DEFINITIONS O F ACIDS AND BASES
51
THE LEWIS XSD THE BROSSTED-LOWRY DEFIKITIOXS O F ACIDS AND B L U E S I. A I . KOLTHOFF School of Chemzstry, Unzversity of Mznnesota, X i n n e a p o l i s , X i n n e s o t a Received September 15, 1943
When the properties of substances become known and it appears that several substances have properties in common, they are usually classified as a group. A definition based upon the compoPition or the common properties of the substances is then given t o characterize such a group. In the interpretation of the properties of such a group of substances it is usually necessary to develop another definition which is based not upon facts but upon conceptions or theories. RIost definition.. are usually more or less limited in scope and sometimes ambiguous. Thi. is especially true of definitions which have been given to characterize the categorieq of acids and bases. I n the early days of chemidry as a n experimental science the terms “acid”, “hase,” and b*vilt”n-ere used in a vague n a y (6). Still, in the middle of the s e i enteenth century acids and bases n e w alre2d)- characterized by the fact t h a t upon interaction of the tn-o a qalt i.i formed. It is of interest to note t h a t the Dutch scientist Johann Baptist 1-an Helmont (1577-1644) made the ubservation and the interpretation that free +ilicioup earth is precipitated from a potassium silicate solution upon addition of a mfficient quantity of acid t o “saturate” the alkali. The formation of a qalt as the result of the interaction of a n acid and a base has been the 1xi.i. of a definition of acids and bases for several centuries. I n 1648, Rudolph Glauber (1604-1G70) stated that alkalies and the alkali carbonates are “opposed” t o the acids, and that salt. are made up of tn-o different components: “Thu. sal ammoniacuni i. derived from sal aciclum commune (hydrochloric acid) and the .a! x-olatile urinae (ammonia) .” Ailbouta century later (17-14, 17511, Rouelle clearly defined a salt as a substance n-hich iq fornierl by the union of an acid :vith a n alkali. *It about the same time Wni. Leu-ic (1746) gave a more elaborate characterization of acids, bases, and saltq v-hich in many respects is similar t o a more modern definition: & k i d sare substances n-hich (upon proper dilution) ta-te sour, n hich effervesce n-ith chalk and alkali cnrbonntes, and n-hich form salts x i t h such substances. -%cidsturn syrup uf violet. led. n-hile alkalies turn this y ; u p hlue ( 7 ) . K e note that the classification of acids and loaqes. up to this point, has been based upor, their mutual interaction and not upon a property related to their conposition. The matter of composition n a s fird brought up by Lavoisier 11777). v h o c o n d e r e d oxJ-gen 3 s the wbstance n hich imparts acidity or acid character + O all acids. Thiq view u - a ~conclusive1)- diqproved by Davy (1810) TT hen he i l i ~ ; ecl \ that hydrochloric acid is composed entirely of hydrogen and ehlorii-ie ,
52
I. LI. KOLTI-IOFF
Contrary to the v i e m of Berzelius, x h o defined an acid as an oxide with certain properties, Davy (1815) rejected the vien- that iodine pentoxide is a n acid and maintained that it beconies nn w i d only by combination n-ith n-ater. I n the same year, Dulong expreswl thc viun- that olalic acid contains hydrogen, and that the formation of aiihydrouq salt? from niefallic oxides gives u-ater as a result of the union of the hvdrogen of t h e a c i d Tvith the oxygen of the oxide, Thus, Dayy and Dulong map be considered as the forerunners of Liebig, n h o in 1838 defined acids as “cornImiiids containing hydrogen, in which the hydrogen can be replaced by metals.” This definition, in n qomewhat extended form, became generally accepted. It is based upon P.qminzentuZ facts referring to composition (acids contain hydrogen) and wucfivity (hydrogen replstced by metals; acid and base react nith formation of salt and water). I n the qualitative and quantitatiye interpretation of acidity and basicity it appeared that the classical definition Tvas too limited. I n order to account for the properties of acids and bases it was necessary to formulate certain conceptions. This was done by Arrhenius, d o s e definition of acids and bases is based on the theory that acids when dissolved in water dissociate into hydrogen (hydronium) ions and anions, and that bases Tvhen dissolved in water dissociate into hydroxyl ions and cations. This important definition, which has contributed so much to the quantitative understanding of acidity and basicity, appeared too limited when phenomena of acidity and basicity Jvere studied in non-aqueous solvents. This led L o w y and Bronsted to the more general definition which is based upon the conception that a n acid may be considered as a combination product of a proton with a base, while a base is defined as a substance which can combine with a proton t o form a n acid. This definition limits the group of acids t o substances which contain a transferable proton. G. N. Lewis JTas the first to point out that many substances which do not contain a proton react in aprotic media with bases Kith the formation of compounds which have some properties similar to those of a salt formed by interaction of a neutral molecule (Bronsted) acid and a base, Lewis, quite rightly, considers the Bronsted definition too narrow and bases his definition upon the view that a base is a substance which can donate a n electron-pair to form a conjugate bond and that a n acid is a substance which can accept an electron-pair from a base t o form a coordinate bond. Xeutralization occurs when an acid and a base combine. Much has been written’ during the last decade concerning the conceptions and terminology regarding acids and bases. Unfortunately, the impression has been established that there is a controversy between the Lewis theory on the one hand and the Bronsted-Lowry theory on the other. This contradiction is not real; it merely appears t o exist through confusion in the terminology and nomenclature used. By adopting a n appropriate terminology it will be seen that it is possible and even advantageous to adopt and teach both definitions or conceptions, even though the Bronsted definition is admittedly more limited in scope than the Lewis definition as far as acids are concerned. 1 For a review and literature references see “Acids and Bases,” published by the Journal of Chemical Education, Easton, Pennsylvania (1941).
