Titrations in nonaqueous solvents - Analytical ... - ACS Publications

Anal. Chem. 1982, 5 4 , 105R-121 R (224) Tsuchida, T.; Furulchi, R.; Ishli, T. Thermochim. Acta 1980, 39, 103. (225) Ulerich, N. H.; Newby, R. A.; Kealrns, D. L. ThermOChh. Acta 1980, 36, 1. (226) Ul'yano, V. Q.; Donetskii, I. A. KhimXarm. Zh. 1980, 14, 98. Chem. Abstr. 93,173794k. (227) van Berkum, J. G.; Hadvoort, G. Thermochlm. Acta 1981, 43, 49. (228) van Dam, J. C.; Hakvoort, G.; Reedijk, J. J . Thermal Anal. 1981, 20, 3. (229) Venvgopal, J. S.; Hlrannaiah, B. V.; Majumder, S. K. J . ThermalAnal. 1980, 18, 15. (230) Warrington, S. B.; Barnes, P. A. Therm. Anal., 6th 1980, 1 , 327. (231) Weeks, J. J.; Sanchez, I . C.; Eby, R. K.; Poser, C. I . Polymer 1980, 21,325. (232) Weill, C. E.; Carroll, B.; Liskowitz, J. W. Thermochim. Acta 1980, 37,

(202) Shultz, A. R.; Young, A. L. Macromolecules 1980, 13,663. (203) Shyamala, M.; Dharwadkar, S. R.; Karkhanavala, M. D.; Deshpande, V. V.; Chandrasekharaiah, M. S. Thermochim. Acta 1981, 44,249. (204) Siegmann, A.; Baraam, 2. J . Appl. folym. Sci. 1980, 25, 1137. (205) Simon. J.; Anclrosits, B.; Szalay, B.; Lantos, S. H S I , Hung. Sci. Instrum. 1980, 48, 1. (206) Siradze, R. V,; Machaladze, T. E.; Dzhokhadze, G. M.; Todria, M. K. Soobshch. Akad Nauk Gruz. SSR 1980, 94,341. Chem. Abstr. 93, 1834 IS. (207) Slrcar, A. K.; Lamond, T. G.; Wells. J. L. Thermochim.Acta 1980, 37, 315. (208) Springer, H.; t3rinkmann, U.; Hinrichsen, G. Colloid folym. Scl. 1981, 259,38. (209) So, C. W.; Barham, D. J . ThermalAnal. 1981, 2 0 , 275. (210) Soboleva, T. N.; Rudnitsky, L. A,; Alekseyev, A. M. J . Thermal Anal. 1980, 18,517. (211) Svetlik, J.; Masarlk, I. Makrotest 1980, 50. Chem. Abstr. 94, 157692r. (212) Szatanlk, J. 1.. Rev. (?en. Caoutch. flast. 1981, 61 1 , 61. Chem. Abstr. 95, 26378s. (213) Szekely, T.; Till, F.; Vnrhegyi, G.; Blazso, M. J . folym. Sci., folym. Symp. 1980, 67, 115. Ch8m. Abstr. 94, 1041952. (214) Szendrei, T.; 'Van Berge, P. C. Thermochim. Acta 1981, 4 4 , 11. (215) Szendrei, T.; 'Van Berge, P. C. Thermochlm. Acta 1981, 4 4 , 11. (216) Sztatisz, J.; Cbl, S.;Komives, J.; Stadler-Szoke, A.; Szejtli, J. R o c . Int. Conf. Therm. Anal.. 6th 1980, 2 , 487. (217) Takahashl, K.; Shirai, IC; Wada, K.; Kawamura, A. Nippon NogelKagaku Kalshi 1980, 5 4 , 357. Cham. Abstr. 93,90458a. (218) Tanaka, H. Thermochirn. Acta 1981, 45, 139. (219) Tanaka, Y.; Nakamura. T. Thermochim. Acta 1981, 4 4 , 303. (220) Tanaka, H.; Dhshima, S.; Ichiba, S.; Negita, H. Thermocim. Acta 1981. 48, 137. (221) Tishkov, N. I. Zavocl. Lab. 1980, 46, 924. Chem. Abstr. 94, 113739g. (222) Tou, J. C.; Whiting, L. F. Thermochim. Acta 1980, 4 2 , 21. (223) Tsai, K. T. Hut-tung HM Kung Hsueh Yuan Hsueh f a 0 1980, 42, 110. Chem. Abstr. 94, 128485~.

65. (233) Wendiandt, W. W. Thermochim. Acta 1980, 37, 117. (234) Wendlandt, W. W. Thermochim. Acta 1980, 39,313. (235) Wendlandt, W. W. NBS Spec. fubl. ( U S . ) 1980, 580,219. (236) Wendlandt, W. W. Thermochim. Acta 1980, 37, 121. (237) Wesoiowski, M. Mikrochim. Acta 1980, 1 , 199. (238) Wesolowskl, M. Thermochim. Acta 1981, 46, 21. (239) Wieczorek-Clurowa, K.; Paulik, F.; Paulik, J. Thermochlm.Acta 1981. 46, 1. (240) Wieczorek-Ciurowa, K.; Paulik, J.; Paulik, F. Thermochim. Acta 1980, 38, 157. (241) Wissler, G. E.; Crist, B. J . Po&m. Sci., folym. Phys. Ed. 1980, IS, 1257. (242) Yamaguchi, J.; Sawada, Y.; Sakurai, 0.; Uematsu, K.; Mizutami, N.; Kato, M. Thermochim. Acta 1980, 37,79. (243) Yuen, H. K.; Grote, W. A.; Young. R. C. Thermochim. Acta 1980, 4 2 , 305. (244) Zeman, S. Thermochlm. Acta 1980, 39, 117. (245) Zsako, J.; Horak, J.; Varhelyl, C. J . Thermal Anal. 1981, 20, 435. (246) Zsako, J.; Varhelyi, C.; Csegedi, B.; Zsako, J., Jr. Thermochim. Acta 1981,45, 11. (247) Zsako, J.; Zsako, J., Jr. J . Thermal Anal. 1980, 19, 333. (248) Zynger, J.; Kossoy, A. D Anal. Chem. 1980, 52, 1538.

Titrations in Nonaqueous Solvents Byron Kratochvii Department of Chemistry, University of Alberta, Edmonton, Alberta, Canada T6G 2G2

BOOKS AND GENERAL REVIEWS Books and reviews1 of relatively broad coverage are included

This review covers approximately the literature reported in Chemical Abstracts over the period December 1979 to November 1981. As in previous reviews, emphasis is on fundamental studies and on methodology, followed by applications. Space limitations have forced deletion of many references; tables have been included this time to provide a somewhat more complek picture of the trends in applications. In general, although the number of applications of nonaqueous titrimetry continues to decrease, interest in the physical and chemical properties of nonaqueous solutions remains a t a high level, especially for solvent mixtures. As can be seen from the sections that follow, increasing effort is being spent on solvation of ions and the resulting effects on reactions involving acid-base and complex formation reactions. The attention paid to electron transfer, though still considerable, is not as great in relative terms. This may be partly because governmental and industrial support for fundamental nonaqueous battery research has dropped off. There have been few new nonaqueous analytical systems reported over the past 2 :years; one area be inning to grow is that of titrations using macrocyclic ligan s for the selective determination of cations, especially the alkali metals and alkaline earths. The expense and unavailability of some of these ligands is a temporary problem that will be relieved in time. Meanwhile, microtitrations allow exploration of the area to proceed. A shortage off precise and accurate information on the physical properties of pure solvents and their mixtures still persists. Collection of this information is tedious, time-consuming, and expensive, but the data are badly needed. Some kind of organized and funded effort in this area would help alleviate the problem.

in this section; those that concentrate more specifically on single topics are listed under the appropriate subject headings of this review. Volume 13 in the series Modern Aspects of Electrochemistry contains a chapter on the temperature dependence of electrolytes in nonaqueous solvents (38)and another on solvent adsorption and double-layer potential drop at electrodes (505). The Compendium of Analytical Nomenclature, published by IUPAC, has a chapter on practical measurement of pH in amphiprotic and mixed solvents, along with recommendations on nomenclature in titrimetry, solution equilibria, and liquid-liquid distribution (204). A book on solvent effects in organic chemistry includes highly useful chapters on solute-solvent interactions, classification of solvents, and solvent effects on the rates and equilibria of homogeneous reactions (405). It is well referenced and contains numerous useful tables and appendices. General reviews in the field of nonaqueous solvents include one on a nonstatisticd approach to solutions (184) and another on a coordination model for nonaqueous solvents (118). Electrolyte behavior in solutions has been reviewed by Barthel (36,37), Renon (407), and Schuster (440). The donor-acceptor approach to chemical reactivity has been treated by Mayer (332). Superacids and protonated solvents have been reviewed briefly (219), as has the general topic of nonaqueous solvents (238). A Japanese review covering both fundamental studies and applications of titrations in nonaqueous solvents to the determination of organic substances has been written @IO), as has one on the use of organic solvents for the dissolution

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1982 American Chemical Society

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TITRATIONS I N NONAQUEOUS SOLVENTS

of samples (240). Other reviews include discussions of the thermodynamics of ion-solvent interactions (58,96,260),of nonstructural ion-solvent interactions (424),and of empirical parameters of solvent polarity as linear free-energy relationships (406). Caldin has considered reaction kinetics as related to solvation in nonaqueous systems (69). The properties of nonaqueous electrolyte solutions in the microwave and farinfrared regions have been surveyed (25). The interactions of solutions of salts in liquid ammonia with polyatomic anions, as studied by Raman spectroscopy,have been reviewed (172).

