and/or alumina of at least 90% by weight.

4. The half-cell of claim 1, further comprising a shaft tube, wherein the carrier element is disposed at least partially within the shaft tube such that the shaft tube and the carrier element define an annular chamber, the annular chamber containing a reference electrolyte, wherein the carrier element defines the inner circumference of the annular chamber.

5. The half-cell of claim 1, wherin the carrier element consists of the zirconia-containing and/or alumina-containing ceramic.

6. The half-cell of claim 1, wherein the ceramic is stabilized with a yttrium compound and/or an alkaline earth metal compound.

7. The half-cell of claim 6, wherein the ceramic is stabilized with a yttrium oxide and/or an alkaline earth metal oxide.

8. The half-cell of claim 6, wherein the yttrium compound and/or alkaline earth metal compound has a content of equal to or less than 20% by weight in the ceramic.

9. The half-cell of claim 6, wherein the yttrium compound and/or alkaline earth metal compound has a content of equal to or less than 10% by weight in the ceramic.

10. The half-cell of claim 1, wherein the ceramic is an all-ceramic.

11. The half-cell of claim 1, wherein the ceramic has a density of: $\frac{{}_{\mathrm{bulk}}}{{}_{\mathrm{true}}}90.Math.\%$

12. The half-cell of claim 11, wherein the ceramic has a density of: $\frac{{}_{\mathrm{bulk}}}{{}_{\mathrm{true}}}95.Math.\%$

13. The half-cell of claim 1, further comprising a reference electrode, wherein the glass membrane and/or the reference electrode is connected to the end section of the carrier element via a diaphragm.

14. The half-cell of claim 13, wherein the diaphragm is an annular molded body, which is disposed in the shaft tube with a bonded joint and in which the glass membrane and/or the carrier element is attached by a form fit and/or adhesion.

15. The half-cell of claim 13, wherein the diaphragm is a plastic molded body or a porous ceramic molded body.

16. The half-cell of claim 13, wherein the diaphragm is a plastic molded body of polytetrafluoroethylene.

17. A method for manufacturing a half-cell for measuring a pH value of a measuring medium, the method comprising: providing an inner tube having an end section; fusing and/or blowing a pH-sensitive glass membrane onto the end section of the inner tube; and assembling the inner tube with the membrane within a shaft tube, wherein at least the end section of the inner tube includes a zirconia-containing and/or alumina-containing ceramic.

18. A potentiometric sensor comprising: a half-cell including: a tube-shaped carrier element having an end section; and a pH-sensitive glass membrane connected to the end section of the carrier element, wherein at least the end section of the carrier element includes a zirconia-containing and/or alumina-containing ceramic.

19. The potentiometric sensor of cliam 18, wherein the the ceramic has a content of zirconia and/or alumina of at least 80% by weight.

20. The potentiometric sensor of cliam 18, wherein the ceramic is stabilized with a yttrium compound and/or an alkaline earth metal compound.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0017] The invention shall be explained in more detail below with reference to a specific exemplary embodiment and with the aid of the enclosed figures. Shown is:

[0018] FIG. 1 shows a pH sensor including a potentiometric single-rod measuring cell, including a half-cell according to the present disclosure.

DETAILED DESCRIPTION

[0019] FIG. 1 shows a potentiometric sensor 1 for pH measurement, which is embodied as a single-rod measuring cell. The sensor 1 includes an outer shaft tube 2, which is connected in a front end section 13 to an inner tube 4 via an annular diaphragm 12, which enables an electro-chemical transition and is spaced apart from this inner tube 4. The outer shaft tube 2 separates the potentiometric sensor 1 from a measuring medium 10 or from the environment. The diaphragm 12 may be manufactured as a plastic molded body, for eaxmple, of PTFE, and may be connected in a form-fit manner, e.g., by force fit or bonded joint, e.g., by gluing or spraying-on, to the outer shaft tube 2. Alternatively, the diaphragm may be a porous, ceramic molded body.

[0020] Via the diaphragm 12, the measuring medium 10, surrounding the sensor 1, in the front end section 13 is in contact with a reference electrolyte 6 of the sensor 1. The inner tube 4 and the glass membrane 3 define a first chamber 17, in which an inner electrolyte 5, e.g., a buffer solution, is arranged. A discharge element 7, which is electrically conductively connected to a measuring circuit 9, is immersed in the inner electrolyte 5. A temperature sensor 15, which may be protected with a capillary tube 16, as shown in FIG. 1, may be immersed in the inner electrolyte 5.

[0021] The inner tube 4 extends coaxially to the outer shaft tube 2, such that an annular chamber 14 filled with the reference electrolyte 6 is disposed between the inner tube 4 and the outer shaft tube 2.

[0022] The reference electrolyte 6 may, for example, be a highly concentrated, e.g., 3 molar, KCl solution solidified by polymer contents, e.g., polyacrylamide, to a cross-linked hydrogel. A reference element 8 that is connected to the measuring circuit 9 in an electrically conductive manner may be immersed in the reference electrolyte 6. In certain embodiments, the reference element 8 may be protected by a capillary tube, which is open at an end, where appropriate. In the present example embodiment, the reference element 8 and the discharge element 7 are chloridated silver wires.

