Aqueous equilibria with acid-base indicators

Apparent Chemical Deviations Apparent deviations from Beer s law- arise when an analyte dissociates, associates, or reacts with a solvent to produce a prttduct with a different iihsorpti<>n spectrum than the analyte. A common example of this behavior is found with aqueous solutions of acid-base indicators. For example, the color change iissociated with a typical iitdicalor tlln arises from. shifts in the equilibrium... 

It is, however, more likely that the discrepancies observed in the periodate oxidation of malonaldehyde concern mainly the hydroxylation step. In the mechanism proposed (5) for this reaction, it is the enol form of malonaldehyde which is hydroxylated. However, titrations of a solution of malonaldehyde, prepared by hydrolysis of an aqueous solution (33) of carefully distilled 1, 3, 3-tri-ethoxypropene (46, 47), both with strong base and with iodine, indicate that only about 80% of the enol form is present in the equilibrium solution. On the other hand, the thio-barbituric acid test (58, 59) gave consistently higher values for the malonaldehyde content of the solution. The fact that only about 80% of the enol form is present in the equilibrium solution is all the more important as it can be shown (56) by titration with strong base that the enolization is slow, and moreover does not seem to go to completion. 

For cryptands in which the molecular cavity is larger than in the case of the [l.l.l]-species [78], proton transfer in and out of the cavity can be observed more conveniently. Proton transfer from the inside-monoprotonated cryptands [2.1.1] [79], [2.2.1] [80], and [2.2.2] [81 ] to hydroxide ion in aqueous solution has been studied by the pressure-jump technique, using the conductance change accompanying the shift in equilibrium position after a pressure jump to follow the reaction (Cox et al., 1978). The temperature-jump technique has also been used to study the reactions. If an equilibrium, such as that given in equation (80), can be coupled with the faster acid-base equilibrium of an indicator, then proton transfer from the proton cryptate to hydroxide ion... 

Copolymer [8o,5-CPPo.5]-Leu(6)o 75-Lys(Bz)o.25 (Table 1) with Mw = 82 000 Da and polydispersity (PDI) = 1.66 was isolated with 76 % yield. As a result, the incorporation of 50% CPP-unit in sebacic acid based PEA raised the Tg from 22 to 44°C. An additional sharp melting endotherm in differential scanning calorimetry (DSC) curves at 286°C was also observed, indicating a semicrystalline nature of the co-polymer. The polymer is soluble in chlorinated nonpolar and aprotic polar solvents, but not in ethanol. Because of its high hydrophobicity, the CPP-co-polymer does not swell in aqueous media, and equilibrium water content is about 2-3% w/w. 

Hydrolysis of Pd(MeCOCHMe)2 in aqueous methanol is considered to involve Pd(0,0-MeCOCHCOMe)(0-MeCOCHCOMe)(MeOH) as an intermediate from which the monodentate acetylacetonate ligand is then solvolyzed.221 Subsequent studies on Lewis base complexes of palladium bis(diketonate) complexes provide ample support for the proposed intermediate. A pulse radiolysis study of the kinetics of aquation of M(MeCOCHCOMe) " (M = Cr, Co) indicates that an 17,-172 equilibrium involving one or more of the acetylacetonate ligands occurs, associated with an acid-catalyzed removal of the monodentate ligand.222 Treatment of Cu(MeCOCHCOMe)2 with picric acid in moist dichloromethane affords a partially hydrolyzed material, Cu(MeCOCHC-0Me)(H20)2[C6H2(N02)30], proposed to contain square pyramidal five-coordinate copper with the oxygen atom from the picrate moiety at the apex.223... 

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