676
R. L. BENOIT ASD P. CLERC
-1.01
///
/
Vol. 65
that below these limits an appreciable error is incurred, the error increasing with increasing T I f 72 as expected. Since, in all cases, the quantity Z / d D T 2 was within the limits for assuming infinite diffusion for cadmium, the error is due to the finite diffusion of thallous ion. Charing Current Distortion of Potential-Time Curves.-Figure 2 allows comparison of the distortions of the potential-time curves using constantcurrent and linear-current-scan techniques. It is apparent that the distortion is nearly the same in the two techniques in the vicinity of the transition time. As expected, however, the distortion in the initial portion of the potential-time curve is much greater n-ith the linear current-scan technique than with the constant-current technique. Acknowledgments.-A portion of this work was supported through Grant #G7333 from tha Nationd Scieiice Foundation. G.W. gratefully acknowledges n fellowship from Phillips Petroleum Company.
: 1.0
2.0
3.0
t i m e (sec.). Fig. 2.-Constant current and linear current-scan chronopotentiograms of 1.00 mM TlXOs. I, constant current, i = 100 pamp.; 11, h e a r current-scan, p = 50 pamp./sec.
CHLOROGERMAKIVX(IV) SPECIES IS ACID MEDIA BY R. L. BENOIT AND P. CLERC Ddpartement de Chimie, Uniaeraitk Laz'al, Qvkbec, Canada Receined November 23, I960
The solvent extraction method has been used to study the germanium(1T')-chloride systen in acid media. GeCL is the main germanium species extracted in CC4. The variation of the Ge distribution ratio with the solution compcsition is interpreted in terms of a series of GeCl,(OH),(H,O)k@++'-4)species. At constant levels of total HCl-HClO4 molality, the average value of z increases from 0 a t low HCl concentration to close to 4 a t the highest HCl Concentration. I n HCl-LiC1 solutions of constant total molality, the average value of j decreases nith increasing acidity from close to 4 to minimum values, these minimum values are in turn lower a t higher molality levels. In HC1 solutions, Ge02(H20)kis the main Ge species up to 5.6 m HCI; a t higher HCI concentrations, anionic complex species are present but complete formation of GeCl? IS not attamed.
stants of the MhL,(OH)j species in solution can be CC14 was chosen as solvent because it is nearly immiscible vith water, n poor solvent for HC1 and likely to extract GeC14 and give ideal solution.
Introduction The stability of the halide complexes of most metal ions decreases in the order I?- >> C1- > Br- > I-; the reverse sequence of stability holds for the complexes of a few transition and heavy metal ions and is attributed to dative T-bonding.'-3 Ge(1V) and Sn(1V) halide complexes apparently fall into the first class whilst Pb(IV) is expected to belong to the second class. As part of a study undertaken to establish the stability trends among the halide complexes of Ge(IV), Sn(1V) and Pb(IV), an investigation of the nature and stability of chlorogermanium(1V) species is reported. So far only qualitative predictions have been made as to the state of germanium(1V) species in hydrochloric solutions. 4,5 The solvent extraction method was selected to investigate the Ge(1V)-Cl- system. By establishing successively the influence of the metal, the ligand and the hydrogen ion concentrations on the metal distribution between solution and immiscible solvent, the formulas and stability coii-
Reagents.-An aqueous 4 X 10-*11f stork solution of germanium(1V) was plepared from 99.gCh germanium dioxide kindly provided by the Tsumeb Corporation. Reagent grade hydrochloric acid, perchloric arid and lithium chloride were used. Reagent grade carbon tetrachloride was employed without purification once it was found that identical results rere obtained vith the purified solvent Procedure.-Aqueous solutions of HCIOa, LiCl and HCl mere mixed and cooled in a separatory funnel, after which the Ge solution was added. A known volume of Cc4, in most eases equal to the volume of the aqueous phase, wa? added and the separatory funnel was shaken mechanjca11y for 60 minutes. Tests showed that distribution equilibrium was reached in 12 seconds. The phase separation was achieved by decantation and when necessary by centrifusation. The experiments were carried out a t 23 f 2.5. .4liquots of the aqueous phase were taken for analysis.
