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1706

Anal. Chem. 1982, 54, 1706-1712

Simultaneous Determination of Copper, Nickel, Cobalt, Chromium(VI), and Chromium(I I I) by Liquid Chromatography with Electrochemical Detection A. M. Bond* and G. G. Wallace Division of Chemical and Physical Sciences, Deakln Unlversiv, Waurn Ponds 32 17, Victoria, Australia

Highperformance reversed-phase liquid chromatography with electrochemical detection based on formation, separation, and subsequent oxidation of dithiocarbamate complexes provldes a method for determination of copper, nlckel, cobalt, chromium( I I I),and chromlum(V1). Electrochemical detection at gold, platinum, and glassy carbon electrodes, the use of dlfferent cells, methods of complex formation and detectlon format have been examined to provide optimization of techniques. Limits of detectlon substantially less than 1 ng can be obtained for ail metals. For Simultaneous determination of ail five species, external formation of complexes prior to injection onto the column is essential. For rapid determination of copper and nickel but not cobalt or chromium, dithlocarbamate ligand may be included in the running solvent with In situ rather than external complex formation.

To date, the majority of work combining high-performance liquid chromatography (HPLC) and electrochemical detection (HPLCEC) has been aimed at the development of sensitive and specific analytical methods for determining organic compounds (1). In a recent article (2), the present authors noted that metal complexes of dithiocarbamates such as I or I1 are separable under conditions of HPLC and that many

I ethyl, [dedtcl-

I1 pyrrolidine, [pydtcl-

[dtcl- = I or I1 of the complexes exhibit well-defined oxidation and reduction reactions under electrochemical conditions (3-5) (at least in nonaqueous media). Therefore, the suggestion was made (2) that after formation of metal dithiocarbamate complexes, HPLCEC could provide a powerful approach to achieving the objective of highly sensitive multielement analysis. In particular, interference-free oxidation pathways for their determination would be available. With most traditional approaches, the majority of metals would have to be determined via reduction steps and with HPLCEC this frequently introduces problems associated with oxygen removal (6). The determination of nickel and cobalt by polarographic techniques has been examined on many occasions in aqueous media (see ref 7-11 for example). Commonly the electrode processes used are the two-electron reduction steps Ni(I1)

+ 2e-

or

+

-+

Ni(0)

(1)

-

Co(1I) 2eCo(0) (2) The nature of both these electrode processes is very dependent on the matrix, and interference from species reduced a t similar potentials has been commonly reported. The determination of chromium(V1) and chromium(II1) is also prone 0003-2700/82/0354-1706$01.25/0

to considerable interference and several workers have devised methods to try and overcome these (12-14). The ability to distinguish between Cr(II1) and Cr(V1) is extremely important (15, 16) because of significantly different toxicities. Chemically, Nin(dtc), can be oxidized to [Niw(dtc)3]+(3-5). Electrochemical oxidation of Ni(dtc)z has also been studied a t length (17-19) and an irreversible electrode process producing [Ni(dtcI3]+ has been reported. The pathway for chemical oxidation of C ~ ( d t cis) ~more obscure (20) although on the electrochemical time scale, Co(dtc)3+appears to be generated in noncoordinating solvents such as dichloromethane (21). Oxidation of Cu(dtc)2 is relatively simple, producing Cu(dtc)z+(2-5). Electrochemical reduction and oxidation of C r ( d t ~have ) ~ also been reported (3-5, 22). All dithiocarbamate complexes can be reduced. The possibility of using oxidation rather than reduction processes has some advantages, particularly with respect to eliminating the difficulties frequently encountered due to the presence of oxygen and its removal. Furthermore, in the present case, reduction of Ni(dtc)z, C ~ ( d t c )and ~ , Cr(dt~),occurs a t relatively negative potentials, and in the buffered media associated with the HPLC separation using reversed-phase chromatography, it was found that the reduction of the complex is masked by reduction of hydrogen ions. In the present work, the determination of nickel, cobalt, chromium(III), and chromium(V1) was therefore investigated at a range of electrodes using the irreversible oxidation steps. Contrasting behavior with the reversibly oxidized copperdithiocarbamate complex (2) was noted and the simultaneous determination of nickel, cobalt, copper, and chromium in two oxidation states was examined to establish the principles for multielement determinations using HPLCEC. The study incorporates the use of a wide range of electrodes, detection methods, and electrochemical cells.