FUNDAMENTAL Solvation. For a reaction to be analytically useful as a titration it must in general proceed quantitatively in a well-defied process at a reasonable rate. The solvent, through interactions with reactants and products, often controls both the rate and extent of reactions between solutes. Information on solvent-solute interactions is therefore important to the development of useful analytical methods. As in previous years, the period covered by this review has seen an increasing accumulation of data on ionic and molecular solvation and on ion-ion and ion-molecule interactions. But overall, understanding of the theory behind the behavior of solvents as reaction media continues to lag. In the area of theory, a dielectric profile for the solvent dielectric constant beyond the primary coordination or solvation sphere shows that neglect of dielectric inhomogeneity by the Born treatment can lead to appreciable overestimation of solvation energies (474). The approach of Covington and co-workers to the preferential solvation of ions in binary solvent mixtures has been reviewed by Covington and Newman (91) and has been extended by Lilley to cover the interaction of two species in one solvent; agreement between the Covington approach and that of Kirkwood and Buff is obtained when all species have the same molar volume (306). Symons has compared the “cavity” model for trapped electrons in polar solvents with a model postulating a solvated solvent anion and concludes that the “cavity” model is more satisfactory (403). Schmidt has examined the interaction between the vibrations of a solvent shell and the ion within the shell and derived the general form of the dynamic interaction between two solvated ions (431). The kinetics of preferential solvation of ions in binary solvent mixtures has been considered (339). A method for the calculation of ion solvation energies has been described for polar media (278) and for gaseous ions (3), and the similarities between the solvated ground states and gaseous excited states of alkali atoms have been pointed out (75). An empirical method for the calculation of the standard mold free energies of solvation of ions has been proposed (326) and extended to many additional species (327). Solvent donor-acceptor properties were used to predict solvent effects on bromine-bromide and iodine-iodide complexation equilibria (53). A variety of experimental techniques continue to be employed for the study of solvation. Solubility and potentiometric methods were used to investigate the solvation of silver bromate and iodate in dimethyl sulfoxide-water (214) and acetonitrile-water (447) mixtures. Potential measurements allowed determination of thermodynamic parameters for the solvation of sodium chloride in mixtures of water and 1propanol (257) and of several alkali metal halides in mixtures of water and acetonitrile (101) and water and dimethylformamide (102). Emf measurements were also used to obtain information on the solvation of hydrochloric acid in N methylformamide-water mixtures (269). Taft and co-workers continue to develop the solvatochromic comparison method. The solvatochromatic parameters A * , a,and p have been studied in relation to Gutmann’s solvent donicity and acceptor number values (491). It was shown that the solvent donicity is linear with p for oxygen bases and nitrile bases, but pyridine does not fit the correlation. In another study the Hildebrand solubility parameter aH was found to show a better linear correlation with free energies of solution of nondipolar solutes than the solvatochromic parameter A* (224). This is suggested to indicate that A* is a better measure of dipolar solute-solvent interactions, while 8H is a better measure of the interactions between solute molecules that must be disrupted to create a cavity for the solute. The polarities of over 40 protic solvents have been estimated by measurements with appropriate betaine dyes and the effect 106 R

ANALYTICAL CHEMISTRY, VOL. 54, NO. 5, APRIL 1982

of solvent polarity on the solvation of anions has been discussed (200). Schmidt has studied the effects of distortion of the primary solvation shell of an external charge on the vibrational spectra of alkali metal cations within the shell (430). Both cause splitting of a single band into two or three new bands. Infrared spectroscopy has also been used to measure the interaction between halide ions and water or N-methylacetamide in carbon tetrachloride (485), the interaction between iodide and protic solvents in acetonitrile (484, and solvation and ion association of magnesium nitrate in acetonitrile over a range of temperatures (381). A dimer of the type [Mg(N0&4MeCNI2, with two bidentate bridging nitrate groups, is thought to form at higher concentrations. Europium(II1) and terbium(II1) nitrate solutions are not dissociated in acetonitrile, but some dissociation occurs in dimethylformamide (65). The effect of water on lithium(1) coordination to acetonitrile was followed by infrared spectroscopy (515),as was the hydrogen bonding between methanol or ethanediol and nitrate ion in dichloromethane (135). Nuclear magnetic resonance continues to be useful for the study of solvation. Symons has reviewed infrared and NMR studies of ionic solvation in water and methanol (482). Ethylenediamine solvates sodium cations more strongly than does tetrahydrofuran (114);a chelate effect was established. Carbon-13 NMR of four tert-butylpyridines in mixtures of methanol and water provided data on hydrogen bonding of solvent to solute and on the motion of the solute in the solvent cavity (198). NMR of quadripolar ions provides solvation information (292), and both NMR and IR have been used to investigate hydrogen bonding of alcohols to anions (487). Thermochemical measurements also provide valuable insight into solvation. The hydrophobic hydration of several substituted quaternary ammonium bromides in water-dimethylformamide mixtures was studied calorimetrically (194), as was the heat of reaction with water of anthracene anion radical ion pairs in dimethoxyethane and tetrahydrofuran (473). Enthalpies of transfer of alkali metal halides from water to water-methanol mixtures were determined by measurement of heats of dilution (113). Heats of solution of alkali metal halides in water-ethanol mixtures (536),of alkali metal iodides and perchlorates in propylene carbonate (263),and of iodine complexes with N,N-dimethylacetamide, pyridine, and benzene in cyclohexane, carbon tetrachloride, and m-xylene (245) have been reported. Solvation numbers for several electrolytes, such as HC1, NaC1, and KC1, were measured in methanol-water and ethanol-water by ultrasonic interferometry (311). The effect of tert-butyl alcohol on the structure of aqueous solutions of alkali metal halides has been studied by measurement of the temperature of the sound velocity maximum (353). The acid dissociation of trichloroacetic acid in 1:l water-dioxane mixtures has been estimated at varying temperatures through thermodynamic measurements (400). Treiner and Fromon have determined the salting constant of acetonitrile for 10 inorganic salts in water by a static vapor pressure method (507). Dash has analyzed solubility data of silver salts in a variety of pure and mixed solvent systems and finds solute-olvent interactions have a more important effect on the solubilities than electrostatic effects associated with changing dielectric constant (107). Conductance remains a useful technique for acquiring information about nonaqueous solutions. Justice and Justice have used data on 1:l salts in water and the alcohols to confim theoretical derivations of excess thermodynamicfunctions and transport coefficients (221),and Gill has calculated ionic radii for four tetraalkylammonium ions in organic solvents of varying dielectric constant from conductivity and viscosity data by a modified form of Stokes law (168). He has also used the modified Stokes equation (170)and a reference electrolyte (169) to estimate single-ion conductivities. A macroscopic treatment of solvated ion dynamics has been developed and applied to calculation of the limiting conductance of cations in several aprotic solvents (82). The conductivity of bromide ion in solutions of nitrobenzene containing bromine is much greater than predicted (415). A hopping mechanism is proposed. Halide anions show little solvent interaction in glycerol (115) or in N-methylpyrrolidone (146). The effect of water on proton migration in hexanol and octanol has been analyzed (111).

T I T R A T I O N S IN NONAQUEOUS SOLVENTS

esls are in mS areas of sampling bulk malerials for chemical ans?As. ~pplicalbnsof rmnsqwow systems la chemical analysis. and memods far determining Substances of clinical interest. He also has a strong interes1 in me leading of anawical Chemistry.

