Freezing-point method

The values of k for some of the common solvents with their melting-points are given in the following talrle — 

It should be remembered that nitrobenzene, phenol, and acetu. acid are hygroscopic. 

A Beckmann Freezing-point Apparatus.—The form of apparatus is shown in the accompanying Fig. 30. It consists of a glass jar standing on a metal tray and furnished with a stirrer. The cover of the jar has a wide slit to admit the stirrer, and a circular aperture with clips to hold a wide test-tube. 

Within the wide test-tube is a narrower one, which is held in position by a cork. The narrow test-tube is sometimes 

Freezing-point Determination. — I n the example to be described, pure benzene (see p. 136) is used as the solvent. Carefully dry the inner tube. Fit it wuth a cork and weigh it together w ith the coik suspended by a wire to the aim of the balance. Intioduce sufficient benzene to cov er the bulb of the Beckmann thermometer when it is pushed nearly to the bottom of the tube. About 10 c.c. will be found to be sufficient. Insert 

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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. 

The method is used to determine the molecular weight primarily of those compounds which are too slightly soluble in the usual cold solvents for the more efficient freezing-point method to be nployed. 

The same considerations with regard to association in solution apply here as in the freezing-point method (p. 435). 

As in the freezing-point method, the molecular weight is calculated from the weight of substance lequired to raise the boiling-point of too grams of solvent i°, and the result multiplied by a coefficient which depends upon the nature of the solvent. The following is a list of solvents commonly employed and their coefficients and boiling-points —... 

Filter-pump, 44 Filtration through cloth, 131 under reduced pressure, 43 with fluted filter, 53 "Fischer s ester method, 133 Fluorescein, 187 Fluted filter, 53 Formic acid, 106 Fractional distillation, 136 Fractionating columns, 137 Freezing-point method, 32 I -riedel-Crafis reaction, 210 furnace, combustion, 4 tube, 23 ... 

Freezing point methods are often applied to the measurement of activities of electrolytes in dilute aqueous solution because the freezing point lowering, 6= T — T, can be determined with high accuracy, and the solute does not dissolve in the solid to any appreciable extent. Equations can be derivedgg relating a to 9 instead of T and T. The detailed expressions can be found in the literature.16... 

For -weak electrolytes the degree of dissociation found by the freezing point method agrees with that found from conductivity within the limits of experimental error and Ostwald s dilution law is obeyed. 

Note Most laboratories use the freezing point method of determining osmolality. However, if the vaporization point method is used, the alcohols may be driven off and their contribution to osmolality will be lost. 

The molecular weight of cobalt chloride as determined by the freezing-point method, with urethane as solvent, corresponds to the double formula, Co2Cl4 (compare ferrous chloride),1 but the results obtained by the boiling-point method 2 indicate that under those conditions the molecule is single, namely, CoCl2. 

A salt which proved to have the simple molecular weight by the freezing-point method was prepared by the action of ethyl hexameta-phosphate dissolved in alcohol on sodium ethoxide —... 

The Determination of Molecular Weight by the Freezing-Point Method. The freezing-point method is a very useful way of determining the molecular weights of substances in solution Camphor, with its very large constant, is of particular value for the study of organic substances. 

The specific rotation of one preparation of the amorphous powder was —28.3 (c 1.5, water) while that of another was —23.2 (c 0.3). The copper reducing power by the Munson and Walker method was 7.4 and 13.9%, respectively, of that of n-fructose. The molecular weight of the second preparation, determined by the freezing point method in water, was found to be 206, an indication that it contained about three parts of the monomer to one of the dimer. 

Tin and lead differ from silicon in forming divalent and tervalent compounds, and tin also forms hydrides of the type SiiR H and The tervalent compounds show true tervalency in dilute solution only, for if the molecular weights by the freezing-point methods are carried out with increasing concentration of solute, association appears to take place. 

In other instances, the values did not correlate with those provided from other sources and in at least one instance no freezing point depression was noted. These effects have not been well explained, although similar anomalies have been observed with other high polymers. For example, in the case of cellulose derivatives, Freudenberg has shown that the freezing point method is very susceptible to error, due both to a tendency of the solute to crystallize and to the probable presence of small amounts of impurities of low molecular weight. 

It will be observed that equation (36.11) is used as the basis of the familiar procedure for the determination of molecular weights by the freezing point method. It is obviously strictly applicable only to very dilute solutions at appreciable concentrations the approximations made in its derivation are no longer justifiable. These are as follows first, that AH/ is independent of temperature second, that ToT is equal to To third, that In ni may be replaced by — n and fourth, that Nj may be set equal to nj/ni. The two latter approximations are avoided in equation (36.8), and this ves somewhat better results than does (36.11) in solutions of moderate concentration, provided the solvent still obeys il oult s law. 

The amount of reliable data available for the purpose of correcting activity coefficients obtained from freezing point measurements is not large. The freezing point method has thus been mainly used for the study of dilute solutions. 

It is seen that the depression of the freezing point is greater than the elevation of the boiling point. On this account the determination of molecular weights by the freezing point method is often preferred. 

Gasser and co-workers °° have obtained activity coefficients from the freezing point method for LiCl in DMSO up to about 0.4m and of Csl up to 0.25m. Skerlak and co-workers ° have calculated activity coefficients for several uni-univalent electrolytes in DMSO by the freezing point method. The measurements reach concentrations greater than Im, but are presented only in graphical form. (For further discussion see sect. 2.10.3.)... 

The vapor pressure depression method has the advantage of smaller sample size. However, it is not as precise as the freezing point method and cannot measure the contribution of volatile solutes such as ethanol. This method is not used as widely as the freezing point depression method in clinical laboratories. 

Above 51 dicobalt octacarbonyl decomposes into tetra-cobalt dodecacarbonyl [Co(CO) s]4, and carbon monoxide. This carbonyl is sparingly soluble in benzene and pentane and can be purified from these solvents to give pitch-black glistening crystals. Its molecular weight by the freezing-point method indicates the tetrameric formula. ... 

Raoult s work attracted widespread attention and the freezing-point method for determining molecular weights came into general use from about 1886. Some early determinations of molecular weights were made by Paterno and Nasini. The use of the method was made easy by the well-known apparatus devised by Beckmann. ... 

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