TITRIMETRY IN THE STUDY OF INORGANIC REACTIONS IN NONAQUEOUS SOLVENTS ORLAND W. KOLLING Friends University, Wichita, Kansas
IN
THE study of weakly acidic or basic organic compounds the practical utility of volumetric methods when applied to nonaqueous solvents is well known. Simple analytical methods of adequate accuracy and precision have been developed for the characterization and determination of many organic compounds by selecting appropriate solvent systems. These have been reviewed by Riddick (35) and by Pifer, Wollish, and Schmall (32). Most of the solvents used are either hydrogen-containing acids and bases or inert hydrocarbons. However, investigations on chemical reactions and on generalized acid-base theories have extended volumetric methods to aprotic solvents as well. In aprotic solvents the behavior of a number of inorganic Lewis acids and bases toward indicators have been recorded (5412). Several titrations of inorganic compounds in both water-like solvents and aprotic solvents have been performed, and these have elucidated oxidation-reduction reactions as well as acid-base phenomena. Most studies on inorganic reactants by titrimetry in nonaqueous systems have been intended to extend our fundamental knowledge of solvents, but the potential analytical importance of these should not be dismissed. I t would seem convenient to summarize the existing sources on the titration of inorganic compounds in nonaqueous media according to the solvent employed, since this correlates the solvent characteristics with a given type of reaction.
ACETIC ACID
The evaluation of dissociation constants for a number of inorganic acids, bases, and salts in glacial acetic acid made by Kolthoff and Willman (23) over twenty years ago formed the basis for several investigations on acidbase behavior in this solvent. They demonstrated a sequence of decreasing acid strengths for the mineral acids perchloric, hydrobromic, sulfuric, hydrochloric, and nitric. The anion basicity of a series of potassium salts was found to increase in the order C104-, I-, Br-, CI-, and NOa-, with potassium nitrate being more basic than acetic acid. The titration of a number of sodium salts with perchloric acid in acetic acid was reported by Pifer and Wollish (51); however, to dissolve the salts, varying and unspecified amounts of water were added. Since water behaves as a base in acetic acid, their results could not he considered conclusive evidence for the basic character of these salts. A similar brief study by Higuchi and Concha (15) reported the titration of several inorganic chlorides, bromides, nitrites, nitrates, and sulfates as bases with perchloric acid, and the visual indicators, or-naphtholbeneein and crystal violet. Later Pifer, Wollish, and Schmall (33) showed that the acetates of ammonium, lithium, potassium, and sodium can be ti-
trated with perchloric acid in anhydrous dioxane and acetic acid-chloroform mixtures. Also, the difference in base strengths of potassium and sodium acetates in such solvents is sufficient to permit their differential titration, using the ordinary glass-calomel electrode pair. The acetates of potassium, sodium, and lithium have been titrated with perchloric acid by Jander and Klaus (19) also. Using sodium, potassium, and lithium acetates as standard bases, sulfuric and hydrochloric acids have been titrated individually and in mixtures in glacial acetic acid (Id). By utilizing the greater sensitivity of conductometric detection of the end point, evidence was obtained for both hydrogens in sulfuric acid being titrated separately. To support the view that water behaves as a base in acetic acid by formation of hydronium acetate, (H,O+)(Ac-), Jander, and Klaus (20) have provided potentiometric and conductometric data for the direct reaction of water with perchloric acid in acetic acid as the solvent. Fritz (7) has described a method for the determination of divalent metals, utilizing their ability to undergo coordination with acetonitrile in glacial acetic acid. The reaction is presumed to be
+ xC&CN
MAC*
+
+
M(CHJCN),-~ 2Ac-
