Freezing-point

The freezing point of a coalescing agent should be low (below -20°C) as materials with a high freezing point may require specialized (therefore more expensive) handling techniques in transport and storage. 

The evaporation rate of a coalescing aid should be slow enough to ensure good film formation of the emulsion coating under a wide range of humidity and temperature conditions  

The odor of a coalescing agent should be minimal. This is especially important for interior coatings ap- 

A coalescing agent should be colorless, to prevent discoloration of the product. 

Freezing poirU of solutions. The freezing point of a solution is always lower than that of the pure solvent. This may be seen at once from Fig. 26, which differs from Fig. 25 only by the addition of the vapour pressure curve CNF of the solid solvent. The abscissa of P, the intersection of CP and is the freezing point Tq of the pure solvent, and the abscissa of the corresponding intersection N is the freezing point of the solution, for at these points the solid solvent is in equilibrium with solvent and solution 

For L=Lq we have L —L — Ls—LQ = Wf the molecular heat of fusion of the pure solvent, and hence 

The formula for the lowering of the freezing point is thus quite analogous to the formula for the raising of the boiling point, and differs from it in that the heat of fusion takes the place of the heat of evaporation. 

For concentrated solutions, for which the heat of dilution has a finite value and for which the lines NP and NQ are always curved, we can deduce a more accurate equation as follows  

We have also the corresponding equation for the solid solvent 

Three test methods are available for determination of the freezing point. All three methods have been found to give equivalent results. However, when a specification calls for a specific test, only that test must be used. 

In the first test (ASTM D-2386, IP 16), a measured fuel sample is placed in a jacketed sample tube also holding a thermometer and a stirrer. The tube is placed in a vacuum flask containing the cooling medium. Various coolants used are acetone, methyl alcohol, ethyl alcohol, or isopropyl alcohol, solid carbon dioxide, or liquid nitrogen. As the sample cools, it is continuously stirred. The temperature at which the hydrocarbon crystals appear is recorded. The jacketed sample is removed from the coolant and allowed to warm, under continuous stirring. The temperature at which the crystals completely disappear is recorded. 

In the second test (ASTM D-5901, IP 434), an automated optical method is used for the temperature range to -70°C (-94°F). In this method, a 

25-min portion of the fuel is placed in a test chamber that is cooled while continuously being stirred and monitored by an optical system. The temperature of the specimen is measured with an electronic measuring 

In the third method (ASTM D-5972, IP 435), an automated phase transition method is used in the temperature range -80 to 20°C (-112°F to 68°F). In this test, a specimen is cooled at a rate of 15 5°C/min while continuously being illuminated by a light source. The specimen is continuously monitored by an array of optical detectors for the first formation of solid hydrocarbon crystals. After that the specimen is warmed at the rate of 10 0.5°C/min until all crystals return to the liquid phase, and that temperature is also recorded. 

A condition for equilibrium between the liquid solution and pure solid solvent 1 is 

Equation (11-149) can be rearranged to yield an explicit expression for log ifiXi) in the form 

The quantities Aj and Aj can be determined from the properties of pure systems containing only component 1. They are functions of pressure alone. Equation (11-155) is an expression for log (./ix,) as a function of pressure p and freezing-point depression 9. Thus, measurement of the temperature at which solid component 1 first appears in the system at pressurep is sufficient to determine/,. Equation (11-155) is useful when the solution is dilute and 9jTi 1. 

In a binary solution, the activity coefficient of component 2 can be determined from Eq. (11-155) by use of the Gibbs-Duhem equation. Since, in the applications of this method, the solution is dilute with respect to component 2 it will be most useful to determine y, the activity coefficient of component 2 with respect to the molality. At constant pressure, the Gibbs-Duhem equation may be written 

It is not possible to apply the Gibbs-Duhem equation at constant T and p to this system because of the requirement of equilibrium between the solution and pure solid 1. Substitution of Eq. (11-147) for pi and Eq. (11-11) for P2 into Eq. (11-158) results in 

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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. 

Uquidus curve The freezing point of a molten mixture of substances varies with the composition of the mixture. If the freezing points are plotted as a function of the composition, the line joining the points is called a liquidus curve. Such mixtures usually freeze over a range of temperature. If the temperature at which the last traces of liquid just solidify (assuming that sufficient time has been allowed for equilibrium to be established) are plotted against composition the resulting line is called a solidus curve. 

Raoult s law When a solute is dissolved in a solvent, the vapour pressure of the latter is lowered proportionally to the mole fraction of solute present. Since the lowering of vapour pressure causes an elevation of the boiling point and a depression of the freezing point, Raoult s law also applies and leads to the conclusion that the elevation of boiling point or depression of freezing point is proportional to the weight of the solute and inversely proportional to its molecular weight. Raoult s law is strictly only applicable to ideal solutions since it assumes that there is no chemical interaction between the solute and solvent molecules. 

The properties of the solids most commonly encountered are tabulated. An important problem arises for petroleum fractions because data for the freezing point and enthalpy of fusion are very scarce. The MEK (methyl ethyl ketone) process utilizes the solvent s property that increases the partial fugacity of the paraffins in the liquid phase and thus favors their crystallization. The calculations for crystallization are sensitive and it is usually necessary to revert to experimental measurement. 

Solubility of water in jet fuels as a function of temperature (Jet A is a variant] of Jet Al, used in the USA for domestic flights. Jet A has a freezing point higher than that of Jet Al). ... 

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... 

Outside of their very high resistance to auto-ignition, the aviation gasolines are characterized by the following specifications vapor pressure between 385 and 490 mbar at 37.8°C, a distillation range (end point less than 170°C), freezing point (-60°C) and sulfur content of less than 500 ppm. 

Paraffins consist mainly of straight chain alkanes, with a very small proportion of isoalkanes and cycloalkanes. Their freezing point is generally between 30°C and 70°C, the average molecular weight being around 350. When present, aromatics appear only in trace quantities. 

Freezing point NF M 07-048 ISO 3013 ASTM D 2386 Temperature of disappearance of cloud on reheating... 

The ammonia gas is used again and the only by-product, calcium chloride, is used to melt snow, prevent freezing of coal in transit and as an antidust treatment since it is hygroscopic and forms a solution of low freezing point. 

The second equilibrium is the more important, giving rise to the nitronium ion, NOj, already mentioned as a product of the dis sociation of dinitrogen tetroxide. Several nitronium salts have been identified, for example nitronium chlorate(VII), (N02) (C104) . If pure nitric acid is dissolved in concentrated sulphuric acid, the freezing point of the latter is depressed to an extent suggesting the formation of four ions, thus ... 

Molecular Weight Determinatioos. Details of the determination of molecular weights on a semi-micro scale by the Freezing-point method are given on p. 436, and by the Boiling-point method on p. 440. 

III. Depression of the Freezing-point of Camphor (Rast s Method). 

If, on the other hand, the thermometer has previously been used at some temperature below the freezing-point of benzene, when the bulb is originally placed in the beaker of water at 7-8 C., the mercury will rise in the capillary and ultimately collect in the upper part of the reservoir at a. When the expansion is complete, again tap the thermometer sharply at R so that this excess of mercury drops down into b, and then as before check the success of the setting by placing the thermometer m some partly frozen benzene. In either case, if the adjustment is not complete, repeat the operations, making a further small adjustment, until a satisfactory result is obtained. 

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