Electrochemical Fractionation: Potentiostatic Chromatography and Elution Voltammetry Counter
Sir: The rapid expansion of separation methods based upon countercurrent fractionation does not appear to have yet included a successful application of continuous partitioning between a flowing solution and a stationary electrode, although the principle was
Electrode
Porous Vycor
Tube
Counter Reference E lectrode -
suggested by Rogers over 12 years ago (6). Electrochemical separations are clearly a part of the general technique based upon heterogeneous equilibria and chromatographic-type operation of an electrochemical cell should be possible. This communication review's some recent developments of column electrolysis both from the literature and
from this laboratory. Two additional techniques are suggested on the basis of analogies between column chromatography and electroanalytical methods. Batch electrochemical separations— i.e., controlled potential coulometry— rely upon very large partition ratios, Kp Cr/C„, w'here Cr is the concentration in the electrode phase and C0 is the concentration in the solution phase. When the deposition is an unalloyed 1. The partition ratio solid metal, Cr is simply Kp E'°). where exp (E is nF/RT, E is the applied potential, and E'° is the formal potential which includes activity and complexation effects. As is well established, KT can be changed over many orders of magnitude by changing E and E'°. When two species are present, the separation factor, =
Figure 1. lector cell
Column electrode and de-
=
=
can
—
usually be made very favorable
so
that quantitative coulometric operation is possible.
Improved electrochemical separations through multiple extractions or countercurrent fractionation does not appear to be necessary from the above considerations, except in occasional cases. Howrever, by analogy with column chromatographic techniques and considering the ease
with which the partition ratios
can
dimension of versatility of operation can be visualized in many electroanalytical methods through the use of column electrodes and flowing solutions. The major difficulty in the design and preparation of the electrolytic column is the electrode w'hich functions as the stationary phase. High surface to volume ratios and dimensionally thin phases must be obtained, in addition to the usual requirements of electrode inertness, electrical contact, and reasonable voltage range. Mercury would be be changed, a new'
ideal material for many systems, but its high surface tension necessitates surface amalgamation of finely divided metals in order to maintain it as a thin layer. A cell which meets many of the requirements is shown in Figure 1. The stationary phase is amalgamated nickel particles of ca. 0.1 mm. average size. Because of the very low solubility of nickel in mercury, this electrode behaves (5) very much like amalgamated platinum (4). Its potentialities as an indicator electrode for general voltammetric use are being examined. The internal resistance of the cell is about 2 ohms with \M NaCl as the flowing phase. The very high dependence of the partition ratio upon electrode potential requires that %R drops be kept to an absolute minimum. Only one counter electrode is shown in the figure, but a second can be positioned around the outside of the amalgamated electrode, separated by an additional porous Vycor tube. Many column cell designs are possible, depending upon application, such as capillary columns with a concentric, amalgamated wire for very small The detector is a simple samples. voltammetric cell with a small mercury pool electrode, over which the eluent flows. It is used to measure limiting currents; it can easily detect 10~10 equivalents/sec. (10“6J/ solution at flow' rate of 10-4 l./sec.). an
A number of alternate electrode materials is available, including lead and noble metal granules as well as other amalgamated metals. Columns filled with crushed graphite have been successfully operated (I, 8) in the fashion of a variable potential Jones reductor. Complications arise w'hen metals are codeposited on graphite (1). However amalgamated metals also can give rise to “irreversible” deposition and dissolution, possibly due to intermetallic compound formation. Crystalline mercury selenide, which has been found to be usable as an indicator electrode (?), is a further possibility. At this time it appears that the major obstacle is the electrode material; the need for studies of the mechanisms of deposition and dissolution is apparent. Methods of operation of column electrodes are indicated in general by chromatographic methods. Similar to ion exchange, metal ions can be deposited and then selectively removed by anodic stripping (elution). Stedman .(8) described the deposition and subsequent removal of copper from crushed graphite. Blaedel and Strohl (1) have further described this technique and w'ere able to extend it to separations involving several metals which were deposited on crushed graphite from a sample (0.1 ml, O.Ol.H in each metal ion) injected into the flow'ing solution. An extension of the above technique, and perhaps analytically more useful, would be the concentration of trace quantities of reducible metal ions from relatively large volumes of solutions. After deposition the elution can be performed with a linear voltage sweep, provided the anodic process is free of the complications noted above. This procedure which can be termed elution voltammetry, effectively increases the signal to noise ratio (faradaic to residual current) compared to coulometry and amperometry. Since flowing electrolyte solutions of high purity are required for elution, they can be pre-electrolyzed in a separate column. This method of preelectrolysis could be of general use in the purification of electrolyte solutions; the very high electrode area to solution volume ratio in columns w'ould expedite the process by several orders of magnitude compared to conventional methods. When the separation factor is small so that adequate separation is not obtained in elution voltammetry, fractionation can be used. The sample is carried through a column electrode held at an intermediate potential. It may prove to be possible to overcome irreversibility due to codeposition by superimposing a VOL. 36, NO. 12, NOVEMBER 1964
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2371
signal on the d.c. potential. The retention volume will be proportional to the partition ratio, as in other chromatographic methods. This method can be given the descriptive name of potentiostatic chromatography. A further possibility is fractionation on electrode through’ adsorption surfaces, since this is potential-dependent. In addition to selective adsorption of organic solutes, which is probably trivial except as a purification technique, separation of anions appears to be an attractive possibility. The
specific adsorption of many anions on mercury has been measured (2, 8), and it is a highly reversible process.
sine wave
LITERATURE CITED
(6) Rogers, L. B., J. Electrochem. Soc. 99, 267 (1952). (7) Sherwood, Peter, S.B. Thesis, Mass. Institute of Technology, 1964. (8) Stedman, H. S., S.M. Thesis, Mass. Institute of Technology, 1960. D. K. Roe
(1) Blaedel, W. J., Strohl, J. H., Anal.