where M is nickel or copper, and the released acetate ions can be titrated with perchloric acid. There seems to be little other information on metal complexes in acetic acid as a solvent. The similar reaction in which mercnry(II), as acetate, complexes halides with the release of acetate ions has been used as the basis for an analytical method for the determination of amine hydrohalide salts with perchloric acid (SO, $4). Oxidation-reduction, as well as acid-base reactions have been studied in acetic acid solutions. Iron(I1) perchlorate reduces, quantitatively, chromium trioxide and sodium permanganate (15). Likewise, a solution of ammonium hexanitratocerate in acetic acid acidified with perchloric acid has been found to be a satisfactory oxidant for the titration of oxalate in the same solvent (16). Bromine, and iodine monochloride have been used to titrete a number of organic compounds, and chromic acid, sodium permanganate, and titanous chloride were titrants for arsenic, antimony, mercury, chromium, bromine, and iodine, according to the work of Tomifek (37). No experimental determinations of oxidation potentials were given for these reactions in acetic acid; however, from polarographic studies in this solvent ($), it appears that oxidation potentials would be slightly more positive in acetic acid than in water. This is consistent with the potenbial for the cerous-ceric couple in acetic acid recently reported by Hinsvark and Stone (16), although it is an JOURNAL O F CHEMICAL EDUCATION
open question whether or not the absolute magnitude of this measured potential is meaningful. ACID CHLORIDES
The neutralization of aprotic acids, such as stannic chloride and aluminum chloride, with pyridine was tried in thionyl chloride solutions by Garher, Pease, and Luder (8). Titrations using the triphenylmethane indicators, crystal violet and malachite green, as well as conductometric titrations, suggested that in this strongly acidic solvent the essential reaction is the neutralization of a weak base and a weak acid. The related solvent, sulfuryl chloride, in which covalent compounds readily dissolve, has been used as a medium for the study of acid-base behavior of aprotic acids (11). The chloride ion donors (hases), tetramethylammonium chloride, phosphorus pentachloride, tellurium tetrachloride, and pyridine, were titrated with the acids, antimony pentachloride, titanium tetrachloride, vanadium tetrachloride, and arsenic trichloride. The stoichiometry of the reactions was determined from conductometric titration curves. In the corresponding solvent, nitrosyl chloride, the conductometric titration of tetramethylammonium chloride with several nitrosyl salts has been attempted hy Burg and McKenzie (5). Only with NOFeC4 did the neutralization reaction NO+ (acid)
+ CI- (bane)
-
NOCl
proceed to completion. A general study of acid-base relationships in selenium(1V) oxychloride has been made by G. B. L. Smith (56, 17, $9). I t has been suggested that this solvent ionizes as Sulfur trioxide reacts as a strong acid in selenium(1V) oxychloride, as demonstrated by potentiometric titration curves for its reaction with the bases potassium chloride, pyridine, and quinoline. For the acids and hases studied, it appears that base strengths decrease in the order isoquinoline, quinoline, pyridine, potassium chloride, and acid strengths decrease in the order sulfur trioxide, ferric chloride, stannic chloride. Pyridine is an excellent conductor in this solvent, presumably due to the existence of selenium oxychloropyridinium ions. Pyridine was used in conductometric titrations of the Lewis acids, stannic chloride, ferric chloride, aluminum chloride, calcium chloride, and potassium chloride. ALIPHATIC AMINES
Although amines have been used as solvents for studying weakly acidic organic compounds, these have been virtually unnoticed as media for inorganic reactions. One significant titration performed by Moss, Elliot, and Hall ($8)was that of boric acid with sodium ethylate in the solvent, ethylenediamine. The potentiometric curve for this reaction showed three distinct inflection points, indicating the titration of the third hydrogen. The need for studying other weakly acidic inorganic compounds in this solvent is readily apparent. Ammonium chloride exhibits greatly enhanced acidity in ethylenediamine, and the acid-base character of other salts would be expected to undergo change in such a basic solvent. VOLUME
34, NO. 4,
APRIL, 1957
ARSENIC TRICHLORIDE
Arsenic trichloride may be viewed as an ampholytlc solvent, quite analogous to water in its behavior as an acid as well as a base. Thus, according to Lindqvist ($7), the solvent ionization mechanism may be pictured as 2AsC13
= AsClt+ + AsC4-
