Metody Analiza Khim. Reaktivov i Preparatov, Moscow, 12, 82-4 (1966). (115) Zbid., pp. 85-6. (116) Said, F., Amer, M. M., George, Z. A., Bull. Fac. Pharm. (Cairo Univ.), 3, 59-71 (1964). (117) Schenk, P. W., Z . Anal. Chem., 216, 279 (1966). (118) Semenov, A. D., Gidrokhim. Mater., 45, 173-88 (1967). (119) Semin’ko, V. A., Farmatsevt. Zh. (Kiev), 19, 11-16 (1964). (120) Sigler, K., Berka, A., Zyka, J., Microchem. J., 11 , 398-403 (1966). (121) Simonyi, I., Kekesy, I., Z . Anal. Chem., 215, 187-90 (1965). (122) Singh, S., Siefker, J. R., Anal. Chim. Acta, 36, 449-53 (1966). (123) Smith, R. C . , Kellum, G. E., ANAL. CHEM.,38,647-8 (1966). (124) Sorochkin, I. N., Tuseev, A. P., Mil’grom, A. E., Tsaryunov, V. T., U.S.S.R. Patent 183,750 (CI. C07c, G Olnl Julv 9. 1966, ADD^. Mav 10, 1965. (125) ‘Spiridonova, ’S.i., Zzv: V’ysshikh Uchebn. Zavedenii, Khim. i Khim. Tekhnol., 9, 156-7 (1966).
-
(126) Srivastava, K. C., Fresenius Z . Anal. Chem., 230, 359-61 (1967). (127) Stelmach, K., Chem. Anal. (Warsaw), 11, 627-8 (1966). (128) Stone, K. G., Treatise Anal. Chem., 13 (Pt. 2), 95-130 (1966). (129) Stransky, Z., Stuzka, V., Ruzicka, E., Mikrochim. Acta, 1966,77-82. (130) Strukova, M. P., Kirilova, T. V., Zh. Anal. Khim., 21, 1236-8 (1966). (131) Suga, M., Bunseka Kagaku, 15, 737-43 (1966). (132) Terent’ev, A. P., Novikova, I. S., Zh. Analit. Khim., 20, 836-41 (1965). (133) Tikhonova, V. I., Izvest. Vysshikh Uchebn. Zavedenii, Khim. i Khim. Tekhnol., 9, 265-9 (1966). (134) Tiwari, R. D., Sharma, J. P., Shukla, I. C., Indian J . Chem., 4, 22122 (1966). (135) Tranchant, J., Mem. Poudres 46-7, 119-28 (1964-65). (136) Trischler, F., Szivos, K., Magy. Kem. Folyoirat, 72, 476-80 (1966). (137) Veibel, S., Wronski, M., Acta Chem. Scand., 20,849-52 (1966). (138) Veibel, S., Wronski, M., ANAL. CHEM.,38, 910-11 (1966).
(139) Vinson, J. A., Fritz, J. S., Kingsbury, C. A., Talanta, 13, 1673-7 (1966). (140) Watanobe, T., Shibuya, M., Bunseki Kagaku, 15, 176-9 (1966). (141) Wheeler, P. P.. Fauth. M.I.. ANAL. CHEM.!38, !970 (1966). ’ (142) Wierzbicki, T., Zeszyty Kauk. Politech. Slask, Inz. Sanit., 5, 17-26 (1964). (143) Wimer, D. C . , Talanta, 13, 1472 (1962). Chem. I., Anal. (War(144) Wronski, &‘ saw), 6, 659-63 (1961). (145) Zbid., 11, 1025-6 (1966). (146) Ibid., pp. 1153-7; Analyst (London), 91, 745-6 (1966). (147) Wronski, M., Talanta, 13, 1145-9 (1066) \-___,. (148) Wronski, M., 2. Anal. Chem., 217, 265-8 (1966). (149) Yalcindag, 0. N., Kim. Sanayi, 14, 47-55 (1966). (150) Zalenetskaya, A. A., Levina, I. V., Borodina, G. P., Tr. Vses. iV4uch.Issled. Inst. Sinfetich. i Nalural’n. Pushistykh Veshchestv., 7, 180-5 (1965). ( 1 0 1 ) Zelenetskaya, A. A., Nikitina, K.N., Borodina, G. P., Zbid., pp. 17580.
Titrations in Nonaqueous Solvents Gerald A. Harlow and Donald H. Morman, Shell Development Co., Emeryville, Calif.
T
PERIOD covered by this review (October 1965 through October 1967) has been a time of considerable activity and advance in the study and exploitation of nonaqueous titrations. Most impressive has been the development of quantitative expressions which satisfactorily describe many of the complex equilibria involved in the more inert solvents. Intensive study has led to a much better understanding of acid-base chemistry in acetonitrile and other solvents of the aprotic dipolar class. The outstanding work of I. M. Kolthoff and his collaborators deserves special mention. HE TWO-YEAR
THEORETICAL
Acid-Base Equilibria and Interpretation of Titration Curves. In protolytic solvents most monobasic acids or bases give little or no indication of intermolecular hydrogen bonding a t conventional titration concentrations. If the dielectric constant is high, dissociation of many salts is extensive and potentiometric and conductometric curves can often be interpreted in much the same manner as with aqueous titrations. The primary equilibrium involved for a weak acid (HA) is: H.4 F? H + AKreshkov et al. (133) have studied the titration of a number of phosphoruscontaining acids in a series of alcohols and compared the calculated pK’s with
+
418 R
ANALYTICAL CHEMISTRY
the pK’s of a reference acid (HCl). The solvents tested included methanol, ethyl alcohol, I-propanol, 1-butanol, and 2-propanol. The results were interpreted without need for acid-base equilibria other than those involved in water. The influence of increasing chain length of the n-alcohols on titration characteristics was primarily on the extent of the above equilibrium. As the molecular weight of the alcohol increases the dielectric constant and the basicity decreases, thus making the acids appear weaker and reducing the “leveling effect.” Consequently l-butanol had the highest differentiating power of these alcohols. The branched alcohol, isopropyl, departed from the consistent pattern shown by the straight-chain alcohols. I t is interesting to consider acid-base equilibria in tert-butyl alcohol (D = 10.9) as compared with n-butyl alcohol (D = 17.1). The use of this highly branched alcohol as a medium for titrations has been recently reviewed by Marple and Scheppers (160). These authors point out that although most aliphatic and aromatic carboxylic acids gave no indication of intermolecular hydrogen bonding, it does occur in exceptional cases such as with p-hydroxybenzoic acid. This acid precipitated during the titration in the form of a trimer (salt). Intramolecular hydrogen bonding is used by these authors to explain conductometric titration curves of dibasic acids where conductance
increases linearly to the first equivalence point, decreases between the first and second, and rises sharply beyond the second equivalence point upon addition of excess titrant. The chelated anion structure distributes the negative charge, with the result that it tends to associate very little with the large cation. Beyond the first equivalence point the cyclic structure is destroyed and the doubly charged anion associates much more strongly. By means of KMR studies Silver et al. (206)have shown that chelate homoconjugation in the mono-anions of polycarboxylic acids occurs even in methanol and water mixtures. Titration curves were obtained by plotting chemical shifts against equivalents neutralized. Puzzling results were obtained for the tetrafunctional pyromellitic acid, Solvent deuterium isotope effects on similarly hydrogen-bonded dicarboxylic acid mono-anions were studied by Eyring and Haslam (63). Also using NMR data Cocivera (42, 43) studied proton exchange involving ion pairs of ammonium salts in tert-butyl alcohol and concluded that the anion was bonded to only one N-H proton. I n the aprotic dipolar solvents the interpretation of titration curves involves additional equilibria. Hydrogenbonded acid-ion complexation may become so strong that inflections are obtained a t the half-neutralization point for a monofunctional acid or base. This tendency of an acid (HA or BH+) to
form a hydrogen-bonded complex with its conjugate base (A- or B) has been termed “homoconjugation.” The equilibria involved are : HA A- s [AHA]-
+ BH+ + B
$
[BHB]+
These equilibria are always in competition with hydrogen bonding by the solvent and ion association involving the titrant co-ion. Conductometric titrations of benzoic acids in acetonitrile (AN) were used by Coetzee and Cunningham (46) to study the “ortho effect” and other structural factors. The titration curves were different from those obtained in water because of additional equilibria. These authors point out that to account quantitatively for the reaction of a Brinsted acid (HA) with a base (B) in acetonitrile (SH), nine independent equilibria must be considered. The most important of these for the particular acids and bases employed were : HA
+B
e BH+A-
e B H + + ABH+A- + HA e AHA- + B H + BH+A-
