WO1997031264A1

WO1997031264A1 - Improved dme voltammetric cell

Info

Publication number: WO1997031264A1
Application number: PCT/IL1997/000068
Authority: WO
Inventors: Chaim Noah Yarnitzky; Elisha Rabinovitz; Yair Baruchi
Original assignee: Verdeco Technologies Ltd.
Priority date: 1996-02-26
Filing date: 1997-02-20
Publication date: 1997-08-28
Also published as: EP0883803A1; IL117273A0; AU1732097A

Abstract

Electroanalytical voltammetric apparatus, which comprises, in combination with a DME voltammetric cell, means for selectively applying an inert gas pressure to one or more of: 1) a space above the level of the mercury in a feed container, containing mercury to be fed to said cell; or/and 2) a space above the level of the mercury in said cell; or/and 3) an inlet leading to said cell.

Description

IMPROVED DME VOLTAMMETRIC CELL

Field of the Invention

This invention relates to an improved voltammetric cell of the dropping mercury electrode (hereinafter DME) type, free from ecological drawbacks and health hazards.

Background of the Invention

Electrochemical detector and voltammetric cells are known in the art and have been used with success for the analysis of trace elements in the laboratory. Two-electrode and three-electrode cells are known. The three- electrode cell comprises a working electrode, a counter-electrode and a reference electrode which has the function of establishing and maintaining a constant potential relative to the working electrode or the sample solution. In principle, the electrodes may be affected by poisoning due to absorption with resulting passivation and loss of signal. In order to avoid such poisoning, the dropping mercury electrode has been adopted in many such cells.

USP 3,922,205 describes the basic structure of a polarographic cell. USP 4,138,322 discloses a structure of shielded dropping mercury cathode. USP 4,260,467 describes a dropping mercury electrode which comprises a reservoir for liquid mercury, a mercury capillary at the outlet end of which mercury drops are formed, and a valve for selective air-purging passage of mercury from the reservoir to the inlet end of the capillary. An automated polarographic cell is described by CN. Yarnitzky in Analytical Chemistry, Vol. 57, No. 9, August 1985, p. 2011-2015. Such cells, however, are not fully satisfactory. The cells which are automatic and also on-line are expensive and not adequately efficient. In some cases, they include solid electrodes which becomes polluted with time. Others are complicated and unreliable or require a very large volume of the sample solution.

An improved voltammetric cell, free from said drawnacks, is disclosed and claimed in PCT application WO 96/35117. It comprises: a) a cell body, housing, in addition to a reference electrode, a working electrode and, in its lowermost portion, a counter-electrode; b) means for removing oxygen from the sample solution; c) means for feeding the sample solution to said deoxygenation means, means for feeding a stream of an inert gas to said deoxygenation means, and means for causing said solution to flow in said deoxygenation means , whereby oxygen is removed therefrom by contact with said inert gas; d) a means for removing said inert gas from said deoxygenation means after deoxygenation of the sample solution; e) an inlet for the deoxigenated sample solution provided in said cell body in the space between said working electrode and said counter-electrode; f) an exit for the sample solution provided in said cell body at a level above said working electrode; and g) vacuum and/or pressure mean¹* for causing said sample solution to flow to said exit, to be discharged from : m cell above said working electrode, thus assuring that the space betwee aid working electrode and said counter- electrode is constantly filled wit . d sample solution.

Still, the use of mercury drop electrodes, while beneficial in many respects, involves health and ecological problems, from which even the aforesaid improved voltammetric cell is not free. The operator, who feeds mercury to the cell, comes into contact with it. The mercury, which has formed the drops, collects in a sump, which must be handled to recover it. The mercury drop forms at the lower end of a capillary tube and this latter becomes clogged at comparatively frequent intervals, so that it must be replaced. In order to replace the capillary tube, the mercuury must be removed from the mercury reservoir. In all these operations and manipulations, the operator comes, to a greater or smaller extent, into contact with the mercury, which contact is ecologically negative and involves a health hazard. These drawbacks are, of course, common to the mercury drop voltammetric cells of the prior art, and this invention has the purpose of eliminating them in any cell in which they exist.

It is therefore an object of this invention to provide an electroanalytical voltammetric cell of the dropping mercury electrode (DME) type, which is free of the said drawbacks.

It is another object of the invention to provide such a cell, which comprises means for feeding mercury to the cell reservoir, which contains it and from which the electrode drops are formed, by means which avoid all manipulation on the operator's part and all contact between him and the mercury.

It is a further object of the invention to provide such a cell, which comprises means for cleaning or replacing the capillary tube, at the lower end of which the mercury drop forms, in case of clogging, without the operator's coming into contact with the mercury. Other objects and advantages of the invention will appear as the description proceeds.

