Solubility freezing point lowering

The description of phase equilibria makes use of the partial molar free enthalpies, i, called also chemical potentials. For one-component phase equilibria the same formalism is used, just that the enthalpies, G, can be used directly. The first case treated is the freezing point lowering of component 1 (solvent) due to the presence of a component 2 (solute). It is assumed that there is complete solubility in the liquid phase (solution, s) and no solubility in the crystalline phase (c). The chemical potentials of the solvent in solution, crystals, and in the pure liquid (o) are shown in Fig. 2.26. At equilibrium, ft of component 1 must be equal in both phases as shown by Eq. (1). A similar set of equations can be written for component 2. By subtracting j,i° from both sides of Eq. (1), the more easily discussed mixing (left-hand side, LHS) and crystallization (right-hand side, RHS) are equated as Eq. (2). 

When the temperature decreases, water becomes less soluble (see Figure 5.15) and deposits as fine droplets that begin to freeze as the temperature reaches 0°C. To prevent this occurrence, it is possible to use anti-freeze additives that absorb the water and lower the freezing point. These products, used at maximum levels of 1500 ppm, are ethers-alcohols for example, 2-methoxy... 

The freezing-point of water in contact with ether is lowered by 80,85 owing to the amount of the latter dissolved. If now a third substance (e.g., benzene) is added to the ether, which does not dissolve in the water, the freezing-point will be raised on account of the diminished solubility of the ether. The diminution of solubility of the ether is quite apparent to the eye if the liquids are contained in a graduated tube. 

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 key feature of ethylene glycol (EG) is the hydroxyl group, -OH, one on each of the two carbon atoms. The hydroxyls are responsible for its reactivity EG is a monomer used in the production of polyester polymers. The hydroxyls also give EG its most important physical property its solubility in water. That, linked with its low freeze point, makes EG suitable as an antifreeze and as a deicer. When EG is sprayed on ice, it combines with the water crystals and lowers the freeze point. This causes the mixture to melt and effectively keeps it in the liquid state. 

ACTIVITY COEFFICIENT. A fractional number which when multiplied by the molar concentration of a substance in solution yields the chemical activity. This term provides an approximation of how much interaction exists between molecules at higher concentrations. Activity coefficients and activities are most commonly obtained from measurements of vapor-pressure lowering, freezing-point depression, boiling-point elevation, solubility, and electromotive force. In certain cases, activity coefficients can be estimated theoretically. As commonly used, activity is a relative quantity having unit value in some chosen standard state. Thus, the standard state of unit activity for water, dty, in aqueous solutions of potassium chloride is pure liquid water at one atmosphere pressure and the given temperature. The standard slate for the activity of a solute like potassium chloride is often so defined as to make the ratio of the activity to the concentration of solute approach unity as Ihe concentration decreases to zero. 

The lower quadruple point Qi (I-Lw-H-V) is located at the intersection of the three-phase Lw-H-V and the I-H-V pressure-temperature loci, usually within a degree of the ice point (273.15 K). The intersection temperature closely approximates the ice point because (with the exception of carbon dioxide and hydrogen sulfide) the solubility of hydrate formers in water is normally too small to change the freezing point of water significantly. 

In the study of the solubility of a solid component in a solution or the lowering of the freezing point of a solvent by the presence of a solute, the solid phase must be considered as the pure component. We indicate this component, the one that is present in both phases, by the subscript 1. The... 

This phenomenon can also be looked at in terms of the dissolution of solid A in the solution (even though A, because it is present at a greater concentration, would usually be considered the solvent of this solution). As the temperature is lowered, the solubility of A in the solution decreases. If the solution is ideal, we can use Eq. (55) with aA = xA to calculate the solubility of A in the solution at temperatures below the freezing point of A ... 

However, the general form of the curve on the left-hand side of Fig. 10 is followed even in nonideal systems. Looking at the right-hand side of Fig. 10, we see that as A is added to pure B, the freezing point of the solution is also lowered. (We have also assumed that B does not form solid solutions.) Alternatively, this can be described as reducing the solubility of B in the solution as the temperature of the solution is lowered below the freezing point of pure B. At an intermediate concentration, the two solubility curves in Fig. 10 meet at point e and a solution with the lowest freezing point is obtained. This solution is known as the eutectic and the concentration and temperature at point e, is known as the eutectic concentration and temperature. 

At 200 bar static pressure the freezing temperature is usually between 4°C and 5°C higher than at normal pressure. Under 200 bar N2 atmosphere the increase is about 40 % lower. Physical solubility of N2 results in a relatively small decrease of the freezing point, so that the effect of static pressure is still dominant. 

For permafrost well compositions, the binder content in the slurry is as high as 72.5 wt%, and the rest is wollastonite and boric acid. The high binder content provides sufficient KH2PO4 solution, which lowers the freezing point of the slurry and allows the acid-base aqueous reaction to continue. With low binder content, the water in the slurry freezes before it reacts. At ambient temperamre, however, one may reduce the amount of binder and substitute it with suitable extenders. Wollastonite, because of its temperature of maximum solubility at 109 °F (43°C) is the preferred choice, but a combination of Class C and F ashes has also been used by researchers [9]. 

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