Isopiestic

Isopiestic or isothermal distillation. This technique can be useful for the preparation of metal-free solutions of volatile acids and bases for use in trace metal studies. The procedure involves placing two beakers, one of distilled water and the other of a solution of the material to be purified, in a desiccator. The desiccator is sealed and left to stand at room temperature for several days. The volatile components distribute themselves between the two beakers whereas the non-volatile contaminants remain in the original beaker. This technique has afforded metal-free pure solutions of ammonia, hydrochloric acid and hydrogen fluoride. 

Silica [7631-86-9 (colloidal)-, 112945-52-5 (fumed)]. Purification of silica for high technology applications uses isopiestic vapour distillation from cone volatile acids and is absorbed in high purity water. The impurities remain behind. Preliminary cleaning to remove surface contaminants uses dip etching in HF or a mixture of HCl, H2O2 and deionised water [Phelan and Powell Analyst 109 1299 1984],... 

If the only external force is a normal, uniform, and constant pressure p, the necessary and sufficient condition for equilibrium is that for all virtual isothermal-isopiestic changes ... 

If SQ is the small amount of heat absorbed in a small reversible isothermal-isopiestic change, we have, if W is the heat function at constant pressure ... 

The horizontal isopiestic cuts the vapour-pressure curve of the solid in the first case, that of the liquid in the second. Melting can be brought about in case (1) by an increased pressure. 

Corollary 1.—Every spontaneous isopiestic change in a univariant system evolves heat if it takes place at a temperature... 

Corollary 2.—If there are two opposite isopiestic transformations possible for a univariant system at two different temperatures, the one occurring at a lower temperature will give rise to an evolution, that at the higher temperature to an absorption of heat. 

Then from (4), (6), (11), and (12) we find Theorem II. Ij an isopiestic p + dp is drawn to cut the three curves of transition (or their prolongations) meeting at a triple point, the central point of section corresponds u-ith the transition involving the greatest change of entropy. This theorem is due to Roozeboom (1901). 

To get an idea of the general trend of dissociation in a gas, we shall consider the isopiestics which represent, at various constant pressures, the density A as a function of temperature. 

The heat absorbed in any reversible isothermal-isopiestic change is ( 55 (10) ) given by ... 

Isentropic change. 75 lsochore, 44, 337 Isolated system, 37 Isomorphous mixture, 417 Isopiestic change, 44, 337 Isopneuma, 442 Isosteres, 442... 

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... 

Rard (1992) reported the results of isopiestic vapor-pressure measurements for the aqueous solution of high-purity NiCl2 solution form 1.4382 to 5.7199 mol/kg at 298.1510.005 K. Based on these measurements he calculated the osmotic coefficient of aqueous NiCb solutions. He also evaluated other data from the literature and finally presented a set of smoothed osmotic coefficient and activity of water data (see Table IV in original reference). 

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). 

Rard, J.A., "Isopiestic Investigation of Water Activities of Aqueous NiCl2 and CuCl2 Solutions and the Thermodynamic Solubility Product of NiCl2-6H20 at 298.15 K", J. Chem. Eng. Data, 37,433-442 (1992). 

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

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