Abstract:
An electroanalytical apparatus defining a Static Mercury Drop Electrode cell which includes a capillary tube at the end of which is formed mercury drops to constitute the working electrode including a container for continuously receiving and collecting mercury that has formed said working electrode and has become contaminated, a purifying vessel positioned in fixed relationship to the capillary tube, conduit means for continuously transferring mercury into the purifying vessel from the container, means for introducing highly oxygenated water into the purifying vessel at a location above the mercury collected in the vessel such that surface contact is established between said highly oxygenated water and the mercury collected in the purifying vessel, and means for continuously drawing mercury from the purifying vessel and feeding it as purified mercury to the capillary tube.

Description:
This application is a continuation of PCT/IL98/00583 filed Nov. 30, 1998. 
    
    
     FIELD OF THE INVENTION 
     This invention relates to an improved voltammetric apparatus of the Static Mercury Drop Electrode (hereinafter SMDE) type, in which the mercury is purified and recycled 
     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. 
     U.S. Pat. No. 3,922,205 describes the basic structure of a polarographic cell. U.S. Pat. No. 4,138,322 discloses a structure of shielded dropping mercury cathode. U.S. Pat. No. 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 C. N. Yarnitzky in Analytical Chemistry, Vol. 57, No. 9, August 1985, p. 2011-2015. 
     Such cells, however, are not fully satisfactory. 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. In others the mercury feed apparatus is complicated, and mercury has to be replaced once a while. 
     An improved voltammetric apparatus, free from said drawbacks, 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 deoxygenated 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. 
     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 mercury 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. 
     Further, prior art voltammetric apparatus are not satisfactory for carrying out for anodic stripping techniques. Therein, the mercury drop remains in place for a time from 3 to 15 seconds, depending to the capillary used. While this lifetime of the drop is sufficient for polarography, it is not sufficient for anodic stripping, which requires a much longer drop lifetime, in the order of minutes, e.g. about 2 minutes. Further, prior art apparatus are sensitive to small particles, e.g. in the range of 25 to 100 μm, which can block the capillary tube. 
     It is therefore an object of this invention to provide an electroanalytical voltammetric apparatus of the Static Mercury Drop Electrode (SMDE) type, which is free of the said drawbacks. 
     It is another object of the invention to provide such an apparatus, which comprises means for purifying the mercury in situ and feeding the purified mercury back to the capillary tube which contains it and from which the electrode drops are formed, by means which avoid all manipulation on the operator&#39;s part and all contact between him and the mercury. 
     It is a further object of the invention to provide such an apparatus in which clogging incidents are reduced and which comprises means that enables the use of improved electroanalytic techniques such as anodic stripping techniques. 
     It is a still further object of the invention to provide such an apparatus in which the capillary tube, at the lower end of which the mercury drop forms, can be replaced, in case of clogging, without the operator&#39;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 SMDE voltammetric cell, means for puriying the mercury and recycling the purified mercury to the capillary tube at the lower end of which the mercury drops are formed. 
     Said means for purifying the mercury is a means for generating surface contact between the contaminated mercury and water having a high oxygen content. Preferably, the water is saturated or nearly saturated with oxygen or air, and its oxygen content is close to 8 mg/L or higher. 
     Accordingly, an aspect of this invention is a process for continuously purifying and recycling mercury in an SMDE cell, which comprises continuously bringing contaminated mercury and highly oxygenated water into mutual surface contact, whereby the contaminating metals are oxidized and migrate from the mercury to the water, and continuously feeding the resulting purified mercury to the SMDE cell. 
     Another aspect of the invention is an apparatus for continuously purifying and recycling mercury in an SMDE cell, which comprises means for continuously bringing contaminated mercury and highly oxygenated water into mutual surface contact, whereby the contaminating metals are oxidized and migrate from the mercury to the water, and means for continuously feeding the resulting purified mercury to the SMDE cell. 
     Preferably, said puriying apparatus comprises a container hereinafter, the “purification container”) for continuously receiving contaminated mercury, said mercury accumulating in said container to form a mass having an upper surface, means for forming a layer of highly oxygenated water in said container above said mercury mass, said layer having a lower surface in contact with said upper surface of said mercury mass, and means for continuously withdrawing purified mercury from said container. 
