Titrations in nonaqueous solvents

(354) Wreaidlo, W., ibid., p 1603. (355) Wreaidlo pol& Prep., A M . Chem. SOC., Polym. Chem., 12 (1) 755 (1971). (356) Wunderlich, B., Bobb, R., Makromol. Chem., 147, 79 (1971). (357) Wydeven, T., J . Catal., 19, 162 (1970). (358) Yagfarov, M. Sh., Gubanov, E.

w.,

h.

F., Vyuokomol.

Soedin, Ser. A, 12, 1155 (1970). (359) Yamamoto, A., Yamada, K.,Mur-

A., Mol. Cryat. Liquid Cryst., 13, 357

Akiyama, J. Therm. Anal., 1988, 1, 105 Proc. Znt. Cmf.,

(1971). (362) Zamyatnin, A. A., Zwod. Lab., 45, 1007 (1971). (363) Zsako, J., J . Therm. Anal., 2, 145 (1970).

Aviram, A., Anal. Calorimetry, h o c . Sym 9 d , 2, 113 (1970). (361) 2;.omg, W. R., Haller, I., Aviram,

(364) Zsinka, L., Szirtes, L. Stenger, V.1 Radiocham. Radioanal. Lett., 4, 257

ate, M.,

d,

(1969). (360;k;;ung, W. R., Barrall, E. M. 11,

(1970).

Titrations in Nonaqueous Solvents 1. 1. Lagowski, The University of Texas af Austin, Austin, Texas

T

the pertinent literature for the period November 1969 through October 1971. As in the last review (174), it is convenient to discuss the subject with reference to individual solvent types. A summary of the titrations reported in various nonaqueous solvents and the methods used to follow the course of reaction is given in Table I. HIS REVIEW COVERS

SOLVENT TYPES

Hydrocarbons and Aprotic Derivatives. Reports have appeared on acid-base behavior in aromatic hydrocarbons (186, 207, 234), their chlorine derivatives (112, 186) as well as in derivatives of aliphatic hydrocarbons, Viz., nitromethane (47, 196), chloroform (96, %'Or), and carbon tetrachloride (807). I n addition, benzene (40, .@, 76, 77, 164, 199, 228), toluene (78), and chloroform (86,139,247,276)have been mixed with other solvents to produce mixtures with more desirable solvent properties. Methylene chloride (279) and chloroform (274) have been used as solvents for redox reactions. Spectrophotometric (96, 96, 274) or indicator (47, 49, 76, 228, 847) methods have been the most widely used techniques to follow the courses of titrations in solvents of this type, but potentiometric methods have also been reported (47, 77, 78, 86, 139, 196, 207, 234, 274, 2'7'6,279). The influence of solvents of this type on the nature of the species present in acid-base systems has been studied potentiometrically (139) and spectroscopically (112, 186). The apparent base strengths for uncharged bases in acetic acid-chloroform mixtures are linearly related to their pKb values in anhydrous acetic acid (139). In contrast to alcohols, benzene does not decrease the basic limit of the acidity scale in a solvent up to a composition of 60% (vol./vol.) (40). The acidity function of sulfuric acid in binary and ternary mixtures of acetic acid, acetic 524R

anhydride, and methylene chloride is inversely related to the dielectric constant of the solvents (224). Ethers. Relatively few reports of interest are available on the use of ethers as nonaqueous solvents for titrations. Mixtures containing dioxane and water (128, 189, 200) or alcohols (73, 20.4) were the most popular for studying acid-base phenomena. The dissociation constants of m-nitroanilinium ion in tetrahydrofuran-water mixtures (229) and of 8-quinolinol as well as its sulfur and selenium analogs in dioxane-water mixtures (200) have been reported; the stability constants of the lanthanide complexes with 2,4-dihydroxybenzaldehyde, 2,4dihydroxyacetophenone, and their oximes also have been determined in the latter solvent mixture (189). A study (73) of the complexes formed between pyridine, ethylenediamine, and glycinate anions with protons, Ni2+, Cu2+, and Znz+ indicates that the protonation constants for pyridine decrease in the order H20 > CHaOH > dioxane; those for ethylenediamine follow the order HzO > dioxane > CHaOH; and those for the glycinate ion, dioxane > HtO > CHaOH. The stability of the metal complexes with ethylenediamine and glycinate ion increases in the order HzO < CHaOH < dioxane, whereas the stability of the pyridine-metal complexes is relatively uniduenced by a change in solvent. Alcohols. Much interest again has been shown in alcohols as nonaqueous solvents for titrimetric methods. Pure alcohols, (7, 16, 32, 48, 106, 111, 116, 129, 169, 160, 179, 180, 187, 197, 207, 210, 230, 834, ,942, 266, 267, 269, 272), mixtures of alcohols (166, 223) or alcohols mixed with hydrocarbons (76, 78, 164), ethers (131), esters (20-22), ketones (8, 9 , 107, l 7 l ) , and water (26, 46,98,180-1%6,167,183,~03,806, ,906, 236, 836) have been used as solvents in the tiration of a variety of compounds. Among these are acids (7-9, 16, 78, 98, 1 1 1 , 116, 120, 122, 183, 126, 189, 166, 169, 160, 183, 187, 806, 206, 210, 223,

