Activity of pure water

In Equation 4.21, the activity of pure water (a) is unity and the activity of the water with the inhibitor (a ) is the product of the water concentration (xw) and the activity coefficient (xw). The water concentration is known and the activity coefficient is easily obtained from colligative properties for the inhibitor, such as the freezing point depression. For instance the activity of water in aqueous sodium chloride solutions may be obtained from Robinson and Stokes (1959, p. 476) or from any of several handbooks of chemistry and physics. 

As Bell (1959d) has pointed out, the absolute values of AF° and AS° for equilibria involving the solvent are of somewhat uncertain significance because these functions depend on the concentration units employed. For consistency the concentration of solvent might be expressed in units of moles liter-1, but the activity of pure water on this basis is an open question. Therefore it is generally more satisfactory to consider relative equilibria of the type... 

In dilute solutions, the concentration of water is very close to that of pure water, and the activity of pure water, by convention, is taken to be 1.0. Furthermore, in dilute solutions, the activity of solutes may be approximated by their concentrations so we may write an expression for a practical acid dissociation constant ... 

The activity of pure water is by convention unity (1), so equation 2 simplifies to ... 

The standard free energies are bases on a standard state of 1m total stoichiometric concentration of reactants and products, except hydrogen ion, and on an activity of pure water of 1.0 b see ref.11 1 c Hydrolysis of ATP and PPj depend strongly on the concentration of Mg2+ in solution and on pH[19 2,). 

If the activity of pure water is unity and Pq = 1 then Equation 4.70 reduces to... 

When both phases exist, liquid chlorine saturated with water is in equilibrium with liquid water saturated with chlorine. The activity of water is by definition of equilibrium the same in both phases. Considering the composition of the water-rich liquid, we see that this activity is very nearly equal to that of pure water. Setting the activity of pure water at unity, we have for the water in the chlorine-rich phase 5 = 1, or... 

In these expressions, ox is the activity of X. If we restrict ourselves to dilute solutions, and take into account that the activity of pure water is unity, the above equilibrium can be written in terms of the acidity constant as mo+ A ... 

If the concentrations are snfficiently low, then the activities of solutes in Equation 11.10 can be replaced with molarities (without units) and the activity of pure water can be assumed to be unity, giving (Equation 11.11)... 

We take the activity of pure water as 1 and obtain the conventional ion product constant for the self-ionization of water. 

The thickness of the equivalent layer of pure water t on the surface of a 3Af sodium chloride solution is about 1 A. Calculate the surface tension of this solution assuming that the surface tension of salt solutions varies linearly with concentration. Neglect activity coefficient effects. 

The reactor coolant pH is controlled using lithium-7 hydroxide [72255-97-17, LiOH. Reactor coolant pH at 300°C, as a function of boric acid and lithium hydroxide concentrations, is shown in Figure 3 (4). A pure boric acid solution is only slightly more acidic than pure water, 5.6 at 300°C, because of the relatively low ionisation of boric acid at operating primary temperatures (see Boron COMPOUNDS). Thus the presence of lithium hydroxide, which has a much higher ionisation, increases the pH ca 1—2 units above that of pure water at operating temperatures. This leads to a reduction in corrosion rates of system materials (see Hydrogen-ION activity). 

Figure 2.4 shows the equilibrium relationships of biological materials between the water content and the water activity, at constant temperatures and pressures. These data were first published in 1971, but did not find much attention in the RM field until now. At equilibrium the water activity is related to the relative humidity cp of the surrounding atmosphere (Equation 2.3) where p is the equihbrium water vapor pressure exerted by the biological material and po the equilibriiun vapor pressure of pure water at the same temperature. 