Conductance has been used to investigatemicelle formation of ionic surfactants in polar nonaqueous solvents such 88 dimethyl sulfoxide and dimethylfommnide (460) and to study micelles of cobalt and cnlciumsoaps in methanol and 1-butanol (518). The interaction of sucrose and urea with symmetrical tetraakylammonium iodides in dimethylfomamide (461),and of lactose with alkali metal halides in formamide (531),was followed by conductance measurements, as was the effect of formic acid on solutions of lithium perchlorate in tetrahydrofuran (110). Conductance measurements made in pure solvents include lithium chloride, lithium perchlorate, and tetraalkylammonium perchlorates in diethylformamide (216),lithium iodide in propylene carbonate (313),uranium(1V) and -(VI) compounds in chlorosulfuric acid ( 3 7 3 ,several lithium salts in 1,3-dioxolane (364),some alkali metal salts in liquid sulfur dioxide (305),tetrapropylammonium iodide in seven different solvents (206). and various salts in methyl isobutyl ketone (both anhydrous and water saturated) (68). Conductivity studies in solvent mixtures, especially water plus an organic solvent, are becoming common. Among the systems reported are lithium chloride in dimethylacetamide, both pure and with water (4551, sodium perchlorate in propylene carbonate-tetrahydrofuran mixtures (329).four salts in propylene carbonatedimethoxyethane mixtures over a temperature of -45 to +25 "C (174), and four salts in 4 1 2-methoxyethanol-water at 25 OC (249). Ionic conductances of six salts in water containing up to 20 mol % acetonitrile (383) or dimethyl sulfoxide (384) have been reported. The limiting equivalent conductivity of potassium chloride in dimethyl sulfoxidewater mixtures shows a minimum at 80% dimethyl sulfoxide (247). A series of sodium and potassium salts have been studied in dioxane containing lC-30% water at 40 "C (103),as have several alkylammonium halides at 25 "C (205).Among the alcohol-water mixtures investigated were I-propanol-water, in which the potassium halides were measured (516),and ethanol-water, in which sodium nitrate was studied (80). The conductivity of magnesium sulfate was determined in binary mixtures of water with methanol, ethylene glycol, glycerol, acetone, and methyl cellosolve (403). An invest' ation of the lactone (ring closed) and ampboterjc (ring opene8) forms of fluorescein and eosin in 11 organic solvents revealed the lactone form of fluorescein to be more stable (137). This is the reverse of the stabilities observed in water or the solid state and is explained on the basis of solubility parameters. Transfer Activity Coefficients. Interest in this area continues. The work reported over the period of this review has been classified according to the experimental technique employed. Mukherjee has reviewed briefly the fundamentals of the common methods for estimation of medium effects for single ions (349). Marcus has reviewed in depth the subject of standard free energy of transfer of ions from water to other solvents (319),as have Lahiri and Aditya more briefly (288). Kolling has continued his interesting contributions with a method for estimating the free energy of transfer of the salt tetraphenylarsonium tetraphenylboride (Ph4AsPh4B)from water to both polar aprotic and hydrogen bond donor solvents by utilizing the Kamlet-Taft parameters T * and a in a linear regression (250). Sen has used a scaled-particle theory to estimate free energies of transfer of various electrolytes in several protic and aprotic solvents (444). The use of Ph,AsPh,B as a reference electrolyte for estimation of standard free energies of transfer of single ions is

increasing. Kim has made a study of the assumption involved, i.e., that the overall free energy of transfer for this salt from water to organic solvents can be divided equally between the cation and anion, and concludes that if a constant asymmetric partition between cation and anion is included, then the assumption is valid for a number of solvents (242). He has used the assumption to calculate values for the transfer of several ions from water to water-dimethyl sulfoxide mixtures (244). Marcus has used the same assumption to correlate the free energy of transfer of chloride (321)and other anions (320)with the solvent polarity index ET. Free energies of transfer of ions from water to a series of chlorinated aliphatic and aromatic solvents have been estimated from experimental hydration free energies and calculated solvation free energies in the organic solvents (4). Solubility measurements of a salt in two solvents allow calculation of transfer activity coefficients. Kolthoff and Chantooni have measured transfer activity coefficients for alkali metal ions, silver(I), and thallium(1) complexed with dibem-1Bcrown-6 (253)and with cryptand 2.2.2 (254). The complexes have a much higher free energy in water than in the dipolar aprotic solvents dimethylformamide, dimethyl sulfoxide, acetonitrile, or propylene carbonate. This method has also been used to obtain coefficients for a series of ions between methanol and propylene carbonate (77),for silver(1) and chloride between water and water-acetonitrile mixtures (2431, for silver(1) and sulfate between methanol and dimethylformamide (4761, and for silver(1) and thiocyanate between methanol and dimethylformamide (411). The free energy of transfer of tetraphenylarsonium tetraphenylboride between ethylene glycol and mixtures of ethylene glycol and acetonitrile has been determined (60).Similarly, solubilities of the pyridine complexes of nickel(I1) and manganese(I1) chloride have been measured in pyridine and binary yridine mixtures with chloroform, 1,2-dichloroethane, anzaprotic solvents and used to calculate standard free energies of transfer (303). Similar studies have been made for tris(1,lO-phenanthroline)iron(II)perchlorate in water-acetone and water-1-propanol mixtures (256). A study of the free energy of transfer of a solute from water into micelles indicates that the value cannot be defined without ambiguity (54). Solute partition between immiscible solvents is also a valuable method for the estimation of free energies of *fer. Values for transfer from water to 1,Zdichlomethane have been reported for nine anions using this approach (99). Coetzee and Istone used the copper(I1) ion selective electrode to estimate free energies of transfer between water and several organic solvents and solvent mixtures (86). Polarography and voltammetry were used by Gritzner and Geyer to obtain the same quantity for lead(I1) from acetonitrile to a series of dipolar aprotic solvents ( 1 77). An electrochemical cell with transference was used to determine the solvent transference number of silver sulfate in mixtures of methanol and N-metbylformamide (223). Cells without transference were employed to obtain the transfer activity coefficient for sodium bromide from water to water-dimethylformamide mixtures (16). Cation and anion transference numbers for potassium chloride were measured in a series of water-ethanol mixtures (131),as was the transport number for the ions of several salts in water-dimethylformamide mixtures (492). Results obtained for HCI by the agarsalt bridge method agreed well with those obtained by the Hittorf and concentration cell methods. The thermodynamic quantities of transfer for HCI from water to watel-1-propanol (258),and for HI from water to aqueous tetrahydrofuran, dioxane, and 1,2-dimethoxyethane (109) were also determined by emf measurements. A moving boundary method was used to determine the transference numbers of tetraalkylammonium bromides in methanol (465,466). Standard free energies of transfer for H+ and OH- have been estimated in aqueous mixtures of tetrahydrofuran and 1,2dimethoxyethane by emf measurements with a hydrogen electrode over the temperature range of 5-35 "C (108). The same group has also reported values for free energies of transfer of alkali metal halides from water to aqueous solutions of 12-dimethoxyethane (51) and 2-methoxyethanol using metal amalgam and Ag-AgCI electrodes (50). The effect of liquid junction potentials on estimations of single ion activities bas been discussed ( Z O I ) , as have the factors affecting the ANALYTICAL CHEMISTRY, VOL. 54. NO. 5. APRIL 1982

