were obtained for 3.6, 17, and 23 mM water, respectively. The increase in the formation “constant” with increasing water coiiceritratioii can be attributed to thc formation of higher hydrates of the proton, which is not uncxpected. c. Reliability of Measurements.-In measurements of this kind, the presencr of even trace amounts of basic impurities, such as m t e r (see above) and ammonia, would be highly objectionable. Hence special care was devoted to the purification,s storage, aiid handling of the solvents. The satisfactory reproducibility of the measurements indicates that> the precautions were adequate. For example, triplicate series of nieasurenicnts iii propionitrile gave p l < ~values ~ + of 2.98, 2.93, and 2.90. Standard Potential of Hydrogen.-In view of the unique importance of the standard potential of hydrogcii iii the e.m.f. series in water, it is highly desirable to obtain values for this quantity (on the vater scale) in other solvcnts. Unfortunately the hydrogcn electrode or predocs ]lot function satisfactorily in acetonitril~,~3 sumably in other nitriles as well. Heiicc iii these solvents iiidirect procedures must be used. Subject to the limitations discused before, the values listed in Table I11 arc related to the diffcwnccs in the standard potential of hydrogen in cach solvent and water, as
( E F I ” ). ~ ~( l $ ~ i ” ) , , ~ t ~=r
(Eb,‘’)0rg=
0.050 ApKsrri (12)
(2.1)
M . F. R l u n q a n d J
1 Coctice, J I’hys Chrm., 66, 89 (1962).
I-Icnce the standard (reduction) potential values of hydrogen in the different solvents arc the following : acetonitrile, +0.30; propionitrile, +O.26; isobutyronitrilc, +0.25; bcruonitrile, +0.29; pheriylacttoiiitrilc, +0.26; and acetone, +0.24 v. vs. the normal hydrogcii electrode in water as reference. Similar calculations h a w becn carried out by StrehIonz4 for methanol and acrtonitrile as solvcnts. Our value for acetonitrile is much more positive than However, Strehlow did not Strchlods (f0.11 allow for the incomplete dissociation of the acid used in his experiments, sulfuric acid. Using the dissociation constaiits reported by Kolthoff, Bruckeiisteiii, and ChaiitooniJ for sulfuric acid (5.5 X lo+) aiid the II,SO,.HSO,- complex (5.9 x lo-“), it can be shown that a t the half-protonation point in Strchlow’s experi.lI sulfuric acid added, total concenment (1.32 X tratioii of o-nitroaniline indicator used = 3 X M), the solvated proton concentration was only 7 X 111 (activity coefficients ignored). The corrected valuc for EIIO is then $0.24 v., which is in better (but still riot very satisfactory) agrecmcnt with our valuc of +0.30 v. The presence of relatively s i l d concentrations of basic impurities iii the solvent can account for thc residual discrepancy. Acknowledgments.--Financial support hy the National Scicncr Foundation under Grant No. NSFG14502 is gratefully acknon-lcdged. We also thank 1Lr. G. P.Cunniiigham of this Laboratory for his assistance in some of this work. 17.).
(24) 11. Strehlow, Z.L’lektrochem., 56, 827 (1932).
The i~ol:tr(igr:~~)iii(~ half-wavp potcritiuls of a variety of nietal ions, of the wlvated pro! 011, and of anions that dcI)olartze rncrvury ariodirally :Lrc rompared in the follonirig solvcnte (dielectrlc ronstmts glveri in parcnttirsrxs): acaetonitrile (86) ropionitrile (%), benzonitrile (%), isobutrronitrile ( Y O ) , phenylacct onitrile (IC)), acetone (21 ), anti wxt cr (79). ’ %he problcrn of liquid junction potentials is avoided by using the revc4l;)lr half-wave po1enti:rl of ruhidium ion as reference in cach solvent. For the majority oi‘ cations, reduvtion potrntittls :ire inme positive in the nit riles than in watcr (with aretone orcupyiiig m intrrrnediatc position), which is iittriblltd t o lower solv:itiori energies of wtions in nitriles than in writer. Variations in the half-anve potential of n given cation among the different nitrilea are rclativelv small and gcneritlly are in the dirrvtion and of the Inagiiitiide cxpcrted on the basis of differcsnws in ion pair formation constants of the clcrtroac.tive spcries us well as the supporting elec-trolyte t~ but partiru1:trl.v in those of lomcr dielectric ronstiirit, :L rhsnge in iri the dilrrrcnt solvents. In all s ( i I ~ mlisted, the nutiire of the (nonroniplexing) supporting electrolyte results in a ninrkcd shift in half-uv:ive potential, att ri1)iitcc-lin p:irt to specific, intt>r:wtiorisbetween supporting electrolyte and solvrnt.
