Buffered acid solvents

In Eq. (6-35), A/Z is the molar heat of ionization of the buffer acid at the conditions (temperature, solvent composition) of the kinetic studies. It happens that for many commonly used acidic buffers this quantity is small. Hamed and Owen give A//2 = —0.09 kcal/mol for acetic acid at 25°C, for example. The very important buffer of dihydrogen phosphate-monohydrogen phosphate is controlled by pK2 of phosphoric acid at 25°C its heat of ionization is —0.82 kcal/mol. 

In situations involving acidic/basic analytes, pH is often the most critical property in the extraction, and buffered aqueous solvents are often necessary. Another important consideration is the stability of the analytes in the extraction medium, and method development should entail analyte stability experiments to demonstrate how long solutions and/or extracts can be stored. 

Various processes separate rare earths from other metal salts. These processes also separate rare earths into specific subgroups. The methods are based on fractional precipitation, selective extraction by nonaqueous solvents, or selective ion exchange. Separation of individual rare earths is the most important step in recovery. Separation may be achieved by ion exchange and solvent extraction techniques. Also, ytterbium may be separated from a mixture of heavy rare earths by reduction with sodium amalgam. In this method, a buffered acidic solution of trivalent heavy rare earths is treated with molten sodium mercury alloy. Ybs+ is reduced and dissolved in the molten alloy. The alloy is treated with hydrochloric acid, after which ytterbium is extracted into the solution. The metal is precipitated as oxalate from solution. 

Sample extraction/deproteinization is usually accomplished with mild acidic solvents to free the noncovalently bound tetracyclines from macromolecules. Mcllvaine buffer, pH 4.0 (286, 287), Mcllvaine/EDTA buffer, pH 4.0 (283, 287-293), succinate buffer, pH 4,0 (278-281,294-296), acidic acetonitrile (297-299), and acidic methanol (14, 199, 300) have all been used successfully. Moreover, trichloroacetic acid, pH 2.0 (301, 302), metaphosphoric acid (303), acetate buffer (126, 280), citrate buffer, pH 4.0 (304), citrate buffer/ethyl acetate, pH 4-5 (305), and hydrochloric acid/glycine buffer (306, 307) have all been employed with varying success to precipitate proteins from the sample homogenates. 

In addition to considering an adjusted buffering range in mixed solvent systems, the effects of dilution on the pH of the formulation should also be considered (Rubino, 1987). As formulations containing mixed solvent systems are diluted, the solvent effects onWIievjil decrease. For example, the a of the buffer acid will decrease as the formulation is diluted due to the dilution of the cosolvent. The consequences of the pqbWnges on the physicochemical stability of the active compounds should be carefully studied. 

Acetic acid Buffer agent, solvent, stabilizer iv, im, sc... 

The pH dependence of the hydrolysis of all compounds studied is, in principle, consistent with the mechanism of Scheme 6 that applies to the alkoxyphenylcarbene complexes, and so are the products of the reactions of 68, 66 and 8. However, the products obtained in the hydrolysis of 144 and the fact that in basic solution the hydrolysis of all the compounds is subject to a substantial kinetic solvent isotope effect are inconsistent with Scheme 6, at least at pH >8.5. The mechanism that accounts best for all experimental observations at pH >8.5, including the isotope effect, is shown in Scheme 17 for the example of 68. It involves rapid deprotonation of 68 followed either by slow protonation of 135 with water ( 2 )) or a buffer acid (fe [BH]) and subsequent rapid conversion of 161 to 162, or slow concerted water (fe2c) or buffer acid catalyzed (fe [BH]) conversion of 135 to 162 (more on these two alternatives below). Complexation between (CO)sCr and the enol ether activates the latter toward basic hydrolysis which rapidly leads to the vinyl alcohol and tautomerization to the aldehyde. Control experiments demonstrated that the kind of complexation indicated by 162 indeed promotes rapid hydrolysis of the enol ether. ° In the reactions of 144 complexation of the enol ether ()8-methoxystyrene) appears to be weak, presumably because of steric crowding, and hence the reaction... 

Controls consisting of the solvents and/or buffers, acids, and so on, without sample, should always be carried out to ensure that observed assay results are in... 

Reverse phase, as its name implies is the reverse of normal phase operation the stationary phase is nonpolar and the eluent is polar. The eluent normally comprises mixtures of water and organic cosolvents (or modifiers). Other additives, such as buffers, acids, bases, and ion-pair reagents may be incorporated into the eluting solvent These are discussed in Subheading 2.1.3. The stationary phases most commonly used in reverse phase HPLC are known as... 

If the chromatographic eluent is a mixture of organic modifier and water and contains no additives such as involatile buffers/acids, then product recovery can be effected simply by removing solvent, e.g., by rotary evaporation. Alternatively, if the solvent volumes are large, the organic modifier can be removed by rotary evaporation and the product recovered by adsorption/elu-tion from a suitable adsorbent such as a reverse phase silica or an organic copolymer resin. This is also a method of choice where additives such as involatile buffers (phosphates, acetates) and acids (orthophosphoric) are present in the fraction and need to be removed. 

Analysis by the uv absorption of citrate buffered solutions enables the I anthocyanin to be determined from the maximum at 520nm. The distribution of component anthocyanins can be found by TLC on cellulose with various strongly acidic solvents (eg. cone HCl, formic acid, water, 19 19.5 61.5) of extracts of plant material obtained with 1% hydrochloric acid in methanol. The Rf values found for 3-monoglucosides of the anthocyaninidins of Vitis vinifera were, for the petunidin compound (0.13), for that of malvidin (0.22) and of peonidin (0.25) (ref. 11). ... 

Many chemicals are available that can be used without further purification. Their selection is made on a trial and error basis, and even different lots from the same supplier may differ in purity. Volatile buffers and solvents such as pyridine, acetic acid, formic acid, and n-propanol, which are employed for chromatography, can be purified by distillation over ninhydrin. Constant-boiling hydrochloric acid is rountinely prepared over sodium dichromate (Schwabe and Catlin, 1974), and water of high purity is obtained from a system composed of a 0.2-fim particle filter, an activated charcoal cartridge, and two deionizer cartridges (Hydro Service and Supplies, Durham, North Carolina). The solvents are tested for purity in the following manner. A sample of the solvent is dried in vacuo. The residue is dissolved in pH 2.2 citrate buffer and injected onto the amino acid analyzer column. The distilled solvents are typically found to contain aspartic acid, serine, and glycine at 20-30 pmol/ml as the major contaminants. 

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