ANALYTICAL CHEMISTRY, VOL. 44, NO. 5, APRIL 1972

230, 234, 242, 266, 267, 269, 272), bases (20, 42, 106,121, 203, $?Or),salts (21,22, 32, 46, 107, 131, 171, 180), metal complexes (46, 7 6 ) , and fluoride ions (26) using potentiometric (7, 9 , 16, 90, 91, 26,42, 78, 98, 106, 107, 111, 129, 166, 167, 169, 160, 187, 203, 906-207, 223, 230, 234, 866, 269); amperometric (32, 116, 180, 267), conductometric (120-123, 129, 131, 810), oscillometric (8, 46, 7 6 ) , spectroscopic (242), and indicator (272) methods to follow the course of the reaction. The use of specific ion electrodes in alcoholic media also has been reported (26,107,124). Titrimetric methods have been developed for the determination of alkylsulfuric and alkylsulfonic acid in their mixtures with sulfuric acid (16), benzene polycarboxylic acids (169,266,267, 269), ketones in phenol (106), amino acid esters and their derivatives (98, 27@, polynitro acids (7, 9 , 183), alkylphosphates (234), the acid constituents of petroleum products (78), metal salts (22, 32, 46, 76, 171, 180), and water in strongly basic amines (197). The effect of water (61, 167, 203, 206, 206, 231, 232), carbon tetrachloride (842), benzene (194), other alcohols (40, 166), and inert salts (126, 187) on acid-base reactions in alcohols has been investigated. The activity function of HClOd in water-ethanol mixtures (238) and of glycolates (H-) in ethylene or propylene glycol-water mixtures (61, 62) have been reported. The pH and buffering capacity of several buffer systems in methanol-water mixtures does not differ substantially from that of the same systems in pure water (261). The results of similar studies in ethanolwater mixtures using potentiometric measurements on the HS-Ag/AgCl couple have been reported (236,136). Several studies of the relative acidities of a series of compounds have appeared. The relative strengths of diamines and amines as measured by their halfneutralization potentials in methanol or isopropanol are linearly related to their pK values in aqueous solution (48).

Calorimetric and oonductimetric studies wing n-butanol suggest that the order of protonjc acid strength in this solvent is: HSOiF > H~S20.r >> &SO, > Cl&COaH > Cl&HCO*H. The pK. values of fluorosilanes in ethanol have been correlated with the cumulative Toft inductive constants (Zo*) according to Equation 1. Data available on pK. = 9.65

- 0.7720*

(1)

the pK. values of weak cationic acids in binary mixed solvents have been critically evaluated (23). It appears that changes in dielectric constants, which are related to electrostatic charging effects, have little influence on the dissociation constants of such acids. Solute-solvent interactions are largely responsible for changes in pK with composition of the solvent. I n solvents containing water, the transfer energy of HC1 was, to a good approximation, related to the activity of water in the system. A method for evaluating ionic enthalpies of solvation in methanol has been reported (248). Several reports containing experimental information of interest have appeared. A detailed study of the characteristics of a calomel electrode using anhydrous CHaOH indicates that it can be used as a convenient reference in this solvent (194). A resin membrane electrode has been described for activity measurement in aqueous alcohol media. A study of the alkaline error, salt effect, and hysteresis effects on glass electrodes in isopropanol has been reported (I 19). Ketones. Although other ketones have been used as solvents, uiz., methyl ethyl ketone (8, 11 , 42, 98, 221 , 226, 242, 269), cyclohexanone ( 2 8 l ) , and n-propyl ketone ( 9 ) , most of the information available involves acetone or its mixtures as solvents. One report describes the use of 30% formaldehyde as a titration medium (24). Substances that exhibit acidic (3, 4, 7-9, 1 1 , 4l , 80, 81, 83, 98, 133, 146, 168, 160, 161, 166, 168, 221 , 283, 241, 242,269) or basic (3, 4, 42, 132, 133, 207, 286, 269, 281) properties as well as salts (107,133,166) and metal complexes (4) have been titrated in pure ketones (3, 4 , 7 , 8 , 1 1 , 41, 81, 83, 98, 132, 160, 161, 166, 207, 221, 223, 241, 259, 269, 281) and mixtures of ketones with chloroform ( 4 2 ) , carbon tetrachloride (a&), alcohols (9, 107, 171, 2 4 l ) , acetonitrile (226),dimethylformamide (S), and water (80, 116,133, 146, 168,168). Potentiometric methods were by far the most popular for following the course of these reactions, however oscillometric (8, $41), spectrophotometric (842), coulometric (269), conductometric (132), chronoconductometric (133), chronopotentiometric (168), and indicator ($41) methods have been reported. Titrimetric methods using ketonic solvents have been described for 8-

nitroalcohols ('7); T N T in the presence of nitric and sulfuric acids (9); amino acids (11, 98) ; the initial, intermediate, and final products in the oxidation of pxylene (4f); organo-phosphorus acids (81); polymer stabilizers (83) ; organic halides (107); carboxylic acids in the presence of their sulfonation products phthalic acid isomers (168); acetic acid and its halogenated derivatives (160); picolines (226); aromatic polynitro compounds (8); total carbon in rocks (941); and mixtures containing acids or weak bases with their salts (133). Ketonecontaining solvents are said to be good differentiating solvents for acids (83, 168, 160, 223). Rare earth nitrates and acetates exhibit acidic properties in ketonealcohol mixtures ( 1 7 l ) , but metal complexes of antipyrine and its derivatives exhibit valiable acid-base properties depending upon the metal ( 4 , 7 ) . Antipyrene complexes of Zn, Cd, Hg, Go, and Mn behave as bases in acetone, whereas those of Bi, Pt, As, Zn, and Sb exhibit acidic properties. Similar behavior obtains for metal complexes of 8-hydroxyquinolines1 8mercaptoquinolines, dithiocarbamates, dithiophosphates, and cupferronates (3). The basic properties of these systems stem from the basicity of the ligand, and their acidity arises from the metal complex. The relative strengths of amines (42) in acetone and methyl ethyl ketone and of acids in acetone (68, 869) or cyclohexanone (68) have been reported. The pK. values of substituted benzenesulfonamides in these solvents are linearly related to the Hammett substituent constants (68). The half neutralization potential of some benzene polycarboxylic acids in acetone are related to their aqueous pK values through

Table 1.

