Acid-Base Relationships at Higher Temperatures L. F. AUDRIETH and THERALD MOELLER Noyes Chemical Laboratory, University of Illinois, Urbana, Illinois
F .
ROM THE very beginnings of chemistry, the "ac~d-base"concept has been the subject of investigation and controversy ( I ) , and even today, in spite of the development of the Br@nstedpoint of view and the broader and more inclusive Lewis representation, our scientific thinking along these lines has been limited largely to phenomena and observations over a rather narrow temperature range. Chemical reactions a t higher temperatures have either been overlooked or tacitly ignored-yet it is significant that many important technical processes operate under conditions where high-temperature acid-base reactions are caused to take place either between solids or in the fused state. Nevertheless, the extent to which our theories have been developed during the past five decades is remarkable. Until just before the beginning of the twentieth century, water was considered to be a unique solvent, and only in aqueous solution did acid-base relationships receive attention. The work of Hantzsch, Walden, and especially Franklin and Kraus directed attention to non-aqueous solvents and resulted in the consideration of many of these as parent substances of solvent systems of compounds. Such systems as those built up from the protonic solvents ammonia, hydrazine, glacial acetic acid, and hydrogen fluoride attracted considerable attention, and interest in these led eventually to the Br@nsteddefinition, according to which an acid is characterized as a proton donor and a base as a proton acceptor. Non-protonic systems such as those based on sulfur dioxide were developed, but these did not fit into the general picture, although efforts were made to set up "proton-like" ions in various attempts to apply the Br@nstedconcept more widely (2). However, since acids and bases are found so widely in chemical systems, relation of phenomena associated with them to some more fundamental property of matter became necessary. It was Lewis (3) who f i s t pointed out that bases are electron pair donors and are therefore capable of adding protons and who finally developed (4) a broad and inclusive definition of acids and bases (6,6). In terms of this concept, a base is characterized as a molecule or ion capable of acting as an electron pair donor and an acid as any electrondeficient molecule or ion, while interaction of an acid and a base results first in the formation of a covalent compound which subsequently may or may not yield ions. Inasmuch as the properties of both original components are completely altered, neutralization again represents the net result of an acid-base reaction, and in this respect we have a complete reversal of thinkmg
+.
from the Br@nstedapproach where salt formation and neutralization have no place in equilibria involving merely proton transfer. It is obvious from the examples listed in Table 1 that the Lewis interpretation makes every case oi coordination an acid-base reaction. The coordination theory, 6rst elucidated by Werner and later given its fundamental basis by the development of the electron theory, therefore becomes synonymous with a rather inclusive acid-base relationship. It is, however, not the purpose of the authors to point out the shortcomings of the Br@nstedconcept or the overextended nature of the Lewis definition. Both have found wide application and have done much to stimulate scientific thinking and research. Yet i t is certainly not desirable to l i n k our efforts a t application of these concepts to reactions which take place a t ordinary temperatures, for through the use of these accepted and well-developed ideas, i t becomes possible to characterize reactions which take place in nonTABLE 1 THELEWISELECTRON THEORY or. ACIDSAND BASES Bases (Electron Pair Donors)
Acids (Acceptor Molecules or Ions)
Neutralization Product (Formation of Cobrdinatc Link; Ionization Not Necessary)
HCI
HCI
SnClr
NHI
Asf
NHa
Cot++
-.
