DE2024008A1 - Detector cell for coulometric analyzes - Google Patents

Description

2024009

Patent attorneys

DIP. R. BSETZsen, DIpI-InD. K. LAMPiίECHT

Dr.-Ing. R. QuETZ Jr. Munich 22, Steinsdorfsir. W.

81-15.698P (15.699H) 5/15/1970

HITACHI, LTD., Tokyo (Japan)

Detector cell for coulometric analyzes

The invention relates to a detector cell for coulometric Analyzes and in particular to a detector cell which is calibrated for coulometric analysis by a liquid chromatograph separate components.

Used between an anode and a cathode, which is in an electrolyte solution immerse, a sufficient voltage is applied, a reduction takes place at the cathode, while at the anode there is a Oxidation is coming.

Although many individual questions regarding the mechanism of action at the electrodes certainly remain open, one can but according to a reliable theory assuming that the electrode is covered with a very thin diffusion layer of a reaction sequence with the following three

81- (POS 21989) -DfBk (7)

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Imply levels?

a) The material contained in the solution migrates from the inside of the solution through convection and electromigration into the diffusion · » shift one.

b) The material in the diffusion layer moves by diffusion and cataphoresis to the electrode "

c) An electronic transition occurs on the surface of the electrode on.

According to Fick ^1's law, there is the following relationship between the number dN of molecules participating in the reaction within an infinitesimally small time interval dt and the concentration gradient dc / dx at the electrode surface

German

= -DA

in which D is the diffusion coefficient of the reacting material and A mean the size of the electrode surface.

The instantaneous current i ^. during the reaction can be expressed through the relationship:

in which η is the number of electrons and F is the measured in Faraday " Mean amount of electricity.

Assuming that the concentration in the diffusion layer is proportional to the distance χ from the electrode surface j one obtains the expression dc / dx - (Cq-cJ / c ', in which c _{q is} the concentration at the electrode surface and (V ^ the thickness of the If the electrolysis proceeds very rapidly, this expression simplifies to dc / dx «c / {/, since then the concentration

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on the electrode surface can be neglected. For the instantaneous current one then obtains the relationship:

i _t = η FDA c // (3)

If the total volume of electrolyte solution is denoted by V, then N = CV, and this results when using Equation (2) the relationship:.

i _t = -nFV (ff-). (3a).

For the speed of the decrease in concentration one obtains the expression:
/ de ν DAc

An integration of the expression (4) with respect to the time between and t leads to the relationship:

c _t = c _o exp (-DAtAcO = C ₀ exp (-Kt) (5), in which the quantity K stands for ~ Dk / i <P .

From the equations (3) and (5) one obtains the expression for the instantaneous current i by combining:

i _t - i _o exp (-Kt) (6),

where i means the initial current at time zero.

The coulometry is a material analysis to which the measurement the number of electrons, i.e. the amount of electricity and the application of the Faraday electrolysis law.

The amount of electricity Q involved in the reaction can be as a function of the weight W of a material involved by the following Play expression:

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in which M is the molecular weight of the material involved ".

If the involved material flows in the electrolyte solution, the following relationship is obtained:

i - —ff- - nFvc (8),

in which ν is the flow rate of the material in the electrolyte solution means.

Now there are two methods for coulometric analysis, viz working with constant current and working with controlled potential.

When applying coulometry to liquid chromatographs, working with a controlled potential turns out to be working with constant current as superior, since momentary electrolysis of the material is required. When using a coulometric Analysis with controlled potential on liquid chromatography can generally be expected to have the following advantages:

1. The sensitivity becomes very high. Accordingly, let sioh also determine materials with very low concentrations.

2. If you can count on a current yield of $ 100, is It is unnecessary to draw up a calibration curve and therefore an absolute quantitative analysis of the material can be carried out.

3 · Since the material is currently being electrolyzed, the Coulometry at controlled potential is particularly good for recognizing each individual separated by the liquid chromatograph Component.

4 · Even materials that the liquid chromatograph cannot can be separated by varying the electrode potential

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analyze quantitatively ·.

