Effects of pH and Solvent

Effects of pH and Solvent Composition Table 3.2 contains data on catalytic activity and selectivity in DHL hydrogenation at different pH and IPrOH content in the solvent. Runs N2-N4 present the data obtained at a fixed pH value (13.4). The selectivity values increase with increasing IPrOH content. For run N2, the catalyst displays practically the same selectivity but the activity increases with increasing IPrOH content, while subsequent alcohol addition leads both to the selectivity and activity increasing (runs N2 and N3). 

The reference gives results at two temperatures and reports the effect of pH and solvent on the rate of decay and the catalytic role of Ag and Cu ions. 

The effects of pH and of mixed solvent systems have been described by Tongaree et al. [13]. 

Biomimetic artificiai membranes-factors Effects of pH and co-solvents on the BAMPA were investigated to determine the optimal conditions for the prediction of oral absorption. The permeability (Pam) of 33 structurally diverse drugs to the PC/PE/ PS/PI/CHO/1,7-octadiene membrane system [bio-mimetic lipid (BML) membrane] was measured at pH 5.5,6.5, and 7.4. The pH dependence of Pam was in accordance with the pH partition theory. The better prediction of oral absorption (fraction of a dose absorbed) was shown under the pH 5.5 condition for determining the permeability of poorly soluble compounds were examined. Dimethysulfoxide (DMSO), ethanol (EtOH) and polyoxyethyleneglycol 400 (PEG 400) were added up to 30% to the transport medium as solubilizers. DMSO, EtOH and PEG 400 decreased Pam of hydrocortisone and propranolol. For example, DMSO (30%) decreased Pam of hydrocortisone and propanol by 60 and 70%2, respectively. DMSO and PEG 400 also decreased Pam of ketoprofen. In contrast, EtOH produced an opposite effect on permeability, that is, an increased Pam of ketoprofen. Therefore, the high concentration of these co-solvents could lead to the under- or overestimation of drug permeability. 

Ritschel Gupta VD, Cadwallader DE, Herman HB and Honigberg IL, Effect of pH and dye concentration on the extraction of a thiamine dye salt by an organic solvent, J. Pharm. Sci., 57, 1199-1202 (1968) cited 1.58 X 10 from Merck Index. 

As with other chiral stationary phases, dispersive interactions with the cyclodextrin structure are controlled with polar solvents, polar interaction controlled with dispersive solvents and, if ionic interactions are present, these will be controlled by both the pH and the type of buffer that is employed. Small changes in pH can be quite critical and a the effect of buffer type on chiral selectivity, under certain circumstances can be quite profound, an example of the effect of pH and buffer type is depicted in figure 8.20. 

Figure 16 Effects of pH and salt type on band spacing in the anion-exchange separation of a protein sample 1, myoglobin 2, transferrin 3, a,-acid glycoprotein 4, a-lactalbumin 5, soybean trypsin inhibitor. Conditions 5 x 0.4-cm WAX column (300A pore) (Shimadzee Shim-pack) 0-100% B in 20min I.OmL/min. (a) A solvent, 0.02 M phosphate buffer B solvent, 0.5 M phosphate buffer (b) A solvent, 0.02 M phosphate buffer B solvent 0.5 M NaCI or Na2S04. (From Ref. 41.)...

Solvents and pH may have a marked effect on stereochemistry as was illustrated in Chapter 1, and the generality given there is useful, A further example of the stereochemical influence that may be exerted by proper choice of catalyst and solvent is shown in the hydrogenation of a complex enamine, By proper choice of conditions high yields of either the cis or trans product could be obtained. Selected results are shown below (52) (data used with permission). 

Sodium diethyldithiocarbamate, (C2H5)2N CS S Na+. This reagent is generally used as a 2 per cent aqueous solution it decomposes rapidly in solutions of low pH. It is an effective extraction reagent for over 20 metals into various organic solvents, such as chloroform, carbon tetrachloride, and ethanol. The selectivity is enhanced by the control of pH and the addition of masking agents. 

Reaction is terminated by acetonitrile quenching, or by liquid-liquid extraction with water-immiscible organic solvent, provided that the extraction efficiency and the effect of the organic solvent on product stability are tested at the small scale. Based on properties of product, the pH of the reaction mixture should be adjusted before termination to allow maximal recovery of the product. For example, acid is usually added to the acyl-glucuronide product mixture at the end of the reaction to minimize acyl migration. 

The above points will now be illustrated with respect to the nitro group. The most convenient data for this purpose are for ionization in 50% v/v Et0H-H20124-126. (Data for other solvent compositions are also in the References124,125.) Ap//a values for the effects of in- and p-N02 on the various acids Ph—G—COOH, along with the corresponding Hammett p values (Section n.A), are shown in Table 4. The values of am (calc) and ap (calc) are obtained as cr(calc) = ApK /p. 

Let us consider now the origin of the effect of varying solvent composition on the hydrogenation rate in diglyme-water mixtures. The key to the explanation comes from the study of the effect of pH on the rate of hydrogenation of maleic and fumaric acids in homogeneous aqueous solutions. Fig. 3.2.a and 3.2.b show these rates as a function of pH together with the concentration distribution of the undissociated (H2A), half dissociated (HA ) and fully dissociated (A ) forms of the substrates [86]. 

Th effect of pH on the rate of hydrogenation of water-soluble unsaturated carboxylic acids and alcohols catalyzed by rhodium complexes with PNS [24], PTA [29], or MePTA r [32] phosphine ligands can be similarly explained by the formation of monohydride complexes, [RhHPJ, facilitated with increasing basicity ofthe solvent. 

Luminescence is often much more sensitive to molecular dynamics than other optical techniques where temperature, viscosity, pH and solvent effects can have a significant influence on the emission response. Analyte degradation for light sensitive fluors and photobleaching for static measurements also influence the emission signal. Because of the wide variety of potential matrix effects, a thorough investigation should be conducted or the sample matrix well understood in terms of its potential impact on emission response. A complete discussion on the fate of the excited states and other measurement risk considerations can be found elsewhere. ... 

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