DEFISITIOSS O F A C I D S .YSD BASES
53
Lenis (1) himqelf states: “The recognition of Bronsted and his school of such ion.: ne. the halide ions and acetatcl ion as true base’, togrther Tvith the developnient of the concept of organic h i e s , tends t o make the present recognized list of 13ase. identical T $ ith mj- 011 11. On the other hand, any similar valuable and instructire eytension of the idea of acids has been prevented by what I a m tempted to call the modern cult of the proton.” Manv chemists. h o w r e r , quite rightly are reluctant to give up the extremely uqeful and simple Bronqted definition This definition leads to a simple and adequate description of the behavil)r of all the common acids, both qualitstirely and quantitatively. I t is posqible t o arrange, for :i gil en qolvent, all the substances meeting Bronsted‘s definition of acids and I x q e S in the order of their acid and basic strength. Furthermore, in all protic so11 ents, p:otons enter strongly into any con+icleration of acid-base properties. Bu? it i. not only upon the basi5 of its pracTica1 usefu1ne.q that tlle Bronqted definition cleserve, general adoption. I n order t o elplain the acid propertie; of Broiisted acid? like perchloric, suifuric, hvdiochloric. and acetic acids in the Imvis terminologv we must consider the pioton as a dihaqic acid (8) Hj-drochloric acid cnn be conqiderecl as the product oht‘tined upon neu:ralization of :t chloride ion TI ith a proton, the proton and the chloiide ion forming a coordinate boncl I n order t o consider hydrochloric acid a- a n acid, it i unied that it react* n i t h :I base by the formation of a hydrogen bond.
.. :Cl:H .. ..
:Cl:H
..
H
..
+ :N:H .. H
-+
+
--+
H :el:H:S:H
..
..
H
9 .
:O:H .. H
: e..I : H : .. z:H H
Thi. hydrogen bonding results in such great electrical dress that practically all of the molecules thus formed split into ions (4). Thuq, the product obtained b y interaction of hydrochloric acid n i t h water yields the ,‘neutralization product” C!H.I-I,O, which in vater as a solvent breaks d o x n into hydronium ions (H,O+) and chloride ions. The hydrogen-bond product obtained b y interaction of one molecule of perchloric acid v i t h one niolecule of ivater is not stable, even in the solid state, and rearrange. into hydronium perchlorate, n hich haq the same crystal structure and lattice dimenqions as ammonium perchlorate. Thus the acids containing replaceable protons diqtinguiqh themselves from other (Leiris) acids by the fact that upon reaction n i t h a base a hydrogen bond is formed firbt. If the hydrogen bond cixild be considered as an ordinary coordination bond there would he no difficulty in interpreting the acid prdperties of acid. which contain a proton on the h - 1 4 ni the Leu-is definition. The hydrogen bond ITS Iormerly believed t o result from the formation of two covalent h n d . by the hydrogen atom. Thi5 vieir is no longer held. Pauling ( 5 ) , f c r example, says: ”It i- non- recognized that t h e hydrogen atom, n i t h one
54
I . 11. KOLTHOFF
stable orbital (the 1s orbital), can form only one covalent bond and that the hydrogen bond is largely ionic in character and is formed only between the most electronegative atoms.” Lewis also recognizes that the hydrogen bond differs from an ordinary coordination bond. Len+, Xagel, and Lipkin (2) made the following statement: “Evidently what has become knon-n as the hydrogen bond differs not only in degree but in k i n d from a true chemical bond.” They refer to experiments made by Levis and Seaborg (3), n-ho noted that trinitrotriphenylmethide ion seems to form coniplexes n-ith substances like alcohol, phenol, or acetic acid by attachment of hydrogen through the nitro groups, ivithout causing any marked change in color. However, v-hen hydrogen ion was similarly attached to a nitro group the color changed from blue t o orange. Probably other strong (Lewis) acids such as stannic chloride or sulfuric trioxide 11-ill have a similar effect. Thus n-e have an example in which the addition compound through the formation of a hydrogen bond between, say, acetic acid and a base has optical properties similar to those of the base, indicating no significant