CH627278A5 - - Google Patents

Description

The invention relates to an electroanalytical cell for distinguishable room parts of existing space from the following perometric measurements, in particular for reasons of oxygen measurement: The surface of the measuring electrode should generally be as close to the inner surface of the membrane as possible for measuring the concentration of one.

electroactive component is used, which are dissolved or in bran, i.e. the "thickness" of the electrolyte space is distributed between other ways in a fluid that is gaseous, such as a membrane and the surface of the measuring electrode should be small, for example a gas mixture, or liquid, such as water, and is e.g. filled with a very thin layer of electrolyte,

can be. which has a thickness in the micrometer range, so that the

Electroanalytical cells, also referred to as transducers or measuring cells at the start of the measurement and / or in the case of change value transducers, and processes for their operation for the analytically interesting measured concentration-quantitative electrochemical analyzes of chemical substances are rapidly addressed.

known, see e.g. US Pat. No. 2,913,386.3,071,530.3,223,608. On the other hand, the electrolyte compartment should also be an electrolyte compartment.

3 227 643.3 272 103.3 406 109.3 429 796.3 515 658 and reservoir with a certain volume, typically in the size

3,622,488. Such transducers or cells are technically classified as milliliters or fractions of milliliters,

3,627,278

sen. Although this storage space is intercepted small in absolute sizes without the measuring current seeming to be between, it is significantly influenced by several orders of magnitude larger than the measuring electrode and counter electrode. Volume of the thin electrolyte layer near the Arbeitsselek- The trade known from DE-OS 1 598 193 and 1 673 261. Protective electrodes are designed as tubes for this purpose

For the purposes of this description, the inner wall with the cylindrical outer wall of the thin-film region of the electrolyte space covered by the measuring branch, the insulator body surrounding the electrode, forms an essentially normally on the surface of the working electrode and a tubular gap; this gap represents the channel to which the adjacent insulator lies, referred to as the «first part of the electrolyte - the only spatial connection between the first and the space», to form it from the remaining part of the electrolyte- the second part of the electrolyte space, and one example -Distinguish space, which includes, among other things, the electronic method for measuring oxygen in the second part of the electrolyte supply and is in contact with the surface of the counterelectrode in the oxygen component present or penetrating there. (“Contamination”) can be on the hollow cylindrical surface

From a structural and functional point of view, that of the known protective electrodes is defined, i.e. from the partial

The surface of the measuring electrode practically «limits» the electrochemical reaction in the area of the first part of the electrolyte compartment. The expression «Limiting- 15 measuring electrode can be held.

tongue »is generally used here to cover all physical, i.e. As explained in more detail below, it has been found that physical parts of the cell (including the membrane) for a high and practically 100% capture probability, which are intended for contact with the electrolyte, denote the protective electrode a relatively good geometric definition

The particular importance of the thin electrolyte layer in the gap or a defined ratio of the width (or length-first part of the electrolyte space is detailed below) of the gap to its thickness is significant.

described. Now the geometry of the tubular electrolyte

When measuring the concentration of an electroactive material gap of protective electrodes according to DE-OS 1 598 193 and fes in a fluid which touches the outer surface of the membrane at 1 673 261 compared to the geometrically undefined of the amperometric measurement method mentioned above, there is a “gap” between a protection designed as a wire loop, the desired current contribution originates only relatively well from the diffusion electrode and the surrounding cell chamber walls according to the ionic active substance through the membrane in the first DE-OS 2 348 263; the thickness of the part of the electrolyte space and the corresponding electro-known tubular electrolyte gaps at the protective electrodes mix reaction of the substance on the surface of the measuring electrodes according to DE-OS 1 598 193 and 1 673 261, however, is relatively trode. In practice, however, additional and undesirable current contributions to the “length” (hereinafter also referred to as “width”) of the column are observed, i.e. such posts that are not large by 30. In addition, this ratio depends on the practical concentration of the electroactive substance to be measured, for reasons such as manufacturing accuracy and overall dimensions of the fluid. These undesirable contributions limit solutions of the cell, and in the case of tubular gaps it does not readily reduce both the accuracy and the sensitivity of the measurement.

systems and also cause problems of stabilization. The object of the invention is to provide an amperometric measurement

the signal in the transient state, the stability of the signal in the 35 zelïe with the steady state mentioned in the preamble of claim 1 and generate unwanted noise signals. Specify characteristics where the capture probability

From DE-OS 1 598 193, an electroanalytical possibility of the protective electrode is significantly increased and bronchial cell is known, which can be achieved to improve the characteristic a higher measuring accuracy.

Response, sensitivity and stability properties of an invention is achieved by the fact that the Ka has so-called “ion deprivation devices” which are either chemically or electrically determined as an ion adjacent to the first part of the electrolyte space Switched off contact with electrolyte. matched surface of the protective electrode and the membrane

In the case of the electrically acting device, it can also be formed.

An external potential is worked on a protective electrode. According to a preferred embodiment, the surface which is in addition to the measuring and counter electrodes is 45 concentric around the protective electrode and is practically turned around. This protective electrode is part of an electrolyte-shaped measuring electrode surface and the channel lying as a gap from the cell space, which is the only spatial channel formed only by the thickness of the insulator layer between a first part of the electrolyte space, which can be seen between the Measuring electrode and the protective electrode from the first of the measuring electrode surface and the adjacent part of the electrolyte space.