Summary of the Invention

The electroanalytical voltammetric apparatus according to the invention, comprises, in combination with a DME voltammetric cell, means for selectively applying gas pressure, preferably an inert gas pressure, to one or more of: 1- a space above the level of the mercury in the cell preferably in a feed container containing mercury to be fed to a mercury reservoir comprised in said said cell; or/and 2 - a space above the level of the mercury in said reservoir; or/and 3 - an inlet leading into said cell.

Preferably, said means for selectively applying inert gas pressure comprise a source of inert gas under pressure, first conduit means connecting said gas source to said space above the level of the mercury in said feed bottle, second conduit means connecting said gas source to said space above the level of the mercury in said reservoir, and third conduit means connecting said gas source to said inlet leading into said cell. More preferably, said means for selectively applying inert gas pressure comprise at least one valve inserted in each of said conduit means for selectively permitting or preventing flow of inert gas through said conduit means.

Preferably, pressure regulating means are interposed between said source of inert gas and said conduit means.

According to an aspect of the invention, the electroanalytical voltammetric apparatus comprises means for feeding mercury to a mercury reservoir comprised therein, which feeding means comprise a pipe line connecting the ass of mercury contained in a feed container to said reservoir, and means for applying pressure to said mercury mass for causing the mercury to flow from said feed container to said reservoir, which last mentioned means preferably comprise a source of inert gas under pressure, and said first conduit means connecting said gas source to said space above the level of the mercury in said feed container and more preferably comprise pressure regulating means interposed between said source of inert gas and said first conduit means and at least one valve inserted in said first conduit means for selectively permitting or preventing flow of inert gas therethrough.

Preferably, the aforesaid means for selectively applying inert gas pressure permit concurrently to apply the same pressure above the level of the mercury in the feed container and above the level of the mercury in the mercury reservoir.

Preferably, said DME voltammetric cell is the cell described in said PCT application WO 96/35117, as hereinbefore defined and hereinafter described, as well as in PCT application WO 96/35118. In this case, said inlet into the cell is the inlet into the deoxygenating means.

According to another aspect of the invention, the electroanalytical voltammetric apparatus comprises a mercury feed container for feeding nercury to the cell and a sump for receiving mercury which has formed the DME and is discharged from the cell. Said feed container, once it has substantially emptied of mercury, can be used as a sump. Preferably, the feed container comprises a metal shell and a bottle housed therein, said bottle having closure means for receiving in gas-tight manner a first pipe having an -6- outlet above the intended level of the mercury therein and a second pipe reaching substantially to the bottom of said bottle.

Brief Description of the Drawings

In the drawings:

- Fig. 1 is a schematic representation of a voltammetric cell according to an embodiment of the invention, seen in vertical cross-section;

- Fig. 2 is a cross-section of a mercury container; and

- Figs. 3a, b and c illustrate the assembly of the mercury reservoir and the mercury capillary, in upright and overturned position

Detailed Description of Preferred Embodiments

The invention is illustrated as apphed to an electroanalytical voltammetric cell such as described in the aforesaid PCT applications WO 96/35117 and WO 96/35118, the content of which is incorporated herein by reference, but it will be understood that it is applicable to any voltammetric cell of the dropping mercury electrode (DME) type, with adaptations that can be easily effected by skilled persons, insofar as any may be required.

Fig. 1 illustrates, in schematic vertical cross-section, an embodiment of the invention, which comprises a DME cell according to said copending patent application. The electroanalytical apparatus according to this embodiment of the invention comprises a cell proper that is generally indicated at 10. The apparatus comprises a mercury reservoir 11, at the top thereof. Numeral 12 indicates a platinum wire used as an electrical contact. From reservoir 11, mercury falls to capillary 15, which passes through a stopper 16 of a suitable elastic matter, preferably Teflon, which closes the top of the cell body, generally indicated at 17, said cell body being preferably made of glass or Teflon. Capillary 15 has an inner diameter from 0.03 to 0.1 and preferably about 0.07 mm. The working electrode is a mercury drop 18 that is formed at the end of capillary 15. Below the zone at which that drop is formed, the cell body 17 forms a pipe portion 19, which is full of sample solution. The sample solution is retained at the end of said pipe portion, because this latter sinks into a standing mercury mass 20. Said mercury mass, together with platinum wire 21, one end of which is immersed therein, constitutes the counter-electrode, and is contained in a reservoir 22, which is provided at its top with a stopper 23 through which pipe 19 passes. The reservoir 22 is connected with an outlet pipe 24. The mercury contained in the drops, which fall through pipe section 19 to reservoir 22, is added to mass 20. Concurrently, mercury overflows from reservoir 22 and is discharged through outlet 24 to sump, only generally and schematically indicated at 25, which, according to an aspect of this invention, has a particular structure which will be described hereinafter. The cell body 17 is provided with an exit 29, which is closed by a porous ceramic body 30 and leads to an auxiliary vessel 31, filled with a potassium chloride solution and containing the reference electrode 32. The porous ceramic body 30 electrically connects the cell to the reference electrode by ion mobility.