and A s C k corresponds to H30+ in aqueous systems. Acids will decrease the amount of C1- present in solution and bases will increase the C1- concentration. By using a simple silver-silver chloride concentration cell, a value for pC1 (by analogy to pH) can be measured in arsenic trichloride solutions of acids and bases. Using such an electrode system, Andersson and Lindqvist (1) determined the potentiometric titration curves for the reaction of the acids, ferric chloride and antimony pentachloride, with the bases, pyridine and tetramethylammonium chloride. In the typical neutralization of tetramethylammonium chloride with ferric chloride, an ionic reaction that has been proposed is Several conductometric titrations have been performed in this solvent by Gutmann (9,lO). Reactions of chlorides of tellurium(IV), antimony(V), tin(IV), and vanadium(IV), with the bases, phosphorus pentachloride and tetramethylammonium chloride, appear to be quantitative and tend to confirm the ionic character of acid-base behavior in arsenic trichloride as a solvent. HYDROCARBONS, CHLORINATED HYDROCARBONS. AND ALCOHOLS
I n his classic experiments on acids and bases in nonaqueous systems G. N. Lewis (85, 26) titrated such acids as boron trichloride and stannic chloride with the base pyridine in benzene and chlorinated hydrocarbon solvents, determining the end point with a visual indicator. He demonstrated that the spectrum of a typical indicator, methylene blue, was identical when acidified by aqueous acids and by stannic chloride in chloroform. Similarly, the color changes of a number of indicators in the presence of different Lewis acids have been observed in the solvents, benzene, chlorobenzene, chloroform, dichloroethane, and carbon tetrachloride (54, 13); however, since quantitative data were not sought, no titrations were performed by these more recent investigators. The titration of stannic chloride with dioxane has, been used as a typical neutralization reaction in the comparison of the thermometric detection of the end point in benzene, carbon tetrachloride, nitrobenzene, and chloroform (48). The heat of neutralization in benzene and carbon tetrachloride solutions is sufficient to make this method analytically useful in evaluating thermodynamic quantities as well as end-point detection in such inert solvents. I n mixed solvents composed of ethylene or propylene glycol and an alcohol or hydrocarbon Das and Palit (6) titrated potentiometrically the acetates of calcium, magnesium, zinc, lead, manganese, and mercury, as bases with standard solutions of hydrochloric acid in the same solvent. The quantitative reduction of cohalt(I1) chloride to
.
metallic cobalt by sodium naphthalide a t the tenthnormal level in tetrahydrofuran solutions has been carried out by Chu and Friel (4). From the conductometric titrations performed in tetrahydrofuran, it was concluded that the reduction of metal salts by sodium naphthalide in this solvent requires the transfer of one or two electrons from the naphthalide ion to the metallic ion, with the resultant sodium ion and naphthalene playing no part in these reductions.
the course of reactions in liquid ammonia has been demonstrated. SULFUR DIOXIDE AM) SIMILAR SOLVENTS
Some investigators have assumed that liquid sulfur dioxide undergoes auto-ionization analogous to the corresponding dissociations of water and liquid ammonia. Thus, according to the equilibrium 2SO2e SOt+
LIQUID AMMONIA
This solvent has been the most extensively studied nonaqueous system for many inorganic and organic reactions. The self-ionization of the solvent is extremely low, butammonia reacts readily with any proton donor. Acid-base reactions in liquid ammonia, as exemplified by the neutralization of ammonium chloride (an acid) with sodamide (a base), usually have not been investigated by conventional volumetric methods, but titrations have been made by adding portions of the acid as a solid (6). However, the potentiometric titrations for a variety of oxidation-reduction reactions, carried out volumetrically below the boiling point of the solvent, have been reported first by Zintl (@), and in recent years by Watt and his co-workers in a series of papers. For example, liquid ammonia solutions of alkali metal sulfides were titrated with an ammonia solution of the corresponding alkali metal, and the titration curves obtained were quite like those obtained for aqueous systems (43). From a study of the characteristics of electrode pairs in the titration of ammonium bromide with potassium amide, Watt and Sowards (&) designed a mercury-mercurous chloride