DE2024008B2 - Flow detector cell for coulometric analysis - Google Patents

Description

German

dx

(1)

where D is the diffusion coefficient of the reacting material and A is the size of the electrode surface.

The instantaneous current /, during the reaction can be expressed by the relationship:

The invention relates to a flow detector cell for coulometric analysis of a flowing sample liquid with a vessel, with a working electrode arranged in the vessel, which is permeable to the sample liquid and in which η is the number of electrons and F is the amount of electricity measured in Faraday.

Assuming that the concentration in the diffusion layer is proportional to the distance χ from the electrode surface, one obtains the expression dc / dx = (c _o - c) / d, in which c _{0 is} the concentration at the electrode surface and <5 is the thickness the diffusion layer means. If the electrolysis proceeds very quickly, this process becomes easier.

pressure to adAx = c / δ, since then the concentration on the electrode surface can be neglected. For the instantaneous current one then obtains the relationship:

i _t = η FDA c / ό . (3)

If the total volume of electrolyte solution is denoted by V, then N = CV applies, and using equation (2) this gives the following relationship:

i ^ -nFvffi. _(3a)

The following expression is then obtained for the speed of the decrease in concentration:
'dc \ DAc

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15th

An integration of the expression (4) after the time between 0 and t leads to the relation:

C = r ₀ exp {-DAt / Vo) = c _a exp (-Kt) , (5)

in which the quantity K stands for DA / V δ .

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

/, = / "Exp (-K /).

(6)

in which; ,, means the initial current at time zero.

Coulometry is a material analysis to which the measurement of the number of electrons, d. H. the amount of electricity and the application of Faraday Electrolysis Act belongs.

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

α, * ,. ₍₇₎ "

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 = - = nFvc , (8)

German

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

There are two methods for coulometric analysis, namely working with constant current and working with controlled potential.

When using coulometry on liquid chromatographs, working with controlled potential is superior to working with constant current, since instantaneous electrolysis of the material is required. When using a controlled potential coulometric analysis The following advantages can generally be expected from liquid chromatography:

1. The sensitivity becomes very high. Accordingly, materials can also be used very low concentration still to be determined.

2. If one with a current efficiency of 100% can calculate, the establishment of a calibration curve is unnecessary, and therefore an absolute perform quantitative analysis of the material.

3. Since the material is currently being electrolyzed, coulometry is suitable for controlled Particularly good potential for recognizing each individual using the liquid chromatograph separate component.

4. Leave even materials that cannot be separated by the liquid chromatograph analyze yourself quantitatively by varying the electrode potential.

5. By introducing a controlled secondary potential even electrically inactive substances can be determined, and therefore a further Area of almost all materials can be examined.

6. Changes in flow rate and temperature of a washout The electrolytes used have almost no influence on the analysis results.

On methods for determining the in a liquid chromatograph separate components are so far the method of light absorption that Refractive index measurement method, fluorescence measurement method, measurement method of electrical conductivity, the method of radioactivity measurement, polarography, etc. are known. Since none of these methods have all the advantages listed above, those of coulometry with controlled potential, this method has recently attracted attention directed towards itself as a method of determining the separated by a liquid chromatograph Components.

The main problem for coulometric analysis is in coulometric or electrolytic Cell. If a coulometric cell is used to identify each one by a liquid chromatograph If a separate component is to be used, it must above all meet the following requirements:

1. If the internal resistance of the cell is denoted by R and the electrolytic current flowing in it with /, the potential drop IR should be negligibly small, which means that the internal resistance R of the cell must be very low.

2. The electrolyzing speed should be very fast and the response should be instantaneous. the end For this reason, as can be seen from equations (4) and (5), the volume of the Cell should be small, and conversely, the surface of the electrode should be as large as any possible.

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

4. The required amount of to be examined! Sample should be small.

5. The structural design of the cell should be such that it has little electrical noise connected is.

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

7. The structural design of the counter electrode should be such that its polarization also be Slromfluss is negligibly small.

