Anal. Chem. 1980, 52, 151 R-161 R (8F) Vire, J.-C.; Patriarche, G. J.: Patriarche-Sepulchre, J. Anal. Lett 1978, 8 1 1 , 681-695. (9F) Johansson, B.-L.: Persson, B. Anal. Chim. Acta 1978, 702, 121-131. (IOF) Jacobsen, E.; Korvald, B. Anal. Chim. Acta 1978, 99, 255-261. ( 1 1F) de Boer, H. S.; den Hartigh, J.: Ploegmakers, H. H. J. L.: van Oort, W. J. Anal. Chim. Acta 1978, 102, 141-155. (12F) de Boer, H. S.; Lansatt, P. H.; van Oort, W. J. Anal. Chim. Acta 1979, 708,389-393. (13F) de Boer, H. S.; Lansatt. P. H.: Kooistra, K. R.: van Oort, W. J. Anal. Chim. Acta 1979, 1 7 1 . 275-279. (14F) Chatten, L. 6.:Moskalyk. R. E.; Locock. R. A,; Schaefer, F. J. Analyst (London) 1978, 103. 837-841. (15F) Smyth, M. R.; Smyth, W. F. Analyst (London) 1978, 103, 529-567. (16F) Smyth, M. R.; Osteryoung, J. G. Anal. Chim. Acta 1978, 96, 335-344. (17F) Smyth, M. R.; Osteryoung, J. G. Anal. Chem. 1978, 5 0 , 1632-1637. (18F) Samuelsson, R. Anal. Chlm. Acta 1978, 102, 133-140; 1979, 108, 2 13-219. (19F) Smyth, M. R.; Lawellin. D. W.: Osteryoung, J. G. Analyst(London) 1979, 104, 73-78. (20F) Fogg, A. G.; Ahmed, Y . 2. Anal. Chim. Acta 1978, 701, 211-214. (21F) Henry. F. T.; Kirch, T. 0.;Thorpe, T. M. Anal. Chem. 1979, 57, 215-218. (22F) van Leeuwen, H. P. J . Electroanal. Chem. 1979, 99, 93-102. (23F) Kirowa-Eisner, E.; Osteryoung, J. Anal. Chem. 1978, 5 0 , 1062-1066. (24F) Elton, R. K.; Geiger, W. E . , Jr. Anal. Chem. 1978, 50, 712-717. (25F) Lowry, J. H.; Smart. R. B.: Mancy, K. H. Anal. Chem. 1978. 50, 1303-1309. (26F) Masschelein, W. J.: Denis, M.; Ledent, R. Anal. Chim. Acta 1979, 707, 383-386. (27F) Kopanica, M.; Stara, V. J . Electroanal. Chem. 1979, 98, 213-221. (28F) Stara, V.; Kopanica, M. J . Electroanal. Chem. 1979, 101, 171-175. (29F) Bosserman, P.; Sawyer, D. T.; Page, A. L. Anal. Chem. 1978, 50, 1300-1303 .- - - . - (30F) Christian, G. D., Vandenbalck. J L ; Patriarche, G J. Anal C h m Acta 1979. 108. 149-154. (31F) Alam, A. M. S.;Vittori, 0.;Porthault, M. Anal. Chem. Acta 1978, 702, 113-1 19. (32F) Hitchen, A. Talanta 1979, 26, 369-372. (33F) Greter, F.-L.; Buffle, J.; Haerdi, W. J . Electroanal. Chem. 1979, 101, 21 1-229. (34F) van Leeuwen, H. P. Anal. Chem. 1979, 5 1 , 1322-1323. (35F) Fogg, A. G.; Osakwe, A. A. Talanta 1978, 2 5 , 226-228. (36F) Alam. A. M. S.: Martin, J. M.; Kapsa, Ph. Anal. Chim. Acta 1979, 107, 39 1-393. (37F) McLean J. D.; Stenger, V. A.; Reim. R. E . ; Long, M. W.; Hiller, T. A. Anal. Chem. 1978, 50, 1309-1314. (38F) Fogg, A. G.; Fayad, N. M.; Burgess, C. Anal. Chim. Acta 1979, 110, 107- 115. (39F) Bruno, P.; Caselli, M.; Monica, M. D.; di Fano, A. Talanta 1979, 2 6 , 1011-1014. (40F) Hu. H.-C. Anal. Chim. Acta 1979, 707, 387-390
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(41F) Hanck, K. W.; McGaughey, J. F. Anal. Chim. Acta 1979, 107, 75-82. (42F) Niki, K.; Yagi, T.: Inokuchi, H.; Kimura, K. J . Am. Chem. SOC.1979, 10 1 , 3335-3340. (43F) Canterford, D. R.; Taylor, R. J. J . Electroanal. Chem. 1979, 98, 25-36. (44F) Schaar, J. C.: Smith, D. E . J . Electroanal. Chem. 1979, 100, 145-157. (45F) Jacobsen, E . ; Bjornsen. M. W. Anal. Chlm. Acta 1978, 96, 345-351. (46F) Nagaosa, Y . Talanta 1979, 26, 987-990. (47F) Canterford, D. R. Anal. Chim. Acta 1978, 98, 205-214. (48F) Ficker. H. F . ; Ostensen, H. N.; Schlossei, R. H.; Scott, F.: Spritzer, M.; Meites, L. Anal. Chlm. Acta 1978, 98, 163-169. (49F) Green, J. B.; Manahan, S. E . Anal. Chem. 1979. 5 1 , 1126-1129. (50F) Hammerschmidt, R. F.: Broman, R. F. J . Hectroanal. Chem. 1979, 99, 103- 110. (51F) Clark, G. C. F.; Moody, G. J.; Thomas, J. D. R. Anal. Chim. Acta 1978, 98,215-220. (52F) Brunt, K. Anal. Chim. Acta 1978, 98, 93-99. (53F) Rao, V. S. N.; Rao, S. B Talanta 1979, 26, 502-504. (54F) Squella, J. A.; Nunez-Vergara, L. J. Talanta 1979, 26, 1039-1040. (55F) Braun, R. D.; LoVerso, M. R . Talanta 1979, 2 6 , 185-188. (56F) Toropova, V. F.; Budnikov, R. G. K.; Ulakhovich, N. A. Talanta 1978, 25, 263-267. (57F) Smyth, W. F.; Smyth, M. R.; Groves, J. A.; Tan, S. B. Analyst(London) 1978. 103, 497-508. (58F) Rozanksi, L. Analyst (London) 1978. 103, 950-954. (59F) Watson, A. Analyst (London) 1978, 103. 332-340. MISCELLANY
(IG) Weber, S. G.; Purdy, W. C. Anal. Lett. 1979, 12, 1-9. (28) Heineman, W. R.; Anderson, C. W.; Halsall; H. B. Science 1979, 204, 865-866. (3G) Flanagan, J. B.; Margel, S.: Bard, A. J.: Anson. F. C. J . Am. Chem. SOC. 1978, 100. 4248-4253. (4G) Saji, T.; Pasch, N. F.; Webber, S. E.; Bard. A. J. J , Phys. Chem. 1978, 8 2 , 1101-1 105. (5G) Fenton, D. E . ; Schroeder. R. R.; Lintvedt. R. L. 1978, 100, 1931-1932. (6G) Ryan, M. D.; Wei, J.-F.; Feinberg, 8. A.; Lau, Y.-K. Anal. Blochem. 1979, 96,326-333. (7G) Hernandez-Mendez, J.; Sanchez-Perez, A.: Rubio-Miron. A. Anal. Lett. 1979, 72,1315-1337. (8G)Malpas, R. E.: Fredlein. R. A.; Bard, A. J. J . Electroanal. Chem. 1979, 98, 171-180. (9G) Malpas, R. E.; Fredlein, R. A.; Bard, A. J. J . Electroanal. Chem. 1979, 98,339-343. (10Gl Nelsen. S. F.; Clennan, E. L.; Evans, D. H. J . Am. Chem. SOC.1978, 100, 4012-4019 (11G) Beztlla, B. M.. Jr.; Maloy, J. T J Electrochem SOC 1979, 126, 579-583 (12G) Tinker, L. A. and Bard, A J J Am Chem SOC. 1979, 101, 23 16-23 19
Titrations in Nonaqueous Solvents Byron Kratochvil Department of Chemistry, University of Alberta, Edmonton, Alberta, Canada T6G 2G2
This review covers approximately the literature reported in Chemical Abstracts over the period December 1977 to December 1979. As in previous reviews, emphasis has been placed on fundamental developments and on methodology, followed by those applications that appear to illustrate new approaches or trends. Space limitations have forced deletion of many references in both fundamental and applied areas, and inevitably many quality papers have had to be omitted. Overall, a small but noticeable decrease in the number of applications of nonaqueous titrations seems to be occurring. This is especially apparent in the area of pharmaceutical analysis, where nonaqueous acid-base titrations are being replaced in many cases by liquid chromatography. On the other hand, an increase in the use of constant current coulometry in nonaqueous solvents is evident. In fundamental studies, considerable work is being done on both ion-solvent interactions and on the behavior of polar neutral solutes. Much of the interest in this area is kindled by the importance of nonaqueous solvents in organic synthesis, where reaction solvents are still selected primarily on an empirical basis. Progress in explaining solvent effects on ionic and molecular reactions is slow, but a large number of experimental tools 0003-2700/80/0352-15lR$Ol .OO/O
are being brought to bear on the problem, and much information is being obtained. This area is likely to remain important to synthetic and solution chemists for some time to come. Its impact on analytical methods has not yet been felt, but is beginning to appear in new methods for organic functional group analysis, for example.
BOOKS AND GENERAL REVIEWS Books and reviews of relative breadth that have appeared over the period covered by this review are included here; those that concentrate on specific areas are included under the appropriate subject headings of this review. Two more volumes, have appeared in the series “The Chemistry of Nonaqueous Solvents”. Volume VA contains a chapter on solvation and complex formation in protic and aprotic solvents ( 1 1 ) ,and one on solvent basicity (33). T h e proceedings of a 1976 symposium on spectroscopic and electrochemical characterization of solute species in nonaqueous solvents (184),and of a 1977 symposium on solvent parameters (142),have appeared. Friedman has reviewed developments in the area of structure of electrol>Te solutions over the period 1952-1977 (88). Another survey treats electrolyte solutions C 1980 American Chemical Society