(1) B. G. F. Carleson and H. Irving, J . Chem. Soc., 4390 (1984). (2) S. Ahrland, Acta Chem. Scand., 10, 723 (196G). (3) 9. Ahrland. J. Cfiatt and N. R. Davies, Quart. Rev., 12, 265 (1958). (4) G. Brauer and H. Muller, 2. anorg. Chem., 287, 71 (195s). ( 5 ) D. A. Everest and J. C. Harrison, J . Ciiem. Soc., 1820 (1957).
( 6 ) II. (19.55). (7) R. ( 8 ) E. Metals,"
Experimental Materials and Methods
@ .
Irving, F. J.
C. Rossotti
and R. J . P. Williams, ?bid., 1906
M. Diamond, J . P h y s . Chem., 61,69 (1957). B. Sandell, "Colorimetrio Determinations of Traces of Interscience Publ., New York, N. Y., 1959, p. 173.
CHLOROGERMAXIUM SPECIESIN ACIDMEDIA
April, 1961
The total acidity was obtained after addition of an excess of sodium carbonate and back titration with standardized HC1; GeOz being a very weak acid9 does not interfere. Chloride was determined by the Volhard method and Ge(IV) by the phe:oylfluorone method,lOJ1 according to the procedure developed by Sandell." A Beckman model DU spectrophotometer was used. The organic phase was analyzed, after ext,raction wit,li water, as indicated above. A germanium inaterial balance provided 3 check of the technique. The germanium recovery amounted to a minimum of 90% when the germanium distribution ratio was less than 102, the recovery decreased to 80% for a distribution ratio .L03.6. Germanium losses appeared to take place during the making up of the germanium aqueous solution and were apparently caused by the high volatility of the germanium tetrachloride. To eliminate these errore, the germanium distribut>ion ratios when high were calculated from the concentration in both phases.
3
+ iC1- + iH+
The value of i can be deduced from the analytical ratios CI-/Ge or H+/Ge. Corrections for the HC1 coextracted in ccI4 were calculated from a value obtained for the HCI partition coefficientI2 and found to be negligible in the concentratioin range studied.
7.00 .. 8.00 .. 10-2 8.96 .. 5X 0.099 6.91 lo-' 0.164 6.84 10-2 0.620 6.74 a Average of 4 extractions.
1.00" 1.00 1.00 l.OOb 1.00 1.00 Average
ratios-
c1-
1
u)
Iu
3. 0 0
v
-I: 0 -2
-3
-4 -5
-3
-2
-1 log
0
1
icr-I '
Fig. 1.-Germanium distribution ratio as a function of C1concentration a t various levels of total molality: open circles, Hcl-HCIO,, filled circles, HCl-H2S04, 3
2
TABLE I AXALYSISOF CC14EXTRACTS Composition of initisl aqueous phase, mole/l. --Analytical Ge ECl HClOi Ge
2
-oq
:Experimental Results Nature of the Ge(1V) Species in CCL-The analysis of ten CC4 extracts corresponding to various hydrochloric solutions of germanium are reported in Table I. When the extracts are equilibrated with water, hydrolysis of the germanium species proceeds according to GeCli(OII)d-i $. (i - 2)Hz0 +GeOz
677
1 Hf
4.03" 3.97" 3.98 4.02 4.16 4.08 3.94b 3.8Ib 4.03 4.11 3.93 3.90 of 2 extractions.
The analyticial ratios CI-/Ge and H+/Ge are in good agreement and come close to 4. The amount of water present in CCl, extracts as determined by the Karl Fischer method using the Townson and Mercier apparatus appeared to be independent of the Ge concentration. Distribution of Ge(IV1 between HC1-HC104-LiCI Solutions and CCi.-The germanium(1V) distribution ratio D = (GeClr)org./Z(Ge)aq. was studied as a function of four concentrations: those of germanium, chloride and hydrogen ions and hydrochloric acid. (1) Influence of the Germanium Concentration.-The following values of D were obtained nThen the germanium concentration was varied in these solutions: 8.90 m HC1 (l), 1.44 m HC16.58 rn HCIO, ( 2 ) , 0.071 m HC1-9.95 m HC104 (3). (9) C. E. Gulesian and J. H. MUler, J . Am. Chem. Soc.. 1 4 , 3112 (1032). (10) H. J. Cluley, Andy88, 76. 517 (1951). (11) Ref. 8, p. 485. (12) Unpublished results by C. Barbeau snd P. Clerc.
