Titrations in Nonaqueous Solvents

monly used nonaqueous titration con- tinues to be the Bronsted acid-base type. The potentiometric method of end point detection continues its populari...
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Titrations in Nonaqueous Solvents Gerald A. Harlow and Donald H . Morman, Shell Development Company, Emeryville, Calif.

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there have been no dramatic “breakthroughs” during the last two years there has been notable advance in several areas. Particularly gratifying is the number of outstanding papers devoted to the fundamentals of nonaqueous equilibria. S o t only are some of the more commonly used solvents such as acetonitrile receiving continued attention, but theoretical studies are being extended to solvents of very low dielectric constant. There appears to be a trend toward utilizing the newer types of measurements such as nuclear magnetic resonance and toward using several different types of measurements in the same investigation. In some of the recent work, for example, one finds spectrophotometric, conductometric, potentiometric, and solubility measurements combined to achieve an understanding not possible with a single technique. I n the area of study concerned with the effect of solvent and acid structure on acidity, new and imaginative ideas have been introduced. The high frequency and thermometric end points are becoming well established in nonaqueous titrimetry, particularly with Lewis acids and bases and solvents of very low dielectric constant. However, the most commonly used nonaqueous titration continues to be the Bronsted acid-base type. The potentiometric method of end point detection continues its popularity with the glass electrode gaining more and more acceptance for both practical titrations and fundamental studies. Perchloric acid remains the most used titrant for bases but the alkali metal hydroxides and alcoholates are being replaced in many cases for acidity titrations by the quaternary ammonium titrants. I n regard to these organic titrants, a surprising number of laboratories continue to use tetraethylammonium alcoholates even though it has been conclusively shown that they are much less stable than either tetramethylammonium or the higher tetraalkylammonium bases in both inert and basic solvents. The present review covers the twoyear period starting where the previous review of Streuli (178) ended and extending through October 1965. However, one long overlooked and neglected paper (120) published in 1944 is included among the references. Our treatment of the fundamental section is meant to be selective rather than comprehensive and critical rather LTHOUGH

than passive. It must inescapably, though inadvertently, reflect some of our biases and special interests. BOOKS AND REVIEWS

Numerous books have been published during the last several years which deal with the subject of this review or are closely related to it. “Titrations in Nonaqueous Solvents” by Kucharsky and Safarik (116) is a translation and up-dating of an earlier Czech edition. “Titrationen in nichtwasserigen Losungsmitteln” by W. Huber (83) has been published in German. Both of these books offer fairly full discussions of nonaqueous titrations including theory, practical aspects, and specific methods. XI. R. F. Ashworth (6) has prepared two volumes on “Titrimetric Organic Analysis.” Much of the information is presented in tabular form as a concise summary of literature methods for determination of functional groups and specific compounds. Underwood has written a chapter on Photometric Titrations in “Advances in Analytical Chemistry and Instrumentation,” editied by C. N. Reilley (163). “Chemistry in Nonaqueous Ionizing Solvents,” edited by Jander et al. (84) includes discussion on purification, solubilities, and titration characteristics of the lower fatty acids and some of their derivatives. Other books of interest, although containing little on titrations, are “Nonaqueous Solvent Systems” by T. C. Waddington (191), “Acid-Base Equilibria” by E. J. King (99), and a chapter entitled Techniques with Nonaqueous Solvents in “Techniques of Inorganic Chemistry” (96). Several review articles have appeared in addition to the last biennial review prepared by Streuli (178). Franswa (66) presented a survey of acid-base, complexometric and redox titrations in organic solvents. Safarik (162) and Helou (80) discussed the application of nonaqueous titrations to pharmaceuticals, while Schute (165) discussed the principles of acid-base titrations in alcoholic medium. Redox titrimetry in nonaqueous media was the subject of a review by Stephen (176). FUNDAMENTAL STUDIES

Hydrogen Bonding and Ion Association. Continuing efforts are being made to clarify the complex equilibria which operate in nonaqueous solvents. A solvent which continues to receive a

great deal of attention is acetonitrile. Kolthoff and Chantooni have calibrated a glass electrode in this solvent and derived an equation for calculating hydrogen ion activity in the neutralization of a weak acid with a tetraalkylammonium base (10%’). I t was shown that despite complications from the formation of the hydrogen bonded acid-anion complex (AHA) from the acid, HA, and the anion, A-, the simple equation paH = pKHa holds at the 50% neutralization point. It was also shown that the value of KHA;, the dissociation constant of the acid-anion complex, can be estimated from the ratio of the experimentally determined values of aH+ from the neutralization curve to the value of ax,;, a t the half-neutralization point. These authors show graphically how the shape of a titration curve changes as the strength of the acid-anion complex, KHA;, increases. A quantitative interpretation is thus given for the perplexing “midpoint inflections” which several investigators have observed in potentiometric titration curves in a variety of solvents. The effect of molecular acid-base dissociation of salts on the shape of the conductometric titration curves obtained with acetonitrile has also been investigated by Kolthoff and Chantooni (101). They found that salts of weak acids and primary, secondary, and tertiary amines may exhibit conductances which are much higher than values calculated from the ionic dissociation constant of the salt. This deviation from the calculated value is particularly great when the tendency to form the acid-anion complex is large and the molecular formation constant is finite. Equations were derived which permit calculation of acid-amine titration curves when the various dissociation constants are known. These curves show a maximum in conductance. I n a study of the effect of ortho substitution on the stability of hydrogenbonded complexes in acetonitrile, Coetzee and Cunningham (33) performed conductometric titrations of various amines with benzoic, salicyclic, and 2,6dihydroxybenzoic as well as picric acid. As would be expected the curves were much different in shape from those obtained in aqueous titrations. I n the acetonitrile equilibria, both the acidanion complex (AHA)- and the basic equivalent (BHB)’, can play an important role, the first when excess acid is V O L 38, NO. 5, APRIL 1966

