Anal. Chem. 1983, 55, 2071-2075
2071
Ion-Selective Electrode with Microprocessor-Based Instrumentation for On-Line Monitoring of Copper in Plant EIect rolyte A, M. Bond,* N.A. Hudson, P. A. van den Bosch, a n d F. L. Walter Division of Chemical and Physical Sciences, Deakin University, Waurn Ponds, Victoria 3217, Australia H. R. A. Exelby Copper Refineries Pty. htd., P.O. Box 5484, M.S.O., Townsville, Queensland 4810, Australia
The contlnuous on-llne monltorlng of Cu( 11) In a purificatlon plant electrolyte can be performed wlth a Cu( II) ion selective electrode. Ascorblc acld Is added to the electrolyte In order to remove interferences from Fe( 111) and other Impurities In the sulfurlc acld matrix. The microcomputer-based system descrlbed in thls paper can be used to monltor concentratons M Cu(I1). By use of two over the range 0.5 M to standards, callbration can be performed as well as detectlon of any malfunction of the electrodes. I n addltlon, one of the standards can be set at the threshold value below which the Cu( 11) concentrationsshould not drop lf potentially hazardous plant operating condltlons are to be avoided. Validation of the approach has been made on plant liquors. Data are In excellent agreement wlth determinatlons based on off-line atomlc absorption spectrometry.
(4,5). However, the Cu ISE whether based on the Cu(1) or Cu(1I) sulfide crystals (6, 7) suffers from a range of interferences particularly with oxidants such as ferric iron Fe(II1) (3-10). Unfortunately, Fe(II1) is present in the copper refineiy plant electrolyte; thus a simple and direct method of determination as an in-line technique is not possible (3). Some studies (4,6,8) indicated that chemical methods may be used to remove the Fe(II1) interference. If this can be achieved, the possibility of on-line monitoring exists ( 3 ) . In this work, we have developed a continuous on-line system for determining copper in concentrated H2S04,which is the plant electrolyte in the copper refinery. The method is based on a cupric sulfide ISE, avoids interference by Fe(III), anid uses microprocessor-based instrumentation to measure anid control the stream process parameters. A new design of a high volume flow cell has been developed for this purpose.
The continuous monitoring of copper in an electrolyte at a copper refinery is extremely important. Inherently, the copper levels are required for control in the process but, in addition, during purification of the sulfuric acid rich electrolyte, arsine, AsH3, may be generated if the copper concentrations are riot maintained above a set value. Because soluble impurities such tis nickel and arsenic accumulate in the electrolyte, semicontinuous removal of liquor from the main electrolyte circuit with subsequent purification in a separate treatment plant is required. In a common purification method (1), copper is initially lowered in concentration by electrowinning of copper. When the copper levels are sufficiently low, arsenic is codeposited ilt the cathode and thereby removed. However, if the copper concentrations become too low, hydrogen is generated at the cathode and reacts with the arsenic to produce the gas, arsine, which i s poisonous. T o prevent the production of hydrogen, and therefore arsine, the copper levels must not drop below a critical concentration. Also, if the purified solution is 1x1have nickel removed at a later stage, copper in solution will reduce the working life OF the steel vessels used. Thus a working range for overflow copper concentrations must be established within these two constraints. Typically, the Cu(I1) must be kept above 0.1 g of Cu(I1) L-l. This is the monitoring problem of interest in the present work. Voltammetric methods should be very suitable (2) but are difficult to install in an on-line mode in the plant, particularly when it is necessary to obtain interference-free data. The same difficulty is true of atomic absorption spectrometry (AAS). Spectrophotometric met hods would be suitable if no interfering element, for example, nickel, overlapped with the copper spectrum (3). The copper ion selectlive electrode (ISE) is extremely attractive for automation. Furthermore, it is very inexpensive and responds to a very wide range of copper concentrations
