Calibration reference element for pH measuring instruments, calibration reference system, holder for calibration reference element, and method for calibrating pH measuring instruments

Abstract

A calibration reference element (10) for a pH measuring instrument comprises a reference liquid (17) with a defined pH value and a tube bag (1) enclosing the reference liquid. The tube bag consists of a foil film sealed along at most three sealing edges (11, 12, 13). A calibration reference system comprises at least two such calibration reference elements (10) with reference liquids of different pH values. A holder (2) for the calibration reference element allows the calibration reference element to stand up so that the reference liquid is conveniently available for calibration. Correspondingly, a calibration set comprises the holder and the calibration reference element (10) or the calibration reference system.

Description

[Technical Field]

[0001] The present invention relates to a calibration reference element of the kind characterized in the claims. The present invention also relates to a calibration reference system using a calibration reference element of the kind described above, to a holder for such a calibration reference element, and to a method for calibrating a pH measuring instrument using the above object. [Background technology]

[0002] When a measurement transducer is used to convert a physical quantity into an electrical signal during measurement acquisition, it is essential to know the relationship between the generated electrical signal and the actual physical quantity. In many measurement transducers or sensors, this relationship must be verified from time to time within the context of calibration, i.e., determined anew, so that the exact value of the physical quantity can be determined, i.e., displayed by the measuring device, based on the measured electrical signal. For this purpose, a defined reference value or, in turn, several defined reference values ​​of the physical quantity to be determined are provided, and the corresponding electrical measurement value is determined, i.e., the measuring device is adjusted so that the exact value for this physical quantity is displayed in each case.

[0003] This also applies, for example, to sensors or measuring instruments for determining pH values, where a pH sensor is successively immersed in buffer solutions with different defined pH values, and the measuring instrument is adjusted, for example, to display the correct pH value in each case.

[0004] Alternatively, the electrical signal output by the sensor can be recorded as a function of the pH value, allowing the pH value of the measured substance to be subsequently inferred from the electrical signal if the pH value is unknown. Using a pH buffer as a reference solution ensures that the reference pH value is maintained. However, handling buffers is not entirely problem-free in practice. For example, some strongly acidic or strongly basic, and therefore aggressive, buffer solutions are preferably provided in containers that prevent spillage as easily as possible during opening and handling, at least with due care. Furthermore, it has proven desirable if the buffer solution container is securely held during calibration and provides as unobstructed an access point for the sensor as possible. For economic reasons and for reasons of disposal of buffer solutions, which are not without problems in terms of wastewater, it is also desirable to provide reference buffer solutions so that as little as possible of each solution is required and that the reference buffer solution can be transported and stored space-savingly. Summary of the Invention [Means for solving the problem]

[0005] This specification relates to the subject matter described at the beginning. According to one aspect of this specification, a calibration reference element or a calibration reference system for a pH measuring instrument is presented, which allows for as simple and reliable handling as possible when calibrating the pH measuring instrument. Furthermore, a holder for such a calibration reference element is presented, which ensures reliable holding of the calibration reference element in a simple manner. Furthermore, a method for calibrating a pH measuring instrument or a pH sensor using the above subject matter is described.

[0006] Further benefits and advantages of the subject matter described herein, whether expressly stated or not, will be apparent in light of this specification.

[0007] This is achieved by the calibration reference element set forth in claim 1 and the subject matter described in the further independent claims. Particular embodiments of the claimed subject matter are evident from the dependent claims.

[0008] Thus, a calibration reference element for a pH measuring instrument is described, comprising a reference liquid having a defined pH value and a tube bag, the reference liquid being enclosed in the tube bag. The tube bag is made of foil film and sealed along no more than three sealing edges, and the tube bag is at least partially filled with the reference liquid. As explained above, a pH buffer solution is particularly used as the reference liquid. It may be provided that the tube bag is only partially filled with the reference liquid, with any remaining volume in the tube bag being filled with air or a protective gas. This prevents spillage of the liquid when the tube bag is opened at the end facing up. The protective gas can improve the shelf life of the reference liquid. Furthermore, it may be provided that the tube bag contains only an amount of liquid such that, when the tube bag is positioned in the configuration achieved during calibration as specified, the fill level in the opened tube bag is between 30% and 90%, preferably between 50% and 80%, of the height of the internal volume of the tube bag positioned in this way. The configuration routinely achieved during calibration can be derived, as explained below, in particular in relation to a holder provided for use with this calibration reference element. The height of the internal volume is known because a line along which the user should open the tube bag can be marked on the outer surface of the tube bag, or such a line, e.g. a tear line, can already be provided by a suitable device.