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present, the second in the presence of excess base. The effect of ortho substitution with hydroxyl groups is to allow the formation of strong intramolecular hydrogen (chelate) bonds with the carboxyl group which effectively compete with the intermolecular hydrogen bonds necessary for complex formation. Incompletely substituted ammonium ions were found to form much more stable ion pairs with substituted benzoic acids than quaternary ammonium ions. This was explained on the basis of hydrogen bonding but one wonders if shielding is not also an important factor. Potentiometric studies of the hydrogen bonded complex of nitrogen bases with their conjugated acids, (BHB)+, were also carried out by Coetzee et al. (57) in acetonitrile. The results indicate that the degree of complexation increases with increasing base strength and with increasing number of hydrogen atoms in the ammonium group of the conjugate acid. Steric hindrance to the formation of the complex is also important. It was pointed out in this paper as indicated also by others (102) that the influence of the hydrogen bonded complex on potentiometric titration curves is to reduce the buffer capacity of the solution (increased slope near the center portion) and to reduce the break a t the equivalence point. It was also pointed out that even the strongest base-conjugate acid complexes are only about one-half as strong, in terms of free energy change, as the corresponding acid-anion complexes. This difference is explained on the basis of a greater tendency for acetonitrile to solvate cations than anions. Very recently Coetzee and Padmanabhan (56) have reported a more extensive potentiometric study involving both mono- and diamines utilizing the calibration method of Kolthoff and Chantooni, mentioned above. Based on the results obtained in the measurement of the dissociation constants of the conjugate acids of 31 amines, the authors conclude that no simple quantitative correlation exists between base strengths in acetonitrile and water. However, there was a correlation between the base strength of an amine and its tendency to form homoconjugated complexes. Coetzee and Padmanabhan (36) also carried out an exploratory potentiometric study of five phenols in acetonitrile and found that all except picric acid formed relatively stable homoconjugate acid-anion complexes. Unsubstituted phenol was found also to form a 2 : l complex. The introduction of a nitro group onto the carbon ortho to the hydroxyl group greatly reduces complex formation while nitro groups in both ortho positions completely eliminates it. No evidence of dimerization of the phenols was obtained. These authors 486 R

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state that a serious limitation of acetonitrile as a medium for acid-base reactions is the unavailability of a relatively strong base for this solvent. Anhydrous solutions of tetraalkylammonium hydroxides are considered to be too unstable to be useful. The nature of this difficulty is not clear since Fonnan and Hume (64) report that it is acetonitrile that is unstable in the presence of strong bases, while Kolthoff and Chantooni (102) reported no serious difficulties in the titration of acids with tetra-nbutylammonium hydroxide in the same solvent. Kolthoff and Thomas (103) have measured electrode potentials in this solvent and measured the liquid junction potential between acetonitrile solutions and the aqueous saturated calomel electrode. Several studies have been made of ionic cocductivities in acetonitrile. Evans et al. (53) measured the conductance of the symmetrical tetraalkylammonium halides and picrates while Coetzee and Cunningham synthesized (31) and evaluated single ion conductivities (32) of tetraisoamylammonium tetraisoamylboride in acetonitrile, nitromethane, and nitrobenzene. This compound is recommended as a reference electrolyte for the evaluation of single ion conductivities. In a study of electrolyte-solvent interaction, Coplan and Fuoss (40) have measured the conductivity of triisoamyl-n-butylammonium iodide, picrate, and tetraphenylboride in the nearly isodielectric mixtures of acetonitrile, nitromethane, and methanol. Ion association and hydrogen bonding have been extensively studied in solvents of low dielectric constant. rllthough most of these studies do not involve titrations, some of them will be briefly outlined here because they provide information which can be of great help in the understanding of the equilibria involved in nonaqueous titrations. Acidbase equilibria in benzene were investigated by Bruckenstein and Saito (25) utilizing infrared and differential vapor pressure measurements. The principal species identified were ion pairs and other uncharged ionic aggregates. Three types of ion pairs were postulated and oligomer formation was reported. Conductance studies of ions in benzene and diethyl ether were carried out by Skinner and Fuoss (172) and in a mixture of benzene and o-dichlorobenzene by Nanney and Gilkerson (135). The same method of study was used for ion pairs in tetrahydrofuran by Bhattacharyya et al. ( 1 9 ) and for the study of ion-solvent interaction by Ralph and Gilkerson (162), and Ezell (67) in o-dichlorobenzene, chlorobenzene, and ethylene chloride. A differential vapor pressure study of self-association of acids and bases in 1,2dichloroethane as well as in some sol-

vents of higher dielectric constant wa5 carried out by Coetzee and ,\lei-Shun Lok (34). I t was found that selfassociation of benzoic acid is markedly decreased by ortho substitution with bromine, hydroxy, and methoxy groups. A similar eflect is seen in phenol with ortho substituted nitro and methoxy groups. Amines undergo little association in this solvent. Various types of spectroscopy have been useful, especially in the study of hydrogen bonding. Hydrogen bonding of alcohols in n-heptane was studied by Chandra and Sannigrahi (28) while Rubin and Yanson (161) used nearinfrared spectrophotometry to determine interaction of phenol with substituted pyridines in carbon tetrachloride. A similar study of phenol interaction with ethers and related compounds was conducted by Powell et al. (193). In a study of hydrogen bonding between hindered phenols and methyl acetamides in the same solvent, Takahaski and Li (182) used nuclear magnetic resonance. Chloroform solvent and NMR spectroscopy were employed by Berkeley and Hanna (17) whereas hIcClellan and Nicksic (121) used the same instrumental technique but made their measurements both on the undiluted samples (haloethanes and halomethanes) and on their solutions in cyclohexane, carbon tetrachloride, and dimethylsulfoxide. Microwave spectroscopy was used by Antony and Smyth (4) to study dielectric relaxation and intramolecular hydrogen bonding in cyclohexane and d’ioxane. The lower alcohols continue to be the subjects of study both as individual solvents and as mixed solvents containing water and/or other components. Coplan and Fuoss (39) measured the single ion conductances of sodium, potassium, and tetra-n-butylammonium picrates as well as tetra-nbutylammonium tetraphenylboride in pure methanol. Ong, Robinson, and Bates (139) have discussed the interpretation of potentiometric titration curves in mixtures of water and methanol. They list the corrections that must be applied to apparent pK’s in this solvent in order to obtain true pK’s. Rosenthal, Hetzer, and Bates (160) also used this mixed solvent in studying the salt effects and medium effects on indicator acid-base equilibria as did Schug and Dadgar (164) in the determination of the relative free energies of some alkali and alkali earth metal ions. Ionic association of potassium and cesium chlorides was studied by Hawes and Kay (79) using conductance measurements and ethanol-water mixtures as solvents. Berge and Jeroschewski (14) measured the potential of buffer solutions in alcohol-water mixtures where the alcohol was methanol, ethanol, n-propyl, or isopropyl. The effect on