EXPERIMENTAL SECTION Reagents and Standard Solutions, All chemicals used were of analytical grade purity unless otherwise stated. Commercially available dithiocarbamate salts were recrystallized from ethanol. Nickel, cobalt, and chromium dithiocarbamate complexes were prepared by standard methods involving reaction of the appropriate amine with carbon disulfide in basic media. Reaction of Cr(V1) (as dichromate) produces Cr(dtc)2(odtc)(23). Chromium(V1) is determined in this form after reaction in an analogous fashion to the work of Tande et al. (24). It should be noted that in the presence of air cobalt(I1) dithiocarbamates Co(dtc)zrapidly produced Co(dtc)3and that determinations of cobalt are based on monitoring the electrochemical response of cobalt(II1). Metal dithiocarbamate stock solutions for use in acetonitrile-buffer solutions were initially prepared by dissolving M(dtc)z (M = Ni, or Cr(dtc)2(odtc)in acetonitrile. SubCu), C ~ ( d t c )Cr(dtc)B, ~, sequently they were prepared in situ by mixing simple metal salts and dithiocarbamate in acetonitrile-buffer. Acetate buffer was -prepared by using the method described by Vogel (25). Liquid chromatographic (LC) grade acetonitrile from Waters Associates was employed in results reported in this paper. Other grades were examined, with various stages of purification, but LC grade proved adequate. 0 1982 American Chemical Society

ANALYTICAL CHEMISTRY, VOL. 54, NO. 11, SEPTEMBER 1982

----

00

02

04

06

08

10

EiVOltS)

Cyclic voltammogram for oxidation of 5 X lo4 M Ni(dedtc), in acetonitrile (0.1 M Et,NCiO,) at a gold working electrode. Scan rate = 500 mV/s. Flgure 1.

Instrumentation. Unless otherwise stated, the instrumentation and liquid chromatographic or electrochemical accessories used in the present study were the same as described previously (2). That is, liquid chromatographic equipment was from Waters, electrochemical instrumentation was based on a PAR Model 174A polarographic analyzer, (andthe electrochemical detector was a Metrohm cell. An additional electrochemical detector using a glassy carbon working electrode was employed in the present work which was a Bioanalytical Systems (West Lafayette, IN) TL4 thin-layer cell. Platinum auxiliary and Ag/AgCl (3 M NaCl) reference electrodes were used with all electrochemical cells. Voltammograms obtained in the flow-through cell were obtained with microprocessor-based instrumentation (26) as were data using normal pulse techniques. Unless otherwise stated, all data were obtained at 21 f 1 OC and solutions were degassed with nitrogen.

I+

RESULTS AND DISCUSSION (a) Electrochemistry in a Conventional Cell. T o optimize the electrochemical detection, one requires details of the electrode processes a t different electrodes. Electrochemical results were as follows: (i) Copper. The electrochemical behavior of copper has been described elsewhere (2)and requires no further comment. (ii) Nickel. In acetonitrile (0.1M Et4NC104),a chemically irreversible oxidation w,avefor N i ( d e d t ~was ) ~ observed under conditions of cyclic voltammetry (scan rate = 500 mV/s) a t gold, platinum, and glwisy carbon electrodes (Figure 1). This is in accordance with the literature on platinum electrodes in acetone and other solvents (17-19). Data are summarized in Table I. The overall electrode process can be assigned to the reaction

-

+

3 N i ( d t ~ ) ~ 2 N i ( d t ~ ) ~ +Ni2+

+ 4e-

(3) but the mechanism is uncertain. A reduction wave was also observed in acetonitrile at relatively negative potentials (Table I). The reverse scan of the cyclic voltammograms, in contrast to the oxidation step, showed partial stability of the product. The reduction process can therefore be described by the equation Ni(dedtc)2 e- G= [Ni(dedtc),](4) with [Ni(dedtc)J being relatively unstable. These data are also consistent with results in different solvents (17-19). The very negative potential of the reduction process precludes its use in HPLCEC, where a buffered aqueous component is necessarily present. Reduction of hydrogen ions from the buffer masks the nickel reduction wave. When free ligand was added to Ni(dedtc)2 in acetonitrile (0.1 M Et4NC104) the response changed markedly at the gold electrode. Figure 2 shows the differential pulse voltammo-

+

30

M

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ANALYTICAL CHEMISTRY, VOL. 54, NO. 11, SEPTEMBER 1982

complexes externally to the column or in situ on the column by including [dtcl- in the solvent as was the case previously for determining copper (2). Optimum results should be obtained with the oxidation process at a glassy carbon electrode. (iii) Cobalt. As with Ni(dtc)zthe electrochemical behavior of Co(dedtc), and Co(pydtc), was initially investigated in acetonitrile (0.1 M Et4NC104). Gold, platinum, and glassy carbon working electrodes were considered. At all those electrodes, C ~ ( d e d t cshowed )~ quasi-reversible oxidation and reduction responses (21) with some instability noted with [ Co(dtc),]-. ~~

0.0

Figure 2.