[0009] Furthermore, the tube bag may have a printing field within it that indicates the pH value of the reference solution, the printing field being located above the upper edge of the holder and below the line along which the user should open the tube bag when the tube bag is positioned in the configuration normally achieved during calibration. The configuration normally achieved during calibration may be derived in relation to a holder specifically designed for use with the calibration reference element, as explained below. Additionally, it is preferred that the printing within the printing field be colored to correspond to the color coding. This allows the user to easily read the pH value of the reference solution during measurement.

[0010] It is clear that in this context, calibration of a pH measuring device can also refer to the process of recording the relationship between the electrical output signal of the sensor described at the beginning and the pH value. Therefore, calibration of a pH measuring device is not limited to adjusting the measuring device so that it outputs the correct pH value, but also includes recording the relationship between the electrical output signal of the sensor described above and the pH value of the measured substance. The chosen expression should be understood broadly to include any process that serves to calibrate a device used to measure pH values.

[0011] Within the framework of this specification, "ein" or "eine" is to be understood as an indefinite article and not as a numeral, unless a different meaning is clearly indicated, for example, by the use of "genau ein" or "genau eine".

[0012] Furthermore, it is contemplated that the tube bag has a sealing edge on each of the two opposing end faces, each of which has a longitudinal extension that runs transversely across the longitudinal extent of the tube bag and is defined by the longitudinal edge of the end sealing edge. In the circumferential direction, the tube bag is particularly seamless or has one longitudinal sealing edge that is circumferentially offset from the longitudinal edge of the end sealing edge or folded circumferentially away from the adjacent longitudinal edge of the end sealing edge. This effectively prevents the longitudinal sealing edge from becoming a cause of shape rigidity that would cause the opened bag to flatten, as is the case with known packaging materials that have two longitudinal sealing edges running on opposite sides of the bag. In contrast, due to the pre-formed material of the tube bag in the region of the longitudinal sealed edges, the tube bag, once opened at the top, has only a relatively low geometrical rigidity in terms of its cross-sectional shape at the top edge. Therefore, when the tube bag is tensioned at the closed bottom and otherwise properly held, it spontaneously assumes an oval to at least approximately circular geometric shape, which allows for easy entry of a sensor into the reference liquid inside the opened tube bag. The circumference of the tube bag and its interior is, in an exemplary embodiment, between 70 mm and 80 mm. This corresponds to an equivalent diameter in the range of approximately 23 to 27 mm, which allows for easy entry of a sensor with a diameter of, for example, 12 mm. The length of the fillable internal volume between the two end faces is, in an exemplary embodiment, between 90 mm and 120 mm, and the liquid filling volume in a certain region is in the range of 15 ml to 30 ml. In particular, the length of the fillable internal volume is between 110 mm and 115 mm, and the filling volume is approximately 20 ml. These parameters result in a reference element that, when the bag is opened and positioned correctly, provides sufficient clearance relative to the upper edge of the opened tube bag to prevent overflow of the liquid when the sensor is immersed, while the closed tube bag is compact enough for storage, transportation, and handling. At the same time, a depth of immersion of the sensor in the reference liquid that ensures correct measurement can be guaranteed.

[0013] It should be noted that the longitudinal sealing edges extend from one end sealing edge to the other end sealing edge in such a way that the tube bag is hermetically closed only if both end sealing edges are present and closed. The tube bag is opened, in particular, by cutting the tube bag across its longitudinal extent near one of the end sealing edges. It should also be noted that in connection with the tube bag, its longitudinal and transverse directions as well as its end faces are implicitly predetermined by its tubular geometry.

[0014] In certain embodiments, the tube bag is made of a foil film having at least two material layers. One of these material layers is on the inside of the tube bag, and the second layer is on the outside of the tube bag. The inner layer is made of a first material, e.g., a first plastic, and the outer layer is made of a second material, e.g., a second plastic, and these materials are particularly different from each other. The first material can be selected, among other things, to ensure stability against the reference solution, while the second material of the outer layer is selected, among other things, to resist environmental conditions and mechanical damage, and may also perform a structural function. In other embodiments, the layer on the inside of the tube bag and the layer on the outside of the tube bag can be made of the same material.

