CHEMICAL POTENTIAL and ALKALINITY WILDER D. BANCROFT Cornell University, Ithaca, New Yark
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N THIS paper I shall show that certain problems of powerful and the oxidizing agent less powerful, while interest to chemists can be solved readily by a non- the acid will act in the opposite way. Whethm the mathematical consideration of the qualitative reaction will take place more readily in an alkaline or in changes in the chemical potential with increasing alka- an acid solution depends on whether the chemical linity or increasing acidity. If we add oxygen to any potential of the reducing agent changes more than the oxide, we are adding a negative element and the oxide chemical potential of the oxidizing agent in passing becomes more acidic and less basic. from an alkaline to an acid solution. I t is well known Roscoe and Schorlemmer (1) say that "it is a general that alkaline pyrogallol is more effective in determining rule that the acid-forming oxides of a metal contain oxygen than neutral or acid pyrogallol and consemore oxygen than the basic oxides of the same metal. quently we deduce that alkali boosts the chemical Thus Bi203is a basic oxide, while Biz06 is a weak acid- potential of pyrogallol more than it lowers the chemical forming oxide; again VO is a basic oxide, while V20s potential of oxygen. This has been confirmed experiyields a strong acid which forms stable salts." mentally by Mr. P. A. Hansen, whose work will be This generalization is repeated in connection with, published later. and amply supported by, readily accessible detailed I t is also well known that organic developers are more discussions of data relating to chromium (2), titanium effective in developing photographic films when used in (3), uranium (4), molybdenum (5), manganese (6), alkaline solution. Consequently we deduce that alkali tungsten (7), vanadium (a), bismuth (9), (lo), anti- affects the redncing potential of hydroquinone, etc., mony (ll), and lead (12). more than it does the oxidizing potential of silver Three oxides of tin are recognized: stannous oxide, bromide. This is also true experimentally. SnO; stannic oxide, SnOz; and tin peroxide, SnOr The From these facts it follows, other things being equal, first two are both basic- andacid-formingoxidesand the that a basic oxide will be oxidized more readily in an third is only acid-forming. We have stannous and alkaline medium. This offers an explanation for the stannic chlorides, and stannites, stannates, and per- surprising f a d discovered by Bancroft and Nugent (16) stannates. The stannates are more stable than the that the degree of oxidation of an oxide of manganese stannites and stannic chloride is hydrolyzed much more dissolved in a borax melt in contact with air increases readily than stannous chloride. continuously with increasing alkalinity. In the acMurray (13) has proved the non-existence of copper companying table are given Nugent's data for the quadrantoxide, '2140, and Mellor is a bit skeptical amounts of available oxygen per gram of manganese in about CuaO, so the oxides in good standing at present melts of different proportions of N a O and BzOa. are CuaO, CuO, Cuz03,and CuOp. Mellor (14) says AYUL~BLB OXYOBN IN BOUT= MELTS A = m o per ~ ent. N ~ S O in borate melt that cupric oxide is a stronger base than cuprous oxide; B = =am. available oxygen per gram manganese but this must be a slip of the pen, because cupric oxide A B A B is more acidic since a sodium cuprite, NazCnOz, seems 0.0 0.000 55.3 0.143 57.7 0.135 to exist (111, 145). Since calcium cuprate, CaCu20a, 16.9 0.020 17.0 0.020 60.4 0.145t is believed to have been prepared, Cuz03is also an acid33.3; 0.059 60.4 0.141t 41.8 0.085 65.5 O.1SOt forming oxide, as it should be. 41.9 0.083 65.7 0.144 48.0 0.102 The change to an acid-forming oxide will take place * Anhydm-3 borax. NsuB~o?. more readily in an alkaline solution than in an acid one t Mm02 = 0.1456 g. available oxygen Per gram rnanzaoese. because the will tend to neutralize the Consequently alkali must raise the chemical potentials I t seems probable that one should use a distinctly of the lower oxides and acid must raise the chemical alkaliie glass with plenty of manganese if one wishes to potentials of the higher oxides. Since the lower oxides duplicate the pink windows of Beacon