     Said highly oxygenated water can be produced in any suitable way. A preferred way of producing it consists in forming a layer of water, e.g. salty water, in surface contact with contaminated mercury, and enriching said layer with oxygen. This may be conveniently done, e.g., by introducing contaminated mercury into a purification container, introducing water above the mercury surface to form a layer, and bubbling through said water layer oxygen or an oxygen containing gas, preferably air. Another way of producing said highly oxygenated water layer is to oxygenate water, e.g., by bubbling through it oxygen or an oxygen containing gas, preferably air, or by mixing a water stream with a stream of oxygen or an oxygen containing gas, preferably air, while said water is out of contact with contaminated mercury, and bringing the resulting, highly oxygenated water into contact with the contaminated mercury. This may be conveniently done, e.g., by continuously introducing contaminated mercury into a purification container, continuously introducing the highly oxygenated water into said purification container above the mercury surface to form a layer in contact with the surface of said mercury, and continuously withdrawing said water from said container, whereby to replace the water of said layer with freshly oxygenated water, at such a rate as to maintain therein the desired oxygen content and to limit its contamination to acceptable levels. Preferably, said SMDE voltammetric cell is basically the cell described in said PCT application WO 96/35117 as well as in PCT application WO 96/35118, with which the mercury purification means of this invention are combined. In this case, said inlet into the cell is the inlet into the deoxygenating means. However, this invention can be carried out with voltammetric cells other than those described in said PCT applications, particularly cells which do not include deoxygenation means. 
    
    
     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 the mercury purification container of the embodiment of FIG. 1, shown in cross-section at an enlarged scale; and 
     FIG. 3 illustrates, in the same way as FIG. 1, another embodiment of the invention. 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     The invention is illustrated in two embodiments as applied 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 (SMDE) 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 PCT patent application WO 96/35117. 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 inlet  11 , at the top thereof. Numeral  12  indicates a platinum wire used as an electrical contact. From inlet  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 non-absorbing material like glass or Teflon. Capillary  15  has an inner diameter equal or greater then 0.08 and preferably about 0.15 mm, which is generally large enough to avoid clogging due to solid particles or surface active materials. 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 a purification unit, only generally and schematically indicated at  25 , which, according to an aspect of this invention, has a particular structure which will be described with reference to FIG.  2 . The cell body  17  is provided with an exit  29 , which is connected to a any suitable reference electrode, such as a conventional electrode or an electrode made as described in WO 96/35117. Exit  29  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 introduced into the apparatus by a peristaltic pump which feeds it to 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 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 , and therefrom to the air. 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 drops  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 a drain  27 . Means, not shown and conventional, are provided for applying a potential between the mercury drop  18  and the reference electrode  31 . Mercury flows out of reservoir  22  through pipe  24  and therefrom into container  25 , in which it undergoes purification, as hereinafter explained. 
     In FIG. 1, a peristaltic pump, schematically indicated at  26 , sucks the pure mercury from purification unit  25 , and pumps it back to inlet  11  for reuse. Thus the mercury is recycled for a theoretically unlimited, and anyway very high, length of time, with no need to empty used mercury bottles or refill the mercury reservoir. 
     A preferred embodiment of the mercury purification unit, only generally and schematically indicated at  25  in FIG. 1, is illustrated in vertical cross-section, at a larger scale in FIG. 2 It comprises a shell  50 , 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 purification container, which is bottle  54 . Bottle  54  is provided with an elastic rubber cap  55 , preferably of silicon rubber, which has gas-tight passages therein for four pipes  56 ,  57 ,  58  and  62 . The mercury from reservoir  20  and pipe  24  accumulates at the bottom of bottle  54  to form as mass, indicated at  60 , and the upper part of the bottle contains a layer of water, preferably salty water (conductivity above 1 mS), indicated at  61 . In this embodiment, the water has to be replaced when it has become excessively polluted with metals, viz. when the metal ion concentration exceeds a limit that can be easily determined in each individual case. Therefore it is preferred that the water layer be deep, so that the metal ions, diffusing out of the mercury to the water, will be diluted and the metal concentration will be low for a long period of time, whereby the water need not be replaced too often. However, this embodiment of the invention can be carried out even with a thin layer of water, e.g. having a depth of 1 cm, or even less, provided that it is replaced at shorter intervals. 