Similarly, concepts of solvation must be employed in the measurement of equilibrium quantities to explain some anomalies, primarily the salting-out effect. Addition of an electrolyte to an aqueous solution of a non-electrolyte results in transfer of part of the water to the hydration sheath of the ion, decreasing the amount of free solvent, and the solubility of the nonelectrolyte decreases. This effect depends, however, on the electrolyte selected. In addition, the activity coefficient values (obtained, for example, by measuring the freezing point) can indicate the magnitude of hydration numbers. Exchange of the open structure of pure water for the more compact structure of the hydration sheath is the cause of lower compressibility of the electrolyte solution compared to pure water and of lower apparent volumes of the ions in solution in comparison with their effective volumes in the crystals. Again, this method yields the overall hydration number. 

The activity of the solvent molecule HS in a single-component solvent is constant and is included in Kus. The concentration of ions is mostly quite low. For example, self-ionization occurs in water according to the equation 2H20— H30+ + OH". The conductivity of pure water at 18°C is only 3.8 X 10"8 Q"1 cm-1, yielding a degree of self-ionization of 1.4xl0"19. Thus, one H30+ or OH" ion is present for every 7.2 x 108 molecules of water. Some values of Kus are listed in Table 1.5 and the temperature dependence of the ion product of water Kw is given in Table 1.6. 

Now we turn our attention to the water and the solids that compose the myriad of fresh and processed foods we consume. When a component is added to water (or coexists with water, as in a fresh food), the overall mobility of the water decreases, compared to that of pure water. The magnitude of the decrease depends on the number, amount, and nature of the component(s) added, as well as the effect of any processing methods used. In the past, researchers focused their attention on the relationship between water (activity, availability, mobility) and food stability. Based on the introduction of the polymer science approach to food stability by Slade and Levine (1985, 1988, 1991), the focus has shifted to the relationship... 

Because the chemical potentials of water distributed in two phases (i.e., solution and vapor) must be equal, the water activity of a food can be measured by bringing the food into equilibrium with the air above it. At equilibrium, under conditions of constant temperature and pressure, the aw values of the aqueous phase of a food (aw l) and of the air (aw v) are equal and can be estimated from the ratio of the partial vapor pressure of water above the food (pv) to the vapor pressure of pure water (p") at the same temperature (Walstra, 2003) ... 

The procedure of Beutier and Renon as well as the later on described method of Edwards, Maurer, Newman and Prausnitz ( 3) is an extension of an earlier work by Edwards, Newman and Prausnitz ( ). Beutier and Renon restrict their procedure to ternary systems NH3-CO2-H2O, NH3-H2S-H2O and NH3-S02 H20 but it may be expected that it is also useful for the complete multisolute system built up with these substances. The concentration range should be limited to mole fractions of water xw 0.7 a temperature range from 0 to 100 °C is recommended. Equilibrium constants for chemical reactions 1 to 9 are taken from literature (cf. Appendix II). Henry s constants are assumed to be independent of pressure numerical values were determined from solubility data of pure gaseous electrolytes in water (cf. Appendix II). The vapor phase is considered to behave like an ideal gas. The fugacity of pure water is replaced by the vapor pressure. For any molecular or ionic species i, except for water, the activity is expressed on the scale of molality m ... 

Water activity (uw) is defined as the ratio between the water vapour pressure exerted by the water in a food system (p) and that of pure water (p0) at the... 

Due to the presence of various solutes, the vapour pressure exerted by water in a food system is always less than that of pure water (unity). Water activity is a temperature-dependent property of water which may be used to characterize the equilibrium or steady state of water in a food system (Roos, 1997). 

Let s calculate the pH of pure water by using activity coefficients. 

Now we must decide what to do about the activity coefficients. We expect that the ionic strength (p.) of pure water will be very low. Therefore, it is reasonable to suppose that yH and y0H are both unity. 

When air is exhaled the small alveoli of the lungs could collapse if it were not for the surface active material (surfactant) present in the fluid that coats the lungs. e In fact, the lack of adequate surfactant is the cause of respiratory distress syndrome, a major cause of death among premature infants and a disease that may develop in acute form in adults. The surfactant material forms a thin film of high fluidity at the air-liquid interface and lowers the surface tension from the 72 mN/m of pure water to <10 mN/mfs (Pay attention to the definition of surface tension.11)... 

---