VOLTAMMETRIC CELL

Abstract: A voltammetric cell for use in stripping voltammetry. The voltammetric cell (11) has a chambre (17) for containing a solution to be analysed. Disposed in the chamber (17) are working and counter and reference electrodes (27, 29, 31) each having an electro-active surface for contacting the solution. The chamber (17) has an inlet means (33) for introducing solution into the chamber and an outlet means (35) for removing solution from the chamber. The outlet means (35) has an outlet port (37) opening into the chamber. The chamber (17) is provided a well (38) for retaining a portion of the solution when solution is removed from the chamber. The working electrode (27) is so positioned that the electro-active surface thereof is disposed in the well (38) so as to be covered by the retained solution. The counter electrode (29) and reference electrode (31) are also positioned to contact solution retained in the well.

Description: "Voltammetric Cell" THIS INVENTION relates to a voltammetric cell for a vol¬ tammetric measuring device. Voltammetric measuring devices are employed in stripping voltammetry which is a technique for analysis of a sample solution to test for trace metals and other electro-active species in the solution. In the technique of stripping voltammetry, ions of all the electro-active species of interest are first deposited on a working electrode to which an electrical potential is applied and thereafter the electrical potential is rever¬ sed and progressively varied to strip the deposited mate¬ rial from the electrode and return it back into the solu¬ tion. The electrolytic current is monitored as the de¬ posited material is stripped back from the electrode. As the deposited material is stripped from the electrode, a change in current results at the respective characteristic potential for the each electro-active species. The magni¬ tude of the current provides a direct measure of the concentration of the electro-active species in the sample. A known voltammetric cell for voltammetric measuring devices comprises a chamber for receiving a sample of the solution to be analysed, the chamber having an open top and a cover sealingly engagable with the top of the cham¬ ber. The working electrode, together with a counter electrode and usually a reference electrode, is mounted in the cover and extends downwardly so as to reach into the chamber and contact the sample solution therein when the cover is fitted onto the chamber. This known voltammetric cell has several deficiencies when a number of samples are to be analysed. One of these deficiencies is that the working area (i.e. the elec¬ tro-active surface) of the working electrode is exposed to air when the sample solution is removed from the chamber and its integrity may diminish if ' the period of exposure to air is prolonged. This is particularly so in the case where the working electrode is in the form of a film of mercury or other electro-active material plated on a substrate of suitable material such as glassy carbon. When the film is exposed to air, it may dry out. Where this occurs, the film is severely distorted (owing to coalescence of the mercury film) and the integrity of the working area is thereby diminished. As a consequence of this, successive analyses are poorly reproducible. Often- this lack of good reproducibility requires either that the working electrode be replated to restore the mercury film or that a. soluble mercury compound be added to the solu¬ tion undergoing analysis so that co-plating of mercury can occur during the stripping analysis. A further deficiency of this known voltammetric cell which has the electrodes located at the upper part of the Cell, is that the electrical potential or voltage applied to the working electrode cannot be continuously controlled; one example of this is in instances where the solution samples are being changed. This is because contact of the various electrodes with the solution is interrupted. For the analysis of some elements such as gold, this lack of electrode contact may lead to the unintentional plating of metal and carry-over contaminant from one sample to the next via the working area of the working electrode. It is an object of this invention to provide a voltamme¬ tric cell in which the working area of the working elec¬ trode can remain wet even during sample replacement. It is a further object to provide a voltammetric cell in which all electrodes remain in contact with a small resi¬ dual portion of solution when the cell is emptied so that a controlling voltage can be maintained between the work¬ ing and reference electrodes whenever the voltammetric measuring device is operational. In one form the invention resides in a voltammetric cell comprising: a chamber for containing a solution to be analysed; a working electrode, a counter electrode and a reference electrode each having an electro-active surface disposed in the chamber for contacting the solution; inlet means for introducing solution into the chamber; and outlet means for removing solution from the chamber, the outlet means having an outlet port opening into the cham¬ ber characterised in that the chamber has a well for retaining a portion of the solution when solution is removed from the chamber, the electro-active surface of the working electrode being disposed in the well so as to be covered by the retained solution, and electro-active surfaces of the counter and reference electrodes being so ■ positioned as to contact the retained solution. The retained solution which covers the electro-active sur¬ face of the working electrode protects that surface from exposure to air. Because the counter and reference electrodes are in con¬ tact with the retained solution, a controlling voltage can be maintained between the working electrode and the refe¬ rence electrode. In one arrangement, the well may be constituted by a recess formed in the floor of the chamber. In another arrangement, the outlet port is located slightly above the floor of the chamber thereby to define the well. In still another arrangement the well may be constituted by a depression formed in a surround about the electro-active surface of the working electrode. In still another arran¬ gement the well may be constituted by a depression formed in the electro-active surface of the working electrode. The outlet means may comprise a drain having a drain entry port (being said outlet port) opening into the chamber, and valve means for opening and closing the drain entry port to the interior of the chamber. The drain itself may comprise a drain passage and a sealing ring, the drain passage having an entry end opening into the chamber at a location immediately adjacent the floor thereof, and the sealing ring surrounding the entry end of the drain pas¬ sage whereby to define the drain entry port, the sealing ring contacting the floor of the chamber. The valve may comprise a poppet valve having a valve head sealingly engagable with a valve seat, the valve seat being defined by the sealing ring. The working electrode may be of any suitable construction and material. It may, for example, be formed of platinum " or stainless steel, or a rod of carbon having an elec¬ tro-active layer deposited on one end face thereof. Preferably, the solution undergoing analysis is circulated in the chamber. Circulation of the solution may be esta¬ blished by stirring or agitating the solution in the chamber or by providing a flow of solution through the chamber. In the case where the solution is circulated by a stirring action, there may be provided a stirring means comprising a rotatable impeller having a liquid flow path formed therein, the flow path having an inlet end, an outlet end and an outer section extending transversely of the rotational axis of the impeller and opening onto the exterior of the impeller at said outlet end, the inlet end of the flow path opening onto the exterior of the impeller at a plane transverse to the rotational axis of the impel¬ ler. The flow path may further include a further section extending axially with respect to the rotational axis of the impeller and opening onto the exterior of the impeller at said inlet end. The impeller is preferably disposed above the floor of the chamber with the inlet end facing the floor. To facilitate good circulation of solution in the chamber, it is preferred that the side wall of the chamber is inclined inwardly and downwardly towards the floor. With this arrangement, circulating solution flows downwardly alongside the side wall of the chamber and turns adjacent the floor to flow upwardly in the central region of the chamber and enter the inlet end of the impeller. This establishes a flow pattern which is non-turbulent in the region of the electro-active surface of the working elec¬ trode and in which solution is substantially stagnant in the zone immediately adjacent the electrode, electro-active surface. This is beneficial in that the electro-active surface is not exposed to the effects of turbulent flow, and a generally constant concentration gradient is main¬ tained in the vicinity of the electrode during plating. The invention will be better understood by reference to the following description of several specific embodiments thereof as shown in the accompanying drawings in whic :- Fig. 1 is a schematic view of a voltammetric measur¬ ing device having a voltammetric cell according to the first embodiment; Fig. 2 is a schematic view, on an . enlarged scale, of the voltammetric cell according to the first embodi¬ ment; Fig. 3 is an isometric view of a stirring element incorporated in the voltammetric cell of the first embodiment; Fig. 4 is a sectional view on the line 4-4 of Fig. 3; Fig. 5 is a cross-sectional view on the line 5-5 of Fig. 3; Fig. 6 is a fragmentary view of the voltammetric cell of the first embodiment, showing schematically the liquid flow pattern in the cell; Fig. 7 is a partially sectioned schematic view of part of a voltammetric measuring device having a voltammetric cell according to the second embodiment; Fig. 8 is a sectional view, on an enlarged scale, of the voltammetric cell according to the second embodi¬ ment; and Fig. 9 is a schematic sectional view of a working electrode of a voltammetric cell according to a third embodiment. Referring to Figs. 1 and 2 of the drawings, the voltamme¬ tric cell 11 according to the first embodiment includes receptacle 13 in the form of a hollow body 15 which de¬ fines a chamber 17 to receive a sample solution for analy¬ sis. The body 15 includes a lower part 19 having an open top and an upper part 21 which forms a closure for the open top of the lower part. The chamber 17 has a side wall 23 which is inclined downwardly and inwardly towards a floor 25. The voltammetric cell has a working electrode 27, a coun¬ ter electrode 29, and a reference electrode 31. The working electrode has a working area (being an elec¬ tro-active surface) which is located at, and defines part of, the floor 25 of the chamber. While the working electrode may be of any suitable con¬ struction, in this embodiment it is in the form of a rod of glassy carbon having on one end face thereof a deposit of an electrode surface layer in the form of a film of mercury. The film of mercury defines the electro-active surface (i.e. working area) of the electrode. The voltammetric cell has a solution inlet means in the form of an inlet port 33 formed in the upper part 21 of the body 15. In addition, the voltammetric cell has a solution outlet means located slightly above the working area of the working electrode. The solution outlet means is in the form of a drain 35 having a drain entry port 37 formed in the inclined side wall 23 of the chamber at a location adjacent but spaced slightly above the floor 25 thereby to define a well 38 in the chamber below the drain entry port. The drain 35 is constituted by a drain passage 39 and a sealing ring 41. The drain passage 39 is formed in the body 15 and has an entry end opening into the chamber 17 at a location immediately adjacent the floor 25 of the chamber. The sealing ring 41 surrounds the entry and of the drain passage 39 and defines the drain entry port 37. A valve 43 is associated with the drain 35 for opening and closing the drain entry port 37 to the chamber. The valve 43 is in the form of a poppet valve having a valve head 45 and a stem 46. The valve head 45 is movable into and out of sealing engagement with a valve seat which is defined by the sealing ring 41. The valve 43 is biassed into a position corresponding to sealing engagement of the valve head with the valve seat by means of a spring 49. A push button 51 is provided on the end of the stem opposite the valve head to facilitate manual operation of the valve. The sealing ring contacts the floor 25 of the chamber and has the effect of providing a barrier between the working area of the working electrode and the entry end of the drain passage 39. The barrier in conjunction with the floor and a portion of side wall of the compartment de¬ fines the well 38 which is arranged to retain a small portion of the solution in the chamber when the drain is opened to discharge the liquid contents from the chamber. The residual solution retained in the well covers' the working area of the working electrode and thereby prevents contact with air. The counter electrode 29 and the reference electrode 31 are positioned in close proximity to the working electrode so as to contact the residual solution retained in the well. More, particularly counter electrode 29 is located on the underside of a stirring means 53, and the reference electrode 31 is located in the side wall 23 of the chamber 17. The stirring means 53 is provided for circulating the solution contained within the chamber 23. The stirring means includes a stirring- element 55 mounted on the lower end of a shaft 57 which- xtends downwardly into the cham¬ ber 23. The stirrer shaft 57 is ro atably supported in a bearing 59 mounted in the upper part 21 of the body and is coupled at its upper end to a motor 61. The motor is mounted on a support assembly 63 which is carried on the upper part 21 of the body. The stirring element 55 may simply be a paddle, but in this embodiment it is in the form of an impeller which is shown in more detail in Figs. 3, 4 and 5. The impeller 55 comprises a body 67 having formed therein a flow path including an axial section 69 and a plurality of radial sections 71. The axial section is disposed axially with respect to the axis of rotation of the body and opens at its lower end onto the exterior of the body to define an inlet end for the flow path. The radial sections 71 are disposed radially of the axis of rotation of the body. Each radial section communicates at its radially inner end with the upper end of the axial section and opens at its radially outer end onto the exterior of the body. On rotation of the impeller, liquid flow is induced through the first and second passages, upwardly along the first passage and then outwardly along the second passa¬ ges. The flow is induced by centrifugal acceleration of liquid in the radial sections of the floor path as the impeller rotates. The rotational speed of the impeller is selected, so as to avoid the formation of vortex in the liquid around the impeller to a depth which would allow air to enter the second passages. The liquid flow generated by the impeller circulates the solution within the chamber, as shown schematically in Fig. 6. In the lower part of the chamber, the circulating- solution flows downwardly alongside the side- wall of the chamber and turns adjacent the floor to flow upwardly in the central region and enter the axial section of the flow path in the body of the impeller, as shown in Fig. 6. This establishes a flow pattern in which there is non-tur¬ bulent flow in the region of the working area of the working electrode and a substantially stagnant zone imme¬ diately adjacent the electrode working area. This gives rise to a concentration gradient in the solution in the vicinity of the electrode working area. Electro-active species in solution are required to diffuse across the concentration gradient prior to being deposited on the working area of the working electrode. With this flow pattern, the working area of the working electrode is not exposed to the damaging effects " of turbulent flow and thus the service life of the working area is prolonged. The electrical controls for operating the voltammetric measuring device to which the voltammetric cell is fitted, are illustrated in Fig. 1 of the drawings. All operations of the device are controlled by a micro-processor 81 with instructions inputted via a keyboard 83. The device is powered by a electrical supply which is preferably in the form of a re-chargable DC supply 87. A digital-to-analo¬ gue converter 89 is used to convert the values to voltages which can be applied via a voltage control circuit 90 to the three electrodes and to the motor 61. Various parame¬ ters for controlling voltages, and plating and stripping times can be inserted into memory and recalled as a speci¬ fic program. A current amplifier 91 amplifies the elec¬ trolytic current, which is sampled at suitable intervals and converted to digital values with an analogue to digi¬ tal converter 92. Peaks in the voltammetric curve are located, and measured. The peak outputs can be displayed on a display device 93 and presented in hard copy form by means of a printer 95. There is also provided an output port 97 to facilitate transference of data to any intelli¬ gent external electronic device. Operation of the voltammetric cell will now be described. A sample of solution for analysis is introduced into the receptacle 13 and a potential which is negative with respect to the reference electrode, is applied to the working electrode so that the working electrode functions in a cathode mode. The solution is circulated in the manner hereinbefore described by the stirrer. The cathode mode of the working electrode causes the electro-active species of interest in the sample solution to be deposited onto the working area of the working electrode. After σathodic deposition on the working area of the working electrode, the working electrode is operated in an anodic mode and the positive voltage applied thereto is progress¬ ively raised to strip the deposited material from the working electrode and return it back into the solution. The electrolytic current is monitored as the deposited material is stripped from the electrode. As each elec¬ tro-active species is stripped from the working electrode, a change in the current results at the respective charac¬ teristic potential for that species. The current-poten¬ tial peaks obtained in the stripping step provide a direct measure of the concentration of the electro-active species of interest in the sample solution. During this stripping phase, the current is sampled at intervals and amplified by the current amplifier, converted via the analo¬ gue-to-digital converter, and, stored digitally for peak analysis. Automatic zeroing and ranging occurs during current sampling to ensure a wide sensitivity range with¬ out the need for a range switch. With the device, various potential ranges of the recorded current-potential curve can be assigned labels according to the chemcial elements or species which may be elec¬ tro-active in that range. The largest peak within the range is then output with the chemical symbol or label assigned during programming:.. The analysis results are displayed on the display device 93 which presents the position and the peak height of each peak' detected during the anodic mode operation of the working electrode. A number of such peaks may be retained in memory and dis¬ played for any run. The printer 95 provides a permanent copy of the results of all samples processed and the output port 97 can be used to transfer data to any intel¬ ligent external electronic device. The voltammetric measuring device has the facility for processing standard samples whereby the results of the standard run can be used for calibration purposes. A calibration factor can be applied to subsequent samples, so that the analytical results can be displayed directly in parts per billion. After analysis of the sample solution, the drain 35 is opened to enable the solution to be discharged from the chamber 17. A small portion of the solution is retained in the well 38 so as to cover the working area of the working electrode thereby to prevent it from drying out. During this process, a controlling voltage is maintained on the working electrode to remove any unstripped material and to prevent any replating. A further advantage of maintaining voltage control occurs when analysis is performed using a medium exchange process (that is, when a different solution is used for the stripping process to that used for the plating process). With the present arrangement, continuous voltage control can be maintained in swapping from one medium to another. In the first embodiment, solution undergoing analysis is circulated within the receptacle by means of the stirrer. In the second embodiment, the solution- undergoing analysis is circulated by providing, a continuous flow of solution through the voltammetric cell * . Referring now to Figs. 7 and 8, the voltammetric cell 101 according to the second embodiment is provided in a vol¬ tammetric measuring device 103. The voltammetric cell includes a receptacle 105 in form of a hollow body which defines a chamber 109 to receive a sample solution for analysis. The body includes a lower part 111 having an open top and an upper part 113 which forms a closure for the open top of the lower part. The chamber 109 has a cylindrical side wall 115 and a floor 117. The floor 97 has a recess therein to define a well 118. The voltammetric cell further includes a working electrode 119, a counter electrode 121 and a reference electrode 123. The working electrode 119 has a working area 124 which is located at, and defines part of, the floor of the chamber; more particularly, the working area defines the bottom of the well 118. The voltammetric cell has a solution inlet means for introducing solution into the chamber, which inlet means includes an inlet port 125 formed in the upper part 113 of the body. The inlet port 125 is located directly above the working area of- the working electrode 119. Circulation means are provided for circulating the sample solution through the chamber via the inlet port 125 and an outlet port 127 also forms in the upper part of the body. The circulation means includes a sample holder 129 mounted in the casing 131 of the voltammetric measuring device. The sample holder 129 has formed in its floor an outlet 133 which is connected to the inlet port 125 of the vol¬ tammetric cell via a delivery line 135. Incorporated in the delivery line 135 is a circulating pump 137. The sample holder 129 also has an inlet 139 for return flow of the sample solution from the voltammetric cell. The inlet 139 is connected to the outlet port 127 of the voltamme¬ tric cell by return line 141, The voltammetric cell also has a solution outlet means 143 is in the form of a drain 145 having a drain entry port 146 formed in the floor of the chamber. A valve 147 is associated with the drain for opening and closing the drain entry port. The valve is in the form of a poppet valve having a valve stem 149 and a valve head 151 which is movable into and out of sealing engagement with a valve seat 153 formed in the floor of the chamber. A reservoir 155 communicates with the drain 145 via a discharge line 157. On opening of the valve, solution in the chamber drains into the reservoir. Operation of the valve 147 is effected by means of a push rod and lever assembly 159. The counter and reference electrodes 121 and 123 respect¬ ively are positioned in close proximity to the working electrode so as to contact the residual solution retained in the well. More particularly, the counter and reference electrodes are located in the side wall of the chamber. Because the working area of the working electrode is located within the well, it is at a level slightly below the drain entry port formed in the floor of the chamber. Consequently, the well will retain a small portion of the solution in the chamber when the drain is opened. The residual solution retained in the well covers the working area, of the electrode and thereby prevents it from being contacted with air. A well to retain solution may be formed in the working electrode itself, rather than in the floor of the chamber. An example of this is illustrated in Fig. 9 of the draw¬ ings in which there is shown a working electrode 161 having a working area 163 having a surround 164 which is dished or otherwise recessed to define a well 165 to retain solution.

Claims: THE CLAIMS defining the invention are- as follows:- 1. A voltammetric cell comprising: a chamber for con¬ taining a solution to be analysed; a working electrode, a counter electrode and a reference electrode each having an electro-active surface disposed in the chamber for con¬ tacting the solution; inlet means for introducing solution into the chamber; and outlet means for removing solution from the chamber, the outlet means having an outlet port opening into the chamber; characterised in that the cham¬ ber has a well for retaining a portion of the solution when solution is removed from the chamber, the elec¬ tro-active surface of the working electrode being disposed in the well so as to be covered by the retained solution, the electro-active surfaces of the counter and reference electrodes being so positioned as to contact the retained solution. 2. A voltammetric' cell according to Claim 1 wherein the chamber has a floor, the well being constituted by a recess formed in the floor. 3. A voltammetric cell according to claim 1 wherein the chamber has a floor, the outlet port being located sligh¬ tly above the floor thereby to define the well in the chamber. 4. A voltammetric cell according to claim 1 wherein the electro-active surface of the working electrode has a surround with a depression formed therein to define the well. 5. A voltammetric cell according to claim 1 wherein the electro-active surface of the working electrode has a depression formed therein to define the well. 6. A voltammetric cell according to any one of the preceeding claims wherein the outlet means comprises a drain having a drain entry port (being said outlet port) opening into the chamber, and valve means for opening and closing the drain entry port to the interior of the cham¬ ber.- 7. A voltammetric cell according to claim 6 wherein the drain comprises a drain passage and a sealing ring, the drain passage having an entry and opening into the chamber at a location immediately adjacent the floor thereof, and the sealing ring surrounding the entry end of the drain passage whereby to define the drain entry port, the seal¬ ing ring contacting the floor of the chamber. 8. A voltammetric cell according to claim 6 or 7 wherein the valve comprises a poppet valve having a valve head sealingly engagable with a valve seat, the valve seat being defined by the sealing ring. 9. A voltammetric cell according to any one of the preceeding claims further comprising a stirring means for circulating solution in the chamber, the stirring means comprising a rotatable impeller having a liquid flow path formed therein, the flow path having an inlet end, an outlet end and an outer section extending transversely of the rotational axis of the impeller and opening onto the exterior of the impeller at said outlet end, the inlet end of the flow path opening onto the exterior of the impeller at a plane transverse to the rotational axis of the impel¬ ler. 10. A voltammetric cell according to claim 9 wherein the outer section extends radially with respect to the rota¬ tional axis of the impeller. 11. A voltammetric cell according to claim 9 or 10 where¬ in the flow path includes an axial section extending axially with respect to the rotational axis of the impel¬ ler and opening onto the exterior of the impeller at said inlet end. 12. A voltammetric cell according to claims 9, 10 or 11 wherein the flow path includes at least one further outer section opening onto the exterior of the impeller. 13. A voltammetric cell according to claims 9, 10, 11 or 12 wherein the impeller is disposed above the floor with the inlet end of the impeller facing the floor. 14. A voltammetric cell according to claim 13 wherein the chamber has a side wall which is inclined inwardly towards the floor, the inlet end of the impeller being disposed directly above the electro-active area of the working electrode. 15. A voltammetric cell substantially as herein described with reference to the accompanying drawings. 16. A voltammetric measuring device having a voltammetric cell according to any one of the preceding claims.

Inventors: MANN ALAN WILLIAM, THRELFALL HARRY JAMES, MANN, ALAN, WILLIAM, THRELFALL, HARRY, JAMES
Applicants: COMMW SCIENT IND RES ORG, COMMONWEALTH SCIENTIFIC AND INDUSTRIAL RESEARCH OR
Application year: 1986
Main classification: G01N27
IPC codes: G01N27