0.2

0.4 0.6 E (Volts)

0.8

1.0

Differential pulse voitammogram for oxidation of 5

Co(dtc) X

M Ni(dedtc), in acetonitrile (0.1 M Et4NCI04)before (1) and after (2) the addition of M [dedtcl- at a gold working electrode. Duration between pulses = 0.5 8 , pulse amplitude = 100 mV.

grams before and after addition of free ligand (dedtc-) a t a gold electrode. As expected the ligand produces a wave a t -+0.20 V vs. Ag/AgCl (3 M NaC1) which arises from the electrode process (27).

(5)

However, as can be seen in Figure 2 the original response for Ni(dedt& at +0.68 V is suppressed and a new response is observed at +0.84 V vs. Ag/AgC1(3 M NaCl) in the presence of [dedtcl-. Oxidation of thiuram disulfide, generated according to eq 5, occurs a t potentials more positive than +1.00 V vs. Ag/AgCl. The electrochemistry at gold electrodes is complicated by the presence of excess ligand. On glassy carbon electrodes the presence of the ligand has no effect on the Ni(dedtc)z oxidation step under conditions of differential pulse or cyclic voltammetry. The electrochemical response a t glassy carbon electrodes therefore has characteristics more favorable for detection of nickel than at gold electrodes using HPLCEC. Glassy carbon is also marginally superior to platinum electrodes in acetonitrile. Gold dithiocarbamate complexes are known (3,4,28)so presumably, the gold electrode is not inert with respect to the nickel electrode process. With [pydtcl- as the ligand, similar electrochemical behavior was observed as with [dedtcl-, e.g., under differential pulse conditions with a pulse amplitude of 50 mV peak potentials E,, were as follows: E, = +OB0 V (in the presence of lo-, M ligand on gold); E, = +0.64 V (in the presence of lo-, M ligand on glassy carbon). For HPLC a buffer is required. At gold and glassy carbon electrodes up to 50% (0.02 M) acetate buffer could be added without significantly affecting the oxidation response of M Ni(dedtc)> The presence of water degraded the platinum response, presumably due to the formation of oxide films a t positive potentials. In the presence of the buffer, 0.1 M Et4NC104was replaced by 0.1 M NaNO, as the supporting electrolyte. Similar results were obtained on addition of buffer and using lo4 M Ni(pydtc),. Ni(pydtc)z is inherently less soluble than the Ni(dedt& in acetonitrile. Studies in a conventional cell indicate that acetate buffer in the pH range 5.5-6.0 was most suitable. Under these conditions the complexes could be formed rapidly by mixing nickel sulfate and free ligand, and they are stable for a t least 6 h. In summary, data in conventional electrochemical cell indicate that HPLCEC should be possible either by forming

