strongly retained species K > 1 is a condensed species or a mixture of condensed species. This is also substantiated by the UV absorption spectra of the fractions and the relative ratio of molybdenum in the two peaks, as it is known that condensation is accelerated by higher acidities. The separation observed cannot be ascribed to gel filtration as was the case for phosphates (3) where the elution pattern followed that shown by organic molecules. Separation into two species appears definite; if this were not so, a single peak should have been obtained under all circumstances. Sorption chromatography appears to be the
principal process. This has also been noted for organic cornpounds such as benzyl alcohol, phenol, and arginine hydrochloride ( I ) . ACKNOWLEDGMENT
The authors gratefully acknowledge technical assistance by Peter Neddermeyer and Ronald Majors in this work.
RECEIVED for review August 7, 1967. Accepted January 8, 1968. Supported in part by U. S. Atomic Energy Contract AT(11-1)-1222 with the Purdue Research Foundation.
Electrochemical Behavior of Cuprous Ion in a Noncomplexing Aqueous Medium Judy A. Altermatt and Stanley E. Manahan' Department of Chemistry, The University of Missouri, Columbia, Mo. 65201
THEFOLLOWING potentials ( I ) : CuH
+ 2e = Cu
Cuf 3. e = C u
+ e = Cu+ Cu2+ + 2e = Cu(Hg) Cu2+
Hg
Cu+ 3. e
Hg =
Cu(Hg)
Eo = +0.095 V
US.
SCE
(1)
IF' = $0.279 V US. SCE (2) Eo = -0.089 V US. SCE (3) Eo = $0.035 V US. SCE (4) Eo = 3.0.146 V US. SCE (5)
explain the electrochemical behavior of copper in a noncomplexing aqueous medium and account for the instability of cuprous ion in such a medium. Because Eo for the Cu+-Cu(Hg) couple is more positive than Eo for the Cu*+-Cu+ couple, cupric ion in water is reduced at the dropping mercury electrode directly to Cu(Hg) by a one-step, two-electron process (Figure 1, curve A ) . Furthermore, the equilibrium constant for the disproportionation reaction 2Cu+ = Cu2+ Cu has the value 1.7 X lo6,which means that the concentration of cuprous ion in equilibrium with cupric ion and copper metal is quite low. Copper(1) as the solvated ion is stable in a number of nonaqueous solvents-e.g., acetonitrile (2) and nitromethane (I). The stability of copper(I) in such solvents is due to the fact that these solvents solvate copper(I1) less strongly than does water and/or they solvate copper(I) more strongly than does water. Methods have recently been developed (3) for the preparation of solutions of copper(I) perchlorate in a noncomplexing aqueous 0.1M HCIOl medium. In such a medium, the disproportionation reaction does not take place immediately. This paper describes the electrochemical behavior of hydrated cuprous ion in aqueous solution. The electrochemical
+
1
Author to whom correspondence should be addressed,
(1) I. V. Nelson, R. C . Larson, and R. T. Iwamoto, J. Znorg. Nucl. Chem., 22, 279 (1961). (2) I. M. Kolthoff and J. F. Coetzee, J. Am. Chem. SOC.,79, 1852 (1957). (3) J. A. Altermatt and S . E. Manahan, Inorg. Nucl. Chem. Letters,
in press.
I
E vs.
