Determination of Trace Elements in Zinc Plant Electrolyte by Differential Pulse Polarography and Anodic Stripping Voltammetry Edwin S. Pilkington” and Christopher Weeks CSIRO Division of Mineral Chemistry, P.O. Box 124, Port Melbourne, Vic. 3207, Australia
Alan M. Bond* Department of Inorganic Chemistry, University of Melbourne, Parkvilie, Vic. 3052, Australia
Rapid determination of a number of trace elements in zinc sulfate electrolyte is essential for adequate plant process control. As a bask for an on-stream monitoring system, differential pulse polarography and differential pulse anodlc stripping voltammetry at a hanging drop mercury electrode have been investlgated, using conventlonal and computercontrolled instrumentation, for the determination of Cd, Cu, Pb, Sb, Co, Ni, TI, and As. Cd and Cu were determined dlrectly In the zinc sulfate solution down to 10 pg/l., and Sb to a similar low level after addition of concentrated hydrochloric acid. Pb, Co, Ni, TI, and As were determined after addition of appropriate reagents in some cases; however, these determinations are generally better suited to hlgher concentration levels than for Cd, Cu, and Sb.
Successful operation of electrolytic zinc plants depends critically on the purity of the cell feed, both for product purity and for high current efficiency in the deposition stage (1-4); therefore, reliable analytical data are very important to the economics of the process. Over the years, reasonable process control has existed (2,5-9) by virtue of manual sampling and rapid analysis for those impurity elements known to be most detrimental. Many elements can affect the efficiency of the process (1-4,8,10-12), particularly Fe, Ni, Co, Cu, Cd, Sb, As, Pb, Ag, Sn, Se, Ge, Te, T1, and C1 a t concentrations in the 10-1000 pgh. range. Fortunately, it is not necessary to monitor all the significant elements with the same frequency. Normal processing technology involves three or four purification stages in which elements are removed in groups, and it therefore becomes possible to assess the purification a t each stage by monitoring a relatively small number of elements. For the present work it was established that determination of Cd, Cu, and Sb had the highest importance for achievement of rapid process control. Considerable importance attaches, also, to Co, Ni, T1, Pb, and As, and these elements have therefore been included. A typical composition of cell feed is given in Table I. This solution is the most significant and the most difficult to analyze, the various trace impurity elements being at their lowest concentrations. Monitoring of solutions at stages earlier in the purification system is a simple extension of techniques established for cell feed.
PRELIMINARY EVALUATION The analytical problem of rapid determination of Cd, Cu, and Sb in concentrated zinc sulfate at levels in the range 10-1000 pgh. is substantial. One possibility was to automate the existing manual spectrophotometric techniques (for Cu and Sb) and the dc polarographic technique (for Cd). Although this would increase the frequency of the data output for Cu and Sb, e.g., using discrete or continuous-flow analyz-
ers, the operational time in these instances would make this approach unacceptable. This limitation does not apply, however, to Co, where the “fast chemistry” available in the Nitroso R Salt procedure can yield results in a continuous flow analyzer (Technicon) every 7 min. Other techniques were eliminated because of inadequate sensitivity at the extremely high concentration of zinc sulfate. Modern polarographic ac and pulse techniques (13) are particularly suited for the rapid determination of low concentrations of selected elements in concentrated zinc sulfate solution. Zinc is reduced at more negative potentials than most of the elements to be determined in this case, and sulfate is inactive at the mercury electrode. Data published on fast sweep dc polarography (14), second harmonic and differential pulse anodic stripping voltammetry ( 1 5 ) ,and a range of anodic stripping techniques (16) indicate the possibilities for polarography with this electrolyte. The technique has, in fact, already been used for impurity determination in zinc plant but few data are available for the low levels solutions (6, 7,17), present after the final purification stages.
EXPERIMENTAL Instrumentation. For convenience, the three forms of instrumentation examined are described as conventional, semiautomated, and computerized. Conventional. Either a PAR Electrochemistry System Model 170 or Polarographic Analyzer Model 174 was modified to perform a range of functions additional to those provided by the manufacturer (18).
Semiautomated. A combination of a PAR Polarographic Analyzer Model 174 and Automated Electroanalysis Controller Model 315 was used. All procedures except the readout were automatically controlled in these experiments. Computerized. The polarographic instrumentation under computer control, essentially a differential pulse polarograph based on the design of Vassos and Osteryoung (19), was interfaced to a PDP8B minicomputer with teleprinter output. The computer controls the degassing and the timing of all stages in anodic stripping voltammetry, initiates the potential sweep, and processes the i-E curve to generate the final print-out in units of concentration. The system is currently being further developed to include automatic cell filling, rinsing, and draining under computer control, which will permit fully automated operation for on-stream industrial use. Further details of this are to be reported in a later paper. Electrode Systems. A three-electrode system was used for all experiments with either a dropping or hanging drop mercury electrode (HDME) as the working electrode,platinum as the auxiliary electrode, and Ag/AgC1(1 M NaCl) or Hg/HgS04 (1 M HzS04) as the reference electrode. For anodic stripping voltammetry, most experiments with conventional or semiautomated instrumentation used a Metrohm BM50-3 HDME. The computerized system used a PAR Model 314 Automated HDME. The HDME was selected in preference to a thin film mercury electrode essentially because linear response is obtained over a wider range of concentration without saturation or interference effects (16), and the automated version is ideal for on-stream work, allowing a fresh working electrode surface to be generated automatically for each analysis. Reagents. Preliminary investigations were undertaken using re-
ANALYTICAL CHEMISTRY, VOL. 48, NO. 12, OCTOBER 1976 * 166s
cu
Table I. Typical Composition of Cell Feed 120
so4
210 10
Mn Mg
Mug/l.
mgll.
g/1.