the apparent pH of adding salts and organic solvents to aqueous solutions of acids has been studied by Schwabe (166). It was reported that the Hammett acidity function and the velocity constants of acid catalyzed reactions show very steep maxima as the water in a solution is gradually replaced by an organic solvent. Conductance measurements have been used by Atkinson and Petrucci (7) to study the association of polyvalent symmetrical electrolytes in acetone-water mixtures. Equilibria have been studied in a few nonaqueous solvents of high dielectric constant. Johari and Tewari (94) employed conductance measurements to study dissociation of some complex salts in formamide and the dissociation of magnesium sulfate in formamide-dioxane mixtures (183). Mandel and Decroly (130) used potentiometry to determine the dissociation constants of carboxylic acids in formamide and explained their results on the basis of specific interactions between the undissociated acid molecules and the solvent. Dawson et al. (44) determined some thermodynamic properties of hydrochloric acid in N-methylacetamide with potentiometric measurements. Sulfolane, a new and interesing solvent of rather high dielectric constant ( D = 44), has been used to study solvent effects by Amett and Douty (5). The third paper in a series on equilibria in ethylenediamine has been authored by Mukherjee, Bruckenstein, and Badawi (fS4). This report is concerned with the determination of absolute p K values of acids and silver salts and the establishment of a p H and pAg scale. The dissociation constant for hydrochloric acid was determined by spectrophotometric and cryoscopic measurements and this value used to calculate the potential of the saturated corrosive sublimate reference electrode and the silver-silver ion electrode. Potentiometric measurements with the hydrogen and silver electrodes were utilized for the determination of pK values. Conductance measurements of some silver and other salts in anhydrous ethylenediamine and propylenediamine were reported by Fowles and McGregor (65) and the conductances and solubilities of alkali metals in ethylenediamine reported by Dewald and Dye (46). Relative Strengths of Acids and Bases. A new approach to explaining the effect of structure and solvent on the apparent strength of acids has been suggested by Grunwald and Price (73) who focus attention on the role of the London dispersion forces. These are van der Waals forces resulting from the interaction of the virtual electronic oscillators that are involved in the optical dispersion of molecules. The authors point out that although it is well recognized that dispersion forces make a

major contribution to the van der Waals forces in liquids, they are often neglected in discussions of the medium effects on ionic organic reactions. Emphasis is usually placed on electrostatic interactions of permanent molecular charge distributions while dispersion forces are thought to cancel out under the assumption that they are about the same for both reactant and product. The dispersion energies involved in acid dissociation should be especially large in the case of compounds such as picric acid which are one-color indicators. I n this study measurements were made of the relative strength of picric, acetic, and chloroacetic acids in the solvents water, methanol and ethanol. It was found that the strength of picric acid relative to acetic acid increased by almost two orders of magnitude in this series of solvents, while the strength of trichloroacetic acid relative to acetic acid remained nearly constant. The results were considered to be qualitatively consistent with a dispersion effect. More quantitative information supporting this view was obtained by actually calculating the dispersion energies for the reaction of picric acid with amines in glacial acetic acid and comparing these with calculations for trichloroacetic acid reactions. The authors feel that strong dispersion effects can account for the differences in relative basicities between primary, secondary, and tertiary aliphatic amines when measured with color indicators in water and in solvents of low dielectric constant. It is also contended that in mixed water-organic systems the dispersion effect opposes and sometimes even overcomes the preferential solvation of ions by water that would result from purely electrostatic interactions. Grunwald and Price have studied ionization and proton exchange of amines in acetic acid, publishing information on kinetic and equilibrium properties for solutions of methylamine ( 7 f ) and on the effect of structure on reactivity as well as the reaction mechanism involved (7%’). ,4 series of studies concerning the effect of structure on basicity has been reported by Feakins et al. Basic strengths were measured by potentiometric titration in nitrobenzene using perchloric acid as titrant. Measurements were made first on a series of common bases (59) and then on fully aminolized (60)and homogeneously substituted (58) cyclotriphosphazatrienes and cyclotetraphosphazatetraenes. Elder and Mariella (50) determined the relative strengths of substituted picric and other acids by nonaqueous titration. Where both aqueous and nonaqueous data were available good correlation was observed. Mai ( f 2 5 ) has studied the effect of solvent dielectric constant on the acidity of 5-bromovaleric acid with p H measurements in water-ethanol mixtures.

Equations have been derived by Popovych (147) which correlate apparent pH with acid or base concentration in a solvent consisting of 50y0 toluene, 49.5% isopropyl alcohol, and 0.5Y0 water (ASTM titration solvent for acid and base determinations). Within moderate concentration ranges, approximately straight lines are obtained. The slopes are a function of the nature of the ionic dissociation while the intercepts depend on the magnitude of the ionic dissociation constant, on the changes in junction potentials, and on the “primary medium effect.” The apparent pH data, in combination with conductometric measurements, show that the sum of the liquid-junction and primary medium effects is roughly a constant characteristic of the medium only. It might be mentioned here that the development of the ASTiM titration solvent nearly 22 years ago (120) was one of the earliest and most successful practical applications of nonaqueous titrimetry. Since this paper was among the first to report use of such innovations as the glass electrode and correlation between aqueous and nonaqueous acidities, it is strange that it is so often overlooked in recent publications. SPECIAL TITRATIONS, TITRANTS, A N D SOLVENTS

Thermometric Titrations. Besides the well-known potentiometric, conductometric, and visual indicator methods, some less common detection techniques have been recently investigated. One of these, thermometric titration, has been applied to the titration of acids and bases in acetonitrile by Forman and Hume (64). Although satisfactory titrations were obtainable for a range of amines and organic acids, the instability of the solvent in the presence of strong bases makes the technique unattractive. However, heat of neutralization data were obtained for a series of acids with lJ3-diphenylguanidine as titrant and the results form- and p-substituted benzoic acids are shown to correlate well with their Hammett sigma values. Glacial acetic acid was used for the solvent by Keily and Hume (97) for titrations of diphenylamine, urea, acetamide, and acetanilide. Traces of water were found to interfere in titrations of extremely weak bases. Thermometric titrations have been found to be particularly applicable to organometallic compounds. Everson (54) has used butanol as the titrant in the determination of butyllithium in hydrocarbon solvent. The reaction is stoichiometric and lithium butoxide which is usually the major impurity does not interfere since it is also the reaction product. Everson and Ramirez (66) used this technique for the determination of aluminum alkyls, alkylaluminum VOL. 38, NO. 5, APRIL 1966

