John T. Stock University of Connecticut Storrs, 06268
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hperometric Monitoring- of iron(M) in bn:~xthanp Separation from Cobd+(ll)
The elution of a solute snecies from a liouid-chromatographic or ion-exhange column can be monitored either continuouslv or bv the collection and examination of successive fractions bf the eluent. Because column separations are commonly introduced by analogy with countercurrent extraction / I ) , the discontinuous approach is pedagogically appealing. However, continuous monitoring allows the elution to be detected while actually occurring, is obviously more rapid, and is the approach used in nearIv all routine annlications. Heilley and'sawyer 12, have described the separation of cobalt(I1) and in1nlII11 in rhloride-containlna solution by passage through a column of anion-exchange resin. his column is formed in the lower portion of a 50-ml buret. Cobalt, which emerges first, and iron are determined by the addition of suitable color-producing reagents to the successive fractions of the eluate. The present modification retains the very simple discontinuous method for cobalt (2), but employs an amperometric flow-through cell for the continuous monitoring of iron. Besides the continuous and the discontinuous approaches, two completely different determinative techniques are thus illustrated. In suitable media, iron(II1) gives a limiting current a t a rotatine ~ l a t i n u melectrode (RPE) that is maintained at zero p&e&ial with respect to thesaturated calomel electrode (SCE) (3). No interference is caused hy cohalt(II), which is not electmactive under these conditions. The RPE is not very convenient for use in a small flow-through cell, but a fixed electrode-stirred solution system can be
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easily constructed. If suitably designed, this type of system can yield current-concentration curves of surprisingly good linearity (4). The compact reference system Ag, AgCl(satd.1 in KCl(satd.) has a potential that is approximately 50 mV more negative than that of the SCE and can be used in place of the SCE. The flow-through system is shown schematically in Figure 1; the four holes in the ~ h b e stopper r lie on a circle, rather than in line. A 20-mm 0.d. specimen tube cut down to 25 mm high forms the cell and contains the 10-mm long micro stirrer bar. This is merely a piece broken from a sewing needle and sealed within melting-point tubing. All of the glass tubes in the stopper are of 4.5 mm 0.d. Inlet tube A terminates a few mm above indicator electrode B. This is a 20-mm length of 0.5-mm diameter platinum wire that is sealed axially through the end of its support tuhe. The projecting 10 mm of wire is then bent at right angles, as shown. Connection is made by a copper wire that dips into a short column of mercury in contact with the inner end of the platinum wire. T o prevent accidental spillage of mercury, the copper wire is wedged with a wood splinter and sealed in with a bead of epoxy cement. The extremities of salt bridge tip C are constricted to 2 mm i.d. This helps to retain the 4% agar-agar in saturated KC1 filling of the tip. By repeated gentle squeezing, all air bubbles are expelled from the rubber tuhe that connects the tip to the saturated KC1 reservoir D. This contains a 15-cm length of approximately 0.7-mm diameter silver wire that is coiled and bent as shown. Before insertion into D, the coil is anodized in KC1 solution to produce a light coating of AgC1. If the usual large-area amperometric SCE (5) is to he used in place of the Ag-AgC1 system, the tip of the SCE salt bridge is merely inserted into the KC1 solution in D. Entrapment of air in the cell is avoid-
I-) Figure 1. Amperametric tlow-through cell.
Figure 2. Elution curveof
iran(ll1). Volume 5 1, Number 7. July 1974
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ed by making the lower end of exit tube E flush with the underside of the stopper. Although the flow-through cell can he permanently attached to the ion-exchange column, it is simpler to attach the system after the cobalt fractions have been collected directly from the column outlet. The cell, which has been filled with 0.1 M HCI, is then attached, and the magnetic stirrer is adiusted until the tinv bar snins smoothlv. Although the Eurrent increases with increase in temperature and with the rate of rotation of the bar. the current is essentially unaffected by moderate changes in the rate of flow through the cell. However, it is desirable to elute a t an essentially constant rate of approximately 1 ml/min. The buret stoocock can be modified to facilitate fine control (6). Figure 2 shows a typical iron(III) elution curve. This was obtained from a 23-cm column length of Dowex 1-X8 50-100 mesh resin and a l-ml sample of 4 M HC1 that contained 1.5 mg of cobalt(II) and 0.5 mg of iron(II1). The cobalt(Il) was eluted with 28 ml of 4 M HCI, then the eluant was changed to water (2) and the flow-through cell
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was attached. Current readings were obtained merely by connecting the electrodes to a microammeter (platinum to positive post). A recorder, useful when the eluant is being pumped at constant rate, is of little value with a simple gravity feed system. If desired the cell may he calibrated, so that the amount of iron can be compared with the amount placed on the column. First, 0.1 M HC1 is allowed to flow through the cell to establish the small and essentially constant residual current. Passage through the cell of a few standard solutions of iron(IJI) (
