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Free Energies of Formation and Aqueous Awrican Mineralogist Vol. 57, pp. 1163-1789 (1972) / FREE ENERGIESOF FORMATION AND AQUEOUS SOLUBILITIESOF ALUMINUM HYDROXIDES AND OXIDE HYDROXIDES AT 25"C GnoncnA. P.e,nrs,Departntant of ApptiedEarth Scienaes,StanJord Uniu ersity, Stanl ord, Cal'it ornia I d305 AssrBAc{ comparison of the standard free energies of formation at 25'c of corundum, diaspore, boehmite, gibbsite, and bayerite estimated from carefully selected uq,ruor6 solubility data with those of calorimetric origin .permits selection of a set which is more consistent with stabilities observed in field and laboratory than the calorimetric set alone. The selection process involves acceptance of -378'2 and -1160 kcal per mole for AG'1 of aALo" and Al"* (aq) respectively and assignmentof -311'0 kcal per mole for the AG'1 of AI(OE); (aq)' Selected values for the solids are: kcal/Mole diaspore -219.5+ 0.5 boehmite -218.7+0.2 gibbsite -275.3+ 0.2 bayerite -274.6+ O.L Solubility-pE curves based on these asignments agree with independent solubility data only if it is assumed that slight supersaturation rezults during many experiments and leads, upon aging toward equilibrium, to precipitation of gibbsite or bayerite, depending on pE. One set of data which suggeststhis inter- pretation is discussed. Isrnopucrrow There is convincing evidencein phase equilibrium investigations that corundum is unsiable with respectto gibbsite in the presenceof of water aL25" and onebar pressure(Kennedy, 1959). Unfortunately, tabulated free energiesof formation (Wagmanet aI., L968,and Robie and Waldbaum, 1968) predict the opposite.During an attempt to find free energiesof formation with which to calculatesolubilities of the aluminum hydroxides,discovery of this discrepancyrevealed the need for critical appraisalof availabledata. It is the purposeof this paper to selectthe most reliable free energiesof formation available for the hydroxidesand oxide hydroxidesof aluminum. For the solids,this is done by critically reviewing the sourcesof tabulated free energiesof formation, which ale largely calorimetric, and. comparing them with independent data derived from solubility measurements.Derivation of free energiesfrom solubility data re- quired review of the free energiesof formation of Als-(aq) and 1163 1164 GEOBGE A. PARKS Al(OH)n (aq). Followingselection of the freeenergies of formationof the mono-hydroxoeomplexes it is shorvnthat polymerichydroxo com- plexescan be ignoredat equilibrium and an attempt is madeto check the validity of the entire set of selectedfree energiesby comparing calculatedsolubility-pH curveswith an independentset of solubility data. Teeur,ertD Fnpn Ewnncros ol' Fonnrerron oF THE Sor,rm Most tabulations of thermodynamic data rely heavily on U.S. Bureau of StandardsCireular 500,Selected Values of ChemicalTher- modgnamicProperties (Rossini et at., Lgb2) andOridation Potentials (Latimer, 7952). The Bureau of Standardsis updating Circular S00 and has releasedinterim tables, omitting references.Data for alu- minum appearin TechnicalNote 220-8 (Wagmanet cll.,1g68), one of the interim series.original referencescited for data attributed to Wagmanet aI. in the followingparagraphs were provided by Wagman in personal communicationsto the author (D. D. Wagman, 1g69, written communication). The u.s. Geologicalsurvey has arsorecently publisheda tabulation of thermodynamicdata (Robieand Waldbaum, 1968).The many publieationsof Pourbaixand his co-workers,col- lected in lhe Atlas of Electro-chemicalEqui.Iibria in Aqueou,ssolu- tions (Powbaix, 196G)comprise still anoiher exhaustivesource of thermodynamicdata. The original sourcesof free energiesof formation in thesetabula- tions have been examinedfor information useful in permitting selec- tion arnongdiffering estimates. Corund,um,aAlzos The free energyof formation of corundumreported by Rossinief al. (1952) is -376.77 kcal/mole. The value given by Wagman et aI. (1968)is -378.2 kcal/mole.The latter is basedon heat capacities listedby Kelley and King (1901)and heatsof combustionof alumi- num reported by Roth, Wolf, and Fritz (1940), Snyder and Seltz (1945),Holley and Huber (1951),Schneider and Gattow (19b4),Mah (1957),and Kocherov,Gertman, and Geld (lgbg). The product of eombustionwas invariably identifiedby X-ray diffraction.Robie and Waldbaum (1968)report -378.082 -L 0.810on the basisof heat ca- pacity data attributed to Kelley and King (1961) and Furukawa et al. (7956)and the heat of formationreported by Mah (lgb7). The differencebetween 1g68 and 1g52estimates reflects new work. It requires revision of free energiesof formation of ail other sub- SOLT]BILITIES OF ALT]MINU M HYDROXIDES 1r65 stancesfor which corundum served as a referencematerial. This adjustmenthas beenmade where necessary in the paragraphsto follow. Diaspore,aAIOOH Rossini et at. (1952) list data for a monohydratenot identifiedby name. The free energy of formation of diasporegiven by Wagman et al., (1968) is -220 kca!