Co(dtc),

+=[Co(dtc),]+

+ e- + [Co(dtc),]-

-

+ e-

Co(dtc)z + [dtcl-

Data for Co(dedtcI3 are summarized in Table I. The electron transfer rate a t glassy carbon electrodes appears to be slower than at gold. Despite the slower electron transfer rate, the glassy carbon working electrode gives the best results with respect to sensitivity to the Co(dtc), complexes because of a lower background (more favorable signal to noise ratio) at potentials greater than f1.0 V vs. Ag/AgCl. Since the reduction process is irreversible and fairly negative it will not be considered with HPLCEC for the same reasons presented for nickel. Up to 50% water could be added to a M Co(dtc), in acetonitrile solution without precipitation. Addition of this aqueous component did not affect the electrochemical response. The possibility of using in situ formation of the Co(dtc), complexes was investigated by adding cobalt(I1) nitrate to a solution of 70% acetonitrile, 30% acetate buffer (0.02 M), [0.005 M NaNO,] and containing the appropriate ligand. Acetate buffers in the pH range 4-6 were considered. The complex was found to be most stable with a buffer of pH 6. The complexes appeared to form quickly as the solution turned green almost immediately. Monitoring of differential pulse voltammograms over a period of 24 h showed the complexes to be stable over this period. (iv) Chromium(II1). Results for Cr(dtc), are summarized in Table I. Data at platinum are similar to those reported by other workers (21),that is, an irreversible oxidation step [Cr(dtc),] =+. [Cr(dtc),]+ + e- (with [Cr(dtc),]+ being unstable). Responses on gold and glassy carbon were similar to those on platinum. Irreversible reduction steps were also observed (221, but at potentials too negative to be useful in HPLCEC. (v) Chromium(V1). Electrochemical responses for Cr(dtc)z(odtc) produced by addition of Cr(V1) to [dtcl- were different on all electrodes (see Table I). A small but reproducible quantity of Cr(dtc), is produced in the reaction (Figure 3) which is allowed for in the calibration procedure (24). The mechanism for this oxidation step is unknown. Reduction waves were also observed (Table I). For both Cr(II1) and Cr(V1) the addition of water as required for HPLC work causes difficulties at gold and platinum electrodes because of the limited potential range available. Glassy carbon is clearly the choice for species such as chromium which are oxidized at very positive potentials. The chromium complexes are extremely stable. (b) HPLCEC Data. For analytical work using HPLCEC for determining all metals considered, two distinct techniques were investigated. In method A, where the complex is formed externally, the following solvent was used: 70% acetonitrile, 30% acetate buffer (0.02 M) pH 5.5, 0.005 M NaN03. With M method B, where the complex is formed in situ, 2 X ligand was included in the running solvent. Method C is essentially the same as method B, but is based on detection of the modified ligand response in the presence of metal rather

ANALYTICAL CHEMISTRY, VOL. 54,NO. 11, SEPTEMBER 1982

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0.1 m A

O!O

I

1

I

1

!

1

0.1

0.2

03

0.4

0.5

0.6

E (Volts)

Figure 4. Flow-through voltammograms obtained for copper determlnatlon: solvent, 70% acetonitrile/30% acetate buffer containing lo-, M [dedtcl- and using a flow rate of 2 mL/min; Metrohm detector cell, glassy carbon working electrode; scan rate = 500 mV/s. injection, 200 pL containing 1.2 pg of copper(I1); curves obtained at point in time where retention volume is 0 mL (l),3 mL (2), 16 mL (3).

00

02

04

06 08 E (Volts)

10

12

14

Figure 3. Cyclic voltammogram for oxidation of Cr(dtc), and Cr(dt~)~(odtc) in acetonitrile (0.1 M Et4NC104)at a glassy carbon working electrode: (A) Cr(pydtc),, (e) Cr(pydtc),(opydtc). Sample contains a small amount of Cr(pydtc), ( 2 4 ) . Scan rate = 500 mV/s. 0 03 mA

Table 11. Retention Volumes for Metal Dithiocarbamate Complexes in 70% Acetonitrile/30% Acetate Buffer k' = (V, - V,)/ compound vr, mL vLla Cu(ded tc ) 2 CU(PYdtc ) 2 Ni(dedtc), Ni(pydtc), Co(dedtc), Co(wdtc ) 3 Cr (dedtc ) 3 Cr(pydtc1, Cr(dedtc),(odedtc) Cr(py dtc),( opycl1;c) a

16.80 11.20

12.40 8.40 14.80 9.80

15.70 10.80 11.20

8.00

I\

6.00 3.67

4.17 2.50 5.17 3.08 5.54 3.50 3.67 2.33

V, = retention volume, V , = void volume = 2.4 mL,

k' = caDacitv factor. than detection of the metal complex (see Table I11 for further details). The concentrations of' supporting electrolyte and ligand have both been reduced from that in previous work (2) to minimize background current and wear on pump seals. Retention volumes for all aietal dithiocarbamates are given in Table 11. (i) E1ectrochemistr:yin the Detector Cell. Dc voltammograms of the flowing mlvent were recorded in the Metrohm cell using the following procedure: A predetermined potential range was swept continuously with a triangular voltage and data output to an x-y recorder. The first scan commenced at the time of sample injection. Data presented in Figures 4 and 5 consist of scans selected a t points of time that are of interest to the discussi(on. The basic principles for determination of copper as a Cu(dtc)z complex have already been reported (2). However,, no voltammetric data in the flowthrough cell were reported. Voltammograms of the flowing solvent with copper inje!ctions (Figure 4) confirm the mechanism proposed in ref 2. For method B thus, a t time 1the ligand oxidation wave ii3 observed. At the time 2 there is a decrease in the ligand 'wave due to formation of Cu(dtc)z. Comparison of the magnitude of this decrease when equal amounts of nickel and copper are injected separately shows that both metals use the same amount of ligand to form M(dtc)* At time 3 there is a shift in the position of the ligand

