eration comes to a halt. The accumulated glass fragments and graphite must then be removed. A simple expedient, which we have used advantageously, consists of placing a trough (Figure 1,D) filled with graphite immediately in front of the coiling tube. As the straight portion of capillary burrows through the trough, small portions of graphite cling to the glass, thus assuring constant, adequate lubrication. To avoid getting graphite inside the glass tubing, the first few coils should be severed. For glass drawing machines operating in the vertical mode, a small cylinder with slots cut into the side from top to bottom and filled with graphite, should serve the same purpose. The trough (Figure l , D insert) consists of a block of aluminum (4.5 X 4.0 X 1.3 cm), with a slot milled 3 mm wide and 6 mm deep. The block is supported by a small laboratory jack (Figure 1,E).
With the former arrangement, starting with borosilicate glass tubing (122 cm long X 0.8-cm 0.d. X 0.3-cm i.d.), wastage is kept to such a minimum that 62-68 meter lengths of coiled capillary tubes (0.95 mm-0.d. X 0.34-mm i.d.) are consistently attained. In addition, since column efficiency increases with column length, longer columns are frequently desirable. By using shrinkable Teflon tubing, capable of operating a t temperatures to 327O, several coiled sections of glass capillaries can be joined together without loss of efficiency.
ACKNOWLEDGMENT Thanks are due to Edgar Biel, Instructional Media Division, CDC, for the drawing which appears in this communication. RECEIVEDfor review June 11, 1974. Accepted July 22, 1974.
Glass-I mmobilized 8-Hydroxyquinoline for Separation of Trace Metals from Base Electrolytes Used for Anodic Stripping Analysis E. D. Moorhead and P. H. Davis Departments o f Chemical Engineering and Chemistry, University ot Kentucky, Lexington, K y . 40506
That anodic stripping voltammetry (ASV) provides a rapid and convenient method for the measurement of trace levels of amalgam-forming heavy metals is well known ( I ) . However, the very high sensitivity of the ASV technique can produce problems arising from the purity of available solvents and reagents when the sample constituents fall into the parts per billion (ppb) or sub-ppb range. This complication is further accentuated if the material to be analyzed contains a mixture of trace metals, since separation of the complex response due to the sample from that produced by contaminants in the base, or supporting, electrolyte is difficult, and substantially compromises analytical speed and convenience. Absolute reagent purity is a virtually unattainable ideal. Even with the purest of available chemicals and conscientious attention to problems of laboratory handling and storage, ad hoc contaminant levels in the ASV test solution may far exceed those in the original sample. A frequently employed, straightforward method of reducing the effect of background interferences involves the use of extremely dilute supporting electrolytes and modern, three-electrode instrumentation with provision for IR compensation. If more concentrated base electrolytes are required for the analysis step, it is customary to clean up such solutions beforehand. using controlled-potential electrolysis at a mercury pool cathode (2). In a recent study of the phase-selective anodic stripping analysis of gallium, we were confronted with the need to substantially reduce trace metal impurities from stock supporting electrolytes 2.OM to 6.0M in NaSCN. Concentrated thiocyanate is required in order to obtain polarographic reversibility of gallium ( 3 ) , which ruled out using a very dilute salt as supporting electrolyte. Purification by (1) E Barendrecht in "Electroanalytical Chemistry-A Series of Advances,'' Vol. 2, A. J. Bard, Ed., Marcel Dekker, New York, N.Y., 1967, p 53. (2) J. J. Lingane. "Electroanalytical Chemistry," Interscience, New York, N . Y . . 1958. (31 E 0 . Moorhead, J . Amer. Chem. SOC..87, 2503 (1965).