change in the bond structure of the base. On the other hand. the neutralization product of the base n-ith a proton or another LeTvis acid has entirely different optical properties. A strict application of Lewis’ definition to acids containing a proton (Bronsted acids) v-ould make it questionable 11-hethersuch substances should be called acids, I n the first place, in their reaction with bases the first reaction (“neutralization”) product formed should be considered as a compound Tvith the hydrogen doubly coordinated. This is not correct according to Pauling. The primary product formed in the interaction of a Bronsted acid with a base differsin the nature of its bonding from the neutralization compound of all other Lewis acids with bases. The fact that Bronsted acids need special consideration in the Lewis terminology contributes an urgent argument for classifying the Bronsted acids as a special group. The Bronsted definition is not concerned with the mechanism. of acid-base reactions. I n this respect it is strictly analogous to the definition of oxidants and reductants. h Bronsted acid in the sense of Lewis is a “neutralization product” of a proton and a base (Cl-, HzO, etc.). I n such a compound the acid (proton) may be replaced by a stronger acid and the base by a stronger base. In the Bronsted terminology we consider only the replacement of the base by a stronger base. The chloride ion, for example, is a very n-eak base. Upon reaction of hydrochloric acid with water the stronger base, water, takes the proton from the weaker base, chloride ion: HC1
+ HZO + H30’ + C1-
From the above it is evident that for practical and theoretical reasons there is a place for both the Lewis and the Bronsted definitions of acids and bases. In order to have a place for both classifications the following terminology is suggest ed . A c i d s which satisfy the L e w i s definition are called L e w i s acids or proto-acids. (Proto is the Greek n-ord for primary.) Substances like the proton, boron trichloride, etc., are proto-acids.
DEFIh-ITIOSS OF ACIDS ; I S D BASES
55
The Bronsted definition remains unchanged. -in acid consists of a prdton and a base, or of several protons with a base. When a distinction is made betn-een a proto-acid and a n acid, there is no conflict between the Lewis and the Bronsted classifications of ncidc and bases. It has been mentioned already that a great advantage obtained in maintaining (Bronsted) acids as a separate group of acids is that in a given solvent the quantitative expression of the strength of acids and bases is unambiguous. It is the affinity of a base for a proton or the tendency of a n acid to give off a proton which determines acidic and basic strength. I n the Levis terminology, acidic and basic strengths become ambiguous expressions. The strength of a given \ m e depends upon the particular proto-acids n-ith Tyhich it combines and the strength of a given acid depends on the particular base lvith n-hich it reacts. Leu-is (9) recognizes this limitation of his generalized t reatmcnt :md itatei: “So in studying acids and bases we find that the relative *trrngth tlcpeiids not only upon the chosen solvent but also upon the particular h q e or acid u.ed for reference.” Luder (4), a n ardent proponent of the Lewis concept, doe\ not agree 11 it h this statement and belie\-e? that proto-ac and 1m.e.; can be grouped quite geneially in the order of :tcid and baqe strength. ITe trie- to .hou, for cxamplc, ili::: the silrer ion behaves as a stronger acid ton-ard hydroxyl ioni t h n ton.ard :iiiiinonia. Hoxever, he bases hi. argument on the fact that qilver ioni prcc‘ipit::tc nit h hydroxyl ions 1.r-it hotit considering n-hether the d y e r form.; a n ionic or :L rwortlinate bond with the hydroq-I. Morecn-er, it is uell knonn t h a r in diliitc ;~queous solution silver ion. do not combine n ith hydroxyl ionb (silver hydroxide is t i strong electrolyte), n herens they do combine \\-ith ammonia. Many examples can be given n-hich subtantiate Lewis’ \t,itcniciit ( 3 ) . For instance, upon addition of mercuric ions to a hydrogen cyanide holutioa the following reaction takes place almost quantitatively: Hg“”
+ 2 H C S -+
HglCS)?