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from the standpoint of a semiphenomenological approach that includes short-range interactions (26). In this article emphasis is placed on the great temperature dependence of many nonaqueous solution properties. Gutmann has reviewed his donor and acceptor number concepts as empirical solvent parameters for the characterization of specific solvent-solute interactions (114),and these two parameters, plus the standard free energy of vaporization of t h e solvent, are proposed for describing t h e influence of solvent effects on chemical reactions (192). In two reviews, t h e use of NMR to study the structure of electrolyte solutions is covered; in one a more general discussion is provided (66),while in the other emphasis is placed on the determination of contact ion-pair formation constants and on the thermodynamics of complexation between alkali metal ions and crown and cryptand ligands (236). Concepts of the leveling and differentiating action of solvents have been reviewed (172),as has the solvation and complexing of mercury ions in nonaqueous solvents (18) and the use of titration microcalorimetry to determine partial molar enthalpies, equilibrium constants, and heats of formation (76). Several reviews dealing with analytical applications have appeared. Two discuss solvents in pharmaceutical analysis; one covers the structure of solvents, data on solvent properties useful in selection of a solvent for a particular application, and problems arising from solvent impurities (85),while the other treats the methodology to be used in finding the optimum solvent for hard-to-dissolve materials (54). Also, a Japanese review on the general topic of titrations in nonaqueous solvents (320) and another on titration of inorganic species (321) have been published.
FUNDAMENTAL Solvation. Solvent-solute interactions determine in large measure the magnitude of solute solubilities, of ion pair, complexation, and other equilibrium constants, and the overall analytical utility of different solvents. Accordingly studies of these interactions are important. Progress in understanding and predicting solvation properties is being made, but slowly, a n d i t is clear t h a t new developments in theory are badly needed t o indicate t h e direction experimental work should take. A general review discusses the various techniques and theoretical approaches t h a t have been developed in solutesolvent studies in pure and mixed solvents, and relates them t o classical thermodynamics (67). Solvent effects on ion pairing have also been reviewed; Bjerrum's theory of ion pairing is reported to be remarkably accurate for the calculation of osmotic coefficients, but is not satisfactory when solvent structural implications related to ion pairs are considered (89). The general topic of current concepts about how solvents affect ionic reactions has been treated by Ritchie (254). Evidence is presented supporting the idea that the motion of nearby solvent molecules along the reaction coordinate of a transition state must be considered in the formation a n d stabilization of the transition state. In another review, t h e measurement of dipole moments of electrolytes in glacial acetic acid to yield information on t h e formation of solvation complexes between ion pairs and solvent molecules is discussed (112). The use of infrared and NMR to investigate ionic solvation in methanol and water has also been the subject of a review (287). More a n d more experimental data are being obtained in solvent mixtures, but it is necessary to treat each mixture as essentially an independent solvent system. For neutral highly polar solutes in polar solvents, reaction field theory has been found to give a satisfactory description of experimental results involving NMR shifts, solvation free energies, activation free energies, T-T* transitions and molecular dipole moments (1). Short-lived clustering of polar solvent molecules around dipolar solutes is considered t o be a significant factor in explaining solvent effects on dipolar solutes. Reaction field theory can be extended to solutes of high polarity if allowance is rhade for dielectric saturation of the polar solvent molecules. In another study, discrete and continuum models have been compared for the solvation of ammonium ion in water and ammonia, and methanol in water (62). Many investigations of ion-solvent systems have been made. A statistical mechanical treatment of ions in a polar nonpolarizable solvent considers the molecular solvent to be on an 152R
ANALYTICAL CHEMISTRY, VOL. 52, NO. 5, APRIL 1980
equal statistical footing with the ions rather than a structureless continuum (122). Extrathermodynamic methods for the evaluation of properties of individual ions in solution have been evaluated and numerical values for various individual ionic functions tabulated (65). Solvation of negative ions by both protic and aprotic solvents has been measured by gas phase ion equilibria. This approach measures enthalpies of ion solvation without the complication of bulk solvent being present. Solvation of halide ions in methanol has been studied by infrared (280). The relation between gas phase acidity and the formation of ion-solvent complexes in solution has been discussed (144). The solvation free energy of electrons produced by electron photoemission in dimethylformamide, methanol, and water has been estimated and correlated with thermodynamic quantities and structural parameters of solvents (127). T h e interaction of electrons with solvent molecules is interpreted as being purely electrostatic; the solvation free energy therefore has been suggested as a criterion of the electrophilic properties of solvents. Solvent effects on ion-ion interactions also continue to receive much attention. Fuoss has shown t h a t t h e simple model of charged spheres in a continuum fails completely when the ions are small compared to solvent molecules (90). In another study, models incorporating a term for short-range forces in calculating the energy of interaction between ions are discussed (25). T h e temperature dependence of the equilibrium constant for ion association permits distinction between different types of short-range forces. The mass action equilibrium expression between free ions and ion pairs in solution, introduced a priori in the chemical model of Bjerrum, has been confirmed by recent results based on statistical mechanical treatments of ionic solutions (141). For 1 1 ionic surfactants in solvents of low dielectric constant, two distinct patterns of aggregation appear to occur, one cyclic dimers and compact oligomers and the other linear dimers and open-chain oligomers (203). NMR continues to be a useful tool for solvation studies. For example, proton magnetic resonance has been used to investigate the interaction of a variety of ions with the solvents 3-methyl-8-oxazolidone (259)and methanol (288). Proton and carbon-13 NMR has been used to study nickel(I1) in mixtures of dimethylformamide and methanol (73),and thallium-205 to study ion pairing between thallium(1) and dimethylthallium(1) with nitrate and perchlorate in pure solvents (46) and in solvent mixtures ( 4 5 ) . Conductance also remains a valuable technique for ionsolvent studies. Justice has developed a new formulation of molar conductance which is similar to the empirical conductance function obtained by grafting the chemical model of Bjerrum onto the equations derived on the basis of Debye's work (139). T h e result allows generalization of the conductance equation to any direct short-range cation-anion energy potential model. Fuoss has revised his 1975 equation to take into account both contact and solvent separated ion pairs (91). Werblan and co-workers have extended the 1933 Fuoss-Kraus equation