The sample solution to be analyzed and which contains the electrolyte, is fed to the apparatus through inlets 40 and 41. It can be drawn into the inlets by the vacuum apphed to the cell, or by a peristaltic pump which feeds it to said inlets, or both. In Fig. 1, a vacuum pump, not illustrated but schematically indicated at 26, creates a vacuum in the cell through an exit pipe 27, which is connected to exit 28 formed in the body 17 of the cell. The vacuum in the cell applies suction to the inlet, drawing the sample solution into the said inlets. Through the said inlets, the solution is led into deoxygenation means. In the embodiment illustrated, this means is constituted by a conduit, indicated in this embodiment as pipe 43. Nitrogen is fed to pipe 43 through pipe 42 and other means, described hereinafter. Thus, the sample solution flows in a thin layer on the inner surface of pipe 43, while nitrogen flows centrally of said pipe; and oxygen is removed from the solution and becomes mixed with the nitrogen. Pipe 43 reaches its highest point, 44, and then continues downwardly to an outlet 45 where it branches out into an upper or gas branch 46 and a lower or liquid branch 47. At the outlet 45, the sample solution becomes separated from the nitrogen stream. This latter flows upwardly through branch 46, while the sample solution flows downwardly through branch 47. The nitrogen flows into the body 17 of the cell, around mercury capillary 15, and out of it through exit 28 and pipe 27, to vacuum pump 26. The sample solution enters the cell body 17 at the inlet 48, situated between the mercury drop 18 and the pipe section 19. It is trapped in said pipe section by the mercury mass 20 and fills it completely, covering platinum electrode 21 and completely filling the space between the mercury mass 20 and the mercury drop 18. It then flows upwards over the mercury capillary 15 and finally out of the cell body 17 through outlet 28 and pipe 27, and therefrom to the drain. Means, not shown and conventional, are provided for applying a potential between the mercury drop 18 and the reference electrode 31.

Mercury is fed to the apparatus from a mercury container, only generally and schematically indicated at 50 in Fig. 1, a preferred embodiment of which is better illustrated in vertical cross-section, at a larger scale, in Fig. 2. Said container comprises a shell, preferably made of plastic, consisting of a body 51 and a cap 52 that can be screwed onto it or screwed from it, as shown at 53, to permit introduction of a feed bottle 54. Feed bottle 54 is provided with an elastic rubber cap 55, preferably of silicon rubber, which has gas-tight passages therein for two pipes 56 and 57. However, different structures of container could easily be devised by skilled persons.

An embodiment of the means for applying gas pressure to one or more of: a space above the level of the mercury in the feed container, a space above the level of the mercury in the mercury reservoir, and an inlet leading into the cell, will now be described, but it should be understood that said embodiment is not limitative and that skilled persons could devise different structures of said means.

As seen in Fig. 1, in this embodiment pipe 57 is connected to or is a terminal portion of a conduit 58 which leads to mercury reservoir 11. Pipe 56, on the other hand, is connected to or is a terminal portion of a pipe 59, which leads to the upper part of said reservoir 11, above the level of the mercury, and in which a valve 60 is inserted. Pipe 59 is connected to a pipe 61 which communicates, through valve 62, with a pipe 63. Pipe 63 is fged with nitrogen from a nitrogen balloon 65 and a pressure regulator 64, which is preferably set at about 0.5 Bar. Pipe 63 is also connected to pipe 66, in which a valve 67 is inserted and which leads to the aforementioned pipe 43. All the aforesaid valves have an open and a closed position, the first one being indicated in full lines and the second one in broken lines.