Homoconjugation was not extensive in the case of the acids which had structures conducive to intramolecular hydrogen bonding, and calculated conductometric curves were in agreement with those experimentally determined. I n the case of benzoic acid, however, where homoconjugation should be more important, the observed titration curves could not be accounted for quantitatively. Coetzee and Padmanabhan (47) determined the homoconjugation and dissociation constants of phenol and four nitrophenols in acetonitrile from potentiometric and spectrophotometric measurements. Except for picric acid the phenols all formed relatively stable acid-anion complexes. Phenol itself formed also a (HA),A- complex. No evidence was found for dimerization of the free acids. Thus, although acetonitrile as a hydrogen-bonding solvent is too weak to prevent ionic homoconjugation, it does compete successfully with molecular dimerization, as indicated also by a nuclear magnetic resonance study by Springer and Meek (21.9). Kolthoff, Chantooni, and Bhowmik (118) have studied the acid-base properties of mono- and dinitrophenols in acetonitrile. Conductometric titrations were carried out as previously (11.3) with triethylamine and the curves interpreted with the aid of dissociation constants for the acid (KdHA), the homoconjugate ( K ~ H A ~ and ) , the salt (KdBHA). The homoconjugation constants were calculated from potentiometric, spectrophotometric, and solubility data. According to these authors
conductometric titration curves can be very misleading as measures of acid strength. For example, one might conclude from such curves that p-nitrophenol is a stronger acid than 2,6dinitrophenol, whereas actually the latter is about lo5stronger. Thus, even for a qualitative interpretation of these curves it is necessary to know the ionic dissociation constant of the triethylammonium salts as well as the dissociation and homoconjugation constants of the acids and the ion mobilities. The authors also conclude that no simple relation exists between the dissociation constants of nitrophenols in water and in acetonitrile, although anion stabilities and intramolecular hydrogen bonding must contribute to the differences. Homoconjugation constants and heteroconjugation constants also reflect steric and intramolecular restrictions. Negligibly small constants were found for 2,&dinitro-, 2,6-di-tertbutyl-4-nitrophenol, and picric acid systems. These are recommended as suitable for the spectrophotometric determination of hydrogen ion activity in acetonitrile. I n another paper (115) two of these authors reported the homoconjugation constants as well as the conventional dissociation constants for a number of substituted benzoic acids in acetonitrile. Over-all dissociation constants, (Kdza~), for the reaction, 2 H.4 e H + AHA-
(117) have also reported on the titration of univalent and uncharged bases in acetonitrile. Visual, spectrophotometric, potentiometric, and conductometric methods were compared for the titration of amines of widely differing strengths. Perchloric acid in various solvents was used as the titrant. The spectrophotometric and potentiometric methods were suitable for weak uncharged bases. For extremely weak bases, ~ K ~ B(HzO) H+ = 1 to 2, the glass electrode was found unsuitable and only the spectrophotometric method gave excellent results. Conductometric titrations were of little or very limited analytical value. These workers compared experimental and calculated potentiometric titration curves and found that the observed break in potential is less than the calculated one because the glass electrode in acetonitrile indicates too high a pH in solutions of perchloric acid. However, the shapes of the experimental titration curves agreed with curves calculated from the dissociation constants K%H+ alone, because the degree of homoconjugation of most amines is very slight compared either to unhindered, unchelated carboxylic or phenolic acids, or to strongly conjugated bases such as the amine oxides described below. Harlow and Morman (86) have carried out a potentiometric and conductometric study of hydrogen bonding and ion association in acetonitrile and other aprotic dipolar solvents. All of the amines tested gave normal curves in for 3,bdinitrobenzoic, salicylic, p-nitroagreement with the results discussed benzoic, m-nitrobenzoic, p-hydroxybenabove. Some amine oxides, however, zoic, and benzoic acids were calculated gave potentiometric curves with two from the characteristics of conductowell-separated plateaus (A400 mv) when metric titration curves using weak bases titrated with perchloric acid. When as titrants. The constants KdHA and hydrochloric acid was used as the tiK ~ H A *were calculated from potentiotrant, only a single inflection was obtained. Conductometric curves for metric measurements with the glass A spectrophotometric electrode. amine oxides in acetonitrile also showed method using p,p’-dimethylaminogreat differences between the two tibenzene as indicator was used to check trants. With hydrochloric acid a prothe K ~ ~ values H A for 3,5-dinitrobenzoic nounced maximum in conductivity apand salicylic acids. Solubility measurepeared near the mid-point, sharply dements were used to check the value of creasing conductance followed the midK ~ H Afor ~ - the same two acids. point, and a sharp conductance miniThe authors concluded that the pomum occurred at the equivalence point. tentiometric and spectrophotometric Perchloric acid gave a smooth curve of methods yielded the most reliable K ~ H A continually increasing conductivity values, and certainly the agreement for without detectable inflections. This the two acids discussed was excellent. titration behavior was explained on the The conductometric values, involving basis of a homoconjugate of extraormany assumptions, were felt to be less dinary stability in competition with reliable than those obtained potentioion association as indicated by the metrically. The values for KfHA2- obequilibria : tained potentiometrically were also conR X -+ 0 H+ + R3NOH+ sidered more reliable than those based on solubility, although again the agreement was satisfactory. The homoconjugation constant for salicylic acid was larger than anticipated, in light of its structure, which would seem to favor chelate homoconjugation over inter(RaNO)zH+ C104- Z molecular homoconjugation. Kolthoff, Chantooni, and Bhowmik R3N -L 0 RaNOH’ f Clod-
+
+
+
+
VOL. 40, NO. 5, APRIL 1968
* 419 R
tonitrile. Homoconjugation is sufficiently strong to cause a mid-point inflection in potentiometric titration curves for unhindered phenols and carboxylic acids. Of special interest is the characteristic behavior of aliphatic amine oxides. These compounds titrate in water as weak bases with a pK of about 5 . I n the sulfolanes as in acetonitrile (above) they titrate as semifunctional bases with pK’s which are widely separated. Because of the unusually strong homoconjugating tendencies of the amine oxides, it has been suggested by the above-mentioned investigators that K ~ R A ~ - one of these compounds might serve as AHA e AHAa reference compound to characterize K ~ AR H AHR $ AHRthe competitive hydrogen-bonding ability of nonaqueous solvents. Thus, the K’A(RR)*- difference in potential between the oneA2HR e A(HR)2fourth and three-fourths neutralization When the amphiprotic substance acts points in the reaction of triethylamine also as an H-bond acceptor (OHR) we oxide with perchloric acid, for example, have : should vary inversely with the tendency AH OHR(B) e -4H. , OHR(BHA) of the solvent to form hydrogen bonds. K ~ B H A Equilibria in inert solvents are even more complex than in the aprotic An equation was derived which allows dipolar solvents. Because potentiothe calculation of the effect of water on metric and conductometric methods are the paH(AN) of mixtures of weak acids difficult to apply, spectral and thermal and their tetraalkylammonium salts. measurements are often used. Few of The equation employs the dissociation, these studies utilize