to calculate the dissociation constant of three-ion clusters as well as of ion pairs in solvents of low dielectric constant (315), while Lee and co-workers have used an adaptation of the 1972 equation of Quint and Viallard for the same purpose in dealing with alkaline earth halides in methanol (176). Through theoretical derivation of excess thermodynamic functions, experimental data have been reduced in terms of parameters characterizing specific short-range cation-anion interactions that represent the energy involved in overlap of the ion solvation cospheres ( 1 4 0 ) . The contribution of noncoulombic forces to ion-pair formation has been investigated by high-precision conductance measurements of alkali metal and tetraalkylammonium salts over a wide temperature range in propanol, acetonitrile, and propylene carbonate (28),and of selected tetrapropylammonium salts in ethanol ( 2 7 ) . Measurements in propylene carbonate of several tetraalkylammonium salts at 25 "C (228) and of a series of alkali metal iodides from 0 to 50 "C (181),have been made. Eight sodium phenolates have been studied in dimethyl sulfoxide (290). Ion-pair association and phenol dimerization constants were measured; both depend on the nature of the substituents on the phenol rings. Other systems studied include long-chain amine hydrochlorides in 2-methoxyethanol (231, alkali metal iodides and thiocyanates in hexamethylphosphotrianiide ( I 161,
T!TRATIONS I N NONAQUEOUS SOLVENTS
Byron Kratochvll, Professor of Chemistry and chairman of the analytical division at the University of Alberta, received his B S M S and Ph D degrees all from Iowa State University He was a faculty member at the University of Wisconsin--Madison from 1961 to 1967, when he joined the chemistry department at Alberta His research interests are in the areas of solvent effects on solute properties and reactions, applications of nonaqueous systems to chemical analysis, and methods for determining substances of ciinical interest He also has a strong interest in the toaching of analytical chemistry
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rubidium iodide in n-alcohols (95).and a variety of salts of large cations in nitromethane (118). Increasing attention is being paid to the properties of ionic solutes in solvent mixtures or in solvents containing molecular compounds t h a t interact preferentially with certain ions. A few examples are given here. Ion pairing and triple ion formation of lithium perchlorate in methanol, tetrahydrofuran, and ethyl acetak have been studied and compared with results in various mixtures of methanol and tetrahydrofuran (212). T h e effect of citric acid on the conductance properties of tetraalkylammonium salts of several anions in acetonitrile has been investigated (124, 125). T h e observed variations in ion-pair association constants with citric acid indicate that chloride and bisulfate ions bind with two molecules of citric acid, and that undissociated ion pairs can complex with one molecule. Additional solvent mixtures that have been studied include water-methanol, methanol -dimet hylformamide, and dimethylformamide--benzene ( I Y 4 ) , :'-hutanone with hexam e t h y 1p hos p h ot r iamide , t r i p hen y 1 phosphine oxide, d i methylsulf'oxide. triethanolamine, and glyme-5 (1:W). acetone--water (%'), and tetrahydrofuran ethylene glycol ( 2 3 4 ) . Triple ion formation has been investigated !'or the LiBr in 1-octanol system; here triple-ion association constants were calculated assuming equal probabilities of forming Ii,Br+ and LiBr2 (97). With weak acids and salts of' their conjugate bases, further aggregation is possible. Thus. in mixtures of 2,,5-dimethylphenol (HA) and its sodium salt ( N a A ) in dirnethylsulfoxide, conductivity data suggest tormation of the salt Nn(HA2) and the anion H2Ad , (291). T h e conductance behavior of solutions of KBr or LiBr and .4I2I3rti in toluene is as expected a t low concentrations, but increases abruptly a t high concentrations (251). From transference number measurements, the predominant ionic species a t high concentrations are though to he tip(AI,Bi;)+ and K(A12Br7)2(250). On the other hand. AICI,, does t i c i t react with alkali metal perchlorates in tetrahydrofuran. while MAICI, and insoluble A1CI,(CI04).211)ME form in 1.2-dimethoxyethane (DME) 1902). 'I'lie dissociation of AlBr and of t!le alkali metal bromides in propylene carbonate has also been studied by conductance (189). T h e alkali metal salts are strong electrolytes as expected: AlHr,{ is only slightly coliducting. Association of LiC1 is high in sulfolane but decreases on addition of methanol even though the dielectric constant decreases (229). A range of other techniques have been employed for solvent-solute studies. Heats uf solutions of' several 1 1 electrolytes in hexamethylphosphoraniide are higher than in the hydrogen-bonding solvents formamide and methylformamide (223). Previously proposed correlations hetu.een overvoltage and ion solvation energies in different solvents are not fullowed for the hydrogen ion in dimethylsulfoxide (LY9). 'I'he hydration of anions in low dielectric solvents such as nitrobenzene and u-dichlorobenzene has h e n studied by Arnett and co-workers ( I S ) . Isopiestic measurements have provided useful information o n hydration o f ions in l o ~ rdielectric constant organic solvents (14). 'I'he moles of excess solubilized water per mole of tetralkylammonium salts can be correlated with a number of anion properties. T h e results support the conclusion t h a t isolated water molecules d o i i o t have an e x ceptional inherent affinity for anions, and that the large anion hydration energies seen in aqueous solutions reflect extensive cooperative interactions in the solvent. Solvation studies o f acids by water and diethyl ether in sulfolane as solvent through vapor pressure and calorimetric measurements indicate that
the species Ht(H20),,*with ri equal to 1 t o 4, can form, while for the corresponding H+(Et,O),, species, n is 1 only. T r a n s f e r Activity Coefficients. Work in this area continues, with a range of experimental techniques being applied. T h e present state of knowledge has been reviewed by Popovych (23Y), and by Brisset ( 4 7 ) . Brisset suggests that none of the extrathermodynamic assumptions used today t o compare acidity scales between two solvents can be selected as best in representing physical reality. 'I'he proposed experimental method for the estimation of absolute half-cell potentials, mentioned in the last review, has aroused coninient (104, 901); meanwhile, another approach involving use .of nonisothermal galvanic cells to estimate ionic activity coefficient functions has appeared (282). T h e relation t)etween standard free ener ies of' transfer between acetonitrile as reference solvent a n 2 other solvents and the Gutmann donor and acceptor numbers of the solvents has been studied (191). 'I'he assumption t h a t t h e Ph4Asf cation and the Ph,B anion each have the same standard free energy of transfer XIo from one solvent to another has been compared with a model in which the electrostatic and nonelectrostatic parts of the free energy are estimated separately and then summed (155). Agreement with experimental values was reported to be satisf'actory. T h e same approach also gave satisfactory results for water-acetonitrile and water-dimet h y 1form a in id e solvet 1t mixt ures ( 156). Measurement of heats of transfer, eith,?r directly or indirectly, can offer much information about solution structure. Thus, all AHt values for the transfer o f electrolytes and nonelectrolytes between water and tert-butyl alcohol are positive, and show a niaxiinum owing to enhaiicement of solution structure ( 1 W . From heats of solution, the eiithalpies of transfer for nickel(l1) perchlorate from water to several polar solvent. were calculated and the values for nickel compared with those previously obtaitied for ~nangant~se(II) (64).A similar study has been reported for lithium perchlorate, in which enthalpies of tratisfer from acetonitrile or propionitrile to ayue~iustnixttires ot t h e solvents were measured (300),and for a number of 1 1 electrolytes from water t o 1-propanol (3). Ion transfer to mult i-site holvents and to solvent mixtures has been studied ti\. I'drker ( 2 1 8 ~ ) . ISlectrochemictrI mimurementh also are valuable sources of informatioil i i 1 1 transfer activity coet'ficients. Amnlgan and glass electrodc>sgave 4milar results for free energies of transfer of alkali nietal chlorides between water and ayiieous tcrt-butyl alcohol (6'3). 