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| '''Specific ion Interaction Theory''' (SIT theory) is a theory used to estimate single-[[ion]] [[activity coefficient]]s in [[electrolyte]] solutions at relatively high concentrations.<ref name=G/><ref name=C/> It does so by taking into consideration ''interaction coefficients'' between the various ions present in solution. Interaction coefficients are determined from [[equilibrium constant]] values obtained with solutions at various [[ionic strength]]s. The determination of SIT interaction coefficients also yields the value of the equilibrium constant at infinite dilution.
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| == Background ==
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| The need for this theory arises from the need to derive activity coefficients of solutes when their concentrations are too high to be predicted accurately by [[Debye–Hückel theory|Debye-Hückel]] theory. These activity coefficients are needed because an [[equilibrium constant]] is defined in [[thermodynamics]] as a quotient of [[Activity (chemistry)|activities]] but is usually measured using [[concentration]]s. The protonation of a [[monobasic]] [[acid]] will be used to simplify the exposition. The equilibrium for protonation of the [[conjugate base]], A<sup>-</sup> of the acid, may be written as
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| :A<sup>-</sup> + H<sup>+</sup> {{eqm}} AH
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| for which
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| :<math>K=\frac{\{HA\}}{\{A^-\}\{H^+\}}</math>
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| where {HA} signifies an activity of the chemical species HA ''etc.''. The role of water in the equilibrium has been ignored as in all but the most concentrated solutions the activity of water is a constant. Note that ''K'' is defined here as an ''association'' constant, the reciprocal of an [[acid dissociation constant]].
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| Each activity term can be expressed as the product of a concentration and an [[activity coefficient]]. For example,
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| :{HA} = [HA] × γ<sub>HA</sub>
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| where the square brackets signify a concentration and γ is an activity coefficient. Thus the equlilbrim constant can be expressed as a product of a concentration quotient and an activity coefficient quotient.
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| :<math>K=\frac{[HA]}{[H^+][A^-]}\times \frac{\gamma_{HA}}{\gamma_{H^+}\gamma_{A^-}}</math> | |
| Taking logarithms.
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| :<math>\log K =\log K^0 + \log \gamma_{HA} - \log \gamma_{H^+} - \log \gamma_{A^-}</math>
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| ''K''<sup>0</sup> is the hypothetical value that the equilibrium constant would have if the solution of the acid were so dilute that the activity coefficients were all equal to one.
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| It is common practise to [[Determination of equilibrium constants|determine]] equilibrium constants in solutions containing an electrolyte at high ionic strength such that the activity coefficients are effectively constant. However, when the ionic strength is changed the measured equilibrium constant will also change, so there is a need to estimate individual (single ion) activity coefficients. [[Debye–Hückel equation|Debye-Huckel]] theory provides a means to do this, but it is accurate only at very low concentrations. Hence the need for an extension to Debye-Hückel theory. Two main approaches have been used. SIT theory, discussed here and [[Pitzer equations]].<ref>{{cite journal|last=Pitzer|first=K.S.|year=1973|title=Thermodynamics of electrolytes, I. Thoretical basis and general equations|journal=J. Phys. Chem.|volume=77|pages=268–277|doi=10.1021/j100621a026}}</ref><ref>{{cite book|last=Pitzer|first=K.S.|title=Activity coefficients in electrolyte solutions |publisher=CRC Press|location=Boca Raton, Fla|year=1991|isbn=0-8493-5415-3}}</ref> | |
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| == Development ==
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| SIT theory was first proposed by [[Brønsted]]<ref>{{cite journal| last=Brønsted |first=J.N. |year=1922 |title=Studies on solubility IV. The principle of the specific interaction of ions|journal=J. Am. Chem. Soc.|volume=44|pages=877–898|doi=10.1021/ja01426a001}}</ref> and was further developed by Guggenheim.<ref name=G>{{cite journal|last=Guggenheim|first=E.A.|coauthors=Turgeon, J.C.|year=1955|title=Specific interaction of ions|journal=Trans. Faraday Soc. |volume=51|pages=747–761|doi=10.1039/TF9555100747}}</ref> Scatchard.<ref>{{cite journal| last=Scatchard |first=G. |year=1936 |title=Concentated solutions of strong electrolytes|journal=Chem. Rev.|volume=19|pages=309–327|doi=10.1021/cr60064a008}}</ref> extended the theory to allow the interaction coefficients to vary with ionic strength. The theory was mainly of theoretical interest until 1945 because of the difficulty of determining equilibrium constants before the [[glass electrode]] was invented. Subsequently Ciavatta<ref name=C>{{cite journal|last=Ciavatta|first=L.|year=1980|title=The specific interaction theory in the evaluating ionic equilibria|journal=Ann. Chim. (Rome)|volume=70|pages=551–562}}</ref> developed the theory further.