5. By introducing a controlled secondary potential • I also determine electrically inactive substances ^ and therefore can a wide range of almost all materials can be examined «

6. Changes in flow rate and temperature an electrolyte used for washing out has almost no effect on the analysis results.

The method of light absorption, the method of refractive index measurement, the method have so far been the methods for dividing the components separated in a liquid chromatograph the fluorescence measurement, the method of measuring the electrical Conductivity) the method of measuring radioactivity, polarography, etc. is known. Because none of these methods all at the same time has gate parts listed above which are intrinsic to controlled potential coulometry, this method has lately attracted attention as a method of identification the components separated by a liquid chromatograph.

The main problem for coulometry before analysis lies in the Eottleeetric or electrolytic cell · If an ooulometric cell is to be used for the determination of every single component separated by a liquid chromatograph they mainly meet the following requirements »\

1. One denotes the in »* the position of the cell with S and the one in it flowing electronic current with I, then the potential drop H T * rnaohläe «igly small, which means that the inner resistance B of the cell must be very low.

2. 5it activation speed should be very high and the response should be momentary, · For this reason, how to get out of the

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™ stakes.

202 * 008

Equations (4) and (5) sees the volume of the cell to be small * and conversely, the surface of the electrode should be as large as only possible.

3. The potential of the working electrode in the cell should be stable ain »and the potential distribution should be uniform.

4. The required amount of sample to be examined should be small

5. The structural design of the cell should be such that it can with only low electrical noise.

6. The structural design of the cell should be very simple.

7 · The structural design of the counter electrodes should be such that their polarization turns out to be negligibly small even when current is flowing.

For some applications, the use of coulometry is ■ It has become known with controlled potential in connection with liquid chromatographs, but fulfilled the ones used ooulometric cells under no circumstances do all of the above

The aim of the invention is therefore to develop a detector cable for ooulometrisohe analyzes su create, which-by a small Own volume, a large surface of your working electrode, one uniform potential distribution in their interior, a low intrinsic resistance and characterized by small electrical noise "

In order to meet all of these requirements listed above, a detector cell according to the invention for coulometry is characteristic

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2Ü24008

Analysis by being in an electrolyte-tight vessel on the one hand a flat working electrode and on the other hand on both sides of it and a counter-electrode is housed separated therefrom by diaphragms, and the vessel has an inlet and an outlet for supply or «, for removing a sample to be analyzed to or from the working electrode.

Either the vessel itself can be filled with electrolyte solution, or there can be an inlet and an outlet for passing through have an electrolyte solution through the counter electrodes. The inlet and outlet for the sample to be analyzed are in place the working electrode, preferably so that there is still a free space between them and the working electrode. the Diaphragms arranged between the working electrode on the one hand and the counter electrodes on the other hand can consist of ion exchange membranes exist, and in addition, these diaphragms can have relatively elastic and electrically insulating layers on both sides be provided with a unified body with the actual Form a diaphragm Tissue is used as the material for these layers in question, which through an impregnation of silicone rubber with the diaphragms consisting of ion exchange membranes to form a uniform Bodies are united.

To further explain the invention and its advantages, some possible embodiments for the construction of an inventive Detector cell described in more detail, which are illustrated in the drawing. Show in the drawing:

1 shows a cross section through a detector cell according to the invention for coulometric analyzes,

FIG. 2 shows a view of a detector cell according to the invention in FIG disassembled state,

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FIG. 5 shows a perspective illustration of a detector cell according to Fig. 2 in the assembled state,

4 shows a circuit diagram for a coulometer using a detector cell according to the invention,

FIG. 5 is a graphical representation to illustrate the Relationship between the flow rate of the sample to be analyzed and the determined amount of dissolved oxygen,

Pig. 6 shows a cross section through a first embodiment a working electrode with sample inlets and outlets for a detector cell according to the invention,

FIG. 7 shows a cross section through a modified one compared to FIG. 6 Embodiment of a working electrode and

8 shows a cross section through a diaphragm for a device according to the invention Detector cell.

