BEHAVIOR OF n-Am4NBr AND KPi
IN
1049
NONAQUEOUS SOLVENTS
ditions are such that the average species is anionic (operating beyond the maximum in D for the complex ion). On the other hand, for the region below the maximum
in D, where the average species is cationic, the value of
D should increase with an increase in organic-solvent mole fraction."
Conductometric Behavior of Tetraamylammonium Bromide and Potassium Picrate in Some Nonaqueous Solvents
by Paolo Bruno and Mario Della Monica* Istituto di Chimica Analitica deU'Universita' di Bari, Bari, Italy
(Received June 88, 1971)
Publication costs assisted by Centro di Chimica Analitica Strumentale, Bari, Italy
Conductance measurements are reported for n-AmdNBr in dimethylformamide, formamide, and sulfolane, for KPi in dimethyl sulfoxide, formamide, and sulfolane, and for KC104 in dimethyl sulfoxide. The data were treated by the Fuoss-Onsager equation. The values of the ion-size parameter of all the salts were empirically corrected for the viscosity effect using the Einstein equation. Potassium picrate in sulfolane and tetraamylammonium bromide in sulfolane and in dimethylformamide exhibited values of the ion-size parameter which were smaller than the crystallographic radii of the salts; for the other salts the values of the ion-size parameter were close to the sum of the crystallographic radii of the salts. The Walden products of the large n-Am4N+ and Pi- ions in the aprotic solvents show a regular increase as the medium viscosity increases. This effect is attributed to the structure breaking of the solvent molecules. I n formamide the conductivity of these ions seems influenced by the protic character of this solvent.
Introduction Tetraalkylammonium salts have been used recently to study the structural aspects of ion-solvent interaction in water and in nonaqueous solvents.' According to Frank and Wen,* large ions in water like tetraalkylammonium ions, because of their hydrophobic character, enforce the water structure, increasing the degree of hydrogen bonding around their hydrocarbon chains. The result is a comparatively lower mobility of these ions in water than in nonaqueous solvent^.^ A different behavior for the other ions has been suggested.* In aqueous solution the presence of ions, for instance the alkaline metal ions, introduces into the normally organized water two regions due to the more or less strong interaction among the ion and the solvent dipoles: close to the ion, the solvent molecules are strongly oriented; after this first zone loosely bound water molecules are found, characterizing a transition region, before the normally organized water which is still apart with respect to the central ion. When ions of different size are dissolved, the greater the ionic radius the larger the transition region becomes to the detriment of the first zone. Thus going along the alkaline series, whereas lithium ion has a net effect of water organizcr because around it there is a broad
region of firmly bound water molecules, rubidium and cesium ions which are not extensively solvated can be considered as structure breakers. On the other hand, in aprotic solvents of different viscosity the comparative conductivities of tetraalkylammonium ions increase with the medium v i s ~ o s i t y . ~ I n these 'media tetraalkylammonium ions lose their peculiarity of enforcing the solvent structure via hydrogen bonding and they should conform to the structure breaker behavior due to their low charge density. They should, therefore, present a different behavior in protic and aprotic media as far as the solvent structure is concerned. Analogous considerations can be made in the case of large oxygenated anions which should present opposite character in these two kinds of solvents. (1) P. G. Sears, E. D. Wilhoit, and L. R. Dawson, J . Phys. Chem., 59, 373 (1955); C. Treiner and R. M . Fuoss, 2.Phys. Chem., 228, 343 (1965); D. E. Arrington and E. Griswold; J . Phys. Chem., 74, 123 (1970) ; D. F. Evans and R. L. Kay, ibid., 7 0 , 366 (1966) ; R. L. Kay, D. F. Evans, and G. P. Cunningham, ibid., 73, 3322 (1969); C. V . Krishnan and H. L. Friedman, ibid., 73,3934 (1969). (2) H. 9. Frank and W. Y . Wen, Discuss. Faraday SOC.,24, 133 (1957). (3) R. L. Kay, D. F. Evans, and G. P. Cunningham, J . Phys. Chem., 73, 3322 (1969). (4) M . Della Monica and L. Senatore, ibid., 74, 205 (1970).