SCE
Figure 1. Polarographic behavior of copper(1) and copper(ZI) in 0.100MHClO~ A . Copper(I1) reduction wave B. Composite anodic-cathodic wave for copper(1) C . Reduction wave for copper(1) in 1.00M acetonitrile D. Oxidation wave wave for copper(1) in 1.00Macetonitrile behavior of copper(1) in solutions containing ligands which complex copper(I) but which do not complex copper(I1) is also discussed. EXPERIMENTAL
Copper(I) was introduced into aqueous 0.100M HCIOI by shaking a deoxygenated mixture of 100 ml of 0.100M HC104, 5 ml of 0.1M copper(1) perchlorate in a methanol solution in which the copper(1) was stabilized by the presence of 1M 1,3-cyclooctadiene, and 50 ml of ether. The 1,3cyclooctadiene is extracted into the ether layer and the copper(I) remains in the aqueous phase. The 1,3-cyclooctadiene complex of copper(I) perchlorate was prepared by electrolysis, at copper electrodes, of a methanol solution of 1Molefin and 0.05Mcopper(II) perchlorate (4). The complex ion is formed at both the cathode and anode by the following reactions: (4) S . E. Manahan, Znorg. Chem., 5,2063 (1966). VOL. 40, NO. 3, MARCH 1968
655
+ x (olefin) + e = Cu (olefin),+ Cu + x (olefin) = Cu (olefin),+ + e
Cathode: Cu2+ Anode:
(6) (7)
Copper(1) was also obtained in aqueous solution by dissolving freshly precipitated copper(1) hydroxide in 0.100M HC104. The copper(1) hydroxide was prepared by adding tetrakis(acetonitrile)copper(I) perchlorate to aqueous NaOH. Disproportionation frequently results when copper(I) hydroxide is dissolved in perchloric acid, but it may usually be avoided by transferring the solid to the acidic solution with a glass spatula followed by rapid stirring of the solution. The copper(1) disproportionates less rapidly at lower temperatures, therefore, the solutions were maintained at 0' C at all times and the electrochemical data were taken on solutions at 0' C . Copper(1) solutions maintained at 0' were stored for up to 10 hours before disproportionation took place. It was necessary that all glassware be very clean, because any trace of metal brings about the immediate disproportionation of copper(1). The polarographic data were taken with a Sargent Model XV polarograph. The hanging mercury drop electrode was prepared by the electrolytic deposition of mercury from a mercuric chloride solution onto a length of platinum wire which protruded just slightly from a glass tube into which it was sealed.
RESULTS AND DISCUSSION Of the two methods for getting copper(1) into aqueous solution, the extraction procedure was the more satisfactory in that it never caused disproportionation to take place. Although some methanol remains in the aqueous phase when the extraction procedure is used, methanol does not complex either copper(1) or copper(I1) and its presence in low concentrations should have a negligible effect upon the electrochemical behavior observed. The electrochemical behavior of the solutions prepared from copper(1) hydroxide was identical in all respects to that of the solutions prepared from the olefin complex. The polarographic reduction wave for cupric ion in 0.100M HCIOa is shown in Figure 1, curve A . It is, of course, a cathodic wave with a half-wave potential of +0.04 V us. SCE corresponding to the one-step, two-electron reduction as follows : Cu2+
+ 2e Hg= Cu(Hg)
The polarographic wave obtained for a solution of cuprous ion in the same medium has the same half-wave potential of $0.04 V us. SCE, but is half anodic and half cathodic (Figure 1, curve B). At the half-wave potential, the potential of the dropping mercury electrode is more negative than Eo for the Cu+-Cu(Hg) couple; hence, cuprous ion may be reduced at the electrode surface. Furthermore, the potential of the DME is more positive than Eo for the Cu2+-Cu+ couple which means that the hydrated copper(I) ion may be oxidized to cupric ion at the electrode surface. Since the half-wave potential corresponds with zero current, the reduction and oxidation reactions Cu+
+ e H=" Cu(Hg)
cu+ H=p cu2+ + e
(9)
(10)
take place to an equal extent, with the net reaction being the disproportionation of copper(1) at the mercury surface as follows : 656
ANALYTICAL CHEMISTRY
2 Cu+ "=" Cu2+
+ Cu(Hg)
(11) At Elizin this case, the concentrations of cupric ion and copper amalgam at the electrode surface, [Cu2+]0 and [Cu(Hg)Jo, are equal. From the Nernst equation