The apparatus according to this embodiment of the invention operates as follows. Feed bottle 54, filled with mercury, is so placed in the apparatus that pipe 56 passes through cap 55 and reaches to a level above the surface of the mercury, while pipe 57 passes through cap 55 and reaches to or almost to the bottom of the mercury mass. When it is desired to feed mercury to reservoir -lO- ll, valve 62 is opened while the other two valves are closed. Pressure is thus supphed to the inside of bottle 54, through pipes 61 and 56, and mercury rises through pipes 57 and 58 and is introduced into mercury reservoir 11. This is desirably provided with electrical contacts (shown in Fig. 3 and omitted in Fig. 1), which serve as level indicators. In this embodiment there are provided two such contacts 71 and 72, which indicate the minimuni and maximum level of the mercury, while an additional one, indicated at 73, which constitutes a contact to the working electrode, would signal an abnormal lowering of the mercury level. These contacts may be a part of an electrical circuit which controls in an obvious manner valve 62, in order to supply mercury whenever and until this is required. Once the mercury has reached the required level in reservoir 11, valve 60 is opened. As a result, the pressure is equalized between the levels above the mercury in bottle 54 and in reservoir 11, and all flow of mercury from the first to the second ceases. Valve 62 is closed at this stage. When a sample solution is to be fed to the voltammetric cell, valve 67 is open and supplies nitrogen to pipe 43, with the consequences that have been described.

When the voltammetric cell is not in operation, valves 62 and 67 are open and therefore pipes 61 and 66 are cut off from the gas supply. Valve 60 is closed, and therefore the pressure is the same in bottle 54 and in mercury reservoir said 11, and at this stage said bottle and said reservoir are at atmospheric pressure. The resistance of capillary 15 to flow prevents the mercury from flowing through it and forming drops at the lower end thereof, as long as reservoir 11 is at atmospheric pressure. Therefore at this stage, there is no flow through the apparatus, either flow of gas or of mercury. When the operation of the voltammetric cell starts, introduction of the hquid sample into the voltammetric cell begins, valve 67 is closed and gas, in particular nitrogen, flows through pipe 66 and through the deoxygenator to the cell. Once the introduction of the sample into the cell has been completed, valve 62 is closed. As a result, the pressure rises in reservoir 11 and bottle 54. The pressure in reservoir 11 causes the mercury to flow through capillary 15 and form drops at the lower end thereof, and the analysis of the sample is carried out.

When the level of the mercury in reservoir 11 decreases, the contact between the level indicator contacts 71 and 72 in said reservoir is interrupted, and, as a result, valve 60, which is controlled by said contacts, is opened, the gas from vessel 11 is discharged to the atmosphere and the pressure in said reservoir is lowered. The lowering of the pressure causes mercury to flow from vessel 54 to reservoir 11 and the formation of mercury drops at the lower end of capillary 15 to cease. As a result of said flow, the level of the mercury in the vessel rises until level indicator contacts 71 and 72 are short- circuited. As a result, valve 60 is closed, the pressure in the two vessels is equalized and the flow of mercury ceases.

Mercury collects, as hereinbefore described, in sump 25. Said sump is actually constituted by a sump bottle 54', which is a feed bottle such as shown in Fig. 2, which has been emptied or nearly emptied of mercury, said mercury having been fed to reservoir 11. The several parts of said sump bottle are indicated hereinafter by accented numerals corresponding to those of Fig. 2. In order to use such a bottle as a sump bottle, the used, empty bottle 50 is transferred to the sump position. Then, sump bottle 54' is so positioned that pipe 27 penetrates through its opening. When a feed bottle 54, from which mercury is supphed to reservoir 11, is sufficiently empty so that mercury can no longer be drawn from it, it is removed and becomes a new sump bottle 54', while the previous sump bottle 54', which has filled meanwhile with mercury discharged from the cell, can be emptied and re¬ used.

Fig. 3 illustrates at an enlarged scale a detail of the apparatus of Fig. 1, and specifically the assembly constituted by the mercury reservoir 11 with the capillary 15, in three positions. In Fig. 3a, the reservoir and the capillary are shown as they are when they are mounted in the voltammetric cell. When it is desired to clean the capillary, the assembly of the mercury reservoir and the capillary is removed from the cell and is overturned, as shown in Fig. 3b. The mercury mass 20 gathers in space 74 at the top of the reservoir. The capillary can then be detached, as shown in Fig. 3c, and cleaned without the operator's coming into contact with the mercury.

It is clear that, thanks to this invention, the feed of the mercury to the cell and its recovery occur without any exposure of operators to contact with the mercury, and therefore without involving any health hazards and in a completely ecological manner. Further, while an embodiment of the invention, which comprises a DME cell such as described in the aforesaid PCT applications WO 96/35117 and WO 96/35118, has been described by way of example, it is clear that the invention may be apphed to other DME cells, having means for feeding mercury to it and preferably recovering mercury from it. It will also be apparent that the invention can be carried out by persons skilled in the art with many modifications, variations and adaptations, without departing from its spirit or exceeding the scope of the claims.

