Spectrophotometric determination of the thermodynamic pK value of picric acid in water at 25 °C

Abstract

Th e ther modynami c pK value of picric acid was determin ed spectrophotomet l'i cally in wa ter co n ta ining hy droc hl oric acid to re press the io ni zation . The pI( value 0.33 ( T< "" 0.46) was obtained from data at 45·0 mIL . A ttempts to determin e t he pI( value by poten t iometric tit r ations of picri c acid and by spectr ophoto metri c m easure ments of picr ic acid solution s in t he n car-saturation ran ge did not y ield satisfactor y res ul ts . The n ew pK va lue is comparcd wit h previously published valucs .

Phenolic Pollutant Uptake Properties of Molecular Templated Polymers Containing β-Cyclodextrin

“…The initial concentration ( C o ) of the adsorbate dye before and after sorption ( C e ) was measured. The molar absorptivity of phenolphthalein (phth), PNP, and TNP was calculated from the calibration curves to be ε = 28 423 L/(mol cm) (pH = 10.5, λ max = 552 nm), ε = 18 478 L/(mol cm) (pH = 9, λ max = 400 nm), ε = 14 880 L/(mol cm) (pH = 4.5; λ max = 355 nm), respectively, in agreement with other reports. ,,, The adsorbate uptake ( Q e ; mmol/g) at equilibrium was determined by eq , where m is the mass of the adsorbent and V is the volume of adsorbate solution. The equilibrium uptake was analyzed by the Sips isotherm (cf. eq ) where Q m (mmol/g) is the sorption capacity of a single-adsorbed monolayer onto the adsorbent, K s (mM –1 ) refers to the adsorption affinity constant at equilibrium, and n s is a heterogeneity constant. …”

“…In general, there is a gradual nonlinear increase in Q e as C e increases with variable uptake for the polymer/phenol (TNP and PNP) systems. At the pH conditions herein, the adsorbates exist as phenolate anions according to the variable p K a (PNP, 7.16 and TNP, 0.38) , due to the electron-withdrawing effects of the NO 2 group. Variable electron density or HLB character of the phenols is the result of substituent effects, where the binding affinity with the polymers can be used to infer the type of adsorption sites involved. ,,, In general, there is an increase in the Q m value as the cross-linker content increases for the polymer/TNP systems as follows: 1:25 > 1:20 > 1:15.…”

“…Substituent effects for PNP and TNP are known to influence the thermodynamic and kinetics of hydration in bulk solution versus adsorption at the polymer-binding sites, in accordance with hydrophobic effects. Although TNP and PNP have similar water solubilities, , differences in their electron density due to the −NO 2 substituent effects influence the polymer adsorption properties for each phenol. The stability of CD host–guest complexes for TNP and PNP can be inferred due to the role of electron donor–acceptor interactions, in agreement with the trends observed herein for the CD–EPH polymer systems (Table ).…”

“…The buildup of such species in ground and surface water supplies relates to the uncontrolled release of industrial effluents from various sources. The widespread use of phenolic additives in textile dyes, explosives, agrochemicals, and pharmaceuticals represent an emerging concern to human and ecosystem health due to buildup in surface and groundwater supplies. − The utility of model systems, such as PNP and trinitrophenol (TNP), relate to their variable solubility profile , and mobility in aquatic environments, where the occurrence of such species has revealed the need to develop advanced materials for remediation of aquatic environments. Recent studies on sorbent materials indicate that favorable host–guest molecular interactions − between cyclodextrins (CDs) and phenols occur, especially in the case of CD polymer-based materials.…”

Unusual Stabilization of an Intermediate Spin State of Iron upon the Axial Phenoxide Coordination of a Diiron(III)–Bisporphyrin: Effect of Heme–Heme Interactions

“…Plots of the pK a values of: A) the phenols versus the E 1/2 [phenoxide-oxidation] couples, and B) the phenols versus the E 1/2 [Fe III / Fe II ] couples. The pK a values [23] are: 2,6-diisopropylphenol: 11, phenol: 9.8, 2-nitrophenol: 7.24, 2,4,6-tribromophenol: 6.8, and 2,4,6-trinitrophenol: 0.38.…”