lO7R


junction itself between dissimilar solvents (264). Spectroscopic measurements have been used to estimate the free energy of transfer of the proton from water to water-dimethyl sulfoxide mixtures (543). Single ion transfer enthalpies have been measured for salts of several anions in methanol-propylene carbonate and in methanol-N-methyl-2-pyrrolidonemixtures (259);similar measurements have been made on cesium iodide in a series of water-propylene carbonate mixtures (535) and on alkali metal halides in 4:l water-dimethylformamide (502). A method using nuclear quadrupole relaxation has been proposed which allows determination of equilibriumconstants for ion solvation reactions and free energies of transfer (340). The method has been applied to the determination of the free energies of transfer of lithium between ten aprotic solvents (338). An accuracy of 5-10% is claimed. A vapor osmometric method has been suggested for the estimation of mean activity coefficients for tetraalkylammonium perchlorates in dimethylformamide at 25 and 45 "C (246). Acid-Base. Activity in the area of acid-base properties in nonaqueous solvents has not been as great as in previous years. Mussini and Covington have reviewed the standardization of pH measurements in nonaqueous and mixed solvents and discussed a universal pH scale with respect to water (354). The correlation between empirical Lewis acid-base solvent parameters and the thermodynamic parameters of ion solvation has been evaluated (545). Ionization constants of 41 weak acids of varying chemical type in 10 water-dimethyl sulfoxide mixtures were treated by multivariate analysis and the results considered in relation to acidity function theory (119). The relation between substituent-induced changes in energy and in charge has been investigated for various proton-transfer equilibria by molecular orbital theory (191). Acid-base interactions in nonaqueous solutions have been considered in terms of the proton-electron-hydride concept of acid-base theory (271),and equations have been derived for the calculation of solvated hydrogen ion concentrations in titrations of acids in dimethylformamide and in dimethylformamide-ethanol mixtures (132). Equilibrium Constants have been measured for protontransfer between variously substituted nitrophenols and substituted anilines and pyridines to form ion pairs in dioxane, benzene, and dioxane-benzene and dioxane-cyclohexane mixtures (299). The constants follow the Hammett q~relation, and make possible the construction of acidity scales in these solvents covering a wide range of acidities. Tanaka and Bates used a sodium ion selective electrode as reference in conjunction with a glass electrode to determine thermodynamic dissociation constants for uncharged weak bases in aqueous 50% (by weight) methanol (498). The pH values of tetraborate buffer solutions at various temperatures in the same solvent system were measured in a cell without transference (268),and acidity scales have been established (267). The effect of organic Rolvents on the Britton-Robinson buffers has been studied (195). The acid-base chemistry of a series of sulfhydryl and ammonium-containing amino acids has been studied in three acetonitrile-water mixtures (390). Both macroscopic and microscopic acid dissociation constants were measured. The dissociation of trichloroacetic acid in three dioxane-water mixtures has been investigated at 25 and 35 OC by conductance measurements (399). A series of nitroaniline indicators has been used to determine Hammett acidity function values in mixtures of sulfuric and phosphoric acid with water (63). Methanesulfuric acid has been investigated as a medium for acid-base reactions (376);conductance measurements and acid-base titrations in it with strong acids indicate that CH3S03H2+and CH3SOp,are the major current carrying ions. The properties of chlorosulfuric acid as a solvent continue to be studied by Paul and co-workers (374). In this solvent selenyl, propionyl, n-butyryl, isobutyryl, and benzoyl chlorides all behave as strong bases, while phosphoryl bromide and acetyl, phenylacetyl, chloroacetyl, and succinyl chlorides behave as weak bases. The behavior of a series of bases, including salts of univalent metals, water, aniline, acetic acid, and various trifluoromethanesulfonate salts has been studied in trifluoromethanesulfonic acid by conductivity measurements (416). Most, except for the alkaline earth metal salts, behave as fully dissociated bases. The acid-base dissociation constant of 108R


water in liquid ammonia has been determined by emf meaat -40 O C (429);this value is 1O1O surements to be 6 X smaller than that reported from conductivity measurements. The small dissociation constant values for ammonia and water in liquid ammonia compared to liquid water are ascribed mainly to differences of entropies of the species in both solvents. The effects of homoconjugation and association of the product salt on the pH at the equivalence point during acid-base titrations in dipolar aprotic solvents have been estimated (239). Benoit and co-workers have measured potentiometrically the dissociation constants of the conjugate acids of aniline, N-methyl- and Nfl-dimethylaniline, and pyridine in dimethyl sulfoxide, along with the heats of solution and heats of neutralization of the bases (45). The order of basicity does not follow that expected from gas-phase data; the difference is ascribed to the decreasing exothermicity of the heat of solution of the BH+ ions as a function of the number of N-H protons available for hydrogen bonding to dimethyl sulfoxide. A similar suggestion was made by Reyes and Scott for several aliphatic amines on the basis of equilibrium measurements between the amines and p-nitrophenol in mixtures of benzene and dimethyl sulfoxide (409). Three forms of protonated water have been found in propylene carbonate containing small amounts of water and perchloric or trifluoromethanesulfonic acids; a proton may be bound to one, two, or three molecules of water (497). Perchloric acid hydrates are extensively dissociated, but those of trifluoromethanesulfonic acid are not. The acidity of oxydipropionitrile, alone and in mixtures with water, was studied by Persin and Gal (382). Using glass and hydrogen electrodes, they measured the dissociation constants of several acids. Very weak bases such as urea, dimethyl sulfoxide, and hexamethylphosphoramide can be titrated in oxydipropionitrile. Homoconjugation was shown to be important. Thermodynamic parameters for the self-association of N-methylformamide and N-methylacetamide in carbon tetrachloride have been measured by infrared spectroscopy (467). Heats of solution and of neutralization of protonic acids (375), and of Lewis acids and bases (370),have been determined in acetic anhydride. Potentiometric and conductometric titrations of melamine in water, dimethyl sulfoxide, and mixtures of the two show that only one NH2 group is protonated in water, but three NH groups are protonated in mole fractions of dimethyl sulfoxiie near unity, and two NH2 groups are protonated in stepwise fashion in 0.38-0.78 mol fraction dimethyl sulfoxide (213). A variety of protonic acids and bases have been shown to form addition compounds in fused phenol at 50 "C (315). Fluorosulfonic acid can be used as a titrant for amides, alkaloids, and their mixtures in this solvent. Dissociation and homoconjugation constants of succinic, glutaric, adipic, and o-phthalic acids in dimethylformamide have been determined potentiometrically and conductometrically (412). The dissociation of phosphoric acid in methanol-water mixtures has been determined by conductivity measurements (52),while dissociation constants of benzoic acid and of dimedone in water-dimethyl sulfoxide mixtures have been obtained by cell potential measurements (441). Dissociation constants of acetic acid and dihydrogen phosphate ion were measured in water-ethanol mixtures at several temperatures by electrochemical cells without liquid junction (39). Similar measurements were made on acetic acid protonated tris(hydroxymethy1)aminomethane in 80 w t % 2methoxyethanol-20 wt % water (448),and on protonated 5-NH 2-CH3-1,3-propanediolin 1:lwater-methanol (40). The acid &ssociation constants for a range of flavonoids were estimated in water, methanol, dimethylformamide, acetone, and dimethyl sulfoxide (151). Titration behavior was poorest in water, and improved in the solvents according to the order listed. Dissociation constants for selected nitrophenol, sulfophthalein, and phthalein acid-base indicators in 4:l dimethyl sulfoxide-water mixtures have been measured (149),as have constants for six sulfophthaleins in acetonitrile, dimethyl sulfoxide, and 1-alkanols (519),and for 12 azo and sulfonphthalein acid-base indicators in 2-propanol (387). The dissociation constant of 5-sulfosalicylicacid has been estimated by conductivity measurements in mixtures of water with methanol and with dioxane (357). In a similar way dissociation constants for malonic and three alkyl-substituted malonic


acids were measured in dimethylformamide (413). Solubility methods were used to obtain the equilibrium constant between basic tetraphenylporpl-iine and its diprotonated form (451). Complexation and Ion-Pair Formation. Interest in this area has picked up since the last review. The number of articles appearing under the general heading of ion pairing and complex formation in solvents other than water is increasing; a significant effect in this area is the result of growing interest in crown and cryptate ligands. Ahrland has reviewed the general area of comlplex formation in protic media, with emphasis on dimethyl inulfoxide (10). Among the more specialized reviews, Lehn has considered in a critical way the effects of cryptate inclusion complexes on solute-solute and solvent-solute reactions and on ionic reactivity (296), and Blandamer and Burgess have surveyed solvent effects on initial and transition states of inorganic complexes undergoing nucleophilic substitution reactions (56). Simonetta has reviewed the structure of ion pairs formed by an alkali metal cation and a paramagnetic organic anion in ethanol (458). Voltammetric studies of metal complexes in nonaqueous solvents have been treated (226) as have NMR studies of the solvolysis and ligand exchange reactions of the dioxouranium(V1) ion (307). Irish has investigated the utility of Raman spectroscopy for the study of several inorganic reactions, including ion pairing, oxidation-reduction, and hydrogen-bonding systems (203). A variety of organic solvents were used. A good deal of effort is being spent on the measurement and interpretation of ion-pairing and other ion-aggregation phenomena. A range of experimental methods have been applied to these studies, with somewhat mixed success. Conductivity measurements remain one of the most powerful and widely used tools in this area (141). Unless otherwise indicated, the work referred to in this paragraph was done by conductance studies. Association constants of several tetraalkylammonium salts were measured in dimethyl sulfoxide (462), in the lower alcohols (I%), and, with silver and lithium halides, in methyl isobutyl ketone (66) and sulfolane (356). The alkali metal hexafluorophosphates were studied in acetonitrile (388). Association was measured of lithium acetate in methanol, water, and water-dioxane mixtures (21, 22), of silver acetate in water-methanol mixtures (ZO),of potassium chloride in water-dioxane mixtures (18), and of lithium perchlorate in four solvent mixtures of low dielectric constant (359) and in water-acetone mixtures (369). The effect of solvent complosition on the conductivity and ion association of tetraalkylammonium and tetraphenylphosphonium salts was studied in acetone, dimethyl sulfoxide, water, and their binary mixtures (427). An ion-pair model was used to analyze data collected in thionyl chloride on lithium chloride and lithium tetrachloroaluminate, and for several tetraalkylammonium salts (423). Both conductance and visible spectroscopy were used to establish the existence in acetonitrile (AN) of species of the type [M(AN),$+.2BF;, where M is cobalt, nickel, manganese, copper, or zinc (302). Ion-pair association constants between thallium(1) and nitrate were measured in liquid ammonia by NMR (196). Infrared spectroscopy was used to measure alkali metal cyanate ion pairing in dimethyl sulfoxide (404). The pKb values of the alkali metal acetates and perchlorates were measured in glacial acetic acid (500). Ion pair association constants have also been reported for tetramethylammonium bromide in dioxane-methanol mixilures (313, sodium iodide in the lower aliphatic alcohob (255),tridhloroacetic acid in water-methanol mixtures (401),sodium tetraphenylborate in mixtures of water and dimethyl sulfoxide, water and acetone, and acetone and dimethyl sulfoxide (428), and sodium nitrate in water-lpropanol (81),all by conductance. Association of cobalt and nickel nitrate in dimethyl sulfoxide, dimethylformamide, and water-alcohol mixtures was followed by NMR (331) and of cobalt, nickel, and copper nitrate in dimethyl sulfoxide by conductance (178). The ion-pairing and dimerization equilibria of lithium isothiocyanate was investigated by vibrational spectroscopy in diethyl carbonate, dimethyl carbonate, and Formation constants for molecular assobutyl acetate (715). ciation complexes between diethylamine and several chlorophenol derivatives were measured in carbon tetrachloride (366). In benzene both 1:l and 2:2 complexes were observed. Lithium-7 NMR studies have been carried out on lithium ion complexes with several crown ethers in a range of nonaqueous solventa (463). Exchange between free and complexed

