278
L. R. DAWSOS, J. W. VAUGHN,G. R. LESTER,?If. E. PRUITT, AND P. G. SEARS
Vol. 67
SOLVEKTS HAVING HIGH DIELECTRIC CBSSTX;”\;TS. XIII. CONDUCTANCES OF MULTIVALEXT ELECTROLYTES IS K-RIETHYLACETAMIDE A T 40°1,2 BY L. R. DAWVSON, J . W. VAUGHK,G. R. LESTER,M. E. PRCITT, AND P. G. SEARS Department o j Chemistry, University of Kentucky, Lexington, Kentucky Received J u l y 10, 1962 The electrical conductivities of solutions of 19 multivalent electrolytes and complex salts of transition elements in N-methylacetamide have been studied a t 40’. Anhydrous and hydrated salts show identical behavior within experimental uncertainty. This is interpreted to mean that in NMA the water of hydration originally associated with the solute loses its identity as such and becomes a negligible part of the solvent. Kohlrausch plots for salts of aluminum and cerium(111)exhibit pronounced maxima a t low concentrations. Lesser downward inflections appear in the plots for other multivalent transition metal cations. These effects are attributed primarily t o displacement reactions between the complexed cations and small amounts of strongly basic anionic ligands such as acetate ions, which occur as persistent impurities in the solvent.
It has been ~ h o w n ~that - ~ many alkali and alkaline earth salts exhibit the properties of completely dissociated electrolytes in dilute N-methylacetamide are (NMA) solutions a t 40’. Linear plots of A us. obtained over the concentration range extending to 0.02 N or more. Generally, slopes slightly less negat,ive than the theoretical were found for the alkaline eart,h salts. This has been attributed to a viscosity effect. Close agreement between the data for corresponding anhydrous and hydrated salts showed that water of hydration loses its identity as such and in effect becomes a negligible part of the solvent. This investigat,ion originally was designed primarily to bring toward completion the accumulation of information about the conductance behavior of typical electrolytes in solvents having very high dielectric constants by studying solutions of multivalent electrolytic species other than the salts of the alkaline earth metals. As the investigation proceeded, it became evident that the plots of A us. l/c for some of the multivalent salts exhibit an unexpected and typical maximum a t very low concentrations. This phenomenon has been observed for some transition metal salts in other solvents. Jones’ and co-workers found a similar maximum N. for cobalt(I1) bromide in formamide at 2 4 X Chaney and il’Iann8 studied several bivalent perchlorates over broad concentration ranges in furfural, ethylene glycol, and pyridine a t 20’. Conductance maxima (A us. de) were observed a t high dilutions for the cupric, nickelous, cobaltous, and manganous perchlorates in furfural. Hydrat~eswere used instead of anhydrous salts. Barium perchlorat,e, which seems to be incompletely dissociated in furfural (dielectric constant = 41.9 a t 20°), exhibited no such maximum. At low concentrations, especially in the case of highly charged cations which may react with t,he solvent or with trace impurities in it, the normal solvent correction may be quite inaccurate (even though it does not exceed (1) Taken in part from an it1.S. thesis by 31.E. Pruitt and a I’h.D. dissertation by J. W. Vaughn. (2) Financial support from the U. S. Atomic Energy Commission under Contract No. AT-(40-1)-2451 is gratefully acknowledged. (3) L. R. Dawson, P. G . Sears, and R. H. Graves, J. Am. Chem. S O L ,7 7 , 1986 (1955). (4) L. R. Dau-son, E. D. Wilhoit, and P. G. Sears, ibid., 78, 1569 (1956). ( 5 ) L. R. Dawson, E. D. Wilhoit, R. R. Holmes, and P. G. Sears, ibid., 79, 3004 (1957). (6) L. R. Dawson, G. R. Lester, and P. G. Sears, ibid., 80, 4233 (1958). (7) P. B. Davis, W. S. Putnam, and H. C. Jones, J . Franklin Inst., 567 (19 15). (8) A. L. Chaney and C. A. Mann, J. Phgs. Chem., 35, 2289 (1931).
a few per cent). An additional consideration for the present work is that S M A is known to be a highly structured s ~ l v e n t . ~ Toward the completion of the experimental work in this investigation, it became desirable to initiate a series of corresponding studies on solutions of acetic acid and its derivatives in SMA. This work seemed necessary because the NMA was synthesized from acetic acid and monomethylamine and consequently acetic acid or monomethylammonium acetate could be a highly conducting impurity. The results of this latter study, which have been submitted for publication,1° provide a substantial basis for interpreting the atypical phoreograms for several multivalent salts in NMA.