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

E (Vol tsl

Figure 5. Flow-through voltammogram obtained for nickel determination. All conditions are as in Figure 3 except that injection is 200 pL containing 1.2 pg of nickel(I1). Retention volume = 0 mL (l),3

mL (2), 12 mL (3).

wave. Monitoring of the changes in the ligand response in the presence and absence of copper form the basis of method C (see Table I11 for further details). With method A, the Cu(dtc)2 oxidation and reduction waves observed were as expected. Figure 5 shows the sequence of events following injection of nickel with [dedtcl- included in the flow-through solvent (method B). In the absence of nickel, the ligand oxidation wave is observed. At time 1 the ligand wave decreases due to formation of Ni(dedtc)z on the column. At the retention time of N i ( d e d t ~a) ~ wave a t +0.65 V vs. Ag/AgC13 M NaCl appears. With the gold electrode a wave appeared a t +0.85 V as expected. All these results are consistent with studies in the conventional cell. At the retention time of N i ( d e d t ~ ) ~ the ligand wave not only shifts but increases in size. When [pydtcl- is employed in the running solvent in method B similar behavior is observed but the ligand wave only shifts and does not increase. If method A is used, the ligand and meta-dithiocarbamate responses are observed at the expected times. With method A (external complex formation), a t the retention volume of Co(dedtc)3 and Co(pydtc)3 well-defined waves were observed, as was the case for Ni and Cu. By use of method B (on column formation), with [pydtcl- similar results to method A were observed, but with [dedtcl- in the solvent the Co(dedtc)3response was difficult to detect (poor sensitivity). Use of method B and monitoring of the free ligand wave at +0.20 V showed that cobalt used more [pydtclthan Cu or Ni for equivalent injections. This is understandable since cobalt forms a tris complex. However, with

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Table 111. Detection Limits for Determination of Metals as Dithiocarbamate Complexes Using HPLCECa metal copper nickel cobalt chromium(II1) chromium(VI)

detection limit (ng of metal) method A method B method C 0.20 0.12 0.50 0.10 0.04

0.20 0.12

0.60 0.14

1.0

-

-

-

-

regions of linearity (ng of metal) method A method B method C 0.20-100 0.12-100 0.50-100 0.10-50 0.04-50

0.20-100 0.12-100

10-100

-

0.60-5 0.60-5

-

a Conditions: running solvent, 70% acetonitrile/30% acetate buffer. Injection volume = 1 0 pL, flow rate = 1.5 mL/min. Detection limit quoted is for signal to noise (background) ratio of 2 : l using a glassy carbon working electrode in a B.A.S. thin-layer detector cell and with [dedtcl- as the ligand. Detection limits with [pydtcl- as the ligand are similar, except for cobalt and method B where the detection limit is 5 ng with [pydtcl- compared with 10 ng for [dedtcl-. [pydtcl- complexes are generally not as soluble as [dedtcl- and linear calibration range is therefore sometimes less than when [pydtcl- is used as a ligand. Methods A and B are described in detail in the text. The dc response was monitored at t 1.20 V vs. Ag/AgCl (Guard column used). For method B the dc response was measured at + 1.00 V vs. Ag/AgCl. Method C is a modified version of method B in which the ligand oxidation wave occurring from the 2 X M [dtcl- always present in the running solvent is monitored. In situ complex formation modifies this response markedly at the retention volume. Dc response is monitored at t 0.20 V vs Ag/AgCl. Standards for method A were prepared in 70% acetonitrile%30% buffer. Standards for method B and C were prepared in distilled water,