lithium was fast on the NMR time scale. The stability of the complexes, with the exception of pyridine, varied inversely with the Gutmann donor number of the solvent. The same trend was found by NMR for the l,lO-diaza-18-crown-6ligand with thallium, lithium, sodium, and cesium (447). Solytions of rubidium metal in the presence of dicyclohexyl-18-crown-6 (18C6) upon pulse radiolysis yield, among other species, an ion pair of the type Rb(18C6)+,e(123). Association constants for the formation of alkali metal cation complexes with 18crown-6 in methanol, dimethyl sulfoxide, and acetonitrile have been determined potentiometrically and conductometrically (513). Association constants have also been measured for crown compounds with lanthanide ions in propylene carbonate (325),and with alkali metal ions in propylene carbonate and mlethanol (493, 494) and in acetonitrile (199, 280). A number of cryptate type macrocyclic ligands have also been studied in nonaqueous solvents. Examples include rate and equilibrium measurements for several cryptands with all~alimetal ions in methanol (92,93) and propylene carbonate (94). Heats of solution and of complexing in methanol, and of transfer from water to methanol, have been determined for these systems (2). A large variation with solvent was observed for the stability constants of alkali metal, silver, and calcium ions with three cryptates (95). Cesium usually forms either inclusive or exclusive complexes with cryptates, depending ori the size of the cavity (230),but with C222 either may form, depending on the solvent and temperature. Both mono- (ML) arid binuclear (M2L)complexes were formed by silver, cadmium, and lead ions with two cryptates in methanol (468). Copper(I1)forms two, three, and four to one complexes with thiocyanate in ethanol-acetone mixtures (73);with chloride in dimethyl sulfoxide the two to one complex predominates (304),but in dimethylformamide the one to one and three to orie species are more important (124). Copper(1) and its halide coimplexes are fairly stable in dimethyl sulfoxide (11). The chloride complexes of zinc(I1) have been investigated in ethylene glycol and methanol-nitromethane and methanolwater mixtures (116)and in water-dimethyl sulfoxide mixtures (143). The stability constant for the two to one complex is larger than for the one to one species. The complexes of cadmium and zinc bromide in dimethyl sulfoxide are more stable in solutions containing lithium, sodium, or tetraethylammonium ions than ammonium (13),probably owing to ion-pair formation between ammonium ion and bromide in this solvent. In methyl isobutyl. ketone silver(1) forms anionic complexes of the form AgX, and Ag3X4-;the neutral salts AgX are highly insoluble (67). Iodine and trieth lamine in chloroform yield an ion pair of the type I(Et3N)2PI3- (1871, and periodic acid in propylene carbonate give four homoconjugate species composed of HI04 and IO4- ions (394). Potassium ion forms weak complexes in acetonitrile with dimethylformamide, dimethyl sulfoxide, hexamethylphosphotriamide, and similar compounds (21.2). Stability constants for the one to one complexes were of the order of 2-7. Equations have been derived for the estimation of the activation energy for the dissociation of metal-ligand complexes in different solventpi as functions of the heat of dissociation of the complex and heat of vaporization of the solvent (499). The solvent dependence of the equilibrium constant and dissociation rate constant for the monocomplex of nickel(I1) with 4-phenylpyridine has been reported (88), as have conditional stability constants for the nickel(II),copper(II), and zinc(I1) complexes of glycine, iminodiacetic acid, and nitrilotriacetic acid in various water-methanol mixtures (134). 0ther complexes of potential analytical utility whose stability cclnstants have been measured include copper(I1)with N-alkyl arid N,N-dialkylethylenediaminein dimethylformamide (14), cadmium(I1)with bypyridine in 50% aqueous ethanol (316), and silver(1) with a series of heterocyclic amines (64). Electron Transfer. Among the reviews covering this area is one on the properties of solvents that influence the potentials of oxidation-reduction couples (85), another on electron solvation (542),and a third on electrode potentials (506). The ferrocene-ferricinium couple has been proposed as an internal standard for electrochemical measurements in aprotic solvents (142). A stable external reference electrode such as the Ag+/Ag system is used to make the electrochemical measurements, but the potentials of the species of interest A.NALYTICAL CHEMISTRY, VOL. 54, NO. 5, APRIL 1982

109 R


are recorded against the ferrocene couple. Reference electrodes prepared by the electrodeposition of poly(viny1ferrocene) films onto platinum are reported to be stable and to undergo rapid reversible electron transfer in acetonitrile (378). Studies of the kinetics of electron transfer between platinum and ferrocene, either in solution or chemically attached to the platinum electrode surface, show that both types conform to Arrhenius behavior (449). The potential of the ferrocene-ferricinium couple is affected by both solvent composition and by the presence of inorganic acids (31);values in binary mixtures of water with acetic acid, acetonitrile, acetone, or ethanol were reported (179). The ferricinium ion is sensitive to trace oxygen dissolved in dipolar aprotic solvents and is reported to decompose rapidly (425). The reduction potential of the iron(II1) / (11) tris(tetramethy1- 1, l o phenanthroline) complex was measured in several dipolar aprotic solvents and found to be nearly independent of solvent (333).

Numerous organic systems have been investigated by a variety of electrochemical tools. The following have been selected because of their interest from the standpoint of possible utility in analytical titrations. Solvent effects on the reduction potentials of four quinones were assessed in eight aprotic organic solvents (217). Equilibrium constants for dimer formation of the one-electron reduction product of the benzenediazonium cation in acetonitrile were measured by titration of benzenediazonium cation or its derivatives with reductants such as the anion radicals of naphthalene, sodium methylate, and tetrabutylammonium hydroxide (472). The copper(1)-copper amalgam electrode has been studied in dimethyl sulfoxide solutions containing halide ions (139). Faradaic impedance measurements at low chloride ion concentrations and at all the bromide and iodide ion concentrations studied indicate a rate-controlling adsorption step. Several metals that are difficult (W, Mo) or impossible (Al, Be, Li) to electrodeposit from aqueous solutions can be deposited from solvents such as methanol (A1 and Be), dimethylacetamide (Li), and formamide (W, Mo) (495). Zerconium(I1) was observed at a ring disk electrode upon anodic dissolution of the metal in dry acetonitrile (235). The dissociation of nitrosyl chloride and dinitrogen tetroxide in sulfolane has been studied at a silver-silver chloride electrode (541). The dissociation constants of these two nitrosyl ion donors were measured. Reference and indicating electrodes continue to be evaluated in various solvents. The hydrogen electrode in propylene carbonate was found to be useful for following titrations of molecular and anionic bases with trifluoromethanesulfonic acid (496). The standard potential for the hydrogen electrode in hexamethylphosphotriamide, prepared from HC1, was estimated relative to the thallium(1) chloride-thallium amalgam couple (90). The solvent decomposes in the presence of HCl. The standard potentials of the silver-silver chloride and hydrogen electrodes were measured in n-amyl alcohol (358). Other silver halide electrodes for which standard potentials have been reported include: silver-silver chloride in 2propanol and its mixtures with water (125) and in mixtures of formic and acetic acid (17); silver-silver bromide in water-dioxane mixtures (127) and in water-2-propanol mixtures (126);silversilver thiocyanate in water-dioxane mixtures (100);silversilver phosphate and arsenate in formamide (106); and silver-silver iodate in formamide (104). The temperature dependence of the standard potentials of mercury-mercury(1) acetate, benzoate, and propionate electrodes in water-dioxane has been evaluated (293),as has that of the chloranil electrode in formamide (105). Standard potentials have been determined in dimethyl sulfoxide for the zinc, cadmium, mercury, and copper couples relative to silver (12). Formal potentials have been obtained for the tris(bipyridine) complexes of iron, cobalt, and chromium, and for the tris(phenanthro1ine) complex of cobalt, in a variety of solvents (421),and also for the tris(ethy1enediamine)complexes of cobalt and ruthenium, the hexammine complex of ruthenium, and the sepulchrate complex of cobalt (422). Solvent Properties. Methods of purifying organic solvents for analytical use, primarily for chromatography (346) and for solvent extraction (537), have been reviewed. A method for the purification of acetonitrile claims high electrochemicaland spectroscopic purity with a yield on the order of 70% (72). The procedure involves two distillations, one rapidly from llOR