Zn
Ca Na K SiO, c1
3
co
500 50 0
Fe Ni cu Cd
400 100
150
Sb Pb As T1
10
Bi Se Ge Sn Ag
600 200
100 300 30 100 < 50 100 1
< 10 3
< 200 < 20
V vs A g / h g C l
Flgure 1. DPASV curves in plant zinc sulfate solution. Deposition potential, -0.75 V vs. AglAgCI, with 2-min stirring and 30-sequilibration time. Pulse amplitude, 25 mV; duration between pulses, 0.5s;scan rate, 10 mV/s. (a) 20 pg/l. Cd, 30 pgll. Cu;(b) 30 pgll. Cd, 10 pgll. Cu
Table 11. Direct Determination of Cd in Zinc Sulfate Plant Electrolyte by DPASV Using Conventional and Computerized Instrumentation Conventional, Pd1.
c
Computerized, Mg/l.
10
20 70
70
110
100
240 270 170
280 270
I
I 0.0
+0,1
v
180
I -0.1
vs A ~ / A ~ C L
Figure 2. Differential pulse polarogram for 160 pg/L Cu in plant zinc sulfate solution Drop time, 2 s;pulse amplitude, -25 mV; scan rate, 1 mV/s
Table 111. Determination of Cu in Zinc Sulfate Plant Electrolyte by DPASV and Spectrophotometry DPASV PgP.
Spectrophotometry, PcglI.
< 10 < 10 < 10
7
30 10
29 6 30
20
8 5
agent grade zinc sulfate solutions at a concentration of approximately 300 g/l. However, to obtain a blank and to prepare calibration curves, further purification was required. Reagent grade material prepared as a 4-1. batch of solution at 440 gh. ZnS04.7H20 in water was acidified to pH 2 and treated with 20 g of powdered reagent grade zinc dust while heating to 50-60 "C. After stirring continuously for 2 h, the solution was filtered into a Buchner flask which had been previously thoroughly washed with dilute sulfuric acid and rinsed. Usually four treatments were adequate to reduce the concentrations of Cd and Cu from around 200 pg/l. to less than 5 pgh., Cu being the more difficult t o remove. Concentrations of other elements were acceptably low. All metal standards were prepared as sulfate solutions with the exception of Pb which was p;epared as nitrate. All other chemicals were of analytical reagent grade. Plant Samples. Plant samples were collected in clean polyethylene containers and acidified to approximately pH 2 with sulfuric acid. Procedures. All solutions were deoxygenated for a minimum period of 3 min using argon or nitrogen. During each potential scan the inert gas was passed over the solution. Elements were determined on aliquots of 20-40 ml at ambient temperatures of 23 f 2 " C . Other details are presented in the Results and Discussion section. Alternative Methods for Cu and Sb Determinations. In view of the significance of Cu and Sb in this work, and as a check on the analytical data at concentrations down to 10 pg/l., separate values were obtained using validated spectrophotometric procedures. An outline of the procedures (20, 21) is as follows. Copper. After acidification and destruction of organic matter, copper is extracted with ammonium 1-pyrrolidine carbodithioate into 1666
carbon tetrachloride. The solvent phase is wet ashed and the copper re-extracted for spectrophotometric determination. Antimony. After conversion to Sb(II1) and extraction in an iodide system by 4-methyl-2-pentanone (MIBK), antimony is stripped from the solvent phase. After removal of thallium and oxidation to Sb(V), the Rhodamine B Complex is determined spectrophotometrically.
RESULTS AND DISCUSSION Cadmium. At a DME cadmium gave extremely well defined reversible fundamental and second harmonic ac and pulse polarographic waves in zinc sulfate with an E112 value of -0.58 V vs. Ag/AgCl. The calibration curves with ac and DPP methods were linear from the limit of detection ( ~ X 510-7 M) to at least M (50-100 000 pg/l.) with the response in plant electrolyte equally satisfactory as that for purified synthetic solution. Fast sweep polarographic methods ( 2 2 ) at a single mercury drop were also satisfactory and have the added advantage of shorter analysis time. For lower concentration levels encountered in cell feed, however, anodic stripping voltammetry is preferable. Figure 1shows DPASV curves for samples of plant electrolyte (acidified to pH 2), including one sample with the lowest concentration encountered (20 kg/l.). As the matrix is essentially constant, the use of a calibration curve based on purified zinc sulfate was investigated rather than a standard addition technique. A linear response extending well into the polarographic range ( M) and passing through the origin was obtained. Results from standard addition agreed well with calibration curves with good reproducibility (