Experimental Apparatus and Procedure.-Adequate descriptions of the apparatus and procedures have been published previously.*-6 Solutes.-Reagent grade salts of the highest purity obtainable were used. All halides were analyzed by the Fajansll method. Analysis of chromium(II1) chloride hexahydrate revealed that it HzO; its behavior in solution conwas actually (Cr(H20)6C1)C12. firmed the results of the analysis. The water content of the hydrates was determined by the Karl Fischer method using Fischer stabilized single-solution reagent after it had been standardized against sodium acetate trihydrate.I2 The molecular weights of the hydrates were adjusted in the calculations, when necessary, to conform t o the analytical data. Potassium trioxalatoferrate(II1) was prepared by Palmer’s method.13 Potassium hexacyanoferrate(II1) was purified by repeated recrystallization from water; the sodium salt was recrystallized three times from ethanol and dried in a desiccator. Both sodium hexacyanoferrate(II1) and disodium pentacyanonitrosylferrate(II1) were analyzed for iron. Solvent.-ZiMA was prepared and purified as has been described previously.3 The quality of the solvent used for most of this work compared favorably to that used in earlier studies; its conductivity ranged from 1 t o 5 X 10-7 ohm-’ cm.-I. The highly purified NMA used in the latter part of this s ~ u d y was prepared by taking it through an increased number of fractional freezing cycles.4 For this purpose the apparatus described by Berger and Dawsonl4 was modified by using a 6-1. Pyrex separatory funnel and placing an internal heater assembly inside the tube previously used for hot water. The heater consisted of 2.44 m. of no. 24 nichrome wire (resistance = 1.66 ohms/ft.) wrapped as 75 turns about 50 cm. of 6-mm. tubing and was connected to a variable resistor, Freezing was allowed to proceed a t room temperature while 3-7 v. (depending on the purity of the sample and the temperature of the room) was applied to the (9) 6. Mizushima, et al.,J. Am. Chem. Soc., 73, 3490 (1950).
(10) L. R. Dawson, J. W. Vaughn, M. E. Pruitt, and H. C. Eckstrom, J. P h w Chem., 66, 2684 (1962). (11) I. M. Kolthoff and E. B. Sandell, “Textbook of Quantitative Inorganic Analysis,” The Macmillan Co., New York, N. Y., 1952, p. 543. (12) G. G. Warren, Can. Chem. Process., 29, 370 (1045). (13) W. G. Palmer, “Experimental Inorgamc Chemistry,” University Press, Cambridge, England, 1954, p. 521. (14) C. Berger and. L. R. Dawson, Anal. Chem., 34, 994 (1952).
CONDUCTANCE OF MULTIVALENT ELECTROLYTES IN E-METHYLACETAMIDE
Feb., 1963
279
heater. After 5-9 days the remaining liquid was withdrawn by suction with a vacuum pump and the procedure was repeated. A t all times the liquid was kept under an atmosphere of purified nitrogen. I n this way solvent having a specific conductance of less than 5.0 X 10-8 ohm-' cm.-l was obtained.
Results Plots of A us. z / C for representative multivalent electrolytes are shown in Fig. 1 and 2 . Not all of the phoreograms are shown but the trend of the plot for each of the other 10 salts is evident from the typical conductance data presented in Table I. In all of these cases the normal solvent correction was made, Le., the conductance of the solution was corrected by subtracting the conductance of the solvent. This maximum correction was 7% and in most cases it was less than 1%.
2
6
4
8
10
loo-\/?. Fig. 1.-Phoreograms
for chlorides of divalent and trivalent metals in h'MA a t 40".