EXPERIMENTAL SECTION Chemicals and Samples. All chemicals used were of analytical reagent grade. Samples of plant electrolyte over a wide range of copper concentrations, for example, 0.01 g of Cu(II) L-l to 33 g of Cu(I1) L-l, were supplied by Copper Refineries Pty Ltd., Townsville, Queenslamd, for use in the pilot study. Instrumentation. a. General Systems. An Orion copper(I1) ion selective electrode, Model 94-29, was used in conjunction wilth a Ag/AgCl (saturated KC1) Orion double junction reference electrode, Model 90-02. The outer chamber of the reference electrode contained 2 M H2S04. As part of routine weekly maintenance the electrode surface of the Cu ISE was polished as recommended by the manufacturer and, if required, the electrolytes of the reference electrode were replaced. Unless otherwise stated, all measurements were made at an ambient temperature of 18 & 1 "C. Determinations by atomic absorption spectrometry (AAS)wiith a Varian Techtron AA6 instrument were performed for crosschecking the copper levels in all solutions. b. The Copper Monitor. (i) The Flow Cell. Figure 1 illustrates the configuration of a typical measuring cell. The capacity of this cell is 200 mL. The electrolyte, via a single line peristaltic pump, enters the cell above a stirrer at the junction of the delta, A, cell and overflow tube. This arrangement enables rapid and effective rinsing of the complete cell and the probes between samples under the control of a solenoid valve for draining. When the cell is full, turbulent flow conditions are produced by the positive action of the stirrer in conjunction with the cell geometry. The circulating electrolyte flow rate is controlled by a variable rheostat in series with the stirrer motor. This arrangement gives flexibility to the flow conditions; circulating flow rates up to 2500 mL min-' may be used. The sensing and reference electrodes are oriented in the same plane and inclined to the direction of flow. The transverse alignment of the electrodes permits both probes to sense the same solution simultaneously. This arrangement prevents any stream disturbance by the electrodes influencing the determination. The position of the electrodes and the cell shape enable the electrolyte to impinge directly on the ISE sensing surface. The solution is pumped around the cell for a suitable time ta ensure thorough mixing and equilibration
0003-27O0/83/0355-2071$01.50/O0 1983 American Chemical Society
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ANALYTICAL CHEMISTRY, VOL. 55, NO. 13, NOVEMBER 1983
ION -SELECTIVE ELECTRODE AND REFERENCE ELECTRODE
,
7
'-4,5cin-
STANDARD SOLUTION
ELECTROLYTE RETURN
L=p
PLANT ELECTROLY
SOLUTION IX
CONSTANT LEVEL TANK
ASCORBIC ACID
I' PUMP
FLOWCELL
Flgure 1.
$"*
The flow-through cell.
of the electrodes. In the present project changes in Cu(I1) concentrations in the plant occur slowly; therefore long equilibration periods could be used to advantage. We routinely used 15 min with plant electrolyte. The potential is measured (e.g., 20 times at 1-s intervals) as the last operation in the cycle and the average of these readings is taken as the accepted value. The exact times for all operations can be field programmed by the operator using the microcomputer (see below). (ii) The Monitoring System. Figure 2 is a schematic flow diagram which incorporates a control diagram for the Cu(I1) monitoring system. A metering pump discharges cooled plant electrolyte from a constant level tank into a mixing chamber. Simultaneously a measured amount of ascorbic acid from an airtight reservoir is pumped to the mixing chamber in which further cooling occurs. The chemically treated plant electrolyte is pumped to the flow cell by a peristaltic pump via the solenoid valve V1. The treated solution rinses the cell of the previous liquor. After drainage the A cell is filled with more treated plant electrolyte of known and measured temperature, T, and then circulated by the stirrer. Following the ISE measurement, the solution is dumped via the solenoid-controlled valve V2. These valves are Asco US 826090. The time control of the valves is based on using opto-coupled triac devices which are connected via control lines on the user PIA (peripheral interface adaptor) of the microcomputer. Optically coupled devices were chosen specifically to provide satisfactory isolation of the microcomputer from mains voltage. The microcomputer handles the control of the analytical procedures, data acquisition and data reporting. It collects, treats, and stores data, monitors the copper concentrations with respect to a predetermined lower limit, monitors temperature, can download the data to a plant computer, and verifies the electrode reading with respect to a Nernstian response. Two standards 2 decades apart in concentration are used in the calibration; thus the microcomputer corrects for any drift in potential. One measuring run consists of the following cycle, assuming the A cell is empty, all valves are closed, and the stirrer and the peristaltic pump are off: open inlet valve, V1, actuate peristaltic pump, actuate stirrer, open outlet valve V2 for rinse, close V2, fill flow cell, stop pump, close V1, equilibrate the system by circulating the liquor, measure the potential, average the readings, store the result, compare the result with the critical or threshold potential. If the value of the potential is greater than the critical value, open V2-dump cell contents. The system proceeds with another sample measurement. If the value of the potential is less than
+WASTE
Flgure 2.