titrations. Acid-base equilibria in the solvent homoconjugation, and hydration constants of the acid-base system. chlorobenzene have been studied by Bruckenstein and Wilson (29) by means fZTV2a2H +C, - ~VWK~HAUH [ (C, CJ of the indicator base dimethylaminoazobenzene and three acids. Constants for kfH~%-(Ca - C,)2/VTf‘l the equilibria involved were obtained V2Kd2H~c, = 0 from spectrophotometric data. In the case of hydrochloric acid one of the surwhere prising results is the lack of participa=: 1 K~AHR-[HR] tion of the bichloride ion (CIHCl-) (62) in the described equilibria. + K ’ A ( H R ) ~ - [ H R ] ~. . . An extensive acid-base study of phe= 1 fKfgn~[B] . . . . nols in benzene has been reported by Steigman and Lorenz (216) who also C,, C, = analytical acid and salt conused spectrophotometric measurements. centrations. The effect of added amine and quaWhen conjugation occurs, the POternary ammonium salts (216) was detentiometric titration curve in the presscribed. Raldstein and Blatz (234) used low frequency Raman spectroscopy ence of HR and B is characterized by to study formic and acetic acid associaC,)K~HA%-/VW instead the factor (C, tion. The pure acids and their aqueous CJKJHA%when H R and B are of (C, and hydrocarbon solutions were used. absent. The paH(AX) at the mid-point Only cyclic dimers were observed in the is smaller by log (V/TP) in the presence of H R and B than in their absence. It hydrocarbon solution. The dimerization of benzoic acid in benzene was inalso follows from the equation that the addition of H R and B causes a decrease vestigated by Shamsul Huq and Lodhi (201). Lloyd et al. (164) reported on in paH(AN) at the beginning and near the intramolecular hydrogen bonding the equivalence point of a titration. in o-substituted benzoic acids. A potentiometric study of acid-base A comprehensive study of the thermoequilibria in dimethylformamide was carried out by Juillard (104). It was dynamics of ion association has been carried out by Pettit and Bruckenstein shown that homoconjugation takes (174). The Denison-Ramsey equations place in this solvent. The titration characteristics of sulfolanes, a family of were extended, taking into consideration the various possible interactions besolvents of the dipolar aprotic type, tween ions and their induced dipoles. have been studied by Morman and Besides ion pair formation, triplets, Harlow (164). The acid-base equilibria quadrupoles, and sexapoles were conencountered in these solvents are simisidered. lar to those described above for aceThe homoconjugate structure is favored by the larger, better shielded perchlorate ion which has much less tendency to form ion pairs than the smaller chloride ion. Kolthoff and Chantooni (112) have studied the effect of hydrogen bond donors and acceptors on paH of acetonitrile solutions of acids and their salts. The effect of water and alcohols on acidity was considered quantitatively, making use of the homoconjugation equilibrium and the two heteroconjugation equilibria shown below. The hydrogen bond donor is designated as HR.
+ + +
+
+
+
v
+
w
+
420 R
+
+
+
+
+
ANALYTICAL CHEMISTRY
A+B=AB AB A e ABA
+ AB + AB + (AB)2 (AB)2 + AB (-iI3)3
In solvents of low dielectric constant such association takes place stepwise with increasing concentration. At low concentrations (10-5Jf) in solvents with dielectric constant of about 4 pure ion pairs can be found but at higher concentrations a mixture of species will always be present. The fact that ion quadrupoles contribute only a slight fraction of the total is used to explain why cryoscopic studies are difficult to interpret in terms of simple equilibria. Of special interest is the conclusion of these authors that in solvents such as benzene, dioxane, anisole, chlorobenzene, and tetrahydrofuran, quaternary ammonium salts do not appear to form solvent-separated ionic aggregates. Mixed ion association for aggregates such as NaC10a~HCI04 is described in a subsequent publication by Uruckenstein and Pettit (27‘). dcid-base equilibria in acetic acid have received considerable study both in the pure solvent and in miature with other solvents. The effects of substituents on the equilibrium constants of aniline were studied by Ceska and Grunwald (36) using glacial acetic acid as solvent. The same solvent was used by Jasinski and Kozlowska (!l7) in the potentiometric determination of equilibrium constants for a series of amino acids. -4general treatment of potentiometric and photometric titrations in this solvent with particular attention to the concept of the ligand buffer has been published by Tanaka and Kakagawa (222). The effect of added p-dioxane on potentiometrically measured basicity of nitrogen and inorganic baies was studied by Kolling and Garber (111). Solvation Effects and Intersolvent Comparison of Acidity. -1 general discussion of the problems of coniparing data obtained in different s 0 1 ~ ~ ~has q t bbeen presented by Strehlom (142). A method for the estimation of medium effects for ions in nonaqueous solutions has been proposed by l’opovych (179). The aswmption is made that an electrolyte composed of large ions of equal size can be ured as a reference electrolyte. The medium effect can then be divided equally between the cation and anion. The particular compound chosen was triisoamylbutylamnionium tetraphenylboride and its behavior in methanol and ASTN solvent was studied along n ith other ions. Relative activities of reference cations in acetonitrile aiid watei ere evaluated by Coetzee and Campion (44)on the basis of a modified Born equation. The usefulness of various reference couples such as rubidium, thallium, ferrocene,
and ferroin systems was compared. The authors prefer a scale based on a rubidium (1-0) couple. The thallium (1-0) potential was placed on this scale and from the solubilities of thallium salts the relative activities of anions such as halides, nitrate, perchlorate, and picrate in water and acetonitrile were determined and reported (46). Of the anions studied, all except perchlorate and picrate favor water over acetonitrile as solvent. The preference of perchlorate for acetonitrile over water is puzzling. The results led to the conclusion that the chemistry of electrolytes in acetonitrile is dominated by differences in the properties of anions. Alexander and Parker (6) used the same two solvents as well as methanol and dimethyl sulfoxide for a study of the effect of solvation on the standard chemical potential of anions. The hydrolysis of some metal cations in acetonitrile containing traces of water was studied by Shirvington (10.2). The hydration of ions in acetonitrile was studied by Chantooni and Kolthoff (37). Equations were developed which permit the calculation of hydration constants from the solubility product of a slightly soluble salt and the total ionic solubility in the presence of varying concentrations of water. The hydration constants for the cations Li, Na, K, and Cs and for the anions perchlorate, periodate, methane sulfonate, nitrate, 3,5dinitrobenzoate, salicylate, picrate, and 3,5 - dinitrophenolate were reported. The values for x in the hydration equilibrium shown below were 1 for K, Cs, iodate, perchlorate, and picrate and 1 and 2 for the other ions. I,* L HZO e I,,*
+
where I,* is a solvated or unsolvated monovalent cation or anion. The individual hydration constants for 3,5dinitrophenolate ion (Kif,*) determined by an independent spectrophotometric method agreed with those obtained by the solubility method. The hydration of undissociated salts in acetonitrile was also studied (114). An equation was derived which made it possible to calculate the concentration of all species present in saturated solutions of potassium salts of three nitrophenols and of potassium salicylate. The dissociation of ammonium ion in methanol-water mixtures has been measured potentiometrically by Paabo, Bates, and Robinson (110). The pK, values show the same trend as other cation acids, decreasing initially as methanol is added to water and passing through a minimum a t about 70 weight yo methanol. Results are interpreted on basis of electrostatic and basicity effects. Korberg (168) measured the potential of a glass electrode against a silver-silver chloride reference electrode in isopropyl alcohol, methyl ethyl ke-