TABLE I TYPICAL COKDUCTANCE DATA FOR SALTSIX N-METHYLACETAMIDE AT
c'/a x
102
A
40' Clh
x
102
A
KB[Fe(CZO~)B] .3Hz0 0.000 23.93 1.37 23.49 2.67 23.00 3.96 22.59 5.92 21.93 7.70 21.32 9.38 20.77
Ba( CsH6.".CsH4S03)2 0.000 17.16 0.952 16.75 1.901 16.68 3.041 16.51 4.193 16.20 5.418 15.97 6.675 15.96
Ce(N 0 ~ ) ~ . 6 H ~ 0 0.840 22.15 1.722 23.91 2.949 23.02 4.176 21.53 5.289 20.24 6.474 19.06
P\'a(0 3 S C s H 4 ~ H C 6 H 5 ) 0.000 15.28 0.891 15.16 1.865 14.99 3.012 14.91 4.292 14.63 5.461 14.46 6.774 14.26
CeCld.7H20 0.892 20.28 I . 653 21.59 3.069 22.06 4.052 21.96 5.121 21.69 6.232 21.31
CoC12.6HzO 1.373 20.40 2.432 20.46 3.881 19.96 5.221 19.15 6.332 18.24 7.453 17.23
Ni( N03)n.6Hz0 1.753 22.58 2.905 22.56 4.546 21.90 6.008 21.06 7,407 20.31 8.751 19.57
FeC13.6Hz0 1,245 19.36 2.625 19.11 4,853 16.28 6.757 14.43 8.380 13.17 9.766 12.34
CO(N03)2.6HzO 22.87 23.06 22.87 22.47 22.08 21.74
Fe(NO3)3.9H20 1.190 22.76 2.809 24.10 4.296 24.41 5.718 24.25 6.877 23.94 7.875 23.61
Al(N03)3,6H20 1.328 22.69 2.631 23.43 3.920 23.51 5.560 23.35 6.922 23.10 8.270 22.81
Cr(N03)3+3H20 1.535 23.41 2.780 23.51 3.847 23.28 5.125 22.94 6.180 22.63 7.173 22.35
1.218 2.220 3.473 4.771 5.866 6.848
y,, ,
20
2
4
,
6
L -
8
10
loo-\/?. Fig. 2.-Phoreograms
for some complex salts in XMA at 40'.
24
3 4 5 6 7 loo*. Fig. 3.-Phoreograms for cerium( 111)chloride 7-water in NMA at 40". Circles, conductivity of solvent = 5 X 10-7 ohm-' I
2
crn.?; squares, conductivity of solvent = 4 X 10-8 ohm-1 cm.-'.
Data for both anhydrous and hydrated barium chloride are plotted in Fig. 1 to show their correspondence within experimental uncertainty. Several other similar results for anhydrous and hydrated salts have been reported earlier.6 Phoreograms and recorded data for solutions of aluminum and cerium(II1) salts show maxima in the conductance curves at low concentrations. Apparently slight downward inflections occur in the conductance curves for CoC12, Co(N03)2.6H20, NiCL .6Hz0, and Fe(N03)3'9H20also. In all of these cases the downward inflections appear at concentrations less than 1 x AT. Information which may have bearing on this behavior in KMA has been obtained from other studies in this Laboratory, the results of which have been submitted for publication.
L. R. DAWSON, J. W. VAUGHN,G. R. LESTER,M. E. PRUITT, AND P. G. SEARS
280 24 r
22
A. 20
I
-
18 -
Fig. 4.-Phoreograms
for cerium(II1) chloride 7-water in
NMA. Circles represent solvent contaminated with potassium acetate to give a solvent conductivity of 5.75 X 10-7 ohm-' cm. -l. Open squares represent solvent contaminated with acetic acid to give a solvent conductivity of 5.08 X lo-' ohm-' cm. -I. Triangles represent solvent contaminated with benzoic acid to give a solvent conductivity of 4.45 x 10-7 ohm-' cm.-'.