+
~ C L ( Z H + ~nc4-1
~ ~ 1 7 /NH$+
c1-
A g y ~ ~ , Co(NH3)~C12+
aqueous, high-temperature systems, regardless of the presence or absence of a liquid phase. To a limited extent, this has already been done in extending the Br@nstedconcept to reactions of "onium" salts a t elevated temperatures, but judicious application of the Lewis definition permits consideration of many nonprotonic high-temperature reactions as acid-base phenomena. The development of the nitrogen system of compounds (7) early revealed the fact that ammonium salts
behaw as acids in liquid ammonia. Subsequent investigations dealing with other nitrogen-containing solvents such as hydrazine and the amines have demonstrated experimentally the general acidic characters of "onium" salts in their parent solvents. With the development of the BrGnsted definitions, these hdmgs, previously related specifically to so-called parent solvents, were given a general theoretical background. It was only natural that the following questions should arise: "Do 'onium' salts, which have been characterized as acids in their respective parent solvents, exhi%it these properties by themselves when undergoing reactions in the dry or fused state?" (8) and "Does the presence of the 'onium' ion give to compounds ordinarily termed salts the chemical properties of acids?" (8). "The answer is most decidedly in the affirmative. The characterization of 'onium' salts as acids, fier se, in the dry and fused state is one of the most useful extensions of the modern Br@nstedtheory of acidity. Indeed, the very simplicity of this concept has caused it to escape investigation until now" (8). The literature contains numerous examples of reactions involving "onium" salts at higher temperatures. Reactions of ammonium chloride with oxides and other compounds at elevated temperatures were studied quite thoroughly by Rose (9) almost one hundred years ago. Ammonium sulfate and ammonium fluoride have been widelv used as fluxes in the o~euinc uo of w ores as has ammonium chloride in the an&& for the alkalies in silicates (10). The use of ammonium chloride to repress hydrolysis in the dehydration of metal chlorides at elevated temperatures is familiar to all, and the acidic characters of ammonium nitrate and the ammonium halides have rendered them useful in the syntheses of anhydrous nitrates and halides (8,ll-15). Fused ammonium nitrate, which can be regarded as a concentrated solution of nitric acid in ammonia, is not only an acid, but a vigorous oxidizing agent as well (8), and its reactions with metals and oxides are in every way analogous to those of nitric acid in water solution. The conversion of cyanamide and of dicyandiamide to guanidine salts by fusion with ammonium nitrate and other ammonium salts (16) involves ammonation catalyzed by the ammonium ion. A
THE LEWIS THEORY AS APPLIED TO ACID-BASE AT HIGHER TEMPERATURES
REACTIONS
There are many high-temperature reactions which are not amenable to treatment in terms of the BrGnsted concept because of the complete absence of any hydrogen-containing ion. However, reactions between the so-called basic oxides, con.. sulfides, .. and fluorides, .. taining the donor ions :0 :-, : S :-, and : F :-, and such .. .. .. coordinatively unsaturated (a) molecules as silica .. or :o .. : titania or (b) ions as the metaphosphate (: 6; : p .. :0 :)-
..
.. :B : 0..:)- are typical high-temor the metaborate (: 0 perature acid-basereactionsfromtbeLewispointofview. Fusion may in some cases take dace. but in other instances it is unnecessary. ~um&arizedin Table 2 are several typical examples of high-temperature acid-base reactions which are fundamental in many metallurgical and ceramic processes. It should be pointed out that many acidic oxides such as silica are giant molecules which must fist undergo depolymerization before the simple ions indicated in Table 2 are formed, if, indeed, they even exist. Reaction of silica with.fused sodium hydroxide, or with other compounds yielding strong anion bases, must involve the intermediate formation of polyanionic silicate
TABLE 2 THELEWISTHEORY AND ACIDBASEREACTIONS AT HIGHER TEMPERATURES Neutralization Applications Barer Acids Produds
Class 1
Other examples of these general phenomena may be cited. Thus fused pyridine hydrochloride (17) is a good solvent for metallic chlorides and reacts with many metals and oxides yielding fused low-temperature melts which are good conductors and from which a number of the less active metals have been electrodeposited. Furthermore, it is of interest that the double "onium"-metallic salts, such as ZnCly2NH&l, also behave as acids in the fused state, many of suchbystems giving stable fused melts at temperatures far above those at which the pure "onium" salt would ordinarily decompose (18). Inasmuch as hydrazine, hydroxylamine, and substituted ammonium salts (18) have also been shown to behave as high-temperature acids, all available evidence lends credence to the premise that "onium" salts do indeed behave as acids.
0- (from MO, MOH, MCO1. MSO,)
O- (from MO) 0- (from MO, etc.)
S
(from NanS)
F- (from alkali fluorides)
SiO, AlnOz BpOi Box- (B2Os) POs-
&&-
(So,.So,-) HS0,(H*O Sza-) FeS CUB BcF2 AIR Tap.