Street. are reducing agepts relatively to the higher oxides and There is no evidence in the color of the melt or in the since the higher oxides are oxidizing agents relatively graph of the data for any substance between manganous to the lower oxides, it follows that alkalies raise the oxide and manganic oxide and yet the pressure relations chemical potential (electromotive force) of a redncing studied by Meyer and RiXgers (17) are conclusive as agent and acids raise the chemical potential (electro- to the existence of M a 0 1 as a stable solid phase. The motive force) of an oxidizing agent. This has been most probable explanation of the apparent discrepancy known for over forty years (15). If we mix a reducing is that M a o r is, as most people have assumed, an inter agent and an oxidizing agent in an alkaline or an acid mediate oxide and its formula should be written MnOsolution, the alkali will make the reducing agent more Mn20s. In the concentrations in question this oxide 267
is dissociated in the melt practically completely into MnO (colorless) and Mnz03 (pink) or into the corresponding horates. Bancroft and Nugent (18) found precisely similar results for precisely similar reasons in the case of copper oxide in borate melts. When cupric oxide was added to a borate melt in air containing 4.6 mol per cent. N%O, 21 per cent. of the copper was present in the cuprous state. With about 21 mol per cent. N a O the cuprous copper fell to about 4 per cent. This accounts for the different behavior of copper oxide in the Egyptian and Persian glazes as opposed to the Italian glazes. The Egyptians used a glaze rich in soda and obtained a blue, because the copper was present almost completely as cupric salt. The Italians used a lead glaze which was much less alkaline, and enough of the cupric oxide changed to cuprous oxide to give a green color. I hope before long to have similar data in regard to other elements so as to show that the phenomena are entirely general. As a matter of fact these data of Nugent were not necessary. There are plenty of data in the literature to show that a lower oxide in an a l h lime solution is oxidized fairly readily by air; but nobody seems to have seen the significance of this. The following quotations will be conclusive. "Chromic oxide is unchanged when heated alone in air, the hydroxide is not so stable, and in presence also of certain metallic oxides under appropriate conditions of pressure, oxygen is readily absorbed. A mixture of chromic oxide with an alkali or an alkaline earth can he completely oxidised hy oxygen a t a red heat, whilst if oxygen is supplied to the alkali-fusion in the form of potassium nitrate or chlorate, the reaction tskes place even more readily, yielding a yellow, soluble mass of an alkali chromate. "Alkaline solutions of chromic salts are easily oxidised t o chromates by chlorine, bromine, hydrogen peroxide, persulphates, and a number of other oxidising agents: Cr.0,
+ lOKOH + 3Bn = 2K2Cr0, + 6KBr + 5H.O.
Acid solutions are also susceptible t o oxidation, though less easily, by potassium permanganate, lead dioxide, manganese dioxide, ceric nitrate, and particularly by persulphates (19)." "Accordine t o %yon traces of chromic acid are formed by the anodic oxidation of chromic oxide in pure water. Much more rapid oxidation ensues in the presence of calcium or potassium ,en,,S nyaroxlae (a,. "when titanium sesquioxide is shaken with milk of lime in the presence of oxygen it is oxidised to the dioxide. More oxygen is absorbed than is necessary for this change, whilst hydrogen peroxide is formed in amount corresponding to the whole of the oxygen absorbed. Water must therefore take part in thereaction. and the phenomenon is probably a case of autoxidation." "The maneanese monoxide obtained bv reducinn .--a hizher . oxide a t low temperature in hydrogen was found by Wright and Menke not to be spontaneously oxidised in air unless free alkali is present; but, if a trace of alkali be present, the monoxide takes up oxygen from the air and acquires a brown or black film. . . While crystalline manganous hydroxide alters very slowly in air, the oxidation proceeds rapidly in the presence of alkali. . . When manganosic oxide is heated with alkali carbonate and nitrate out of contact with air, the nitric oxide is given off: 6MmOa 2KNOs = 9Mn10s KzO 2NO (23)."