tone, and mixtures of the two solvents. Autoprotolysis constants were reported. Dimethylformamide, an aprotic dipolar solvent which has been used extensively for nonaqueous titrations, continues to receive attention. A comparison of acid dissociation constants in dimethylformamide, dimethyl sulfoxide, and methanol as measured by indicator studies were reported by Clare et al. (41). Rates of reactions were used to estimate hydrogen-bonding activity coefficients which predict closely the effects of anion solvation on acid-base equilibria. The molecular association of formic and acetic acids both as pure liquids and in solutions of water and hydrocarbons has been investigated by Waldstein and Blatz (234) using lowfrequency Raman spectra. Gregory et al. (78) have studied the solvation of amines in organic solvents. The hydration of benzoic acid in diphenylmethane has been studied by Wood et al. (239). -4solute vapor pressure method was developed for measuring hydrogen bonding. Glover (7.9) has investigated ion solvation in mixed solvents containing varying ratios of dioxane and water. The pK and solvation numbers for the same compounds were reported (74). Gilkerson and Ezell (71) have studied ion-solvent interaction and discussed the importance of the dipole moment and the basicity of the ligand. Christian et al. (39) have developed a method for predicting the effect of solvation on hydrogen-bonding association equilibria. The method is based on the solubility of water in the various solvents. Acidity Functions. The significance of the Hammett acidity function (H,)as measured in toluene has been evaluated by Sanders and Berger (193). The results obtained using four different indicators showed that H, values cannot be regarded as an absolute index of acid strength. The measured strengths of the acids varied with the indicator used as a reference point and with acid concentration. Aggregate formation was thought to be a complicating factor. The acidity of hydrocarbon acids in dimethyl sulfoxide was measured by Ritchie and Uschold (186) using potentiometric data obtained with the glass electrode. The electrode was found to respond reversibly to changes in hydrogen ion activity over a range of 25 powers of 10. An acidity scale was set up for this solvent (187). Acidities of extremely weak acids were measured in the same solvent by Steiner and Starkey (217), who also discuss the H- acidity scales. Bowden, Buckley, and Stewart (26) have set up an acidity scale ( H 2 - ) for four strongly basic systems containing quaternary ammonium hydroxide-water, pyridine-water, sulfolane-water, and
dimethyl sulfoxide-water. The relative acidities of hydrocarbons towards cesium cyclohexylamide have been determined by Streitwieser et al. (219). Acidity functions of some organophosphorus acids in ethylene glycol and dimethylsulfoxide were measured by Cook and Mason (48). Basicity of phosphine oxides and sulfides has been calculated from measurements of their chemical shift in sulfuric acid solutions. I n this study Haake, Cook, and Hurst (85) found that phosphine oxides are 1/106 as basic as amine oxides, arsine oxides, and stilbine oxides. An NMR method for the determination of relative acidities of weak acids in liquid ammonia has been developed by Birchall and Jolly (19). The acidity of some anilines covering a range of 6.6 pK units were found to fit a Hammett plot. Conductance and Other Fundamental Data. Gilkerson and Ralph (72) measured the conductance of trin-butylamine N-oxide picrate as a function of salt concentration in the solvent o-dichlorobenzene. The addition of picric acid gave a large depression in the conductance of the salt. Addition of pyridine had the opposite effect. The data were interpreted in terms of acidbase dissociation of the salt to yield free amine Ar-oxide and picric acid and the formation of a 1 to 1 complex between the cation from the salt and the amine N-oxide (homoconjugation). The amine N-oxide was found to be a stronger base toward picric acid than the corresponding amine (tributylamine). The reverse order is observed in water. The conductances, viscosities, and densities of solutions of tetra-n-butylammonium thiocyanate in nitromethane were measured by Longo et al. (156). Transport properties of the tetraethanolammonium ion in various nonaqueous solvents were determined by Cunningham, Evans, and Kay (51) from precise conductance measurements. The same method was used by Carper and de hlaine (36) to study solutions of the group IIA metals in methanol and methanol-carbon tetrachloride. Conductance measurements of solutions of electrolytes in nitrobenzene (165) and anhydrous acetone (194) have also been reported. A method for the conductometric determination of solubility and solubility products of silver salts in the ASTM solvent (toluene-isopropyl alcohol-water) was described by Popovych (178), who also measured the solubility of various salts in methanol and water (180). Heats of solution of salts in propylene carbonate and water as well as enthalpies of transfer have been measured by Wu and Friedman (240, $41).
The ionization constant of acetic VOL. 40, NO. 5, APRIL 1968
421 R
acid in ethanol-water mixtures a t three temperatures has been reported by Spivey and Shedlovsky (221). The same authors reported conductivity data for hydrochloric acid, sodium chloride, and sodium acetate in similar solvents (212). Goffredi and Shedlovsky published data for the conductivity of sodium chloride (76) and hydrochloric acid (76) in mixtures of water and n-propyl alcohol. Luksha and Criss (156) reported on the free energies, entropies, and activity coefficients of some alkali metal halides in anhydrous N-methylformamide as calculated from potentiometric data. Dissociation of tetra-n-hexylammonium iodide in dichloroethane was studied spectrophotometrically by Matheson (162). Electron paramagnetic resonance data were used by Hirota and Kreilick (89) for studying ion-pair equilibria in tetrahydrofuran. Transference numbers for potassium chloride in formamide and n-methytacetamide were measured by Johari and Tewari (1OS). Padova and Abrahamer (172) have reported the apparent and partial molal volumes of some symmetrical tetraalkylammonium halides in anhydrous methanol solutions. APPARATUS AND TECHNIQUES
A simple photoelectric device (208) for the titration of small amounts of carbon dioxide uses a photoresistor to obtain automatic shutoff of the titrant flow. A pyridine-monoethanolamine medium and sodium methoxide titrant are used. Young et al. (247) have described a new accessory to convert a commercial automatic potentiometric titrator with digital readout to an automatic photometric titrator. The unit utilizes a wedge interference filter to obtain monochromatic light and fiber optics immersed directly in the solution to eliminate the need for special titration cells. The instrument was applied to aqueous and nonaqueous titrations, including some complexometric studies. Two applications of membrane electrodes have appeared. A previously described membrane electrode was applied to potentiometric titrations of some bases and sulfuric acid in anhydrous acetic acid (77). Some results were better than those obtained with a glass electrode. Blackburn and Greenberg (21) illustrated the analytical possibilities of an easily fabricated membrane electrode in the coulometric generation of hydrogen ions for titration in water and in methanol. An interesting technique for determining weak organic acids in the effluent from a chromatographic partition column (108) involves titration with an indicator. The effluent is mixed with an excess of an indicator salt solution. When an acid emerges, the indicator is 422 R
