Spectrophotometric Determination of Dissociation Constant of Picrates in 4-Methyl-2-pentanone

Abstract

Solvent extraction of lithium, sodium, potassium and tetrabutylammonium(tba⁺) picrates in aqueous solutions into 4-methyl-2-pentanone(MIBK) was determined at 298 K. The absorption spectrum of the MIBK solutions was measured. When the salt concentration was low, the distribution ratio of picrate was constant but it increased by an increase in the concentration. This was explained in terms that the salts were extracted as ion pairs and dissociated nearly completely in the organic phase in the lower concentration range but the dissociation became incomplete as the concentration increased. The extraction as well as the dissociation in the organic phase of the three alkaline picrates were nearly similar to each other. However, the extraction of tba⁺ picrate was much better and the ion pairs dissociated a little more than the alkali picrates. The absorption spectrum of MIBK solutions of tba⁺ picrate was similar to that of its aqueous solutions and the spectrum did not change even in the higher concentration range where a partial association of the ions was assumed from the solvent extraction data. The spectrum of MIBK solutions of alkali metal picrates agreed with that of tba⁺ salt when the concentration was low but the peak shifted to the lower wavelength when the concentration increased. This shift was explained in terms of incomplete dissociation of the ion pairs and the dissociation constants were determined by a dual-wavelength method. The dissociation constants of the alkali picrate ion pairs in MIBK obtained from the spectrophotometric data well agreed with those obtained from the solvent extraction data.