/mole. It is basedon an estimateof the enthalpy changeassociated with dehydrationderived from calibrated differential thermal analysis work (Foldvari-Vogl and Kiburszky, 1958)and on low temperatureheat capacitydata reportedfor diaspore by King and Weller (1961).Robie and Waldbaum (1968)list only the entropy of diaspore,citing King and Weller (1961) and Kelley andKing (1961). Fyfe and Hollander (1964) reporbthe free energyof formation of diasporeas -219.5 kcaf/mole.This estimateis basedon the tempera- ture at which diaspore and corundum coexist at equilibrium with liquid water and an estimateof the entropy neededto correctto 25'C' The estimatedprobable error from all sourcesis :t0'44 kcal/mole' Fyfe and Hollander took extremepains to insure equilibrium, but DTA methods are dynamic hence unlikely to produce equilibrium reaction products,thus -219.5 I 0.5 is probably the best estimate of the free energyof formation of diasporefrom thesesources. Boehmite,YAIOOH The free energyof formation of boehmitegiven by Wagman et al' (1963) is -218.15 kcal/mole.It is basedon measurementsof the enthalpy changeassociated with its dehydrationreported by Calvet (1962),Foldvari-Vogl and Kiburszky (1953),Michel (1957b),and Eyraud et aL. (1955) and heat capacity data originally reportedby Shomateand Cook (1946). Robie and Waldbaum (1968) report -217.67:L 3.5 kcal,/mole,basing their selectionon Rossini et oZ' (1952) and the same heat capacity data. The heat of formation reported by Rossini et al., is a calculatedestimate for an AIOO'H which is not identifiedas boehmite. There are severalsources of uncertainty in the data for boehmite. Most important is ambiguity in identificationof the solid usedin the heat capacity ureasurementsby Shomateand Cook. The purity of boehmitesused in dehydration experimentsand the characterization of dehydrationproducts are also doubtful. Shomateand Cook (1946)used a monohydrateof correctchemical compositionbut reportedthat "X-ray examinationshowed . the 1166 GEONGEA. PARKS monohydrate[to have] a structuresimilar to bayerite (AITOB.BH2O) differingfrom the latter principally in the intensitiesof rines.,,Neither they nor Rossiniet al. (1g52)identified the solid as boehmite.Beeause the x-ray diffraction patterns for bayerite and boehmite differ in the presenceor absenceof major lines as well as in relative intensities of lines, identificationof the solid as boehmiteis not justified on the basis of their data alone. Kelley and King (1g61) were the first to to usethe data of shomateand cook referringto the soridas boehmite. may be uncertain by as much as a few percent.They are adequare, however,for ordinary heat balance calculations.,,Kennedy (lg5g) refers to low temperature heat capacity measurementsmade by Kelley and King on a boehmitehe suppliedbut there is no mentionof new measurementsin publicationsafter lgbg. Eyraud et al. (1955) reported 4 percent crystalline trihydrate in their boehmite. Eyraud et al. also point out that the dehydration product is, at least in some instances,a mixture of polymorphsof AlzOa.Since the other authors reporting dehydration did not repoft characterizationof the dehydration product, some doubt must be admitted about the phasepurity and identity of the oxide. In view of thesecriticisms, little or no confidencemay be placedin the validity of tabulated free energiesof formation of boehmite. Gibbsite,dAl(OH)s TIre free energy of formation of gibbsite given by Wagman et aI. (1968) is -273.35 kcalmole. It was selectedby considerationof heat pacity and the heat of solutionin B0 percentNaoH solutionsprovided by Roth, Wirths, and Berendt (1942) and the heat of formation derived from HF solution calorimetry by Barany and Kelley (196l). 'Waldbaum The free energy of formation reported by Robie and (1968)is -273.486 -t 0.310kcal/mole. It is basedon the heat capacity data of shomateand cook (lg4o) and the heat of formation reported by Baranyand Kelley (1961). The free energy of formation reported by pourbaix (1g66) is -277.32 kca/mole. It is based on the solubility measurementsof SOLT]BILITIESOF ALUMINUM HYDROXIDES 1167 Fricke and Jucaitis (1930)and a freeenergy of formationof Al (OH) + . The free energyof formation of AI(OH)+ was derivedfrom the solu- bility of boehmite (Fricke and Meyering, 1933) and a free energy of formation of boehmite attributed to Latimer (1952). Latimer's soutcewas Rossiniet al. (1952),hence his flgure is invalid. This, to- gether with the high concentrationsof the solvents used by Fricke and Jucaitis (1930) and Fricke and Meyering (1933), which make activity corrections doubtful, unfortunately invalidates Pourbaix's
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