Membrane is located, forms with a second part of the electrolyte space in connection with the preferred region of the counterelectrode just mentioned. Structure of the cell according to the invention is to be noted that

Protective electrodes with a similar effect for electroanalytical concentric arrangements of three electrodes e.g. from the membrane cells are also from DE-OS 1 673 261 and DE-OS 2 129 414 for open, i.e. not known with membranes - the DE-OS 2 348 263. covered cells, possibly in addition to the measurement and

The effect of such protective electrodes is based on the fact that 55 counterelectrodes have a reference electrode, which are in themselves potentially influencing the measurement, but are not known.

desired electrochemical component according to a further preferred embodiment which can be intercepted by the protective electrode, e.g. In the case of the cell according to the invention, the surfaces of the measuring electrode oxygen measurement are the oxygen which is arranged practically coplanar as the remainder in the second trode and the protective electrode. Electrolyte space is present or through cell walls or 00 Also preferred is a cell according to the invention, in the case of insufficiently dense cell connections in the second electrolyte or the gap (viewed in cross section in a radial plane, space penetrates. In order to influence the measurement, the latter would have to begin extends outwards on the axis of the cell head, which is concentrically constructed and is not connected to the desired measurement size)) has a width dimension of material in the first part of the electrolyte space and there, which practically pass through the width of the ring-shaped measuring electrode surface; However, the protective surface of the protective electrode is now determined, and has a thickness electrode opposite the counter electrode of a suitable dimension, which is determined by the distance between the upper and e.g. same or similar potential as the measuring electrode surface of the protective electrode and the inner surface of the membrane, so the unwanted oxygen on the protective electrode bran is determined, the ratio of the width dimension

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for the thickness dimension is at least 7.20 corresponding to a capture probability of at least 99.999%.

The radial distance between the circular surface of the measuring electrode and the annular surface of the protective electrode can be kept very small, e.g. not more than 100 microns and even less than 50 microns e.g. 10 to 30 microns.

Furthermore, the size of the annular surface of the protective electrode exposed to the electrolyte can be less than 50%, e.g. less than 20%, the size of the practically circular surface of the measuring electrode exposed to the electrolyte.

It has been found that the channel of a cell according to the invention acts as a diffusion gap and offers a surprisingly simple and effective means of eliminating undesired contributions to electricity which are caused by diffusion and / or leakage of electroactive substances into the electrolyte space in regions remote from the first electrolyte space part.

Obviously, at a boundary of the diffusion gap, such a protective electrode is capable of electrolyzing "impurities" (this expression also includes the measured electroactive substance, such as oxygen, if it penetrates into the electrolyte away from the first electrolyte space) at or near the edge of the Switch off the measuring electrode surface when the system is in a steady state. The cell according to the invention enables the measuring range to be expanded to very low concentrations, e.g. below the ppm range; preferred embodiments of cells according to the invention offer quantitative oxygen detection down to concentration values of 5 parts per billion (5 * 10 ~ 9), which offers an extension of the range of measurement in the low concentration range by about a decade compared to the most sensitive known detectors.

In addition, the decay times (start-up time and delay in the response to changes in concentration), probably as a result of the simplified transient response, can be significantly reduced, e.g. by more than 90%.

The invention is explained in more detail below using exemplary embodiments with reference to the drawings. Show it:

1 shows the schematic of the section through an electrolyte area on and near the protective electrode of an electro-analytical cell,

2 shows a semi-schematic cross section of a preferred embodiment of a cell according to the invention,

3 shows a top view of the cell shown in FIG. 2, FIG. 4 shows a simplified equivalent circuit diagram of the cell,

5 shows a semi-schematic sectional illustration of the boundaries of the electrolyte space in a special embodiment of the cell according to the invention,

6 shows a current / voltage curve,

7 shows the diagram of the time dependence of the current on the protected electrode according to the invention and an unprotected electrode for comparison purposes, and FIG. 8 shows the circuit of an electronic measuring instrument suitable for use in cells according to the invention.

1 shows, in a schematic and broken representation, a vertical cross section of an electrolyte space area for explaining important terms: this area is defined by the surface 110 of a measuring electrode 11 exposed to the electrolyte, the surface 120 of a counter electrode 12 exposed to the electrolyte, and the surface 130 exposed to the electrolyte a protective electrode 13, the surfaces 150 and 170 of a first insulator 15 and a second insulator 17 exposed to the electrolyte and the inner, ie surface 140 of a semipermeable membrane 14 exposed to the electrolyte.

The outer surface 141 of the membrane 14 is in contact with a fluid, not shown, which contains the electroactive substance, the concentration of which is measured. The electrolyte space part 191 between the surface 110 and 150 on the one side and the same area of the membrane surface 140 on the other side represents the first electrolyte space part.

The surface 130 of the protective electrode 13 forms a boundary of a diffusion gap 193, which forms the only spatial connection between the first electrolyte space part 191 and the entire remaining electrolyte space part 199. The other limitation of the diffusion gap 193 is the closest area of the inner membrane surface 140 with the same area.

On the other hand, the term “diffusion gap” is intended to contain the condition that the “width” dimension B3 is normally at least as large and preferably larger than its “thickness” dimension D3, in order to ensure a high probability of trapping contaminants on the protective electrode surface 130 if it is kept at a controlled potential, preferably at the same potential as the measuring electrode surface 110. It should be emphasized in this connection that the protective electrode device is electrically separated from both the measuring electrode and the counter electrode device by insulators and represents a conductive component of the cell that is to be distinguished from the other electrodes. The term "electrode device" on the other hand is intended to mean both one-piece structures, e.g. those in which the surface exposed to the electrolyte is a surface part of an electrode body, as well as composite structures, e.g. those in which the surface exposed to the electrolyte of a given electrode device is formed by two or more segments which are connected to one another via a metallic conductor 35 and / or in which the surface exposed to the electrolyte is a relatively thin flat structure on the top of a carrier structure , which consists of another, electrically conductive or else electrically non-conductive material.