As shown in FIG. 1, in which the basic structure of the detector cell of the invention is shown shows, has the detector cell ψ a vessel 1 made of electrically insulating material. The interior of the vessel 1 contains a working electrode 2, which can consist, for example, of a metal wire mesh, a charred tissue or the like, diaphragms 3 and J ', which can consist, for example, of ion exchange membranes or the like, and counter electrodes 4 and 4, which, for example, consist of a metal wire mesh, charred fabric or the like. On one side of the working electrode 2, the counter electrode 4 is arranged with the diaphragm J interposed, and the counter electrode 4 is arranged on the other side of the working electrode 2 with the diaphragm J 'interposed. The working electrode 2 is therefore between the diaphragms J and 5 ¹ and

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the counter electrodes 4 and 4 ¹ inserted. At the top and bottom, the vessel 1 is each provided in its middle with a sample inlet 5 and a sample outlet 6, respectively. A sample to be analyzed is introduced into the working electrode 2 through the sample inlet 5 and sucked off again through the sample outlet 6. In addition, the vessel 1 ″ is provided with lines 7 and 8 on its left side and lines 9 and 10 on its right side. The lines 8 and 9 are connected to one another via a connecting line not shown in the drawing Electrolyte solution passed through the counterelectrode 4 to the lines 8 and 9 and then sucked off via the counterelectrode. ^ Through ^ the line 10. With the working electrode 2 or . 1 £: gerbunde.n, ·

V Is the vessel 1 filled with an electrolyte solution andTVii'd '* ~ ·,: ^ -— • «wipe the working electrode 2 and the counter electrodes 4 and 4 ' a sufficient voltage is applied, a test material in of a sample to be analyzed while the sample is being electrolyzed by the sample inlet 5 through the working electrode 2 to the sample outlet 6 flows.

In Pig. 2 is a detector cable according to the invention with basically the same structure as shown in FIG. 1 in the disassembled state. This detector cell has two suction plates 1 and 1 ", each made of plastic such as polyvinyl chloride, each with an area of $ 0 x 40 m" and a thickness of a few millimeters. The holding plates

1 and 1 are provided with lugs 7 and 8 or 9 and 10, which also from an electrical isolator, such as polyvinyl chloride exist and an inner diameter of for example

2 »have β. These tubes are for insertion or drainage. an electrolyte such as saline. The counter electrodes 4 and 4 '' * are made of metal wire with about 40 to 200 liases per unit area. When assembling the

009 8 4 7/1683 orK3 »l imspected

Detector cell find the counter electrodes. 4 ^ d 4 ¹ recording in recesses in isolator plates 4A or 4 ¹ A ¹ , which consist, for example, of silicone rubber. The recesses in the insulator plates 4A and 4 ¹ A ^{1 are in} the form of the counter electrode !! 4 or 4 ¹ adapted exactly «In the illustrated exemplary embodiment, the isolator plates 4A and 4'A ^! the same size as the holding

panels 1 and 1Ί i.e. an area of 90 χ 40 mm and a thickness

of about 1 mm. The working electrode 2 can consist of a metal made of platinum wire or silver wire or a charred mesh or the like and has the same shape as the counterelectrode, and the working electrode 2 has electrically insulating tubes. ιιηα * Ό v, ereejhes £ "(lie" consist, for example, of alytetrafluoroethylene and have an internal diameter of "FTmIHf and -tur supply or - disposal acid; pure brob solution are used during assembly in their outer shape corresponding openings in electrically insulating sheets 2A ^: and 2A ¹ fitted, which are, for example, plates aas silicone rubber 'and the insulating plates 4A and 4 ¹ A ¹ gleiolu & u The size of the' plates 2A and 2A ¹ is the same as that of the retaining plates 1 and 1', -