it may be seen that El/t is essentially equal to EO for the Cu2f -Cu(Hg) couple. At potentials more positive than El,*, the Nernst equation requires that the concentration of cupric ion exceed the concentration of Cu(Hg) and, therefore, more of the copper(1) is oxidized than is reduced and the anodic portion of the wave results. At potentials more negative than E L ~the Z , concentration of Cu(Hg) must exceed the concentration of cupric ion, the reduction of copper(1) predominates, and the cathodic portion of the polarographic wave is produced. The polarographic behavior of copper(1) in a noncomplexing aqueous medium is, therefore, analogous to the polarographic behavior of copper(I1) at the amalgam electrode in the same medium. A polarogram identical to the one shown in Figure 1, curve A , may be obtained from polarographic analysis at a dropping copper amalgam electrode of a solution of cupric ion in 0.100MHC1O4 where the relative concentrations of cupric ion and amalgam are such that the cathodic and anodic portions of the wave are of equal height. The above explanation of the polarographic behavior of hydrated copper(1) ion requires that hydrated copper(1) disproportionate at an uncharged mercury surface. If a solution of copper(1) is shaken with a small quantity of mercury for a short time, no evidence of copper(1) can be found in solution. Further evidence of the reaction may be obtained from stripping analysis. A mercury drop electrode was suspended under open circuit in a stirred solution of copper(1) for 5 minutes. The electrode was then transferred to a solution of pure electrolyte, and a potential of +0.1 V us. SCEsufficiently positive to strip copper from the mercury surfacewas applied. A high stripping current was observed from the copper amalgam formed at the electrode surface. The disproportionation of copper(1) at a platinum surface was observed by rotating a platinum electrode in a solution of copper(1). The presence of a copper deposit was confirmed by visual observation under a microscope and by stripping analysis. Further evidence for the presence of copper(1) in solution was obtained by theaddition of ligands which complex copper (I) but do not complex copper(I1). Acetonitrile forms complexes with copper(1) and in sufficiently high concentrations stabilizes the +1 oxidation state in aqueous solution (5). Acetonitrile was added to a copper(1) solution to make it 1.OM in acetonitrile. The polarogram of the resulting solution is split and consists of a cathodic wave at -0.12 V us. SCE (Figure 1, curve C) and an anodic wave at +0.21 V us. SCE (Figure 1, curve D). It is identical in form to the waves obtained (5) from the reduction of copper (11) at the DME or from the oxidation of Cu(Hg) at the dropping copper amalgam electrode in a 1.OM acetonitrile medium, with the exception that in the former case, both waves are cathodic, whereas in the latter case both waves are anodic. The reason for the split polarographic wave in the presence of acetonitrile is obvious from examination of the Nernst equation for the Cuz+-Cu+ couple (Equation 13) and for the Cu+-Cu(Hg) couple (Equation 14). ( 5 ) S. E. Manahan and R. T. Iwamoto, J. Electroanal. Chem., 14, 213 (1967).
Anodic:
Acetonitrile complexes copper(1) but not copper(I1); therefore, the potential of the Cu(1)-Cu(Hg) couple is shifted negatively to the same extent that the Cu(11)-Cu(1) couple is shifted positively and the result is a split wave at sufficiently high acetonitrile concentrations. The cathodic and anodic waves obtained for copper(1) solutions in the presence of acetonitrile are due to the following reactions : Cathodic: Cu(CH3CN),+
+ e “=“ WHg)
+ x(CH3CN)
(15)
Cu(CH3CN),+
”=” Cuz+ + x(CH8CN) + e
(16)
The split wave, half anodic and half cathodic, was obtained for solutions in which the acetonitrile was added up to 8 hours after the preparation of copper(I), which is further evidence that hydrated copper(1) ion is stable in aqueous solution for prolonged periods of time. A similar split wave was observed from the solution obtained by saturation of the copper (I) solution with ethylene. In this case the +1 oxidation state of copper is stabilized by complexation of the copper(1) with the olefin. RECEIVED for review September 22, 1967. Accepted December 11, 1967. This work was supported by a grant from NASA awarded through the University of Missouri Space Sciences Research Center. Miss Altermatt gratefully acknowledges the support of an NSF Undergraduate Research Participation Fellowship.