Equation 2. A study of the ionieation pK = 4.264

constant of benzoic acid in acetonewater mixtures revealed a minimum in the pK u8. I / D (D = dielectric constant of the mixture) plot suggesting a strong interaction between the solvent and the acid (80). Similar studies of the stability of cadmium chloride complexes indicate that these complexes are more stable in acetone solutions (126). From an experimental standpoint, the interfacial cell for potentiometric titrations was successfully used with acetone (207), and the uw of cells incorporating fixed cell con jtants with different solution volumes TJas described for conductometric titrations in the same solvent (132). The sign and magnitude of the liquid junction potential between a saturated aqueous KC1 solution and acetone were determined by a measurement of the EMF'S of cells with transference (12). Titrations involving coulometrically generated hydrogen ions in acetone give sharper breaks for amines when the solution contains LiClOd (64). Acids. The most extensively investigated solvent in this class is acetic acid (27, 66, 86, 91, 98, 110, 148, 163, 179, 824, 252,268,262, 266) and its mixtures with acetic anhydride (138, 170, 224, 260, 264), ketones (263), alcohols (197), chloroform (139), and benzene (49). The use of acetic anhydride (218, 249) and its mixtures with chloroform (247, 273) and acetonitrile (163,243)has also been reported. Some interest has been shown in the use of the ternary systems acetic acid-acetic anhydridechloroform (1 70, 171 , 276) or -acetronitrile (261). Other solvents in

Solvent/Titrant" HOAc/K, [Fe(CN)SI MeOH/MOH EtOH/(CH.hCO/NaOH

.

-

NMP -.- ,/-

Aliphatic polycarboxylic acids

Alkaloids

(2)

Titrimetric Methods in Nonaqueous Solvents

Compound Aldehydes Aliphatic monocarboxylic acids

- 0.00436 ( E l / * )

PC /KOH (MeOH ) DR4F/HClO4(EtOH) MeOH/EtcNOH CsHb(0H)a :Go-PrOH /EtrNOH (CHa)zCO/KOH(iso-PrOH) (CHs)zCO/EtrNOH (CHs)&O/NMP/PC/KOH(MeOH) MeNO9/HC10. H-oAc-/-H cio4 AczO: CHCls/HClO4 various/PA

Method* amp. cond.

Reference

pot.

cc pot.

pot. CP

pot. pot.

pot. pot. pot. pot.

pot. CP

pot. pot. pot., ind. pot. pot. ind.

pot.


525 R

Table 1. Titrimetric Methods in Nonaqueous Solvents (Continued)

Compound

Solvent/Titrant"

Method6

Reference

Amides SmP.

AGO)-

pot. pot.

pot. Amines, primary

tm cond. pot. pot.

cond. pot. pot. pot. pot. pot. pot. pot. pot.

Amines, secondary

Amines, tertiary

Amino acids

Amino acid esters Ammonium salts

Anthraquinones Aromatic hydrocarbons N-Arylhydroxamic acids Barbiturates Benzoic acids

pot. tm cond. pot. pot. pot. pot. pot. pot. pot. pot. tm cond. tm pot. tm pot. pot. pot. pot. pot. cond. pot. pot. pot. ind. ind. cc pot. ind. pot. pot. pot. pot.

pot.

pot. pot. Ind. ind. ind.

pot., ind.

pot. pot. pot. amp. pot.

pot.

spec. pot.

pot. pot.

osc.

pot. pot. osc.

pot. ind.

pot.

526R

ANALYTICAL CHEMISTRY, VOL. 44, NO. 5 , APRIL 1972

(866, 169, 370) (8441) (36, 164) (161, 833, 866) (866, 869)

this classification that have attracted study are trifluoroacetic acid (SO, 101, 817, NO),formic acid (807), methanesulfonic acid (813), and fluorosulfuric acid (818). As was usually the case in the period covered by this review, potentiometric methods were the most popular techniques for studying acidbase reactions in this class of solvents although coulometric (860, 861, 866), amperometric (163, 179, 868, 264) and indicator (4.9, 66, 847, 863) methods have also been employed. As might be expected most of the titrations reported involve bases, i.e., amines and metal acetates; however, strong acids have been titrated in trifluoroacetic acid (SO). Titrimetric methods have been developed using these solvents to determine epoxy groups (&); mixtures containing ca!Teine and theobromine (273); barbiturates (247); 2-beneothiaeolylsulfenamides (163); unsaturated and sulfur-containing compounds (86, 179, 279); formaldehyde and formic acid in acetic acid (868);rare earth salts (171, 276), gallium, thallium, and indium (170); organic selenoxides (249) ;lactams (91, 218); isoniaeed (243); mixtures of primary, secondary, and tertiary amines (87, 868, 263); nitriles (98); and metal 8-hydroxyquinolates (110). I t has been suggested that the autoprotolysis constant can be used as a basis for selecting a solvent for acid-base titrations (163). Solvents with small autoprotolysis constants have a higher differentiatingeffect for certain electrolytes; the principle has been applied to the determination of the isomers of aminobenzoic acid in the acetic anhydrideacetonitrile solvent system. A study of conductometric and potentiometric titrations in mixtures of acetic acid and acetic anhydride indicates that pure acetic acid is the most useful solvent for bases with pK > 13. The apparent base strengths for uncharged bases in acetic acid-chloroform mixtures are linearly related to their pKb values in pure acetic acid (139); the relative order of basicity of such bases is unchanged by the addition of acetic anhydride (138). The acidity function of HvSOd in binary and ternary mixtures of acetic acid, acetic anhydride, and methylene chloride has been determined ( S S 4 ) . Studies establishing the relative strength of mineral acids and other strong acids are available for trifluoroacetic acid (30) and methanesulfonic acid (913). Relatively few reports are available which shed light on the species present in solutions prepared from these solvents. Spectroscopic evidence is available which indicates that solutions of HClOd in acetic anhydride contain the mixed anhydride GHaCOC104; the latter is probably ionized into Clod- and CHsCO+ (16). A study of the effect of salts on indicator equilibria in acetic acid indicates that the acid-base proper-