Introduction 1)uiiig rcwnt yca1.s thc nitriles have iwcired eonsiderable attention as mcdia for clectrocheinical studies. These compounds provide an interesting series of solvents, with a smooth gradation in dielectric constants and acid-base propertics, in which the influence of the solvent o n the properties of electrolytic solutes can be studied. This communication is concerned with a comparison of the polarographic half-wave potentials of inorganic compounds in a series of nitriles, acetone, aiid water (11 Address all correspondence t o this author.
as solvents. Acetone is iiicludrd in this comparison, brcausc in rc’rtaiii iniportaiit rvspccts it occupics an iiitci*mediate positioii t)ct\veeri the “iioiiwaterlikc” nitriles and water. X therniody~ianiicallyexact comparison of potential values in different solvents is inipossiblc, since inclusion of liquid juiictioii potentials caiiiiotj be avoided. IIowever, b y invoking certaiii cxtra-thcrmodyiiamic evideiicr, it t)econies possible to inakc cornparisoris that are sufficiently exact for many purposes. In this (mimunication half-wave potentials in different solvents are compared by assuming that the reversible half-
Sept., 1963
POLAROGRAPHIC HALF-WAVE
wave potential of rubidium is constant in all solvents compared, for reasons described below The voltammetry of inorganic substances has been studied in considerable detail in acetonitrile by three independent groups, 2-4 and somewhat less extensively in propio-, acrylo-, benzo-, and phenyla~etonitrile,~ isobutyronitrile,6 and acetone.7 I n order to compare the results obtained in these solvents and in water on the rubidium scale, it was necessary to determine the half-wave potential of rubidium in several solvents for which this information was not available, to evaluate the effect of a change in the nature and concentration of the supporting electrolyte, and to redetermine several half-wave potentials that appeared suspect on the basis of comparison with other solvents. Experimental For new measurements reported in this communication, experimental conditions were the following. Purification of Solvents.-Practical grade acetonitrile (Sohio), propionitrile (obtained through Union Carbide), and isobutyronitrile (Eastman) were purified by shaking with silica gel, followed by fractional distillation, first from a small amount of phosphorus pentoxide, then from calcium hydride, as described elsewhere.8 Benzonitrile and phenylacetonitrile (both Eastman Highest Purity grade) first were shaken with silica gel, then stirred with calcium hydride for 2 hr., decanted, and vacuum distilled twice, first alone and then from a small amount of phosphorus pentoxide (distillation from calcium hydride appeared to introduce a basic impurity, possibly ammonia). Technical grade acetone was purified as described elsewhere.7 All organic solvents were stored and dispensed as described before.6t7 Supporting Electrolytes and Other Salts.-Tetraethylammonium perchlorate was prepared as described before.6 Tetrabutylammonium perchlorate was prepared in an analogous manner from tetrabutylammonium iodide and was purified by dissolving it in a minimuin amount of hot acetone, followed by precipitation with water. Tetraethylammonium iodide was prepared by neutralizing hydrogen iodide (Baker Analyzed Reagent) in isobutyl alcohol with 10% by wt. aqueous tetraethylammonium hydroxide (using indicator paper), followed by boiling until the first crystals appeared. The product was recrystallized from ethanol. Tetrabutylammonium iodide was recryatallized from ethyl acetate. Tetrapropylammonium iodide was purified by dissolving it in hot ethyl acetate to which the minimum amount of ethanol was added gradually, followed by boiling until the first crystals appeared, cooling in a Dry Ice-acetone bath, filtering, and washing with ethyl acetate. All tetraalkylammonium salts used were Eastman White Label products. ,411 recrystallized tetraalkylammonium salts finally were dried in vacuo near 50”. Thallium(1) perchlorate was prepared by heating thallium(1) nitrate (Fisher) to dryness with an excess of perchloric acid. Sodium perchlorate and rubidium iodide were the same products as used before.’ Instrumentation.-The dropping mercury electrode was the same as that used before; as were the reference electrode (an aqueous saturated calomel electrode, “s.c.e.”) and salt bridge (aqueous potassium chloride-agar gel), as well as the polarograph and other experimental details. The salt bridge was never kept immersed in the organic solvent for more than a few minutes a t a time, in order to avoid possible complications of the type described elsewhere9 for acetonitrile as solvent. (2) S. Waweonek and M. E. Runner, J . EZeclrochem. SOC.,99,457 (1952). (3) I. M. Kolthoff and J . F. Coetzee, J . Am. Chem. Soc., 19, 870, 1852, 6110 (1957). (4) A. I. Popov and D. H. Geske, ibid.,79, 2074 (1957); 80, 1340 (1958). ( 5 ) R. C. Larson and E. T. Iwamoto, ibid., 82, 3239, 3526 (1960). (6) J. F. Coetzee and J. L. Hedrick, J . Phye. Chem., 67, 221 (1963). (7) J. F. Coetzee and W.4. Siao, Inorg. Chem., 2 , 1 4 (1963). (8) J. F. Coeteee, G. P. Cunningham, D. K. McGuire, and G. R. Padmanabhan, Anal. Chem., 8 4 , 1139 (1962). (9) J. F. Coetsee and G. R. Padmanabhan, J . Phys. Chem., 66, 1708 (1962).