In relation to the gap dimensions B3, D3 shown in Fig. 1, a high value of the ratio B3: D3 is preferred, e.g. a B3 dimension in the millimeter range and a D3 dimension in the micrometer range.

For the reasons explained in more detail below, the width dimension D5 of the insulator surface 15 and accordingly the lateral distance between the surfaces 110 and 130 of the measuring electrode 11 and the protective electrode 13 is preferably small, e.g. 100 microns or less. Other dimension parameters, such as Dl5 D2, D7, Bj and B2 can be selected in the manner known per se for measuring cells. According to a preferred embodiment, D! practically the same as D3. The "length" parameter of the diffusion gap 193, i.e. whose dimension perpendicular to the plane of the drawing is determined by the length dimension of the surface 130 of the protective electrode 13 55. According to a preferred embodiment, surface 130 is endless like a ring-like structure, i.e. closed like a round or polygonal ring. Accordingly, the diffusion gap preferably has a correspondingly endless or annuloid, i.e. ring-like, developed shape. 60 The special cross-sectional shape or the profile of the diffusion gap 193, i.e. the shape of the area between the boundaries 130, 140 and the lines A and C does not have to be «rectangular» or «regular» and can be irregular, curved or different if the D3, 65 perpendicular to the main longitudinal dimension B3, is considerably smaller latter.

A preferred diffusion gap is one between an annular or annular surface of the protective

5

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electrode and a uniformly shaped surface area which is in a distance - if the protective electrode is at protective potential, the nem of the desired thickness dimension of the diffusion gap contamination from the protective electrode surface 230 is adjacent or parallel to the upper surface, i.e. for the measuring electrode surface 210 is electro-protective electrode. The corresponding preferred made inactive. The “captive” and practically coaxial structure of the measuring cell that is “captured” in this way is then no longer explained with reference to the inclination of the measuring electrode 21 FIG. 2. measured current at.

2 shows a measuring cell 20 with a generally coaxial structure. FIG. 3 shows a plan view of the coaxial measuring cell structure, which is shown in a semi-schematic cross section in a plane lying parallel to the structure of FIG. 2 through a transparent membrane axis K. The measuring electrode 21 24, and shows the disk-like shape of the first electrolyte is shown as a cylindrical body with a circular-shaped part of the space on the top of the measuring electrode surface 210 th surface 210 exposed to the electrolyte. and the adjacent first insulator 25, the annular A thin sleeve 25 made of insulation material, e.g. An organic formation of the diffusion gap on the top of the protective polymer is sealingly inserted between the body of the electrode electrode surface 293, the adjacent second insulator 21 and the protective electrode 23, the latter the 27, the counter electrode surface 220, the surface 260 of the

Has the shape of a tubular body with an annular, 15 tubular housing 26 and the pinched part of the electrode surface 230 exposed to the electrolyte. Membrane 24 and the 0-ring 28.

As can be seen from FIGS. 2 and 3, the first insulator is intended to be thin on one side in the first electrolyte space part 291 and the actual thickness or radial width is essentially defined and limited by the measuring electrodes - an insulator 230 should be made from the surface 210 explained in more detail below, because the surfaces exposed to the electrolyte are as small as possible. In practice, the area of the thin insulator 25 can be neglected, the actual radial dimension of the electrode surfaces, which can, and the adjacent identical inner surface insulators and the housing in a coaxially constructed measuring range of the membrane 24. The diffusion gap 293 is defined by the cell shown in FIG 2 and 3 are smaller, and is generally determined in the annular surface 230 of the protective electrode 23 by the desired size of the surface of the measuring electrode which is exposed to the part and the same area of the membrane 2s electrolyte. If inner surface 24 delimits and represents the only spatial connection, for example, a diameter of the circular first manure between the first electrolyte space part 291 and the rest of the electrode surface is in the range of about 2 to about 25 mm of electrolyte space part 299. The rest of the electrolyte assumes (where neither the lower nor the upper value as critical

space part 299 comprises the electrolyte reservoir on the surface of the table) and a corresponding size of the 220 of the counterelectrode 22, which can be seen as a tubular body between the surface exposed to the electrolyte in the range from about 10 to the tubular insulator 27 and a tubular to about 2,000 mm 2, the radial distance between the jacket or housing 26 should be shown, which usually consists of an electrode surface 210, 230 and, accordingly, the insulating material. "Thickness" of the intermediate layer of insulator 25 is preferred

The membrane 24 lies on the surface 260 of the wall of which should not be larger than 100 micrometers, a value of we housing 26 being on and a well fitting 0-ring 28 being around 35 micrometers less than approximately 50 micrometers, e.g. about 10 to 30 micrometers, recess 266 in the outer wall of housing 26 is particularly preferred. A lower limit of the thickness can be used to hold the membrane 24 in place. This type of membrane determined by the electronic conductivity of the insulator is known per se. this condition, which should preferably be observed, is dependent on

The electrical connections 215 and 225 are of particular importance for oxygen measurements with high

known way of applying a potential between the 40 sensitivity. Although the protective electrode practically switches off at the protective potential of the first and the second electrode and for the amperometric current contributions, this is provided by oxygen diffusion measurement of the chemical reaction by diffusion or penetration of oxygen through leaks in the ion of the electroactive substance from the fluid, not shown Electrolyte space removed from the first electrolyte space part 291 on the outside of the membrane 24 through the membrane and things like, the infiltration of oxygen by the first subsequent electrooxidation or electroreduction of this 45 insulator 25, for example from the «back», i.e. The lower part of the substance on the surface 210 of the measuring electrode triggered cell 20, causing a current contribution that is not completely captured by the protective electrode 23.