ties of the solvent are virtually unchanged (29). Gaseous HC1 dissolved in HSOiF is a nonconductor, but solutions of this substance in the HSOsFSbFrSOs system (super acid) appear to contain H&l+ (212). Conductivity studies on solutions of ammonium and alkali metal trifluoroacetates indicate that solvation of cations is of minor importance in this medium and that the conductance of the trifluoroacetate ion does not appear to involve a proton transfer mechanism (101). Several reports containing useful experimental information are available. A method for the determination of water by the Karl Fischer method in strongly basic amines using an acetic acidmethanol mixture has been described (197). A reference electrode system for trifluoroacetic acid is available (217); the use of the activated carbon indicating electrode for potentiometric acidbase titrations in acetic anhydride has been reported (37). Potentiometric titrations can be conducted in acetic acid using the interfacial cell (207). The details for titrating bases in acidic solvents with electrolytically generated H + have been reported (260, 261, 264, 266). Derivatives of dibensophenoxasin-5-one are reported to act as useful visual indicators in acetic acid (66). Esters. I n contrast t o the period covered by the last review, esters have received markedly less attention as nonaqueous solvents for titrations. Propylene carbonate and its mixtures (19-22, 271), butylphosphate (69, 76), ethylacetate (8S), and pentylacetate (207) have been used in this capacity. Potentiometry has been employed in all investigations save one where indicator methods were used (69). Using this type of solvent, titrimetric methods have been reported for alkali metal salts of carboxylic acids and their mixtures (21); acetates and nitrates of divalent metals (22); aliphatic nitrocompounds (76); and covalent metal chlorides (69). The differentiating ability of propylene carbonate was studied potentiometrically (19); 2-, 3-, and 4- component mixtures of bases, the pK.(HZO) values of which differ by 1 4 units, can be successfully titrated. Mixtures of mono- and di-carboxylic acids, and 2 component mixtures of dicarboxylic acids can be differentiated in propylene carbonate. However the range of acidity attainable in propylene carbonate is markedly decreased by the addition of methanol or water (271). Acetates and nitrates of divalent elements can be differentiated in propylene carbonate-methanol solutions (22). Acetates are acidic in this medium, whereas nitrates can be acidic, basic, or neutral. The order of Lewis acid strength of several covalent metal halides was established in tributyl-

Titrimetric Methods in Nonaqueous Solvents (Continued)

Table 1.

Compound Bene thiazoles Boric acid

Solvent/Titranta

Method* amp. cond.

pot.

pot.

Carbon dioxide Carbon monoxide Esters Fluorogermanic acid Halides

pot. ind. ind. cond. cond. pot. pot. pot. pot.

redox cond. redox cond.

Hydrazines Hydrogen sulfide Ketones Metal derivatives NaI PbCln

pot.

pot. redox. cond. cond. cond. cond.

SbCls Al(II1) Aim

osc.

tm redox esr pot. pot. pot. tm tm

(241) Cu(I1) Fe(I1)

Ga(1V)

3[3 Sn(I1)

pot.

Ti(II1)

redox pot.

E!&

M(II)r M(II)* M(I1)' M(II)i M(1I)h M(II)t M(I1)' M(1IP Acetates Antipyrine complexes Carboxylates" Cupf erronatesn Dithiocarbomateso Dithiophosphinatesp Dithiozonates Ferrocenes Fluorides 8-Hydroxyquinolatesc

cond. cond. amp. osc., tm

osc.

ind. spec. spec. ind. pot. pot. pot. pot.

...

pot. pot. pot. pot. pot. spec. pot.

pot. spec.

pot.

&Hydroxyquinolates' 8-Mercaptoquinolatesc Nitrates' Oxslatest Thiocyanates" Thiocyanates" Mineral acids HCI

pot. pot. pot. pot. pot. pot. amp. pot. pot. pot. tm cond. pot.

cc

...

pot. pot.

HNO, HBO4

cc

...

pot. ind.

(9)

'

(Continued)


527 R

~

Table I.

Titrimetric Methods in Nonaqueous Solvents (Continued)

Compound Nitrites Nitroalcohols Nitroalkanes Nitrobenzenes Phenols

Phosphates, alkyl Phosphoric acids Phosphorus heteropoly acids Selenoxides Silanes, fluoro Silanols Sulfones Sulfonic acids

Sulfur compounds Sulfuric acids Sulfur dioxide Tetrasoles Thiols Trinitrotoluene Unsaturated compounds

Solvent/Titrmta HOAc/HClO4 (CH8)&O/MeNOH DMF/Me4NOH (BuO)~PO/M~NOH (CH, )zCO/Et4NOH MEK/Et,NOH MeOH/NaOH EtOH/(CHs)&O/NaOH (CHs)zCO/KOH (CHs)zCO/EtrNOH NMP/EtOAc/KOH EtOAc/EtiNOH DMF/NaOMe DMF/Bu,NOH DMF/CE.H~N/KOH TMS/DMSO/Bu,NOH various/KOH (CHa)zCO/KOH DMF /KOH MeC&/KOH iso-PROH/varioua MEK/various NMF/various AczO/HClO4 EtOH /NaOEt

M~OH~KOH

MeOH jR*N-oH /MOH MeoH CsHsN/R,NOH TMS/MeOH/Brz HOAc/Brz MeOH/KC

Pr&O /Me.NOH .

Methodb pot. pot. pot.

Reference

pot. OSC.

osc. cond. amp. cc pot. pot. pot. pot. pot. pot. pot. pot. pot. pot. pot. pot. pot. pot. pot. pot. pot. pot.

pot. pot. pot. pot. pot. pot. pot. ind. cond.