Figure 3 shows the change in the curve for cerium(II1) chloride heptahydrate where the more highly purified solvent is used. I n Fig. 4 the effects of adding potassium acetate, acetic acid, or benzoic acid, t o the highly purified solvent may be seen. The conductance data were analyzed by both the Debye-Huckel-Onsager equation and the Shedlovsky modification of this equation. Maxima in some of the graphs prevented meaningful extrapolations to infinite dilution and calculation of theoretical slopes. Slopes and intercepts in the other cases were evaluated by the method of lea'st squares. Data from duplicate or triplicate sets of measurements were used for these calculations. The results are summarized in Tables I1 and 111. TABLEI1 TESTOF TIONS
DEBYE-HUCKEL-ONSAGER EQUATION FOR SOLUELECTROLYTES IN N-METHYLACETAMIDE AT 40"
THE
OF
-Exptl. slope
-Theor. dope (SE- ST) 100
Electrolyte
AD
(SE)
(ST)
Ba(Cl2H1~NS0& Na3Fe(C N ) 6 x € 2 0 NaC2H302.3H~0 NazFe( CN)5N0.2Hz0 NaC6H5NHC&4S03 K3Fe(CN)6 K8[Fe(C Z O ~.3Hz0 )~]
17.16 24.90 17.94 23.91 15.31 25.20 24.02
23.7 31.7 14.1 24.1 15.3 31.1 35.2
24.8 40.3 13.2 25.8 13.1 40.7 40.3
ST
- 4 -21
7 - 7 17
- 24 - 13
TABLE I11 DATADERIVEDFROM THE SHEDLOVSKY MODIFICATION OF DEBYE-HUCKEL-ONSAGER EQUATION Electrolyte
B~(CIZH,ONSO& NaaFe( C N ) B ~ H I O Na2Fe(CN)5NO.2H20 N~C~H~NHC~HISO~ K3Fe(CN)6 Ks[Fe( CnO~)a].3HzO
AO'
17.20 25.08 23.94 15.27 25.40 24.05
B
+ 8 +go +I1 - 24 62 67
+ +
THE
Vol. 67
Discussion The marked decrease in the maximum in the conductance curve for cerium(II1) chloride heptahydrate where more highly purified solvent is used (Fig. 3) is strong evidence that this atypical phenomenon is caused by the presence of an impurity in the solvent. Since it has been shownl0 that some acetate ions (from dissociated acetic acid or monomethylammonium acetate) persist tenaciously in NMA and are present in small quantities at a solvent conductance of 2-4 X lo-' ohm-' cm.-', it may be assumed that the more strongly basic acetate ions displace weaker (neutral or anionic) ligands associated with the acidic multivalent cations. This would decrease concomitantly the amount of conducting impurity in the solvent and the currentcarrying capacity of the solute. This effect would be most pronounced at very low solute concentrations but would be negligible a t higher concentrations. It does not appear in solutions of the alkali and alkaline earth salts in NMA because of the lower acidities or noncomplexing characteristics of these cations. Figure 4 shows that although there is little inflection in the conductance curve where highly purified solvent is used, the maximum reappears where potassium acetate, acetic acid, or benzoic acid is added to the solvent. The presence of small quantities of acetate ion in the solvent could produce the maxima in the conductance curve for solutions of aluminum salts and salts of other highly charged cations also. The possible influence of the changing nature of the solvent environment with increased concentration of multivalent cations, in such a highly structured solvent as KMA, cannot be neglected completely. Measurements did not reveal any marked change in the macroscopic viscosity of the solution within the concentration range of the conductance maxima. However results of diffusion studies15 provide strong evidence of some unusual solute-solvent interaction and indicate that significant variations between the macroscopic viscosity of the solution and the microscopic viscosity in the immediate vicinity of the ionic species may be operative. Further evidence of pronounced short-range viscosity effects in NMA appears in the decreasing value of the Walden product (noto)with increasing temperature for solutions of alkali salts. Solutes containing multivalent anions such as the trioxalatoferrate(II1), hexacyanoferrate(II1) , or pentacyanonitrosylferrate(II1) ions exhibit conductance behavior normal for strong electrolytes in KMA. That is, the solutes are dissociated completely as expected for stable complex ions and their phoreograms are linear throughout the most dilute solutions. This suggests that no ligands, including the acetate ion, outside the cyano complexes have sufficient basicity t o displace the highly basic cyano group. (15) W.D. Williams, J. A. Ellard, and L. R. Dawson, J . Am. Chem. Soc., 1 9 , 4652 (1957).