The surface 230 of the protective electrode 23 is over the For this reason (a) the seal at the interface

electrical connection 235 is kept at a controlled potential between the first insulator 25 and the neighboring ones, which electrodes 21, 23 are as effective as possible for protecting the surface 210 of the measuring electrode 50, e.g. according to which 21 is effective against undesirable contributions to the amperometric measurement pressure described in the above-mentioned DE-OS 2 710 760. Such a potential will also be achieved here as a sealing method, and (b) that referred to the "protective potential" for the insulator 25 and is preferably practically the material selected should have a low permeability for gas-equivalent potential as that of the measuring electrode, i.e. possess the respective oxygen

Potential against the counter electrode 22. 55 The second condition (b) just mentioned is special

The potential applied to the protective electrode is therefore of importance if an organic, corresponding limitation of the diffusion gap 293 to polymer is used for the first insulator. Has a protection potential for the present purpose. If a contamination, e.g. Acid-insulating polymer then contains a “low” oxygen permeant or another electroactive substance, in the electrolytic ability, if the oxygen permeability value of the insulator 299 is permeated, for example by the overlying it is considerably smaller than that for the semipermeable membrane 24, due to a “leak” between the membrane 24 used polymer.

bran 24 and the recess 266, over an interface of the oxygen permeability values can according to ASTM measurement

Measuring cell structure or a similar leak, so method D 1436-63 could be determined and in units with the dimension and would such contamination with a usual measuring ml • (24 hours) -1 • (100 inches2) -1 • (atm) - 1 • 0.025 mm, cell in the first electrolyte space on the measuring electrode surface - measure at 25 ° C. Special values diffuse from the butt surface and are used for amperometric measurement, for example, in Mod. Plastics Encyclopedia. In the measuring cell according to the invention, the contamination (1968-1969) must be published (see “Film Chart”, page 526 ff., However pass through the diffusion gap 293. of the publication just mentioned).

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6

Polytetrafluoroethylene, a preferred material for membrane 24, which is not shown in the circuit 30 for the sake of simplification, has, for example, an oxygen permeability of several functions, which in a practical embodiment was 1,000 ASTM units and a preferred polymer for the invention Measuring cell are present, include the first insulator 25 has a considerably smaller oxygen-distributed capacitance parallel to the current generators 36 permeability value, for example 10% or less of the permeability 5 and a leakage value of the membrane not detected by the protective electrode 33, i.e. 100 ASTM units or less. Oil resistance parallel to element 34. The non-detected polymer, which is particularly suitable for the first insulator, the leakage resistance essentially determines the lower possess oxygen permeability values of less than 10 limit of the sensitivity of the measuring cell according to the invention. ASTM units. With reference to the special components of the circuit

Of course, a small thickness of the first insulator (Ab 10 30:

Measurement D5 in Fig. 1) is preferred, among other things, to reduce the acid (i) counterelectrode 32 as a direct contact material permeation through this insulator, which can be represented from the electrolyte. In order for this to be the case, a considerable length-electrode reaction on the counter electrode should preferably be unstable for this reason, i.e. has a measurement (in the axial direction), e.g. have at least about insignificant resistance, and the electrode area should be ten times, and preferably at least about fifty times longer, 15 i. there is an infinite parallel capacitance than the «width» (D5). point.

The «thickness» (i.e. D3 in Fig. 1) of the diffusion gap at (2) The solution resistance (or ô Rs) is over the upper

2 and 3 is typically distributed in the area of about 1 area of the working electrode and accordingly in up to about 50 micrometers and the corresponding preferred circuit 30 as a series of resistance elements 38

"Width" (i.e. B3 in Fig. 1) ranges from about 0.1 to 20. The value of ö Rs can be calculated from the specific

about 5mm. the level of the electrolyte o (ohm • cm), the width ôr (cm) and

Since the use of a recessed first electrode of the cross-sectional area 2jt r • d (cm2) of the resistance element normally the surface (i.e. if Dj is greater than D3), through which the current flowing through offers no advantage, must be exposed to the electrolyte. Here, "r" means the radial distance coordinate, the surface of the measuring electrode and the protective electrode pre-measure from the central axis of the measuring electrode, and "d" means the coplanar pattern; This coplanar design includes the thickness of the electrolyte between the surface of the measuring electrode and preferably also the surfaces de exposed to the electrolyte and the membrane. The relationship of the first and possibly the second insulator applies accordingly. The mountain r

Drawing «coplanar» should have regularly shaped planes ÖRS = q -

including curved or convex shapes, 30

i.e. if the surfaces mentioned in the common Ebe- (3) The combinations of constant current generators 36

ne, which by the convex side of a spherical segment and diodes 37 correspond to the electrode reaction of the

is defined. In any case, the main requirement for the standing electroactive substance on the measuring electrode 31.

general design of the end face of the measuring cell near the

Membrane in that any polymer films or foils as 35 surfaces between two limits, namely a minimum and a

Semipermeable membrane is smooth and even on the one with the maximum, is the size of the element

Electrolyte-provided end face of the measuring cell can be placed on surface 2ra: ôr flowing current regardless of the potential. and corresponds to the relationship:

The function of the third electrode explained above,

which represents a critical feature of the measuring cell 40 §j = r 'm according to the invention, means that the cell according to the invention as a three A.

pol (three-pole network) can be understood, in which Im means the desired total current and A the total of the usual electroanalytical cells corresponds to a two-pole surface. Im can in turn chen by the permeability; Accordingly, the measuring cell according to the invention in pm and the thickness xm of the membrane and the outer concentration - its most general definition - is a three-pole electroanalysis of the electroactive substance C by the formula see cell, which has a protective electrode in contact with the electrolyte between the measuring electrode and the counter electrode seated in ■ = n FAPm C / xm.