Schematic flow diagram showing microcomputer control.
the critical value, open V2-dump the cell contents, raise an alarm. The microcomputer part of the monitoring system is based on a Motorola MEK 6800 D2 microcomputer kit. The program runs in one of two modes-data aquisition or equipment control with data acquisition. The programs are written at assembler level by using a cross-assembler on a main frame computer (DEC 20/60) and down-line dumped to tape either for EPROM programming or debugging. The microcomputer may be interfaced to industrial control equipment via a 4-20 mA current loop. Details of the hardware and software are available from the authors on request. At the industrial plant the electronic components of the monitoring system and the cell are housed in a stainless steel box (Faraday cage) and the microcomputer is isolated from the main voltage by using an isolation transformer (3).
RESULTS AND DISCUSSION 1. Elimination of Fe(II1) Interference. Before an online analytical system could be developed, the Fe(II1) interference problem had to be solved. Possible treatment methods include reduction of the Fe(II1) to the Fe(I1) state which does not interfere, or complexation of Fe(II1) to an unreactive form with respect to the copper sulfide crystal. The highly acidic sulfuric acid medium applicable to the plant electrolyte prohibits any approaches which require substantial increases in pH. In addition, heat problems and the precipitation of hydroxides caused by addition of a base are difficult to handle in the plant liquor for an on-line mode. Methods investigated, but found to be inadequate were (i) increasing the pH, (ii) fluoride complexation, (iii) citrate buffering, (iv) formaldehyde addition, and (v) sulfite addition. T o simplify the operation in an on-line mode, a reagent which would work ,in concentrated H2S04and could be added directly to the plant electrolyte was required. Furthermore, the method had to be based on direct calibration rather than standard additions. Smith and Manahan (8) have used successfully a complexing antioxidant buffer. Unfortunately, this buffer operated a t pH 5, used sodium fluoride and formaldehyde in
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ANALYTICAL CHEMISTRY, VOL. 55, NO. 13, NOVEMBER 1983
Table I. The Inflluence of the Addition of Fe(II1) on the Potential of the Cu(I1) ISE concentration of Cu(1I) 10-1 (AE,,
exptno.
10-2
2073
ACID ADDED
230
(AE,, 10-3
potential M mV)C M mV)C M
1. no Fe(II1) E , (mV) 229 199 170 no AsAca (30) (29) 225 2. with Fe(III)b' E , (mV) 239 227 (2) no AsAca (12) 171 200 3. with Fe(II1)" E , (mV) 231
and then (29) with AsAca (31) 4. equilibration 0.5 3 time for expt 3 (min) a AsAc refers to ascorbic acid. 0.028 g of Fe(II1) in 50 mL of solution. AE, refers to the changes in potential for a decade change in Cu(I1) concentration. Table 11. The Influence of the Addition of Ascorbic Acid and Fe(II1) on the Potential of the Cu(I1) ISEa concentration of Cu(I1) lo-'
expt no.
potential
1. no AsAc
M
10-3
(AE,) M
E , (mt') 229 no Fe(II1) (29) 2. with AsAc E , (mV) 232 no Fe(II1) (31) 2'. as for expt E,' (mV) 233 no. 2 but (31) 24 h later 3. with AsAc E , (mV) 232 and with Fe(II1) (31) a Symbols in this table are as defined text.
