Activity in concentrated solutions

Both modifications affect the analysis of dilute solution behavior, and it is difficult to judge how much the e/e0 term is actually needed. In any case, as the authors themselves point out (41), the e/e0 term makes an entirely negligible contribution to solvent activity in concentrated solutions. For example, simple calculations yield a contribution of approximately l%ina 10% solution of natural rubber in benzene at 30° C (M = 500000, [t/]g = 250, y=0.4). It is therefore clear that thermodynamic measurements can furnish no evidence for or against continued collapse in concentrated solutions. 

Activities in Concentrated Solutions.—For relatively concentrated solutions it is necessary to use the complete Hiickel equation (62) by choosing suitable values for the two adjustable parameters a and it has been found possible to represent the variation of activity coefficients with concentration of several electrolytes from 0.001 to 1 molal, and sometimes up to 3 molal. The values of C seem to lie approximately between 0.05 and 0.15 in aqueous solution. At the higher concentrations it is necessary to make allowance for the difference between the rational and stoichiometric activity coefficients the latter, which is the experimentally determined quantity, is represented by an extension of equation (62) thus (cf. p. 135),... 

Pollard, A. and Wyn Jones, W.R.G. Enzyme activities in concentrated solutions of glycinebetaine and other solutes, Planta, 144, 291,1979. 

As with reactions in concentrated solutions of electrolytes (Chapter 5) the rate of a catalytic reaction increases exponentially when the concentration of catalyst exceeds about 5 X 10" M. The phenomenon is particularly marked with strong acids and basis, which become extremely active in concentrated solution. 

The increased acidity of the larger polymers most likely leads to this reduction in metal ion activity through easier development of active bonding sites in siUcate polymers. Thus, it could be expected that interaction constants between metal ions and polymer sdanol sites vary as a function of time and the sihcate polymer size. The interaction of cations with a siUcate anion leads to a reduction in pH. This produces larger siUcate anions, which in turn increases the complexation of metal ions. Therefore, the metal ion distribution in an amorphous metal sihcate particle is expected to be nonhomogeneous. It is not known whether this occurs, but it is clear that metal ions and siUcates react in a complex process that is comparable to metal ion hydrolysis. The products of the reactions of soluble siUcates with metal salts in concentrated solutions at ambient temperature are considered to be complex mixtures of metal ions and/or metal hydroxides, coagulated coUoidal size siUca species, and siUca gels. 

Fig. 9. Activation parameters for resole methylolation in the presence of NaOH in concentrated solutions [80,90].

Sometimes the term normal hydrogen electrode (and respectively normal potential instead of standard potential) has been used referring to a hydrogen electrode with a platinized platinum electrode immersed in 1 M sulfuric acid irrespectively of the actual proton activity in this solution. With the latter electrode poorly defined diffusion (liquid junction) potentials will be caused, thus data obtained with this electrode are not included. The term normal hydrogen electrode should not be used either, because it implies a reference to the concentration unit normal which is not to be used anymore, see also below. 

When a solute is added to an acidic solvent it may become protonated by the solvent. If the solvent is water and the concentration of solute is not very great, then the pH of the solution is a good measure of the proton-donating ability of the solvent. Unfortunately, this is no longer true in concentrated solutions because activity coefficients are no longer unity. A measurement of solvent acidity is needed that works in concentrated solutions and applies to mixed solvents as well. The Hammett acidity function is a measurement that is used for acidic solvents of high dielectric constant. For any solvent, including mixtures of solvents (but the proportions of the mixture must be specified), a value Hq is defined as... 

Therefore, the activity coefficients in solutions are determined primarily by the energy of electrostatic interaction w j between the ions. It is only in concentrated solutions when solvation conditions may change, that changes in (but not the existence of) solvation energy must be included, and that nonelectrostatic interactions between ions must be accounted for. 

The nitrilase activity of Arthrobacter sp. F-73 retains substantial activity in aqueous solutions containing a significant concentration of organic co-solvent [84]. More than 10% of nitrilase activity remains at acetone concentrations up to 60%, whereas no activity... 

Since the deposit contains halite and anhydrite, the brines should be saturated with respect to these minerals and hence provide a good test of the activity models. Table 8.8 shows analyses of brine samples from the deposit. Note that the reported pH values are almost certainly incorrect because pH electrodes do not respond accurately in concentrated solutions. Hence, there is little to be gained by calculating dolomite saturation. 