To simplify a more detailed explanation of this, where n is the number of electroactive

4 is a simplified equivalent circuit so substance added or subtracted during the electrode reaction

(function-equivalent circuit) of the electrons according to the invention and F is the Faraday constant.

troanalytic cell shown with membrane cover. Ob- (4) The leakage resistance RL, which corresponds to the nonetheless the equivalent circuit to the element 34 shown in FIGS. 2 and 3, is intended to make all those contributions to the specific embodiment of the coaxial type and the current in a common and accordingly not represent protected special use for the measurement of oxygen (02) as an electrical cell, which according to the invention relates to additional rust-proof material, the arguments of a general nature using the protective electrode 33 can be suppressed, and can easily be used for a different structure of the measurement - These contributions include the diffusion of cell contaminants and to be transposed to other electroactive materials. or oxygen from the electrolyte reservoir or from the

The three contacts of the replacement formwork solid components of the measuring cell shown in FIG. 4 to the edge of the measuring elec-

device 30 correspond to the measuring electrode 31, the counter electrode 60 trode. Electronic leak contributions can also be suppressed

32 and the protective electrode 33 in between. They will, however, to a relatively lesser extent.

Circuit 30 comprises an infinite number of constant (5) The function of the protective electrode 33 is determined by the

Current generators 36, diodes 37 and resistor elements 38 shown diode 35 to express the fact

(Ô Rs). Furthermore, the circuit 30 comprises a resistance electrode that it has a practically unlimited element 34 (RL) in one direction and a diode 35 in the connection to the protective current, but practically no electrode in the opposite direction. If the measuring electrode is connected as an anode, current passes. When the potential of the protection electrode 33

Conversely, the diodes 35, 37 in the circuit 30 must be kept at the same value as that of the measuring electrode 31. the whole through the leakage resistance 34

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current trapped by the protective electrode 33 and thereby the passage of the undesired current contributions Va / Rl through the measuring electrode 31 is switched off.

General considerations regarding the preferred width dimension of the diffusion gap (B3 in FIG. 1) and the corresponding delimitation of the gap defined by the surface of the protective electrode exposed to the electrolyte have already been mentioned above. The probability that a “contaminant” molecule that enters the diffusion gap is caught by the surface of the protective electrode exposed to the electrolyte was calculated as a limit value problem in a manner known per se from diffusion theory. The results of this calculation are summarized in Table I below.

Table I

Probability of catching

(O)

Spall

0

0

10th

0.05

20th

0.1

30th

0.15

40

0.25

50

0.32

60

0.48

70

0.65

80

0.9

90

1.35

95

1.8

97

2.1

98

2.4

99

2.8

99.9

4.26

99.99

5.75

99.999

7.20

Gap width (ratio

Accordingly, a capture probability of practically 100% can be achieved with a B3: D3 ratio of approximately 10: 1.

However, two criteria have to be considered for the upper limit of the lateral dimension B3 of the protective electrode surface. The first of these criteria relates to the size of the electrolytic resistance in the cell through which the measuring current flows. The ions generated on the measuring electrode must pass through the diffusion gap above the protective electrode surface in order to reach the counter electrode surface where they are neutralized. Since this electrolytic channel of the diffusion gap is preferably narrow or narrow, it represents a not insignificant resistance to the flow of the ionic current and causes a potential difference between the counter electrode and the electrolyte in the first part of the electrolyte space. Expressed by the quantities shown in replacement diagram 30 of FIG. 4, the potential is then smaller than the applied potential VA. The permissible size of this potential shift depends on the particular type of measuring cell according to the invention, but in general this requires a limit beyond which the cell no longer measures precisely, because the potential at the measuring electrode becomes insufficient to maintain a diffusion-controlled reaction of the electroactive substance and then the proportionality between the current output at the cell and the concentration of the electroactive substance is no longer guaranteed. This can be seen from the current / voltage curve shown in FIG. 6, which shows schematically the dependence of the magnitude of the current ôi (ordinate) flowing through an element of the area ôA under the effect of the effective potential er (abscissa). Such a current / voltage curve 60 is generally typical for electroanalytical cells. Of particular interest here is the current plateau 61 at medium voltages; in the area of this plateau the current is independent of the applied voltage and this is the area in which an amperometric cell including cells according to the invention should work in order to ensure a linear relationship between current output and concentration. A potential loss caused by uncompensated internal cell resistance can cause the potential to become smaller than the io minimum voltage e; which is required to maintain a diffusion-controlled reaction of the electroactive substance, i.e. can cause a reduction in current output, even if the concentration of the electroactive substance remains constant.

15 The second criterion to be considered for the lateral dimension of the protective electrode surface relates to the total operating time of the measuring cell. This operating time should generally be as long as possible, but depends in part on the size of the total current flowing through the counter electrode, since this current causes the consumption of the counter electrode. In a cell according to the invention, this current is the sum of the currents passing through the working electrode and through the third electrode, the protective electrode. In order to compensate for this, the surface of the counterelectrode exposed to the electrolyte can be increased accordingly.