2 - -ι - .- "I '," is 90 χ 40 mm, and the thickness of the plates 2Jk. land 2A * --- 'f is about 2 mm. The insulating plates 2A and 21 'have ah .'- ■ the position of the tubes 5 and 6 for the, Brofoenöinlaa Tjzw- occasion _; Corresponding place excavations ton jiaasender QroSa *. Between,; the insulator plates 4A and 2A bair ·. 4'4Γ wad "ZA * " iad JSiaphra ^ nien. 3 baw. 3 ¹ arranged, the certain ions in an electrolyte solution allow passage ^ jdajijpjiufmaterial and the electrolysis products at the counter electrodes 4 and 4 'on the other hand at a passage To reduce the electrical resistance of the detector cell, the diaphragms should be constructed in such a way that the ions in the electrolyte solution can penetrate them easily, and their diodes should also be as small as possible. '' which leads to a large amount of ions passing through per unit area. For this reason, the use of

GINAL. INSPECTED

0 0 9 8 4 7 / ilfi-f '^ *> £ ■' ν

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Ion exchange membranes for diaphragms 5 and 3 'The size of diaphragms 5 and 3 ¹ is less than the dimensions of the holding

2 plates 1 and 1 «, is also less than 90 χ 40 mm.

Fig. 3 is a perspective view of a detector cell which. . ctareh the assembly of the items shown in Fig. 2 results. The insulator plates 2A and 2A ¹ , the insulator plates 4A and 4 ¹ J. ¹ and the holding plates 1 and 1 'are assembled in this order from the inside out and combined with clamping bolts 1J to form a liquid-tight vessel. The wires 11 and 12 connected to the working electrode 2 or the counter electrodes 4 and 4 serve as connecting terminals during the electrolysis.

For performing a coulometry with controlled potential are a potentiostat, i.e. a voltage source for electrolysis with controlled potential tmd a coulometer required. at an electrolytically prepared one for liquid echography,! However, it is cell. Possible to have a controlled electrolysis To carry out potential «by using a voltage source, which provides a substantially constant voltage on the order of 0-3 YoIt and has a low inherent impedance.

An example of a circuit for an automatic coulometer using modern electronic techniques is shown in FIG. 4 illustrated.

The circuit shown in FIG. 4 contains an arithmetic amplifier as an integrator which is suitable for measuring very small amounts of electricity. This circuit also proves to be very favorable in terms of its simplicity of use and its stability. The output voltage e _{o of} the circuit directly gives the measured amount of electricity wieiers

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This output voltage is indicated by a potentiometer or a digital voltmeter. For an automatic evaluation of the liquid chromatography, the voltage across the resistor R _T on the input side and the output voltage e _{Q of} the integrator can be displayed individually or together

For the determination of a test material in a sample solution with the help of a coulometry with controlled potential it is desirable to a current efficiency and an electrolysis rate of tOO $ to achieve. In other words, this means that the test material on its way from the sample inlet to the sample outlet in its body is electrolyzed. In Fig. 5 is the relationship. between the flow rate of a sample and the slectrolysis rate for an electrolytic cell, the latter being tapped by the amount of oxygen dissolved in the sample » In the illustration in FIG. 5, the number of meshes per unit area for the working electrode 2, which is between 40 and 200, chosen as a parameter. As can be seen from the curves in Fig. 5, a complete electrolysis of the sample with regard to oxygen takes place at a flow rate of 4 ml / min or less and a mesh count of 80 for the working electrode 2 and at a flow rate of. 1 ml / min or less and a number of meshes of 40 for the working electrode 2. Since each

Although the limits for the flow velocity are of course influenced by the number of meshes of the counter-electrodes, the volume of the detector cell and similar sizes, it is obvious that the values given above are only a rough indication of the order of magnitude. As far as the wire meshes for the construction of the electrodes are concerned, the larger they are, the better they are for a detector cell according to the invention. Masohenansahl per unit area is. However, a net with a mesh count of 200 or more is difficult to produce and is correspondingly expensive, and it is also subject to increased risk of clogging by precipitation or contamination.

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For these reasons, metal wire nets with a number of meshes turn out to be from 40 "to 200 for making the working electrodes for detector cells for coulometric work with flowing electrolyte with cell sizes corresponding to those given above Examples as best suited.