Analysis of mass transfer in ternary media, until now, has mainly involved experimental studies of model and real food. Phenomenological models could be applied to obtain a more detailed description of the mechanisms involved. However, this would require an understanding of factors such as mass transport properties and transfer dynamics of different active compounds in concentrated solutions, which have yet to be characterized. 

The alpha amylase of malted barley, the amylase of Aspergillus oryeae and pancreatic amylase all are thermolabile proteins that rapidly lose their amylase activities upon exposure to unfavorable temperatures, to unfavorable hydrogen ion activities, or to other unfavorable chemical environments. The loss of amylase activity in aqueous solutions increases with increasing temperatures and is exceedingly rapid for each of these amylases at 50°. The inactivation of each of these amylases at unfavorable temperatures or at unfavorable hydrogen ion activities may be retarded by the presence of suitable concentrations of calcium ions. 

In principle, this system of 20 equations can be solved provided the equilibrium constants, activities, Henry-constants and fugacities are available. While some results for most of these properties are available, there exists no approved method for calculating activities in concentrated aqueous solutions of weak electrolytes therefore, several approximations were developed. ... 

Activity coefficients in concentrated solutions are often described using Harned s rule (l ). This rule states that for a ternary solution at constant total molality the logarithm of the activity coefficient of each electrolyte is proportional to the molality of the other electrolyte. The expressions for the activity coefficients are written ... 

The p/<, of a base is actually that of its conjugate acid. As the numeric value of the dissociation constant increases (i.e., pKa decreases), the acid strength increases. Conversely, as the acid dissociation constant of a base (that of its conjugate acid) increases, the strength of the base decreases. For a more accurate definition of dissociation constants, each concentration term must be replaced by thermodynamic activity. In dilute solutions, concentration of each species is taken to be equal to activity. Activity-based dissociation constants are true equilibrium constants and depend only on temperature. Dissociation constants measured by spectroscopy are concentration dissociation constants." Most piCa values in the pharmaceutical literature are measured by ignoring activity effects and therefore are actually concentration dissociation constants or apparent dissociation constants. It is customary to report dissociation constant values at 25°C. 

Halcon has developed a new non-noble metal catalyst for methanol reductive carbonylation (32). It is formed under more moderate conditions (1200 psi, 120 C) and permits a selective reaction at only 1200-1800 psi of reaction pressure. Under these conditions, the catalyst s activity is comparable with noble metal catalyzed carbonylations. The conversion rate is 1.5-3.0 mol/l.hr. and acetaldehyde selectivity is 85%. In concentrated solutions, a considerable portion of product acetaldehyde (20-40%) is converted to its acetal. The acetal can be readily hydrolyzed back to acetaldehyde at 100-150 without catalyst (33). Acetal formation is actually beneficial through prevention of undesirable acetaldehyde condensation reactions. 

The solvent activity in a solution of polybutadiene in benzene was determined by measuring the vapor pressure / , of benzene over solutions containing various concentrations of poly-mer.f A plot of ln(p,// ) — In 4>, — (1 — /ri) 2 versus — in which/ is the vapor pressure of the pure benzene —yields a straight line having an intercept of zero and a slope equal to 0.33. Evaluate the interaction parameter x from this result. Is the 0 temperature above or below the experimental temperature Explain. 

Just as it is customary to consider the concentration of water to be constant (or at unit activity) in dilute solutions of weak acids, so we shall consider that the water concentration remains constant in dilute solutions of weak bases [H20] will not appear in any K-i expression. 

An alternative method has been used by Morton (73, 74) which involves an independent measurement of the degree of association of polyisoprenyllithium. Measurements were made of the concentrated solution viscosity of active and terminated polymer solutions. An approximately ten-fold decrease in viscosity was observed on discharging the ion-pair. This corresponds to two-fold association for polyisoprenyllithium in hexane, since in concentrated solutions v) = KMffl. A careful study established that the association numbers were slightly less than two and decreased with temperature. From the data, K2 can be calculated and hence k . In this way it was found that in hexane kp — 3.4 X 103 exp. (— 4100/RT) at 30°, kp = 4.7 litre/mole sec. For practical reasons,... 

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