strong base to remove acrylonitrile and one slowly from sulfuric acid to remove allyl alcohol, amines, and benzene. Water is removed before use by passage of the doubly distilled material through a column of activated alumina. A discussion of reference materials for accurate viscosity calibration, with suppliers, is available (324). A nonstatistical approach to solvent structure proposes a hierarchy of types of solvent molecules, ranging from those on the solvent surface to those interacting with or influenced by solutes to those comprising "normal" solvent (408). The solvatochromic comparison method proposed by Taft and Kamlet in 1977 for estimating solvent polarities continues to receive attention. The method has been extended by the original authors to correlate non-hydrogen-bonding solvent effects with several chemical properties (490). The KamletTaft generalized scale of solvent polarity is felt by Kolling to be best interpreted by consideration of dispersive and dipolar interactions between solute and solvent rather than by a form of reaction field theory (252);the same worker suggests phenol blue as a solvatochromic indicator for the estimation of the a* and a terms, especially for solvents that are weak hydrogen bond donors (251). The a* scale correlates better with a combination of simple individual-moleculedipole reaction field models plus a continuum model than with either separately (120). A model expressing solvent polarity as a function of shifts in relative permittivity and refractive index has been found to correlate well with the Taft-Kamlet parameter a* (44). A calorimetric method relating solvent basicity to the enthalpy of reaction between the solvent and the Lewis acid BF3gives results comparable to the Gutmann method, which uses SbClb (121). The heat of mixing of a solvent with chloroform has also been correlated with the Gutmann method (314).

New data on a range of solvent properties have been provided. Autoprotolysis constants have been measured for anhydrous liquid hydrogen fluoride (183) and N-methylformamide (365). Protonated solvent concentrations in pure methanol, ethanol, and water, measured by study of NMR line shapes (193), agree fairly well with KS1I2values. The autoprotolysis constants for ethylene glycol in various mixtures with acetonitrile were measured at 25 "C and the free energies of ionization calculated (59). The purification of acetylacetone and its use as a solvent for electrochemicalstudies have been discussed (140, 295). Dielectric relaxation studies of dimethylformamide reveal the percentage of molecules present as dimers to be about 7% at -60 OC, 3% at +20 O C , and 0% above 80 "C (144);for dimethyl sulfoxide about 22% exist as dimers at 20 "C. Propylene carbonate is decomposed by chlorine if traces of water are present (554). Sulfolane reacts with hydrogen bromide to form bromine, water, and C4H8S0 (128). Both the rate and extent of degradation are affected. The solubility of water in C4 to C121-alkanols has been reported over the range of 15-40 "C (504). The properties of solvent mixtures are being investigated with increasing frequency. Much of this interest is undoubtedly the result of greater use of solvent mixtures in liquid chromatography, but an important factor is the additional information about solvent properties that can be gained when techniques such as infrared spectroscopy and NMR are used to obtain data on mixtures at the molecular level. Polarity measurements of binary solvent mixtures have been made using six polarity scales (290). An equation that describes deviations of the mixtures from linearity was developed. Empirical solvent parameters for several binary mixtures have also been measured in which the parameters generally varied nonlinearly with composition (28, 29, 122). A minimum in the autoprotolysis constant values for mixtures of water and diethylene glycol was found at 25 mol % glycol (49). For mixtures of methyl ethyl ketone with formic or acetic acid the autoprotolysis constant depends, as expected, on the fraction of protic component (272). The structures of mixtures of methanol with carbon tetrachloride and dichloromethane (486) and with four dipolar aprotic solvents (488)have been studied by infrared and NMR spectroscopy. In the chlorinated solvents fully hydrogenbonded clusters of methanol molecules predominate through most of the mole fraction range. The NMR measurements agree with the assumption that for strongly basic aprotic solvents hydrogen bonding to them is complete over the entire mole fraction range. Dielectric constant measurements of


mixtures of dimethylformamide or dimethyl sulfoxide with water suggest the formation of species of the type DMF.3H20 and DMS0.2H20 (529). The effect on the solubility of polar organic solutes of adding polar organic solvents to a nonpolar solvent such as isooctane has been investigated (23). The bulk physical properties (viscosity, density, dielectric constant, refractive index) of mixtures of 3-methylsydnone with water have been reported (298). Viscosities and often other physical properties such as densities and dielectric constants have also been reported for binary mixtures of dimethyl sulfoxide with six other components (6), for propylene carbonate with dimethoxyethane (328) and with tetrahydrofuran (330),anld for acetone with dimethylformamide (171). Mixtures of dimethylacetamide with water show maximum solvent viscosity a t a 1:3 mol ratio; this suggests the presence of a 1:3 complex (26). Determination of Impurities in Solvents. Methods for evaluating the purity of dimethylformamide have been reviewed in Polish (83). Trace organic compounds have been determined in organic solvents by gas chromatography following preconcentration (46,61). A method for the detection and identification of trace organic impurities in binary solvent mixtures employs reversed-phaseliquid chromatography (62). Trace metals in methanol and 2-propanol have been measured by atomic absorption Eipectrometry (318). Dissolved oxygen has been deterinined in polar aprotic solvents by gas chromatography (5),and in both aqueous and organic solvents as well as in various binary mixtures with a modified oxygen electrode (175).

EXPERIMENTAL TECHNIQUES As in previous reviews, this section covers apparatus and systems useful for work in nonaqueous solvents. It includes various methods of end-point detection, new standards, and developments in analytical instrumentation. New methods for end-point detection in the Karl Fischer method for water, and new instrumental methods for water determination, are covered in the wection on the determination of water. Coulometric titrations are useful in nonaqueous analytical work because water is more easily excluded when the need for addition of external titrant is eliminated. The method has been reviewed by Ewing (133) and used by Glab and Hulanicki to measure the autoprotolysis constants of methanol and ethylene glycol (173). An automatic coulometrictitrator that compensates for drift in the end-point region has been patented (129). Computers and microprocessors are increasingly utilized for control of (automatictitrators (148) and for simulation and interpretation of titration curves (414). The factors affecting the shape of titration curves in nonaqueous media have been discussed by Stransky and Dostal (475). Use of the Gran method for the estimation of end points in the potentiometric titration of long-chained alkylaminopropylamines in methyl ethyl ketone was found to increase precision (323). Titration curves in phase solubility analyses where ion-pair formation is present can be interpreted by nonlinear regression methods, and the results used to obtain about both amount of material present and the extent of ion-pair formation (456). The magnitude of the error introduced into end-point location in conductometric acid-base titrations in water-methanol mixtures from conductance of the dissociated fraction of weak electrolvtes has been assessed by Karlik (227). The stability and suitability of a number of sulfonphthalein indicators for acid-base titrations in glacial acetic acid have been evaluated (420),m has the relatioGship between the color transition of methylrosaniline chloride and the equivalence point in the titration of sodium barbital and phenobarbital with strong acid in nonaqueous solvents (548). Methylene blue and malachite green were found applicable as indicators for the titration of organic compounds such as p-aminophenol and ascorbic acid with hexanitratocerium(IV)(525). Greenhow has continued his studies of ionic polymerization as a means of end-point indication in nonaqueous thermometric titrations with an investigation of the mechanism of the reaction at the end point when acrylonitrile is used as indicator for the thermometric titration of acids with alkoxides in alcohol solvents (176). Bark has discussed the thermometric and enthalpimetric determination of organic compounds in industry (35). 2,4-Dinitrobernzenesulfenylchloride has been found to give