ANALYTICAL CHEMISTRY
converted to its hydrogen form and the absorbance is measured by a recording photometer. Boardman and Warren (22) have developed techniques and a cell to permit the in situ conductometric titration of acidic and basic components separated by thin-layer chromatography. Bruckenstein and Vanderborgh (28) have described the principles of a new method of end point detection, cryoscopic titrations. .4n apparatus was constructed which continuously records the variation in freezing point depression during a titration and was applied to titrations in water and benzene solutions. Rogers et al. (189) have published another in a series of papers on phase titrations. In part V, the use of nitroalkanes is discussed and the use of nitrobenzene to improve phase-titration end points. A so-called “chronopotentiometric” method (156) for titrating acids in a nonaqueous solution is simply a potentiometric titration which utilizes a constant rate of titrant addition from a Mariotte bottle accompanied by automatic recording of the electrode potentials. Spiridonova (210) has based a method for determining furfuraldehyde in nonaqueous solutions on a turbidimetric titration with water. TITRANTS AND SOLVENTS
Two studies of the use of sulfonic acids as titrants in nonaqueous media have been made. Twelve aromatic sulfonic acids were compared with some inorganic acids by titration in several nonaqueous solvents with basic titrants. 2,4-Dinitrobenzenesulfonic acid, reported to be close in strength to perchloric acid, was further evaluated as a titrant for a number of organic bases (17 6 ) . The acidic behavior of perchloric and several sulfonic acids in acetic anhydride-acetic acid mixtures was reported (17 5 ) . The increased potentiometric break in this solvent system is discussed, as well as the comparison of perchloric and trinitrobenzenesulfonic acids as titrants. Kelluni and Uglum (107) have developed a method for the determination of silanol and water using lithium aluminum di-n-butyl amide as a direct acid-base titrant. The use of a number of less commonly used solvents has been reported. The sulfolanes (184) have been shown to have an extremely wide potential range suitable for the differential titration of both acids and bases. They are nonleveling for strong components, yet permit the titration of very weak components, and their low hydrogen-bonding capability makes them interesting solvents for studies involving homoconjugation. Jasinski et al. (102) found that methyl thiocyanate can be used for potentiometric titrations of strong
acids with tetramethylammonium hydroxide (TMAH) and of very weak bases with perchloric acid. A number of indicators were also tested. Adipodinitrile (81) has been suggested as a good differentiating solvent for the potentiometric titration of organic acids with tetraethylammonium hydroxide. The relative strengths of a number of weak acids have been studied. I n a paper on acid-base equilibria in tert-butyl alcohol, Marple and Scheppers (160) discuss the conductometric titration of a variety of acid types. They have also investigated chelation reactions of a number of metal ions with the ditetrabutylammonium salt of EDTA in this solvent. Paul et al. (278) carried out conductometric titrations in formamide to establish the acidic and basic character of Lewis and protonic acids and organic bases in this polar solvent. Some tetraalkoxysilanes have been studied for the potentiometric titration of monosubstituted aniline derivatives (188) with perchloric acid, and of benzoic acid derivatives (96). Butler (88) has reviewed the literature on electrochemistry in dimethylsulfoxide (DMSO). Although no applications are described, Jasinski and Kirkland (85) reported the purification by fractional distillation of propylene carbonate, which is of current interest in electrochemistry. Titrations in inorganic nonaqueous media usually present a greater challenge than do titrations in the more commonly used organic solvents. Schenk (195, 196, 197) has described an apparatus for carrying out potentiometric and conductometric titrations in liquid ammonia which provides for both evacuation of the cell and working under a nitrogen atmosphere. Its operation is demonstrated by titrations of ammonium chloride and sulfide with potassium amide, and of ammonium sulfide with silver iodide. Baumann and Simon (13) found that glass electrodes behaving as “protodes” in aqueous systems respond to the protonated solvent in liquid ammonia and can be used to follow the activity of ammonium ion. They performed titrations of the potassium salts of very weak acids, such as alcohols and sterically hindered phenols, using ammonium chloride as titrant and a glass indicator electrode. Acids may be titrated by in situ formation of potassium amide by stepwise addition of metallic potassium. A fused salt mixture containing equal parts of sodium and potassium nitrates was used as a solvent by several workers. Bombi and Fiorani (23) studied the titration of 13 cations with 02- produced by constant-current reduction of the nitrate melt at a platinum electrode. Schlegel(198) titrated Crz07*- by the addition of weighed amounts of sodium
carbonate which produced 02-in the melt. A platinum indicator and silversilver nitrate reference electrode system was used. Mountford and Wyatt (165) carried out some conductance titrations of oleums with KH$OI or KDSOd in both the hydrogen and deuterium sulfuric acid systems. END POINT TECHNIQUES
I n addition to the more commonly used potentiometric and conductometric titrations in organic solvents, which constitute much of this review, some recent work has been done with the less frequently used techniques. Thermometric Titrations. Vaughn and Swithenbank have performed thermometric, or enthalpimetric, titrations of both acids and basic nitrogen compounds. They have made the interesting observation that if acids (232) are titrated in acetone solution with a nonaqueous base (KOH in IPA), the acetone will serve as an “indicator,” causing the evolution of a large amount of heat after the acids have been titrated. This is attributed to formation of diacetone alcohol in the presence of excess base. The method can be applied to a wide range of acids including carboxylic acids, phenols, keto-enol tautomers, and imides. For the titration of bases (233), these authors have made use of the large endothermic heat of dilution when a strong HC1 solution in isopropyl alcohol is added to many organic solvents other than alcohols. The 5N HC1 solution first produces a temperature rise due to the neutralization of the bases present, which is then followed by a sharp temperature drop due to dilution once the end point is reached. Aliphatic and aromatic bases can be resolved. Tertiary amines and salts of organic acids have been determined in acetic acid by a “catalymetric-thermometric” titration (229). Quilty (183) suggests the thermometric titration of new and used petroleum oils, which offers some advantages over the commonly used potentiometric method for acidity. Wasilewski and Miller (235) have applied direct injection enthalpimetry in the determination of moisture with Karl Fischer reagent. Cryoscopic Titrations. A new method of detecting end points in nonaqueous (as well as aqueous) titrations has been developed by Bruckenstein and Vanderborgh (28). It consists of continuously measuring the freezing point depression during the course of the titration. End point deflections are due to changes in the rate of production of solute particles before and after the equivalence point. This method should not be confused with the older thermometric titration which is dependent on the heat generated by the reaction.