In general and in particular in the case of a measuring cell with the coaxial structure shown in FIGS. 2 and 5, the surface of the protective electrode 30 exposed to the electrolyte is generally less than 50% and preferably less than 20% of the surface of the measuring electrode exposed to the electrolyte.

The measuring electrode of the measuring cell according to the invention generally consists of a noble metal, such as gold, stainless steel, platinum, palladium and iridium, at least in the surface area which is exposed to the electrolyte, since these surfaces are polarized by the applied voltage in the absence of electroactive material should. In any case, the surface of the first electrode should be chemically inert under the working conditions.

40 The counter electrode is generally made from a suitable reference electrode metal, i.e. this electrode should not be polarized by the applied voltage and should be able to react with ions or molecules present in the electrolyte. Silver is a typical example of the Me-45 tali of the counter electrode. The protective electrode generally and at least on its surface exposed to the electrolyte consists of a material that is suitable for the surface of the measuring electrode. Examples are gold and high quality stainless steel.

50 The general requirements which are decisive for the selection of suitable materials for the insulators arise for the person skilled in the art in view of the basic requirements (a) the insulation effect, (b) the ability to permanently seal the electrodes, preferably 55 under pressure, and (c) the resistance to changes in both the mechanical design and the electrical properties of the parts of the measuring cell under the ambient conditions, ie the operating conditions of the measuring cell during the entire operating period. Creep-resistant insulators are preferred for those insulator parts that are sealed by pressure.

Suitable electrically insulating materials for the insulators of measuring cells according to the invention, in particular the second insulator, include organic and inorganic materials from the large group of solid organic polymers (thermoplastics and thermosets), silicates, molten oxides, glasses, etc. Special examples are Epoxies, polypropylene, nylon-66, polyethylene terephthalate, acrylic polymers including

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8th

borrowed from polymethacrylic acid esters, polystyrene, polyvinyl chloride (not softened), polyvinyl fluoride, high-density polyethylene, polyvinylidene fluoride, polyvinyl carbazole, polyvinyl acetate, polysulfones, polycarbonates including polybisphenol carbonate, polyphenylene oxide, polyurethane, polyacetals including polymethylene copolymers, such as various polyoxymethylene polymers from styrene and acrylonitrile or from styrene, acrylonitrile and butadiene, glass, quartz (molten silicon dioxide), ruby, diamond, granite, ceramic, hard rubber (ebonite), ivory, etc.

Mixtures including multicomponent materials, such as polymers from the groups mentioned above, with a disperse phase of a filler, which may have a reinforcing effect, e.g. Polyester or polyepoxide materials with glass in particulate or fibrous storage can also be used for the second insulator.

Organic polymers from the above groups are preferred for the first insulator, taking into account the low oxygen permeability explained above. Polyvinyl fluoride is a preferred example of the first insulator. The housing can be made of any insulating material, preferably an organic polymer. If the housing is made of metal, an insulator is required between the housing and the closest electrode.

All electrolytes known for amperometric analyzes, including aqueous and non-aqueous solvents together with all desired dissolved components of electrolytes, are suitable for using the measuring cell according to the invention.

The following examples serve to explain further, but not to limit the invention.

example 1

A measuring cell was built in a circular cylindrical, symmetrical shape. The structure of the parts of the end face (without membrane) exposed to the electrolyte can be seen from FIG. 5, which represents half of the symmetrical electrolyte space in a semi-schematic axial cross section.

The measuring electrode 51 lies in the central axis of symmetry X and has a radius of 3.16 mm. The protective electrode 51 is surrounded by a first insulator 55 in the form of a layer of polyvinyl fluoride with a radial thickness (radial distance between the two adjacent electrode surfaces 510 and 530) of 0.0127 mm. The protective electrode 53 is made of gold and has a radial width (radial distance between the adjacent insulators 55 and 57) of 0.4 mm. The second insulator 57 is a ring made of poly (trifluoromonochlorethylene) with an outer diameter of 9 mm and an inner diameter of 7.145 mm, i.e. a radial width of 0.928 mm. The counter electrode 52 is a tubular structure (outer diameter 20 mm, inner diameter 9 mm) made of silver with a recessed part 599 for receiving the electrolyte reservoir. The housing 56 is a tubular structure made of polyacetal with an outer diameter of 24 mm.

The front of the cell 50 (the part intended for the absorption of electrolyte) was provided with an aqueous alkaline electrolyte (1 n KOH in water) with a specific resistance q of 8 ohm • cm and with a membrane made of fluorinated ethylene / propylene copolymer sealed to a thickness of 0.0254 mm. An electrolyte gap (D3, Fig. 1, D3 = D,) of 0.003 mm was formed between the membrane and the electrode surfaces 510, 530 of the measuring cell. The coplanar surfaces of the face of the measuring cell were on a common spherical plane with a radius of curvature of 100 mm. The electrolyte reservoir 599 formed by the depression had a capacity of 0.2 ml.

A “valve seat” of the type described in DE-OS 2 710 760 was formed in the protective electrode and the measuring electrode was pressed against the protective electrode by means of a spring, the contact area being insulated by a thin film of polyvinyl fluoride between the two metallic electrodes.

s This measuring cell was used to analyze oxygen dissolved in water. For this purpose, a voltage of 800 mV was applied between the counter electrode 52 (as an anode) and the measuring electrode 51 (as a cathode). This potential is sufficient for the electroreduction of oxygen on the

loGold cathode according to the equation

0, + 2H, 0 + 4e-

■ 40I-T

in which «e» means an electron in the surface of the measuring electrode-15 de. At 25 ° C, a current of 1.6 jxA per ppm of dissolved oxygen flowed through the measuring electrode and a current of about 1.0 [xA per ppm of dissolved oxygen through the protective electrode.