+
SO8- or Si04A10,- or A 1 0 i B O or~ BOF BoaPO,' SO,-
Fe&CuSBeF4AIFC TaK-
Manufacture of glass, cement, and ceramic products. Slag formation Borax bead tests Metaphospbate head tests Opening of ores
Oxford process for nickel concentration Electrolytic melts
complexes, the average ionic sizes of which must depend both upon the temperature and upon the quantity of added base. Just as isopoly and heteropoly anions in aqueous solution can be degraded to simple ions by raising the pH, so can the giant molecules of silica be degraded by the metallic oxides, hydroxides, or carbonates because of the basic natures of the latter. It is interesting to note, however, that this depolymerization proceeds most rapidly in the presence of fluorides, a fact whicli is undoubtedly due to the impossibility of the bridging of two silicon tetrahedra a t a point where the fluoride occupies a comer. Certainly this explanation is in line with the practice of adding fluorides to silicate melts to increase their fluidities (19). The sintering reactions involved in the manufacture of cement can also be looked upon as involving hightemperature acid-base behavior wherein both alumina and silica function as acids. The second series of reactions listed in Table 2 involves the bead tests, formerly so much used in analytical chemistry. In each of these the anion of the low melting fused salt functions as an acid in accepting an electron pair from the basic oxide ion. Here again the over-all reactions may not result in the formation of simple anions, for in fact the existence of a discrete metaphosphate ion, either in solution, in the fused state, or in the crystalline condition, is highly improbable. Rather it is more probable that the metaphosphate ion is a polymeric aggregate and reacts with oxides to form an entire series of intermediate polyphosphate ions of indefinite compositions. Such a process might be formulated as yo-
(Po,-).
--1 (POa-).
0-.
(z-y-l)o-
x.POi
with the formation of definite ionic aggregates such as the tripoly- and pyro-phosphate as intermediates with the ortho-phosphate as the coordinatively saturated end-product. While borate reactions have been best characterized in terms of oxides as bases (20),it is common practice to detect cobalt, for example, by means of a bead test upon the precipitated sulfide and fluoborates have been obtained by fusing boric oxide and potassium fluoride (21). Furthermore, the fact that a t elevated temperatures boric oxide will decompose nitrates, sulfates, and other salts (22, 23) with the production of borates and more volatile acids or their decomposition produds certainly agrees with the Lewis interpretation. The same is, in general, true of the phosphates. The formation of sodium monothio-orthophosphate by a high-temperature reaction between sodium sulfide and sodium metaphosphate (24) is representative, and a consideration of the electronic structure of phosphorus pentoxide makes it apparent that the formation of fluophosphates as a result of heating this oxide with ammonium fluoride (25) is only another example of acid-base behavior. The well-known solubiiiing effects of fusion *th alkali bisulfate or pyrosulfate, especially in the treatment of such refractory ores as those of titanium, tanta-
lum, and columbium (26),are merely other examples of high-temperature acid-base behavior in which sulfur trioxide, a real or intermediate decomposition product of either flux, because of its electron unsaturation behaves as an acid. The literature is replete with references to reactions of this type. In such melts as are furnished by molten sodium sulfide, the sulfide ion is a strong base. Thiosomplexes are common, and their formation in the molten state is accomplished in an interesting series of metallurgical processes, for instance in the refining of nickel where the cuprous and ferrous sulfides dissolve in fused sodium sulfide and are thus separated from the nickel sulfide which undergoes little or no reaction (27). Molten mixtures of sodium and cuprous sulfides show electrolytic conduction and the existence of CuSions has been demonstrated (28). Many metallic fluorides and oxides dissolve in an excess of molten alkali