[0015] Furthermore, it is also possible to provide for an intermediate layer of metal foil film to be disposed between the layer on the inside of the tube bag and the layer on the outside of the tube bag. The metal foil film is preferably an aluminum foil film. The metal foil film improves the stability of the reference solution, especially during storage and transportation. In particular, the foil film layer on the outside of the tube bag and the intermediate layer of aluminum foil film can be perforated, particularly by laser, resulting in a perforation line. This allows for easy opening of the tube bag along a defined line, from which the height of the internal volume can be determined. In contrast, at least the foil film layer of the tube bag, at least the layer on the inside of the tube bag, can be perforated completely or only incompletely.

[0016] Also disclosed is a calibration reference system including at least two of the above-mentioned types of calibration reference elements. At least two of the total number of all calibration reference elements of the calibration reference system contain reference solutions with different pH values. In particular, the calibration reference elements with reference solutions with different pH values ​​can be coded with different colors. This allows for quick recognition of the pH values ​​of the individual reference elements during application. In a specific embodiment, the calibration reference system consists of three to five calibration reference elements with different pH values.

[0017] A suitable holder for a calibration reference element of the type described above has at least a triangular prism shape. A triangular prism is a prism with a triangular base. The prism has at least one recess extending from one side of the prism, through a side surface of the prism adjacent to the side, toward the side surface of the prism opposite the side. In this regard, the side defines the upper edge of the holder, and the side surface of the prism opposite the side defines the lower surface of the holder. The recess is open at the upper edge. At the upper edge, the recess has a maximum width parallel to the upper edge, while this width decreases from the upper edge to a minimum width from the upper edge toward the lower surface. This minimum width remains constant in a lower region of the recess distal to the upper edge along a direction from the upper edge toward the lower surface. Due to the downward tapering of the upper region of the recess, a reference element of the type described above inserted into the recess can be guided and supported reliably in the lower region, while still allowing the upper open end to open widely. The maximum width of the recess parallel to the upper edge corresponds, in particular, to 75% to 100% of the diameter of a circle whose circumference is the same as the inner circumference of the tube bag. The holder is placed on the base with its underside facing downward and its upper side facing upward, and the opened reference element is inserted into the recess with its closed underside, with the edge of the recess supporting the opened tube bag in a direction parallel to the upper edge. In this regard, the height of the holder, measured from the lower surface to the upper edge, is determined in certain embodiments so that the inserted opened tube bag protrudes from the holder at its upper side. This height may be determined, for example, so that the opened tube bag protrudes from the upper edge of the holder by 10% to 50% of its length. This geometry ensures that the open upper end of the tube bag fitted into the holder has the above-mentioned oval or at least approximately circular cross-sectional shape. In particular, two or more recesses for accommodating reference elements with liquids of different pH values ​​can be arranged along the extension of the upper edge of the holder. The number of recesses can be the same as the number of reference elements with different pH values ​​in a set or system of reference elements. If the reference elements with different pH values ​​are color-coded as described above, the recesses in the holder can also be color-coded accordingly.

[0018] The calibration set comprises a calibration reference element and a holder according to the invention, which allows a particularly easy and reliable calibration of pH measuring instruments.

[0019] The calibration set preferably includes a calibration reference system and a holder, and it is particularly preferred that the holder has at least as many recesses as there are calibration reference elements with different pH values ​​in the calibration reference system, so that the user can easily and reliably use all reference elements for the multi-point calibration.

[0020] A method for calibrating a pH measuring device or pH sensor using the above-described object includes placing the holder on a base with its upper edge facing up. At least one calibration reference element is opened near one end of the tube bag. The opened calibration reference element is inserted into at least one recess in the holder with the closed end of the tube bag facing down, with the wall of the tube bag supported by the edge of the recess. This forms an open-topped container within the recess. A pH sensor is immersed in a reference solution through the opening at the top of the tube bag. The measured value (as the electrical output of the sensor or already converted to a pH value in the measuring device) is then compared with the specified pH value of the reference solution. The measurement value assigned to the reference pH value can then be noted, or the measuring device can be adjusted to display a value corresponding to the reference pH value. This process can be repeated for additional different reference pH values. To this end, multiple reference elements with different reference pH values ​​can be inserted into the holder, and the sensor is immersed in these reference elements one after the other.

[0021] The individual embodiments listed above may be combined with one another, and further, not individually disclosed, embodiments of the teachings of this document can be readily inferred by those skilled in the art.