If the measuring cell was immersed in oxygen-free water and no current was drawn through the protective electrode, a constant residual current of about 8 nA flowed through the measuring electrode. However, if the protective electrode 53 was kept at the same potential as the measuring electrode and internal leakage currents were trapped by the protective electrode, the residual current immediately dropped to about 0.8 nA. Accordingly, the range of oxygen concentrations that can be measured with this protected measuring cell can be expanded by about a decade as a direct result of the use of the protective electrode.

An additional advantage of the protective electrode 30 was shown when comparing the measuring cell 50 against vibrations and shock-like vibration in a protected and unprotected state, i.e. with the protective electrode 53 switched on on the one hand and with the protective electrode 53 switched off on the other hand. Impact shocks that caused the measuring current to "deflect" 35 when the protective electrode 53 was switched off - due to the convective transport of oxygen dissolved in the electrolyte reservoir to the cathode - had no effect on the measuring cell protected by the switched-on protective electrode, because such oxygen transport by the interception effect the

40

Protective electrode is prevented.

Example 2

A simplified mathematical model of the measuring cell according to the invention was analyzed in order to show the protective effect 45 of the protective electrode in relation to its distance from the measuring electrode and to investigate time factors which occur when the measuring cell is not in a steady state. For this purpose, the decay of the current was calculated on a flat electrode, to which a potential is suddenly applied at time t = 0, for two different situations. It is assumed that the diffusion takes place from a semi-infinite area to the electrode at x = 0 in the first case and from a finite area in the second case, the finite area being limited by parallel electrodes which are at x = 0 and at x = a lie.

The first case gives an estimate of the transient current at the edge for a conventional unprotected electrode, while the second case approximately determines this for a protected working electrode according to the invention with a mutual electrode spacing a. The diffusion-theoretical limit value problems can be formulated as follows:

50

55

60

First case (state of the art

65

ÔC

"ôT

= D

ô2c w

, c = cGat t = 0, c = c „atx - c = 0 atx = 0, tO

9

627 278

Second case (according to the invention)

ôc ô2c

"ôT = D

t.-2 ", c = coatt = 0, c = 0atx = 0 ^ andx = a, t> 0

Here «c» means the concentration of the impurities dissolved in the electrolyte and D their diffusion coefficient. In any case, the current i at the electrode at x = 0 is calculated from the formula i = nFAD

in which n is the number of electrons added or removed to the impurity molecule during the electrode reaction. F is the Faraday constant and A is the effective electrode area.

The solutions to these limit value problems are as follows:

i "= nFACo and ig 4i0

D

itt nFACc jiDt itDt exp-

n = o

(2n + l) 2 Jt2

Dt where iu is the current at the unprotected electrode and ig is the current at the protected electrode. It can be shown that with very short times, i.e. if t «a2 / D, ig = iu and that for all later times ig <iu.

These results are shown graphically in Figure 7, which shows the time dependence of the current on the protected and unprotected electrodes (plotted as ig / i0 and iu / i0, relative to the left ordinate, and the relationship between them) Pour on the right ordinate).

The time parameter is shown on the abscissa in a logarithmic scale.

Curve 70 shows the time function of iu / i0 for the unprotected electrode, curve 71 shows the time function ig / i0 for the protected electrode, in each case in the units indicated on the left ordinate. Curve 72 represents the time function of the ratio of the current at the electrode protected according to the invention to the current at the usual unprotected electrode and relates to the units indicated on the right ordinate of FIG. 7.

7 shows the much faster decay of the edge current in the electrode protected according to the invention compared to the decay on a conventional non-protected electrode. For example, at time t = 0.62 a2 / D, the edge current at the protected electrode according to the invention is only 1% of the value that can be observed with a conventional non-protected electrode.

The role of the distance between the protective electrode and 5 of the measuring electrode also results from FIG. 7. Since the advantages of the protective electrode are significant at times greater than t ~ a2 / D, it is advantageous to calculate the distance «a» (D5 in Fig. 1) keep it low so that x represents a short period of time.

Assuming the diffusion coefficient D of the contamination molecules in the order of magnitude of about 10_6 cm2 / sec, it follows that if the protective electrode is arranged at a distance of about 30 [xm from the measuring electrode, the edge currents effectively within 1 sec are switched off. The time period x is the time period that the measuring electrode together with the protective electrode needs to consume the total concentration of impurities in the volume A • a. The unprotected electrode alone consumes the impurities from an unlimited volume and therefore the speed of the decay of the current is considerably lower, 2c than that of the electrode protected according to the invention.

The amperometric cell according to the invention requires few changes with respect to the measuring instruments. An example of such a modified measuring circuit is shown schematically in FIG. 8. For simplification, the scheme 80 contains the assumption that the resistance of the galvanometer 85, which is used to measure the current flowing through the measuring electrode 81 of the cell 89, is sufficiently small and can be neglected, so that the potential of the measuring electrode 81 and Protective electrode 83 remain practically the same, 30 regardless of the current passing through the cell 89.

The cell voltage generator 86 maintains a constant potential difference between the cell contacts 82/81, so that the cell 89 operates in a steady state. The protective electrode 83 is kept at the same potential as the measuring electrode 35 81 and is connected via the line 84 between the generator 86 and the calvanometer 85 at the connection point 87. The galvanometer is arranged in such a way that only the current passing through the measuring electrode 81 is measured, since only this current is proportional to the concentration of the electroactive substance which is to be measured in the fluid with the semipermeable membrane of the cell, not shown 89 is in contact.