0 e
n
-8-1 -2
-3 -4
-2
-1
log
0
ktJ ,
1
Fig. 2.-Germanium distribution ratio as a function of H f concentration at various levels of total molality. (1) log 2(Ge) aq.: -5.2 to -2.9, logD = f0.80 f 0.03
( 2 ) log z(Ge) aq.: -3.4 to -1.7, logD = -1.00 f 0.06 (3) log Z(Ge) aq.: -5.6 to -2.1,logD = -1.34 f 0.04
These results show no significant effect of the germanium concentration on the distribution ratio. (2) Influence of the Chloride Ion Concentration at Constant Acidity and Total Molality.-Data are txesented for solutions in which the HC1 concentration was varied while acidity and total molality were maintained constant by means of HClO4 a t 6.22, 8.02, 9.92, 11.94, 14.88 and 17.26 m, and with Ha04 at 8.02 m. The germanium con-
R. L. BENOITI N D P. CLERC
678
iog a c t i v i t y HCI,
Fig. 3.-Dependence
of the germanium distribution ratio
and the solubility of GeCb on HCI activity: 0, Brauer’s
solubility values; 0 , Allison’s solubility values.
centration was kept between 5 X and 5 X loe3. Log D is plotted in Fig. 1 against the logarithm of the “free chloride” equilibrium concentration (Cl-) in aqueous solution. This concentration was equal to the HC1 concentration when the latter was not too low. A corrective term equal to the concentration of chloride bound to germanium, Le., a times the germanium concentration was wbtracted from the HC1 concentration a t lorn was obtained by using successive HCI levels; approximations, from the value of the slope of the curve. The distribution ratio increases with the chloride concentration. For total molality 9.92 and 11.94 m, D reaches a maximum value and then decreases. D increases markedly with increasing total molality a t constant chloride concentration. (3) Influence of the Hydrogen Ion Concentration at Constant Chloride Concentration and Total Molality.-The distribution ratios corresponding to solutions of variable HC1 concentration and constant HC1 LiCl total molality of 6.86, 8.05, 10.0 and 12.0 m are plotted against log (H+) in Fig. 2. D increases but less and less rapidly with increasing Hf concentration. At constant acidity there is also a large increase of D with increasing total molality. (4) Influence of the HC1 Concentration.Figure 3 presents the data for HC1 solutions from 0.1 to 16 rn HCl. The germanium initial concentration varied between and 3.6 X lo-*. The values of log D are plotted against the HCl activity logarithm calculated from Akerlof’s data.13 D 1-aries a t first slowly with the HC1 concentration til 5 m HC1 then D increases rapidly to reach a maximum value for 14 m HC1. Interpretation of Results The experimental results in Table I indicate that the germanium species extracted in CC1, is germanium tetrachloride. Recent determination of GeC1, vapor pressureI2 when compared with solvent extraction data support this conclusion and shorn further that the GeC14 solutions in CC14 are ideal. Irvine and c o - ~ o r k e r sreport ~ ~ values of Cl-/Ge
between 3.0 and 3.9 for extracted germanium species but no specific data are quoted so that it is difficult to discuss their results. The lack of dependence of the distribution ratio on the metal concentration can be shown mathematicaliy6e7to imply identical degrees of polymerization for the metal species in the aqueous and organic phases. GeC14 being present in CC14, the germanium species in the aqueous solutions studied are therefore mononuclear. i and j , respectively, the number of C1 and OH bound per Ge atom in the GeCl,(OH),(H20)k species in aqueous solutions, charges being omitted for convenience, are related to the slope of the curves plotted in Figs. 1 and 2. For sake of simplification it is assumed first that GeCl,(OH),(H20)k is the only species present in a given concentration iiiterval for C1- and OH-. Calling KI1 the mass action constant for the equilibrium GeCl,(OH),(HzO)k + j H + 4-( 4 - i)Cl-= GeC1,
+ ( j -t-
k)H%O
and introducing the distribution ratio D,,and P the GeC1, partition coefficient, the following expression is found where 1 1 means activity and fiJk the activity coefficient of GeCl,(OH), (H20)k. Assuming now that various GeCl,(OH), (H20)kspecies are present simultaneously the distribution ratio is such that 1 _ D -
1
’by
Taking the derivative with respect to log(Cl-) dlogD 1 d log D,, - D 2 (21 d log (C1-)
D,, d log (C1-)
Combining equations 1 and 2 and int,roducing Z the average number of C1 bound per Ge atom
+
(13) a. AkerMf snd J. W. Tcare. J . Am. Chem. Soc., 69,1855 (1937).