A Micro Method for Determining Silicon in Organosilicon Compounds by a Modified Oxygen Flask Method James E. Burroughs, William G. Kator, and Alan I. Attia R . C . Ingersoll Research Center, Borg- Warner Corp., Des Plaines, Ill. 60018
REVERCHON AND LEGRAND ( I ) described a method for the microdetermination of silicon in organosilicon compounds by the use of a modified Schoniger technique. This procedure required specialized apparatus and was too time-consuming for routine analyses. Furthermore, the use of sodium peroxide as a flux was undesirable because many research-type samples often contain components which could spontaneously explode when mixed with this powerful oxidizing agent. To circumvent these disadvantages, as well as those of other methods for determining silicon (2,3), a simple, unique, and practical technique was developed. It involves the direct conversion of the silicon to a volatile, readily-hydrolyzed, fluorinated species by direct combustion of the organosilicon compound in the presence of a fluorocarbon. EXPERIMENTAL The combustion flask consisted of a 1-quart polyethylene bottle (Fisher Scientific Co., Cat. No. 2-923). No deterioration of this container was observed at any time during these studies. To support the platinum sample carrier basket (A. H. Thomas Co., Cat. No. 6471-Q), a length of 24-gauge platinum wire (ca. 12 cm) was inserted in the bottom of a No. 4 rubber stopper such that the sample basket would be centered in the bottle during combustion. A Beckman Model B spectrophotometer was used at Sensitivity 2. Reagents. The 6N NaOH solution was prepared by dilution of a 50% NaOH reagent solution (Fisher Scientific Co., Apparatus.
(1) R. Reverchon and Y. Legrand, Chim. Analytique, 47,194 (1965). (2) I. M. Kolthoff and P. J. Elving, “Treatise on Analytical Chemistry,” Vol. 11, Part 11, Interscience, New York, 1965, pp 107-206. (3) J. A. McHard, “Anal. Chem. of Polymers,” XII, Interscience, New York, 1959, Chapter XIV.
Cat. No. So-S-254) which was found to be essentially free of silicon. A 10% (w/v) ammonium molybdate solution was prepared from reagent grade ( N H ~ ) ~ M o ~ O ~ ~ . All ~HZO. reagents were prepared and stored in polyethylene bottles to prevent silicon contamination. Polytetrafluoroethylene powder (Halon TFE, Type G80, Allied Chemical Co.) was used as received because fluorine analysis (4) indicated a high degree of purity. Reagent grade NazSiOa+9Hz0was used for preparing the calibration curve, while octaphenylcyclotetrasiloxane (NBS Standard No. 1066), containing 14.1 silicon, was used in establishing the accuracy of the method. Procedure. Solid samples were transferred into the center of tared, prefolded Schoniger-type sample wrappers and liquids into tared No. 5 gelatin capsules (available from Arthur H. Thomas Co., Philadelphia, Pa.). Sample weights were obtained by difference. Sample sizes were taken such that the amount of silicon was between 0.2 and 1.5 mg. TO the sample was added an amount of PTFE powder such that a weight ratio of approximately 50:l existed between the amount of PTFE powder and the silicon present in the sample. The sample was loosely wrapped in the conventional manner (5) in the sample wrapper and mounted in the platinum basket. Ten milliliters of 6N NaOH and 2 ml of 3 z H ~ O were Z introduced into the polyethylene bottle, and the bottle was immersed into a large stainless steel beiker containing cold water. The bottle was then flushed rapidly with pure oxygen for 30 to 60 seconds. With the sample carrier supported on the platinum wire, the paper sample flag was ignited manually,
z
(4) R. N. Rogers and S . K. Yasuda, ANAL.CHEM., 31, 616 (1959). (5) C. N. Reilley, “Advances in Analytical Chemistry and Instrumentation,” Vol. IV, Interscience, New York, 1965, pp 75-1 16. VOL. 40, NO. 3, MARCH 1968
657