... ...

pot. amp. amp. pot. ind. cond. pot. pot. redox. pot. pot. amp. amp.

o The following abbreviations are used: Ac~0,aoetic anhydride; Bu, butyl; DCTA, disodium l,>diaminocyclohexane N,N,N',N'-tetraaceta~; DEDTC, sodium dieth 1dithiocarbamate; diox, dioxane; DMF, dimethylformamide; DMSO, dimeth Isdfoxde; Et, ethyl; HOAc, acetic acid; %H&, Shydrox quinoline; Me, methyl; MJK, methyl ethyl ketone; NMF, N-methylformamide; NMg, N-methylpyrrolidinone; NTA, nitrilotriacetic acid; PA, picric acid; PC, propylenecarbonate; Go-Pr, iso ropyl; PTSA p toluenesdfonic acid; R, alkyl; red, anthraquinone radical anion; T d S , tetramethyiensuflone; TMU, tetramethyl urea. b The following abbreviations are used: amp, amperometric titration; cc, chronoconductometric titration; cond., conductometric titration; c chronopotentiometric titration; esr, titration followed by electron spin resonance; in$;, visual indicator; osc., oscillometric titration; pot., potentiometric titration; redox, potentiometric titration involving redox cou les; spec., spectrophotometric titration; tm, thermometric titration. O M = Ge Sn T I Zr. *EDTA kTA. # M = d. Na. 1 M = Ch,Hg, Co, Pb, Cu, Zn, Mn, Mg, Ca, Sr. g M = Hg, Pb, CO, Cd, NI, Mg. h M = Zn, Cd, Mg. i M = Zn, Cd, Mn, Fe, 'Co, Ni, Cu. j M = Ba, Sr. t M = rare earth nitrates or acetates. M = As, Sb, C1. " M = Na. M = Bi, Ai. 0 M = Bi, Pb. p M = Na, Nil Cd. * M = Na, Mn, T1, Mg, Cd Al, Bi, Fe, Th. M = Co, Zn, Cd, Nil Be, br, Ca ,Me, Ga, T1, AI, In. * M = K, Co, Cd, Zn, Mn, Mg, di. M = Ni, Co Ca, Mg, Sr, Ba. M = Zn, cd, Mn, Fe, Co, Ni, Cu. e M = Hg, Zn, Cd, Mn, Fe, Ni, Cu, Cr, Mg, Bi, Sn. Y

528R


phosphate, benzylacetate, and butyl acetate (69). Acid Amines. A large majority of investigations in which amides are used as titrimetric solvents involved N,N,dimethylformide (7, 8, 18, 28, 31, 61, 66, 81, 89, 98, 106, 109, 117, 1&, 1.46, 186, 207, 234, 267-270, 277, 278) and its mixtures with water (162, 219), butylamine (167), and benzene (228). Other solvents investigated in this class involve formamide (216), N-methylformamide (221), acetamide (209), Nmethylacetamide (70), tetramethyl urea (69), and N-methylpyrrolidinone (36, 84). For the most part, potentiometric methods were employed in these investigations; however, there are also 109, reports describing indicator (69,94, 228),amperometric (267),spectrophotometric (169, 167),oscillometric (8, 268), conductometric (18,277, 278) and coulometric (31) techniques. Titrimetric techniques incorporating acid amides as solvents have been developed for the determination of covalent metal chlorides with chelating agents (278) ; benzene polycarboxylic acids (267-270); aromatic nitro compounds (8); Ba2+ and Sr2+ as sulfates (162) ; divalent metal ions as &hydroxyquinolates (I67) ; alkylphosphates and other organophosphorus compounds (81, 234) ; complex thiocyanates (66, 1.46); inorganic substances capable of being oxidized by PbCL (106), KaCrzO,, or IZ (216);amino acids (98);anthraquinones (89); 8-nitroalcohols (7); phenobarbitone (228); carbon in steel (31); hydroxamic acids (109); and sulfa drugs or barbiturates (94). Several papers containing information relating to the strength of acids in acid amides have appeared. A detailed study of acid strengths in N,N-dimethylformamide as determined by a combination of conductometric, spectrophotometric, and potentiometric methods is available (l44), and acidity scales for this solvent have been suggested (61, 117). The acidity (as measured by the half-neutralization potential, E l / $ of a series of acids in N,Ndimethylformaniide is related to their aqueous pK. values through Equation 3 (269), pK.(HzO) = 3.996 - 0.00525 Ells (3) whereas the pK values of acids in N,Ndimethylformamide-water mixtures are related to both the pK. of the acid in water and the mole fraction of water (Equation 4); the values of the coeffipK = A

+ BpKa(Hz0)

(4)

cients A and B in Equation 4 are dependent on the mole fraction of water (219). A relationship similar to Equation 3 has been obtained for N-methylpyrrolidinone (84); the degree of d8erentiation of acids in this solvent was

found to increase in the following order: +substituted benzoic acids < monocarboxylic aliphatic acids < phenols < 3,4-disubstituted benzoic acids < dicarboxylic acids. Solvation effects for halides, trihalides, and halogens (946) ; metal-containing Lewis Acids (278) ; the proton (62); and a series of common anions (86) in N , h ,-dimethylformamide have been reported. A method for the evaluation of ionic enthalpies of solvation in formamide, N-methylformamide, N-methylacetamide, and N,N-dimethylformamide has been presented (248). Several papers contain information of experimental interest (61, 144). The Cd/CdClz electrode has been suggested as a reference in N,N-dimethylformamide solutions (190), and the use of the interfacial cell for titrations in this solvent has been reported (207). The use of carbon-indicating electrodes in acid-base titrations has also been described for this solvent (28). Amines. Pyridine (34, 66, 83, 97, 109, 111, 146, 234, 238); its mixtures with benzene (199), isopropanol (199), aniline (272), butylamine (103), water (7), and N,N-dimethylformide (167) are among the solvents in this category which have been used to study acid-base reactions. Potentiometric (66, 82, 83, 97, 103, 111, 116, 167, 199, 234, 238), spectrophotometric (34), and indicator (109, 114, 272) methods have been reported to be useful. Titrimetric procedures have been described for complex metal thiocyanates (66,146); phenols (83); naphthyl esters of carboxylic acids (97); 8-hydroxyquinolates of divalent metal ions (167); alkylphosphates (234) ; silanols and cyclosiloxanes (238); weak acids in petroleum fractions (199); amino acid esters (872); and hydroxamic acids (109). A detailed electrochemical study of solute-solvent interactions in pyridine indicates that anions influence the position of equilibria involving the pyridinium species through its chemical type, the contribution of the dispersion effect on the free energy of the process in question, and their interaction with other ions in the system (72). A report of a study on the dissociation of ion pairs in pyridine is available (33). Butylamine exhibits a marked differentiating effect on acids (103). A detailed review of solution phenomena in liquid ammonia has appeared (176). An acidity scale based upon the hydrogen electrode in butylamine (82) has been suggested. An acidity scale for liquid ammonia using a combination of spectrophotometric and NMR data, has been described (266); the ionization constants for other acids have been determined in this solvent using an electrochemical method (102). A technique for evaluating ionic enthalpies of solvation in ammonia has been reported (5.48) *