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hydrides, and alkylaluminum halides and it was found to be applicable to these as well as all other compounds tested in the c 1 - C ~range. Selection of titrants permits the determination of one compound in the presence of others. For example, under certain conditions ketones are specific titrants for alkylaluminum hydrides. The use of potentiometric titrations for the determination of organoalumimum compounds has been reported by Chniat and Zawada (187). Benzene was used as solvent and an aluminum-silver alamgam electrode pair for endpoint detection. Everson and Ramirez (56) found that solutions of diethylzinc in hydrocarbons could be thermometrically titrated with o-phenanthroline or 8-quinolinol. The ophenanthroline appears to be specific for EtnZn in the presence of its oxidation and hydrolysis products. High Frequency Titrations. The use of the high frequency endpoint in nonaqueous titrations appears t o be receiving increased attention. .i review of its use in organic analysis, covering the years 1956 to 1962 has been prepared by Zarinskii and Gur’ev (198). Two general papers have been published concerning the influence of water ($02) and the change in electrical characteristics during reactions in nonaqueous media (77). Much of the recent work is devoted to the titration of acids. Zarinskii and his associates have continued their active study of this technique utilizing a variety of solvents. Zarinskii and Gur’ev (199) studied the titration of strong mineral acids in acetic acid solvent and found that the best titrant was a solution of pyridine in acetic acid. Titration data yielded quantitative measures of the amounts present as well as information on relative strengths. Bokina et al. (21) showed that in the same solvent, perchloric acid can be determined in binary mixtures with nitric acid to about 5y0 when the molar ratios are 1:lO to 10:l. Diphenylguanidine as well as pyridine could be used as titrant. An aqueous dioxane media was used by Zarinskii and Gur’ev for both direct (201) and back titration (200) of various acids. Riolo and Soldi (157) investigated the use of mixed solvents of benzene and various alcohols and found the benzenet-butanol solvent best for the titration of weak and very weak acids with triethylbutylammonium hydroxide. A measure of the relative strengths of a series of acids (largely phenolic) was obtained and the possibility of resolving acids with similar pK,(H20) discussed. The high frequency titration of nitrogen bases with Lewis acids in the solvent acetonitrile was also studied by Riolo and Soldi (156). Aliphatic and aromatic amines, amino-alcohols, and heterocyclic compounds were titrated with aluminum, gallium, and boron tribro488 R

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mides. A specific interaction between gallium tribromide and the solvent influenced the shape of the titration curves obtained with this titrant. Riolo and Perago (155) further studied these titrations and found that only 1:1 adducts were formed. The complexing power of the titrants appears t o be in the order BBr3, GaBr3, AlBr3, but N B r 3 reacts with the largest number of bases. Riolo et al. (158) used the same technique but a different solvent, nitrobenzene, in a study which compares high frequency and potentiometric titrations for the determination of organic nitrogen bases. In this investigation ferric chloride, as well as the above mentioned Lewis acids, was used as the titrant. The high frequency method was found to be more sensitive, revealing the presence of even the less stable complexes, and usable a t a lower concentration. The same authors (159) came to a similar conclusion in a study involving a different set of Lewis acid titrants. Titrations with tin, zirconium, and titanium tetrachlorides as well as antimony, tantalum, and neobium pentachlorides indicated that antimony pentachloride was the most useful titrant because it forms 1: 1 complexes with all of the bases tested over a wide concentration range. High frequency methods have also been used to follow precipitation titrations. Grey and Cave (68) titrated halides and thiocyanate with silver nitrate titrant and methanol solvent. The method permits the determination of milligram amounts of each halide in a mixture of alkali metal halides. The same authors (69) extended this technique to the micro range by employing a smaller cell and compensating for cell wall effects. Liquid ammonia, an unusual solvent for high frequency titrations, has been studied by Hileman (82). Titrations in Fused Salts. Molten salts provide a rather unusual and interesting type of solvent for nonaqueous titrations. Shams El Din and El Hosary (168) used fused potassium nitrate as solvent in the potentiometric acid-base titration of the reaction products which resulted when the trioxides of chromium, molybdenum, and tungsten were dissolved. An oxygen (Pt) indicator was used with sodium peroxide as the titrant. The kinetics of the reactions between the oxides and the solvent were also studied. Shams E l Din et al. (169) also used a metal-metal oxide electrode in a potentiometric study of acid-base equilibria in the same solvent. Bombi et d.(22) used a lithium nitrate-potassium nitrate eutectic mixture as the solvent in the amperometric and potentiometric titration of halides and cyanide with electrolytically generated silver ion. Measurements were made at 150OC. with silver electrodes. Instability of cyanide in this solvent was

noted but the halides could be titrated to I to 2%. Somewhat better precision was reported by Tien (185) for halide titrations in the same solvent in which a glass reference electrode was used. Guenther (?4) used fused mercuric chloride as solvent and conductometric endpoint detection in acid-base titrations. Titrants. -4 few reports mention the use of less commonly employed titrants. Lead tetraacetate in anhydrous acetic acid has been used by Piccardi (144) for redox titration of inorganic ions. Acetic acid also serves as the media and endpoints are detected either potentiometrically or amperometrically. Piccardi et al. (145) also reported the use of iodine trichloride in glacial acetic acid as an oxidizing titrant for similar determinations. Some further studies on the use of fluorosulfuric acid for the titrations of bases has been published by Paul and Pahil (143). The solvents used were methanol and a 1: 1 mixture of methanol and ethanediol. Both potentiometric and indicator end points were utilized. Coulometric Titrant Generation. The coulometric method of adding titrant has been utilized in several nonaqueous titrations. Streuli et al. (179) have studied the potentiometric titration of organic acids in acetone solvent with the basic titrant being electrolytically introduced. The acids titrated were p-toluene-sulfonic, benzoic, and 2,4-dinitrophenol, Glass and calomel electrodes were used for indication while two platinum electrodes served for generation. Tetra-n-butylammonium perchlorate and bromide electrolytes were employed with 0.5% water necessary in the latter solution. A glass-calomel indicating system was also used by Johansson (93) in an investigation of weak acidity titrations. A series of solvents was used which utilized isopropyl alcohol either alone or mixed with other solvents such as ketones. The strong base was generated from sodium or potassium bromide or sodium perchlorate solutions. Separate electrode compartments proved to be more successful than a single cell. Cotman et al. (41) carried out similar titrations but in a single cell and with an electrolytesolvent system consisting of sodium perchlorate and tetrabutylammonium iodide in benzene-methanol or t-butyl alcohol-methanol. The argentometric titration of orthophosphate in 80:20 ethanol-water was reported by Christian and Knoblock (29). The silver ion titrant was introduced either volumetrically or by electrolytic generation. Sodium acetate (0.131) served as the electrolyte. Badoz-Lambling and Stojkovic (8) used potentiometric and amperometric detection in titrating phenothiazine to the phenazothionium salt in acetonitrile.