similar results to benzoic acid in the standardization of solutions of sodium methoxide in benzene-methanol (350). A method for the determination of autoprotolysis constants of amphiprotic solvents is based on the potentiometric titration of two weak protolytes (520). A procedure for estimation of the ratio of solvent separated to contact ion pairs for ZnSOl in methanol by high field conductivity measurements has been developed (192). The method is likely to be oif general applicability. More and more information is appearing on selective ellectrodes in nonaqueous solvents. Pungor and Toth have reviewed the applications of ion-selective electrodes to analysis in nonaqueous solvents (396);a similar review in Japanese has bleen written (211). The behavior of the fluoride electrode in oirganic solvents has been studied (89). The effect of ethanol concentration on the sulfide ion-selective electrode as a sensor for measurement of lead(I1) ions in water-alcohol mixtures has been investigated (398). The causes of both acidic and alkaline errors of glass electrodes in the measurement of pH have been studied (397). Agrawal has reported correction factors for the conversion of pH readings into hydrogen ion concentrations in aqueous solutions of methanol (7) and of ethanol (9). A miniature glass pH electrode suitable for micropuncture techniques is more stable when a nonaqueous internal reference solution, such as silver nitrate in acetonitrile, is employed (426). The nonaqueous electrolyte solution prevented excessive hydration of the glass membrane, maintained tip dimensions, and gave Nernstian electrode response. The copper(I1) selective electrode was found to respond satisfactorily in a wide range of solvents, but not in acetonitrile (87).The failure of the electrode with certain copper(I1) salts in acetonitrile is considered to be caused by replacement of copper(1) and silver(1) in the AgzS-CuzS membrane by copper(I1). The effect of several water-immiscibleorganic solvents on the response of a lead ion selective electrode was studied (79). A silver bromide membrane prepared from AgBr and PVC dissolved in tetrahydrofuran was found to function satisfactorily as an indicating electrode for precipitation titrations of bromide with silver nitrate in nonaqueous solvents (225). The influence of varying water-acetone ratios on the potential of cation and anion exchanger membranes has been investigated (334). Acetonitrile appears to adsorb strongly on the surface of platinum and palladium electrodes (385). The behavior of lithium metal as an electrode in SOClzhas been studied (231). Sulfur, in several oxidation states, and lithium chloride are found in the passivating film on the electrode surface. The ellectrochemical generation of solvated electrons has been proposed to be a rather general cathodic process occurring in solutions of tetrabutylammonium tetrafluoroborate in ethanol ( ~ ' 9as ) well as other solvent systems. A cathode of mercury os amalgamated copper was used. The general subject of reference electrodes in nonaqueous solvents has been reviewed (209).

An electrochemical cell with an integral drying column of allumina is described by Kiesele as capable of reducing water content to below lV5 M (241). Conditions under which vanadium(II1) can be generated with 100% current efficiency from vanadium(IV) in acetic acid have been established. The procedure is reported to be applicable to the coulometric titration of organic and inorganic substances (262). The vanadium(V)/(IV) couple has been proposed as a catalyst for oxidation-reduction titrations in nonaqueous solvent,s, but actual applications have not been provided (368). The electrochemical characteristics of trifluoromethanesulfonic acid and its salts in nonaqueous solvents have been reviewed (208). Organic compounds separated by thin-layer chromatography on silica gel can be determined by isolation of the spot and microtitration (190). Basic compounds were titrated directly with perchloric acid without separation of the silica gel. For acidic compounds an extraction was necessry before titration with tetrabutylammonium hydroxide. Halogenated organic compounds could be decomposed by sodium in liquid ammonia and the liberated halide titrated with silver nitrate.

APPLICATIONS Because of space limitations all the work involving nonaqueous solvents in analytical titrimetry cannot be included ANALYTICAL CHEMISTRY, VOL. 54, NO. 5, APRIL 1982

111R


Table I. Some Substances Determined by Acid-Base Titrations in Nonaqueous Solvents substance

titrant

substituted phenylbenzimidazoles diazepam, napoton, nitrazepam aminophenazone, caffeine, Na salicylate thiamine-HC1 amino acids and their ammonium salts amino acids

HClO, HC10, HC10, HClO, HClO, HClO,

organic sulfides

HClO, in HOAc

pyrazole and derivatives glaphenine and glaphenine .HCl

HCIO, inHOAc HClO, inHOAc

meclozine .2HC1 berbamine bemosat trimet hoprim dabequine oxaminiquine pholcodine

HC10, HC10, HClO, HCIO, HClO, HClO, HClO, HClO, HClO,

trimethoprimsulfamethoxazole mixtures N-Me-N-(2-acetoxyethy1)morpholinium iodide is0niaz ide maleic anhydride

mixts. of n-BuNH, with PhNH,, PhNHCH,, PhN(CH,),, Py, 2-NH, Py, or picolines N-arylhydroxamic acids phenobarbitol-papaverine HCl

mixtures 5,5-dimethylhydantoin carbazoles, nitrocarbazoles amino acids with N-tert-BuOCO derivatives 1,2,3-benzotriazole-N-hydroxy -1,2,3benzotriazole cinnamic acid-chalcone mixtures organic acids and anhydrides L-carnotine .HCl stearic acid and Ca stearate in butyl rubber ampicillin, tetracyclineaHC1, doxycycline .HCl nicotinic acid phenolphthalein ammonium perchlorate-perchloric acid mixtures

in dioxane inHOAc in dioxane inHOAc inHOAc in HOAc

inHOAc inHOAc inHOAc inHOAc inHOAc inHOAc inHOAc inHOAc in HOAc

end-point method

Me,CO HOAc (EtCO),O-d'ioxane H0AC-AC,O 2:3 HOAC-EtOAc HCOOH-MeCOEt or HOAc-MeCOEt HOAc or Ac,O, Hg(0 Ac 1, HOAc, Hg(OAc), HOAc (plus Hg(OAc), if HCl salt) HOAc HOAc, H,O

pot., glass ind. electrode pot., glass ind. electrode pot., glass ind. electrode pot., glass ind. electrode pot., glass ind. electrode pot., glass ind. electrode

ref 417 393 443 363 402 273

pot., glass ind. electrode 34 pot., glass ind. electrode 503 pot., glass ind. electrode 501

CHCl, HO A&AC,O-C,H, HOAc HOAc Me,CO-HOAc HOAc, Hg(OAc),

conductance crystal violet ind. crystal violet ind. crystal violet ind. crystal violet ind. quinaldine red ind. conductance pot., glass ind. electrode crystal violet ind.

Ac O , DMF

double green ind. I1 pot., glass ind. electrode 517

HOAC-CHC1,

pot., glass ind. electrode 469, 470

Bu,NOH in C,H,MeOH Et,NOH in EtOH

DMF

o-N0,aniline ind.

8

DMSO

thymol blue ind.

98

Et,NOH in C,H,MeOH Et,NOH in C,H,MeOH Me,NOH in MeCN

DMF-L, H,

pot., glass ind. electrode 345

DMSO, DMF, MeCN, or Me,CO MeCN-H,O


KOH in 2-PrOH

2-PrOH-C, H,


KOH in 2-PrOH KOH in EtOH KOH in EtOH KOH in EtOH

Me,CO PrOH-C,H 14-H,O EtOH Me,CO t excess std HClO, (Me,N)*CO

pot., glass ind. electrode pot., glass ind. electrode pot., glass ind. electrode pot., glass ind. electrode

419 43 481

pot., glass ind. electrode or thymol blue ind. resazurin ind. conductance pot., glass ind. electrode, 2 end points

410

HClO, in dioxane HCl in MeOH (back titn of excess morpholine) chlorosulfonic acid in dioxane