'I'he silver, silver(1) [ 2 2 J cryptate couple has been investigated tis a solcent-iiidepetid~iitreference couple, and gave wtisfiic.tory results in water, Itiethanol, acetotiitrile, diriiethylsultoxide. tetrcitnethylurea, and,nitroinethane ( 177). ,4cornbinati,in of ii Strehlou-type oxidation- reduction couple and ion selcctiw electrodes has been proposed as the basis for a new met hod o f estimating single ion activity coefficients by ;l-electrcidr polarography ( 2 8 9 ) . Ube o!' cesiiin~(l)and a cesium aniolgani elect rode as a solvetit-independelit oxidation-reduction couple to estimate liquid junctioii potentials between solventh \vas studied by Kondinini and w-workers (2.76. 2.57). Single.iiil1 values of the transfer free energies and entropies f o r ii number of .siniple cations and anions from water to nit~thanol,ethanul. mid 1 - p r o p u i i o l were obtained from solubility ~iie:isurements (2). 'I'runsfer tree energies increase with higher tnolecular weight of the alcohi~l.Another tern that has herti proposed for coniparing potentials between solvents is the osidation reduction couple consisting ( i f a niono- o r polynuclear arornat ic hydrocarbon and either its radic'il cation or anion (IN)). T h e difference between cation-neutral and neutral-anion potentials for severid hytlrocarhonz is constant in various sol\'ents. Solvent distritiution eyuilibria between w a t e r iiiid several organic solvents such a s 1-hexanol and chhoforrti were measured tor ii nuintier of 1%'-acetyl ethyl esters uf amino acids (_"OH). The therniod>mim;c paranietcJrsfor translbr were found t o be little affected b y iiitroduction ot' an ester group. 'The transt'er activity cort't'icients of Hiimmett neutral indicators from a ret'erence solvent to mixtures of water, acetic acid, and strong inorg:binic.iiiids were tiieasiiretl direc.tly tiy solvent dia t r i t i u t io ti t q 11i 1 i tir i uni met hods ( i , 5 ), 'I'he general are2i of reaction kinetic5 and s o l ' i ~ ~ i t i o in t i nvnaquetrus soIvtLtits has heen reviewed 1)y ('aldiii (.53). Solvent el't'ec~s011 rates m d ~necha~tisnis of t ratisition n i r a t a l conipleses have 1 ) r ~ (,itiisidt'r(d n by Hurgess (,TO): IVatts a130 has re\.icwed ANALYTICAL CHEMISTRY. VOL. 52. NO. 5. APRIL 1980
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this topic, and points out t h a t the role of solvents in determining inorganic mechanisms is much smaller than in carbon chemistry owing to solvolytic effects and to the counteracting of changes in anion solvation by changes in solvation of the cationic residue (314). T h e effect of ion pairing on the reactivity of solvated eiectrons in ethanol has been studied (219). The rate of reaction of solvated electrons with nitrate ion is comparatively slow, but i s increased when ions such as magnesium or calcium are added because ion pairing of nitrate with the divalent cations reduces coulombic repulsion between the electrons and nitrate ion. Studies of the effect of' solvent donor strength on the substitution kinetics of nickel(I1) have been extended t o the lower alcohols (58). Electron transfer rates of tetraphenylporphyrins and their iron complexes have been investigated in dimethylsulfoxide and in dimethylacetamide (214). Electron Transfer. T h e use of nonaqueous solvents in organic electroanalytical chemistry has been surveyed (329). Other general discussions include one on solvent dipoles a t the electrode-solut,ion interface (1 2.51, and another on ionselective electrodes in nonaqueous solvents (244). The acidbase and oxidation-reduction properties of liquid ammonia have been conipared with water (48). T h e propert,ies of solvated electrons and their generation have been outlined in liquid ammonia (2961, hexamethylphosphotriamide (187,327),and dimethylsulfoxide (327). The superoxide anion radical, O,-, is highly stable (t,,? = 20 h) in dimethylformamide ( 4 ) . It was determined by reaction with butyl bromide or benzoquinone. The stability of alkali metals in many aprotic solvents has made these systems attractive for incorporation into high energy density batteries. Thus, much interest has been shown in such systems. Reactions of tetrahydrofuran and lithium hexafluoroarsenate with lithium metal have been evaluated (258). The AsF; anion is reduced t o A S F , ~by lithium, while tetrahydrofuran is reductively cleaved to butoxide anion. ethylene, and t.he enolate a n i m of acetaldehyde. The 2-methyl derivative of tetrahydrofuran is reported to be stable t o reduction by lithium, and warrants study as a medium for reactions involving strong reducing agents (159). Solvent effects on the electroreduction of the alkali metal cations were found to be marked for several aprotic dipolar solvents and methanol (19). Sodium ion--sodium amalgam concentration cells were studied over a range of temperatures in dry propylene carbonate ( 1 6 ) . Thermodynamic quantities were determined. The standard potential of the potassium ion-potassium amalgam couple was determined in ethylenediamine (182). Several workers have explored the hydrogen halides in ems. A hydrogen electrode was used to nt interactions between hydrogen chloride and t h e lower alcohols (238):the interaction increases with increasing molecular weight of the solvent. A similar study was reported in 2-methoxy ethanol (193). T h e silver- silver bromide and hydrogen electrode combination provided information on the thermodynamic behavior of hydrobromic acid in mixtures of water and N-methylacetamide over a 40 "C temperature range ( 5 1 ) . Five studies have appeared on the effect of solvent on the standard electrode potentials of the silver halide electrodes in alcohol--water mixtures (79-83). T h e standard potentials of the silver chloride and hydrogen electrodes in ethanol, propanol. and hutanol also have been measured (286). Values .for the standard potential of the silver--silverchloride couple in most of the common solvents have been tabulated (279). Among the metal ion couples and metal complexes whose oxidation--reduction potentials have been reported are the ferricinium- ferrocene couple in mixtures of water and inorganic acids with acetic acid. acetonitrile, acetone, dimethyl sulfoxide, and dimethylforrnamide ( 17 , 270). the hexacyanomanganese(1V--111) couples in 13 dipolar solvents ( 2 1 I ) , the gold--gold (--I)couple in liquid ammonia (297),the vanadate-vanadyl couple in dioxane-water. acetone-water, and dimethylforrnamide-water (247).a i d the (1V-111) and (111 -11) couples of the lanthanides and actinides in and two fused salt mixtures ( 7 8 ) . Other are the (SeCN),/SeCN couple in aceton redox reactions of a series of' substituted liipyridiiie and phenanthroline complexes of manganese(IV),( I I I ) , and (11) in acetonitrile (201. 202). A tabulation of pularoyraphic 154R