Solvent and Titrant Purification. ai simplified procedure for the purification of acetonitrile bas been developed by Forcier and Olver (63). Tur'yan and Aliferova (186) have used potentiometric titrations as it control method on the purity of acetonitrile and give methods for the determination of the hydrolysis products (ammonium acetate, acetic acid, and ammonia). An ion exchange method for removing alkali metal ions from quaternary ammonium titrants has been developed by Olsen and Poole (138). Traces of these ions greatly influence the potentials of the glass electrode in highly basic nonaqueous solutions, especially under conditions used to titrate extremely weak acids.

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APPARATUS AND TECHNIQUES

Cluett (30) has demonstrated the adverse effect of low input resistance on potentiometric titrations in nonaqueous holvents by a series of titrations in acetone using an electrometer a t various input resistance settings. ri full scale automatic conductance titrimeter was developed by Boardman and Warren for titrations in nonaqueous media (20). Several modifications of commercial calomel reference electrodes have been described for use in nonaqueous solvents. Deily and Donn (45) added an agarK K 0 3 bridge to a Coleman plunger-type electrode to avoid interference from chloride ion. Adler (2) placed a conventional sleeve type electrode inside a jacket containing a second glass-frit type junction and a saturated solution of KC104 in acetic acid. The electrode system was recommended for potentiometric titrations in acetic acid or acetic anhydride. -4novel technique for performing titrations by following the changes in the transmittance curve of a microwave cavity in which the reaction occurs has been reported by Adema and Schrama (1). Their apparatus and some applications are described. Aluminum halides in hydrocarbons can be titrated with ethers giving a strong 1 : l complex. Isopropanol was titrated with AlBr3 giving 1:6, 1 : 3 and 1 : l complexes. Sharp end-point breaks were obtained in nonpolar solvents such as benzene, heptane, and carbon disulfide. Kuwana (117) has described the apparatus and technique for the photonometric titration of copper(I1) and dissolved oxygen in organic liquids. The reducing titrant was generated a t a constant rate by the photochemical reduction of anthroquinone in an alkaline methanol solution. The total number of photons consumed was proportional to the amount of oxygen or copper(I1) in solution. METHODS AND APPLICATIONS

Acids. An interesting method for determining fatty acids in the presence of anhydrides has been developed by

W-harton (195). The acidities of monoacidic acids with pK. values up to 5.5 are enhanced in acetonitrile by the addition of lithium chloride. Wharton has suggested that the acidic proton is displaced by the lithium ion to form HCl. This permits titration of the fatty acids with tri-n-propylamine without interference from anhydrides. Fatty acids have also been determined by micro (98) and ultramicro (149)techniques. The third paper in a series on the potentiometric and conductometric titration of polynuclear phenolic compounds in pyridine was presented by iMitra and Chatterjee (133). A method has been reported for the titration of 2,4-dichlorophenoxyaceticacid and 2,4dichlorophenol in isopropyl alcohol (197). A number of methods have been given for determining acid mixtures. Diethyl sulfate, ethyl hydrogen sulfate and sulfuric acid mixtures (10) were titrated in pyridine with tetra-n-butylammonium hydroxide. Evidence was obtained for an unusual reaction between neutral diethylsulfate and pyridine to form a titratable complex. However, this was not completely satisfactory for analytical purposes, so the diethylsulfate was determined in water following hydrolysis to sulfuric acid. Bezrogova (18) determined hydrofluoric, sulfuric and fluosilicic acids in their mixtures by both potentiometric and high frequency titration. Kreshkov et al. have detemined multicomponent mixtures of dicarboxylic acids (lor),the phthalic acid isomers (104, and mixtures of nitrotoluidine isomers (113). A method has been developed (51) for the analysis of production samples of chlorendic anhydride containing chlorendic acid, maleic acid, and maleic anhydride. Fijolka (61) has determined acid anhydrides, free acids and carboxyl end groups in polyesters both conductometrically and potentiometrically by titration first in acetoneethanol solution and then in acetonewater solution. A rapid method for the determination of acid sites on catalyst surfaces by automatic potentiometric titration has been reported (180). The potassium methoxide titrant is added a t a constant rate to the covered titration cell in which the finely ground catalyst is dispersed in dimethylformamide a t 8OOC. A platinum indicator and a calomel reference electrode with a nonaqueous bridge were used. Kreshkov et al. have titrated methyl phosphinic acid (109) and methyl phosphonic acid (108) along with some of their derivatives in methylethyl ketone. Wetzel and Meloan (194) investigated a wide variety of solvents and quaternary ammonium titrants for the titration of aromatic sulfinic acids. UacDonald (122) determined a number

of sodium aminoethylthiosulfates and aminoethylthiosulfuric acids in pyridine with tetra-n-butylammonium hydroxide. Dimethylsulfoxide mas used as the solvent for the determination of ammonium and substituted ammonium ions as acids with tetra-n-butylammonium hydroxide titrant (11 ) . Alkylthiocyanatosilanes were titrated in acetonitrile or methanol with sodium methoxide (110). Hydroxy-, nitro-, and amino-derivatives of benzoic acid were titrated in several nonaqueous solvents with sodium methylate (111). Bykova et ul. (21) determined hydroxynaphthoic acids and naphthols in production melts, after decomposition with HCl, by titration in scetone-isopropyl alcohol without priliminary separation. Two procedures have been reported for the titration of hexahydro-1,3,5trinitro-1,3,5-triazine (RDX) and octahydro-l ,3,5,7-tetranitro-l,2,4,5-tetrazocine (HRIX) as acids. Fauth et al. (57) found that the compounds had negligible basic properties but could be titrated in a number of solvents with tetra-n-butylammonium hydroxide. Sinha et ul. (171) titrated potentiometrically in methylisobutyl ketone-isopropyl alcohol media with sodium methoxide. The acidic groups of lignins were titrated in pyridine with potassium methoxide. Sharp potential breaks were obtained using platinized platinum and calomel electrodes. The acidic properties of the lignins varied with their source and method of isolation (184). Bases. Several procedures have been reported for determining mixtures of different amine types. Okuno et al. (137) determined five types of nitrogen of differing basicity in crude oils by potentiometric titration in acetic anhydride with perchloric acid before and after reduction of the sample with lithium aluminum hydride. They found strongly basic, three types of weakly basic, and nontitratable nitrogen. Strepikheev et al. (177) determined primary, secondary, and tertiary amine groups in polyamines by titration of aliquots in acetone directly and following reaction with acetic anhdride or salicylaldehyde. Similar reactions were used to resolve mono-, di-, and triethanol amine mixtures (70). Bournique and Neuser (23) used differential titration in ethanediol-isopropyl alcohol solvent before and after acetylation to resolve butylamine, N-methylaniline and N , N dimethylaniline, while Miller and Keyworth (131) used phenylisothiocyanate to permit the determination of tertiary amines in the presence of primary and secondary amines. Kreshkov et al. have resolved mixtures containing diamines (106), heteroand nicyclic nitrogen compounds (105), tro-derivatives of primary amines (I 1 2 ) . VOL. 38, NO. 5, APRIL 1966