For some applications, the use of coulometry is associated with controlled potential became known with liquid chromatographs,

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2 024 0 \) 8

5 ^ 6

However, the coulometri- and discharge of the sample liquid used thereby met so that

Cells by no means see all the forms just listed - there is hardly any interference, with the detector cell

changes. does not become clogged even when there is rainfall

The object of the invention is therefore to provide a detector in the sample liquid,
To create a cell for coulometric analyzes, the 5 The sealing of the detector according to the invention is due to a small intrinsic volume, a large cell is improved by the fact that on each side of the surface of its working electrode, a uniform ion exchange membrane, an elastic potential distribution in its interior, a small layer is provided, which is formed in one piece with the membrane's inherent resistance and by small electrical interference, and in particular because it is characterized by noise. ^lo the elastic layer consists of a fabric,

In order to meet all of these requirements listed above, the outer sides are covered with silicone rubber.

The detector cell mentioned at the beginning is the changes.

th type according to the invention characterized in that a leakage of electrolyte liquid

that every diaphragm both on the working elec- tricity that could generate disturbing noises is practical

trode as well as on one of the counter-electrodes, 15 made impossible.

that the electrodes the vessel essentially completely To further explain the invention and its

fill out and that the vessel with a further advantage are now some possible designs

Inlet and another outlet is provided. tionb examples for the construction of an inventive

The flow detector cell according to the invention is described in more detail, which is shown in the drawing the following advantages: are illustrated as follows. It shows in the

1. Because of the compact arrangement of the slide drawing

phrases, the working electrode and also the F i g. 1 shows a cross section through an invention

Counter electrodes can be the surface of the appropriate detector cell for coulometric analyzes,

Working electrode with a smaller volume of the F i g. 2 a view of a de-

Make the detector cell larger than before, so that the detector cell in the disassembled state,

the electrolysis speed or the start. 3 a perspective view of a de-

speak of the detector cell is increased. detector cell according to FIG. 2 in the assembled

2. The compact design leads to a tied,

A circuit diagram for a coulometer is shown below for a moderate potential distribution within FIG

Detector cell. 3 »Use of a detector cell according to the invention,

3. As a result of the small volume of the invention F i g. Fig. 5 is a diagram showing the convenience Detector cell is its internal representation of the relationship between the was low, and a small amount of the flow velocity to be analyzed is sufficient of sample liquid. Sample and the determined amount of dissolved

4. Through the further inlet and the further 35 oxygen,

Outlet it is possible to get the electrolyte solution to F i g. 6 a cross section through a first Auszw, lead away from the counter electrodes, lead form of a working electrode with samples without that the supply and discharge of the sample inlet and outlet for an inventive fluid is disturbed. Detector cell,

In contrast, so far there is only one measurement. 7 shows a cross section through an opposite

cell for monitoring the concentration of oxy- Fig. 6 modified embodiment of an ar-

dated forms in a flowing liquid and working electrode

become known (see. DT-OS 1 498 607), in which the F i g. 8 shows a cross section through a diaphragm

Counter electrode is also permeable to liquid. for a detector cell according to the invention.

45 As in FIG. 1, in which the basic structure of the

hold close to each other a cathode, a membrane, detector cell according to the invention is reproduced,

an anode and a porous substrate arranged, shows the detector cell has a vessel 1, which consists of

there is also a low resistance of the measuring electrically insulating material. The GE-

cell a quick response of the entire measuring vessel 1 contains a working elec-

arrangement favors concentration differences - 5 ° trode 2, which for example consists of a metal wire mesh,

gen should. a charred tissue or the like can exist,

To further reduce the internal resistance, diaphragms 3 and 3 ', which for example consist of ionic

in the flow detector cell according to the invention or the like exchange membranes can exist,

It is advantageous that the diaphragms of ion and counter electrodes 4 and 4 ', for example

Exchanger membranes exist. 55 made of a metal wire mesh, made of charred fabric

A particularly simple design will be able to exist as a result or the like. On one side of the achieves that the vessel consists of a central part of the working electrode 2, the counter electrode 4 is below and two lateral parts, that the middle interposition of the diaphragm 3 is arranged, and Part of the working electrode and each of the side on the other side of the working electrode 2 is under Parts of one of the counter electrodes and that the 60 interposition of the diaphragm 3 'contains the counter cell arranged by combining these three parts with the electrode 4 '. The working electrode 2 is intermediate diaphragms is formed. so between the diaphragms 3 and 3 'and the

It is also useful that the on and off counter-electrodes 4 and 4 'are inserted. Above and

Let the sample liquid on opposite bottom, the vessel 1 is in each case in its center

Sides of the working electrode are so that between 65 a sample inlet 5 and a sample outlet 6

the inlet and outlet and the working electrode Zwi-. A sample to be analyzed is through

between rooms are provided. the sample inlet 5 into the working electrode 2

In this way there is enough space for the supply and drainage again through the sample outlet 6.