POTEKTIALS IN VARIOUSSOLVENTS
1815
Results and Discussion Reference Electrodes.-Three different t’ypes of reference electrodes have been used for voltammetry in nonaqueous solvents. (1) Internal reference electrodes of the second kind, such as silver-silver halide and mercury-mercury (I) halide (mercury pool) electrodes have the important advantage of providing relative half-wave potentials in a given solvent that are free of liquid junction potentials. However, apart from several inherent limitations of internal reference electrodes in any solvent, these electrodes are of particularly limited utility in solvents such as the nitriles and ketones since in these solvents (a) the depolarizing anions required to maintain a constant reference potential form relatively stable complexes or insoluble salts with many cations, (b) the solubility of silver and particularly mercury(1) halides in solutions containing excess halide ion is quite high (since the halides form relatively stable complexes in these solvents with silver and mercury(II), the latter produced by disproportionation of mercury(I)6s10), giving rise to objectionable voltammetric waves. (2) By using an external reference electrode containing the same solvent as in the electrolysis solution the above complications can be avoided. Silver-silver i011Q811 and silver-silver chloride4 electrodes are suitable in acetonitrile. It is common practice to minimize the liquid junction potential introduced by the use of an external reference electrode by employing in the salt bridge a relatively high concentration of an electrolyte which has a Cation and anion of approximately equal mobilities. However, in nonhydrogen-bonding solvents, such as the nitriles and ketones, which solvate anions only weakly, mobilities of common anions generally are much higher than those of cations, and no electrolyte has been found that is ideally suitable for general use in a salt bridge.g (3) Among external reference electrodes containing a solvent different from that in the electrolysis solution, the aqueous saturated calomel electrode has been used extensively in nonaqueous voltammetry for reasons of convenience3 and with satisfactory results, since the liquid junction potential is reproducible to an adequate degree. All results reported in this communication have been obtained with an aqueous saturated calomel electrode as reference. Comparison of Half-Wave Potentials in Different Solvents.-Irrespective of the nature of the particular reference electrode used (whether it is internal or external, and whether or not it contains the same solvent as in the electrolysis solution), any direct comparison of potentials in different solvents by referring all values to a common working reference electrode (such as the (10) K. Cruse, E. P. Goertz, and H. Petermoller, 2. Elektroehen., 66, 405 (1951). (11) V. A. Pleskov, Zh. Fiz. Khim., 22, 351 (1948). (12) Results obtained in acetonitrile with an external silver-silver chloride reference electrode in the same solvent&and with an aqueous saturated calomel electrode-agar salt bridge combination5 generally were i n good agreement. However, i t is important t o note t h a t prolonged immersion of an aqueous potassium chloride-agar gel salt bridge in acetonitrile causes formation of a plug of solid potassium chloride and dehydrated agar, resulting in a significant change in liquid junction potential.9 We did not observe a similar ohange with a Beckman Model 1170 fiber type calomel electrode, indicating t h a t dehydration of agar may be the main cause of the complication described above. Larson and Iwamotos used a calomel eleotrode with a Rowing type salt bridge, thereby avoiding formation of a solid Plug.
J. F. COETZEE, D. I