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Malone et al. (127, 128) and Serencha et al. (167) have described procedures for determining mixtures containing various hydrazines. Tetracene was determined in a formicacetic acid solvent mixture by titration with perchloric acid. The formic acid is required for solubility, the acetic acid for a satisfactory potentiometric endpoint (81). Small amounts of N-isopropyl-"-phenyl- p -phenylenediamine were titrated visually in chloroform with toluene-p-sulfonic acid (43). Jasinski and Szponar titrated weak bases photometrically in propionic acid and its anhydride (91) and in acetic anhydride (90).

Acetic anhydride was the solvent choice of a number of other authors with perchloric acid used as the titrant. Berger and Uldall (16) titrated amides while Cowell and Selby (42) found the method applicable to the titration of derivatives of carbonyl compounds such as semicarbazones and various phenylhydrazones. The titration characteristics of various bis(disubstituted phosphiny1)alkanes and trisubstituted phosphine oxides were reported by Parker and Banks (141). Puthoff and Benedict (161) titrated high moleculer weight quaternary ammonium halides and acetates, and the salts of fatty tertiary amines. As part of a study on the polarographic oxidation of the droppingmercury electrode in acetic acid media containing increasing amounts of acetic anhydride, Durand and Tremillon (48) carried out the amperometric and potentiometric titration a t constant current of acetate in 95y0acetic anhydride. Other Functional Groups. Dijkstra and Dahmen (47) have developed a convenient method for the determination of a-epoxy compounds. The titration is carried out directly in acetic acid medium with perchloric acid titrant. The presence of excess cetyltrimethylammonium bromide provides a rapid reaction with the oxirane ring. Crystal violet serves as the indicator. A similar technique was used by Jay (92) for the direct titration of epoxy compounds and aziridines in chloroform. Gunther (76) titrated adsorbed ethylene and propylene oxides with hydrobromic acid following release from the adsorbents by distillation with chlorobenzene. Room temperature esterification with 3-nitrophthalic anhydride, followed by titration of the excess anhydride, was used by Floria et aZ. (62) for hydroxyl determination of primary alcohols up to a molecular weight of 20,000. Higher alkoxy1 groups have been determined (49) following reaction with hydrogen iodide and then aniline to give aniline hydroiodide which is titrated visually with sodium methoxide. Magnuson (123) determined alkoxy-groups in alkoxysilanes by acid-catalyzed acetylation and titration of the excess acetic an490R

ANALYTICAL CHEMISTRY

hydride. Berger and Magnuson (15) used the same approach to determine silanethiols. They also report a second procedure based on reaction of the silanethiol with mercuric acetate. A novel method for titrating unsaturation in olefins was devised by Guenther et aZ. (75). Ozone is generated electrically in an air stream which is then split into two equal streams. The sample stream is passed through the olefin in chloroform solution and then into a potassium iodide solution. The reference stream goes directly into a potassium iodide solution. The liberated iodine in each case is titrated with sodium thiosulfate, Patek (142) determined styrene by bromination in anhydrous acetic acid followed by addition of potassium iodide and titration with sodium thiosulfate. A potentiometric method (9) for determining vinyl monomers in methanol was reported. The acetic acid liberated following reaction with mercuric acetate reagent is titrated with standard alkali. Several methods for mercaptans have appeared in the literature. Hammerich and Gondermann (78) determined thiolsulfur in hydrocarbons down to 0.1 p.p.m. by potentiometric titration in a mixed solvent containing sodium acetate. +Modifications of iodimetric and mercurimetric methods for application on a submicro scale were reported by Belcher et al. (13). A mercurimetric titration of mercaptans in organic solvents after forming an emulsion with aqueous ammonia was described (196). Acrylonitrile was used to distinguish between aliphatic and aromatic mercaptans. Belcher and Fleet (12) developed an improved method for the submicro determination of the carbonyl group. Following an oximation reaction, the sample is titrated with perchloric acid in nonaqueous medium to avoid interference from the oxime. Error from the decomposition of free hydroxylamine is minimized by using a nitrogen atmosphere. Highly hindered esters were saponified quantitatively with sodium hydroxide in hexanol containing sodium perchlorate as a catalyst. Esters previously considered hydrolytically stable, as well as simple esters, were successfully saponified by Jordan (96). The excess caustic is titrated visually with hydrochloric acid. Latour et al. (118) have determined carboxylic acid hydrazides following reaction with 2,4-dichlorobenzaldehyde to form the acidic hydrazone which is then titrated potentiometrically in pyridine with tetrabutylammonium hydroxide. Surface-active derivatives of alkenylsuccinic acids have been titrated in acetone with ethanolic KOH after treatment with excess 0.1N HCl (129). Halosilanes (124) have been titrated