(AE,)
200 201 202
M 171
(29) (31)
I70 171
(31) 201
171
(30) in Table I and the
-.
addition to an acetate/acetic acid system. This system is unacceptable for use in concentrated H2S04. The same authors attempted the use of ascorbic acid as an antioxidant, but this was reported to reduce even low levels of Cu(11). In the different medium of concentrated sulfuric acid it was believed that ascorbic acid may specifically reduce Fe(II1) and not Cu(I1). Therefore, we examined the possible use of this reagent, despite the negative result reported in the literature. In distilled water, we confirmed that ascorbic acid reduces Cu(I1) to Cu(1) and metallic copper. However, in 2 M H2SC14 (approximately the concentration used in the refinery) no reaction occurred with Cu(I1) over a period of 18 h as evidenced by the constancy of the potential of the ISE. These experiments in H2S04weire performed w i t h lO-l, and lo* M Cu(1I) solutions. Rubchinskaya et al. (11) have shown that ascorbic acid is stable in 2-3 M IIzS04.]However,it does react with oxygen; this had to be taken into account in the design of the on-line system. The results of experiments which were performed to investigate the effectiveness in removing Fe(II1) interference are summarized in Tables I and 11. The copper concentrations examined cover the range of interest in the practical problem associated with this project. Table I shows the effect on the potential of the Cu(I1) ISE caused by the addition of Fe(II1) followed by ascorbic acid to solutions of Cu(I1). El refers to the potential if both Fe(II1) and ascorbic acid are absent, E, is when Fe(II1) only is added, and E3 is when aqcorbic acid is added to the solution in which Fe(II1) is present. AE, (where n = 1 , 2 , or 3) are changes in potential for a decade change in Cu(I1) concentration. The
I: 180
. 10
\ 20
-
30 40 TIME(rnins.)
50
60
Figure 3. Effect of adding ascorbic acid to refinery plant electrolyte. copper solutions were prepared in 2 M H2S04and the potential El was measured. To 50 mL of each of the respective copper solutions 0.5 mL of 1 M Fe(II1) was added and E2 measured. To this same 50 mL solution 0.5 g of ascorbic acid was added and E3 measured. Data in Table I show that thie presence of Fe(II1) shifts the potentials to more positive values and decreases the slope of plots of potential against log concentration. The ratio of Cu(I1) to Fe(II1) is important. In the limiting case of low copper and relatively high Fe(II[) concentration, the electrode potential using the Cu(I1) ISE becomes almost independent of copper. Table I1 shows that the addition of ascorbic acid makes veiy little difference to the electrode potential. Furthermore, if Fe(II1) is added subsequently to the ascorbic acid, no interference is noted provided the ascorbic acid concentration is high enough. El refers to the potential in a 2 M H2S04matrix if both ascorbic acid and Fe(II1) are absent, Ezis when 0.5 g of ascorbic acid per 50 mL is added, and E; is the potential of the same solution 24 h later. E3 is the potential of the solution on adding 0.5 mL of 1 M Fe(II1) solution to the solution containing ascorbic acid. AE, (where n = 1,2, or 3) values are changes in potential for a decade change in Cu(I1) concentration. The values of E, and AE, in Tables I and 11 imply that the Nernst equation is obeyed even in the presence of Fe(II1) if ascorbic acid is present. Figure 3 shows the effect of adding ascorbic acid to refinery electrolyte. The potential reaches a very stable value which is comparable with the expected value for the concentratioln determined by atomic absorption spectrometry. Solutions containing ascorbic acid give an extremely stable potential over long periods of time (see Table I1 and Figuire 3). Calibration curves of potential against log [Cu(II)] aire linear with the Nernst slope over the concentration range to 10-1 mol of Cu(I1) L-l. No adjustment for ionic strength is necessary because the con,centrated HzS04maintains the ionic strength even after addition of the copper. The response time is excellent over this concentration range. Much lower copper concentrations could be determined but are not required for the application described in this study. Since strong oxidants such as Fe(III), Ce(1V) and Mn04attack the electrode and damage the surface (12-14), it is preferable always that ascorbic acid be present at the electrode surface. The data ih Figure 3 are consistent with the suggestion that substantial amounta of oxidant are present in the plant electrolyte. Never should the electrode be in contact with untreated plant electrolyte. This fact was taken into account in the flow-through cell design. In detailed studies of ISE surface morphology, Pungor et al. (12) and Ebel et til. (13, 14) showed that the electrode response is drastically influenced by MnO; and Ce(1V). Also, they showed that the
