Solvents nonaqueous, acidity measurements

In industrial production of acid-modified starches, a 40% slurry of normal com starch or waxy maize starch is acidified with hydrochloric or sulfuric acid at 25—55°C. Reaction time is controlled by measuring loss of viscosity and may vary from 6 to 24 hs. For product reproducibiUty, it is necessary to strictly control the type of starch, its concentration, the type of acid and its concentration, the temperature, and time of reaction. Viscosity is plotted versus time, and when the desired amount of thinning is attained the mixture is neutralized with soda ash or dilute sodium hydroxide. The acid-modified starch is then filtered and dried. If the starch is washed with a nonaqueous solvent (89), gelling time is reduced, but such drying is seldom used. Acid treatment may be used in conjunction with preparation of starch ethers (90), cationic starches, or cross-linked starches. Acid treatment of 34 different rice starches has been reported (91), as well as acidic hydrolysis of wheat and com starches followed by hydroxypropylation for the purpose of preparing thin-hoiling and nongelling adhesives (92). 

Hydrogen-iodine reaction, 13 770 Hydrogen-ion activity, 14 23-34 nonaqueous solvents, 14 32 pH determination, 14 24-27 pH measurement systems, 14 27-31 Hydrogen ion concentration (total acidity), 14 23... 

In addition, lactams can be prepared by the present technique under heterogeneous conditions although most amino acids are barely soluble in nonaqueous solvents (Table II). Interestingly, (S)-(-)-proline selectively gives the cyclic dimer with no measurable loss of enantiomeric purity. 

RP-HPLC with nonaqueous solvents and UVD at 246 nm was developed for the determination of low level POVs of vegetable oils. These measurements are specific for conjugated diene peroxides derived from vegetable oils with relatively high linoleic acid content. These measurements may be supplemented by nonspecific UVD at 210 nm and ELSD for detection of all eluted species. The elution sequence of the triglycerides in a nonaqueous RP-HPLC is linearly dependent on the partition number of each species, Vp, which is defined as = Nq — 2Ni, where Nq is the carbon number and is the double bond number. In the case of hydroperoxides = Nq — 2Nd — Vhpo, where Vhpo is the number of hydroperoxyl groups in the molecule (usually 1 for incipient POV). For... 

Semiaqueous or Nonaqueous Solutions. Although the measurement of pH in mixed solvents (e.g., water/organic solvent) is not recommended, for a solution containing more than 5% water, the classical definition of a pH measurement may still apply. In nonaqueous solution, only relative pH values can be obtained. Measurements taken in nonaqueous or partly aqueous solutions require the electrode to be frequently rehydrated (i.e soaked in water or an acidic buffer). Between measurements and after use with a nonaqueous solvent (which is immiscible with water), the electrode should first be rinsed with a solvent, which is miscible with water as well as the analyte solvent, then rinsed with water. Another potential problem with this type of medium is the risk of precipitation of the KC1 electrolyte in the junction between the reference electrode and the measuring solution. To minimize this problem, the reference electrolyte and the sample solution should be matched for mobility and solubility. For example, LiCl in ethanol or LiCl in acetic acid are often used as the reference electrode electrolyte for nonaqueous measurements. 

A potentiometric method for determination of ionization constants for weak acids and bases in mixed solvents and for determination of solubility product constants in mixed solvents is described. The method utilizes glass electrodes, is rapid and convenient, and gives results in agreement with corresponding values from the literature. After describing the experimental details of the method, we present results of its application to three types of ionization equilibria. These results include a study of the thermodynamics of ionization of acetic acid, benzoic acid, phenol, water, and silver chloride in aqueous mixtures of acetone, tetrahydrofuran, and ethanol. The solvent compositions in these studies were varied from 0 to ca. 70 mass % nonaqueous component, and measurements were made at several temperatures between 10° and 40°C. 

Direct conductivity measurements do not provide a satisfactory index of added water in milk. However, it has been reported (Rao et al. 1970) that measurement of conductivity in nonaqueous solvents can be useful in detecting adulteration. The conductivities of extracts using two different solvent systems were correlated with the percentage of added water in the original milk. One solvent system consisted of 10 ml acetone and 90 ml methanol plus 3 g sodium chloride, and the other contained 2.65 g formic acid in 100 ml acetone. 

Sulfuric acid has received considerable study as a nonaqueous solvent (see Chapter 5). It is, of course, a strongly acidic solvent, and it has a Kf value of -6.15 °/molal. In 100% H2S04, conductivity measurements and cryoscopic studies show that protonation of many substances occurs even though they are not normally bases in the usual sense. For example, organic compounds such as acetic acid and ether function as proton acceptors ... 

