Temperature freezing-point depression

Beckmann thermometer A very sensitive mercury thermometer with a small temperature range which can be changed by transferring mercury between the capillary and a bulb reservoir. Used for accurate temperature measurements in the determination of molecular weights by freezing point depression or boiling point elevation. 

This is an expression of Raoult s law which we have used previously. Freezing point depression. A solute which does not form solid solutions with the solvent and is therefore excluded from the solid phase lowers the freezing point of the solvent. It is the chemical potential of the solvent which is lowered by the solute, so the pure solvent reaches the same (lower) value at a lower temperature. At equilibrium... 

Freeze Point Depression. The slight heat-transfer penalty incurred when an antifreeze is added to the aqueous heat-transfer fluid is necessitated by the need for increased operating temperature range in most internal combustion engines. Because most parts of the world achieve temperatures below freezing during some time of the year, an antifreeze fluid is required to keep equipment operational in these subfreezing temperatures. 

If we calculate the H values for various water temperatures, we see results as shown in Table 4.4. The importance of the information content encoded in the H value in these studies is that it is a single-numerical description of the system, water in this case, that can be used to relate to physical property changes occurring at different temperatures. This approach can be used to evaluate a property change such as the freezing point depression. 

Using the information content, H, to describe the structure at any temperature, it is possible to estimate the new temperature of water when a solute has been added. An increase in this temperature corresponds to the freezing point depression because the water must experience a greater decrease in temperature in order to arrive at the point of solidification. 

A = Kj- Cflj A 7b = Ki) Cflj We use molality in these equations because they describe temperature changes. The constant Zf is called the freezing point depression constant, and is called the boiling point elevation constant. These constants are different for different solvents but do not depend on the identity of the solutes. For water, Zf is 1.858 °C kg/mol and is 0.512 °C kg/mol. 

In the freezing point depression method, one measures the temperature lowering AT/ required to render the activity of the solvent in the solution equal to that of the pure crystalline solvent (referred to the pure liquid as the standard state see above). Then... 

The solvent s activity can be determined by measuring the saturation vapor pressure above the solution. Such measurements are rather tedious and their accuracy at concentrations below 0.1 to 0.5M is not high enough to produce reliable data therefore, this method is used only for concentrated solutions. The activity can also be determined from the freezing-point depression or boiling-point elevation of the solution. These temperature changes must be ascertained with an accuracy of about 0.0001 K, which is quite feasible. This method is used primarily for solutions with concentrations not higher than 1M. 

Special formulations have been developed for cementing operations in arctic regions or for deep water applications [206,208,256,720,739,1792]. In low-temperature formations, wherein the cement is subjected to freeze-thaw cycling, freezing-point depressants must be added. Salts may serve as such, but traditional organic freezing-point depressants, such as ethylene glycol, also may be added [1022-1024]. 

In addition to the surfactant, a freezing-point depressant can be added for low-temperature transportation. Possible depressants include salts, sugars, and alcohols such as glycerol [736]. 

Mixtures of aqueous emulsions of oil can be more effectively transported through pipelines if certain antifreeze formulations are added to the system. Stable oil-in-water emulsions for pipeline transmission by using 0.05% to 4% ethoxylated alkylphenol as an emulgator and a freezing-point depressant for water enable pipeline transmission at temperatures below the freezing point of water [736]. 

This expression can be modified to apply directly to any of various techniques used to measure the interaction parameter, including membrane and vapor osmometry, freezing point depression, light scattering, viscometry, and inverse gas chromatography [89], A polynomial curve fit is typically used for the concentration dependence of %, while the temperature dependence can usually be fit over a limited temperature range to the form [47]... 

The freezing point depression of a solvent is proportional to the concentration of solute particles and may be used to measure the extent of ionization once the new particles have been identified qualitatively as ions. The method has the obvious disadvantage of not allowing measurements over a range of temperatures in a single solvent. It is almost certainly not worth while to compute an enthalpy of ionization from ionization constants at two different temperatures in two different solvents. Usable solvents are limited not only by the requirement that the melting point be at a convenient temperature but also by the requirement that the solvent be capable of producing ions yet not be sufficiently nucleophilic to react irreversibly with them once they are formed. For this reason most cryoscopic work has been done in sulfuric acid or methanesulfonic acid.170... 

An inherent property of osmolytes is that they lower the freezing point depressions of cells (Kirst 1996). Therefore, DMSP may also be acting as a cryoprotectant in algae exposed to freezing temperatures. However, Kirst (1996) argues that cellular concentrations of DMSP are too low to produce a significant freezing point depression. 

In freezing-point depression and boiling-point elevation problems, to find the actual freezing/boiling point, calculate the AT (change in temperature), then subtract that amount from the solvent s freezing point, or add it to the solvent s boiling point. 

Calculate the molality of the solution by dividing the change in temperature (AT) by the freezing-point depression constant (A)). 

Measuring The colligative property of freezing point depression can be observed in a simple laboratory investigation. You will measure the temperatures of two beakers and their contents. 

For a solution with a liquid as solvent, the temperature at which it freezes to a solid is slightly lower than the freezing point of the pure solvent. This phenomenon is known as freezing point depression and is related in a simple manner to the concentration of the solute. The lowering of the freezing point is given hy... 

By taking the freezing point constant for water as 1.86 from Table 9-3 and then substituting the values into the equation for freezing point depression, you obtain the change in freezing temperature ... 

A solution also exhibits a depression in its freezing point. The freezing point depression is the decrease in the temperature of the freezing point due to the addition of a solute. It is calculated using the equations ATj. = Kjm, where ATj. is the decrease in freezing point for the solution, Kj. is the molal freezing point depression constant, and m is the molality of the solution. Water s K. value is 1.86°C/m. 

Adding an impurity to a solvent makes its liquid phase more stable through the combined effects of boiling point elevation and freezing point depression. That s why you r irely see bodies of frozen salt water. The salt in the oceans lowers the freezing point of the water, making the liquid phase more stable and able to sustain temperatures slightly below 0°C. 

Phase diagrams from freezing point depressions show true compound formations for simpler amides—e.g., water-N-methylacetamide forms a compound at a mole ratio of 2 to 1, water-N,N-dimethylacetamide at 3 to 2 and 3 to 1, and water-N-methylpyrrolidone at 2 to 1. The heats of mixing and heat capacities at 25°C. of a number of water-amide systems were determined. All mixing curves were exothermic and possess maxima at definite mole ratios, while the heat capacities for the most part show distinct curvature changes at the characteristic mole ratios. Both experimental results point to the stability of the particular complexes even at room temperature. This is further supported by absolute viscosity studies over the whole concentration range where large maxima occur at these same mole ratios for disubsti-tuted amides and N-substituted pyrrolidones. 

---