Mixed Aqueous-Organic Solvents

In this experiment the effect of a mixed aqueous-organic solvent on the color transition range of common indicators is investigated. One goal of the experiment is to design an appropriate titrimetric method for analyzing sparingly soluble acids and bases. 

Investigations in aqueous systems have established many of the fundamental principles of ion exchange as well as providing useful applications. The scope of the ion exchange process has, however, been extended by the use of both organic and mixed aqueous-organic solvent systems.32,33... 

A brief discussion of the systematics of solvent effects on the p/, pr, and values of Tables II and III is presented in the discussion section. However, it is worthy of note here that sets 7, 37, 38, 39, 40, and 41, which involve nonhydroxylic solvents, are fitted with comparable precision to that for reaction series in aqueous or mixed aqueous organic solvents. The present analysis does not support the previous assignment (7b) of ion-pair formation of benzoic acids... 

The greater than unit value of Kf in pure solvents is the result expected by the field effect model (33) on a distance basis. The lower than unit value ofKj in the mixed aqueous organic solvents appears to be related to preferential hydration of the reaction center, which results in an increased effective dielectric constant from the m- compared to the p- position. The fact that values fall into two separate categories for pure organic and mixed aqueous organic solvents does not support the treatments of Exner (20) or Yukawa and Tsuno (16). 

The exchange reaction between U(VI) and U(IV) in aqueous sulphate chloride and perchlorate media and in various mixed aqueous-organic solvents... 

The selection of proper mobile phase in TLC exerts a decisive influence on the separation of inorganic ions. With a particular stationary phase, the possibility of separation of a complex mixture is greatly improved by the selection of an appropriate mobile phase system. In general, the mixed aqueous-organic solvent systems containing an acid, a base, or a buffer have been the most favored mobile phases for the separation of ionic species. The mobile phases used as developers in inorganic PLC include ... 

For some elements, there are a variety of possible precursors, sulfur for instance (Figure 11). Use of metal-organic precursors, used in MOMBE or MOVPE, are possible if they are soluble in water, or a nonaqueous solvent is used. Mixed aqueous-organic solvents could improve solubility. Overall, there would be an increased probability of carbon contamination, however. One of the benefits of using inorganic salts in aqueous solutions is that the number of constituents is limited, and thus the... 

These results suggest that the crystallographic determination of the structure of a productive enzyme-substrate complex is feasible for lysozyme and oligosaccharide substrates. They also provide the information of pH, temperature, and solvent effects on activity which are necessary to choose the best conditions for crystal structure work. The system of choice for human lysozyme is mixed aqueous-organic solvents at -25°C, pH 4.7. Data gathered on the dielectric constant, viscosity, and pH behavior of mixed solvents (Douzou, 1974) enable these conditions to be achieved with precision. 

The lowered temperature approach has been linked to flow, temperature jump, photolysis, and nmr methods. Cryoenzymology allows the characterization of enzyme intermediates which have life-times of only milliseconds at normal temperatures, but are stable for hours at low temperatures. Mixed aqueous/organic solvents or even concentrated salt solutions are employed and must always be tested for any adverse effects on the catalytic or structural properties of the enzyme. 

Solntions in organic solvents or in mixed aqueous-organic solvents, on the whole, behave not very differently from purely aqueous solutions. In particular,... 

A further complication that sets in when organic or mixed aqueous-organic solvents are used, which is aggravated when the relative permittivity of the medium, e, falls below 40, is ion pairing. This phenomenon does occur in purely aqueous solutions, mainly with higher-valence-type electrolytes 2 2 and higher, and with 2 1 or 1 2 electrolytes only at high concentrations. Ion pairs may also form in aqueous solutions of some 1 1 electrolytes, provided the ions are poorly hydrated and can approach each other to within <0.35 nm. Such ion pairs are of major importance in solvents that are relatively poor in water or that are nonaqueous. 

We have recently devised a rapid and convenient method for determination of the ionization constant for water in mixed aqueous organic solvents (11-16). The method utilizes glass electrodes and gives results in satisfactory agreement with earlier work. 

Hefter, G., Marcus, Y., and Waghome, W. E. (2002) Enthalpies and entropies of transfer of electrolytes and ions from water to mixed aqueous organic solvents, Chem. Rev. 102, 2773-2835 and references cited therein. 

Photoreduction of viologens in mixed aqueous/organic solvents has been reported but the results compare badly with those obtained in... 

In most of its reactions periodate is a two-electron oxidant, and it is reduced to iodate. Periodate oxidations are normally carried out in aqueous solution, but organic or mixed aqueous-organic solvents can be used (Qureshi and Sklarz ) and in section 1.8 the very limited amount of kinetic data available for mixed solvents is reviewed. 

Periodate oxidations are much slower in non-aqueous solvents or in mixed aqueous-organic solvents. Very few kinetic studies have been made of the solvent effect. Taylor et al. found that the rate of oxidation of ethane-1,2-diol by periodic acid is reduced by a factor of approximately 12 by the addition of 38 % of ethanol to water. Guthrie studied the oxidation of tra 5-cyclohexane-l,2-diol in dimethylformamide/water mixtures, and found a sharp reduction in rate when the DMF content was increased beyond 20 %. 

The apparent pH of the mixed aqueous organic solvent when the aqueous component was pH 4.75. 

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