THERMODYNAMICS OF ELECTROLYTES. XI. PROPERTIES OF 3-2, 4-2, AND OTHER HIGH-VALENCE TYPES

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Various thermodynamic properties are considered for very high-valence 3-2 and 4-2 electrolytes in water at room temperature. These solutions show the behavior described by Davies in which ion pairing arises as the concentration increases follow by re-dissociation at still higher concentrations. Heat of dilution data, which extend below 10{sup -4} M, are interpreted with the same form of equation used earlier for 2-2 electrolytes. Activity and osmotic coefficient data do not extend to low enough concentration for independent, interpretation, but they are treated with the aid of conductance data in the more dilute range. Parameters are reported for A{ell}{sub 2}(SO{sub ... continued below

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15 p.

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Pitzer, Kenneth S. & Silvester, Leonard F. December 1, 1977.

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Various thermodynamic properties are considered for very high-valence 3-2 and 4-2 electrolytes in water at room temperature. These solutions show the behavior described by Davies in which ion pairing arises as the concentration increases follow by re-dissociation at still higher concentrations. Heat of dilution data, which extend below 10{sup -4} M, are interpreted with the same form of equation used earlier for 2-2 electrolytes. Activity and osmotic coefficient data do not extend to low enough concentration for independent, interpretation, but they are treated with the aid of conductance data in the more dilute range. Parameters are reported for A{ell}{sub 2}(SO{sub 4}){sub 3}, La{sub 2}(SO{sub 4}){sub 3}, In{sub 2}(SO{sub 4}){sub 3}, and several cyanoferrates. High-valence electrolytes show a special behavior at very low concentrations which was recognized by Bjerrum who showed in 1926 that purely electrostatic forces would yield an ion association. Davies showed that this association commonly reached a maximum at an intermediate concentration above which there was a re-dissociation. From one viewpoint, this ion association is an artifact of the linearization approximation in the Debye-Hueckel theory since a more exact statistical treatment yields agreement with experiment without assuming a separate associated species. If an association equilibrium is assumed for these electrolytes, it is found that the value of the association constant depends on the assumptions about the activity coefficients of the ions. To the extent that these effects are important for 2-2 electrolytes, they are discussed in paper III{sup 6} of this series. While in 3-2 and 4-2 electrolytes the effects are of the same nature as those in the 2-2 solutes, they occur at much lower concentration in the higher-valence solutes; consequently new problems arise in treating experimental data. Indeed, it is only the conductance and heat-of-dilution measurements, which extend down to 10{sup -5} M, that show clearly these effects for the 3-2 and 4-2 electrolytes in water. Thus it was not possible to give an adequate treatment of osmotic and activity coefficients for such solutes without considering these other properties which can be measured at lower concentrations. While conductance data have been reasonably interpreted following Davies and others, the reports of enthalpy data for very high valence electrolytes left the behavior as apparently anomalous. Thus Nathan, Wallace, and Robinson stated that the heat of dilution of La{sub 2}(SO{sub 4}){sub 3} solutions fails to approach the Debye-Hueckel limiting law even at 2 x 10{sup -5} M and Lange and Miederer reported similar conclusions for several 3-2 and 4-2 solutes. We shall show that these data may be given a reasonable interpretation consistent with the limiting law and that these results assist in estimating activity coefficients for high-valence electrolytes.

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15 p.

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  • Journal Name: Journal of Physical Chemistry

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  • Report No.: LBL-7021
  • Grant Number: DE-AC02-05CH11231
  • Office of Scientific & Technical Information Report Number: 1004894
  • Archival Resource Key: ark:/67531/metadc837478

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  • December 1, 1977

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  • May 19, 2016, 3:16 p.m.

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  • July 26, 2016, 5:49 p.m.

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Pitzer, Kenneth S. & Silvester, Leonard F. THERMODYNAMICS OF ELECTROLYTES. XI. PROPERTIES OF 3-2, 4-2, AND OTHER HIGH-VALENCE TYPES, article, December 1, 1977; Berkeley, California. (digital.library.unt.edu/ark:/67531/metadc837478/: accessed September 21, 2018), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.