DK158680B - ION SELECTIVE POTENTIOMETRIC MILLED CHAIN - Google Patents

Description

! DK 158680B

The invention relates to temperature-compensated potentiometric measurement chains suitable for systems for selectively measuring the activity of particular ions in solutions.

So-called ion-selective potentiometric electrodes are, in principle, a half-cell with an ion-selective membrane that can be contacted with at least a portion of its outer surface with the sample solution in which the activity of the particular ion species is to be measured. At least part of the inner surface of the diaphragm is electrically conductive connected to a conductive conduit, which in turn is connected to a measuring device with high internal resistance. At the electrodes to which the invention relates, the electrical connection of the membrane to the feed takes place over an internal electrolyte solution which exhibits a predetermined concentration or activity of the particular ion species. Typically, the feed is in communication with the inner electrolyte solution over an internal Ag / AgCl or calomel reference element, thereby providing a stable, well-defined internal contact potential. When this electrode is brought into contact with the sample solution, an electric ΡΠ potential occurs at the membrane, which depends on the difference between the ionic activity on the two sides of the membrane. The system is completed to a measuring chain by contacting an additional half-cell which provides a fixed potential and serves as a potentiometric reference electrode with the PC sample solution. The sum of the potentials of the two half-cells can be determined by connecting the two half-cells in series with each other and with an electrometer.

The membranes of such ion-selective potentiometric electrodes consist of either solid or liquid ion exchangers or neutral masking agents; their application possibilities are as diverse as the known glass membranes, each of which is selective for e.g.

H +, Na +, K +, of crystalline, e.g. of LaF ^ consisting of 35 membranes selective for F, and of liquid 2

DK 158680 B

substances such as dodecylphosphoric acid, or antibiotics such as Trinactin, which is enclosed in a porous inert solid matrix.

5 Corresponding to the Nernst equation E = k + RT / nx Flnfa ^ / aj), where 10 k = a constant, RT / nx F = the so-called Nernst factor, where n is the valence of the measured ion (at 25 ° C and 15 n = 1, the factor is 59.16 mV) a ^ = the activity of the measured ion in the sample solution and 20 aj = the activity of the measured ion in the internal electrolyte solution, the potentiometric measured potential E is linearly proportional to the logarithm of the activity aM of the 25 ionic species of interest, and usually over several orders of magnitude of the ionic activity, e.g. for fluoride ions -5 of about 1 M to below approx. 10 M when measured with a LaFg electrode. The increase of corresponding semi-logarithmically reproduced curves is given by the Nernst factor and therefore changes in temperature dependent. Theoretically, the semi-logarithmically represented curves for different temperatures at a given electrode intersect at a single point referred to as the isopotential point. Ideally, the commercial pH electrodes are arranged such that the isopotential point is close to the pH 7; the temperature compensation circuits in the commercial appliances are designed to take this custom into account.

DK 158680B

3 In practice, however, the behavior of the ion-selective electrodes as well as the reference electrodes differs from the theoretical one; the different isotherms intersect within a fairly wide range and not at a single point. These 5 deviations from ideal conditions can be traced back to two causes: first, to the time needed to obtain the temperature equilibrium, and second, to the non-linearity of the temperature / ECH characteristic of the isotherms, respectively.

10 In the above Nernst equation, the second term RT / n x Flnia ^ / a-j.) Accounts for the potential increase due to the ion-selective membrane.

The constant k represents the sum of all other potential sources in the measurement range in question, including the external reference electrode potential, the internal reference electrode potential, the liquid boundary layer potential, and the thermal potential in solutions which are in contact with the reference elements. Therefore, potential / activity curves for different temperatures intersect at an isopotential point only when the sum of all potentials contained in the term k constitutes a linear function of temperature, and a.j. is not temperature-dependent. However, these conditions are not unequivocally met by the known measurement systems.