[dedtcl-, cobalt only used up the same amount of ligand as Cu and Ni, suggesting that [dedtcl- complex does not form as well uon the column" as does the [pydtcl- complex. Presumably, oxidation (oxygen) is not as efficient or alternative reactions occur. Method A is therefore recommended for the determination of cobalt. With method B and injecting Cr(II1) (as chromium(II1) nitrate) or Cr(V1) (as potassium dichromate) no response was observed for Cr(dtc), or Cr(dtc)2(odtc). The kinetics of complex formation are too slow for method B to be applicable. For method A the expected electrochemical response is observed a t the retention volume of the respective complexes. The formation of Cr(dtc)Band Cr(dtc)z(odtc) was confirmed to be extremely slow as has been reported elsewhere (24) and the lack of response when using method B is not surprising. (ii)Use of Thin-Layer Flow Cell. All HPLCEC results described above refer to the use of the Metrohm detector cell which operates on the wall-jet principle. It therefore seemed appropriate to compare results with a detector based on the other most common principle, the thin-layer flow cell. The Bioanalytical Systems detector was employed and results are presented in Table 111. In general lower detection limits were observed. However, it was also noted that smaller linear ranges of plots of peak height vs. concentration were obtained. A comparison of current-voltage curves for Ni(dtc)z or Cu(dtcIz indicates a higher ohmic (iR)drop in the Bioanalytical Systems cell, presumably caused in part by the fact that the reference electrode is not situated as close to the working electrode detector as in the Metrohm counterpart. The higher iR drop would explain the decreased regions of linearity. A study of background current to noise ratios vs. the applied potential are more favorable in a Bioanalytical Systems cell, particularly at very positive potentials, and an enhanced Faradaic signal to noise (background) ratio enables the lower limits of detection to be obtained. An investigation of peak current responses vs. flow rate showed that the response with the thin-layer flow cell to be virtually independent of flow rate for both Ni(I1) and Cu(I1) over the range 1-4 mL/min. This result can be contrasted with data reported for the Metrohm wall jet electrode (2). For trace metal determinations, the thin-layer flow cell appears to be superior to the wall-jet electrode, although for higher concentrations iR drop effects are undesirable. Dilution of sample is readily undertaken if concentration is too high for the optimum response. All subsequent data refer to the thin-layer cell. (iii) Determination of Metal Complexes by HPLCEC. Determination of Copper. Reference 2 can be consulted for

"1

1

2

I

Improvement of chromatograms obtained with guard column: running solvent, 70% acetonitrile/30% buffer; flow rate = 1.3 mL/mln; detector cell, B.A.S., glassy carbon working electrode; dc response monitored at 4-1.20 V vs Ag/AgCI; injection 10 pL of 70% acetonitriie/30% buffer M [dedtcl-) containing 5 ng of nickel(II), 10 ng of cobalt(II),10 ng of copper(II),10 ng of chromium(V1); (A) method A without guard column Co(dtc), (peak I), Ni(dtc), (peak 2), Cu(dtc),(peak 3), [dedtcl- (peak 4); (B)method A wtth Amberlite guard column, residual [dedtcl--solvent front (peak l),thiuram disulfide (peak 2), Cr(dtc),(odtc)(peak 3),Co(dtc)3(peak 4), Ni(dtc), (peak 5), Cu(dtc), (peak 6). Flgure 6.

ANALYTICAL CHEMISTRY, VOL. 54, NO. 11, SEPTEMBER 1982

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3

2

1

5 0 nP,

-

1

2 T,

8

4

0

(mid

Figure 7. Determinatlon of chromium by method A: running solvent as in Figure 6 except flow rate = 1 mL/min; detection as in Flgure 66, injection, 10 pL containing 6 ng of chromlum(II1) (peak 4) and 6 ng of chromlum(V1) (peak 31) in acetonitrile/acetate buffer M [pydtcl-); residual [pydtcl--solvent front (peak 1); thluram disulfide (peak 2).

details on the determination of copper. The only improvement is that greater sensitivity is achieved with the thin-layer cell (Table 111). Determination of Nickel. From Consideration of the voltammograms in the flow-through cell, there are a t least three ways (methods A, B, and C) in which the nickel response could be monitored. Table I11 summarizes the methods and results. All data confirm the previous suggestion that glassy carbon electrodes are preferable to gold. Determination of Cobalt. Method A By measurement of the dc response a t +1.00 V vs. Ag/AgC1. detection limits of 500 pg for Co as Co(pydik), and 1ng for Co as C ~ ( d e d t cwere )~ found. Table I11 provides additional details. Both gold and glassy carbon electrodes were considered with glassy carbon being slightly more sensitive. Unfortunately, monitoring the response at more positive potentials than +LOO V causes problems. This potential is sufficient to cause the second oxidation of the free ligand (27) leading to deleterious effects. Hence the excess ligand must be removed. This could be achieved by extraction but a more convenient method ha,s been devised. An anion exchange guard column, packed with Amberlite CG400 is inserted between the injector and the reversed-phase separating column. The ion exchange resin chosen has the desired properties of high capacity and good physical strength. Since the free ligand has a net negative charge, it was retained on the guard column and the neutral compleires passed through unretained (Figure 6). The Cu, Co, Ni, and Cr dithiocarbamate responses were unaffected. With this particular resin several hundred injections of standard can be performed before repacking of the guard column is necesrrary. Method B: Whereas method B (on column formation) has proven convenient for Cu and Ni analysis, the results for Co are unsatisfactory. Firstly, as noted previously, the C o ( d e d t ~complex )~ does not form as well on the column as Cu or Ni, probably because of the need for the oxidation (with oxygen) step. As mentioned above, monitoring a t potentials more positive than +1.0 V results in the second oxidation step of the free ligand. With method B, this results in very large background currents and also electrode contamination which interferes with the Co determination. In fact as the potential was made more positive, the Co(dtcI3 response decreased until eventually it was completely masked. With method A, the Ni, Cu, Co, and Cr dithiocarbamates are separated. However, with method B, the retention times of the Co complexes are) altered in such a way that they elute with the Cu complexes, making simultaneous determinations

