Isopiestic method

Raid, J. A. and Platford, R. F., 1991, Experimental methods isopiestic, in K. S. Pitzer, ed.. Activity Coefficients in Electrolyte Solutions, 2nd edition. Boca Raton, FL CRC Press, pp. 209-77. 

The activity of a volatile solvent in a solution that contains a nonvolatile solute can be obtained from an experimental technique known as the isopiestic method .19 An apparatus is constructed similar to that shown in Figure 6.17. The mixture in container A is a solution of a nonvolatile solute in a solvent in which A], the activity of the solvent, has been accurately determined in other experiments as a function of concentration. Containers B and C hold solutions of other nonvolatile solutes in the same solvent. These are the solutions for which the activity of the solvent is to be determined. 

The isopiestic method is often applied to electrolyte solutions, since volatility of the solute is not a problem. Sulfuric acid is often used as the... 

The isopiestic method is based upon the equality of the solvent chemical potentials and fugacities when solutions of different solutes, but the same solvent, are allowed to come to equilibrium together. A method in which a solute is allowed to establish an equilibrium distribution between two solvents has also been developed to determine activities of the solute, usually based on the Henry s law standard state. In this case, one brings together two immiscible solvents, A and B, adds a solute, and shakes the mixture to obtain two phases that are in equilibrium, a solution of the solute in A with composition. vA, and a solution of the solute in B with composition, a . 

The isopiestic method was first introduced by W. R. Bousfield. "Isopiestic Solutions." Trans. Faraday Soc., 13, 401-410 (1918) and refined by D. A. Sinclair. A Simple Method for Accurate Determinations of Vapor Pressure of Solutions". J. Plus. Chem., 37.495-504 (1933). 

Park has also obtained osmotic coefficient data for the aqueous solutions of NaOH-NaCl- NaAl(OH)4 at 25°C employing the isopiestic method (Park and Englezos, 1999 Park, 1999). The solutions were prepared by dissolving AlCl r6H20 in aqueous NaOH solutions. The osmotic coefficient data were then used to evaluate the unknown Pitzer s binary and mixing parameters for the NaOH-NaCI-NaAl(OH)4-H20 system. The binary Pitzer s parameters, [3(0), P0). and C9, for NaAI(OH)4 were found to be -0.0083, 0.0710, and 0.00184 respectively. These binary parameters were obtained from the data on the ternary system because it was not possible to prepare a single (NaAl(OH)4) solution. 

Thiessen and Wilson (1987) presented a modified isopiestic apparatus and obtained osmotic coefficient data for KC1 solutions using NaCl as reference solution. The data are given in Table 15.4. Subsequently, they employed Pitzer s method to correlate the data. They obtained the following values for three Pitzer s... 

Park, H., and P. Englezos, "Osmotic Coefficient Data for Na2Si03 and Na2Si03-NaOH by an Isopiestic Method and Modelling Using Pitzer s Model", Fluid Phase Equilibria, 153, 87-104 (1998). 

Thiessen, D.B., and Wilson, "An Isopiestic method for Measurement of Electrolyte Activity Coefficients", AIChE J., 33(11), 1926-1929, 1987. 

In the isopiestic method two condensed phases are equilibrated via the gas phase [104,105]. The composition and pressure of the gas phase are determined by use of a reference compound for which the partial pressure of a volatile component... 

The isopiestic method measures a difference in vapour pressure while the isothermal distillation technique depends upon a difference in volume. Despite the specific changes being measured in the techniques each change is proportional to the colligative property of the solution - the lowering of the vapour pressure. 

The isopiestic method has been used frequently to measure the vapor pressure of aqueous solutions of nonvolatile solutes. 

The experimental basis of sorption studies includes structural data (SANS, SAXS, USAXS), isopiestic vapor sorption isotherms,i and capillary isotherms, measured by the method of standard porosimetry. i 2-i44 Thermodynamic models for water uptake by vapor-equilibrated PEMs have been suggested by various groupThe models account for interfacial energies, elastic energies, and entropic contributions. They usually treat rate constants of interfacial water exchange and of bulk transport of water by diffusion and hydraulic permeation as empirical functions of temperature. 