Useful experimental details concerning acid-base reactions in n-butylamine (103) or liquid ammonia (114, 116) have appeared. The Cu+/Cu2+ couple has been suggested. as a reference electrode in pyridine (37). Nitriles. Acetonitrile (3, 27, 62, 64, 81, 98, 110, 161, 226) and its mixtures with water (162), chloroform (86), methyl ethyl ketone (226), and acetic acid (10) were the solvents of choice for acid-base studies in this class of compounds. Titrimetric methods were reported for amino acids (8, 98); picolines (226); ferrocene derivatives (226); metal chelates (3, 110); organophosphorus compounds (81); isoniazed (64); sulfur compounds (86); and Ba2+ and Sr2+(162). Several reports have appeared which contain extensive summaries of solvation phenomena in acetonitrile (141, 142). The hydration constants of the proton in acetonitrile have been reevaluated on the basis of mono- and dihydrate formation for the species B H + in the solvent (60). Extensive hydrogen bonded interactions have been reported in solution of acetonitrile containing ethanol (81). An attempt was made to relate acidity scales in acetonitrile and N,Ndimethylformamide using the Strehlow method (62). The addition of acetonitrile to ethylene propylene glycol leads to an increase of the acidity function H- (61). Useful experimental information has been made available on hydrogen ionsensitive electrodes in acetonitrilewater mixtures (90). Activated carbon rods serve as suitable indicator electrodes for acid-base studies in acetonitrile (27). Sulfones. Reports on sulfolane (tetramethylenesulfone) (26, 66, 71, 282) and dimethyl sulfone (39) as nonaqueous solvents have appeared. Electrode potentials for a series of couples were established in the latter investigation. Indicator methods for weak acids or bases (282) and potentiometric techniques using a hydrogen indicator electrode for a variety of acids (71) have been described. Conductivity and NMR data suggest that the order of strengths of mineral acids in this solvent is: HC104 > HSOaF > H2S207. A report has appeared of preliminary attempts to establish a practical acidity scale in sulfolane (66). Dimethyl Sulfoxide. This substance has been used pure and mixed with benzene (77, 228) as solvent for titrations using potentiometric (66, 77, 146, 173), indicator (104, 136, 228), and thermometric (134) methods. Titrimetric techniques have been developed for the determination of divalent cations with chelates (134, 136); acids (104, 228) ; alkylphenols (77) ; amines (173) ; thiocyanate complexes (66,146). Dimethyl sulfoxide solvates anions

slightly, but cations are very strongly solvated (68, 181). A discussion of proton transfer reactions is available (118). A correlation of the relative acidities of phenols and benzoic acids in water and in dimethyl sulfoxide shows that the former class of acids exhibits a greater increase in relative acidity in dimethyl sulfoxide than in water (196). It is possible to differentiate acids in dimethyl sulfoxide which exhibit halfneutralization potential differences greater than 140 mV. A study of the heats of solution of Lewis acids in dimethyl sulfoxide shows that they decrease in acid strength in the following order: SbCls >SnCl4 > SnBrc > SnI4 > ABr3 > AlC& > AsCla (213). The order of base strengths obtained from heats of neutralization is: benzylamine > a-picoline > quinoline. Molten Salts and Other Aprotic Solvents. A variety of fused salt systems has been investigated with respect to establishing potentials for many useful redox couples. The following are among the systems studied in this respect: K2S20rK2S04 (67);NaCl2-ZnClrPbCl2 (14) ; NaPOdKPO4 (48); NaC1-KCl (66, 149); MgClrKCl (87, 88) ; LiF-BeFz-ZrF4 (113); LiF-NaF-KF (113); ZnClr InCl (136) ; SrC12-NaC1-KC1 (140). Noteworthy is the description of an electrode sensitive to oxide ions in molten salts (66) and the use of glass electrodes in fused alkali metaphosphates (48). A method for establishing a unified E M F series in molten salts has been suggested (36). An acidity scale has been suggested for fused potassium nitrate (178) and an optical scale of Lewis basicities in inorganic oxyacids, molten salts, and glasses proposed (67). The theory of acidity of oxides and salts of oxyanions has been discussed (63). One report is available on acid-base behavior in selenium oxychloride (63). Potentiometric measurements with a chlorine gas electrode were used to follow neutralization reactions between chloro bases and chloroacids. A pC1scale was established for this solvent. RELATIVE ACIDITY SCALES

Continued interest has been expressed in the problem of establishing acidity scales within a given solvent system as well as in relating acidity scales among solvents. Papers concerned with this problem for a single solvent generally are discussed in the appropriate section; however, several reports are available which are also discussed in this section. The relative acidities of a variety of compounds have been established in ethers (93), alcohols (42, 166, 188), ketones (42, 68, 81, 188, 269), acids (SO, 158, 224, 276), esters (21, 93), acid amides (18, 61, 84, 117, 144, 188, 269),