I

I

10

5

'

I

0

T, (min)

Flgure 8. Multielement deterrnlnation using HPLCEC: synthetic sample uslng conditions described in Figure 6B 'but with a flow rate of 2 mL/min; injection, 10 pL containing 10 ng of copper(I1) (peak 6), chromium(II1)(peak 5), cobalt (peak 4), chromium(V1)(peak 3), and 5 ng of nickel (peak 2). Peak 1 is due to oxidation of thluram disulfide.

u 8 4 0 T,lmin)

Flgure 9. Slmuitaneous determlnation of copper and nickel: method B with 2 X lo-' M [pydtcl- in solvent; flow rate of 1.5 mL/mln; detection, B.A.S. detector cell, glassy carbon working electrode; dc response monitored at +0.70 V vs. Ag/AgCI; Injectlon, 10 pL sample

obtained from Queensiand Nickel Refinerles, Townsville, Queensland, Australia, without pretreatment; determined, 0.70 ppm nickel (peak l), 0.03 ppm copper (peak 2). of cobalt and copper difficult Method A is the only feasible approach for Co determination. Determinution of Chromium. Method A is the only method feasible for determination of chromium as noted previously. The use of the ion exchange guard column to remove excess [dtcl- is also highly recommended. The solvent employed for the investigation was 70% acetonitrile:30% acetate buffer (pH 6). Standards were prepared by adding chromium(II1) or chromium(V1) to a solution of [pydtcl- of [dedtcl- in 70/30% acetonitrile/buffer. Complex formation was slow, and it is recommended that standards are left overnight before injection. Results are summarized in Table 111.

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ANALYTICAL CHEMISTRY, VOL. 54, NO. 11, SEPTEMBER 1982

results in only peak 3. Simultaneous determination of both chromium(II1) and chromium(V1) is readily achieved.

Simultaneous Determination of Cu, Ni,Co, Cr(III),and Cr(Vr). Simultaneous determination of Cu, Co, Ni, Cr(III), and Cr(V1) has been examined on a wide range of industrial effluents and other solutions. Figures 8-11 show that for many solutions results are extremely successful. Data are essentially in agreement with results from atomic absorption spectroscopy, implying total metal ions are being determined. As with most analytical techniques, the concentration ratios of the elements are critical. Additionally, many peaks have been observed in addition to those accounted for in the above survey. Such peaks may of course cause interference by producing overlapping peaks. Conversely, it is also clear that many additional metals may also be determined by the method described in this work.

Flgure 10. Determination of copper In the presence of very high concentrations of nickel: method C with loa3M [dedtcl- In solvent; flow rate of 1.5 mL/mln: detection, normal pulse wave form applied +0.10 to 4-0.20V; duration between pulses = 0.5 s; pulse width = 20 ms; Metrohm detector cell: injection, 10 pL sample of electrolyte obtained from Copper Refineries, Townsville, Queensland, Australia, after 100-fold dilution; determined, 3 ppm copper (peak 2)In presence of 0.02 g/L nickel (peak 1) with values referred to diluted sample.

u 10

5 Trlminl

0

Flgure 11. Simultaneous determination of chromium(II1) and chromium(V1): (method A) using [dedtcl- as ligand; flow rate, 2 mL/mln: detection, B.A.S. detector cell; glassy carbon working electrode; dc response monitored at 4-1.2 V vs. Ag/AgCI; Injection, 10 pL sample supplied by Ordnance Factory, Maribyrnong, Victorla, Australia; determlned, 1.0 ppm chromium(II1) (10 ng) (peak 2), 0.05 ppm chromlum(V1) (0.5 ng) (peak 1).