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half-wave potentials of inorganic substances in dimethylformamide, along with information on dimethylformamide as a solvent, provides a useful guide to couples of possible analytical utility (128). T h e charge transfer rate and mechanism of anodic complex formation between mercury and [222] cryptate was investigated in propylene carbonate and in dimethylformamide (240). An interesting solvent-stabilized species is the free-radical anion of azobenzene in dimethylformamide; when lithium ions are present, however, the lithium ion pair with the azobenzene anion (stability constant = 9) causes rapid disproportionation of the anion (143). Electrochemical reactions in rigorously dry N-methylpyrrolidone, dimethylacetamide, and dimethylforniamide were examined with the goal of studying reactions involving strong bases (44). Electron exchange occurs normally over the whole activity range, but the hydrogen electrode functions only in the acidic half of the p H range; this is analogous to previous observations in tetrahydrofuran and dimethoxyethane. Finally, the oxidizing power of the periodate ion in four solvents has been determined polarographically (241). Conductance and UV spectral studies show the existence of several forms of periodate, depending on the composition of the solvent. T h e effect of solvent on the rate of oxidation of pinicol and ethanediol by periodate was also studied. Acid-Base. The general topic of proton transfer in solution has been considered in a fundamental way by Erdey-Gruz and Lengyel ( 8 4 ) and by T a f t ( 2 9 I a ) . Volume 2 of the second edition of the "Treatise on Analytical Chemistry" includes five chapters on acid-base equilibria in various classes of solvents (269, 164, 237, 238, 276). Kolthoff has reviewed solvent acid-base properties and hydrogen-bonding effects on acid-base titrations, and has discussed the use of dipolar aprotic solvents in analysis (162). Other reviews include one on the so-called superacids and their applications in organic synthesis (2181, and another on pH values of calibration buffers in most common solvents (279). Acid-base properties of various compounds in a variety of solvents can be predicted on the basis of proton-electron-hydride interactions (171). The properties of 2-propanol and tert-butanol as solvents for acid-base systems have been studied (56, 167,168),as have the equilibria of hydrochloric and hydrobromic acids in these so1vent.s (166). The basicity of several organomagnesium and organosodium compounds in the presence of hexamethylphosphotriamide or cryptate 222 has been measured in tetrahydrofuran with a hydrogen electrode (601, a?has the acidity of several compounds in dimethylsulfoxide (62). In other acid-base studies, 34 nitrogen-containing asphaltenes with a wide range of basicities were titrated in acetophenone and nitrobenzene (70). T h e compounds, isolated from coai tars, could he divided into five classes on the basis of acidity. Homoconjugation constants for a number of substituted acetates and benzoates have been determined in acetonitrile by potentiometric titration (135), as well as for several weak acids in propylene carbonate by conductance, spectrophotometric, and potentiometric measurements (2311. Dissociation constants for several weak organic acids in dimethylsulfoxide were measured by spectrophotometric and potentiometric methods and compared with published constants in dimethylsulfoxide and water (268a). Among the dissociation constants measured by spectral and potentiometric methods are those of several nitroanilines and picric acid in acetone (86), of HS04 and H C 0 , 3ions ~ i n formamide over a range of temperatures ( 7 1 ) , and of tetrafluoroboric acid in a series of alcohols and dipolar aprotic solvdnts (284). Equilibrium constants for the reaction of four nitrophenols with mono-, di-, and trihutylamine in the solvents chlorobenzene, ethyl acetate, and carbon tetrachloride have been measured spectrophotometrically (236), as have the dissociation constants of a large number of azo and sulfophthalein dyes in dimethylsulfoxide (233). Iiolling has tabulated dye transition energies for three indicators in representative nonpolar. polar aprotic, and amphiprotic solvents (261). Transition energies were correlated with other solvent parameters such as the Gutmann acceptor number and the Taft scale for hydrogen bond donor solvents. Acid dissociation constants for a large number of systems have been determined spectrophotometrically in 90% methylene chloride--l07~ methanol, and found to parallel the values seen i r i pure methanol (38). Georgieva has studied the acid- base
TITRATIONS I N NONAQUEOUS SOLVENTS
properties of several carboxylic acids in 80% dimethylsulfoxide20% water, and considers that the mixed solvent system offers better titration conditions for the determination of these neutral acids than either water or other nonaqueous solvents (94). A generalized acid-base treatment has been proposed for solvolysis of salts of weak acids or bases in various solvents, and titration curves have been calculated by the treatment and compared to experimental ones (10). In a related paper, a mathematical description is given of the equilibria occurring on reaction of mixtures of monoprotic acids with bases of varying strength in nonaqueous media (149, 150). A disadvantage of the approach is that homoconjugation equilibria are not taken into account. A linear titration plot procedure has been developed to calculate the equivalence point in the determination of partially neutralized weak acids or bases, and has been programmed in FORTRAN (195). Another computer program for calculating curves for weak base-strong acid titrations in various solvents uses a third degree polynomial expression (9). Complexation. Ahrland has discussed complex formation in both protic and aprotic media (12);a less accessible review has been written by Burger (49). Considerable interest has been shown in the properties of macrocyclic ligands and their complexes with metals. Potentiometric and NMR studies have been reported on complex formation in methanol of the naturally occurring antibiotic macrocycle monensin with silver(I), thallium(I), and the alkali metal ions (1201,on cesium complexes with four cryptands in six aprotic solvents (193), and on cesium, potassium, and sodium complexes with dibenzo-30-crown-10 in five solvents (267). Protonation, copper(II), and zinc(I1) equilibria with five cryptands in anhydrous methanol have been measured potentiometrically and spectrophotometrically (34),while the interaction of a series of lanthanide(II1) chlorides with 18-crown-6, also in methanol, has been studied by titration calorimetry (130). On the basis of an absence of detectable heat of reaction, the lanthanides past gadolinium do not appear t o form complexes to any measurable extent. T h e stability constants of the monofluoride complexes of the alkaline earths in methanol and methanol-water mixtures have been measured and related to other systems (40). The fluoride-ion selective electrode was used for the experimental measurements; some difficulties with use of this electrode are described. Complex formation between the ammonium ion and such ligands as water. methanol, dimethylformamide, dimethylsulfoxide, and dibenzo- 18-crown-6 in acetonitrile was followed by means of a cation-sensitive glass electrode (132). From one to five ligands were found to interact with the ammonium ion. 8-Hydroxyquinoline forms 1 to 1 and 2 t o 1 complexes with lead(I1) and zinc(II), and 3 to 1 complexes with copper(I1) and cadmium(I1) in water-saturated propylene carbonate (245). Stability constants for a series of substituted pyridines with iodine were found to give a linear correlation with solvent solubility parameters (305). Solvent Properties. An earlier scheme for the classification of solvents according to their polarity and specific interaction properties has been refined and extended (269). Although designed