Direct Spectrophotometric Measurements of Acid-catalyzed Complexation of Palladium(II) with 2-(5-Bromo-2-pyridylazo)-5-diethylaminophenol at the Heptane/Water Interface by a Centrifugal Liquid Membrane Method

Abstract

Chemical reactions at the interfaces between two immiscible liquids have become a more interesting subject in various fields, including solvent extraction chemistry, biological chemistry and electrochemistry. 1,2 Although the interfacial complexation has been studied in some solvent extraction systems, catalytic reactions at the interface have not been reported enough compared with those in the aqueous phase. We have previously clarified the various reaction mechanisms proceeding at the liquid-liquid interface in solvent extraction systems by a highspeed stirring method 3-7 that can simultaneously measure the extraction rate of a metal complex and the interfacial concentration of a ligand or the complex from the change of the absorbance in the organic phase. However, this method has one drawback: it cannot directly measure the interfacial species. When the complex was not extracted into the organic phase and exists only at the interface, the complexation rate was indirectly determined from the consumption rate of the ligand in the organic phase in the high-speed stirring method. 6 Recently, in our laboratory we developed a new in situ spectrophotometric method, the centrifugal liquid membrane (CLM) method, to measure the species adsorbed at the liquid-liquid interface. 8,9 This method utilizes an ultra-thin two-phase liquid membrane in a rotating optical cell by a centrifugal force and can directly measure the interfacial species by transmission spectrophotometry or other spectrometries. But the previous rotating cell did not permit us to inject the sample solution rapidly while the cell was rotating. Therefore, there were some problems: fast reactions were difficult to measure and it was impossible to change the solution conditions in the course of the measurement. To solve these problems, in the present study we made a modified rotating cell that had a hole in the bottom.2-(5-Bromo-2-pyridylazo)-5-diethylaminophenol, 5-Br-PADAP or HL, has been known as a highly sensitive colorimetric reagent that forms colored complexes with various metal ions and has been used to detect trace amounts of various metal elements. [10][11][12] It was reported that 5-Br-PADAP formed a complex with Pd(II) with the mole ratio of 1:1. 13In the present study, we directly measured the formation kinetics of Pd(II)-5-Br-PADAP complex, which was not extracted into the heptane phase and existed only at the interface, from the spectral change in the heptane/water system. We clarified the kinetic mechanism of the catalytic effect of acid on the complexation reaction. Also, the advantage of the modified centrifugal liquid membrane cell in the kinetic study was demonstrated. ExperimentalReagents 5-Br-PADAP (Dojindo Lab.) was used as purchased and was dissolved in purified heptane. For the purification, heptane (G. R., Katayama Chemical) was stirred with concentrated sulfuric acid for one day (repeated three times), washed successively with distilled water and 5% sodium hydroxide aqueous solution, dehydrated by calcium chloride dehydrate overni...

Structural Evaluation of the Aggregate of Palladium(II)-Pyridylazophenol Complex at the Heptane-Water Interface by the Centrifugal Liquid Membrane Method

“…The absorption maximum wavelength of the aggregate was blue-shifted about 92 nm with respect to that of the PdLCl monomer complex. This may be attributed to the PdLCl-PdLCl interaction and suggests the formation of H-aggregates at the heptane-water [7]) of the PdLCl monomer at the heptane-water interface, it was a relatively large value. This suggested that the charged quinone structure was retained in the PdLCl aggregate.…”

“…In this method, the ultra-thin two-phase liquid membranes are produced in a rotating optical cell. This method enables us to measure absorption spectra of adsorbates effectively at the liquid-liquid interface [6][7][8], Figure 1. Structure of PdLCl.…”

Chemical Kinetics and Reaction Mechanisms in Solvent Extraction: New Trends and Applications

“…24 Liquid membranes combine extraction and stripping, which are normally carried out in two separate steps in conventional processes such as solvent extraction into one step. When a substance of interfacial absorptivity is dissolved in the organic phase and the interfacial area becomes wider by vigorous stirring, the substance concentration in the organic phase decreases according to the principle of mass balance.…”

“…Recently, detailed mechanism of the complexation reaction at the heptane/water or toluene/water interface was proved in the solvent extraction of metal ions. [24][25][26] …”

Catalytic Effect of Ionic Surfactants on the Extraction Rate of Nickel(II) and Palladium(II) with 2-(5-Bromo-2-pyridylazo)-5-diethylaminophenol in the Toluene/Water System

“…The centrifugal liquid membrane (CLM) method has some advantages in the evaluation of the interfacial reaction in extraction kinetics [11][12][13][14]. This method utilizes an ultra-thin two-phase liquid membrane created by centrifugal force in a rotating optical cell.…”