Injection of the chromium(V1) standard prepared by addition of potassium dichromate to [dtcl- resulted in the observation of three peaks (Figure 7). Peak 1is due to oxidation of the dimer, thiuram disulfide, which occurs during the reaction of Cr(V1) and [dtcl- to produce Cr(dtc)2(odtc)(23). Peak 2 is due to oxidation of Cr(dt&(odtc) and peak 3 is due to oxidation of Cr(dt&. Although some Cr(dt& is formed in the chromium(V1) reaction, this fraction is constant and can be corrected for (24). Injection of chromium(II1) alone

ACKNOWLEDGMENT Samples provided by Queensland Nickel Refineries, Townsville, Queensland, Australia, Copper Refineries, Townsville, Queensland, Australia, and Ordnance Factory, Maribymong, Victoria, Australia, were used in evaluation of the work described in this paper. The provision of samples and the assistance of staff from these companies is gratefully acknowledged by the authors. LITERATURE CITED Kisslnger, P. T. Anal. Chem. 1977, 4 9 , 447A-456A. Bond, A. M.; Wallace, G. G. Anal. Chem. 1981, 5 3 , 1209-1213. Coucouvanls, D. Prog. Inorg. Chem. 1970, 7 7 , 233-371 and references clted therein. Coucouvanls, D. Prog. Inorg. Chem. 1979, 2 6 , 301-469 and references clted therein. Bond, A, M.; Hendrickson, A. R.; Martin, R. L., to be submitted for publication In Coord. Chem. Rev. Hanekamp, H. B.; Voogt, W. H.; Bos, P.; Frei, R. W. Anal. Chlm. Acta 1980, 778, 81-86. Reo, V. S. N.; Rao, S. B. Fresenius' 2. Anal. Chem. 1979, 294, 4 14-4 15. Saraswathl, K.; Sreenivasulu, R. Indlan J . Chem.. Sect. A 1977, 75A, 758-760. Bloom, H.; Nolier, B. W.; Richardson, D. E. Anal. Chlm. Acta 1979, 709, 157-160. Iackett, S. L.; Ong, P. T. Anal. Len. 1970, 3 , 169-175. Temmerman, E.; Verbeek, F. Anal. Chlm. Acta 1970, 5 0 , 505-514. Llngane. J. J.; Kolthoff, 1. M. J . Am. Chem. Soc. 1940, 6 2 , 852-858. Zanello, P.; Giorglo, R. Anal. Chlm. Acta 1977, 8 6 , 237-243. Gilbert, T. R.; Clay, A. M. Anal. Chlm. Acta 1973, 6 7 , 289-295. Tandon, S. K.; Sazena, D. L.; Gaur, J. S.; Chandra, S. V. Envlron. Res. 1978, 75, 90-99. Mertz, W. Phys. Rev. 1969, 49, 163-239. Hendrlckson, A. R.; Martin, R. L.; Rohde, N. M. Inorg. Chem. 1975, 74, 2980-2985. Lachenal, D. Inorg. Chem. Nucl. Lett. 1975, 7 7 , 101-106. Van der Linden, J. G. M.; Dix, A. H. Inorg. Chlm. Acta 1979. 3 5 , 65-71. Hendrickson, A. R.; Martin, R. L.; Taylor, D. J . Chem. Soc., Dalton Trans. 1975, 2182-2188. Bond, A. M.; Hendrickson, A. R.; Martin, R. L.; Moir, J. E.; Page, D. R., unpublished work, Deakin University and Australian National University 1978-1981. Chant, R.; Hendrickson, A. R.; Martin, R. L.; Rhode, N. M. Aust. J . Chem. 1973, 2 6 , 2533-2538. Hope, J. M.; Martin, R. L.; Taylor, D.; White, A. H. J . Chem. Soc., Dalton Trans. 1977, 99-100. Tande, T.; Petterson, J. E.; Torgrimsen, T. Chromagoraphla 1980, 73, 607-610. Vogel, A. "A Textbook of Quantltatlve Inorganic Analysis"; Longmans, London, 1968; p 869. Anderson, J. E.; Bond, A. M.; Heritage, I.D.; Jones, R. D.; Wallace, G. G. Anal. Chem. 1982, 5 4 , 1702-1705. Cauquls, 0.;Lachenal, D. J . flectroanal. Chem. 1973, 4 3 , 205-213. Van der Linden, J. G. M.; Van de Roes, H. G. J. Inorg. Chlm. Acta 1971, 5 . 254-256.

RECEIVED for review December 21, 1981. Accepted May 7, 1982. Part of the work described in this paper was undertaken with financial support from the Ordnance Factory, Maribyrnong, Victoria, Australia, as part of a collaborative Research Project between the authors from Deakin University and T. Crosher and L. McLachlan, Ordnance Factory, Maribyrnong.