The curves obtained in cryoscopic titrations are somewhat similar to conductometric curves. Analytical accuracy was reported to be within about 1%. One of the advantages of the new method is that it is applicable to inert solvents where electrometric methods are difficult to apply. As an example, the authors titrated a number of amines in benzene using trichloro-, trifluoro-, and acetic acids as titrants. High Frequency Titrations. Berges and Perez have investigated the high frequency (HF) titration of sulfides (16) and thiols (17). Hg(SCN)2, and HgC12 were compared as titrants in acetone, ethanol, or methanol-benezene solution. Sulfides and thiols in mixtures can be resolved. Various organolithium reagents (236) in hydrocarbons have been determined by H F titration with a standard hydrocarbon solution of acetone using commercially available equipment. H F titration has also been used for the determination of dicarboxylic acids in dimethylformamide (DMF) (131) and for the determination of carboxylic and phenolic acid groups in lignin preparations (231). Indicators. Fritz and Gainer (67) have given the acid-base transition ranges of 13 indicators covering almost the entire range for the titration of acids in pyridine with tetrabutylammonium hydroxide. The ranges are given in terms of potentials obtained with a glass-modified calomel reference electrode system, permitting the selection of a suitable indicator from a potentiometric titration curve for a given acid or mixture. Titrimetric and equilibrium studies of Nile blue A and several related indicators in nonaqueous media were reported by Davis and Hetzer (52). Legradi has published a series of three papers on new acid-base indicators which he has used in nonaqueous media (acetone and ethanol) : phenylhydrazine derivatives (145); 2-acetyl-l-(4-nitropheny1)hydrazine (146); and 1-(2,4dinitrophenyl) pyridinium chloride (147). The latter indicator was used in the titration of some electronegatively substituted phenols. A study (248) of the behavior of crystal violet and methyl violet indicators in anhydrous acetic acid medium concluded that an arbitrary choice of either indicator for a titration was satisfactory, since they give identical color changes. A bisazoderivative of chromotropic acid (53) has been used for the titration of sulfate ion with barium chloride in acetonewater (4 to 1) media. It was also used as a metallochromic indicator for the direct complexometric titration of barium, strontium, and lead (11). Kolthoff, Chantooni, and Bhowmik (116) have determined indicator constants and spectrophotometric characteristics of a number of indicators in
acetonitrile. These data are compiled together with previously published measurements to yield a list of 39 indicators in order of increasing dissociation constant. Dissociation constants in water are also listed for most of these indicators. The data show that the ApK’s of these indicators between water and acetonitrile vary from 4.6 to nearly 20 (phenolphthalein). Indicator acidbase equilibria for 0-, p-, and m-nitroaniline in three mixed solvents were studied by Boni and Strobe1 (24). Dioxane-water, acetic acid-water, and formic acid-water mixtures were used. METHODS AND APPLICATIONS
Acids. Titration in pyridinebenzene solvent with tetrabutylammonium hydroxide (TBAH) was used by Buell (31) to differentiate phenols from carboxylic acids in petroleum. Lee (144) has determined dibenzoylmethane and other weak acids by microtitration in ethylenediamine or pyridine with tetrabutylammonium methylate. Polarized bimetallic electrodes were used for end point detection. A study (55) was made of the titration of various types of acids in dimethylformamide (DMF) using potassium methoxide titrant and an antimonycalomel electrode pair. Organic acid salts (68), such as acetates, benzoates, naphthenates, palmitates, and stearates, were determined in acetone or D M F with tetraethylammonium hydroxide (TEAH). A series of dicarboxylic acids and some tricarboxylic acids were titrated in pyridine or acetone with TBAH and sodium methoxide (100). Terephthalic acid and p-toluic acid can be resolved by titration in D M F with KOH (628). Diphenic acids (119) can be determined in the presence of acetic acid and diphenaldehydic acid by potentiometric titration in acetone with TEAH. If phthalic acid is present, it is corrected for following polarographic determination. Jasinski et al. (98) titrated a series of phosphinic acids potentiometrically in methanol, pyridine, DMF, and dimethylsulfoxide (DMSO) with sodium methoxide. Kreshkov et al. (132) determined individual inorganic and organic phosphorus-containing acids (mostly phosphonic) and their mixtures with HCI by titration in methyl ethyl ketone (MEK), tert-butyl alcohol, and pyridine with TEAH. These same workers (133) titrated other phosphoruscontaining acids (mostly phosphinic) in the lower aliphatic alcohols and calculated their dissociation constants. Aksenenko et al. determined several 2-nitroalcohols (5) and nitroguanidine (4) as acids by potentiometric titration in acetone medium with quaternary ammonium hydroxides. Trusell (226) VOL 40, NO. 5, APRIL 1968
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determined the extent of acid hydrolysis of N-methylpyrrolidone by potentiometric titration of the reaction mixture in ethanol with T13hH. Three inflections were obtained showing the starting material and reaction product. Stamey and Christian (214) investigated nine solvents and three different indicator electrodes for the titration of hydroxamic acids with TBhH. They found D l I F and a platinum-platinum electrode system best. Several nionobasic perfluoro-acids (98) hare been titrated in the presence of HF in acetone containing 5 to lOy0water with KOH as titrant. h tungsten indicator electrode mas uqed. Ricinoleic acid ( 1 2 ) n a s differentiated from H2S04 plus ethyl hydrogen sulfate by potentiometric titration in either X E K or ethanol with 0.8N S a O H . Kre\hkov and his coworkers have resolved mixtures containing naphthoic and hydroxynaphthoic acids (130) by titration in M E K with TBAH. They also titrated some naphthyl acid derivatives (137) spectrophotometrically in methanol-2-propanol solvent mixture i l to 2) wing sodium methoxide. Crowell and Burnett (50) modified the .ISTM procedure to determine fatty acids and rosin in tall oil on a niicro scale. a-Olefin-derived sodium sulfonates (SO) were analyzed by titration of the acids in acetone with T B d H follo\ring ion exchange. Drushel and Sommers (54) have studied type, number, and strength of acid qites on the surface of cracking catalybts using vi4ble and fluorescent indicators. Ruskul (191) titrated weak and very weak acids amperometrically in alcohol-glycerol or alcohol-formamide solvent mixtures with diethylamine and ethylenediamine as titrant.. The amperometric measurements were made with pyrogallol as an electronietric indicator and without an externally applied potential. Fritz and Gainer (66) describe the coulometric titration of mineral, sulfonic and carboxylic acids, enols, imide., and phenols in tert-butyl alcohol mid acetone by internal generation of tetrabutylamnionium hydroxide or alkoxide. Both visual and potentiometric detection were used. Bases. 13uell (SO) has set up a classification system for nitrogen bases in petroleum dependent upon the apparent base strengths of the components according t o titrations in acetonitrile and acetic anhydride solvents with perchloric acid. Acetic anhydride mas also used a. the solvent for the determination of the amide, ecaprolactam (3). Wimer (838) has commented on procedures permitting the direct titration of amides with perchloric acid in several solvent systems including acetic anhydride. The latter solvent ma5 used by Georgievskii and Dzyuba (70) for the resolution of mixtures of the weakly basic pyrazolone 424 R
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derivatives and was found to be better than acetic or formic acid solvents. The titration of the amino groups in some amino acids and their mixtures has been accomplished potentiometrically in acetonitrile-acetic acid solvent mixture ( 5 to I) with perchloric acid (125). Kalidas (105) obtained two inflections in the potentiometric titration of benzidine in propylene glycol-chloroform (1 to 3) with HCI. ,Jasinski and Pawlak (99) reported the conductometric titration of weak bases such as brucine, pyridine, p-toluidine, benzidine, quinoline, and aniline in several organic solvents and their aqueous mixtures. h sharper end point was generally obtained with an increase in water content. Visual titration in ethanol-ethyl ether solution with thymol blue indicator has been used to determine acetate and free alkali in the presence of other sodium and potassium salts (61). The free sodium hydroxide is first titrated with HC1. Then an excess of HC1 is added and the solution is back-titrated with KOH, observing the titrant volume consumed between color changes for the titration 01' the excess HC1 and the acetic acid present. , Other Functional Groups. l f c Cullough and Hartinger have presented two interesting papers on the analysis of organophosphites. Tertiary alkyl phosphites [ f 5 7 ]were found to be sufficiently strong bases to be titrated in acetic anhydride with perchloric acid. Results were said to agree with the iodimetric method. Some of the weaker dialkylphenyl phosphites could be titrated also and were differentiated from the tertiary alkyl phosphites. I n a second procedure the total phenoxy content (158) of triaryl and alkyl aryl phosphites was determined by potentiometric titration in pyridine with TB;IH. The mechanism was believed to be a base-catalyzed transesterification involving the methanol in the titrant solvent, with subsequent titration of the liberated phenol. The method has also been applied to phenyl phthalates and phenyl carbonates. Using both titration procedures it was possible to determine the composition of mixtures of trialkyl, dialkyl phenyl, and diphenyl alkyl phosphites. Belcher et al. (15) examined various procedures for the determination of hydroxyl groups on a submicro wale. Conventional acetylation methods followed by titration could not be used for aliphatic alcohols, but a satisfactory esterification-spectrophotometric procedure is reported. Considerable discussion is given on the titration of phenolic hydroxyl groups for which the widely used pyridine-TBAH system was found best. Bromination methods for phenols were not feasible on a submicro scale.