[0023] At a rear end opposite the front end section 13 connected to the glass membrane 3, the outer shaft tube 2 and the inner shaft tube 4 are sealed in a liquid-tight manner (not shown in FIG. 1). The liquid-tight seal may be achieved, for example, by a stopper that is bonded to the inner tube 4 and the outer shaft tube 2 or by using a polymer cast section. In an alternative embodiment, the glass and ceramic components may be fused together in rear end section.

[0024] The measuring circuit 9 may be accommodated in an electronics housing attached to the rear end of the outer shaft tube 2. The measuring circuit 9 may be configured to detect a difference in potential between the discharge element 7 and the reference element 9 and to generate a measuring signal that represents this difference in potential. The measuring signal can be output via a cable connection 11 to a higher-level data processing unit (not shown in FIG. 1), e.g., a transmitter, transducer, processor, computer, or programmable logic controller.

[0025] In the present example, at least one section of the inner tube 4, which is hereafter also called a carrier element, consists of a zirconia-containing and/or alumina-containing ceramic.

[0026] The sensor according to the present disclosure can preferably have a sensor impedance in the range of 50 M to 1 G.

[0027] In the context of the present disclosure, a multitude of other design variants are possible. In another embodiment, a short ceramic tube may be arranged as a carrier element injected and/or glued into a plastic tube. In such an embodiment, for example, in an embodiment having a sensor impedance of 1 G, a bypass can be avoided. For example, the zirconia-containing and/or alumina-containing ceramic may have a composition according to the following table:

TABLE-US-00001 TABLE 1 Ceramic Composition by Embodiment Embodiment Component of the Ceramic Proportion (in wt %) Composition 1 ZrO.sub.2 85-99% Y.sub.2O.sub.3 1-15% Composition 2 ZrO.sub.2 93-97% Y.sub.2O.sub.3 3-7% Composition 3 ZrO.sub.2 88-92% ZrO.sub.2 8-12%

[0028] Certain embodiments may include other components as stabilizers (e.g., MgO and/or CaO), the ZrO.sub.2 component may have a proportion of at least 80%. In such an embodiment, the ZrO.sub.2 component may have a proportion of 87-92%. The remaining proportion in the ceramic may be the respective stabilizer.

[0029] For example, Y-stabilized ceramics are particularly chemically stable and mechanically and thermally resistant and have a suitable expansion coefficient, which enables the material connection to the glass membrane 3, even with respect to a sufficient thermal shock resistance. In alternative embodiments, the Al.sub.2O.sub.3 component may have a proportion of more than 85% (in wt %).

[0030] The proportion of the total mass of the ceramic can thus be expressed as follows:

[00001]$\frac{m_{\mathrm{ZrO}.Math..Math.2}+m_{\mathrm{Al}.Math..Math.2.Math.O.Math..Math.3}}{m_{\mathrm{total}}}80.Math..Math.\mathrm{wt}.Math..Math.\%-\mathrm{preferably},90.Math..Math.\mathrm{wt}.Math..Math.\%$

[0031] Embodiments including Y.sub.2O.sub.3 amy have a content of 10% of stabilizers and a content of 20% of alkaline earth metals.

[0032] Ideally, the ceramic should have a preferred density, in order to avoid diffusion losses of the electrolyte. The preferred porosity of the ceramic is specified in relation to the true density. In certain embodiments, the specification may be:

[00002]$\frac{{}_{\mathrm{bulk}}}{{}_{\mathrm{true}}}90.Math.\%-\mathrm{preferably},95.Math.\%$

[0033] The specification corresponds to the comparison of the ceramic density (i.e, true density) to the maximum theoretical density (i.e., bulk density).

[0034] In embodiments for the reduction of material transitions, the inner tube 4 may consist completely of the zirconia-containing and/or alumina-containing ceramic.

[0035] The pH-sensitive glass of which the glass membrane 3 is formed may include a multi-component glass comprising a prespecified lithium oxide proportion.

[0036] The zirconia-containing and/or alumina-containing ceramic may be formulated as an all-ceramic, as generally known from other technical fields, e.g., ceramic engineering.

[0037] In comparison to carrier elements of lead glass, the zirconia-containing and/or alumina-containing ceramic has the special advantage of widespread use and sustained availability as a result of the various fields of application in ceramic engineering, filter ceramics, and medical engineering. Carrier elements of zirconia-containing and/or alumina-containing ceramic are, moreover, significantly tougher (i.e., break-proof) than lead glass. In addition, as in the embodiment of FIG. 1, no contamination of the glass membranes occurs as a result of the formation of mixed zones due to fusing.

[0038] The half-cell with the glass membrane 3 and the carrier element, i.e., the inner shaft tube 4, may be used as a component of the potentiometric sensor, in which undesired measuring effects are prevented as a result of the low contamination of the glass membrane.

[0039] The zirconia-containing and/or alumina-containing ceramic is, moreover, shatterproof and non-toxic, such that the pH half-cell may also be disposed of more easily in case of accidental damage and may be used in food applications where appropriate.