1.01. 65
and j and
E are likewise defined. Then
If activity coefficients and water activity are kept constant, equation 3 becomes
Although the values of log D plotted against log (Cl-) in Fig. 1 are for solutions of constant HC1HCIO, total molality, activity coefficients are likely to vary if the replacement of one constituent say HC104, by the other one HC1 takes too large proportion^.^^ The applicability of equation 4 to calculate i is therefore restricted to relatively low chloride concentrations. This point is illustrated (14) Q. 0 Brink, P. Kafalas, R. 4.Shaip, E L \Teiss and J. TV. Irvlne, J r . , zbcd.. 79, 1303 (1957). (15) P. G, Murdocb and R. C . Barton, %bid.,SO, 4074 (1933).
April, 1961
CHLOROGERMhNIUM SPECIES IS
by the results of the experiments made using H$Oc in place of HCLO, a t the same total molality 8.02 m. The corresponding curves giving log D against log (Cl-) are at first parallel and unexpectedly close but then slopes become different for 1.6 m (Cl-). I n this particular case, a 1.6 m C1- concentration, Le., a replacement of 20% of HClOd by HC1, is thus an upper limit for the applicability of equation 4. Values of d were calculated for each total molality, from the slope of the curve giving log D against log (Cl-) in Fig. 1, according to equation 4. For the lowest chloride concentrations, .i was close to or equal to zero and then increased gradually with (Cl-). The values of log (Cl-) corresponding to 5 = i '/z which are tabulated below for total molality 11.94 xm give an indication of the stability of the chloro-complexes.
+
lOg(Cl-) 2.
-2.3 0.5
-1.2 1.5
-0.6 2.5
-0.1 3.5
At 11.94 m total molality, D goes through a maximum leading to Z = 4.0, for 1.5 m (Cl-), a chloride concentration for which equation 4 is still thought to be applicable. On the other hand, the value i = 4.0 for 7.0 m (Cl-) a t 9.92 m total molality may not be significant. An expression giving [d log D/d log (H+)](ci-) is obtained by following steps similar to those used for establishing relation -3. The application of the simpler form
ACIDM E D I A
679
log c o n cn.H+. Fig. 4.-Variations of the average number of OH bound per Ge atom, with H + concentration at various levels of total molality: 1, 6.86 m; 2, 8.05 m; 3, 10.0 m; 4, 12.0 nz.