The saponification of difficultly saponifiable esters was greatly accelerated by the use of dimethylsulfoxide (DMSO) as the solvent (200). Greive et al. (79) stress the importance of the presence of water in the saponification of organic esters. Low and variable results were obtained with an essentially anhydrous alcoholic KOH solution, while good recoveries were obtained with the addition of mater. Feuge et al. (65) have reported low recoveries in the determination of cyclopropenes by previously reported methods using a modification of the Durbetaki procedure for oxirane groups (acetic acid-benzene solvent and HBr in acetic acid titrant). They suggest substituting benzene for acetic acid in both sample and titrant solvents. Acrylonitrile has been determined by adding an excess of mercaptoacetic acid in ethanol solution, after which the excess thiol is titrated with 0 . 1 5 iodine solution (122). Alarquardt and Luce (161) have reported an efficient, accurate method for the determination of vinyl unsaturation in styrenes, cyclohexene, allyl, and methallyl alcohols based on addition of mercuric acetate. Kreshkov et al. (126) have developed a related method for determining the purity of styrene. A modified methylene blue titration for the determination of styphnates (141) has been presented. Sturzer (221) has determined acetyl chloride and acetic anhydride in acetic acid medium by titration of the chloride with qilver acetate and the acetic anhydride with aniline. -1method for the analysis of trialkyldifluorophosphatosilanes and bis(trialkylsily1)monofluorophosphates (134) is based on potentiometric titration in methanol with sodium or potassium methoxide. An inveotigation (49) has shown that the isoquinoline conductometric titration of mixtures of triethylaluminum and diethylaluminum is unsuitable for the determination of the individual components. Redox. Cerate oxidimetry in nonaqueous media, using 2NH4S03 Ce(NOJi as oxidant, has been reported in two papers. Ferrous chloride (184) was used to titrate the oxidant in acetonitrile solution with a platinum indicator electrode and a glass or antimony reference electrode. In the second paper (69),the oxidant was used to titrate a-hydroxy- and a-oxocarbouylic acids in acetic acid medium in the presence of perchloric acid. Alternatively, an excess of ouidant can be added to the sample and back-titrated with ,odium oxalate solution. Hladky (90) performed some potentiometric redov titrations in pyridine and dimethylformamide media involving cupric chloride and acetate, ferric chloride, bromine, or iodine for titration of mercapto groups and some other compounds.
Trischler and Szivos have carried out potentiometric titrations of phenol (224) and its dihydric derivatives (225) in methanol containing KBr and HgClz with N-bromosuccinimide in acetic acid as titrant. I t was found that although the titration of quinol and catechol involves oxidation, the titration of resorcinol involves a substitution reaction with bromine. Benzothiazole-2-sulfenamides have been titrated potentiometrically (121) in acetic acid containing hydrochloric acid with 0.1N bromidebromate titrant, Alfaro and Dolezal ( 7 ) found that Pt(IV), Pd(II), and Ir(1V) can be titrated more effectively with Fe(I1) as a reductometric agent in a complexing medium such as triethanolamine. Redox titrations have seldom been carried out by coulometric generation in nonaqueous solvents. Streuli (220) has described the cathodic generation of 1,4-benzosemiquinone from hydroquinone in dimethyl sulfoxide and acetonitrile. The reductant reacts quickly and quantitatively with anodically generated iodine, but is destroyed by hydrogen ion. Controlled acidity conditions are required. Dichloramine-T (DCT) has been proposed as an oxidimetric titrant in anhydrous acetic acid medium (167). I t is thought to be the first time a metal-free organic compound has been so used. DCT in acetic acid containing 10% acetic anhydride was used to determine iodide, ferrous, and stannous ions, ascorbic and thioglycollic acids. Kratochvil et al. (125) have found Cu(I1) in acetonitrile to have a high reduction potential] good stability] and the ability to oxidize a number of compounds rapidly. Hydroquinone, thiourea] and tetrabutylammonium iodide were titrated. Pharmaceuticals. Lin and Blake have published several papers (148, 1-49] 150) on the determination of acetylsalicylic acid and phenacetin in dosage forms by titration of the former in D M F or methyl isobutyl ketone (MIBK) with sodium methoxide, and the latter in acetic anhydride-chloroform-benzene (1: 1:9) with perchloric acid. -4method for phenacetin in drug mixtures with titration after a separation procedure was given by Posgay and Zoltai (181). Ellert and Sell have reported the titration of antihistamines after the addition of mercuric acetate both conductometrically (57) in acetic acid with perchloric acid and visually (58) in acetic acid-acetic anhydride with methanesulfonic acid. Ellert et al. (66) also titrated tetracycline hydrochlorides potentiometrically in acetic anhydride with perchloric acid. The application of nonaqueous titrimetry to the analysis of drugs containing a variety of acidic functional groups was investigated (40). A number of different titrants and solvents were
used. Safarik (192) described a method for the microdetermination of alkaloids in tablets by a visual titration in chloroform with perchloric acid following an extraction procedure. Schutte and Maussen (199) assayed tablets containing barbiturates by automatic conductometric titration in acetone with sodium hydroxide in ethanol. Kalidixic acid in Negram tablets (94) was titrated in ethylenediamine or DMF-methanol with sodium methoxide. The latter titrant was also used for the determination of glutethimide and bemegride (1) in D M F solvent. Elste et al. (59) investigated the accuracy of the analysis of suppositories involving the application of a number of acid-base titrations in nonaqueous media. Various mixtures of amidopyrine, phenazone, and caffeine (124) were determined by titration with perchloric acid after separation of the compounds on the basis of their different solubilities in benzene and chloroform. Inorganic. Nonaqueous titration has been used for the determination of a number of inorganic compounds. The halide and nitrate salts of some divalent metal ions were titrated as acids in DMSO with sodium methoxide (102). Yarovenko et al. (242) determined potassium salts in mixtures with acids and bases by nonaqueous titration of the liberated acids following ion exchange on a cation exchange resin in the hydrogen form. Henrion and Pungor (88) have studied the precipitation titration of some alkali and alkaline earth metal ions in acetone and MIBK. Lithium chloride was used as the titrant and end points were detected conductometrically and oscillometrically. Potassium (204) in mixtures with sodium or lithium can be determined by titration of the acetate salt in acetic acid-chloroform with perchloric acid. Kreshkov et al. (139) titrated rare-earth nitrates as acids in methanol-acetone (1 to 4) with TEAH or T l I A H . Some mixtures with other nitrates and nitric acid were resolved. A similar method (158) was also developed for the analysis of rare-earth element alloys with magnesium by titration of the bromide salts after solution with HBr. Calcium (109) was determined by turbidimetric titration in ethanol using sodium molybdate titrant. iimmonium perchlorate in double-base propellants (10) was titrated potentiometrically with KOH after solution in acetone. Visual (52) and conductometric (2) procedures for the titration of sodium sulfate in sodium alkyl aryl sulfonates with barium ion have been presented. The recently developed fluoride ionsensitive electrode has been reported in two papers for the potentiometric titration of fluoride. Lingane (152) examined the use of thorium, lanthanum, and calcium ions as precipitants in