     In order to achieve and maintain a desired oxygen content of the water, an appropriate gas, preferably air, is bubbled, in this embodiment, through the water layer, by feeding it below the surface of the water and slightly above the level of the mercury. One way of doing this, is to feed air through pipe  58 , which extends downwardly to a level close to the bottom of bottle  54 , whereby the air or other oxygen containing gas, admitted through pipe  58 , bubbles through the mercury and produces a mixing action, to maintain the concentration of polluting metals substantially uniform throughout the mercury mass  60 . Pipe  62 , which ends at a level above the surface of the water layer  61 , permits the discharge of air or other oxygen containing gas that has not dissolved in the water. The contaminated mercury flows in from pipe  24  (FIG.  1 ), only the lowermost portion  56  of which is visible in FIG. 2, and which extends downwardly to a level close to, but below the upper surface of the water mass  61 . The upper surface of the mercury mass is exposed to the oxygen dissolved in the water layer. Surprisingly the contact of the upper surface of the mercury, which mercury contains the metal impurities that it is desired to remove, with the lower surface of the oxygen containing water is sufficient to cause the metals to undergo a rapid oxidation and migrate and dissolve into the water. The oxidized and dissolved metal ions are replaced by other metal ions which migrate from the lower levels of the mercury mass to the surface and also undergo oxygenation followed by dissolution, and this process continues until the mercury is entirely purified. The pure mercury is pumped out of bottle  54  through pipe  57 , which reaches to a level close to the bottom of the bottle, and is connected to pump  26 , which returns the mercury back to the upper part of the cell inlet  11  through pipe  59  (see FIG.  1 ). 
     Since the oxygen content of the water in the purification container is decreased by the oxidation of the metals, and these latter become dissolved in the water and contaminate it, the water must be periodically replaced to keep the oxygen content high enough and contamination low enough. 
     When the operation of the voltammetric cell starts, introduction of the liquid sample into the voltammetric cell begins, and gas, in particular nitrogen, flows through the deoxygenator  43  to the cell. Once the introduction of the sample into the cell has been completed, pump  26  starts operating. As a result, the mercury flows through capillary  15  and forms drops at the lower end thereof. When the drop reaches the desired size, the pump stops running and the analysis of the sample is carried out. 
     In another embodiment of the invention, illustrated in FIG. 3 ,the voltammetric cell is the same as in FIG. 1, but the purification unit  65 , which is illustrated here in schematic cross-section, has a different structure, though it is based on the same principle of oxidizing the metallic impurities In FIG. 3, the parts that are the same as in FIG. 1 are indicated by the same numerals. 
     In this embodiment, the working electrode is still constituted by mercury drops, formed as illustrated in FIG.  1 . The fallen drops sink into a mercury trap  66  which, together with platinum wire  21 , one end of which is immersed therein, constitutes the counter-electrode. The mercury is discharged from trap  66  through outlet  67  to a purification unit  65 , where it forms a mass  68 . Fresh, salty water (drinking water, for example) previously enriched with oxygen in any suitable way, is introduced by an inlet pipe  70  which reaches close to the bottom of the mercury mass  68 , whereby to exert a mixing action to assure that the concentration of metals be substantially uniform throughout the mercury mass, and forms a layer  69  above and in contact with the upper surface of said mercury mass  68 . The contact between said upper surface and the lower surface of layer  69  provides the oxidation condition necessary for the purification of the mercury, as described in connection with the purification unit  25  of FIG.  2 . The water, which has lost oxygen through the oxidation process and has been contaminated by dissolved metals, is continuously withdrawn through outlet  71  above the level of inlet  70 . The outlet and inlet water flow rates are, of course, equal, and are determined, for each particular cell, in such a way that the concentration of oxygen in the water remains high enough and the contamination by metals does not reach too high a level. 
     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 embodiments of the invention, which comprise a DME cell such as described in the aforesaid PCT applications WO 96/35117 and WO 96/35118, have been described by way of example, it is clear that the invention may be applied to other DME cells, having means for feeding mercury to it and preferably recovering mercury from it. 
     Further, the use of a peristaltic pump for recycling the mercury, by drawing it from the purification unit and pumping it back to the working electrode, permits to stabilize the mercury drop at the tip of the capillary tube after it reaches the desired size, viz. to produce a static drop. Thanks to the longer lifetime of the drop, the device according to the invention is adapted to apply improved electroanalytic techniques, such as anodic stripping techniques. Further, the ability to stabilize the drop enables the use of larger capillary tubes, e.g. up to 200 μm, thus reducing the sensitivity of the apparatus to small diameter particles; and in the event that a particle penetrates the tube, it is forced out by the pump which drives the mercury through the tube. 
     An important parameter, in the process of this invention, is the ratio between the concentration of metal ions at the surface of the mercury and their concentration in the bulk of the water. Such a ratio should preferably be at least 1:100, viz. the metal concentration in the water bulk should be 100 times or more lower than the concentration at the mercury surface. 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.