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ANALYTICAL CHEMISTRY, VOL. 55, NO. 13, NOVEMBER 1983
electrode can be restored to Nernstian behavior by ascorbic acid after an hour or so of treatment. We find the same to be true with Fe(II1). Thus, ascorbic acid has the dual benefit of eliminating Fe(II1) interference and restoring the damaged (oxidized) CuS surface of the electrode. Pungor et al. (12-14) show by X-ray photoelectron spectroscopy that chemical oxidants damage the copper sulfide/ silver sulfide electrode surface through the formation of copper sulfate monolayers. The addition of ascorbic acid in a 0.5 M sulfuric acid medium reduces the sulfate surface to copper sulfide. In alkaline media, Gulens et al. (15,16) have shown that in the presence of ascorbic acid, metallic silver is deposited from a Ag/Ag2S electrode. Reactions of this kind could be expected to cause difficulty with a copper sulfide/ silver sulfide electrode as used in this work. In weakly acidic media, for example pH 4-5, erratic behavior of the potential was noted after the addition of ascorbic acid to copper solutions. Highly acidic media clearly decrease the reducing power of ascorbic acid. For example, as noted above, in 2 M H2S04 ascorbic acid does not reduce Cu(I1) to Cu(1) or Cu(0) as it can in higher pH solutions of copper. In the highly acidic media no problem with deleterious attack on the electrode surface occurred. The speed with which ascorbic acid restores the functioning of the copper electrode is in marked contrast to the report by Matsuda et al. (17)in which several weeks were necessary to restore the ideal response if an electrode was immersed in dilute C U ( N O ~solution )~ following prolonged desiccation. 2. On-Line Determination of Cu(I1). (i) Requirements for On-Line Cu(I1) Determination. Cell design, related electronics, and operational procedures described in the Experimental Section were designed to meet the following requirements: (a) The flow-through cell, Figure 1, must be able to operate under high volume flow rates. Previous work (3) has shown that with turbulent flow greater long term stability is achieved and the electrode surface kept clean. In the presence of ascorbic acid a stable response is found for lower flow rates than is required without ascorbic acid (3). (b) The cell design must permit rapid drainage and flushing facilities. The electrodes must remain in solution for the maximum possible time. At no stage must the electrodes become dry or come in contact with untreated plant electrolyte. (c) Mixing of the acidified ascorbic acid solution with plant electrolyte must occur externally before entering the cell (see discussion earlier). (d) The acidified ascorbic acid solution must remain “air free”. This solution is degassed with nitrogen before being placed in an airtight “bag in a box”. As the solution is used the “bag” collapses thus preventing any air inflow to the solution. (ii) Standardization-Calibration Procedure. Standarization of the cell should occur after a preselected number (e.g., six) of determinations on the plant electrolyte. All subsequent sample results are referred to the last standardization. Two standard solutions 2 decades apart in concenand 10-1 M Cu(II), should be used tration, for example, in the calibration. Ideally, the lower value should correspond to the threshold value, below which the microcomputer actuates the plant warning system or initiates a control valve to rectify the problem. The reference solutions should be made up to mimic the plant electrolyte, a similar matrix will remove any inherent offset which could result if less realistic standards were prepared. In the present case, the standards would be comparable to the plant liquor with respect to nickel, arsenic, and acid, say, 12 g of Ni L-l and 5 g of As L-’ in 2-3 M HzS04.
186.
ACID ADDED
/ASCORBIC
I\
,/ /,
17*i 174 1701
q ,
,
,
,
2
6
10
14
___
CURVE A
J \
18
22
TIME(rnins.)
26
30
34
38
42
Flgure 4. Influence of ascorbic acid concentration on the potential of the Cu(I1) ISE in the presence of plant electrolyte: (curve A) at t = 0, 12, and 24 min, 2, 1, and 2 g of ascorbic acid per 50 mL of electrolyte were added, respectively; (curve B)at t = 0, 5 g of ascorbic acld per 50 mL of electrolyte was added.