Io

sucked. In addition, the vessel 1 is arranged on its left or 3 ', the particular ions in a On the side with lines 7 and 8 and on the right electrolyte solution, allow passage, the test Provide the side with lines 9 and 10. The material and the electrolysis products are on the opposite side the lines 8 and 9 via a electrodes in the drawing 4 and 4 'on the other hand at a Durchganj Prevent connecting line (not shown) to each other 5. To reduce the electrical tied together. From the line 7, an electrical resistance of the detector cell should be dii Trolyte solution through the counter electrode 4 to the Lei diaphragms 3, 3 'be built so that the ions ii lines 8 and 9 and then easily penetrate them via the counter- the electrolyte solution] Electrode 4 'sucked through the line 10. With can, and besides, their thickness should be so small the working electrode 2 or with the counterelectrode as possible, resulting in a large amount of pr 4 and 4 'are electrical connecting wires 11 unit area leads through ions. Au or 12 connected. for this reason it is recommended to use vo

If the vessel 1 is filled with an electrolyte solution, ion exchange membranes for the diaphragms are placed between the working electrode 2 and the and 3 '. The size of the diaphragms 3 and 3 ', if the counter-electrodes 4 and 4' have a sufficient span 15 below the dimensions of the holding plates 1 and 1, a test material is used in a size that is less than 90X40 mm ² ,
analyzing sample is electrolyzed, while FIG. 3 is a perspective view

Sample from the sample inlet 5 through the working detector cell, which is de electrode 2 to sample outlet 6 flows. in Fig. 2 shown items results. The iso

In Fig. 2 is a detector cell 20 according to the invention, lator plates 2 A and 2 A ', the insulator plates 4 / with basically the same structure as in FIG. 1 and 4'A ' and the retaining plates 1 and 1' are shown in a disassembled state. inside outwards together in this order - this detector cell has two retaining plates 1 set on the outside and combined with clamping bolts 13 to form a liquid and 1 ', each made of plastic such as, for example, a tight vessel. Those with the working polyvinyl chloride and each have an area as electrode 2 or the counter-electrodes 4 and 4 'ver of 90 X 40 mm ² and a thickness of a few multi-bonded wires 11 and 12 are used during the meters. The holding plates 1 and 1 'are with electrolysis as connection terminals.
Tubes 7 and 8 or 9 and 10 provided, the To carry out a coulometry with ge

A potentiostat, ie a polyvinyl chloride, for example, and a voltage source for electrolysis with a controlled internal diameter of 2 mm, for example, and a coulometer are also required from an electrical insulator such as a potential. With a wise man. These tubes 7, 8, 9, 10 are used to lead an electrolyte prepared for liquid chromatography or to draw off an electrolyte such as a trolytic cell, but it is possible to use an electrolyte saline solution. The counter-electrodes 4 carry out trolyse with controlled potential, in and 4 'consist of wire meshes made of metal with 35 which one uses a voltage source, which is about 40 to 200 meshes per unit area. When the voltage is essentially constant in the size of the assembly of the detector cell, the opposite order of 0 to ± 3 volts and a low! electrodes 4 and 4 'recording in recesses in self-impedance.
Isolator plates 4A or 4Ά ', for example from An example of a circuit for an automa

Consist of silicone rubber. The recessed coulometers are made using modern conditions in the insulator plates 4A and 4'A ' the form of electronic techniques is shown in Fig. 4, the counter-electrodes 4 and 4' exactly adapted. In the light
In the illustrated embodiment, the die in FIG. 4 includes a circuit shown

Isolator plates 4 A and 4Ά ' the same size as the computing amplifier as an integrator, which is suitable for measuring the holding plates 1 and 1', ie an area of 45 smallest amounts of electricity. This circuit 90 X 40 mm ² and a thickness of about 1 mm. This also proves to be very favorable in the single working electrode 2 can be operated from a network of platinum and in terms of its stability. The wire or silver wire or a charred output voltage e _{0 of} the circuit gives immediate weave or the like and has the same shape as the measured amount of electricity:
like the counter electrodes 4 and 4 '. In addition, the 50