The problem of the temperature equilibrium adjustment can be found as follows: when an electrode pair forming a potentiometric measuring chain is suddenly immersed in a sample solution of aberrant temperature, a considerable time passes until all the components of the electrodes have assumed the new temperature. This period depends on the electrode construction, the ambient temperature and the temperature drop. In a typical commercial pH electrode combination, e.g. are subjected to a temperature change of 10-20 ° C, e.g. go 5-10

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4 minutes before the internal temperature drop has dropped to a few tenths of a degree or less. During the time needed to achieve the temperature equilibrium, the measured potential changes. This problem is compounded even more by the use of internal reference elements in the reference electrode as well as in the ion-selective measuring electrode, where both reference elements depend on saturated solutions of highly soluble salts (eg Ag / AgCl). Such reference elements have a "temperature hysteresis" because, due to the low dissolution and precipitation rate of the heavily-soluble salts, there is also considerable time to attain a chemical equilibrium state alongside the new temperature setting. The slow adjustment of the chemical equilibrium in temperature change is the main factor that influences the iT: r * electrode operation and thus determines the measurement accuracy and makes frequent re-adjustments necessary.

If one waits long enough (in many cases for hours or days), the chemical and thermal equilibrium can settle at the known commercial electrodes upon which reproducible reliable data can be obtained; but these conditions may not always be met by fixed-time measurements or process control and are therefore not applicable in practice.

US Patent No. 3,445,363 discloses a glass electrode measuring chain in which all isotherms intersect at the same point, preferably in the vicinity of pH 7 and at the electrical zero of the measuring chain potential. For this purpose, the inner electrolyte solution contains a buffer of a mono- or polybasic acid, such as p-nitrophenol, a mono- or polyvalent base, such as morpholine, and a supplier of at least one of the ionic species which determines the potential of it. internal reference element which is in contact with the buffer in one

DK 158680B

5, such as water or glycerol. The internal reference element is constituted by a conventional metal / metal salt element, e.g. an Ag / AgCl or Hg / Hg2Cl2 reference element.

5

When appropriate selection of the buffer, the temperature dependence of the relevant pH value is used to compensate for the other temperature dependencies. Nowhere in this print is there any reference to the fact that specially soluble redox systems could be used for this purpose, and without the need for reference elements of the second kind.

The object of the invention is therefore to improve the prior art related ion-selective potentiometric measurement systems by specifying a potentiometric measurement chain which avoids the disadvantages associated with the use of second-type internal reference elements, such as Ag / AgCl, and which at high temperature independence of the EMK-20 value within a wide temperature range at the same time exhibits high impact velocity and long-term stability, even at measured values in the submillivolt range, due to higher setting speed of the thermal and chemical equilibrium than previous electrodes of this type.

25

This potentiometric measuring chain must also be adjustable during manufacture and usable without temperature compensation circuits. Finally, it must also have a simple structure and be manufactured at a favorable price of 30.

This is achieved with a measuring chain, which is peculiar to the characterizing part of the claim.

35 In H.Ebert, 'Electrochemistry kurz und Biindig', Vogel-Verlag, 1972, Chapter 2.8.4, pages 87-95 are the basic types of conventional potentiometric measuring and reference electrodes 6

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and their function and potential relationships described in detail. On page 89, right column, first paragraph, states that all electrochemical reactions are redox processes. However, the concept of redox potentials can only be used for semiconductors or measurement chains built up, the potential of which depends on the movement of ions of one and the same element, ie on substances which can occur in several oxidation steps or valences. Examples of such redox systems include 10 Sn4 + / Sn2, [Co (CNg] + - / [Co (CNg) 4-, reversible organic redox systems such as vitamin K, pyocyanins and flavins, organic redox catalysts such as cytochrome, flavin enzymes, catalases, copper proteins, peroxidases and pyridine enzymes, the chinhydron electrode, and the antimony and bismuth electrode, which are similarly based on ions or compounds present in several oxidation steps and can be reversibly transferred therein.