controlled-potential electrolysis proved to be an unsuitable solution in this case because of the appearance of anode-generated thiocyanate oxidation products. The long electrolysis period required for purification produced yellow oxidation products even when the anode compartment was well isolated using a fine porosity glass frit diaphragm. To circumvent these difficulties, an alternative purification approach was adopted in which the concentrated thiocyanate, contaminated primarily with Zn, Pb, Cd, and Cu, was passed through a 6-mm X 41-mm glass column packed with 7.16 cm3 (wet) of Corning CPG-550 microporous glass beads, the surface area of which contained a coating of immobilized 8-hydroxyquinoline ("oxine") as metal chelating agent (Pierce Chemical Co.). The CPGoxine had a pore diameter, average particle diameter, and surface area of 550 A, 177-840 micrometers, and 70 m2 @-I, respectively; its exchange capacity in milliequivalents of amine per gram was 0.03. As in the case of bulk separations of metals using oxine, retention of metals by immobilized oxine appears to be strongly pH dependent. Following the manufacturer's recommendations, the column packing was first conditioned by elution with 1.OM HC1 in an attempt to displace any residual metal, and then flushed with ca. 50 ml of quadruply-distilled water. In a trial run, a neutral solution 2.OM in NaSCN was passed through the column. Analysis of the effluent using the phase-selective ASV technique ( 4 ) revealed a substantial increase in Cu content, with Pb, Cd, and Zn essentially unchanged. It is the manufacturer's recommendation that alkaline solutions, especially strongly alkaline solutions, should not be employed with the immobilized oxine if quantitative regeneration of separated metal is to be expected. As this was not an important consideration in the present application, it was decided to operate the column at pH > 7 to maximize retention of the trace metal constituents. ( 4 ) E. 0. Moorhead and P. H . Davis, Anal. Chem.,45, 2178 (1973)
A N A L Y T I C A L C H E M I S T R Y , VOL. 46, NO. 12, OCTOBER 1974
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GO
-320
-040
-063 -360 VOLTS v s A g / A g C I
-,c
Figure 1. The effect of glass-immobilized 8-hydroxyquinoline on
removal of trace metals from concentrated NaSCN as measured by phase-selective anodic stripping Solution: 2.OM NaSCN. Frequency = 50 Hz; applied voltage = 10 mV (rms); plating time = 5.0 min at - 1 . 1 5 volts vs. Ag/AgCI; rest period = 30 sec; electrode area = 0.041 cm2; T = 303°K
Elution Procedure. After elution with a 25-ml volume of 1.OM HCl, the fresh column packing was washed with a 100-ml volume of quadruply-distilled water. An unbuffered solution 6.OM in NaSCN was then adjusted to pH 9 with NaOH and placed in an aspirator bottle. This reservoir was then connected to the head of the column through a glass needle-septum arrangement. Initial aliquots of effluent gave a measured pH of 6.4. However, the pH gradually increased with time, and after passage of 450 ml of 6.0M NaSCN, the pH had risen to 8.5. In this initial step of purification, the first 25 ml of effluent were discarded and the remaining 425-m1 volume collected in a receiver protected from laboratory air and dust. After a second adjustment with NaOH to increase its pH to 9.0, the solution collected from the first run was passed through the column again-and once again the first 25 ml of effluent were discarded. The level of trace impurities in both the original and twice purified NaSCN was monitored [after dilution from 6.OM to 2.OMI with ac phase-selective anodic stripping
1880
voltammetry using a Kemula-type micrometer hanging mercury drop electrode [Metrohm E-4101. Figure 1 provides a comparison of results obtained for the diluted solution. Curve A depicts impurity peaks which were obtained for the unpurified NaSCN. The addition to this solution of known metal ions tentatively identified the grouping of peaks a t -0.5 volt us. Ag/AgCl as due to Pb, Cu, and Cd, while the peak at -1.0 volt is due to zinc. In view of the care taken with these solutions and the use of quadruply distilled water, the curve A peaks are probably representative of the intrinsic heavymetal impurity in the NaSCN salt. From a standard addition experiment, the zinc impurity in the solid salts was estimated to be 81 ppb. On the other hand, curve B shows the result of passing this same solution twice through the bed of CPG-oxine beads. Virtually no evidence remains of the trace impurities a t -0.5 volt us. Ag/AgCl, and there is evidence of considerable reduction in the trace zinc at - 1.0 volt. Unlike ion exchange resins whose swelling characteristics normally vary considerably with ionic strength, the microporous glass packing appeared in this sense to be unaffected physically by use of concentrated salt. The material, which ordinarily appears a light ruby red in color, turned darker red in the presence of alkaline thiocyanate. Elution with HC1 restored the material to its original color. The packing seemed unaffected by alkaline pH, although its ability to exchange metal quantitatively was untested. In the present study, elution with 6.OM NaSCN resulted in the very slow flow rate of 25 ml h r 1 . Rather than working as a disadvantage, this enabled a system to be set up to process large quantities of material unattended, and obviated the usual necessity to commit electrochemical cells and potentiostats for prolonged periods. From our exploratory use of the glass-immobilized oxine, we conclude that the material represents a simple, convenient, alternative device for clearing supporting electrolytes of unwanted trace metals, and should prove useful for preparing stocks of alkali metal base electrolytes. RECEIVED for review February 22, 1974. Accepted April 15, 1974.
ANALYTICAL CHEMISTRY, VOL. 46, NO. 12, OCTOBER 1974