Working electrode 2 with electrically insulating tube _e = ^L f / df = · Q

chen 5 and 6 provided, for example from Poly- ° RC RC

tetrafluoroethylene and have an inside diameter of 1 mm and for the supply or. This output voltage is used by a potential discharge of a sample solution. For an assembly the working electrode 2 is shown in its automatic evaluation of the liquid chromatographic external shape corresponding openings in the electrical graph, the voltage across the resistive insulating plates 2A and 2A ' , which was A ₁ on the input side and the output are, for example, plates made of silicone rubber and equal to the voltage e _{0 of} the integrator individually or jointly on insulating plates 4A and 4Ά ' . Show the size of the 60.

Plates 2A and 2A ' is the same as that of the holding For the determination of a test material in a

plates 1 and 1 ', is thus 90 X 40 mm ² , and the sample solution with hooves of a coulometry with ge thickness of plates 2 A and 2 A' is about 2 mm. steuertem potential is it desirable to have a power, the insulating plates 2A and 2/4 'have on the yield and an electrolysis rate of 100 ⁰ / o aim to he position of the tubes 5 and 6 for the sample inlet 65th In other words, this means that the test or outlet corresponding point cavities material on its way from the sample inlet initially of the appropriate size. Between the isolator plates sample outlet in its entirety are electrolyzed 4A and 2A or 4Ά ' and 2A' are diaphragms 3 is. In Fig. 5, the relationship between i

2 02-08

ίο

the flow rate of a sample and the rate of electrolysis for an electrolytic cell are shown, the latter being tapped by the amount of oxygen dissolved in the sample. In the illustration in FIG. 5 is the number of meshes per unit area for the working electrode 2, the between 40 and 200 is selected as a parameter. How to get out of the curves in F i g. 5 sees, takes place complete electrolysis of the sample for oxygen at one flow rate of 4 ml / min or less and a mesh count of 80 for the working electrode 2 and one Flow rate of 1 ml / min or less and a mesh number of 40 for the Aibcit electrode 2. However, since the limits for the flow velocity are of course determined by the number of meshes the counter electrodes, the volume of the detector cell and similar parameters are influenced, it is obvious that the values given above are only a rough ao Represent information. What wire nets do to the house As far as the electrodes are concerned, they are all the more suitable for a detector cell according to the invention their number of meshes per unit area becomes larger. However, a network with a mesh number as of 200 or more is difficult to manufacture and is correspondingly expensive, and moreover it is in increased Measures of the risk of clogging due to precipitation or contamination exposed from these Metal wire nets with a number of meshes from 40 to 200 prove to be suitable for production the working electrodes for detector cells for coulometric work with flowing electrolyte as best suited for cell sizes according to the examples given above

When the ends of the tubes 5 and 6 for the inlet and the outlet of the samples touch the working electrode 2 directly and precipitates are present in the sample solution, the cell becomes clogged very quickly. Therefore, as shown in FIG. 6 and 7 is illustrated between the ends of tubes 5 and 6 and the Working electrode suitable gaps can be left, which makes these disadvantages practical can be avoided completely. In Fig.6 and 7 are the Working electrodes 2 each made up of three sub-electrodes 2 ', 2 "and 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, it has proven difficult in practice to construct such a detector cell in such a way that no electrolyte solution escapes from the ion exchange membrane Source for Störangen. A construction which avoids this is shown in Fig. 8 this case, the diaphragm 3 and 3 'are externally fabric 3 A and 3 B are arranged wherein by impregnation with a chemically resistant, tacky and sufficiently elastic electrically insulating material such on both sides For example silicone rubber, the fabric 3 A and 35 and the diaphragms 3 and 3 'are combined into a single body. The assembly of the detector cell is then very simple, and at the same time any leakage of electrolyte solution is prevented. In addition, since the fabrics 3 A and 3 B surround the diaphragms 3 and 3 ', they do not touch the counter electrodes 4 and 4' directly, which results in a longer service life. The reference number 15 in FIG. 8 denotes an insulating layer which can consist of silicone rubber, for example .

From the above description of preferred embodiments for detector cells according to the invention, the following advantages are clearly evident

1. As the flat working electrode with the interposition of the diaphragms between the Counter-electrodes is inserted, their surface can be with a smaller volume of the Make the detector cell bigger than before. The electrolytic speed becomes accordingly high, and there is a quick response.