Furthermore, from page 89, right column, second paragraph of this publication 20 it can be deduced that ions of this dissolved in the surrounding electrolyte can be used instead of an electrode of redox electrodes instead of an electrode of the same metal. the reduced form of the redox pair may be used platinum or another foreign metal. In this case, however, the platinum or foreign metal does not even participate in the redox reaction as it is not ionized during electron release. The electrode is thus - unlike e.g. Ag / AgCl electrode with Ag lead - not part of the redox-dependent 30-relay system, but only for pure potential lead. A typical example of this is the quinhydron electrode illustrated on page 90, left column, the middle, in which the two components of the redox system, the quinhydron-formed quinone and hydroquinone compounds are present in the electrolyte solution and there for potential derivation is placed a platinum wire.

7

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Moreover, a platinum wire immersed in a redox system is not generally useful as a reference electrode, because depending on the redox system chosen, very long potential setting times are obtained and measurement errors of up to 5 to ± 25 mV can occur. For this reason, reference elements within the state of the art so far are used without exception reference elements of the second kind, ie. electrode systems whose potential depends indirectly on the concentration of the ions of their electrolytes, which originates from a heavy soluble substance present as a bottom material.

Electrodes or reference elements of the second kind can be produced with more than three orders of magnitude better reproducibility in the µV range and exhibit an accurate potential of four decimal places (see also H.Ebert, above, page 15 92, left column, second section).

The iodide / triodide electrode is also described in K. J. Vetter 'Electrochemical Kinetics', Springer-Verlag, 1961, pages 370-374. In this publication, 20 is a purely scientific monograph on the kinetics of electrode reactions, which are involved in the elucidation of reaction mechanisms. The temperature dependence of the redox potential of such electrodes is as little mentioned in this publication as the importance of the interchange current density 25 for the influence ratio of electrodes, so that one of ordinary skill in the art would not be able to obtain any information on the suitability of the system iodide triodide for redox electrodes.

DE presenting specification No. 1,297,359 relates to an electrochemical measuring chain for potentiometric determination of ion concentrations and in particular to measurement having both a measuring electrode and a reference electrode, the glass electrode serving as a reference electrode exhibiting an inferior steepness in relation to the measuring electrode. This prior art assumes the task of avoiding the main source of disturbance for electrochemical chains

DK 158680B

8 there with transfer, namely the diffusion potential. The problem of the temperature dependence of the potential of such electrodes or a temperature compensation of the ECHR value is not discussed in this publication.

5

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1: a schematic cross section through a potentiometric measuring chain according to the invention; FIG. 2: a schematic longitudinal section of the measuring chain of FIG. 1 along line 2-2, FIG. 3: an idealized graphical representation of the potential / time curve of temperature changes of a typically known membrane electrode; FIG. 4: a graphical representation of the potential / time curve 20 for temperature changes of the measuring chain according to the invention; FIG. 5: a graphical representation of the dependence on the contact potential for a number of concentrations and proportions of redox-paired iodide / triodide; FIG. 6: a graphical representation of the voltage / temperature curves for consecutive buffer / redox pairs and FIG. 7: a graphical representation of the temperature coefficient of a buffer solution and the temperature dependence of the pH value measured with an electrode device according to the invention and with a known electrode device.

The potentiometric measuring chain according to the invention comprises a measuring and a reference electrode, each comprising a housing with an electrolyte internal solution which is electrically 9

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connected a lead for the external connection of the electrode to a measuring device. The electrolyte, which preferably does not react with the sample solution, according to the invention contains a thermodynamically reversible redox pair 5 with high exchange current density. The charge consists of platinum, which is in direct contact with the electrolyte and is inert to it. The term "potentiometric measuring cloth" shall refer to all measuring chains used for electrochemical measurements of the type in question and, in particular, include measuring chains with ion-sensitive measuring electrodes.

The term "redox pair" refers to a system which, due to various valence steps of a given element or compound, contains both oxidized and reduced steps, these steps being in a reversible thermo-dynamic equilibrium, ie. each step can be transformed into the second stage from an infinitely small change of potential by an infinitely small change in potential. Hereby, an inert metal which is in contact with the redox system of the electrolyte has a defined and reproducible potential, depending on the activity conditions of the two stages.