particularly for use in the selection of phases for gas and liquid chromatography, the approach may be useful in solvent selection for titration systems. Solvent basicity has been reviewed from a thermodynamic point of view (34). Other reviews have treated hydrogen bonding and solvent effects (121),properties of ionizing liquids (I 74),and intermolecular interactions contributing to the three-dimensional structures of solvents and solutions (286). Solvents that have been surveyed as media for chemical reactions include the organic solvents pyridine (213),alkylsubstituted and cyclic ureas (23),N-methylacetamide (1781, phenol hemihydrate ( 3 1 ) , and trifluoroacetic acid (198). Among the inorganic solvents considered are the halosulfuric acids (209),the inorganic halides and oxyhalides (227),hydrazine (301, and hydrogen fluoride (217). Chlorosulfuric acid has been investigated by Paul and co-workers as a solvent for a range of reactions, including the acid-base behavior of several selenium compounds (226) and of a variety of other solutes (225). A number of oxidation-reduction titrations in this solvent a t 93 "C have been explored (183). Methods for the purification of dimethylformamide have been published by the IUPAC Analytical Chemical Division (129),along with details on physical properties and safety
considerations in handling. A procedure for the determination of autoprotolysis constants of amphiprotic solvents has been proposed based on potentiometric acid-base titrations and computer treatment of the data (306). The autoprotolysis constant of ethanediol has been measured; the value of pK,, is 15.6 (42).
EXPERIMENTAL TECHNIQUES As in previous reviews, this section considers tools and techniques used in the study of nonaqueous systems. Included here are various indicating electrodes for potentiometry, visual indicators for titrations, proposed new standards, and developments in coulometric and gravimetric titrimetry. Formation of an adsorbed layer of acetonitrile molecules on the surface of platinum in acetonitrile solutions affects electrode behavior by preventing solvated inorganic species from reaching the electrode surface (230). Adsorption-polarized electrodes are reported to give sharp end points in precipitation titrations in acetic acid-water and acetone-water mixtures (260),as do membrane graphite electrodes in potentiometric titrations of bases with perchloric acid, and of reducing substances with lead(1V) acetate, in acetic acid (222). Dual reference electrodes are recommended to counter the high resistance of some solvent systems in electrochemical measurements (93). For biamperometric end-point detection of titrations with iodine in acetonitrile and methanol a platinum-graphite electrode pair is suggested (242),while an electrode composed of compressed powdered graphite and Kel-F fluorocarbon plastic is reported to offer significant advantages over many other materials for general voltammetric applications in nonaqueous solvents (6). The chloranil electrodes can be used for p H measurements in anhydrous hydrogen fluoride and in superacid mixtures (72). An unusual electrochemical system involving single crystal semiconductors as working electrode materials has been investigated by Kohl and Bard (160). Cyclic voltammetric results differed depending on whether the electrodes were dark or illuminated. The glass electrode is surprisingly broad in its applicability to hydrogen ion measurements in a range of solvents. A procedure for testing its applicability in various solvents has been described (261),and it has been evaluated in propylene carbonate (293) and in water-dioxane mixtures (190). A general review on ion-selective electrodes in nonaqueous solvents is available (244). The sodium and iodide ion-selective electrodes can be used satisfactorily in 4-heptanol and l-octanol (223);the polycrystalline fluoride electrode was found to function better in solvents containing 80% acetone or dioxane and 2070 water than in those of higher water content (185). Greenhow and co-workers have extended the end-point detection technique of measuring the heat produced by an indicator reaction a t the end point, termed catalytic thermometric titrimetry, to acid-catalyzed acetylation reactions (107). The effect of cyanoethylation on end-point sharpness in titrations of phenol in cyanoethylene with potassium hydroxide or tert-butoxide has been assessed (10.9). The best solvent-titrant combinations for weak acids are potassium n-butoxide in n-butanol as titrant in a 65% cyanoethylene35 % dimethylformamide solvent system, and potassium tert-butoxide in tert-butanol as titrant in a 50% cyanoethylene-3570 dimethylformamide-15% n-butanol solvent (69). Applications of the method to studies of the molecular structure and reactivity of organic hydroxy groups have been made (108). Other methods of titration end-point detection include high frequency conductance for polycarboxylic acid determinations in dimethylformamide or methanol (15) and dilatometry for the titration of amines in methanol (318). Visual indicator studies for acid-base titrations include alizarin-9-imine in acetic acid for titration with perchloric acid (211,nitrophenoxazines in acetone for titrations of sulfonamides with tetrabutylammonium hydroxide (32).and several common indicators such as bromocresol green and neutral red in dimethylacetamide, N-methylpyrrolidone, or ethanediol for a variety of acid-base systems (43). Among proposed new acid- base titrimetric standards are benzylthiuronium chloride as an acid for solutions of sodium methoxide in 75% benzene-25% methanol (249)and as a base after addition of mercury(I1) acetate for solutions of perchloric acid in acetic acid (146),and bis(l,5-dimethyl-2-phenyl-3ANALYTICAL CHEMISTRY, VOL. 52,
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pyrazo1one)perchlorate for solutions of perchloric acid in acetic acid and tetrabutylammonium hydroxide in several aprotic solvents (263). Replacement of benzene by less toxic toluene as solvent for titrations with tetrabutylammonium hydroxide has been evaluated (265). Precision of the analytical results is unaffected, but end-point breaks are slightly smaller. A significant increase has occurred over recent years in the use of constant current coulometry for titrations in water-free or mixed aqueous organic solvent systems. Two reviews of this area have appeared (68, 123). The anodic dissolution of a chromium electrode in dimethylformamide, acetonitrile, and pyridine produced chromium(I1) or (IV) for analytical reductions (169);with vanadium in glacial acetic acid, vanadium(V) could be generated quantitatively (1686). Bromine can be electrogenerated quantitatively from bromide in acetic acid and used for oxidation of organic compounds (221);it can also be generated in propylene carbonate and used for the determination of unsaturation (59). Both bases and acids can be determined in glacial acetic acid by coulometry (92). Biamperometric end-point detection with quinhydrone electrodes was found satisfactory. Biamperometric end-point detection with a pair of bismuth electrodes was used for the coulometric titration of several acids in a variety of solvents (313). Gravimetric titrations involve measurement of titrant by weight rather than volume. Many of the disadvantages encountered in handling nonaqueous solvents by conventional volumetric methods are diminished or eliminated by weight methods using top-loading balances. Polyethylene bottles can be used as burets (253);automatic gravimetric titrators have been described (113, 200, 295). An algorithm and computer program for computation of species concentrations and conductimetric and potentiometric acid-base titration curves, in solvents where homoconjugation must be taken into account, has been developed (148).
