502. Standard potentials in aqueous organic media : a general discussion of the cell H₂(Pt)|HCl|AgCl–Ag

Abstract

Relations for the standard potential of the cell H,(Pt)IHClIAgCl-Ag in various aqueous organic media, based on the Born equation, are briefly reviewed, and the limitations of this method noted. A new stoicheiometric relationship is developed, based on a simple treatment of ion solvation in liquid mixtures. The resulting equation shows that a plot of the standard potential E," on the molar scale should be linearly related to the logarithm of the volume fraction #w of water in the solvent medium. The correlation achieved experimentally is considerably better than anything obtained with plots based on the Born equation, and is free from marked solvent effects. Certain observed deviations are discussed.FOLLOWING work on aqueous ketone and glycol solutions (cf. J., 1957, 2284), we can discuss the standard potentials of the cell H,(Pt)[HClIAgCl-Ag in a large number of aqueous solutions of the comrnoner water-soluble organic substances, particularly where the water content is high.In Table 1 are given all the available standard potentials of the cell for such systems on the molal (Em'), molar (E," = Emo + 2k log do), and mole fractional [EN' = Emo -2k log (lOOO/M,,)] scales ( k = 2.3026 RTIF, do = density of pure solvent mixture M , = lOO/[X/M, + (100 -X)/My] = mean molecular weight of solvent ( X = weight yo of organic substance of molecular weight Mx, and M y = molecular weight of water). Values of the dielectric constant E, and of the density do of each mixture are recorded. Where not given by the original author, the values of E are taken from Hikerlof's data,l and the do values from International Critical Tables or from GriEths's data.2*3 The mole fraction of water, N,, has been calculated for each mixture.The difference between the standard potentials, "E" in water and "E" in another solvent, is related to a freeenergy difference AG by the equations Discussion of the CeU H,(Pt) 1 HClI AgCl-Ag.-AG = F("E' -*E") = -2RT In aJa, = -2RT In flcl/f2c, AG is the difference in solvation energies of hydrogen chloride in the two solvents, referred to standard conditions in the gas phase and the standard state in the given solvent, andf. are mean ion-activity coefficients referred to a standard state of unity at infinite dilution in pure water, i.e., fi = 1. By the definition of standard potential, interionic attractions are eliminated, and we are concerned solely with ion-solvent interactions.It would be convenient to choose the standard state such that the work done in transferring the ions, considered uncharged, is zero; in other words, that terms of the type 2RT In cl}c, vanish. Classically this is assumed to be the case if the mole fraction of

Ionization constants ofo-phthalic acid in (propylene carbonate + water) and (ethylene carbonate + water), and thermodynamics of the cellPt|H2|HCl(m)| Hg2Cl2|Hg

“…From figure 2 it is evident that in water-rich mixtures the primary medium effect varies linearly with w s , and it is •, ethylene carbonate (present work); , propylene carbonate; (17) •, 1,4-dioxane; (19) , acetonitrile; (22) , methanol; (19)(20)(21) , ethanol; (25) , glycerol; (19)(20)(21) +, ethylene glycol; (23,24) ×, methylcellosolve; (26) * , 2-propanol. (19)(20)(21) S = W denotes the hypothetical cosolvent S equivalent to water.…”

“…Another application of the concept of primary medium effect is found in Feakins and French's (20) method of determining for HCl the primary hydration number n(hydr) (number of water molecules firmly bound to one HCl molecule). In such a context, the primary medium effect is plotted as (E o W −E o Z ) c (on the amount-of-substance concentration scale) against lgϕ w according to the linear relation:…”

“…Figure 2 shows the linear dependence of the primary medium effect, in terms of the standard potential difference (E o W − E o Z ), in keeping with Owen's definition, (16) on the cosolvent mass fractions w s of both EC and PC, comparison being made with literature results for other cosolvents. (19)(20)(21)(22)(23)(24)(25)(26) The (E o W − E o Z ) is a measure of the standard Gibbs energy change upon transferring the neutral pair of pivot H + and Cl − ions relevant to the reversible cell (III) from the m o = 1 mol · kg −1 standard state in water to the m o = 1 mol · kg −1 standard state in Z . From figure 2 it is evident that in water-rich mixtures the primary medium effect varies linearly with w s , and it is •, ethylene carbonate (present work); , propylene carbonate; (17) •, 1,4-dioxane; (19) , acetonitrile; (22) , methanol; (19)(20)(21) , ethanol; (25) , glycerol; (19)(20)(21) +, ethylene glycol; (23,24) ×, methylcellosolve; (26) * , 2-propanol.…”

The extrapolation of EMF data to infinite dilution in non-aqueous and mixed solvents

“…Pt 1 M 1 M'+x'-, v+ v-m , i n an appropriate solvent s 1 x 1 Pt ( 1 ) and i t s emf E can be expressed by the Nernstian equation a s : E = Eo -(vllT/v+z+F)ln(Zmy+) - (2) where m /(mol. {kg solvent}-') is the molality of the electrolyte E(:Xtr, y+ is the correv-l l v sponding mean molal a c t i v i t y coefficient, v = v+ + v-, V+Z+ = v-/z-I, Z L (vy' v-) , R = 8.31451 J.K-l.mo1-l is the gas constant, F = 96485.3 J*V-'*mol-' is Faraday's const a n t , and T is the absolute temperature.…”

“…I t is well hown t h a t E o is strongly dependent on the solvent of the supporting e l e c t r o l y t e , and t h i s f a c t was rationalised i n terms of the so-called primary medium e f f e c t ( r e f s . [1][2][3][4]. Among the various solvents, needless t o say, wat e r has the greatest importance and aqueous electrolyte solutions t a k e t h e l i o n ' s s h a r e o f prac t i c a l cases.…”