aqueous alcoholic solvent bystems. Orenberg and Morris (169) used the fluoride electrode to monitor the aqueous phase during an extractive titration into chloroform with tetraphenylantimony sulfate. They studied the effect of various possible interferences and developed steps to eliminate them. A potentiometric titration with silver nitrate for chloride ion in lubricants (64) has been proposed in which the sample is dissolved in a complex mixture of organic solvents. Kreshkov et al. have reported several methods in which both amperometric and visual end point indication were used. Chloride, bromide, and thiocyanate ions (128) were titrated in acetic acid with cadmium nitrate, while nitrate ion (127) was titrated in acetic acid medium with lead acetate. Biruri et al. (20) titrated nitrates of various elements in methanol or ketone medium with basic titrants] with or without ion exchange, depending on the periodic group of the elements. Nitrites were titrated with perchloric acid. The spectrophotometric titration of cyanate (227) with Co(I1) perchlorate has been studied in DMF, DMSO, and N methylpyrrolidone. I t has been found (177) that heteropolyacids containing a phosphorus central atom can be titrated in nonaqueous solvents, generally giving a better inflection than obtained in water. Several methods have been reported for the determination of CO, following absorption in nonaqueous media. Braid et al. (26) studied the absorption of COz by various amines in D M F and compared the titration with TBXH using several indicators. A rapid method for determination of total carbon and carbon-14 in small samples (230) involves the absorption of COz in scintillation solution containing ethanolamine and an indicator. The C 0 2is titrated during combustion with sodium methoxide. In a microcoulometric method (23?), base titrant is generated in acetone0.5% methanol solution saturated with potassium iodide. Visual end point indication was used with integration of the current used. Yoshimura et al. have published three methods for the determination of antimony pentachloride in nonaqueous media. Two conductometric methods involve titration in D M F with EDTA (246) and in chloroform with ferric oxinate (245). The third procedure (244) is a titration in ethyl Cellosolve with titanium trichloride using potentiometric or photometric indication of the end point. Water. Nonaqueous titration by the Karl Fischer method is probably the most widely used technique to determine water. Tranchant (225) has presented a review of the developments, improvements, and applications of this VOL. 40, NO. 5, APRIL 1968
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method. Campiglio (34) has given a critical review of micromethods using the Karl Fischer reagent followed by a description of an apparatus and method of his own. Some difficulties in determining less than 100 p.p.m. of water in organic liquids (8) have been overcome by the addition of N-ethylpiperidine as an accelerating catalyst and the use of a special technique. Smith and Kellum (206, 207) have published two papers on determination of water in silicone materials in which certain interferences are minimized by the use of high molecular weight alcohols as diluents. Two applications of Karl Fischer reagent to determination of water in gases have been published: one for hydrocarbon gases (110) and one for hydrogen chloride (14). Xethods have also been presented for determination of water in gelatin (ad), molasses (60), and grease (166). Yasumori et al. (243) have modified a previously described coulometric method for Karl Fischer titration, and Panteleeva (172) has given a coulometric method for determination of microgram amounts of mater in Freon, carbon tetrachloride, furan, and tetrahydrofuran. Lindbeck (161) has applied controlled potential coulometry to the Karl Fischer determination of water in DLISO. Two procedures for determining water which do not involve Karl Fischer reagent have been developed. The first (153) is based on the water hydrolyzing 4-nitrobenzoyl chloride in pyridine solution, which is then followed by titration of the excess 4-nitrobenzoyl chloride. In the method by Markevich (I&?), the water reacts with a n alkali metal alcoholate in the presence of a readily saponifiable ester. The liberated alkali hydroxide then reacts quantitatively with the eqter, and the excess alcoholate is determined by titration with a nonaqueous acetic acid solution. Miscellaneous. Snyder and 13uell (209) have tabulated data to provide an index of acidity, basicity, and adsorptivity on alumina of various petroleum compound types. The data form the basis for separation and/or classification of compound type by using ion exchange or adsorption chromatography, and titration. Acidity data are from titration in pyridine with TBAH and basicity data are from titration in acetic anhydride with perchloric acid. Huber (93) has given a paper on new developments in nonaqueous titrations. He discusses the titration of very weak bases in acetonitrile, particularly with regard t o the perchloric acid titrant solvent which can introduce buffering components. He also describes the titration of nonbasic nitrogen compounds following catalytic hydrogenation t o the amines in glacial acetic acid, 426 R
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and the analysis of mixtures of aliphatic sulfonic acids and primary and secondary sull‘onic acid amides. A method (140) for determination of some nitrogen-containing derivatives of carbonic acid (urea, calcium cyanamide, melamine, etc.) has been based on their reactions with acids, bases or certain salts having a n acid-base character in nonaqueous solution. This is followed by titration of the resulting ammonium salts with perchloric acid. Pyridine and quinoline bases (120) have been titrated in glacial acetic acid and pyridine carboxylic acids in MEK-methanol with TEAH. Banick and Francis (11) analyzed dimethyl sulfate samples by the addition of 2-(dimethy1amino)ethanol in IPA and titration of the excess reagent with perchloric acid. The methyl hydrogen sulfate and sulfuric acid present were determined by potentiometric titration in pyridine with tributylethylammonium hydroxide. Kreshkov et al. (136) determined acetic acid in acetylation mixtures by two potentiometric titrations. The sample reacts with a measured excess of aniline and is titrated with KOH to determine total acetic acid, including that resulting from reaction of the acetic anhydride with aniline. The latter acetic acid is then determined by titrating the unreacted aniline with perchloric acid. A turbidimetric titration of atactic polypropylene modifed by styrene was developed by Romanov (190) using the binary solvent heptane-MEK. Gutmann and Keyzer (82, 83) have published two papers on the conductometric titration of charge-transfer complexes in which a discussion of the theory is presented. Some complexes studied were chloropromazine-iodine and anthracene-iodine in AN, phthalocyanineiodine in DMSO, benzene-chloranyl and anthracene-chloranyl in methanol. BOOKS A N D REVIEWS
A book by Huher (92) entitled “Titrations in Nonaqueous Solvents” has come out recently. This appears to be a direct English translation, without any updating, of the German edition which was published in 1964. It contains a fairly full discussion of nonaqueous titrations including theory, practical aspects, and applications. Kreshkov, Bykova, and Kazaryan (1Zt9) have written a book on “Nonaqueous Titration of Inorganic and Organic Compounds.” Its availability outside of Russia is uncertain, but it has been reviewed by Shkodin (203). I t apparently deals primarily with the practical apects of the field and contain< little on theory. The second volume, Part 11: Indirect Methods, of Ashworth’s “Titrimetric Organic Xnalysis” ( 9 ) has a compilation of over one thousand page> on ~
methods. It is systematically organized in tabular form according t o reactions and reagents used, functional groups, and compound classes to be determined. The preceding Part I dealt with direct methods. “Amperometric Titrations” by Stock (218) is an extensive monograph on the subject covering principles, apparatus, techniques, and methods. Another monograph which includes work in nonaqueous solvents is Pungor’s book on “Oscillometry and Conductometry” (182) discussing theoretical, instrumental, and practical aspects and applications. Berka et al. (18) have written a book on “Newer Redox Titrants” which contains applications in nonaqueous media. “Inorganic Chemistry in Non-Aqueous Solvents” (91) deals with chemistry in liquid ammonia and amines, protonic and nonprotonic solvents, and high temperature solvents (fused salts and oxides). Some information on potentiometric titrations is included. Although no titrations are discussed, the book “Advances in Organic Chemistry,” Vol. 5 (28.5) contains some information of interest in Chapter I by A. J. Parker entitled “The Use of Dipolar Aprotic Solvents in Organic Chemistry.” The series “The Chemistry of Nonaqueous Solvents” edited by J. J. Lagowski (142, 143) contailis material of value in interpreting behavior in nonaqueous media. Volume I on principles and techniques contains chapters on Lewis acid-base reactions, solvation of electrolytes and solution equilibria, acidity functions for amphiprotic media, and experimental techniques for low boiling solvents and fused salts. Volume I1 on acidic and basic solvents includes discussions on liquid HCl, HBr, HI, HF, and KH3, H J S 0 4 , HXOo, and amides. Several rcviews have appeared in addition to the, last review (87) in ANALYTICAL CHEMISTRY.Kashima and Okeda (106) have covered the entire area of titrations in nonaqueous solvents, while Meulenhoff (163) surveyed titrations with perchloric acid and alkoxides in various solvents. LITERATURE CITED
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