Water activity, calculation

The activity of water in PHRQPITZ is computed from the osmotic coefficient, and represents a substantial improvement over water activity calculations in PHREEQE (2). 

The water activities calculated in this manner could then be combined using equation (5.31). 

Water activity in foods is usually determined from knowledge of the equUib rium relative humidity or can be measured using various hygrometers. In some foods it can be calculated from various theoretical and empirical models that take into account the food chemical composition, the content of electrolytes such as sodium chloride and non-electrolytes such as saccharose, respectively. Equations, varying in their levels of compUcation, are numerous, but their use is limited to certain commodities. One of the simple empirical equations for water activity calculation in jams has the form = 1/(1 + 0.21n), where... 

Besides the aforementioned descriptors, grid-based methods are frequently used in the field of QSAR quantitative structure-activity relationships) [50]. A molecule is placed in a box and for an orthogonal grid of points the interaction energy values between this molecule and another small molecule, such as water, are calculated. The grid map thus obtained characterizes the molecular shape, charge distribution, and hydrophobicity. 

Figure 3. Time series of nitrate (Slagle and Heimerdinger 1991) and dissolved, particulate, and total in surface water at 47°N, 20°W (Atlantic Ocean) in April-May 1989. activity calculated as 0.0686 salinity (Chen et al. 1986). The production of biogenic particles during the bloom enhances the scavenging of Th, resulting in growing disequilibrium with time due to sinking of particles.

Tables 3 and 4 contain values of the log water activity and log sulfuric acid activity in molarity units. These can be obtained at any temperature by using the polynomial coefficients supplied by Zeleznik,45 which are based on all of the preexisting thermodynamic data obtained for this medium. The numbers were converted to the molarity scale using the conversion formula given in Robinson and Stokes 46 Molarity-based water activities are given for HCIO4 in Tables 5 and 6. These are calculated from data obtained at 25°C by Pearce and Nelson,17...

For a first chemical model, we calculate the distribution of species in surface seawater, a problem first undertaken by Garrels and Thompson (1962 see also Thompson, 1992). We base our calculation on the major element composition of seawater (Table 6.2), as determined by chemical analysis. To set pH, we assume equilibrium with CO2 in the atmosphere (Table 6.3). Since the program will determine the HCOJ and water activities, setting the CO2 fugacity (about equal to partial pressure) fixes pH according to the reaction,... 

The program produces in its output dataset a block of results that shows the concentration, activity coefficient, and activity calculated for each aqueous species (Table 6.4), the saturation state of each mineral that can be formed from the basis, the fugacity of each such gas, and the system s bulk composition. The extent of the system is 1 kg of solvent water and the solutes dissolved in it the solution mass is 1.0364 kg. 

Fig. 24.8. Evolution of fluid chemistry during the simulated evaporation of seawater as an equilibrium system at 25 °C, calculated using the Harvie-Mpller-Weare activity model. Upper figures show variation in salinity, water activity (aw), and ionic strength (/) over the reaction path in Figure 24.7 bottom figure shows how the fluid s bulk composition varies.

In contrast to the effects of temperature, the effect of pressure on c/w is relatively small and can be neglected for reasonable pressure differences. Based on thermodynamics, a change in total pressure of a system affects the vapor pressure. The change in water activity with pressure, at constant moisture content, can be calculated using Eq (8) (Bell and Labuza, 2000) ... 

These hypothetical water activities are labeled A, with appropriate subscripts, and can be used to calculate a. the total r w,mix... 

Molecular dynamics free-energy perturbation simulations utilizing the empirical valence bond model have been used to study the catalytic action of -cyclodextrin in ester hydrolysis. Reaction routes for nucleophilic attack on m-f-butylphenyl acetate (225) by the secondary alkoxide ions 0(2) and 0(3) of cyclodextrin giving the R and S stereoisomers of ester tetrahedral intermediate were examined. Only the reaction path leading to the S isomer at 0(2) shows an activation barrier that is lower (by about 3kcal mol ) than the barrier for the corresponding reference reaction in water. The calculated rate acceleration was in excellent agreement with experimental data. ... 

Trolard and Tardy (1987) calculated an equilibrium water activity of ca. 0.6 at 5 °C, 0.78 at 15 °C, 0.88 at 25 °C and 0.9 at 40 °C. At higher temperatures, hematite should be the stable phase even at an an2o of unity. Experimental confirmation of the importance of the relative humidity on the direction of the reaction is provided by the work of Torrent et al. (1982). This study also demonstrated that at aH2o < tho transformation offerrihydrite into more stable phases is very slow. 

Figure 7.18 gives the ratio (K /K)s4 s4 of the calculated equilibrium constants for solution-phase ammonium nitrate compared to the solid salt product at various temperatures and water activities. As the water activity, i.e., water vapor pressure above the solution, increases, the equilibrium constant falls. That is, at higher relative humidities, relatively less HNO, and NH, are found in the vapor phase at equilibrium. This may be why relatively more ammonium nitrate in particles collected on filters evaporates at lower RHs compared to higher ones. 

The water activity of food samples can be estimated by direct measurement of the partial vapor pressure of water using a manometer. A simple schematic diagram is shown in Figure A2.4.1. A sample of unknown water activity is placed in the sample flask and sealed onto the apparatus. The air space in the apparatus is evacuated with the sample flask excluded from the system. The sample flask is connected with the evacuated air space and the space in the sample flask is evacuated. The stopcock across the manometer is closed and temperatures are read. The equilibrium manometer reading is recorded (/, ). The stopcock over the sample is closed and the air space is connected with the desiccant flask. The manometer reading in the legs is read to give h2. The water activity of the sample is then calculated (Labuza et al., 1976) as ... 

Calculate the water activity, aw, of the sample from ht and h2 values using Equation A2.4.1. 

Calibration is performed by measuring the water activities of various saturated salt solutions and comparing them to values in the literature or values calculated from Table A2.3.2. Literature values at a range of temperatures are found in Table A2.3.1. Values at intermediate temperatures can be predicted from the equations in Table A2.3.2. 

Record the water activity displayed by the equipment and calculate the correction factor by using the value given in Table A2.3.1 for the solution. 

The dependence of solubility on % fill in 0.5M (OH) solution is shown in Fig. 5.(10) The solubility in pure water is an order of magnitude smaller under similar conditions. In pure water, activity coefficient (actually fugacity calculated from appropriate compressibility) estimates enable one to get reasonably accurate values for the equilibrium constant. This treatment suggests that the solubizing reaction in pure water is ... 

As an alternative to laboratory solubility measurements, solubility product constants (KSp), which are derived from thermodynamic data, can be used to calculate the solubility of solids in water (Table 2.9). Each solubility product constant describes a disassociation of a solid in water and calculates the activities or concentrations of the dissolution products in the saturated solution. The solubility product constant or another equilibrium constant of a reaction may be derived from the Gibbs free energy of the reaction (AG"K) as shown in the following equation ... 

The lifetime tf may be thus calculated as the product of three terms, respectively dependent on T, RH (or water activity), and polymer structure. 

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