[0022] The facts described here will be explained in more detail below on the basis of selected exemplary embodiments shown in the drawings. [Brief explanation of the drawings]

[0023] [Figure 1] 1 shows two opposing faces of an example of a reference element as described herein, in this case with the tube bag of the reference element closed. [Figure 2] FIG. 2 is a top view of the reference element from FIG. 1, in which the tube bag of the reference element has been opened near the top end. [Figure 3] FIG. 10 is a top view of another embodiment of the reference element with the tube bag opened. [Figure 4] FIG. 1 is a side view of a holder prepared for use with reference elements of the type described herein, in this case with several reference elements fitted in place; [Figure 5] FIG. 5 is a front view of the holder from FIG. 4 with the reference element properly fitted. DETAILED DESCRIPTION OF THE INVENTION

[0024] The drawings are highly simplified. Details not necessary for understanding the subject matter being described have been omitted. Furthermore, the drawings show only selected exemplary embodiments and should not be considered to restrict the subject matter rewritten in the claims. Embodiments not shown may be fully covered by the claims.

[0025] Figure 1 shows a reference element 10 of the type described here with a tubing bag 1 closed thereon, Figures 1a and 1b showing two opposite faces of the reference element 10. The tubing bag 1 comprises two end sealing edges 11 and 12.

[0026] The sealing edge 11 will hereinafter also be referred to as the upper sealing edge, and the sealing edge 12 will hereinafter be referred to as the lower sealing edge. The end sealing edges 11 and 12 each have a longitudinal extension that runs transversely to the longitudinal extension of the tube bag. The longitudinal extension of the tube bag therefore extends between the end sealing edges 11 and 12. This designation of the orientation or direction of the tube bag is informal and is given in relation to Figure 2, which shows a top view of the reference element 10 in which the tube bag has been opened along the tear line 14. The end sealing edges 11 and 12 are delimited in the direction of their longitudinal extension by their longitudinal edges 111 and 121.

[0027] The tubing bag 1 or reference element 10 further includes a longitudinal sealing edge 13, which, as is only visible in the representation of FIG. 1b, is folded away from the adjacent longitudinal edges of the end sealing edges 11 and 12, as will be further explained in connection with FIG. 2. The tubing bag 1 shown in the example is made of a foil film element that is folded over and sealed along its sealing edges, e.g., welded or glued. Because the tubing bag 1 is made of only one foil film element, it can be hermetically sealed by only three sealing edges. In a further embodiment (not shown), the tubing bag can be manufactured from a closed cylindrical foil film element, in which case only end sealing edges are required. A reference liquid having a defined pH value, e.g., a pH buffer solution, is filled into the tubing bag 1 before it is completely sealed. It is also possible for the tubing bag to be filled not entirely with the reference liquid but also contain air or a protective gas. This allows the reference element to be opened without spilling the reference liquid. The foil film forming the foil film element can be multi-layered. The foil film layer on the inside of the tube bag can be made of a material that is chemically resistant to the reference solution, while the foil film layer on the outside of the tube bag can be made of a material that is resistant to environmental and mechanical influences. Between these two layers, a further layer, for example, made of aluminum foil film, can be arranged. This intermediate layer serves, among other things, to mechanically reinforce the tube bag. At least one of the outer layers of such a multi-layer tube bag is perforated, in the illustrated exemplary embodiment, along perforation line 14 near the upper sealing edge 11. This line allows the tube bag to be opened when applying the reference element, creating an access hole for the contained reference solution. One side of the tube bag 1 also has a printing field 18, in which information about the reference element, such as the pH value of the reference solution contained in the tube bag, the batch number, and similar information, can be written.

[0028] 2 shows a top view of the reference element 10, with the tube bag 1 opened along the cut line 18 in FIG. 1 and the sensor 3 inserted into the reference liquid 17 through the resulting opening at the top of the tube bag. As can be clearly seen in this view, the longitudinal sealing edges are folded in the circumferential direction so that this joint does not, or at least does not excessively, contribute to the rigidity of the tube bag, which would otherwise flatten the opened bag. This allows the opening to assume an oval or at least approximately circular geometric shape, as can be seen, so that the sensor 3 can easily pass through the opening and be inserted into the reference liquid 17. FIG. 3 shows an embodiment in which the longitudinal sealing edges 13 are offset in the circumferential direction relative to the longitudinal edges of the end sealing edges, which has a similar effect, i.e. the rigidity of the tube bag is likewise not, or not significantly affected in an undesirable way by the longitudinal sealing edges 13.