APPLICATIONS Many papers describing specific applications of nonaqueous solvents to analytical titrimetry have appeared over the past two years. Only a fraction of these can be included in this review owing to space limitations. The criteria for selection has been an indication of trends in t.he use of nonaqueous systems, or of possible new directions. Acid- Base. As usual, acid base applications dominated the other methods. This field, though mature, still remains active. Potassium tcrt-butoxide dissociates much more extensively in tert-butanol if potassium-complexing ligands such as 18-crown-6 or [222] cryptand are added to the solution (166‘). This prevents precipitation of potassium salts during titration of mono- and diprotic acids. The titration of very weak acids in dimethylsulfoxide with potassium t e r t -butoxide complexed with 18-crown-6 is recommended. Guanidine carbonate has been suggested as a titrant for organic acids in acetonitrile (252). Dimethylsulfoxide was less satisfactory as solvent. Lithium diisopropylamide in tetrahydrofuran is a strong base capable of titrating as acids such compounds as phenylureas, anilides, and alkylcarbamates (117). Phenoxycarbamates undergo lission of the 0 C O bond during titration and so cannot he determined. End points can be obtained potentiometrically with a tungsten indicating and a silver--silverchloride reference electrode pair. Weak bases such as thiocyanate and iodide have been determined in dimethylsulfoxide by titration with p-nitrophenyldiazonium tetrafluoroborate (232). This and other diazonium salts can be determined. in turn, in the same solvent by titration with sodium 2-propoxide in ‘.‘-propanol (232). A platinum indicator electrode was used. Compounds forming products with the diazonium cation such as pyridine and triethylamine can also be titrated. Organolithium reagents such as butyl and phenyl lithium in ethers or hydrocarbons have been determined by addition of excess N-benzylidene benzylamine. which is reduced to a deep crimson anion ( 7 7 ) . ‘I’he anion is then titrated with an acid such as benzoic acid t o the disappearance of the crimson. A procedure tor the analysis of products of phosphorylation of higher fatty alcohols involves potentiometric titration with a base such as sodium methoxide in ethanol ( 2 7 7 ) . Bismuth(II1) nitrate can replace mercury(I1) acetate as a halide ion complexing agent i n the titration of halides and hydro halides of organic t m e s with perchloric acid i n glacial acetic 156R
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acid (207). A bismuth to halide mole ratio of 0.35 t o 0.42 is sufficient for good results. T h e first end point corresponds to neutralization of the dialkyl and half of the monoalkylphosphoric acid. Continuation of the titration after addition of 20% water yields a second end point corresponding to the second acidic hydrogen on the monoalkyl ester. Cephalosporin antibiotics can be titrated either as bases with perchloric acid in glacial acetic acid or dioxane, or as acids with tetrabutylammonium hydroxide in 90% benzene-10% methanol (29). Glass and modified calomel electrodes were used. Copolymers of p-aminobenzoic acid, formaldehyde, and p-bromophenol could be analyzed for acid carboxyl and phenolic groups by titration with sodium methoxide in pyridine, and for basic amino groups by titration with perchloric acid in acetic acid ( 5 7 ) . Interestingly, it is reported that with tetramethylammonium hydroxide in pyridine as titrant, complete neutralization of the acidic groups did not occur. Some of the groups were more acidic or basic than predicted on the basis of monomer properties; this is suggested to be the result of hydrogen bonding and complexation. Adenosine can be titrated accurately with perchloric acid in dioxane either potentiometrically or by use of indicators ( 7 ) . Nitroguanidine can be determined in acetic anhydride by titration with perchloric acid in glacial acetic acid (264). Nitramines do not interfere. Substituted dipiperidylbenzamines give two sharp potential breaks when titrated in nitrornethane by perchloric acid in dioxane ( 3 0 4 ) . A variety of drug bases, including caffeine, theobromine, theophylline, and ethyl morphine, can be titrated either potentiometrically or with visual indicators by perchloric acid in acetic anhydride-chloroform or acetic anhydride- benzene mixtures (238). Chlorosulfonic acid can be employed as a titrant for 1,2-, 1,3-, and 1,4-benzenediamines in acetic acid methyl ethyl ketone mixtures (206). 1-Amino-4-methylpiperazine and methylpiperazine were determined in mixtures by titration with hydrochloric acid in 50% glycol--50% 2-propanol before and after reaction with salicylaldehyde ( 2 3 5 ) . Mixtures of ribonucleosides have been determined by differential potentiometric titrations in dimethylsulfoxide with tetraethylammonium hydroxide in 10 % methanol-90% 2-propanol (196). Substituted 8-hydroxybenzophenone oxime compounds, used as extractants in mineral processing, can be determined by potentiometric titration in tert-butanol with tetrabutylammonium hydroxide; the syn and anti isomers of the oximes can be differentiated ( 2 4 ) . Tetramethylguanidine as solvent gave larger potential breaks than did pyridine or hexamethylphosphotriamide in titrations of the nitramine explosives RDX and H M X with tetrabutylammonium hydroxide in methanol--toluene as titrant (266). Use of two polarized platinum electrodes improved precision over potentiometric titrations. Three triphenylmethane dyes showed twb welldefined breaks in several aprotic solvents such as acetonitrile when titrated with methyltributylamrnonium hydroxide ( 4 1 ) . Platinum is not satisfactory as an indicator electrode. In an inorganic application, the carbon dioxide in several rocks has been determined by titration with sodium methoxide (303). The carbon dioxide produced in combustion of organic compounds can be measured by absorption in dimethylformamide containing ethanolamine and titration with tetrabutylammonium hydroxide in benzene-methanol using a potassium indicating electrode or thymolphthalein visual indicator (322). Sodium methoxide was also used as the titrant for the drugs dichlorophen, in dimethylformamide as solvent (It?), and indomethacin, in dimethylformamide, acetone. or chloroform as solvent (292). Potassium hydroxide in ethanol is recommended as titrant for the conductometric titration of several opium alkaloids i n dimethylsulfoxide (21.51, and sodium hydroxide in methanol for a variety of fluorophosphoric and related acids in binary or ternary mixtures in methanol~wateras solvent ( 1 7 9 ) . The effect of isomerism on the shape of conductometric titration curves of diamines and dicarboxylic acids in aqueous- organic and nonaqueous solutions has been assessed (f.51). Maleic and phthalic acids and p-phenylenediamine give normally shaped titration curves, while fumaric and terphthalic acids and ophenylenediamine do not. Oxidation -Reduction. Cobalt(II1) acetate is stable in glacial acetic acid and can be used as an oxidimetric titrant (2668). An excess is added and the amount remaining is determined by titration with iron(I1) sulfate i n the presence of
TITRATIONS I N NONAQUEOUS SOLVENTS