half-cell to be used with a platinum indicator electrode for titrations in ammonia. This electrode pair has been used to determine the relative acid strengths of chloride, acetate, and guanidinium ions in titrations with potassium amide (45). As a method for the detection and characterization of unusual oxidation states of various elements, Watt and his co-workers (41) investigated the potentiometric titration of liquid ammonia solutions of salts of higher oxidation states of several elements with standard solutions of alkali or alkaline earth metals in the same solvent. The reaction of potassium with potassium hexacyanocobaltate(II1) appears to be a two electron reduction when the salt is in excess, and a one electron reduction when potassium is in excess (42). The reduction of tetrammineplatinum (1I)bromide withpotassium shows the reduction to proceed directly to platinum metal without the intermediate state being formed (59). I n corresponding titrations of ammines of other platinum group metals, rhodium(II1) exhibited no change in oxidation state. From the titration of the iodides of zinc(II), cadmium(II), and mercury(I1) with potassium in ammonia, a single reduction step to& metals is observed without formation of the intermediate state (40) The reduction of bismuth(II1) iodide is similar, hut the reactions of ferrous(I1) bromide and cobalt(I1) nitrate with potassium yield intermediate states and complex intermetallic compounds ($8). The general utility of potentiometric titrations in the elucidation of
+ SO8--
any source of SOC+ ion in solution would behave as an acid, and any source of SOa-- would be a base. However, exchange reactions have shown the absence of the thionyl ion in the solvent, but do not exclude an ionic mechanism for neutralization reactions in sulfur dioxide. Jander, Wickert, and Ruppolt (21, 22) carried out a number of typical acid-base titratious using sulfites and thionyl compounds, following the course of the reactions conductometrically. The reactions of thionyl chloride with halide salts investigated in this solvent (18) suggest ionic reactions involving SOCl+ and C1ions. In fluorosulfonic acid the predominant ionic equilibrium is according to the conductance measurements of m700lf (46). The neutralization of the base potassium fluorosulfonate with the acid antimony(V) fluoride appeared to be the reaction of a strong acid with a strong base. Consequently, solutions of the fluorides of gold, tantalum, platinum, arsenic, bromine, and iodine, were classified as acidic or basic in relation to potassium fluorosulfonate and antimony(V) fluoride. It is interesting to note that perchloric acid is titrated as a base with antimony(V) fluoride in fluorosulfonic acid as a solvent. In similar studies by Woolf and Emel6us (47), acidbase titratious, followed conductometrically, were performed in the ionizing solvent bromine trifluoride. For this solvent, 2BrFa
= BrF*++ BrF,-
and the reaction of a base, silver bromofluoride (Ag+BrF4-), with bromofluoridic (BrFs+)-containing acids was studied. The titration curves indicated the neutralization to be an ionic reaction. SUMMARY
Numerous examples may be found in the literature demonstrating the importance of titrimetric methods in the study of reactions of inorganic compounds in nonaqueous solvents. For neutralization reactions, visual indicators and potentiometric methods of end point detection have been used, hut conductometric titrations are most frequently employed to follow the course of unknown reactions. Acid-base reactions have been studied in such solvents as acetic acid, ethylenediamine, hydrocarbons, chlorinated hydrocarbons, arsenic trichloride, liquidammonia, sulfur dioxide, fluorosulfonic acid, bromine trifluoride, thionyl chloride, nitrosyl chloride, and selenium(1V) oxychloride. Titrations involving oxidation-reduction reactions have been carried out in only a few solvents, namely, acetic acid, tetrahydrofuran, and liquid ammonia. As an analytical approach in the examination of inorganic reactions in nonaqueous JOURNAL OF CHEMICAL EDUCATION
systems, titrimetry has been shown to be particularly valuable in acetic acid and liquid ammonia, and it would he expected that volumetric methods might be applied fruitfully in other nonaqueous systems having current interest as reaction media. LITERATURE CITED (1) ANDERSSON, L., A N D I. LINDQVIW, Ada Chem. Scand., 9, 79-83 (1955).
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