+ 211~
Regarding its affinity toward the cyanide ion the proto-acid mcrciiric ion is tt stronger acid than the proton. Even in it. reactivity toivard the estrr.mcly weak bases iodide, bromide, and chloride the mercuric ion behnvt.. like tt s t r m g a oicl, or the halide ions behave like strong bases with regard to the proto-aciJ mercuric ion. Qualitatively, several other cations behave like merciuic ions. If we consider mercuric iodide as a “neutralization compound” of the +trong proto-acid mercuric ion with the base iodide ion, the i,dide ion is ;t much stronger base than, for example, the acetate ion (referred to mercuric ion). ‘The “acid strength” then of the mercuric ion is not determined by the properties of this ion alone, but is also determined by the properties of the lxiw \vith which it reacts. I n the Bronsted system the difficulty iq eliminated, becawe the 5trength of bases aln-ays refers to their affinity for the proton. Objections hcve been raised to the Bronsted terminology 1)y p inting ,)tit tiiat several metal cations are called acids although they do n:)t contain oiit’ or more proton< in the anhydrouc state. Thiz objection i. valid. In the vnw oi 1,cn-iq
56
I. RI. KOLTHOFF
several of the inorganic anhylrous. citions are proto-acids. Th2v can react, for example, n-ith the base n-afer. with the formation of a n acid. The aluminum ion, for instance, is not an acid, but i f is a protn-acid. Upon reaction with the base m t e r it forme the heswaquo aluminum ion, 77-hich is 211 acid. I n this respoct the formation of the acid hexaaquo aluminum ion is c?rnsarahle to the formstion of sulfuric acid by the interaction of the prato-aci$ qulfur trioxide with the base u-at er , The follon-ing example is illustrative of the uqef tilness of the simultaneous application of both the Bronsted and the Lewis concept-. 9solution of sulfur diovide in n-attr contains the protn-acid SO*, part of which has reacted v-ith n-ater Jvith the formation of the acid H2SOa. When the properties of w c h a qolution are studied n e have to consider that the proto-acid S O z , and thP acids H2S03, HSO,, and H,O-may eshibit acid properties. Whether the proto-acid SO2 contribute. t o the acid catalysis of the inversion of sugar in aqwou5 medium has. not been investigated t o my knolrledgp. Indicator bases of siiitahle qtrength in aqueoiis medium seem t o react only n-ith the acid constituents and not with SO2. Recently, Dr. L. S.Guqs and the author have been investigsting the behavior of sulfur dioxide dissolved in ethanol toward the indicator thymol blue. -1quantitative study revealed t h a t the red color imparted t o the thymol blue is due partly t o the interaction of the acid C2H,0P020H (or C2H,0HH-) x i t h the indicator and partly t o that of the “neutralization product” of the proto-acid SOz v-ith the indicator. The results of this study nil1 be communicated in a subsequent publication. In conclusion the following statements are made: 1. The Brbnstcd acids should be reprewited as a special class in the group of substances which art. wid? according t o the Lewis concept. 2. The Bronqted acids deserve special designation, because this class comprises the great number of inorganic and organic acids which contain replaceable protonaltb of primary alcohol sulfates containing from tnelve to eighteen carbon atom>, The minima are as much as 5 dynes per centimeter l o w r than the value reached at higher concentrations. These “anomalous” results have presented apparent disxgreement Tvith Gibbs’ adsorption equation. The hypotheses p r o p o d t o reconcile such incongruities have been frequently discussed (1 t o 12). Usually the explanations proposed fall into two groups: First, it ha? been claimed that there are experimental difficulties which result in errors in the methods used for measuring surface tension (3, 3 , 7). On the other hand, the possible existence of abrupt changes in colligative properties of iolutioni of certain surface-active substances has been emphasized (12) and this could lead to minima in the surface tension-concentration curves. The data which we present here deal with solutions and procedures not covered by either of thc abovc e ~ p l n nations. PROCGDURC
The sulfated alcohols used in this work were of a particularly high dcgree of purity. The details of the niethods used in their preparation arc given i n a report entitled “Pome Properties Involving Surface &Ictivityof Sodiuni S d t s (ti Primary and Secondary -4lcohol Sulfates” ( 5 ) . Particular care 1~~ a- twrcitcd in