Numerous changes to the information mentioned below are apparent to the person skilled in the art. For example, instead of assembling the measuring cell from rotationally symmetrical parts and, for example, machined parts, a measuring cell according to the invention can also be made from molded or injection-molded components with electrodes previously attached to them, and / or the electrodes can be sprayed, vapor deposited or electrolytically applied, for example suitable metals are applied to preformed bodies. Further modifications within the scope of the invention relate, inter alia, to the design of the diffusion space, the limitations of which could also be formed, for example, by two segments of the protective electrode lying opposite one another.

4 sheets of drawings

Claims (6)

627 278 2 PATENT CLAIMS especially for the measurement of the oxygen (02) content in water 1. Electroanalytical cell made for amperometric measurements, media or gases. Details on the structure and conditions, in particular for oxygen measurements, with an electric drive of such known transducers can be found in the literature, lyt space, a measuring electrode surface, a counter electrode, see e.g. Monograph by Fatt, Irving, "Polarographic Oxy - a protective electrode and a sensor close to the measuring electrode 5", CRC-Press, Inc., USA, 1976, to which particular reference is made here, which closes off the electrolyte space. Membrane, the protective electrode being part of a channel located in the electrolyte-conventional transducer for amperometric use with space in the cell, which is the only space that generally has a working electrode, which is also connected as a measuring electrode, connecting a first part of the electrolyte space, the or Determination electrode is referred to and has a surface that is exactly determined between the measuring electrode surface and the adjacent part 10, and a counter electrode. Both of the membranes are located, with a second part of the electrolyte rough electrodes being exposed to an aqueous or non-aqueous electrometer in the area of the counterelectrode, thereby being exposed. For performing amperometric analysis, the channel is applied as a ring gap (193, 293) adjacent to the first part (191, 291) between the counter electrode and the measuring electrode in the form of a constant DC voltage of the electrolyte space, see the contact surface determined with electrolyte so that the latter is polarized to supply a current (130, 230), the protective electrode (13, 23) and the membrane (14), the size of which, in the uniform operating state of the cell, 24) is formed. proportional to the activity of the electroactive to be determined 2. Cell according to claim 1, characterized in that component. that the surface (130,230) of the protective electrode (13,23) oxygen is a preferred example of an electroactive concentric around the practically circular measuring electrode component, but other electro-che (110,210) are arranged for the invention and that that as a gap (193,293) active substances of interest and include other and more normally formed channels only by the thickness of the insulator layer (15, white gaseous (this also includes vaporous) elements 25) between the measuring electrode (11, 21) and the protective electrodes or connections that are more easily oxidized or reduced (13, 23) in the cell from the first electrolyte space (191, 291) than the electrolyte (solvent and Solvat). det. 25 The electrodes usually consist of different 3. Cell according to claim 1, characterized, tallen so that the counter electrode is "consumed" during operation by the surfaces (210, 230) of the measuring electrode (21) and reaction with the ions, which practically on the protective electrode (23) are arranged practically coplanar. measuring electrode that does not consume. Iso- 4. Cell according to claim 1, characterized in lators, i.e. electrically non-conductive non-metallic, anorga-that the gap (293), viewed in cross-section in a radial 30 niche or organic solids, are provided between the electrodes, which are concentrically provided starting at the axis (K) of the that any current that extends outward from an electrode head (20) built up can pass through a slurry to another, due to the electrochemical phenomena dimension (B3), which is practically generated by the width of the ring-shaped electrodes on the electrodes exposed to the electrolyte formed surface (230) of the protective electrode (23) is ionic current in the electrolyte. is correct, and a thickness dimension (D3), which is defined by the 35 stood between the surface (230) of the protective electrode (23). In the ready state, such cells have one and the inner surface of the membrane (24) is determined, semipermeable membrane, i.e. has a thin film with one, the ratio of the width dimension (B3) to the thickness of the dik being in the range from 10 to 30 micrometers from an organic ken dimension (D3) at least 7.20 corresponding to an insight polymer, such as polytetrafluoroethylene, which is suitable for the catch probability of at least 99.999%. 40 send gaseous component permeable but for the electrical 5. Cell according to one of the claims 1-4, thereby trolyte is practically impermeable. The membrane is generally characterized in that the radial distance (D5) between the circular is attached to the cell after its electrolyte on the surface (210) of the measuring electrode (21) and the receiving part is filled with the electrolyte, and can be on the annular surface (230) of the protective electrode (23) cannot be removed replacing the electrolyte. In any case, more than 100 microns, preferably less than 50 microns, the membrane separates the electrolyte from the fluid that the anameters, e.g. 10-30 microns. contains gaseous component to be detected, and provides 6. Cell according to one of the claims 1-5, thereby representing a limitation of the electrolyte space in the cell. Characterizes that the size of the other limitation parts of the electrolyte space exposed to the electrolyte are from the annular surface (230) of the protective electrode (23) if the electrode surfaces, the isolator exposed to the electrolyte is formed as 50%, preferably less than 20% of the size of the gate parts and possibly the practically circular surface of the holder exposed by parts of the housing or egg electrolyte. (210) of the measuring electrode (21). Although the electrolyte space, i.e. the electrolyte-receiving and electrolyte-holding part of the cell, a coherent gender space in the sense that it is not through physical 55 locks is divided, it can be as one of different and