Freezing-point cryoscopic method

Freezing-point (Cryoscopic) Method If w gm. of a substance when dissolved in W gm. of a solvent lower the freezing-point of the latter by the molar weight of the solute is obtained by means of the expression 

The method can be employed only when the solute does not form an isomorphous mixture (solid solution) with the solid solvent, i.e. only when the solvent crystallizes out pure. 

Apparatus.—In order to find the value of d in the above expression, it is necessary to determine the freezing-point of the pure solvent and of a solution of known concentration. This determination is carried out in the apparatus shown (Fig. 42).2 The cooling bath consists of a glass or stoneware vessel, A, on the top of which rests a lid of brass, L. Through a hole in the centre of the lid there passes a wide glass tube, B, which is fixed in place by means of a cork and through 

The freezing-point tube C, which is furnished with the side tube D, is supported in the neck of tube B by means of a cork or asbestos ring, so that the freezing-point tube is surrounded by an air-mantle. This ensures a slower and more uniform 

In order to keep the temperature of the cooling bath more uniform, it is well to surround the latter and to cover the lid with thick felt. 

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Freezing-point depression method (cryoscopic method)... 

The theory of the cryoscopic method, which is also called the freezing point lowering method, has been given by Rossini [44-tay/ros, 44-tay/ros, 50-ros]. For the equilibrium between a crystalline phase consisting of the major component alone and a liquid phase corrsisting of the major component and minor components, the thermodynamic relation between the temperature of eqitilibriirm and the composition of the liquid phase, for an ideal or sufficiently dilute solution, is [similiarto equation (1.4)] ... 

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... 

Henningson, R. W. 1970. Regulatory agency acceptance of the interpretation of the freezing point value of milk as part of the official cryoscopic method. J. Assoc. Offic. Anal. Chem. 53, 539-542. 

The polyformal (I) was characterized by wet chemical and infrared analyses, and by cryoscopic molecular weight determinations. For the latter, the preferred method was the freezing point depression of an ethylene bromide solution. The other polyacetals were characterized primarily by infrared analysis and molecular weight determinations. 

An osmometer is an instrument which measures the osmolality of a solution, usually by determining the freezing point depression of the solution in relation to pure water, a technique known as cryoscopic osmometry. A small amount of sample is cooled rapidly and then brought to the freezing point (Fig. 6.1), which is measured by a temperature-sensitive thermistor probe calibrated in mosmol kg . An alternative method is used in vapour pressure osmometry, which measures the relative decrease in the vapour pressure produced in the gas phase when a small sample of the solution is equilibrated within a chamber. 

The cryoscopic method was used in Hantzsch s original work 53, 54) and was later developed by Hammett and Deyrup 56) and by Gillespie et al. 48). Sulfuric acid freezes at 10.371 C and has a relatively large molal freezing-point depression or cryoscopic constant fc = 6.12 32). It is thus a convenient solvent for cryoscopic measurements provided that adequate precautions are taken to prevent absorption of water from the atmosphere. From freezing-point measurements the number, v, of moles of particles (ions and molecules) produced by one mole of any solute may be obtained. Some examples are given in Table IV. 

Starch acetate solutions have so far proved unsatisfactory for molecular weight measurements by either the cryoscopic or ebullioscopic methods. Cryoscopic measurements in phenol or glacial acetic acid have generally given abnormally large freezing point depressions indicative of impossibly low molecular weight values for the starch acetate. 

Schardinger himself did not attempt molecular-weight determinations of the crystalline dextrins. The first molecular-weight determinations were based on freezing-point depressions in water" or on osmotic-pressure measurements. " " Other measurements were based on dialysis rates or on cryoscopic measurements rvith the acetates," nitrates, and methyl ethers. In some cases microisopiestic methods were used."-... 

Another method of determining the molecular mass of a substance is to measure the freezing-point depression of a solvent that is brought about when a known quantity of test substance is added. This is known as a cryoscopic method. Another method, which is used occasionally, is vapor pressure osmometry, in which the molecular weight of a substance is determined through an examination of the change in vapor pressure of a solvent when a test substance is dissolved in it. 

The accirrate determination of freezing points by time-temperature analysis was extensively used by the American Petroleirm Institute Research Project 6 for the determination of the pirrities of hydrocarbon samples. Also, the method has been developed for determining the cryoscopic constant, from which the molar enthalpy of fusion can be calculated. Accirracy of freezing points determined by this method is estimated to be 0.01 K. However, inaccirracy of enthalpy of fusion estimated from the method is more than +10 percent, or even 50 percent in some cases. Especially, the method can not separate the enthalpy of a solid transition from the enthalpy of fusion if the temperatures of these phase transition are within a few Kelvin. 

The average molecular weight of MAO can be measured by a cryoscopic method. The MAO solution in toluene is dried up, dissolved in dioxane, and subsequently the temperature of the freezing point can be measured. A small amount of TMA strongly coordinated with the MAO is a contamination to this solution and can cause some error to the molecular weight of the MAO, but the optimum molecular weight of MAO is around 400g/mol (Fig. 3.5). 

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