If the ends of the tubes 5 and 6 for the inlet and the outlet of the samples are in direct contact with the working electrode 2 and there are precipitates in the sample solution, the cell will very quickly become clogged. Therefore, as indicated in I ¹ Xg. 6 and 7, suitable spaces are left between the ends of the vials 16 and 6 and the working electrode, by means of which these disadvantages can practically completely be avoided. In Fig. 6 and 7, the working electrode 2 are each composed of three T eilelektroden 2 ¹ constructed 2 ", 2" ·.

Although it is advisable to use ion exchange membranes as a diaphragm in order to reduce the internal resistance of the detector cell, as explained above, in practice it proves to be difficult to construct such a detector cell in such a way that no electrolyte solution escapes from the ion exchange membrane. However, such leakage of electrolyte is a source of interference. A construction which avoids this is shown in FIG. In this case, on both sides of the diaphragm or 3 'tissue ^ L or JB are arranged on the outside, whereby the tissues 3A and 3B and the diaphragms 3 and 3 are impregnated with a chemically resistant, adhesive and sufficiently elastic electrically insulating material such as silicone rubber 'are united into a unified body. The assembly of the detector cell is then very easy, and at the same time any leakage of electrolyte solution is prevented. In addition, since the tissues 3A and 3B surround the diaphragms 3 and 3 ·, they do not touch the electrical electrodes 4 and 4 · directly, whereby a higher one

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-H-

Service life results in ■> The reference number 15 in FIG. 8 denotes a Insulating layer, which can consist of silicone rubber, for example.

From the above description of preferred embodiments the following gate parts are clear for detector cells according to the invention emerge:

1. Since the flat working electrode is inserted between the counter electrodes with the interposition of the diaphragms, can their surface area with a smaller volume of the operator cell make it bigger than before. The electrolysis speed becomes accordingly hochjund it comes to a quick response. "

2. For the same reason, even if the volume is small, the Detector cell and large electrode surface, the potential distribution evenly within the detector cell.

3 · Again for the same reason, the small one results Volume of the detector cell also a small amount of sample and a low internal resistance of the detector cell

4 · In addition, the structural design of the detector cell is very simple and compact

5 · As between the working electrode and dep tube for the feed and removal of the samples, if sufficient spaces remain, there are hardly any disturbances ^ and the cell will even then not clogged if there is precipitate in the sample.

6. As the diaphragms are inserted between layers of tissue and with this connected to a unitary body by a suitable, sufficiently elastic and adhesive electrical insulating material are both a leakage of electrolyte solution and auoh a direct contact between the diaphragms and the electrodes

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avoided.

7 ο Since ion exchange media are used as diaphragms, the internal resistance of the detector cell is very low compared to previously known cases *

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Claims (7)

2Ü2A008 - 16 claims (Iy detector cell for coulometric analyzes, d a du rc h marked that in an electrolyte-tight vessel on the one hand a flat working electrode and on the other hand to both Sides thereof and separated therefrom by diaphragms each have a counter electrode and the vessel has an inlet and an inlet Has outlet for supplying or removing a sample to be analyzed to or from the working electrode. 2. Detector cell according to claim 1, characterized in that the vessel is filled with an electrolyte solution. 3. Detector cell according to claim 1, characterized in that the Vessel has an inlet and an outlet for passing an electrolyte solution through the counter electrodes. 4. Detector cell according to one of claims 1 to 5 »characterized in that that the inlet and outlet for the sample to be analyzed of the working electrode are on opposite sides, respectively face at a distance. 5. Detector cell according to one of claims 1 to 4, characterized in that that the diaphragms consist of ion exchange membranes. 6. Detector cell according to one of claims 1 to 5, characterized in that that relatively elastic and electrically insulating layers are arranged on both sides of the diaphragms, which are allow to unite with the diaphragms to form a single body. 7. detector cell according to claim 6, characterized in that the Diaphragms made from ion exchange membranes and the electrically insulating 0098 U / 1683 2Ü24008 - .17 - Ending layers are made up of fabrics and impregnated made of silicone rubber to form a unified body. 0 9 8 4 7/1683 Blank page