Chem. 36, 1245 (1964). (2) Grahame, D. C., J. Am. Chem. Soc. 74, 4422 (1952). (3) Grahame, D. C., Soderberg, Barbara A., J. Chem. Phys. 22, 449 (1954). (4) Ramaley, L., Brubaker, R. L., Enke, C. G., Anal. Chem. 35, 1088 (1963). (5) Roe, D. K., Toni, J. E. A., unpublished data, 1964.
Department of Chemistry and Laboratory for Nuclear Science Massachusetts Institute of Technology Cambridge, Mass. 02139
This work was supported in part by the U. S. Atomic Energy Commission under Contract AT(30-l)-905.
X-Ray Fluorescence Spectrometric Analysis of Rubidium(l) and Cesium(l) Salts of 5-Nitrobarbituric Acid (Eastman Kodak Co.) (/). Dilituric acid, 0.014/, was prepared by diluting the 0.054/ solution with 50% ethanol.
Sir: Previously the analysis of a mixture of several metal-organic compounds employing x-ray fluorescence analysis has been reported (2, 8). It was of interest to investigate whether this method could be extended to the determination of a mixture of rubidium and cesium ions which are normally difficult to separate and determine. In a previous paper (4) it has been reported that 5-nitrobarbituric acid (dilituric acid) quantitatively precipitates rubidium (I) and cesium (I) as RbC4H205N3 and CsC4N205N3 and that the salts are thermally stable. Since the precipitation is quantitative, it was thought that the two elements could be determined simultaneously by using the ratio plot technique developed previously (2).
Metal Ion Solutions. A primary standard solution of 0.005004/ rubidium was prepared from c.p. rubidium chloride, Fairmount Chemical Co., which had been dried at 105° C. A primary standard solution of 0.005024/ cesium was prepared from c.p. cesium nitrate, Fairmount Chemical Co., which had been dried at 105° C. Preparation of Rubidium and Cesium Salts. These standard solutions were used to prepare solution mixtures of the proper mole perThe rubidium centages (Table I). and cesium mixtures were precipitated with dilituric acid, heated and weighed on the thermo balance (4). Analysis. The x-ray apparatus has been described before (2, 3). The x-ray spectrometer was operated at 50 kv. and 50 ma. with a LiF analyzing crystal and a scintillation counter operated at 1130 volts. The analyses were conducted as described previously (2, 3). The Ka¡Ka2 line of rubidium was selected for analysis at 26.64° 29 and the Leu line of cesium at 91.77° 29.
EXPERIMENTAL
Acid. SatuReagents. Dilituric rated dilituric acid solution (0.054/) in 50% ethanol was prepared from recrystallized White Label dilituric acid
Analysis by X-Ray Fluorescence Method Thermogravimetric fluorescence analysis _X-ray analysis, total weight of diliturate Areas of peaks, of metal Quantity taken_ Ratios of peak areas mg. of paper Mole % Mg. Mg. Mg. of Mg. of found Rb Rb caled. Cs Cs Cs Rb/Cs Cs/Rb 30.3 2.38 30.2 9.2 21.9 0.43 95.0 12.7 0.4 2.1 21.6 0.48 30.0 30.0 45.3 90.0 12.0 0.9
Table
Mole % Rb 5.0 10.0 15.0 20.0 25.0 30.0
2372
·
RESULTS AND DISCUSSION
The Ka¡Ka¡ peak of rubidium was selected for this analysis because it had the strongest peak and had no interferences. The La¡ peak of cesium was selected because there were no interferences and also because the high atomic number of cesium makes the use of the K lines impractical. The same procedure as used previously (2) was followed in obtaining experimental data and calculating results. The proper settings of the electronic circuit panel were made to keep the peaks on the chart paper when scanned at 0.25° per minute. The areas under the rubidium and cesium peaks were evaluated by cutting out the peaks and weighing the paper. The paper weights of the peaks and the rubidium/ cesium and cesium/rubidium peak ratios are recorded in Table I. The ratios were plotted on semilog paper vs. the mole % rubidium and the mole % cesium. Curved lines were obtained. From these data the mole fractions of rubidium and cesium in a sample were
85.0 80.0 75.0 70.0
1.3 1.7 2.1 2.6
ANALYTICAL CHEMISTRY
I.
11.3 10.7 10.0 9.3
29.9 29.6 29.4 29.2
29.8 29.6 29.4 29.2
64.0 40,6 97.6 165.7
12.9 7.4 11.8
12 0
4.9 5.5 8.3 13.8
0.20 0.18 0.12 0.07