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In alkaline solutions manganese hydroxide in mass or colloidally dispersed is oxidized quantitatively (22) to MnZOaor to Mn02 depending on conditions. By the action of potassium cyanide on manganous carbonate
one gets potassium manganocyanide, which oxidizes in the air to potassium manganicyanide, from which hydrous manganic oxide, Mn203, can be obtained by hydrolysis. We know also that a ferromanganese anode in caustic potash goes direct to permanganate. This is not quite analogous because the anode may be equivalent to highly active oxygen. "Mitscherlich prepared potassium manganate, KsMnO.. by heating a mixture of any oxide of manganese with potassium hydroxide, carbonate and nitrate, or a mixture of potassium hydroxide and chlorate. . . According to Chevillot and Edwards, a mixture of 44 parts of manganese dioxide with four times its weight of potassium hydroxide absorbs 9.4 t o 10.4 parts of oxygen when it is heated in oxygen gas. . . .According to Mitscherlich, if manganese dioxide is heated with the alkali hydroxide in a closed vessel, the oxygen required for the formation of the manganate is derived from the manganese dioxide: 3Mn01 = MnlO. , MnO.. ---.-" - " . .. "According to Askenasy and Klonowsky, the potassium manganate of the highest degree of oxidation which can be prepared by the oxidation of an excess of the lower oxides of manganese in the presence of potassium hydroxide, in an atmosphere containing oxygen, contains a small excess of that alkali. The results obtained hy starting with manganese dioxide or with manganese hemitrioxide are almost the same. In both cases the quantity of manganate farmed increases as the temperature rises up t o the point a t which the pressure of the oxygen is equal to the dis~ciationpressure of the manganate; a t higher temperature the manganate is, of course, decomposed. In air the best temperature is about 600'; in oxygen it is near 700'. Using an excess of manganese oxide, some 60 to 65 per cent of the potassium hydroxide employed is converted into manganate under the best conditions" (23).
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Bismuth tetroxide, BizOl, can he obtained by melting the trioxide with an excess of alkali hydroxide while exposed to the air. According to Rose, F r h y , and Mitscherlich the alkalime solution of antimony trioxide is oxidized by exposure to the air, and the antimonites are oxidized when fused with alkali hydroxide in air. Since oxygen is a weaker oxidizing agent in alkaline solutions and since the reaction between the basic oxides and oxygen take place more readily in alkaline solutions. we are not accurate theoreticallv in saving that oxidizes these basic oxides, It.is the dasiG oxides which are activated and which reduce oxyzen. Under ordinary coniiitions it does not make much difference which one says; but there are a number of cases -~~ in which it is the that is activated " ap-ent and it will simplify matters a good deal if we are clear on the theory. The organic chemists do a good deal of oxidation with alkaline &manganate, wh&h seems to the physical chemist at first sight to be a barbarous thing to do, because alkaline permanganate is a much weaker oxidizing agent than acidified permanganate. The matter is of course determined in part by the question of possible secondary reaction due to alkali or acid; hut there will be advantages in the use of alkaline permanganate in case the organic substance is activated sufficiently by alkali. Donath and Ditz (24) say that, in general, sulfuric acid solutions of potassium permanganate act less strongly on organic substances than does alkaline per-
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manganate and they give the true reason, without understanding it-that the oxidation products are mostly of an acid nature. On the other hand, benzene is oxidized much more readily by acid permanganate than by alkaline permanganate. According to Mellor, acyl compounds are oxidized more readily by acid permanganate than by alkaline permanganate. The alkaloids are often oxidized with acid permanganate. Chromic acid is an oxidizing agent of an entirely different type from permanganate. Potassium dichromate does not react with hydrogen set free electrolytically a t a platinum cathode, while a mixture of chromic acid and sulfuric acid is a powerful oxidizing agent. We oxidize organic substances with alkaline permanganate solutions; but nobody would thmk of substituting an alkalme chromate solution. On the other hand, a strongly alkaline solution of permanganate will oxidize green chromic oxide to chromate. While the metallic oxides are oxidized more readily by air in an alkaline medium, that is not true of iodides. Air will oxidize hydriodic acid to iodine; but it will not, under ordinary conditions, oxidize neutral or alkaline potassium iodide to iodine, hypoiodite, or iodate. Alkaline permanganate will not oxidize oxalic acid; but acidified permanganate will. The theory as outlined in this paper makes it possible to put the chemistry of hydrogen peroxide on a more rational basis. This will be discussed in a later paper. I hope also at some time to show that Lenssen (26) would probably have cleared up the whole subject nearly seventy-five years ago if he had done electromotive force measurements on his reducing and oxidizing agents under conditions of varying alkalinity and acidity. The general conclusions of this paper are: 1. The lower oxides of a metal are more basic and the higher oxides are more acidic. 