FIG. 1 and 2 illustrate a potentiometric measuring chain 25 of the invention. It comprises a measuring cell 21 with a housing in the form of an elongated tubular container 22, which typically consists of a liquid impervious, essentially mechanically solid, electrically insulating material, e.g. of one of the known high molecular weight polymerisates 30 or glass which is substantially chemically inert to the electrolyte.

The end of the container 22 is closed with a membrane 24, which is typically in the form of a sphere and consists of an ion-sensitive material. In the embodiment of FIG. 1 and 2, which serve as an example, membrane 24 is a pH membrane; however, it may also consist of various others per se

DK 158680B

10 known materials, each of which is sensitive to a particular ion species. Preferred dimensions as well as the shape and strength of the membrane 24 are obvious to those skilled in the art and depend on the nature of the material from which the membrane is made.

5 The close connection between the membrane 24 and the end of the container 22 also depends on the material and the method known per se.

Potentiometric electrodes of this kind, as is known per se, typically contain an ion conductive internal electrolyte electrically connected to an external conductor. The measuring chain 20 contains e.g. in the container 22 an electrolyte solution 26 of e.g. a phosphate buffer (e.g., a solution of 0.05 mol / l Na 2 PO 2 and 0.05 mol / l 15 ^ 2 ^ 0 ^) to determine the activity of the hydrogen ions, i.e. of the pH value, and a particular redox pair to be described in more detail. The electrolyte can thereby also be present as gel or sol. The electrolyte solution 26 is in direct physical and electrical contact with at least one end 20 of the conduit 28.

The combination of the electrolyte solution 26 and the conduit 28 serves to electrically connect the electrode externally while maintaining stable internal potential 25 arising from the various connections or transitions between the various materials in contact with each other in the electrode structure. The measurement electrode described above is an ion-sensitive electrode which differs from the usual ion-sensitive membrane electrodes at the redox pair added to the electrolyte. However, unlike the known pH electrode devices, the inner electrolyte solution 26 is also not connected to the electrolyte solution over an internal standard reference element, such as the usual 35 Ag / AgCl electrode, but directly over the platinum 28 which has a high electrical conductivity and is chemically inert to the electrolyte. 1 a preferred 11

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In embodiment, the conduit 28 merely consists of a platinum wire.

The electrolyte solution 265 used in accordance with the invention necessarily contains a redox pair having a relatively high exchange current density of about 1 x 10- 10 A / cm or more. The exchange current is as known a kinetic property of redox pairs; it is a measure of the reversibility of the redox pairs reversible According to the invention, the redox-paired iodide / triiodide (31 -> 1 + + 2e) is used.

As is well known, a practically inert electrically conductive material, such as platinum or a similar precious metal, contacts a solution containing both the oxidized and the reduced form of the redox pair with a given and reproducible contact potential. At a temperature change of the solution, the equilibrium state between the reduced and the oxidized form of the redox pair undergoes rapid displacement to produce a new potential which, while achieving the new equilibrium state, assumes a given reproducible magnitude. Since no material of the feed needs to be dissolved or precipitated, the change of potential takes place very quickly, namely within seconds or less. Preferably, the redox pair in the electrolyte solution has a very low temperature coefficient for the redox potential, thus overcoming the problem of the slow transition to the temperature equilibrium state. An example of a favorable suitable redox pair is iodide / triiodide having e.g.

I at a concentration of 5.68 mol / l and IQ at a concentration of 3.6 x 10 mol / l. Using such a system, temperature changes of the electrode have component parts of the device, including the interior electrolyte or sample solution to be measured, none relevant 12

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influence on the cell potential so that accurate and reproducible cell potentials can be measured even before reaching the equilibrium state.

5 The measurement cell 21 described in the electrode assembly of FIG.

1 and 2 can be schematically reproduced as follows:

Pt / electrolyte; In ", Ig" / glass / sample solution.