The monitoring system is assumed to be operating correctly if the potential difference between the two standards is (2 X 2.303RTfnF) mV f error. The error is determined by the operator, typically f 2 mV. The operating and analytical procedure for the system during calibration follows a similar routine to that of the normal on-line sampling of the plant electrolyte, except that by microcomputer control, the standard solutions instead of plant electrolyte are introduced in the flow-through cell stream circuit. During all determinations the temperature of the solution is measured to ensure that standards and plant liquor are acceptably close in temperature, for example f 2 OC. Normal plant operating and control procedures, in which electrolyte analyses using AAS are performed on a routine basis by the control laboratory, can be used as a daily off-line check of the flow-through monitoring system. 3. Recommended Operating Procedure for Monitoring Cu(I1) in Plant Electrolyte. The system for Cu(I1) monitoring has been designed to provide extreme flexibility. Changes in operating parameters are readily implemented by changing the appropriate microcomputer instructions. On the basis of 2 years’ experience with ion-selective electrodes for monitoring Cu(I1) in plant electrolyte, the following recommendations are made: (i) Ascorbic Acid Concentration. In synthetic solutions 10 g of ascorbic acid L-’ is sufficient to remove interference. In most plant electrolytes, this value is adequate, but for those liquors in which very low levels of Cu(I1) occur, this is inadequate a t times. Apart from Fe(III), other species, for example, permanganate, oxygen, etc. may be present in the electrolyte and react with the ascorbic acid. To ensure a high probability of valid analytical data, 100 g of ascorbic acid L-l is recommended. Figure 4 illustrates the stability of the potential reached using ascorbic acid in plant electrolytes containing 0.08 g of Cu(I1) L-l. Curve A illustrates the change in potential by separate additions of ascorbic acid to a 50-mL sample. The two significant discontinuities represent the addition of a further 1g and 2 g, respectively, of ascorbic acid to the electrolyte which initially was dosed with 2 g of the reagent. Curve B shows the effect of initially adding 5 g of ascorbic acid to 50 mL of the same electrolyte, when stability was reached after 10 min. Correct data can be correlated with
ANALYTICAL CHEMISTRY, VOL. 55, NO. 13, NOVEMBER 1983 .~
Table 111. Comparison of the Results for the Determination of Cu(I1) lay AAS and the ISE Methods rnethod of determination AAS AAS (Copper (Deakin Refineries) University) ISE
-~
plant electrolyte sample A B C
D E
g of
g of
g of
Chl(I1) L-l
Cu(I1) L-1
Cu(I1) L"
4.4
4.5 0.34
4.6 0.34
0.10
0.09 0.03 0.02
0.33 0.09 0.02 0.01
0.03 0.01
using an excess OS ascorbic acid. When interference occurs the potential is a function of the ascorbic acid concentration. When the interference is removed, the addition of excess ascorbic acid leadfn to measurement of a constant and stable potential. (ii) Reproducibility. Long term reproducibility is excellent. Data obtained during a 17-h run, during which 102 readings were taken at 10-min intervals, gave a mean potential of 185 mV, with a standard deviation of 1.4. The maximum spread was 4 mV, which would be acceptable under most circumstances. During this period no Cu(I1) concentration changes occurred and no recalibration was made. However, we recommend that calibrations be performed every 4 h. With this procedure, data should be in agreement to better than 10-15% of the off-line AAS method mentioned in the Experimental Section; this corresponds to an uncertainty of 2-4 mV, respectively, in potential readings. In a laboratory situation, uncertainties of less than 1mV are readily obtained with frequent calibration, very careful temperature control, and low noise levels. Under plant conditions in a harsh environment (3), the 1-mV level is not readily achieved. In the absence of turbulent flow, significant potential drift was frequently obinerved within an hour. After several hours non-Nernstian behavior was noted. Occasionally, the deposition of material on thie electrode surface slowed the response time dramatically. These deleterious effects could be removed by polishing the electrode sensor surface as recommended