the germanium solutions used were not in equilibrium with respect to hydrolysis. The experimental values of D obtained for HC1 solutions are in good agreement with those of Sandelll' and Fischerls which are less complete but differ somewhat from those given by IrvineI4 particularly a t low HC1 concentrations. Although the variations- of log D against log (HC1) are related to d, j and k as d log D/d log (HCl) = [d log D/d log (H+)lccl-) f [d log D j d log Cl-)(=+)
the unknown values of the activity coefficient derivatives do not permit a complete quantitative interpretation of d log D/d log(HC1) in terms of to the calculation of J should be restricted to low solution equilibria. However, the maximum value H + concentrations to ensure the constancy of the 4.1 found for the slope of the curve giving log D activity coefficients. However, for solutions of versus log activity HC1 (Fig. 3) near 5.6 m HC1 is constant total molality LiCl HC1 = 4 and 6 m, consistent with the presence of Ge02, possibly the HC1 activity coefficient has been shown to be hydrated, as the main germanium species in this nearly independent of the HC1 concentration.lB solution; the value of d log D/d log /HCl/ deduced If the constancy of the activity coefficient could from the equilibrium GeO2 4HC1 GeCla be extended to higher total molality and to ionic 2H20 would be somewhat above 4 because of two activity coefficients, equation 5 could be used to activity correction terms. Confirmation of the calculate j over the whole range of HC1 concentra- presence of Ge02 up to 5 m HC1 is found in the tion. The values of j are plotted in Fig. 4, against linear decrease of the Ge02 solubility logarithm log(H+) for total molality 6.86 (l),8.05 (2), 10.0 with the HC1 c o n c e n t r a t i ~ n ~which ~ ~ J ~ suggests (3), and 12.0 m (4). j is seen to decrease regularly a salting-out effect. The formation of a series of with increasing Hf concentration from close to 4.0 GeCl,(OH), (H20)k species, where 5 increases toto minimum values which in turn decrease with wards 4 and j decreases to low values, with increasincreasing total molality. ing HC1 concentration, accounts qualitatively for Approximate values of Z and j for the GeCL- the observed decrease of d log D/d log /HC11 (OH)j(HZO)s species present in HC1 solutions are from 4.1 to 0. The constancy of D a t the highest obtained from the limit slopes of the curves given HC1 concentrations indicates a t least that comin Figs. 1 and 2. For 12.0 m HCl, Z = 4.2 and3 = plete formation of GeCls2- does not take place even 0.7 are found. This would indicate germanium though the existence of GeC162- in a solid salt apspecies with an average charge -1 and suggest pears probable." The formation of GeCls2- is a coordination number 6 for germanium, water also reported not to take place in liquid anhybeing coordinated. Migration experiments give drous HC1 in contrast with that of SnC162-.2D also evidence that anionic Ge species are p r e ~ e n t . ~ , ~The ~ small variation of the distribution ratio up Using the resin loading method, Everests concludes to 5 m HCl appears to be due to the presence of that CeClf,-,(OH),- species with 3 < j < 4 are another CC1,-soluble Ge species. That this species formed between 7.5 and 11 m HCl. However, this is likely to be GeOzis indicated: the GeOzsolubility conclusion is not too dependable because the resin in CC1, was determined as 3.3 X lo-' m giving loading method is subject to criticism and most of (18) W. Fischer, W. Harre. W. Freese and K. G. Hackstein, Angew. [51
+
+
(16) J. E. Hawkins, {bid., 64, 4480 (1932). (17) A. W. l~aubengayer,0. B. Billin88 and A. sa, 546 (1940).
E,Npnklrk ibid..
+
+
Chem., 66, 165 (1954). (19) W. Pugh, J . Chem. Soc., 1637 (1929). ( 2 0 ) T.C. Waddington and F. Klanberg, Nolurwiee., 16, 578 (1059).
-5.1 for log solubility GeOz in CClr/log solubility GeOz in H20, in fair agreement with -5.5, the value of log D extrapolated to zero HC1 concentration. The slow rise of D with HCI concentration up to 5 m would result from a salting-out of GeOz. It is of interest to compare the variations of distribution ratio and GeC14 solubility between 9 and 16 m HC1 as D and SGeClr are related IOg D = A - log S G e C l d (6) The experimental values of - log 8 G e C l r as determined by Brauer4 and Allisonz1are plotted in Fig. 3, against the HCl activity. Although the points are somewhat scattered, they iall approximately on a line parallel to the log D curve as indicated by expression 6. Thes.:: results also confirm that (21) E. R. Allison and J. H. Muller, J . A m . Chem. Soc., 64, 2833 (1932).
GeC14 is the extracted species and that the GeC14 solution in CCl, is ideal. An exact definition of the nature and stability of the chlorogermanium species has not proved possible. As OH- is much more firmly bound to Ge than C1-, measurable substitution takes place only in concentrated H+CI- solutions where the unknown values of the activity coefficients make the interpretation difficult. The existence of a series of species with similar stabilities further complicates the situation. Acknowledognent.-One of the authors (P.C.) gratefully acknowledges the aid given him in the form of a fellowship sponsored by the Consolidated Mining and Smelting Co. The authors wish to thank the National Research Council of Canada for a grant.