The second half-cell, which is the reference electrode, can be reproduced as follows: sample solution / electrolyte;

15 The electrode arrangement illustrated by way of example in FIG. 1 and 2, a reference electrode 29 comprises a housing in the form of an elongated tubular container 30, typically of a liquid impervious, substantially mechanically solid, electrically insulating material, preferably of the same material as the container 22. The container 30 has for example, an aperture provided with a diffusion stretch known per se, known as fiber wall 32. However, this diffusion stretch may in a manner known per se also be a porous frit, a liquid-leaking seal or a porous polymer. The diffusion line 32 is to form a free diffusion line for an electrolyte flow between the inner and outer sides of the container 30. An inner electrolyte solution 34, which can typically consist of any suitable filling solution, such as 3.5 mol / l KCl, is provided in the container 30. It constitutes an ion source of similar conductivity and, according to the invention, necessarily contains the selected thermodynamically reversible redox pair iodide / triiodide with the required exchange current density, as does the electrolyte solution 26. In the reference electrode 35 29, no buffer is required in the electrolyte.

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13

Finally, the reference electrode 29 as electrically conductive, to the chemically inert conduit 36 to the electrolyte solution 34 contains a platinum wire which is in physical contact with the electrolyte solution 34.

5 The measuring electrode 21 and the reference electrode 29 are integrated into an integrated structure. According to FIG. 1 and 2, for this purpose, a third housing 38 is provided which encloses the containers 22 and 30 and is preferably sealed 10 around the container 22 or welded thereto immediately above the connection between the container 22 and the membrane 24. The housing 38 is preferably also filled with a electrolyte solution 40, e.g. a 3.5 mol / l KC1 solution compatible with the electrolyte solution 15, from which there can be a diffusion to the electrolyte solution 34. across the diffusion line 34, to complete the measuring chain and to obtain an ion conducting line between the reference electrode 29 and a sample solution in contact with the outer surface 20 of the membrane 24 is arranged another narrowed flow connection 42 in the form of a small opening in one wall of the housing 38, preferably close to the membrane 24.

To clarify the differences between the invention and the state of the art, a typical conventional commercial pH electrode combination (Modeli 91-02, Fa. Gebr. Moeller, Zurich) was examined in two subsets of one to a pH value of 4.01 buffered sample solution. One solution was thereby first brought to a temperature of 80 ° C, while the other solution was kept at ca. 26.4 ° C. The usual electrode combination was alternately immersed in the two solutions, and it took place with the shortest possible time between the withdrawal of one solution and the immersion in the other. The electrode device was left in each solution until the potential value read on a high potential commercial potential measuring device.

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impedance, was reproducible. According to FIG. 3 required the known electrode device at both temperatures for about 1-2 minutes. for the delivery of a practically reproducible potential E (mV). Furthermore, small changes in the temperature of the two solutions could be observed, presumably due to the heat transfer between the solutions and the electrode device.

An electrode device according to the invention having a pH-sensitive membrane and an internal electrolyte solution of a phosphate buffer solution with an iodide / triiodide redox pair was subjected to the same test as the known electrode device. By periodic exchange between the two solutions with temperatures of 25 ° C and 80 ° C respectively, according to FIG. 4 achieved reproducible potentials in less than ca. 30 s, thereby confirming the virtually complete temperature independence of the electrode device described.

The simultaneous use of the redox pair in the inner electrolyte solution and its electrical coupling with a chemically inert feed of platinum results in a faster reaction to temperature changes than to the known device. Also important is the value of the temperature coefficient of the redox pair, ie. the change of its contact potential with the conduit due to equilibrium shifts depending on the ambient temperature.

As an example, the temperature coefficient of the redox-paired iodide / triiodide, which has the necessary high exchange current density, was determined for different concentration ratios of the oxidized and the reduced step. The results obtained are shown in FIG. 5. The various 35 curves of FIG. 5, in which the contact potential E (mV) of a platinum wire is plotted against the temperature (° C), was measured experimentally for those listed in the following table.

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different concentrations and ratios of triiodide / iodide.