visually with KOH after alcoholysis in isopropyl alcohol. Pharmaceuticals. iMethods for the determination of alkaloids in nonaqueous solvents were reported by a number of authors. Small amounts were determined potentiometrically in several solvents using an adsorption micro-electrode. Picric and trichloroacetic acids were used as titrants (192). Alkaloids in injection solutions were determined by visual titration with toluene-p-sulfonic acid after extraction from basic solution with chloroform (174). Another method for alkaloids was presented by Sobiczewska (173). Alkaloid salts have been titrated with perchloric acid (148). Connors and Swanson (38) determined mercurials in acetic acid by converting them to titratable bases with methylammonium chloride. Dilaudid (hydromorphone hydrochloride) was determined after conversion to a basic acetate (26). ilcetonitrile (126) was studied as a solvent for the titration of various organic medicinals consisting of local anesthetics, sympathomimetics, antihistamines and phenothiazine derivatives. Both visual and potentiometric indication were used. Rink et al. (154) titrated sulfonamides visually in dimethylformamide using tetramethylammonium hydroxide as the titrant. Acetylsalicyclic acid was determined by constant current coulometric titration in ethanol-water solution with sodium sulfate electrolyte and phenolphthalein indicator (136) and also by potentiometric titration in ethanol (170). Organic and Complex Salts. Amine picrates have been titrated potentiometrically and visually both as acids in pyridine solvent with sodium methoxide titrant and as bases in acetic acid and acetic anhydride with perchloric acid titrant (88). Vajgand and Pastor determined tertiary amines and salts of organic acids by a derivative polarographic titration (potentiometric titration a t constant current) (189) and by a dead-stop method (190). Various diazonium salts have been determined potentiometrically in organic solvents with sodium methoxide (87). Complex salts, such as the hexachloroand hexabromotellurites and +tannates, and hexabromoselenites of ammonium, butylammonium, and pyridinium cations, were titrated by Jasinski et al. (89). Pyridine or dimethylformamide solvents were used with sodium methoxide as the titrant and a glass- or antimony-calomel electrode pair. Inorganic Samples. Nonaqueous titration has been used in the determineration of a number of inorganic compounds. Kreshkov et al. have determined inorganic salts by nonaqueous ion exchange followed by titration

of the liberated acidh ( f f b ) , and by reaction of metal salts with excess tetraethylammonium hydroxide followed by back titration with perchloric acid (114). Resolution of some mixtures is demonstrated by both methods. Jasinski et al. (86, 86) have determined inorganic salts in acetic acid by photometric titration. Metal ions have been titrated spectrophotometrically in 2 : 1 acetone : water medium using dithizone solution as the titrant (119). A scheme for the separation and conductometric titration of inorganic ions in acetic acid was developed by Sanqoni and Stolz (163). Acetic acid was also used as the solvent for the titration of beryllium with perchloric acid (3). Trace carbon in metals may be determined by absorption of the COz combustion product in acetone where it is continuously titrated with standard sodium hydroxide to a thymol blue endpoint (24). Small amounts of chloride and organic chlorine in epoxy resins were determined before and after alkaline hydrolysis by potentiometric titration in dioxane-acetic acid media with silver nitrate (185). Knight and Schlitt (100) titrated alkyl and hydroxyalkyl ferrocenes potentiometrically with standard ferric chloride. Several applications of cyanometric titration in nonaqueous media have been reported by Erdey et al. (62). Elemental sulfur was titrated directly in benzene-acetone solution with potassium cyanide in isopropyl alcohol. Elemental selenium was dissolved in excess standard potassium cyanide which way then back titrated with standard elemental sulfur in benzeneacetone solution. Salts of quaternary ammonium bases can also be titrated. A method for determining dissolved oxygen in organic solvents and active hydrogen in antioxidants was presented by Paris, Gorsuoh and Hercules (140). Tri-tert-butylphenoxy free radicals were used as the titrant with either potentiometric or photometric endpoint detection. Miscellaneous. When conducting studies in nonaqueous solvents, it is frequently necessary t o determine the amount of water present in the solvents used. Two coulometric procedures have been presented along with special sample handling techniques. Swensen and Keyworth (181) determined water below 10 p.p.m. in benzene and related solvents by coulometric generation of iodine from potassium iodide. The solvent contained pyridine, formamide and sulfur dioxide. A dead-stop endpoint was used with an accuracy of less than 1 p.p.m. Prybyl and Slovak (160) used the Karl Fisher principle for the coulometric determination of 10 to 155 micrograms of water in small samples of liquids.

The normal Karl Fisher technique was used by Pollio (146) to determine moisture in ion exchange resins by direct titration of the resin in methanol or pyridine. hiitchell (132) has given a survey of aquametric methods used in organic analysis. Sorokin and Latov (175) have described the turbidimetric titration of polymer solutions. The titration is begun with the polymer in a solution of a solvent-nonsolvent pair. The polymer is then precipitated by addition of the nonsolvent while following the light transmitted with a photoelectric colorimeter. Polymethyl methacrylate titration is given as an example. Xot 1 0 0 ~ oof the polymer is precipitated. LITERATURE CITED