by the manufacturer. However, in our opinion, the use of turbulent flow conditions in conjunction with the electrode configurehion is essential in the colpper refinery plant electrolyte, although this is achieved at the expense of increased complexity of the monitor. (iii) Check on Interfesence. The calibraton procedure of using two standards, 2 decades of concentration apart, in addition to providing the c'alibration, also ensures that a simple check is available to verity that the electrlode is obeying the Nernst equation. If the check fails, the operator should assume there is a fault, such as a malfunctioning reference electrode or surface contaminated ISE, etc. If there is any indication of interference arising from the matrix of the electrolyte, for example, slow electrode response or drifting, which is not observed in the standards, then the ascorbic acid concentration should be increased. We recommend that a clomparison against the off-line AAS method be retained as a routine procedure performed during normal working hours. Our experience shows that the mon-
2075
itoring system can be used maintenance free for longer than a week; however, we prefer the ISE surface and the reference electrode be inspected and cleaned as part of the routine weekly maintenance (see Experimental Section). 4. Comparison of Results for Cu(I1) Determination by Different Techniques. Table I11 gives a comparison of the results obtained for the determination of Cu(I1) in samples of Copper Refineries plant electrolyte by (a) the control laboratory, Copper Refineries Pty. Ltd., using atomic absorption spectrometry, (b) the chemistry laboratory at Deakin Univ,. ersity, using atomic absorption spectrometry, and (c) the ion-selective electrode technique described in this paper, that is, using the equivalent of 100 g of ascorbic acid L-l of sample with an equilibration time of 15 min and microcomputer control. The Cu(I1) determinations using ISE show compatibility with both the control laboratory and the chemistry laboratory AAS figures. The significant point is that the integrated ISE: and microcomputer would alarm the operator and initiate an automatic control valve in each of the samples if the critical Cu(I1) concentration was set as 0.1 g of Cu(II) L-I. The simplicity of the ion-selective electrode method in which only potential needs to be monitored offers a distinct advantage compared with other possible automated methodti based on spectrometry. ACKNOWLEDGMENT The technical assistance from the laboratory staffs art Deakin University and Copper Refineries Pty. Ltd., is greatly appreciated. Registry No. Cu, 7440-50-8. LITERATURE CITED Cutmore, J. P. Proo., Australas. Inst. Mln. Metall. (North West Queensland Branch Regional Meeting) 1974, 101-109. Koshchei, A. M.; et 8.1. Zavod. Lab. 1981,4 7 , 10-12. Bond, A. M.; Hudson, H. A.; van den Bosch, P. A.; Walter, F. L.; Exelby, H. R. A. Anal. Chlm. Acta 1982, !36, 51-59. Vesely, J.; Welss. D.; Stulik, K. Analysis with Ion-Selective Electrodes"; Ellis Horwood Limited: Chlchester, 1978; Chapter 4, pp 194-197. Orion Research, Instruction ManuaCCuprlc Electrode Model 94-29, 1981. Hulanicki, A.; Krawcrynski, T.; Trojanowicz, M. Chem. Anal. (Warsaw) 1979,2 4 , 435-441, and literature cited therein. Rice, G. K.; Jasinski, R. J. NBS Spec. Pub/. ( U S . ) 1974,No. 422, 899-915. Smith, M. J.; Manahan, S. E. Anal. Chem. 1973,4 5 , 836-839. Llght, T. S. NBS Spec. Publ. (U.S.) 1969, No. 374, Chapter 10, 349-374. Fung, Y. S.; Fung, K. W. Analyst (London) 1878, 103,149-155. Rubchinskaya, Yu. M.; L'vova, M. Sh.; Kozlov, E. I. Pharm. Chem. J . (Engl. Transl.) 1976, IO, 123-127. Pungor, E.; Toth, K.; et al. Anal. Chlm. Acta 1979, 709,279-290. Ebel, M. F.; Toth, K.; Polos, L.; Pungor, E. SIA , Surf. Interface Anal. lW0, 7 , 197-198. Ebel, M. F.; Groger, W.; Polos, L.; Toth, K.; Pungor, E. HSI, Hung. Sci. Insfrum. 1980,49,41-45. Gulens, J.; Ikeda, B. Anal. Chem. 1978,50, 782-787. Gulens, J.; Shoesmith, D. W. J . Electrochem. SOC. 1981, 728, 811-816. Matsuda, N.; Nakagawa, G.; Ikeda, S.; Ito, K. Denkl Kagaku 1980,419 (3), 199-202.
RECEIVED for review December 28,1982. Accepted June 10, 1983. The authors thank the Copper Refineries Pty. Ltd. for their financial contribution to the project. A provisional patenit has been obtained for the monitoring system described in this paper.