NaOH in MeOH

HZo,

C6H6

NaOH in MeOH MeOH-C,H, NaOH in EtOH EtOH-H,O R,NOH or KOH in 2-PrOH or Me,CO EtOH or 2-PrOH

in this review. An increase in the use of mixed solvent systems, often with water as one component, seems to be evident. A few new solvent-titrant combinations have been introduced, but most of the work reports refinements on previously used systems or extension of known systems to the determination of additional substances. The use of dipolar aprotic solvents as media for organic synthesis will continue to be important, and useful analytical methods for organic compounds will evolve from quantitative studies of these reactions. Applications of nonaqueous solvents to other areas, such as battery technology and mineral and metallurgical extractions (371), can also provide ideas for new procedures. Acid-Base. A wide range of organic compounds, such as alcohols, esters, anilides, carbamates, acetophenones, and lactams, can be titrated as very weak acids in tetrahydrofuran with lithium [ l,l,l-trimethyl-N-(trimethylsilyl)]silanamide (84). End points can be obtained either potentiometrically with a platinum indicating electrode or visually with Nphenyl-p-aminoazobenzene as indicator. The titrant was stable a t least a week under a nitrogen atmosphere. Organic acids, phenols, and aromatic nitro compounds were determined by titration with diphenylguanidine in Z-methoxy112R

solvent


360 218 237 78

236 261 39 2 335 367


445

70 555

532

ethanol (395). End points were obtained conductometrically and were comparable in precision and accuracy to conventional nonaqueous titrations. The end point with nitrobenzene is as sharp as with phenol; no explanation for this behavior is available, but donor-acceptor complex formation seems likely. The weak organic base pholcodine was titrated conductometrically with tungstosilicic acid in acetic acid or in aqueous ethanol. The titration curves were more analytically suitable than those obtained with perchloric acid (392). A study of bases in acetic acid, and their differential titration, has been made (74).Differential potentiometric titration is feasible if the difference in the p& values is four or more. The color changes of indicators in nonaqueous systems have been considered, and the indicator transitions compared with theoretical titration curves (549). Organic amine hydrochlorides have been determined by conversion to the acetates by passage through a strong base anion exchanger in acetic acid as solvent (30); the acetate salt is then titrated with hydrochloric acid in acetic acid. The pK, values in water for 13 hydroxyanthraquinones are linearly related to the values in acetone, dimethylformamide, or dimethyl sulfoxide (150). Dimethyl sulfoxide was the most


satisfactory solvent of this group for analytical applications. Of four methods tested for the determination of penicillamine in a drug formulation, titration with perchloric acid was generally preferred, although somewhat less precise than titration with mercury(I1)acetate (534). Mixtures of polybasic benzenecarboxylicacids could be separated into fractions by dissolution in acetone, titration to the corresponding salts with sodium hydroxide in ethanol, and then stepwise back-titration with hydrochloric acid in acetone to intermediate pH values, each time stopping to extract the neutral acids present (514). Hydrochloride salts of peptides and amino acids were titrated, after addition of mercury(I1) acetate, with perchloric acid in nitromethane as solvent. This solvent suppressed interference from sulfur-containing groups by keeping them from reacting with mercury(I[) (112). The solubility of the peptides was increased by the addition of up to 10% formic acid; the formic acid did not affect the removal of sulfur interference by the nitromethane. Binary mixtures of methyl isobutyl ketone with methanol, dioxane, benzene, or acetic acid gave smooth titration curves with sharp inflections when used as solvents for the titration of organic bases with perchloric acid in methyl isobutyl ketone (207). Nitroaminobenizanilides can be titrated as bases in glacial acetic acid with perchloric acid, or as acids in dimethylformamide, N-methylpyrrolidone, or acetone with tetraethylammonium hydroxide in benzene-ethanol (145). The potassium alcoholates of the C3 to C5 alcohols were investigated as titrants for weak acids (57). Solutions of potassium butoxide andl potassium 2-propoxide in 15mixtures of the corresponding alcohols with toluene were found to be less sensitive to atmospheric carbon dioxide than solutions of potassium methoxide. A potentiometric study of the dissociation equilibria of substituted benzoic acids in cationic and anionic micellar systems showed pK shifts of less than unity in the cationic micelles and between 0.5 and 3 in the anionic micelles (380). Visual titrations af otherwise insoluble acids in cationic micelles gave satisfactory results. Oxygen in steel was measured by oxidation of the CO extracted from a steel sample with copper(I1)oxide, absorption of the resulting C 0 2 in 5 vol % monoethanolamine in dimethylformamide, and titration of the C02with tetrabutylammonium hydroxide in benzene-methanol (553). Sucrose was found a suitable standard reference material for the system. Mixtures of chlorinated acetic acids could be titrated potentiometrically in 41 acetone-acetonitrile with potassium 2-propoxide in 2-propanol as titrant if phthalic acid were present as a differentiating electrolyte (464). For complete analysis a separate aqueous titration on another aliquot after decarboxylation of the trichloroacetic acid was necessary. Tetrahydrofuran is not suitable as a solvent for the differential potentiometric titration of mixtures of nitrophenols with tetrabutylammonium hydroxide in benzene-methanol as titrant, but in 1:l mixtures with acetone, dimethyl sulfoxide, dimethylformamide,or methanol the differentiating properties were superior to any of the solvents individually (312). The free acid and anhydride content of samples of aliphatic or aromatic anhydrides can be obtained by dissolution in acetone and conductometric titration with sodium methoxide in methanol (457). A procedure for the determination of basic organomagnesium andl organolithium reagents is based on deprotonation of N-phenyl-1-naphthylamineto form the highly colored magnesium or lithium derivative, which is then titrated with 2-butanol in xylene (47). Boric acid can be titrated after conversion to tetrafluoroboric acid with sodium or potassium acetate in glacial acetic acid (197). N,N-Diethylaniline or pyridine could also be used as titrants; end points were obtained by potentiometry or conductometry. Some applications of acid-base titrimetry are given in Table I. Ion Pair and Complex Formation. The interest apparent in fundamental studies of ion pairing and complex formation is gradually carrying over into such analytical applications as titrations and solvent extraction systems. In this section selected applications indicate the types of reactions being studied. Macrocyclic ligands such as 18-crown-6 can be determined in methanol by thermometric titration with appropriate metal cations-bariuni(II), silver(I), otassium(1) and so on (289). The technique can be used to oEtain distribution coefficients of macrocycles between water and chloroform. The reverse

titration, of metal ions with ligand, can also be performed. Kuznetsov and co-workers estimated several metals and boron by spectrophotometrictitration of their complex fluoro anions with potassium in 50-90% dimethyl sulfoxide in water. Among the elements determined were boron, aluminum, gallium, indium, and thallium (285), antimony (286), and niobium and tantalum (284). In a somewhat related system the alkaline earth metals and aluminum were titrated conductometrically with fluorosilicic acid in dimethyl formamide-water mixtures (551). Aluminum was also determined b y titration with sodium fluoride in 50% aqueous ethanol arnperometrically using a platinum electrode (511) and potentiometrically using a glass indicating electrode (138). Polyamines such triethylenetetramine were investigated a13titrants for various metal ions in acetonitrile (15). Best results were obtained for copper(II), but cobalt(II), manganlese(II), nickel(II), iron(III), and magnesium(I1) were also dleterminable. A platinum indicating electrode gave the sharpest end points. Gevorgyan and co-workers have used EDTA in 2-propanol containing 10% water to titrate biamplerometrically cadmium, nickel, copper, iron(III), calcium, and magnesium (161);replacement of the water by up to 0.05 M monoethanolamine provided improved solubility of the EDTA and did not interfere in the titrations of the transition metals (154). Palladium(I1) has been titrated amperometrically with sodium diethyldithiocarbamate in mixtures of ethanol and propanol (156), and with lead diethyldithiocarbamate in toluene (155). In both cases the sample is dissolved in acetic add. Mixtures of palladium(I1)salts with other metal cations such as bismuth, silver, nickel, and copper have been titrated biamperometrically in glacial acetic acid (159). Titrants included thioacetamide, rubeanic acid, and soidium diethyldithiocarbamate. A similar titration of mixtures of palladium and gold with sodium diethyldithiocarbamate was reported in dimethylformamide (153);addition of 20-40% by volume of a second solvent such as chloroform, carbon tetrachloride, toluene, or benzene improved the shape of the amperometric titration curves. Bismuth was also titrated with sodium diethyldithiocarbamate in propanol (157). Best results for the titration of metal ions with 8quinolinethiol, 2,3-quinoxalinedithiol, rubeanic acid, and sodiium diethyldithiocarbamatewere obtained with a silvemilver sulfide indicating electrode (389). Mercaptans have been titrated conductometricdy in methanol, ethanol, acetone, and binary mixtures of these solvents with water (229). The titrants included mercury(I1) chloride in 60-80% methanol, copper(I1) sulfate in 80-100% methanol, cadmium(I1)sulfate in ammoniacal 60-100% methanol, and silver diammine hydroxide. Two chromogenic crown ethers have been synthesized and their use in the extraction and photometric determination of alkali metal ions assessed (355). Such compounds might be dleveloped as indicators for the compleximetric titration of metal ions with more stable ligands. An electrochemical study of the complexation of iron(lI1) with organic dicarboxylic acids such as oxalic, malonic, and tartaric in dimethylformamide has been made (130). A list of some specific analytical complexation titrations is given in Table 11. Two-phase titrations continue to find application in the determination of Surfactants, long-chain aliphatic compounds, and other hydrophobic ions. Thus berberine has been titrated with sodium tetraphenylboron, the ion pair being extracted into chloroform from an acidic aqueous solution (512). The ethyl ester of eosin was used as an extraction indicator. By using a two to three ratio of chloroform to nitropropane as the organic phase and by carrying out multiple extractions, the two-phase titration of anionic surfactants having short hydrophobic roups (

Titrations in nonaqueous solvents - Analytical ... - ACS Publications

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