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(29) Chribtian, G. D., Knoblock, E. C., ANAL.CHEM.35, 1869 (1963). (30) Cluett, hl. L., Zbid., 36, 2199 (1964). (31) Coetzee, J. F., Cunningham, G. P., J . Am. Chem. SOC.86, 3403 (1964). (32) Zbid., 87, 2529 (1965). (33) Ibid., 87, 2534 (1965). (34) Coetzee, J. F., Mei-Shun Lok, R., J . Phys. Chem. 69, 2690 (1965). (35) Coetzee, J. F., Padmanabhan, G. R., J . Am. Chem. SOC.87, 5005 (1965). (36) Coetzee, J. F., Padmanabhan, G. R., J . Phys. Chem. 69, 3193 (1965). (37) Coetzee, J. F., Padmanabhan, G. R., Cunningham, G. P., Talanta 11, 93 (1964). (38) Connors, K. A., Swanson, D. R., J . Pharm. Sci. 53, 432 (1964). (39) Coplan, h1. A., Fuoss, R. AI., J . Phys. Chem. 68, 1177 (1964). (40) Zbid., p. 1181. (41) Cotman, C., Shreiner, W., Hickey, J., Williams, T., Talanta 12, 17 (1965). (42) Cowell, D. B., Selby, B. D., Analyst 88, 974 (1963). (43) Davies, J. R., Analyst 90, 216 (1965). (44) Dawson, L. R., Zuber, W. H., Eckstrom, H. C., J . Phys. Chem. 69, 1335 (1965). (45) Deiley, J. R., Donn, L., ANN.CHEM. 36. 952 (1964). (46) ‘DeWald, R. R., Dye, J. L., J . Phys. Chem. 68, 128 (1964). (47) Dijkstra, R., Dahmen, E. A. h4. F., Anal. Chim. Acta 3 1 , 3 8 (1964). (48) Durand, G., Tremillon, B., Bull. SOC. Chim. France 1963, 2867. (49) Ehrlich-Rogozinski, S., Patchornik, A., ANAL.CHEM.36, 840 (1964). (50) Elder, J. W., Mariella, R. P., Can. J. Chem. 42, 1106 (1964). (51) Emelin, E. A., Smyslova, N. F., Tsarfin, Y. A., Zavodskaya Lab. 29, 1169 (1963). (52) Erdey, L., Gimesi, O., Rady, C., Talanta 11, 461 (1964). (53) Evans, D. F., Zawoyski, C., Kay, R. L., J . Phys. Chem. 69, 3878 (1965). (54) Everson, W. L., ANAL.CHEM.36.854 (1964). (55) Everson, W. L., Ramirez, E. M., Zbid., 37, 806 (1965). (56) Ibid., p. 812. (57) Fauth, haf. I., Frandsen, hl., Havlik, B. R., Zbid., 36,380 (1964). (58) Feakins, D., Last, W. A., Neemuchwala, N., Shaw, R. A., J . Chem. SOC. 1965. 2804. (59) Feakins, D., Last, W. A., Shaw, R. A., Zbid., 1964, 2387. (60) Zbid., p. 4464. (61) Fijolka, P., Plaste Kautschuk 10, 582 (1963). (62) Floria, J. A., Dobratz, I. W., McClure, J. H., ANAL. CHEM. 36, 2053 (1964). (63) Forcier, G. A., Olver, J. W., Zbid., 37, 1447 (1965). (64) Forman, E. J., Hume, D. N., Talanta 1 1 , 129 (1964). (65) Fowles, G. W. A., MeGregor, W. R., J. Phys. Chem. 68, 1342 (1964). (66) Franswa, C. E. M., Chem. Weekblad 59, 249 (1963). (67) Gilkerson, W. R., Ezell, J. B., J . Am. Chem. SOC.87, 3812 (1965). (68) Grey, P., Cave, G. C. B., Can. J . Chem. 42, 770 (1964). (69) Zbid., p. 980. (70) Gribora, E. A., Khmel’nitskaya, E. Y., Zavodskaya Lab. 31,417 (1965). (71) Grunwald. E.. Price. E..’ J . Am. Chem. SOC.86, 2965 (1964). (72) Ibid., p. 2970. (73) Zbid., p. 4517. (74) Guenther, K. F., Znorg. Chem. 3, 295 (1964). (75) Guenther, K. F., Sosnovsky, G., Brunier, R., ANAL. CHEM. 36, 2508 (1964). ’

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(76) Gunther, D. A,, Zbid., 37, 1172 (1965). (77) Gur’ev, I. A., Zarinskii, 5‘. A,, Zh. Anal. Khim. 18, 698 (1963). (78) Hammerich, T., Gondermann, H., Erdoel Kohle 17, 20 (1964). (79) Hawes, J. L., Kay, R. L., J . Phys. Chem. 69, 2420 (1965). (80) Helou, J. H., Anais Farm. Quim. Sao Paulo 14, 161 (1963). (81) Hetman, J. S., Chem. Znd. (London) 1964, 232. 182) Hileman. J. C.. Dissertation Abstracts 24, 63 (1963). ’ (83) Huber, W., “Titrationen in Nichtwasserigen Losungsmitteln,” Akademische T7erlagsgesellschaft, Frankfurt am Main, 1964. (84) Jander, G., Spandau, H., Addison, C. C., “Chemistry in Nonaqueous Ionizing Solvents,” F’ol. 4, Interscience, New York, 1963. (85) Jasinski, T., Chem. Anal. (Warsaw) 9, 475 (1964). (86) Jasinski, T., Hippe, R., Zbid., 9, 315 (1964): (87) Jasinski, T., Korewa, R., Smagowski, H., Ibid., 9, 655 (1964). (88) Jasinski, T., Misiak, T., Zbid., 9, 113 (1964): (89) Jasinski, T., Smagowski, H., Korewa, R., Zbid., 9, 343 (1964). (90) Jasinski, T., Szponar, Z., Zbid., 10, 619 (196.5). ( g i j j d i d . , 6. 655. (92) Jay, R. R., ANAL.CHEM.36, 667 (1964). (93) Johansson,. G.,, Talanta 11, 789 (i964). (94) Johari, G. P., Tewari, P. H., J . Phys. Chem. 69,2862 (1965). (95) Jonassen, H. B., Weissberger, A., “Technique of Inorganic Chemistry,” Vol. I, p. 37, Interscience, New York, 1963. (96) Jordan, D. E., ANAL.CHEM.36,2134 (i964). (97) Keily, H. J., Hume, D. N., Ibid., 36, 54.1 - ~ (1964). (98) Kelly, T . F., Zbid., 37, 1078 (1965). (99) King, E. J., “Acid-Base Equilibria,” iVIachlillan, New York, 1965. (100) Knight, D. M., Schlitt, R. C., ANAL. CHEM.37, 470 (1965). (101) Kolthoff, I. M., Chantooni, M. K. J . Am. Chem. SOC. 87, 1004 (1965). (102) Zbid., p. 4428. (103) Kolthoff, I. M.,Thomas, F. G., J . Phys. Chem. 69, 3049 (1965). (104) Kreshkov, A. P., Plasticheskie Massy 1964, 49. (105) Kreshkov, A. P., Aldarova, N. S., Zh. Analit. Khim. 19, 537 (1964). (106) Kreshkov, A. P., Bykova, L. N., Pevzner, I. D., Zbid., 19, 890 (1964). (107) Kreshkov, A. P., Bykova, L. N., Smolova, N. T., Zbid., 19, 166 (1964). (108) Kreshkov, A. P., Drozdov, V. A., Kolchina, N. A., Zavodskaya Lab. 31, 160 (1965). 109) Kreshkov, A. P., Drozdov, y. A., Kolchina, N. A., Zh. Analit. Khzm. 19, 1177 (1964). 110) Kreshkov, A. P., Drozdov, V. A., Tarasyants, R. R., Zavodskaya Lab. 30, 413 (1964). 111) Kreshkov, A. P., Mikhailenko, Y. Y., Tumovskii, L. A., Zh. Analit. Khim. 19, 1293 (1964). 112) Kreshkov, A. P., Vasil’ev, V. I., Zavodskaua Lab. 31. 30 (1965). (113) Kreshkov, A. P., tasil’ev, V. I., Zh. Analit. Khim. 19, 1508 (1964). (114) Kreshkov, A. P., Yarovenko, A. N., Nevskaya, V. N., Zavodskaya Lab. 31, 274 (1965). (115) Kreshkov, A. P., Yarovenko, A. N., Sayushkina, E. N., Zelenina, L. N., Zh. Analit. Khim. 19, 409 (1964). ~

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