VOLTAMMETRY IN LIQUID SULFUR DIOXIDE. I. TECHIC'IQUE AND THEORETICAL PROBLEMS B Y PHILIP J. ELVING, JOSEPH M. MbRKOWITZ AND ISADORE ROSENTHAL Departments of Chemistry, The University of Michigan, Ann Arbor, Michigan, and The Penmylvania State University, University Park, Pa. Received December 15, 1960
The feasibility of voltammetry and polarography in solutions of a totally non-protonic solvent, sulfur dioxide, has been investigated. Apparatus, procedures and orientative work with various inorganic and organic solutes are described for two indicating electrodes, the dropping mercury electrode and the stationary cylindrical platinum electrode, and two reference electrodes, the mercury calomel pool and the silver-ailver chloride electrode. The specific methodological findings are discussed critically, aa are the theoretical aspects and practical effects for the general practice of voltammetry of the nonexistence of a suitable background electrolyte, the possibility of reference electrode polarization, and the presence of a characteristic high solution resistance.
Although non-aqueous media2 have been investigated as solvents for polarography and, more generally, voltammetry, use of a totally non-protonic solvent has not been reported. Consequently, the feasibility of voltammetry in liquid sulfur dioxide was studied. Sulfur dioxide was selected because of its exceptional effectiveness as a reaction medium3+ in stabilizing free radicals; this would permit investigation of the electrochemical behavior of free radicals and of the frequent postulation of free radical intermediates in the electroreduction of organic species. This paper covers the development of experimental apparatus and procedures, orientative studies with various solutes, and exploration of certain theoretical problems involved in voltam(1) Abstracted from the Ph.D. theses of I. Rosenthal. The Pennsylvania State University, 1951, and J. M. Markowitz, The Gniversity of Michigan, 1958. (2) For example, the excellent studies in acetonitrile (I. M. Kolthoff and J. Coetzee, J . A m . Chem. Soc., 79, 870, 1852, 6110 (1957) and in ammonia (H. 8. Laitinen and C. J. Nyman, J . A m . Chem. Sac., 7 0 , 2241, 3002 (1948); H. A. Laitinen and C. E. Shoemaker, ibid., 71, 663, 4975 (1950); A. D. McElroy and H. A. Laitinen, J . Phys. Chem., 57, 664 (1963)). (3) L. F. Audrieth and J. Kleinberg, "Non-Aqueous Solvents," John Wilev and Sons, New York, N. Y., 1953. (4) G. Jander "Die Chemie in Wssserahrilichen Losungsmitteln," Springer-Verlag Berlin, 1949. (5) J. M. Markowitz. Ph.D. Thesis, The University of Michigan, 1958. (6) K. Cruse. Z . Eleklrochem.. 46, 571 (1940).
metry in sulfur dioxide. A subsequent paper' describes the behavior of triphenylchloromethane. Discussion The fundamental experimental requirements for successful voltammetry of (a) a reliably responsive indicating electrode, (b) a totally non-polarizable reference electrode, and (e) the availability of soluble electrolytes with relatively high decomposition potentials to serve as background electrolytes and provide solutions with appreciable conductivity, are only partially satisfied in sulfur dioxide. The effects of this situation can be evaluated from the following considerations of electrode systems, observed electroactivity of various solutes, and factors which may produce error in the determination of characterizing potentials, e.g., presence of a large migration current component in the limiting current, reference electrode polarization and solution iR drop. Electrode Systems.-The dropping mercury electrode (DME) is relatively inferior to the cylindrical platinum electrode (CPE) under the experimental conditions used. Compared to its behavior in aqueous solution, the DME is unstable, showing non-reproducible drop-rate changes and other unfavorable electrokinetic characteristics ; frequent clogging or streaming also occurs. Elec(7) P. J. Elving and J. M. Markowite, J . Phge. Chem.. 65, 086 (1961).