The Journal of Physical Chemistry, Vol. 76, No. 7 , 1978
1050 I n this occurrence two salts, n-Am4NBr and KPi, have been selected to study the conductometric behavior of n-Am4N' and Pi- ions in four solvents characterized by a large differencein viscosity (-0.0080.1 P) and in the ability to form a hydrogen bond,
RAOLOBRUNOAND MARIODELLAMONICA
General Radio tuned amplifier Type 1232-A was used as a null detector. The bridge of the Wheatstone type consisted of two General Radio Type 500 precision resistors plugged in the fixed arms. The chosen values of these resistors had to match the measured resistances as closely as possible. The cell, with a variExperimental Section able condenser in parallel, and a precision decade resisDimethylformamide (BDH for chromatography) tance box (121,111 ohms in 0.1-ohm steps) composed was allowed to stand in contact with potassium hythe other two arms. The bridge was properly screened droxide pellets for 3 days; the liquid was decanted and and all connections were made with shielded coaxial then fractionated a t atmospheric pressure. The midcables. A corner of the bridge was properly grounded. dle fraction obtained was distilled a t 15 mm pressure. Using a dummy cell, the dispersion of the components A fraction with a specific conductance 2 X of the impedence was about l/lo,ooo in the frequency ohm-l cm-l was used. range 800-8000 Hz. The bridge was calibrated Reagent grade (Rudi Pont) dimethyl sulfoxide was against General Radio Type 500 fixed precision resisrefluxed for 24 hr over calcium oxide and then fractors and the values were found to be within the specificationally distilled a t 50" under reduced pressure (4 mm). tions. The middle fraction had a specific conductance of 3 X Two cells of the Daggett, Bair, and Kraus typeg IO-* ohm-1 cm-l. were used whose constants (about 0.5 and 0.05 cm-l) Sulfolane (tetrahydrothiophene 1,l-dioxide), kindly were determined with aqueous KC1.1° For the 0.05supplied by Shell Italiana, was first distilled at reduced cm-' cell the value of the constant was also determined pressure (15 mm). Before heating, the sulfolane was at 30" using a 0.01 demal aqueous solution of KC1 and pumped overnight to remove the volatile organic imthe Bremner and Thompson equation.ll The cells purities. The final product was obtained by distillawere equipped with a side arm through which dry Torr) over tion under reduced pressure (1 X nitrogen was passed to prevent the immission of air sodium hydroxide pellets. when stock solution was added. According to the method of Notley and S p i r ~ , ~ The four solvents used were distilled directly in the Baker Analyzed Reagent formamide was dried with cell and the resistance of the pure solvents was meamolecular sieves 3A (Union Carbide) in the form of sured with 10,000 and 20,000 ohms in shunt to the 1/16-in.pellets. The column was heated electrically to cell. Part of the pure soIvent was used to prepare a 60" and the percolation rate was 1 1. in 4 hr. The stock solution which was added to pure solvent using a formamide obtained in this way was used for the weight buret. All operations were performed in a dryfollowing operations: to wash and dehydrate the box saturated with dry nitrogen; the weights were resins and to prepare sulfuric acid and sodium form* corrected to vacuum. I n the calculations of the conmide solutions. ductance of each solution allowance was made for the Deionization was performed by means of a mixed conductance of pure solvent. Since the conductance bed of Amberlite ion-exchange resins loaded, respecof pure formamide was continuously changing it was tively, with H+ and HCONH- ions and treated with measured in a separate cell and the actual value was formamide previously obtained. The final product subtracted from the conductance of each solution. had a specific