fluoride to form complex ions (29),the fluoride ion acting as a strong base in bringing about these reactions. The small radius of the fluoride ion undoubtedly contributes to the formation of extremely stable complexes of relatively low ionic weights which yield melts of high fluidity and excellent conductivity. Fluorides have long been recognized as mineralizers when found with certain classes of ore deposits, and fluorspar, together with other fluoride containing minerals, is found in the end fractions of magmatic intrusions as well as in the surrounding and sometimes highly metamorphosed country rock. The opening up of ores in general necessitates the use of a wide variety of fluxes, the choice of flux in each instance being governed by a knowledge of the chemical composition of the raw mineral. For acidic ores the high-temperature bases such as sodium hydroxide, carbonate, or fluoride, calcium carbonate or oxide, and potassium fluoride are widely used, whereas for basic ores such acids as silica, borax, bisulfate, or pyrosulfate are important. CONCLUSION
In presenting these few examples of acid-base reactions a t higher temperatures, no effort has been made to be specific in elucidating reaction mechanisms. While qualitative evidence leaves little doubt conceming the validity of extending both the Brflnsted and Lewis definitions to systems of this sort, it is to be regretted that quantitative physico-chemical studies are almost completely lacking. Certainly the extensions proposed should be useful particularly to the inoi-ganic chemist interested in new syntheses, the metallurgist, and the ceramist. It is also hoped that recognition of these simple acid-base relations in high-temperature systems may serve to interest the theoretical chemist in quantitative definitions of the phenomena which occur. LITERATURE CITED
(1) For a discussion of the historical aspects of acid-basetheory, see WALDEN, "Salts, acids, and bases," George Fisher
Baker Non-Resident Lectureship Series, McGraw-Hill Book Company, Inc., New York City, 1929. (2) HALL, BRISCOE,HAMMETT, JOHNSON,ALYBA,MCREYNOLDS. HAZLEHURST. AND LUDER,"Acids and bases." J o m N A ~OF EDUCAT1ON? Pa., (3) LEWIS,"Valence and the structure of atoms and molecules," The Chemical Catalog Co., 1% New York City, 1923,P. 142. (4) LEWIS,J. Franklin Inst., 226,293 (1938). (5) LUDER,Chem. Rcv.,27,547 (1940). (6) LUDER.J. %EM. EDUC.,19,24 (1942). R&(7) FRANKLIN, The nitrogen system of hold Publishing Company, New York City, 1935. (8) AUDRIETH A m SCHMIDT, P ~ c Nat. . A d . Scj. U.S., 20, 221 (1934). (9) ROSE,Pofg. Ann., 73,582(1848);74,562 (1848). AND HALL, "Analytical chemistry," J O ~ " (10) TREADWELL Wiley and Sons, New York City, 9th Edition, Vol. 11. (1942).p. 416. (11) AUDRIETH, JURKOLA, MEINTS.AND HOPKINS. J. Am. Chem. Soc., 53, 1805 (1931). (12) HOPKINSAND AUDRIETH, Trans. Am. Ebdrochem. Soc., 66, 135 (1934). AND AUDRIETH, J. Am. Chem. Soc., 57,1159 (13) REED,HOPKINS, (1935). (14) SCHMIDT AND AUDRIETH, Tans.Illinois State Amd. Sci., 28, 133 (1935).
(15) TAEBELAND HOPKINS,Z. (inorg. ellgenr. Chenr., 235, 62 (1937). (16) SMITII,SABEITA,AND STEINBACH, Ind. Eng. Chem., 23,1124 (1931). (17) A U D R I ~ H LONG, , AND EDWARDS, 1.A m . Chem. Sot., 58,428 (1936). (18) A ~Unpublished ~ observations, ~ ~ ~ ~ (19) MACHINAND VANECEK. Illinois State Geol. Survey, Rept. Investigations No. 68,1940. Z.anorg. Chem., 40, 225,337 (1904).i (20) GWRTLER. (21) S c n r AND ~ ~ SESTINI,Ann., 228, 72 (1885). (22) TATE,Quart. I. Chem. Soc., 12,160 (1859). (23) ROLLET AND ANDRBS, Bull. soc. chim., 49, 1065 (1931). (21) ZINTLAND BERTRAM, Z. anorg. allgem. Chem.. 245,16 (1940). (25) LANCE.Ber.. 62B,786 (1929). (26) SEARSANDQUILL,I. Am. Chem. Sac.. 47,922 (1925);48,343 (1926). (27) THOMPSON, D.R.P.,91, 288 (1893). See also ABEGG'S "Handbuch der anorganischen chemie. Die elemente der achten grnppe. Vierter teil. Nickel und seine verbindungen." Verlag van S. Hirzel, Leipzig, 1939, p. 113. Z.Elektrochem., 46,379 (1940). (28) SAYELSBERG, (29) For a list of complex fluorides see Gmelin's "Handhuch der anorganischen chemie," Verlag Chemie Leipzig-Berlin, System-Nummer 5, 1926,p. 59.
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