Water activity isopiestic equilibration

Alternate Protocol Isopiestic Determination Water Activity by Equilibration... 

ISOPIESTIC DETERMINATION WATER ACTIVITY BY EQUILIBRATION WITH STANDARDS OF KNOWN WATER ACTIVITY (aw)... 

ISOPIESTIC DETERMINATION OF WATER ACTIVITY (aw) BY EQUILIBRATION WITH SATURATED SALTS... 

Fig. 29. Enzymatic activity of lysozyme as a function of water content (grams of water per gram of sample), at pH 8, 9, and 10. , O, A, Measurements on powders hydrated by isopiestic equilibration ,, A, solvent added to powder. Powder samples were the 1 1 (GlcNAc)6-lysozyme complex, obtained by lyophilization. The reaction rate (no sec ) was determined by product analysis. From Rupley etcd. (1980).

Fig. 17.7. Two experimental techniques for measuring stoichiometric ion activity coefficients in hydrothermal solutions, (a) Determination by measuring difference in vapor pressures between solutions of known total salt concentration and pure water, (b) Isopiestic measurement in which sample solutions containing known weights of salts are equilibrated with a standard solution for which activity coefficients have been independently measured. In a sealed system water activity is everywhere the same at equilibrium measured salt concentrations in each sample container give the desired activity coefficients from the known activity of water in the standard salt solution.

Isopiestic studies involve equilibration of the water in a small sample with that in a large reference mixture of known water activity, via transport through the vapor phase. Because of its poor solubility at room temperature, the activity of water in saturated solutions of C12MG will be very close to 1 therefore, the reference mixture selected was pure water. Elemental analytical data established that the starting sample was dry. Isopiestic... 

For a two-component solution with a volatile solvent such as water and a nonvolatile solute, values of the activity of the solvent can be determined for several values of the solvent mole fraction between unity and the composition of interest. Integration of the Gibbs-Duhem relation can then give the value of the activity coefficient of the solute. The activity of the solvent is usually determined using the isopiestic method. The solution of interest and a solution of a well-studied nonvolatile reference solute in the same solvent are placed in a closed container at a fixed temperature. A solution of KCl is usually used as the reference solute for aqueous solutions, since accurate water activity coefficient data are available for KCl solutions. The solutions are left undisturbed at constant temperature until enough solvent has evaporated from one solution and condensed into the other solution to equilibrate the solvent in the two solutions. 

Isopiestic measurements. High-temperature isopiestic method could be considered as an instrumental method among other methods of sampling without temperature and pressure drops to study hquid-gas equilibrium (Tsopiesf in Table 1.1). One of the electrolyte solutions present in the isopiestic vessel is a reference since its vapor pressure is known imder the conditions of the experiment. The compositions of the solutions placed in the same vessel change during equilibration due to a redistribution of water between liquid solutions to reach the common vapor pressure at the constant temperature. The equilibrium composition of the isopiestic solutions could be measured at the experimental or at the room temperature if the samples of isopiestic solutions carefidly preserved and analyzed in order to determine the concentration (isopiestic molality) at the equilibrium, isopiestic vapor pressures and activity coefficients for the electrolytes. 

---