conductance of 2 X lo-' ohm-l cm-', The thermostat was an oil-filled bath kept at 25" but its value changed in the course of experiments. for the measurements in dimethylformamide, dimethyl Eastman Kodak tetraamylammonium bromide was sulfoxide and formamide; in sulfolane the measurerecrystallized three times from benzene and was ments were carried out at 30".l 2 The concentration dried for 48 hr in a vacuum oven at 40°.6 of each solution in equivalents per liter was calculated Fisher Certified Reagent potassium perchlorate was assuming that the density of the solutions was the twice crystallized from conductivity water and dried same as the pure solvent. in a vacuum oven a t 130" for 24 hr. Picric acid was recrystallized from benzene; the (5) J. M. Notley and M. Spiro, J . Chem. SOC.B, 362 (1966). details of this purification are reported in the literature.? (6) A. C. Harkness and H. M. Daggett, Can. J. Chem., 43, 1215 The picric acid obtained in this way was allowed to (1965). react with potassium hydroxide and the product was (7) S. R. Benedict, J . BWZ. Chem., 54, 239 (1922). recrystallized three times from an ethanol-water mix(8) M. A, Coplan and R. M.Fuoss, J . Phys. Chem., 68,1177 (1964). (9) H. M. Daggett Jr., E. J. Bair, and C. A . Kraus, J . Amer. Chem. ture and dried under vacuum over phosphoric oxide a t Soc., 73,799 (1951). 100"*8 (10) G. Jones and B. C. Bradshaw, (bid., 55,1780 (1933). Apparatus. A Wavetek Function Generator Model (11) R. W. Bremner and T. G. Thompson, &id., 5 9 , 2372 (1937). 102 was coupled with a home-made Conductivity (12) M. Della Monica, L. Jannelli, and U. Lamanna, J . Phys. Chem., bridge via, a transformer in a grounded metal box. A 72, 1068 (1968). The Journal of Physical C h e w d r y , Vol. 76, No. 7, 1972
BEHAVIOR OF n-AmrNBr AND KPi
IN
NONAQUEOUS SOLVENTS
Results and Discussion The measured equivalent conductance and corresponding electrolyte concentration have been filed with the ACS Microfilm Depository Service. l 3 Conductance data were analyzed using the FuossOnsager cquation14in the form A
= A0
- SC”’+ E C log c + (J - F&)c
1051
from the J values. I n this occurence precise values of distance u j cannot be calculated, and the a, values, as shown in Table I, are in all cases, except for KC104 in dimethyl sulfoxide, too low. An attempt to calculate more correct a, values can be tentatively made with the equation’3
(1)
F
for unassociatcd electrolytes and in the form A
= A0
+
- S(C~)”’ E c log ~ (cy) (J
+
- FAdcr - KASCYA (2)
for associated electrolytes, where y is the degree of dissociation, f is the mean activity coefficient, and KA is the association constant, all other symbols having their usual meaning. In the calculations the densities, viscosities, and dielectric constants of the four solvents were taken as: dimethylformamide, d = O.9439,ls 7 = 0.00796,16E = 36.71;’’ dimethyl sulfoxide, d = 1.0956,18q = 0.01992,18E = 46.6;19 formamide, d = 1.1296,20q = 0.0330,21E = 109.5;** sulfolanc, d = 1.2618,23q = 0.1029,24E = 43.3.26 Kay’s least-squares program26 was used to solve eq 1 and 2 for the parameters of interest, A0 and 8, and for associated electrolytes, K A . The results are shown in Table I. Equation 2 gave positive association constants K A for n-AmJYBr in dimethylformamide and for KPi in sulfolane. For the other salts studied eq 2 gave negative association constants, so that conductance parameters have been calculated using eq 1. Table I : Conductance Parameters Solvent
Sulfolane Formamide Dimethylf ormamide
J
A0
aj
KA
n-AmlNBr 11.416 f 0.002 30.0 1.60 f 0.03 ... 22.95 i 0.01 28.4 4.4 f 0.6 . .. 76.46 i. 0.08 691.9 3 . 1 & 0.6 4 f2
KPi Sulfolane Formamide Dimethyl sulfoxide
Dimethyl sulfoxide
9.36 f 0.03 21.88 f 0.01 31.43 =t0.02
46.2 3 f 2 25.2 3.9 & 0.5 223.8 4.7 f 0 . 1
KClOa 38.63 f 0.01 235.9 4.08 f 0.07
9 * 7
...
...