2. Since the higher oxides are more acidic the presence of alkali will facilitate the oxidation of the lower oxides if other things are the same. 3. When added to fused boric acid in contact with air. everv oxide of manzanese goes down to manzanous
chemical potential (electromotive force) of a reducing aeent and deaeaqes that of an oxidizine aeent. 8. It has been known for over forty years that an alkali increases the electromotive force of a reducing agent and deaeases the electromotive force of an oxidizing agent; but the reason for this has not hitherto been clear. 9. Whether the reaction between a reducing agent and an oxidizing agent will take place better in an alkaline or an acid solution depends on the relative displacements of the reduction potential and oxidation potential by alkali and acid. 10. Alkaline pyrogallol is more effective than acidified pyrogallol in reducing oxygen or the silver bromide of a photographic film. 11. Acyl compounds and benzene are oxidized more readily by acidified permanganate than by alkaline permanganate; but the reverse is true for many organic compounds. On the other hand, acidified permanganate oxidizes oxalic acid and alkalime permanganate does not oxidize sodium oxalate. 12. Alkaline chromate solutions have practically no oxidizing power under ordinary conditions, so oxidations of organic compounds, excluding photochemical oxidations, are practically always carried out with some form of acidified dichromate solution. 13. Permanganate and dichromate represent two extreme types of oxidizing agents. Permanganate can be used in many cases either in alkaline or acid solutions, dichromate only in acid solutions. Under the influence of light dichromate is an oxidizing agent, but not an alkaline chromate solution so far as I know. 14. The first draft of this paper was written before Lenssen's papers were discovered The fundamental principles as developed here are those of Lenssen.
4. Wh& cupric oxide is added to fused boric acid in contact with air, there is more cuprous oxide formed than when more alkali is present, bas been pointed out previously this is why the alkaline glazes of the Egyptians and Persians are cololed blue by copper, while the less alkaline lead glazes of the Italians are colored 'preen. , 5. A similar behavior is to be expected with other metallic oxides, such as vanadium, uranium, etc. 6. Air oxidizes hydriodic acid to iodine; but will not under ordinary conditions oxidize neutral or alkaline sodium iodide to iodine, hypoiodite, or iodate. 7. Since the lower basic oxides are reducing agents to the higher of the metal, we may say generally that alkali raises the
(7) (8) (9) (10) (11) (12) 13) 114) (15) (16) (17) (18) (19) (20) (21) (22)
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LITERATURE CITED
(1) Roscoe AND SCHORLEMMER, "A treatise on chemistry," 6th pd:, Macmillan Co., New York City, Vol. 11, 1923, pp. 11b5.
(2) M e ~ ~ o"A n , comprehensive treatise on inorganic and t h o retical chemistry," Longmans, Green & Co., New York XI, ... . City, . . . Vol. . . . . . .1931, .._p._176.
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23) 124) (25) (26)
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MELLOR,I O C ~ CV~~~ I~ . .,1 , 1 9 3 1p, , 745. ROSCOEAN^ SCHORLEMMER, ~ O C . it., VOI. 11, 1923, P. 949. Ibid., Vol. 11, 1923, p. 1037. MELLOR, 106. cit., Vol. IX, 1929, p: 589. ROSCOEAND SCHORLEMMER. loc. ctt., Vd. 11. 1923, p. 1008. bid., vol. 11, 1923, p. 915. Munnay, J. Phys. Chem., 35, 1011 (1931). MELLOR,lac. cil., Vol. 111, 1923, p. 139. B~NcRom,Z,physik. Che(ll., 387 (1892). BANCROW AND NUGENT.J . Phys. Chcm., 33, 481 (1929). Meyen AND R ~ T G E R Z.~anow-Chnn., , 57, 104 (1098). BANCRORT AND NUGENT. J. Pkys. C h m . . 33, 729 (1929). Roscoa AND S C I ~ O R L E ~lot, E Rcil., , VOI. 11, 1923, p. 1059. MELLOR,loc. cit., vol. XT, 1931, p. 180. MELLOR,lac. ~ $ 1 . .Vd. XIl, 1932, PP. 223, 234. METER,Z. enorg. Chcm., 81, 385 (1913); Z. enorg. ellgcm. Chem., 116, 117 (1921): 155,66 (1926). MELLOR,loc. zit., Vol. XII, 1932, P. 283. DONATH AND DITZ.J. pprakl. Chew., 121. 60, 566 (1899). WANKLXNAND COOPER, Phil. Mag., [ 5 ] ,7, 138 (1879). LENSSEN.1. prakt. Chenz.. 78, 193 (1859); 81, 276 (1860).