The activity of the solvent often can be obtained by an experimental technique known as the isopiestic method [5]. With this method we compare solutions of two different nonvolatile solutes for one of which, the reference solution, the activity of the solvent has been determined previously with high precision. If both solutions are placed in an evacuated container, solvent will evaporate from the solution with higher vapor pressure and condense into the solution with lower vapor pressure until equilibrium is attained. The solute concentration for each solution then is determined by analysis. Once the molality of the reference solution is known, the activity of the solvent in the reference solution can be read from records of previous experiments with reference solutions. As the standard state of the solvent is the same for all solutes, the activity of the solvent is the same in both solutions at equUibrium. Once the activity of the solvent is known as a function of m2 for the new solution, the activity of the new solute can be calculated by the methods discussed previously in this section. 

A great deal of information on activities of electrolytes also has been obtained by the isopiestic method, in which a comparison is made of the concentrations of two solutions with equal solvent vapor pressure. The principles of this method were discussed in Section 17.5. 

Static Methods for Measurement of Vapour Pressures 4.2.2.2 The Dew-point and Non-isothermal Isopiestic 68... 

The dew-point and non-isothermal isopiestic methods. These are methods where a temperature gradient is maintained between a sample held at a high temperature and a pure metal which is held at a lower temperature. In the dew-point method the vapour pressure of the volatile component is determined by direct observation of the temperature at which it condenses. 

Alternatively, aw may be measured by isopiestic equilibration. In this method, a dehydrated sorbent (e.g. microcrystalline cellulose) with a known moisture sorption isotherm (section 7.4) is exposed to the atmosphere in contact with the sample in an enclosed vessel. After the sample and sorbent have reached equilibrium, the moisture content of the sorbent can be measured gravimetrically and related to the nw of the sample. 

Activity Coefficients of Electrolytes in Water-Polyethylene Glycol Mixed Solvent by Isopiestic Method... 

With the use of thermodynamic relations and numerical procedure, the activity coefficients of the solutes in a ternary system are expressed as a function of binary data and the water activity of the ternary system. The isopiestic method was used to obtain water activity data. The systems KCl-H20-PEG-200 and KBr-H20-PEG-200 were measured. The activity coefficient of potassium chloride is higher in the mixed solvent than in pure water. The activity coefficient of potassium bromide is smaller and changes very little with the increasing nonelectrolyte concentration. PEG-200 is salted out from the system with KCl, but it is salted in in the system with KBr within a certain concentration range. 

The isopiestic method described elsewhere (8, 9,10) was used to obtain the experimental data of water activity of the aqueous solutions. In our arrangement, eight gold-plated silver crucibles were embedded into hollows of a copper block and placed in the glass vacuum desiccator. The crucibles were provided with covers which can be handled from inside. Three desiccators were placed in a water bath (maintained at 25 0.01 °C) on a brass construction which moved from one side to the other. This movement, aided by a moving glass bead in each crucible, mixed the solutions. The desiccators with the samples were evacuated to the pressure of about 20 torr and the evacuation after a time interval was repeated 2-3 times. The time needed for the establishment of equilibrium was... 

A2.1 Factors to Consider When Estimating Water Vapor Pressure A2.2 Dew-Point Method for the Determination of Water Activity A2.3 Measurement of Water Activity Using Isopiestic Method A2.4 Direct Manometric Determination of Vapor Pressure A2.5 Measurement of Water Activity by Electronic Sensors... 

A2.3 Measurement of Water Activity Using Isopiestic Method A2.3.1... 

Isopiestic determination is one of the most commonly used methods for measuring food aw. In this method a sample of known mass is stored in a closed chamber and allowed to reach equilibrium with an atmosphere of known ERH (or equilibrate with a standard of known aw). In the first protocol (see Basic Protocol), a standard salt solution, for which aw is well established, is used to control this atmosphere. The aw of the sample is then determined by equilibration with the resulting atmosphere. In the second protocol (see Alternate Protocol), the isopiestic determination of aw is accomplished by equilibration of the sample with a reference material, for which the relationship between water content and aw is known. The condition of equilibrium is determined by reweighing the sample at intervals until constant mass is reached. The moisture content of the sample is then determined either directly or by calculation from the reference material s original moisture content and change in mass. Unsaturated salt solutions of known ERH can also be used to equilibrate the samples however, this requires estimation of the ERH of the jars at the end of the equilibration by measuring the exact concentration of the salt solution, which may be tedious. 

In the isopiestic method, often the purpose is to characterize how the aw of a sample depends upon its gravimetric water content. This curve, which relates aw and water content, is known as the Sorption isotherm. Knowledge of the sorption isotherm allows estimation of aw if a samples moisture content is known. This is useful, as it is often easier and more rapid to determine moisture content than aw (Rahman, 1995). 

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