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| The activity coefficient of the ''j''th ion in solution is written as γ<sub>j</sub> when concentrations are on the [[molal]] [[concentration]] scale and as ''y''<sub>j</sub> when concentrations are on the [[molar concentration]] scale. (The molality scale is preferred in thermodynamics because molal concentrations are independent of temperature). The basic idea of SIT theory is that the activity coefficient can be expressed as
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| :<math>\log \gamma_j= -z_j^2 \frac{0.51 \sqrt I}{1+1.5 \sqrt I}+\sum _k \epsilon_{jk} m_k</math> (molalities)
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| :<math>\log y_j= -z_j^2 \frac{0.51 \sqrt I}{1+1.5 \sqrt I}+\sum _k b_{jk} c_k</math> (molar concentrations)
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| where ''z'' is the electrical charge on the ion, ''I'' is the ionic strength, ε and ''b'' are interaction coefficients and ''m'' and ''c'' are concentrations. The summation extends over the other ions present in solution, which includes the ions produced by the background electrolyte. The first term in these expressions comes from Debye-Hückel theory. The second term shows how the contributions from "interaction" are dependent on concentration. Thus, the interaction coefficients are used as corrections to Debye-Hückel theory when concentrations are higher than the region of validity of that theory.
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| The activity coefficient of a neutral species can be assumed to depend linearly on ionic strength, as in
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| :<math>\log \gamma = k_m I\,</math>
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| where ''k''<sub>m</sub> is a Setschenow coefficient.<ref>{{cite journal| last=Setchenow| first=I.M.| year=1892|journal=Ann. Chim. Phys. |volume=25|pages=226}}</ref>
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| In the example of a monobasic acid HA, assuming that the background electrolyte is the salt NaNO<sub>3</sub>, the interaction coefficients will be for interaction between H<sup>+</sup> and NO<sub>3</sub><sup>-</sup>, and between A<sup>-</sup> and Na<sup>+</sup>.
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| == Determination and application ==
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| Firstly, equilibrium constants are determined at a number of different ionic strengths, at a chosen temperature and particular background electrolyte. The interaction coefficients are then determined by fitting to the observed equilibrium constant values. The procedure also provides the value of ''K'' at infinite dilution. It is not limited to monobasic acids.<ref>{{cite journal| last=Crea| first=F.| coauthors=De Stefano, C.; Foti, C.; Sammartano, S.|year=2007|title=Sit parameters for the dependence of (poly)carboxylate activity coefficients on ionic strength ...|journal=J. Chem. Eng. Data| volume=52| pages=2195–2203}}</ref> and can also be applied to metal complexes.<ref>{{cite journal|last=Ciavatta|first=L.|year=1990|title=The specific interaction theory in equilibrium analysis. Some empirical rules for estimate interaction coefficients of metal ion complexes|journal=Ann. Chim. (Rome)|volume=80|pages=255–263}}</ref> The SIT and Pitzer approaches have been compared recently.<ref>{{cite journal|last=Elizalde |first=M. P. |coauthors=Aparicio, J. L. | title=Current theories in the calculation of activity coefficients—II. Specific interaction theories applied to some equilibria studies in solution chemistry |year=1995| journal=Talanta| volume=42| issue=3|pages=395–400 |doi=10.1016/0039-9140(95)01422-8|pmid=18966243 }}</ref> The [[Bromley equation]]<ref>{{cite journal |last=Bromley |first=L.A. |year=1973 |title=Thermodynamic properties of strong electrolytes in aqueous solutions |journal=AIChEJ|volume=19|issue=2|pages=313–320|doi=10.1002/aic.690190216}}</ref> has also been compared to both SIT and [[Pitzer equations]].<ref>{{cite journal|last=Foti|first=C.|coauthors=Gianguzza, A.; Sammartano, S.|year=1997|title=Comparison of equations for fitting protonation constants of carboxylic acids in aqueous tetramethylammonium chloride at various ionic strengths |journal=Journal of Solution Chemistry|volume=26|issue=6|pages=631–648|doi=10.1007/BF02767633}}</ref>
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| == References ==
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| {{reflist}}
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| == External links ==
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| *[http://www.acadsoft.co.uk/aq_solutions.htm SIT program] A PC program to correct stability constants for changes in ionic strength using SIT theory and to estimate SIT parameters with full statistics. Contains an editable database of published SIT parameters. It also provides routines to inter-convert MolaRities (c) and MolaLities (m), and lg K(c) and lg K(m).
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| [[Category:Equilibrium chemistry]]
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| [[Category:Thermodynamics]]
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