529 R

amines (33, 81, 102, 103, 188, 226), nitriles (81), sulfolanes (26, 66), dimethyl sulfoxide (104, 166, 196, 2 1 4 , and fused salts (63,169). Where the compounds in question were of similar structural types, the order of acidity is related to the Hammett substituent constants (68, 93, 166). Acidity functions have been established for organic bases in perchloric acid (280), deuterium sulfate (246),and a review has appeared on this subject for protic solvents (876). Much useful information on this subject appears in a recently published book on acids and bases (233) and a review (43). A potentiometric method for the definition of a pH scale in ethanolwater mixtures has been presented (236, 236). A number of reports on the effect of water on the course of acid-base reactions in nonaqueous solvents have appeared; solvents such as ethers (200, 229), alcohols (23, 126, 167, 183, 203, 206, 206, 231, 261), ketones (80, 1 6 4 , acid amides (184, 819), amines (sa), and sulfolanes (184) are among those investigated. Interest is still strong in the relationship of acidity scales among different nonaqueous solvents. An attempt has been made to relate acidity scales using the concept of absolute acidity as derived from thermochemical cycles (176). Spectrophotometric (242) and potentiometric (44, 218) methods have been used to establish acidities in different solvents. The ionization constants for salicyclic acid in nonaqueous solvents-carbon tetrachloride mixtures depend upon the nature of the nonaqueous solvent and decrease in the order: CHaOH > CzHsOH > CaH,OH > SOCsH70H > iso-C4HpOH > sec-C4HeOH > CsHnOH > iso-CsHiiOH > CHsCN > dioxane > CHaCOC2Hs > (CHa)&O (242). In another study, the pK values of a series of isomeric phthalic acids and other derivatives of benzoic acid were determined potentiometrically in alcohols containing two to six carbon atoms (169). For a given acid, linear relationships exist between the pK of the acid and the autoprotolysis constant (pK,) for the alcohols, e.g. for toluic acid Equation 5 is valid. A comparison

+

pK = 0.36pK. 2.55 (5) of acid-base properties in N,Ndimethylformamide using Strehlow’s nonthermodynamic assumptions and Hammett’s approach has appeared (68). The compounds within a class appear to correlate better with the former approach. A Born-like expression has been used to relate the thermodynamic ionization constants of the medium (240). The nucleophylic character of a series of amines has been estimated from a potentiometric study of their reactions with alkyl halides in N,Ndimethylformamide and dimethyl sulfoxide (147). Several investigations yielding in530R

formation on the nature of complex species in nonaqueous solvents have appeared (3, 6, 6, 73). The stability of the complex and the nature of the ligand, metal, solvent, and titrant affect the acid-base properties of complexes in nonaqueous solvents (6). A study of the acid-base properties of metal complexes of 8-hydroxyquinoline, 8-mercaptoquinoline, dialkyldithiophosphates, and cuperferron indicate that the anion chelate ligands bound in these complexes behave as acids or bases in nonaqueous solvents (3) Acidity is determined by the metal whereas basic properties are governed by the ligand (3, 4). The relative ionizing power of aprotic donor solvents has been established from conductivity and NMR studies of solutions of (CH&SnI (99); a review on this subject is available (100). Acid-base conductometric titrations in nonaqueous solvents have been described on a theoretical basis; that is, equations which give the equilibrium concentrations of all species present during titration have been derived (180, 131). The validity of this interpretation is shown by comparison with the results obtained for the titration of acids, the salts of weak bases, and their mixtures in CHaOH and 45% dioxane (131 ) . I

SOLUTION PHENOMENA

Extensive reviews covering the chemical and physical properties of protic solvents (276), liquid ammonia (176), and acetonitrile (141, 142) have appeared, as have a critical analysis of the available data on autoprotolysis constants for nonaqueous solvents (43) and a review concerning ionic equilibria in protic and aprotic solvents (208). A review of contributions from Russian laboratories to the theory of nonaqueous solvents has been published (166). A comprehensive description of the use of radioactive tracers to study acid-base phenomena in the aprotic solvents S02, SOa, SOC12, SO2C12, SOBr2, OPCh, SeOC12, Ac20, and S2C12 has appeared (202). The results suggest that the solvent system concept for interpreting such phenomena in these solvents is less useful than the broader Lewis approach. Acid-base behavior of acid anhydrides in disulfuric acid has been described (211). Considerable interest in solvation phenomena in nonaqueous solvents has been in evidence since the last review. Solvation numbers in several nonaqueous solvents have been estimated from a compressibility method (201). I n N,Ndimethylformamide, the energy of solvation of common anions decreases in the order c104- > SCN- > I- > Br- > C1- (86); this order for the halogens has been verified by an independent investigation (246) which also indicates