[0029] 4 and 5 illustrate the application of the described reference element 10 within a holder 2 prepared and adapted for use in conjunction with the reference element described above. As is apparent from FIGS. 4 and 5 relative to one another, the holder 2 has the basic shape of a triangular prism, i.e., a rectangular prism with a triangular base. A side surface 29 of the prism defines the lower surface of the holder, while a side edge 28 opposite the side surface 29 defines the upper edge of the holder. As can be seen in FIG. 4, the exemplary holder has three recesses 21a, 21b, and 21c, each extending from the upper edge 28 of the holder toward the lower surface. Each recess has an extent 23 parallel to the upper edge 28 of the holder, which decreases in the upper region of the recess toward the lower surface but remains constant in the lower region 22 of each recess. It can be seen that the recesses do not extend completely to the lower surface of the holder. In the recesses 21a and 21b, the reference elements 10a and 10b are shown, with the top of the tube bag unsealed. These reference elements are supported by the bottom sealing edge 12 of the tube bag against the underside of the holder and are guided in the relatively narrow lower region 22 of the respective recess. In contrast, in the upper region of the recess, the reference elements are supported in a direction parallel to the upper edge 28 of the holder by the recess edge, which flares upward. Due to the avoided shape rigidity, they can expand upward like a funnel. In this regard, the maximum width of the recess in the region of the upper edge 28 is determined and adapted to the geometry of the tube bag so that an ellipse is formed at the top of the tube bag, with an axial ratio between the minor and major axes of the ellipse ranging, for example, from 0.5 to 1. This results in an opening in the tube bag at the top, allowing a sensor to pass through the opening and reach the interior of the reference element without any problems. Within the tubing bag there is a volume of reference liquid 17, and it may be provided in particular that the reference liquids in the tubing bags 10a and 10b have different pH values. A tubing bag thus used as specified and fitted into the holder 2 has a maximum available filling height marked 16. As can be seen in the reference element 10b, the volume of reference liquid 17 is selected so that the actual filling height 15a, at which the liquid level 15 of the reference liquid 17 is present, is lower than the maximum filling height 16.In the illustrated exemplary embodiment, the volume of the filled reference liquid 17 is determined, for example, so that the actual filling height 15a when the reference element is positioned correctly in the holder 2 is approximately 60% to 75% of the maximum filling height 16. When the sensor is immersed in the reference liquid 17, the displacement action of the sensor causes the liquid level 15 to rise, as indicated by the reference element 10a. However, the safety margin between the actual filling height 15a and the maximum filling height 16 is selected to be sufficiently large so that even in this case, the liquid does not spill over the upper edge of the tube bag. Figure 5 shows the holder 2 in a front view with the reference element 10 fitted in and the sensor 3 immersed in the reference liquid 17 of the reference element.

[0030] Although the subject matter herein has been described with reference to selected exemplary embodiments, these exemplary embodiments are not intended to limit the invention as set forth in the claims, which include embodiments not expressly set forth, and embodiments that differ from the examples set forth are still covered by the claims.

Claims

1. 1. A calibration reference element (10) for a pH measuring instrument, comprising a reference liquid (17) having a defined pH value and a tube bag (1), wherein the reference liquid is enclosed in the tube bag, the tube bag being made of foil film and sealed along one or more sealing edges (11, 12, 13), the number of said sealing edges being at most three, the tube bag being at least partially filled with the reference liquid, and the calibration reference element (10) being opened by cutting the tube bag across its longitudinal extent near one of the end sealing edges.

2. 2. Calibration reference element according to claim 1, wherein the tube bag (1) is only partially filled with the reference liquid (17), the remaining volume being filled with air or a protective gas.

3. 2. The calibration reference element according to claim 1, wherein the tube bag (1) has one sealing edge (11, 12) on each of its two opposite end faces, each of the sealing edges running transversely across the longitudinal extent of the tube bag and each having a longitudinal extension defined by a longitudinal edge (111, 121) of the end sealing edge, and the tube bag is seamless in the circumferential direction or has one longitudinal sealing edge (13), which is circumferentially offset from the longitudinal edge of the end sealing edge or is folded in the circumferential direction away from the adjacent longitudinal edge (111, 121) of the end sealing edge (11, 12).

4. 2. A calibration reference element according to claim 1, wherein the tube bag (1) is made of a foil film having at least two material layers, the layer on the inside of the tube bag being made of a first material and the layer on the outside of the tube bag being made of a second material.

5. 2. A calibration reference element according to claim 1, wherein the tube bag (1) is made of a foil film having at least two material layers, and an intermediate layer made of a metal foil film is arranged between the layer on the inside of the tube bag and the layer on the outside of the tube bag.

