A Flow-Through Cell for Continuous Reduction with an Electrochemically Prepared Alkali Amalgam Fritz Scholz and Fritz Pragst Humboldt University, Hessische Str. 1-2, Berlin 1040, GDR The reduction of organic and inorganic compounds with alkali amalgams is a well-established technique (1-2). Recently, a new kind of mercury electrode was introduced for electrochemical detection in liquid chromatography and flow injection analysis 13.41, sirnultaneoua electrochemistry and ESR specrroscopy (5,6 ) , electrolytic purification (71, and oreilnic 18). The nrincioles of these so-called - - - - ~ -svnthesis ~~~ bubble electrodes cons& in the flow of a solution in the form of bubbles through a pool of mercury with the surface of the bubbles forming the active electrode surface. Here we describe a cell that operates on essentially the same basis and that uses an electrochemically generated alkali amalgam for the reduction step. Many principles of chemistry can be taught from the experiment. The cell (Fig. 1) consists of two combined parts: part A houses the electrolysis section of the experiment. The mercury (-60 mL) in section A is covered by an aqueous sodium hydroxide solution. A platinum counter electrode is placed in the solution and a constant current electrolysis provides continuous formation of sodium amalgam. In part B of the cell the reduction proceeds by pumping the solution in the form of hubbles through the porous glass frit and then through the amalgam. This part of the cell includes a pump and ensures (1) circulation of the liquid amalgam between the left (A)
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and right (B) part of the cell via glass tubes (see Fig. 1) and (2) a very efficient reduction of the solute. The cell design ensures that no mixing occurs between the cycled solution and the sodium hydroxide solution. This is especially important when a nonaqueous solvent is used to dissolve the compound to be reduced. Figure 2 illustrates the decrease of the amount of startinr! material for the reduction of D-hvdroxvhenzaldehyde to~l,2-di-1~-hydroxyphen~l~-l,2-dih;arox;~ethaneu,ith time (R1.A 10 M NaOH solution u,as used in nart A of the cell, and a constant current of 1 A provided formation of the amalgam. The broken line in Figure 2 represents the theoretical dependence assuming that all current is consumed for the one-electron-reduction of p-hydroxybenzaldehyde. The coincidence of theoretical and experimental points (concentration of p-hydroxybenzaldehyde was followed by dc polarography) is an indication of a very fast reaction between the sodium amalgam and the p-hydroxyhenzaldehyde. When sulfuric acid is cycled through the amalgam instead of the methanol solution, a simple neutralization of the acid occurs. A plot similar to Figure 2 reveals that the measured concentration of sulfuric acid is always
Figure 2. Dependence of concentration of phydraxybenzaldehydeIn methanol Fig~re1 F ow-thra~ghcell tor cont8n~ousred~ctton w man eisnrachemcal y prepared aika 8 amalgam 1 smtered glass d s*. Ppex. poror ry 3 or 4 For further explanation see the text.
(15.6 g/L) on eiectrdysis time. The solution was cycled through the amalgam with aflow rate of 1.6 mL s-'. The sodium amalgam was formed by electrolysis of 10 M NaOH with i = 1 A. The broken line gives the theoretical dependence according to Faraday's law.
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September 1990
805
lower than the theoretical values, which can be attributed to a slow reaction of the sodium with protons (high overpotential of hydrogen generation a t mercury). In summary, the cell is very suitable for the efficient continuous reduction of compounds with alkali amalgams. The cell can be used in the undergraduate laboratory offering a range of possibilities to teach the fundamentals of electrolysis, amalgam reduction, and an opportunity to work with flow-through systems. Since mercury vapor is very poisonous the cell should be handled with appropriate care (9, 10) and used in a fume cupboard. Furthermore the mercury is always covered by solutions, which minimizes dangers of health hazard.
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Journal of Chemical Education
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Baize., M. M.: L""d, H., Ed. orgonic Elsclrochmistry-A" Infmduction and Guide: Dekker: New Yurk, 1983; ~ 8 2 9 . Examples in which sodium amalgam is "Bad as the reducingagent in organic swynfhesis are described in la) Hollemen. A. F. Or#. Synfh. Coil. 1941. 554: (bi Deulolen, V.: Guerrero. T. J. Orp. Synlh. Coil. 1955.111,586: (cl Renlow. W. B., Jr.; Hauser. C. R. Org. Synlh. Coil 1943,II. 607: Id1 Brason. W. R.; Hauser, C. R. Orp. Synlh. Coil. 1963,IV, 508;Chanp, C.T.: Roaenblum, M.; Simms. N. O w Synth. 1988.66, 95. Seholz. F.;Henrion, G.Z. Chem. 1985.24, 121-125. Schuln. F.; Henrion,G.: Wolll, M.: Kersten.D.Z. Chem 1984.24.39. Sfnesse?,R.;Schoh. F.:Pragsf. F.; Henrion, G.Z. Chem. 1985.25, 167. Bagchi.R.;Bond. A. M.:Schnh. F.;Stoesser.R. J . Eieclruanol. Chsm. 1988,245,1057,"
7. Sehulr. F.:Henrion.G.:Pragrt. F. Z. Chem. 1984.24, 197-198. 8. Pmgst, F.: Scholz, F.: Wuifke, P.; Kollek, V.: Heniion. G. J. Prakl. Chem 1985.327,
1028-1032. 9. Steore. N. V . . Ed. CRCD Handbook ofLoborotorySo/eis:Chem. Rubber Co.: Cleveland, 1971;pp334-341. lo. Pipitme. D. A . Ed. So/< Storape of Labornlory Chemicola. Wilcy: New York. 1984.