Claims

1. Electroanalytical voltammetric apparatus, which comprises, in combination with a DME voltammetric cell, means for selectively applying an inert gas pressure to one or more of: 1- a space above the level of the mercury in a feed container, containing mercury to be fed to said cell; or/and 2 - a space above the level of the mercury in said cell; or/and 3 - an inlet leading into said cell.

2. Apparatus according to claim 1, comprising means for selectively applying an inert gas pressure to one or more of: 1- a space above the level of the mercury in a feed container, containing mercury to be fed to a mercury reservoir comprised in said cell; or/and 2 - a space above the level of the mercury in said said reservoir; or/and 3 - an inlet leading into said cell.

3. Apparatus according to claim 2, wherein the means for selectively applying inert gas pressure comprise a source of inert gas under pressure, first conduit means connecting said gas source to the space above the level of the mercury in the feed container, second conduit means connecting said gas source to said space above the level of the mercury in the reservoir, and third conduit means connecting said gas source to the inlet leading into the cell.

4. Apparatus according to claim 2, wherein the means for selectively applying inert gas pressure comprise at least one valve inserted in each of the conduit means for selectively permitting or preventing flow of inert gas through said conduit means.

5. Apparatus according to claim 4, wherein the pressure regulating means are interposed between the source of inert gas and the conduit means.

6. Apparatus according to claim 2, wherein the means for selectively applying inert gas pressure permit concurrently to apply the same pressure above the level of the mercury in the feed container and above the level of the mercury in the mercury reservoir.

7. Apparatus according to claim 1 or 2, wherein the DME voltammetric cell is a cell as described in PCT applications WO 96/35117 and WO 96/35118.

8. Apparatus according to claim 1, which comprises a mercury feed container and means for feeding mercury from said container to a mercury reservoir comprised in the DME voltammetric cell, which feeding means comprise a pipe line connecting the mass of mercury contained in said container to said reservoir, and means for applying pressure to said mercury mass for causing the mercury to flow from said container to said reservoir.

9. Apparatus according to claim 8, wherein the means for applying pressure to the mercury mass comprise a source of inert gas under pressure and conduit means connecting said gas source to the space above the level of the mercury in the feed container, pressure regulating means interposed between said source of inert gas and said conduit means and at least one valve inserted in said conduit means for selectively permitting or preventing flow of inert gas therethrough.

10. Apparatus according to claim 7, wherein the DME voltammetric cell comprises deoxygenating means and the inlet leading into the cell is the inlet into said deoxygenating means.

11. Apparatus according to claim 1, wherein the DME voltammetric cell comprises a mercury capillary fed with mercury from a mercury reservoir, whereby a mercury drop is periodically formed and separated from the lowermost end of said capillary.

12. Apparatus according to claim 11, wherein the mercury reservoir and the mercury capillary form an assembly detachable from the remaining parts of the DME cell, said reservoir being provided with a space at the top thereof that is adapted to collect the mercury when said assembly is overturned, and said capillary being detachable from said reservoir, when said assembly is overturned, without contacting the mercury collected in said space.

13. Apparatus according to claim 1, wherein the DME voltammetric cell comprises a) a cell body housing, in addition to a reference electrode, a working electrode and, in its lowermost portion, a counter-electrode; b) means for removing oxygen from the sample solution; c) means for feeding the sample solution to said deoxygenation means, means for feeding a stream of an inert gas to said deoxygenation means, and means for causing said solution to flow in said deoxygenation means, whereby oxygen is removed therefrom by contact with said inert gas; d) a means for removing said inert gas from said deoxygenation means after deoxygenation of the sample solution; e) an inlet for the deoxigenated sample solution provided in said cell body in the space between said working electrode and said counter-electrode; f) an exit for the sample solution provided in said cell body at a level above said working electrode; and g) vacuum and/or pressure means for causing said sample solution to flow to said exit, to be discharged from the cell above said working electrode, thus assuring that the space between said working electrode and said counter- electrode is constantly filled with said sample solution.

14. Apparatus according to claim 1, wherein the inert gas is chosen from among nitrogen and helium.

15. Apparatus accprding to claim 1, which comprises a mercury feed container for feeding mercury to the cell and a sump for receiving mercury which has formed the DME and is discharged from the cell.

16. Apparatus according to claim 15, wherein the feed container comprises a metal shell and a bottle housed therein, said bottle having closure means for receiving in gas-tight manner a first pipe having an outlet above the intended level of the mercury therein and a second pipe reaching substantially to the bottom of said bottle.

17. Electroanalytical voltammetric apparatus, subastantially as described and illustrated.