6. 2. The calibration reference element according to claim 1, wherein the material of the tube bag in the region of the longitudinal sealed edges is pre-shaped so that the tube bag opened at the top has a relatively low shape rigidity in terms of its cross-sectional shape at the top edge and therefore, when tensioned at the closed bottom and otherwise properly held, will spontaneously change from an oval to an at least approximately circular geometric shape, which allows for easy entry of a sensor into the reference liquid inside the opened tube bag.

7. 2. The calibration reference element of claim 1, wherein the perimeter of the tube bag and its inner surface is between 70 mm and 80 mm.

8. 2. The calibration reference element according to claim 1, wherein the length of the fillable internal volume between the two end faces is 90 mm or more and 120 mm or less, and the liquid filling amount in a certain region is in the range of 15 ml or more and 30 ml or less.

9. 2. A calibration reference element according to claim 1, wherein a line is marked or provided by a suitable device on the outer surface of the tube bag along which the tube bag should be opened by the user.

10. 1. A calibration reference system for a pH measuring instrument, comprising at least two calibration reference elements, each comprising a reference liquid (17) having a defined pH value and a tube bag (1), wherein the reference liquid is enclosed in the tube bag, the tube bag is made of foil film and is sealed along one or more sealing edges (11, 12, 13), the number of sealing edges being at most three, the tube bag is at least partially filled with the reference liquid, at least two of the calibration reference elements contain reference liquids with different pH values, and the calibration reference elements with the reference liquids with different pH values ​​are coded with different colors.

11. 10. A holder (2) for a calibration reference element according to any one of claims 1 to 9, wherein the holder has at least partly the shape of a triangular prism, the prism having at least one recess (21a, 21b, 21c), the recess extending from one side (28) of the prism through a side surface of the prism adjacent to the side in the direction of a side surface (29) of the prism opposite the side, the side (28) defining an upper edge of the holder, and the side surface (29) of the prism opposite the side (28) defining a lower surface of the holder, A retainer (2) in which the recess is open at the upper edge, the width (23) of the recess parallel to the upper edge (28) decreases from the upper edge toward the lower surface (29) to a minimum width, and the minimum width is maintained in a lower region (22) of the recess distal from the upper edge along a direction from the upper edge toward the lower surface.

12. The holder according to claim 11, wherein the maximum width of the recess parallel to the upper edge is 75% to 100% of the diameter of a circle whose circumference is the same as the inner circumference of the tube bag.

13. 12. A calibration set comprising: a calibration reference element for a pH measuring instrument, the calibration reference element comprising a reference liquid (17) having a defined pH value and a tube bag (1), the reference liquid being enclosed in the tube bag, the tube bag being made of foil film and sealed along one or more sealing edges (11, 12, 13), the number of sealing edges being at most three, and the tube bag being at least partially filled with the reference liquid; and a holder as claimed in claim 11.

14. 14. A method for calibrating a pH measuring instrument or a pH sensor using at least one calibration set according to claim 13, comprising the steps of: placing the holder (2) on a base with the upper edge (28) facing up; opening at least one calibration reference element (10) near one end face of the tubing bag; fitting the opened calibration reference element into the at least one recess (21 a, 21 b, 21 c) of the holder with the closed end face of the tubing bag facing down, with the wall of the tubing bag supported by the edge of the recess, so that the tubing bag in the recess becomes an open-topped container; and immersing a pH sensor (3) into the reference liquid (17) through the opening at the top of the tubing bag; and comparing the measured value with the specified pH value of the reference liquid.

15. 1. Use of at least one calibration reference element for a pH measuring instrument, comprising a reference liquid (17) having a defined pH value and a tube bag (1), the reference liquid being enclosed in the tube bag, the tube bag consisting of a foil film and sealed along one or more sealing edges (11, 12, 13), the number of said sealing edges being at most three, the tube bag being at least partially filled with the reference liquid, for calibrating a pH measuring instrument or a pH sensor, wherein the material of the tube bag in the region of the longitudinal sealing edges is pre-shaped so that the tube bag, opened at its top side, has a relatively low shape rigidity in relation to its cross-sectional shape at its upper edge, Use in which the calibration reference element is tensioned at the closed lower side and otherwise held in place, so that the tube bag, when opened at the upper side, automatically changes from an oval shape to an at least approximately circular geometric shape at the upper edge, and the geometric shape allows for easy entry of a sensor into the reference liquid inside the opened tube bag.

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