sodium acetate as catalyst and a t 40 to 50 "C. Copper(I1) perchlorate can be used to titrate ascorbic acid in acetic acid-acetonitrile solvent mixtures, the copper being reduced to acetonitrile-stabilized copper(1) (310). Diphenylbenzidine is a suitable indicator. Grignard reagents such as methylmagnesium chloride and diethylmagnesium can be titrated with silver perchlorate in tetrahydrofuran (145). Thiourea has been suggested as a primary standard for solutions of ammonium hexanitratocerium(1V)ate in acetonitrile (307). Cyanide can be determined by titration with dichloramine-?' in glacial acetic acid (106). The same titrant and solvent have been used to measure cinnamic alcohol and crotyl alcohol (246)and several sulfonamides (243). A series of substituted trithiocarbonates were titrated with Nbromosuccinimide in acetonitrile (3111, and N-bromoacetamide in acetic acid has been recommended for the oxidation of a large number of indoles (105). A number of halogen oxidants, including iodine trichloride and N-bromosuccinimide, were found suitable as oxidizing titrants for mercaptans, dithiocarbamates, and xanthates in acetonitrile-methanol solvent systems (312). Manganese(", prepared by dissolving potassium permanganate in anhydrous phosphoric acid at room temperature, then heating for 2 to 3 h at 50 to 60 "C, can be used as a titrant for the determination of the terpenes citral and perillartine in 25% H3P04-75% dimethylformamide at 45 "C (324). The powerful reducing agent naphthenide, the anion radical of naphthalene, has been employed to measure hydroxyl groups and adsorbed water on the surface of suspensions of heterogeneous catalysts in tetrahydrofuran (153). Another reducing agent, sodium borohydride, is satisfactory for the titration of a variety of weakly acidic drugs in several aprotic solvents (39). Ferrocene is a mild reducing agent under most conditions. Solomatin and co-workers have studied in detail the applications of this reagent to the estimation of metals in alloys. Usually as solvent a mixture of water, an inorganic acid such as hydrochloric, sulfuric, or perchloric, and an organic solvent such as acetic acid, acetone, or ethanol is employed. Among the metals that have been determined with ferrocene are silver(1) (273),copper(I1) (1261, mercury(1) and (11) (157), lead(1V) (272),selenium(1V) (271),and vanadium(1V) and (V) (274). A variety of oxidizing or reducing titrants can be generated by coulometric means. Those of a more specific applied nature, and so not discussed previously, are collected here; included are a few that involve electrochemical generation of acids or bases. For example, Glab and Hulanicki have investigated the coulometric titration of weak bases in ethylene glycol, propylene glycol, and their mixtures with 2-propanol (103). A 3% solution of sodium perchlorate monohydrate was found to be a satisfactory supporting electrolyte. In the other direction, the possibilities for constant-current coulometric determinations of mono- and polyprotic organic acids by generation of hydroxide ion have been explored (137). As solvent, methanol with a small quantity of water was recommended. Carboxylic acids can also be titrated in ethanol containing a trace of lithium chloride (216). Hydrazine can be titrated by constant-current coulometric generation of bromine in acetic acid containing about 0.9 M sodium or potassium acetate (220). Up to 4% water does not affect the results, but even traces of acetic anhydride cannot be tolerated. Instead of bromine, manganese(II1) or lead(IV) can be generated from the divalent salts for the same hydrazine determination (219). A number of substituted ferrocenes have been determined by oxidation with coulometrically generated copper(I1) from copper(1) tetrafluoroborate in acetonitrile (152),and several water insoluble aromatic nitro compounds have been reduced with coulometrically generated chromium(I1) after dissolution in acetonitrile or mixtures of water and acetonitrile or ethanol and injection into a catholyte of aqueous hydrochloric acid (5). Phenoxazones also aye quantitatively reduced by electrogenerated titanium(II1) in a n ethanol-water mixture (283). Trace levels of aldehydes in 30 to 95% ethanol can be determined by coulometric titration with electrogenerated iodine (154). Complexation. An increasing number of reports are appearing on the use of organic solvents or solvent mixtures to improve equilibrium conditions or increase selectivity in analytical complexation reactions. The explosion of interest in macrocyclic ligands, for example, will likely soon carry over
into their use as titrants for metal ions in alcohols. Meanwhile, many aqueous systems have been adapted; for example, thiols can be titrated conductometrically with mercury(I1) in either water or dimethylformamide ( 7 4 ) . Mercury(I1) perchlorate in acetic acid can serve as titrant for alkali metal chlorides, acetates, formates, and propionates (1751, while silver(1) in mixtures of acetic acid, acetone, and water gives excellent potentiometric titration breaks in the determination of dialkyl phosphorodithioates (258). Silver nitrate in 2-propanol is an effective titrant for cysteine in ammoniacal 2-propanol (278). Dithizone is used as indicator. Uranium(V1) forms a stable 1 to 1 complex with oxalate in 50% ethanol-50% 2-propanol that can serve as the basis for a determination of the uranium(V1) ion (205). Cobalt in alloys can be estimated by titration with sodium diethyldithiocarbamate (1 70). T h e solvent is 80% dimethylformamide-20 % water, and a silver ion selective electrode is used as the indicating electrode. EDTA, the nearly universal complexing titrant for metal ions in water, has limited solubility in most aprotic solvents. Solubility data for the free acid and the disodium dihydrate salt, which is the usual commercial form, have been tabulated in mixtures of water with dimethylformamide, dimethylsulfoxide, acetone, dioxane, and ethanol (110). Yoshimura has studied the effect of masking agents on the conductometric titrations of a number of transition and alkaline earth metals with EDTA in dimethylformamide (323, 325). Gevorgyan and co-workers used the magnesium salt of EDTA as titrant for the determination of metals in acetic acid and its mixtures with other solvents. Moderate to high concentrations of alkali metal acetates are often added to increase the solubility of the magnesium EDTA complex. Biamperometric end-point detection is used at an applied potential on the order of 1 volt. Examples of metals titrated include indium and copper (102),cobalt(I1) ( 9 8 ) ,mixtures of cobalt with nickel, copper, neodymium, or gadolinium (95),copper, bismuth, gallium, indium, and cadmium (99),thallium(II1) (97).the lanthanides, lead, and thorium (101). scandium (100. 294), and manganese(I1) (96). Karl Fischer Method. The Determination of Water. Several Japanese reviews on the determination of water have appeared. One surveys the determination by the Karl Fischer method (204),another by coulometry (298),and a third by . has continued his thermometric titrimetry ( 1 5 1 ~ ) Verhoef studies of the Karl Fischer method. On the basis of earlier work indicating the oxidizable species in Karl Fischer reagent to be CH,SO,