=
NT -(R+3 300
+ RR3)
(3)
where N is the Avogadro number and R+ and R- are the crystallographic radius of the cation and anion, respectively. This equation holds for large ions compared to the size of the solvent molecules and was derived for unhydrated solute species in a continuous medium. Table 11: Correction for t h e Viscosity Effect Solvent
FA0
JCO,
aCOr
@+
+ R-)
n-Am,NBr Sulfolane Formamide Dimethylformamide
11.2 22.5 75
Sulfolane Formamide Dimethyl sulfoxide
7.1 17.6 25.2
41.2 50.9 655.8
2.1 8.4 2.9
7.24 7.24 7.24
53.3 42.8 249
3.2 7.0 5.4
6.3 6.3 6.3
4.16
4.25
KPi
Dimethyl sulfoxide
KClOa 3.9 239.8
(13) Equivalent conductance data will appear following these pages in the microfilm edition of this volume of the journal. Single copies may be obtained from the Business Operations Office, Books and Journals Division, American Chemical Society, 1155 Sixteenth St., N.W., Washington, D. C. 20036, by referring to author, title of article, volume, and page number. Remit check or money order for (3.OOfor photocopy or $2.00 for microfiche. (14) R. iM. Fuoss and F. Accascina, “Electrolyte Conductance,” Interscience, New York, N.Y., 1959. (15) J. E. Prue and P. J. Sherrington, Trans. Faraday Soc., 57, 1795 (1961). (16) D. P. Ames and P. G. Sears, J . Phys. Chem., 59, 16 (1955). (17) G. R. Leader and J. F. Gormley, J . Amer. Chem. Soc., 73, 5731 (1951). (18) D. E. Arrington and E . Griswold, J . Phys. Chem., 74, 123 (1970). (19) P. G . Sears, G. R. Lester, and L. It. Dawson, {bid.,60, 1433 (1956) (20) J. Thomas and D. F. Evans, ibid., 74,3812 (1970). (21) G. F. Smith, J . Chem. Soc., 3257 (1931). (22) G. R. Leader, J . Amer. Chem. Soc., 73, 856 (1951). (23) U. Lamanna, 0. Sciacovelli, and L. Jannelli, Gam. Chim. Ital., 94, 567 (1964). (24) R. Fernandex-Prini and J. E. Prue, Trans. Faraday soc., 62, 1257 (1966) ; J. W. Vaughn and C . F. Hawkins, J. Chem. Eng. Data, 9 , 140 (1964). (25) M. Della Monica, U. Lamanna, and L. Jannelli, Gam. Chim. Ital., 97, 367 (1967). (26) R. L. Kay, J . Amer. Chem. Soc., 82,2099 (1960). I
...
I n the calculations, no allowance was made for the term F which takcs into account the variation of viscosity for thc added electrolyte. This fact does not alter the A, values, but only the value of &, the maximum approach distance of the ions, which is deduced
The J O U T W of ~Physical Chemistry, Vol. 75, .Vo. 7 , 1972
RAOLOBRUNO AND MARIODELLA MONICA
1052
Table I11 : Conductances and Walden Products of n-Am,N + and Pi- Ions in Formamide, Dimethylformamide, llimethyl Sulfoxide, and Sulfolane Solvents Dimeth ylformamide Ion
XO
XOV
n-AmrN +
22.9
P1-
37. .5"
0.182 0.299
Dimethyl sulfoxide XO XOV
10.41' 16.99
0.206 0.335
Formamide
Sulfolane
XO
XOV
XO
XOV
5.81 9.13
0.192 0.301
2.50 5.28
0.257 0.544
' This mean value has been deduced from the A0 of NaPi and KPi salts (P. G, Sears, It. K. Wolford, and L. It. Dawson, J. Electrochem. the the values of KClO, and KPi salts reported in this work. Reference 18. Soc., 103,633 (19>6))and the i o values of Na+ and K + ions reported in the literature (ref 15). This value has been calculated by i b value of C10,- ion (M. Della Monica, D. Masciopinto, and G. Tessari, Trans. Faraday SOC.,66, 2872 (1970)) in combination with
There conditions can be assumed for n-Am4?:+ and Pi- ions and in some' respects for Br- and c104-ions, but not for t h r I