TABLE

5 --—— -——— - -

Concentration Concentration Relationships

Curve of I3 "(mol / l) of I ~ (mol / l) (Ι3" / (ΐ “) 3) 10 A 1.835 x 10-2 2.84 8.0 x 1 (T4 B 8.25 x 10 ~ 3 2.84 3.6 x 10-4 C 4.58 x 10 "3 2.84 2.0 x ΙΟ-4 15 D 1.0 x 1 (T3 2.84 4.37 x 10" 5 E 3.67 x 10 "2 5.68 2.0 x 10-4 20 -: -

Curve E shows that the temperature coefficient is not only a function of the mass-effect ratio triiodide / iodide, but also depends on the absolute concentration. From the curves of FIG. 5 it can be seen that the slope, shape and symmetry of the temperature dependence characteristic of the given redox pair can be selected in the range of about one at approx. The 25 ° C isopotential point corresponded to the concentration of the redox pair steps and their mass-action ratio. This fact is significant considering that the electrolyte solution filled into pH measurement electrode 21 constitutes a mixture of the redox pair and a buffer for determining the activity of the ion of interest, in the present case of 35 H +.

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16

The temperature dependence of the potential E of a typical buffer, such as the phosphate buffer used in the measuring electrode 21, constitutes, as curve A in FIG. 6 shows essentially an upward concave of the second degree when the dependence of the potential 5 or pH value on the temperature is graphically depicted. In the same view, on the other hand, the redox pair of the invention selected according to the invention gives a temperature dependence according to curve B, which is downwardly concave, so that the two temperature dependencies are substantially opposite of each other. Thus, according to the invention, an electrolyte solution 24 is provided for pH measurement chains and other ion sensitive measurement chains using a buffer for internally determining the activity of the interesting ion in the measuring electrode, at which the two temperature dependencies approximate each other and thus the respective electrode becomes practically temperature independent.

The effect of the practically inverted adaptation of the temperature coefficients of redox pairs and electrolyte in an ion-sensitive electrode or reference electrode in measurement chains of the invention can be shown particularly clearly by means of measurements with a described pH measurement chain versus a sample solution. Thus, e.g. prepared a sample solution of a standard phosphate buffer whose actual published temperature characteristics are represented by curve A of FIG. 7. The pH of this sample solution was determined at numerous temperatures in the range of ca. 5 and approx. 90 ° C with the aforementioned commercial pH electrode device. The pH measurement values thus obtained are represented as curve B in FIG. 7. Remarkably, the mismatch between curves A and B. Furthermore, upon further repetition, curve B was not very well reproducible. Using a disclosed pH electrode device, whereby the electrolyte solution 26 loaded in the measuring electrode 21 was a mixture of buffer and redox pairs of 17

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on the other hand, curve C was obtained, which on several repetitions proved to be highly reproducible. The agreement between curves A and C illustrates the very significant difference between the temperature characteristics of conventional electrode devices and electrode devices of the invention, and the virtually complete temperature independence achieved with the latter.

10 15 20 25 30 35

Claims (1)

GB 158680B Patent Claim: Ion selective potentiometric measurement chain for determining the activity of a particular ion species in a sample solution, comprising two potentiometric electrodes optionally joined to an integral structure consisting of a measurement electrode having an end for an ionic ion sensitive membrane in question. and is filled with an internal electrolyte solution having a predetermined concentration or ionic activity and over a lead of an inert electrically conductive material contacts a reference electrode such that the two electrodes are connected in series with each other and with an electrometer and the reference electrode, in turn, is filled with an inner electrolyte solution which is also in contact with an inlet of an inert, electrically conductive material and provides a determined potential, and is enclosed by a further container which is also filled with a 20 electrolyte from which a diffusion can occur over a compound the reference electrode, while this container also has a narrowed flow connection to the sample solution, characterized in that the inner electrolyte solutions in the measuring and reference electrode contain redox-paired iodide / triodide (3I -> I2 + 2E-) dissolved in a concentration of 2.84 to 5.68 mol / l I and 1.0 x 10 "3 to 3.67 x 10-2 mol / l Ig ~. 35