Their unique relation to water systems favors the inclusion of acid-base reactions in deuterium oxide with aqueous acid-base equilibria, even though some aspects of the chemistry suggest inclusion with nonaqueous solvents. In studies such as those of deuterium isotope effects, it is desirable to be able to measure pD as an index of acidity in heavy water. Glass electrodes respond in a nemstian way to changes in deuterium ion concentration, and therefore the usual combination of glass and calomel electrodes can form the basis of an operational definition of pD ... 

In addition to ion-pair formation, other types of association reactions are important in nonaqueous solvents. Evidence for triple and quadruple ion formation in nonaqueous solvents is obtained from conductimetric, solvent extraction, calorimetric, or cryoscopic measurements. Self-association reactions, that is, equilibria such as 2HA (HA)2, have been reviewed " and have been studied by dififer-ential-vapor-pressure techniques. Frequently the anion A obtained from an acid HA is poorly solvated stabilization then may occur by interaction (homoconjugation) with a second molecule of acid to give HAj . Homoconjugation can be studied by techniques such as spectroscopy or conductimetry. Thus, the homoconjugation... 

The meaning to be attached to pH values obtained in different solvents and under different conditions needs interpretation. For example, solutions of pH 5 in the three solvents water, ethanol, and acetonitrile do not denote the same acidity (acidity defined as a measure of the tendency for protons to be donated to basic substances). In view of what is known about transfer activity coefficients (Section 4-1), an acetonitrile solution of pH 5 is far more acidic (higher absolute activity) than an ethanol solution of pH 5 and, in turn, one in ethanol is more acidic than one in water. The significant point is that a solution of, say, pH 5 in a given solvent has an acidity 10 times the acidity of another solution of pH 6 in that same solvent. A single universal scale of pH for all solvents does not exist instead there is a different scale for every solvent of differing composition. To denote pH values in nonaqueous solvents, the... 

Recent studies have made it possible to classify water-organic solvent systems in CCC for separation of organic substances on the basis of the liquid-phase density difference, the solvent polarity, and other parameters from the point of view of stationary-phase retention in a CCC column [1,3-9]. Ito [1] classified some liquid systems as hydrophobic (such as heptane-water or chloroform-water), intermediate (chloroform-acetic acid-water and n-butanol-water) and hydrophilic (such as n-butanol-acetic acid-water) according to the hydrophobicity of the nonaqueous phase. Thirteen two-phase solvent systems were evaluated for relative polarity by using Reichardt s dye to measure solvachromatic shifts and using the solubility of index compounds [6]. 

Dielectric constants cannot explain, quantitatively, most physicochemical properties and laws of solutions, and we shall soon see that they can become unimportant. The molecules of more polar solvents, which tend to cluster around the ions and dipole ions, produce a preferential or selective solvation that is reflected in measurements of such properties as solubility, acid—base equilibria, and reaction rates. Nonelectrostatic effects, such as the basicity of some solvents, their hydrogen-bonding, and the internal cohesion and the viscosity of mixtures, probably interfere with the electrostatic effects and thus reduce their actual influence. On the other hand, mixtures of water and nonaqueous solvents are enormously complicated systems, and their effective microscopic properties may be vastly different from their macroscopic properties, varying with the solute because of selective attraction of one of the solvents for the solute. 

The salt titration method [666] is a modification of the salt addition method. A portion of salt is added to a dispersion, and the ApH is recorded. When ApH > 0, base is added to shift the pH to an even higher value. When ApH < 0, acid is added to shift the pH to an even lower value. Once a constant pH has been established, a new portion of salt is added, and ApH is recorded again. The series of salt additions followed by acid or base additions is continued until ApH = 0. The advantage of the salt titration method as compared with the classical salt addition method is that only one portion of dispersion is used. Thus, PZC determination requires a smaller amount of solid and only one reaction vessel. Moreover, a series of measurements (e.g., at different temperatures or at different concentrations of a nonaqueous co-solvent) can be carried out with the same portion of solid, and effects due to a difference in surface properties between different portions of solid are avoided. The number of consecutive salt additions in the salt titration method is limited, because the sensitivity of ApH to an addition of the same amount of salt decreases as the initial salt concentration increases. 

In general catalytic measurements in aqueous solutions are easier to interpret with certainty than those in other solvents, since our knowledge of the properties of solutions (especially of electrolytes) is very limited outside water. However, it is often necessary to use nonaqueous solvents for practical reasons e.g., solubility and chemical inertness, and the use of different solvents has elucidated a number of points of interest in the general theory of acid-base catalysis. 

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