that the solvation coefficients for halogen-containing species are in the order X- > XZ> Xa-. A discussion is available on the effect of N,Ndimethylformamide on the dissociation of acids and the solvation of the reactants and products (117). Dimethyl sulfoxide solvates anions slightly, behaving like other dipolar aprotic solvents; however, cations are very strongly solvated by this solvent (181); similar results are obtained for dimethyl sulfoxidewater mixtures (68). Solvation effects for alcohol-water mixtures have been examined with respect to the protonation CHVCOZ-, polyconstants for ”8, amines, and polyaminocarboxylate for complexes of Niz+, anions (MI), Cuz+, and Zn2+ (73), for cations (a), and for cation acids of the type B H + (23). Solute-solvent interactions have been studied in pyridine, using electrochemical means (72), and in trihoroacetic acid, using conductivity data (101). Similar techniques were used to study the solvation of alkali metal salts in 1,1,3,3-tetramethylurea (46). Evidence was presented to indicate that acetic acid preferentially solvates ionic bases in its mixtures with chloroform (139). Magnetic resonance methods were used to study the solvation of Ala+ and Be2+ in solutions of hexametapol (60). An analysis of the polarographic half-wave potentials in a variety of solvents yields information on ion-solvent interactions for the alkali metals ions (38). Halide and pseudohalide ions are more strongly solvated in protonic solvents than in aprotic donor solvents (192). The relative donor properties of a ligand anion decrease in aprotic solvents and are no longer useful guides for estimating the relative stabilities of complex ions in such solvents. Solvating properties decrease in the order H20 > ROH >> (CH&SO N C2H4SOa > CHsCN N (CH2)rSOz ‘V CHaPiO2 E (CHa)zNCHO > CHaC02C2Hs OP[N(CH3)2]a. A method has been suggested for the evaluation of ionic enthalpies of solvation of a variety of ions in HQNCO, (CHs)HNCO, (CHa)zNCO, (CHr NHCOCHa, and IQH3(848). Numerous discussions of solute-solvent or solute-solute interactions involving hydrogen bonding (191) are available for acetonitrile (62,81, 141, 143), dimethyl sulfoxide (166),N , N dimethylformamide (62,I & ) , pyridine (33, 1861, mixtures of N-methylacetamide and N ,Ndimethylformamide (SW),and alcohol-water mixtures (29). The effect of neutral salts on indicator equilibria in nonaqueous solvents with low dielectric constants has been investigated (29). It appears that such salts do not change the acid-base properties of the solvent but rather affect the nature and number of indicator species in equilibrium. 8-Quinolinese-

lenol exists as a zwitterion in a wide range of water-dioxane mixtures (90). Interest continues in the thermodynamic properties of nonaqueous solutions. Electrode potentials have been established for a molten sodium, zinc, and lead chloride mixture (14))a molten mixture of potassium sulfate and pyrosulfate (67)) and liquid ammonia (17). In the last named instance, detailed information on the hydrogen electrode as a primary standard for electrode potentials in liquid ammonia is available. The EMF of the system glass electrode IHCl,MH/AgCl,Ag(I) was investigated in aliphatic esters, ketones, amides, methyl alcohol, hexamethylphosphoramide, and ethylene sulfone (164). The data were used to evaluate the solvation energy of HCl in these and other solvents (13). The solvation energy, ZU,, is related to the activity coefficient through Equation 6; an inlog 70 = -O.3648ZU8 - 120.77 (6) verse linear relationship was also obtained between log yo and the bulk dielectric constant of the solvent for members of a group of solvents with similar structures. An indirect method, based on the Gibbs-Duhem expression, can be used to calculate the activity coefficient for a substance dissolved in a mixed solvent (13‘7). The thermodynamic solubility product for AgCl is reported for methanol-water and dioxanewater mixtures (74). Several studies of the stability of cadmium halide complexes in nonaqueous solvents have appeared (186, 189). The stability of cadmium bromide complexes in different solvents increases in the order &O < CHaOH < CzHsOH < (CH&CO < iso-CaHsOH (187). Potential measurements have established the chloro basicity constants, Kb = [chloro acid] [Cl-]n/[chloro base] for a variety of systems in selenium oxychloride (63). EXPERIMENTAL TECHNIQUES

Reviews containing useful experimental information have appeared on the theory and practice of using nonaqueous solvents in analytical chemistry (168)) thermometric titrations in nonaqueous solvents (172, $60)) and conductometric titrations (816). An analysis of the use of conductometric methods to many-component mixtures of acids and bases in nonaqueous solvents (131) and a description of the differential thermometric titrations of aldehydes and ketones (188) also contain much information of experimental interest. It has been suggested that a solvent for acid-base titrations can be selected on the basis of its autoprotolysis constant (163). Solvents with low autoprotolysis constants exhibit a broad range of acidity and give a higher differentiating effect with respect to groups of acids or bases. A method is available

for calculating the photometric titration curve for mixtures of acids and bases (99). Details of the use of coulometric techniques in acetic acid have been reported (961,964,966). A variety of electrode systems useful for titrations in nonaqueous solvents have been reported. Ion selective electrodes for a variety of cations (177, $H), anions ( 1 , $6, 184, 177)) and hydrogen ions (963,964) have been described; conventional hydrogen electrodes have been used in dimethylformamide (61))tetramethylene sulfone (71), fused alkali metal chlorides ( I @ ) , and acetonitrilewater mixtures (90). The response of glass electrodes has been studied in isopropanol (119)) methanol (108), and fused alkali metaphosphates (48). Carbon-indicating electrodes, activated by oxidation with KMnO, followed by treatment with SnC12, were used for acid-base titrations in N,Ndimethylformamide, acetonitrile, and acetic anhydride (87,28). An electrode responsive to oxide ions in molten salts has been reported (66). Suitable reference electrodes have been suggested for trifluoroacetic acid (817 ) ) dimethylformamide (190)) pyridine (97)) isopropanol (77))and methanol (194), The action of a variety of indicators in liquid ammonia ( 1 1 4 ) , acetic acid (47, 66))sulfolane (888), dioxane (47)) and other nonaqueous solvents (867) has been described. A novel optical scale for evaluating Lewis basicity in a variety of oxide media including oxyanion glasses and oxyacids, involving nephelauxetic effects on the probe cations T1+, Pb* k, and Bi*+, has been developed (67). Mixtures of acids in nonaqueous media can be titrated potentiometrically using a semi-automated method (160, 161, 169). An interfacial cell can be used for acid-base titrations in CHaOH, CzHsOH, (CHa),CO, CsHsCHzOH, CzHdC12, CHCl3, and CH&OzCsH,, even though precipitates are formed in the process (807). A method for performing conductometric titrations in cells which maintain a fixed cell constant with different volumes of solution has been described (138). A cautionary note for the preparation of solutions of HClO, in CHzCl in CHZC12 has appeared (198). A detailed account of the experimental difficulties in determining small quantities of water in organic solvents using the Karl Fischer method has been published (837)) and the use of this technique for determining water in strongly basic amines has been reported (197). Impurities in N,N-dimethylformamide can be estimated by a differential titration technique (193). LITERATURE CITED

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