2. For the same reason, even with a small volume of the detector cell and a large one Electrode surface, the potential distribution within the detector cell evenly.

3. Again for the same reason results from the small 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 becomes very simple and compact.

5. As between the working electrode and the tubes for supply and discharge If sufficient spaces remain between the samples, there is hardly any interference, and the cell will not clog even when there is rainfall in the sample there

6. As the diaphragms are inserted between layers of tissue and with these through one suitable sufficiently elastic and adhesive electrical insulating material to one unitary bodies are both a leakage of electrolyte solution as well direct contact between the diaphragms and the electrodes avoided.

7. Since ion exchange membranes are used as diaphragms, the internal resistance falls of the detector cell is very low compared to previously known cases.

For this purpose 3 31att drawings

Claims (6)

Patent claims: 1. Flow detector cell for coulometric analysis of a sample liquid flowing through with a vessel, with a working electrode arranged in the vessel, which is opposite to the Sample liquid is made permeable and plate-shaped, with two counter electrodes, of each one arranged on opposite sides of the working electrode in the vessel is, wherein the counter electrodes are designed to be permeable to an electrolyte solution, with Diaphragms that are impermeable to the sample liquid and to ions of the Electrolyte solution are permeable and each lie between a counter electrode and the working electrode, and with an inlet and outlet of the vessel in order to move the sample liquid to be analyzed to or from the working electrode ao to lead, characterized in that each diaphragm (3, 3 ^ both at the Working electrode (2) as well as one of the counter electrodes (4, <f) is applied so that the electrodes (2, 4, 4 ') fill the vessel (1) essentially completely and that the vessel with another Inlet (7) and a further outlet (10) is provided. 2. Flow detector cell according to claim 1, characterized in that the diaphragms (3, 3 ') consist of ion exchange membranes. 3. Flow detector cell according to claim 1, characterized in that the vessel (1) consists of a middle part and two side parts, that the middle part is the working electrode (2) and each of the side parts contains one of the counter electrodes (4, 4 ') and that the Cell by combining these three parts with the diaphragms in between (3, 3 ') is formed. 4. Flow detector cell according to claim 1, characterized in that the inlet and outlet (5, 6) of the sample liquid are on opposite sides of the working electrode (2) so that that gaps are provided between the inlet and outlet and the working electrode are. 5. flow detector cell according to claim 2, characterized in that on each side of the Ion exchange membrane (3, 3 ') an elastic layer (3/4, 3B) is provided, which is in one piece is formed with the membrane (3, 3 '). 6. Flow detector cell according to claim 5, characterized in that the elastic layer (3/1, 3B) consists of a fabric which is impregnated on its outer sides with silicone rubber. is plate-shaped, with two counter electrodes, one of which is on opposite sides the working electrode is arranged in the vessel, the counter-electrodes facing an electrolyte solution are designed to be permeable, with diaphragms that are impermeable to the sample liquid and to ions of the electrolyte solution are permeable and each between one Counter electrode and the working electrode are, and with an inlet and outlet of the vessel to the to lead the sample liquid to be analyzed to or away from the working electrode. Detector cells of the type mentioned are except for the liquid permeability of the counter-electrodes essentially already known (cf. Analytical Chemistry, Vol. 36, 1964, pp. 2417 bis 2419, and Vol. 37, 1965, p. 1647). The continuous operation of a detector cell is also known (See U.S. Patent 3,208,926). For a better understanding of the invention, a theoretical outline is given below: There is sufficient space between an anode and a cathode immersed in an electrolyte solution When voltage is applied, a reduction takes place at the cathode, while there is a reduction at the Anode comes to an oxidation. Although there are certainly still many individual questions regarding the reaction mechanism at the electrodes remain open, one can, according to a reliable theory, under the assumption of coverage the electrode with a very thin diffusion layer of low concentration a reaction process with the following three levels: a) The material contained in the solution migrates from the interior of the solution by convection and Electromigration into the diffusion layer. b) The material in the dillusion layer moves to the electrode by diffusion and cataphoresis. c) An electron transfer occurs on the surface of the electrode. According to Fick's law, the following relationship exists between the number dN J ^ r within an infinitesimally small time interval di molecules participating in the reaction and the concentration gradient dc / ax on the electrode surface: