GB2117120A

GB2117120A - Anodic stripping voltameter

Info

Publication number: GB2117120A
Application number: GB08305516A
Authority: GB
Inventors: John Vincent Dobson
Original assignee: UK Secretary of State for Defence
Current assignee: UK Secretary of State for Defence
Priority date: 1982-03-01
Filing date: 1983-02-28
Publication date: 1983-10-05
Also published as: GB8305516D0; GB2117120B

Abstract

An anodic stripping voltameter comprises a flow-through sample cell 10 including a working electrode 13, a counter electrode 15 and optionally a reference electrode 14. A cyclically changing voltage waveform is applied to the working electrode by a generator 21. A logic circuit 27 with parameters set by a control 26 is arranged to measure the dissolution current in resistor 22 in a specified voltage window (V2-V3) in the dissolution portion of the coating and stripping of ionic species in the sample. The logic circuit 27 controls an integrator 28 to integrate the dissolution current during the voltage window and to reset the integrator at a third voltage (V1) on the voltage waveform. Alternatively, the rate of change of the dissolution of deposited ions with time may be measured. <IMAGE>

Description

SPECIFICATION Improvements in or relating to anodic stripping voltammeters This invention relates to an anodic stripping voltammeter.

Anodic stripping voltammeters are known in the prior art, and comprise a cell having two inert electrodes between which a current is passed through an electrolyte. An ionic (normally metallic) species in the electrolyte is deposited on one electrode, and is subsequently removed or stripped by reversing the cell current. The coulombic charge transferred during total removal of the deposited species is proportional to the original concentration of the ionic species in the sample.

Prior art anodic stripping voltammeters do not however lend themselves to continuous flow sampling as required for on-stream measurement purposes. It is necessary to provide a discrete sample for each measurement.

It is an object of the present invention to provide an anodic stripping voltammeter adapted for measurements on continuously flowing samples.

The present invention provides an anodic stripping voltammeter comprising a sample cell having a working electrode and a counter electrode, means for cycling the working electrode potential repetitively to produce successive deposition/dissolution cycles, and means for detecting the dissolution current.

Repetitively cycling the working electrode potential provides cyclical coating and stripping of the working electrode from a sample flowing in the cell, so that the electrode surface is repeatedly refreshed. Detection of the dissolution current during the stripping portion of the cycle then provides a measure of the relevant ionic species concentration in the sample of interest. In view of its repetitive measurement, the invention is responsive to fluctuations in ionic concentration, unlike prior art voltammeters, and is accordingly suited to onstream analysis.

Conveniently, the cell includes a reference electrode with respect to which the working electrode potential is cycled. The electrodes may be arranged within a flow-through sample cell.

The working electrode potential is preferably cycled by means arranged to effect at least incipient hydrogen evolution during a negative half-cycle and at least incipient oxygen evolution during a positive half-cycle.

In a preferred embodiment, the dissolution current detecting means is responsive in a voltage interval of the stripping portion of the cycle and is arranged to integrate the dissolution current to provide a measure of coulombic charge transfer during that interval. The measured charge is repeatedly refreshed on successive deposition/dissolution cycles to provide continuous monitoring of ionic species having dissolution potentials with the said voltage interval.

The detecting means is preferably arranged to initiate dissolution current integration at a first working electrode potential and to terminate integration at a second such potential. Means may be provided for storing the integrated current and for indicating its magnitude. Conveniently, the detecting means includes zeroing means operative at a third working electrode potential to reduce t9 zero the stored integrated current value obtained on each cycle prior to iniating current integration on the respective successive cycle.

The detecting means may alternatively be arranged for current measurement or for current differential measurement.

The detecting means may conveniently include calibration and zeroing circuits whereby the anodic stripping voltammeter may be calibrated with reference to a first sample having a known concentration of dissolvable ionic species and zeroed against a second sample having a zero concentration of such species.

In order that the invention might be more fully understood, one embodiment thereof will now be described with reference to the accompanying drawings, in which: Figure 1 is a schematic drawing of a sample cell, Figure 2 is a functional block diagram of a dissolution current detection circuit Figure 3 is a graph of cell current against working electrode potential for a deposition/dissolution cycle, Figure 4 is a diagram of a current detection buffer circuit, Figure 5 is a diagram of a voltage detection buffer circuit, Figure 6 is a diagram of a logic circuit for controlling current measurement, Figure 7 is a circuit diagram of a measurement zero control, Figure 8 is a diagram of a current integrator circuit, and Figure 9 is a diagram of a calibration circuit.

Referring to Figures 1 and 2 a flow-through sample cell indicated generally by 10 consists of a perspex (R.T.M) block 11 having a central sample flow channel 12. A working electrode 13, a reference electrode 14 and a counter electrode 1 5 are longitudinally arranged within the channel 12. A sample flow indicated by arrows 16 enters an inlet region 17 of the channel 12 and moves past the electrodes 13, 14 and 1 5 to an outlet region 18.

The electrodes 13, 14 and 1 5 are connected to a potentiostat 20 which controls the potential of the working electrode 1 3 with respect to the reference electrode 14 in response to the output of a waveform generator 21. A current measuring resistor 22 in series with the counter electrode 1 5 is connected across the input of a current sensing or "Y" input buffer circuit 24. The working electrode/reference electrode potential difference VWR is fed to a voltage sensing or "X" input buffer circuit 25. The "Y" and "X" buffer circuits 24 and 25 and a sample window set/reset level circuit 26 are connected to a logic circuit 27 controlling the operation of an integrator 28. The logic circuit 27 controls integrator sampling, resetting and output via leads 29,30 and 31 respectively.The output of the "Y" input buffer 24 is fed to a zeroing circuit 32 and thence to the integrator 28. The output of the integrator 28 is connected to a calibrator 33 having an output 34.

The operation of the circuit of Figure 2 will now be described with reference to Figure 3, in which the cell current Ic flowing between the working and counter electrodes 1 3 and 1 5 is plotted as a function of the potential difference VWR between the working and reference electrodes 1 3 and 14.

Three negative voltages V1, V2 and V3 are set on the circuit 26, V, being the voltage at which the integrator 28 is reset or zeroed, and V2 and V3 the "sample window" voltages at which initiation and termination of cell current integration occurs. The waveform generator 21 supplies a sawtooth waveform having a period of 5 to 10 sec and a peak-to-peak amplitude of 3.0 volts. The waveform controls the potentiostat 20 to produce alternate forward and reverse current half-cycles in the cell 10. The "Y" input buffer 24 detects the cell current in the resistor 22, and produces an output referred to the circuit earth proportional to current. The "X" input buffer 25 detects the potential difference between the working and reference electrodes, V,,, and converts it to a voltage output also referred to circuit earth.

The logic circuit 27 is arranged to initiate the action of the current integrator 28 via line 29 when VWR is equal to V2 and is positive-going (i.e.

1c is positive), and to terminate integration when VWR subsequently reaches V3. The integrator 28 is reset or zeroed via line 30 by the logic circuit 27 when VWR reaches V1 positive-going. Beginning at the point 41 in the 1C,,VwR plot of Figure 3, the integrator 28 is storing the integrated cell current value obtained on the previous cycle. VWR then passes through points 42 and 43 during which no logic levels are detected by the circuit 27 until the point 44 is reached, at which lc changes sign.

When the change of sign occurs, the logic circuit 27 supplies a central pulse via line 31 effecting transfer of the stored integrated current value through the calibrator 33 to the output 34. The logic circuit 27 is arranged such that no sample or reset response to the preset voltage levels V1, V2 and V3 is made while Ic is negative in the regions 45 and 46. After Ic becomes positive at the point 47 and VWR has reached V, at 48, the logic circuit 27 supplies a reset pulse via line 30 to zero the integrator 28 and remove the stored current value. The logic circuit subsequently enables integration to initiate when VWR is equal to V2 at 49 with Ic positive, this being achieved via the sample line 29.Integration is terminated at 50 when VWR reaches V3, and the integrated current is stored for subsequent calibration and output when lc next swings negative at 44.

The chain line 51 in Figure 3 illustrates generally the effect on lc of metal ions in the sample cell 10. Metal ions are deposited on the working electrodes 13 at 52 with lc and VWR negative. As indicated by the lc peak between 49 and 50, metal is subsequently dissolved when VWR is within the current sampling window or region V2 to V3. To calibrate the instrument, the sample cell 10 is supplied with a flow of liquid having as low a concentration as possible of metal or other ions depositable during the cell cycle. The zeroing circuit 32 is operated to provide zero output of the integrator 28 while this liquid is flowing in the cell 10.The cell is then supplied with a solution having a known concentration of metal ions, and the calibrator circuit 33 is adjusted to give an output level at 34 appropriate for a chart recorder or other recording device. The instrument is then ready for operation on liquids having unknown metal ion concentrations.

Individual circuits illustrated schematically in Figure 2 are shown in detail in Figures 4 to 9.

Referring to Figure 4, the "Y" input buffer or cell current detection circuit 24 comprises two current amplifiers 60 and 61 each connected to a respective end of the current measuring resistor 22. The outputs of the amplifiers 60 and 61 are subtracted by a differential amplifier 62 driving a further amplifier 63 arranged for voltage offset adjustment via a resistor chain 64. The output 65 of amplifier 63 is proportional to cell current lc.

The "X" input buffer or VWR detection circuit 25 is shown in Figure 5. The working electrode is connected at 70 to earth and the reference electrode at 71 to the non-inverting input of a current amplifier 72. The amplifier 72 operates with unit voltage gain to transfer VWR to the noninverting input of a comparator amplifier 73. A high-frequency triangular wave-form generator 74 supplies the inverting input of the comparator 73. The comparator output is a series of pulses having a mark/space ratio dependent on the amplitude of VWR. The output of the comparator 73 passes via an opto-isolator 75 to an integrator 76.The opto-isolator 75 is included to avoid earth-loop difficulties, and the output of the integrator 76 is referred to circuit earth and is directly proportional to VWR despite possible differences between earth potentials either side of the isolator 75. The output of the integrator 76 isffed to an amplifier 77 having offset adjustment provided by a resistor chain 78, and thence via an output 79 to the logic circuit 27.

Referring now to Figure 6, there is shown the logic circuit 27 and (enclosed in chain lines) the "sample window" set/reset level circuit 26. The voltages V2 and V3 defining the current integration interval or window are set on potentiometers 80 and 81 with polarity adjusted by a switch 82 appropriate to the ionic species of interest. The integrator reset level V, and is polarity are set by a resistor chain/switch network indicated generally by 83.

The cell current or lc signal from the "Y" input buffer 24 is compared with earth potential by a comparator 84, to provide an lc polarity signal.

The integrator reset and "sample window" voltages V1, V2 and V3 are each fed to a respective comparator 842,843 and 844 and compared with the VWR signal from the "X" input buffer to indicate when VWR passes each voltage level. The output of each of the comparators 84, to 844 changes state when lc changes sign or VWR changes from less than to greater than V1, V2 or V3 as appropriate. The output of each of the comparators 84, to 844 is fed to a respective switching transistor 85t to 854 each connected to an array of logic circuits indicated generally by 86. In addition, the output of comparator 84, indicating ic polarity is connected via transistor 851 to an integrator output enable terminal 87.The logic array 86 is connected to output terminals 88 and 89 for integrator reset and current sample logic signals respectively. The arrangement of the logic array 86 is such that the current sample logic output 89 is in a logical "on" state only when VWR lies between V2 and V3 and lc is positive, otherwise an "off" state is present. The integrator reset output terminal 88 provides a logic pulse when lc is positive and VWR is equal to V1, and the integrator enable output 87 is in a logical "on" state when lc is negative due to polarity inversion at transistor 85,.

The zero control circuit 32 is shown in Figure 7, in which the output terminal 65 (see Figure 4) of the "Y" input buffer circuit 24 is connected to the non-inverting input of a comparator amplifier 90 having unit gain. The inverting input of the comparator 90 is connected to the slider 91 of a potentiometric resistor chain 92. The output of the comparator 90 is the Ic signal less the voltage on the slider 91, adjustment of the slider providing zero control. The output 93 of the comprator 90 is connected to the integrator 28.

Referring now to Figure 8, the lc signal from output 93 of the zero control circuit 32 is fed to a potential divider network 100 including a solid state switch 101, and thence through a second solid state switch 101 2to an inverting amplifier 102. The output of the amplifier 102 drives an integrator 103 via a third solid state switch 1013.

The integrator 103 has a reset switch 104 and is connected through an isolating switch 105 to a sample and hold amplifier 106 having an output 107. The switches 101 " 1012 and 1012 are connected to the current sample logic terminal 89 (see Figure 6) of the logic circuit 27 via in the case of switch 101, a transistor 108. When the current sample logic terminal 89 is in a logical "on" state, the switches 101, to 1013 are switched on to allow the integrator 103 to integrate the current signal from the zero control output 93. As previously described, terminal 89 is on only while VWR is between V2 and V3, and integration begins and ends at these limits. When Ic changes from positive to negative, the integrator output enable terminal 87 (see Figure 6) changes to a logical "on" state switching on the isolating switch 105.This allows the integrated current signal at the output of the integrator 103 to pass to the sample and hold amplifier 106. The integrator 103 is reset by the switch 104 when the reset logic signal on terminal 88 changes from a logical "off" state to an "on" state.

Referring now also to Figure 9, the output terminal 107 of amplifier 106 is connected to the calibration circuit 33 comprising an amplifier 110 with a variable feedback resistor 111. Varying resistor 111 alters the gain of the amplifier 110 so that the output voltage at terminal 11 2 may be adjusted to a convenient value for a calibration sample having a known ionic concentration. The output terminal 11 2 is connected to a chart recorder (not shown).

Whereas the detection circuit described above has employed an integrating technique for sample current measurement, it is also possible to detect the current directly or to detect the time differential of the current with appropriate circuitry. The advantage of the integration technique is that very low ionic concentrations may be detected, whereas detecting the current or its derivative is less sensitive due to inferior signal to noise characteristcs. However, where low signal to noise performance can be tolerated at comparatively high ionic concentrations, measurement of current or its derivative will be advantageous since different ionic species can be detected as they are stripped from the working electrode at different values of VWR.

An anodic stripping voltammeter of the invention has been found to be very sensitive in detection of metal ions in solution, such as for example lead, zinc and copper. A working electrode of glassy carbon has been found to be suitable, but a platinum working electrode is preferred because of superior resistance to degradation. It is anticipated that the invention will also be appropriate from detection of organic species in solution. It is well-known in the art that electrochemical measurements on organic materials give rise to difficulties associated with electrode contamination. The invention provides a repeatedly refreshed working electrode surface which is less susceptible to contamination. In particular, the invention has been found capable of good detection performance in respect of glucose and urea solutions.

Claims

1. An anodic stripping voltammeter comprising a sample cell having a working electrode and a counter electrode, mcans for cycling the working electrode potential repetitively to produce successive deposition/dissolution cycles, and means for detecting the dissolution current.

2. An anodic stripping voltammeter as claimed in claim 1 wherein the cell includes a reference electrode with respect to which the working electrode potential is cycled.

3. Anodic stripping voltammeter as claimed in claim 1 or 2 wherein the electrodes are arranged within a flow-through sample cell.

4. An anodic stripping voltammeter as claimed in any one preceding claim wherein the working electrode potential is preferably cycled by means arranged to effect at least incipient hydrogen evolution during a negative half-cycle and at least incipient oxgyen evolution during a positive halfcycle.

5. an anodic stripping voltammeter as claimed in any one preceding claim wherein the dissolution current detecting means is responsive in a voltage interval of the stripping portion of the cycle and is arranged to integrate the dissolution current to provide a measure of coulombic charge transfer during that interval.

6. An anodic stripping voltammeter as claimed in claim 5 wherein the measured charge is repeatedly refreshed on successive deposition/dissolution cycles to provide continuous monitoring of ionic species having dissolution potentials within the said voltage interval.

7. An anode stripping voltammeter as claimed in claim 5 or 6 wherein the detecting means is arranged to initiate dissolution current integration at a first working electrode potential and to terminate integration at a second such potential.

8. An anodic stripping voltammeter as claimed in any one of claims 5 to 7 wherein there is provided means for storing the integrated current and for indicating its magnitude.

9. An anodic stripping voltammeter as claimed in any one of claims 5 to 8 wherein the detecting means includes zeroing means operative at a third working electrode potential to reduce to zero the stored integrated current value obtained on each cycle prior to initiating current integration on the respective successive cycle.

1 0. An anodic stripping voltammeter as claimed in any one preceding claim wherein the detecting means includes calibration and zeroing circuits whereby the anodic stripping voltammeter may be calibrated with reference to a first sample having a known concentration of dissolvable ionic species and zeroed against a second sample having a zero concentration of such species.

11. A method for detecting ions within a fluid comprising the following steps: a. placing a working electrode and a counter electrode in a flow of the fluid; b. applying a continuously changing cyclic voltage to the working electrode such that the ions are cyclically deposited then removed from the working electrode; and c. measuring the rate of change of the dissolution of the deposited ions with time.

12. An anodic stripping voltammeter substantially as described with reference to Figures 1 to 9 of the accompanying drawings.