Salts freezing point depression

Here, i, the van t Hoff i factor, is determined experimentally. In a very dilute solution (less than about 10 3 mol-I. ), when all ions are independent, i = 2 for MX salts such as NaCl, i = 3 for MX2 salts such as CaCl2, and so on. For dilute nonelectrolyte solutions, i =l. The i factor is so unreliable, however that it is best to confine quantitative calculations of freezing-point depression to nonelectrolyte solutions. Even these solutions must be dilute enough to be approximately ideal. 

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]. 

Antifreezing agents for cement consist mainly of salts such as calcium chloride, magnesium chloride, sodium chloride, and soda. Calcium chloride is highly corrosive and very restricted in use. Some salts, especially potassium chloride, will affect the curing time of cement. The latter chemical is in fact used to increase the pot life of cement. Likewise, alcohol freezing-point depressants, such as ethylene glycol, can be also included in the composition [1022]. 

We must appreciate, however, that no chemical reaction occurs between the salt and the water more or less, any ionic salt, when put on ice, will therefore cause it to melt. The chemical identity of the salt is irrelevant - it need not be sodium chloride at all. What matters is the amount of the salt added to the ice, which relates eventually to the mole fraction of salt. So, what is the magnitude of the freezing-point depression ... 

Salt is a strong electrolyte that produces two ions, Na+ and Cl, when it dissociates in water. Why is this important to consider when calculating the colligative property of freezing point depression ... 

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. 

Fish living in Arctic and Antarctic waters may encounter temperatures as low as -1.9°C. The freezing point depression provided by dissolved salts and proteins in the blood is insufficient to protect the fish from freezing. As winter approaches, they synthesize and accumulate in their blood serum a series of eight or more special antifreeze proteins.a d One type of antifreeze glycoprotein from winter flounder contains the following unit repeated 17-50 times. 

The method can be extended to salt mixtures, if the freezing point depression of water is known for the mixture. Patwardhan and Kumar (1986) suggest a simple extension to determine water activities for mixed salts from single salt activities, such as in Equations 4.9 and 4.10. 

Table A.5 is the output file for salts in the 4.5- to 5.0-km layer, where the system pressure is 484.5 bars (102 bars km-1 x 4.75 km). The temperature of 268.28 K is the freezing point depression for this particular composition and pressure at 268.27 K, ice forms. The pH of this system is 8.02. The number of independent components is seven. This example deals with lithostatic pressures on solutions dispersed in a regolith, which is fundamentally different from the previous examples (Tables A.2-A.4) that dealt with seawaters.

Free Energy Perturbation Monte Carlo Simulations of Salt Influences on Aqueous Freezing Point Depression... 

Using the freezing point depression expression we can determine the molality of the salt-ice solution that will reach -15°C. 

Various salts can cause freezing point depressions. These depressions are the results of the ions colligative properties within solution. A significant freezing point depression is created not by any particular type of material, but rather by the number of particles you have in solution. The effects can be enhanced by achieving a supersaturation of material. For example, if you mix ice with a salt such as NaCl, you will end up with two particles within solution (Na+ + Cl ) (the temperature of ice water supersaturated with sodium chloride (23% by weight) is -20.67°C). If you place a salt such as CaCl2- H20 into ice, you will end up with three particles within the solution (Ca+ + 20 ) [the temperature of ice water... 

A practical example of freezing point depression is the use of salt on icy roads. Salt does not melt the ice it just lowers the freezing point. 

Figure 2-9. Relationship between concentration and osmotic pressure at 20°C for a nonelectrolyte (sucrose) and two readily dissociating salts (NaCl and CaCl2). The different initial slopes indicate the different degrees of dissociation for the three substances and are consistent with the Van t Hoff relation (Eq. 2.10). Data for osmotic pressure are based on the freezing point depression. (Data source Lide, 2008.)...

What happens in snow, in fact, if the snow is falling, is that there will be in snow the different salts, mainly ammonium sulphate salts — and where there are temperature changes, of course there will be in the system a freezing point depression, so finally during the season everything of these particles will be collected just over the soil, down there, and now when the melting period comes, you have the shock, and the shock, especially when the snow is lying on a frozen lake, can kill all the fish population in a few days. 

You may be familiar with the property illustrated in the first demonstration, freezing-point depression, if you have ever used an ice cream maker. In ice cream making, the cream mixture is poured into a container and the container is placed in a bath of salted ice. The salt-ice mixture cools to a point below the freezing point of water, so when the water in the cream comes into contact with the sides of the container, it freezes. A paddle to scrape ice crystals off the sides of the container is turned constantly until the cream and ice form a uniform semisolid. The fi eezing point of the salt-ice mixture is lowered because of the presence of a dissolved solute (salt) in the solvent (water). Antifreeze works on the same principle. Chosen to be noncorrosive, antifreeze is a material added to radiator water to lower the freezing point so the radiator water will not readily freeze. 

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