On the Interfacial Potential Differences for the Extraction of Alkaline- Earth Metal Picrates by 18-Crown-6 Ether Derivatives into Nitrobenzene

Abstract

ndividual distribution constants ( K D,A ) of picrate ion (Pic − ) into nitrobenzene (NB) were determined at 298 K and given values of ionic strength, together with the determination of an extraction constant ( K ex ) for the extraction of alkaline-earth metal picrates, MPic 2 , by 18-crown-6 ether (18C6) and benzo-18C6 (B18C6). Here, K D,A and K ex were defined as [A − ] NB /[A − ] and [MLA 2 ] NB /[M 2+ ][L] NB [A − ] 2 , respectively: A − shows Pic − , L does either 18C6 or B18C6 and the subscript “NB” refers to the NB phase. Interfacial potential differences (∆ φ eq in a V unit) at extraction equilibria were evaluated from differences between the experimental log K D,Pic and its standardized values (log KD , Pic S ) which have been determined by electrochemical measurements at a water/NB interface. By a combination with the K ex values, other extraction constants, K ex± =[MLPic + ] NB [Pic − ] NB /[M 2+ ][L] NB [Pic − ] 2 , were also determined. Electrochemically- standardized log K ex± values, log K ex± S ones, were calculated from the relation of log K ex± = (∆ φ eq /0.05916) + log K ex± S at 298 K. Using the log K ex± S values, their correlation with the log K ex± ones was discussed. Additionally, ion-pair forma- tion constants ( K 1,NB & K 2,NB ) for the stepwise reactions of ML 2+ with Pic − in the NB phase were evaluated from K ex , K ex± and another constant ( K ex2± ) reported before at ∆ φ eq = 0. Here, K 2,NB was evaluated from K ex / K ex± and K 1,NB is defined as [MLPic + ] NB /[ML 2+ ] NB [Pic − ] NB , which was evaluated from the relation of K 1,NB = K ex± / K ex2± . Moreover, reproductions of the electrochemically-standardized K ex± and K ex2± values were tried, using the ∆ φ eq values and relations between K ex± or K ex2± and component equilibrium constants constituting their extraction ones. Consequently, a functional expres - sion of K ex± with ∆ φ eq was extended into the MPic 2 -L extraction systems, in addition to the AgPic-L extraction ones reported previously