Osmotic Methods

Dynamic osmotic methods have been used occasionally, the rate of permeation being measured as a function of the pressure difference. The interpolated pressure for zero rate should equal the osmotic pressure. In practice, however, more reliable results usually have been secured by establishing an equilibrium pressure which remains unchanged over an extended period of time. 

The osmotic method is most useful, of course, in dilute solutions where the theory which we have developed above is invalid. Disregarding this deficiency for the moment, we may expand the logarithmic term in series with the retention only of terms in low powers of V2. Then... 

Methodically, there is no great difference between measuring the mean activity coefficient in a solution of one electrolyte and measuring this quantity in a mixture of electrolytes. Binary mixtures have been studied most extensively. If osmotic methods are used, then the coefficients ocx and... 

For the success of the osmotic experiment the availability of a membrane through which solvent but not solute molecules can pass freely is essential. Existing membranes can only be considered as of approximate ideal semi-permeability. The major limitation of the osmotic method is the diffusion of low-molecular-weight species through the membrane. 

Molecular weights measurable by the osmotic method is about 106. 

Edsman, K., Sundelof, L.-O. (1987). Determination of thermodynamic interaction in mixed polymer solutions by a rapid and precise osmotic method the system dex-tran/polyvinyl-pyrrolidone/water. Polymer, 28, 2261-2214. 

Of the osmotic methods I would mention only two that we have examined and used at Aberystwyth. The indirect determination of vapour pressure lowerings can be made simply, quickly, and precisely using thermistors an accuracy equivalent to one-half per cent in AP is readily attained for M/20 solutions in benzene this is equivalent to a... 

Unfortunately such "effective molecular weight studies involve the major assumption that the whole deviation from ideal solutions is due to association and this assumption can only be controlled by some non-osmotic method such as the spectrophotoraetric one. Dolezalek s papers of the 19)0 era provide ample warning of the falsity of this assumption when indiscriminately applied however, where associative interaction is pronounced, comparison of spectroscopic and osmotic data shows that the careful evaluation of the latter leads to essentially correct results. 

The most extensive investigations were those of Medvedev [105]. Commercial collodion cotton of 11.08%N was divided by Medvedev into four fractions. The viscosity of the acetone solutions as well as the molecular weight (osmotic method) of these were determined. The nitrogen content in all the fractions was approximately the same (Table 57). 

Man substances can now be produced in a pure condition by electro-osmotic methods, that is, by causing electrolytic separation to take place by employing diaphragms between which the substance to be treated is placed while under the influence of the current passing between electrodes situated outside the diaphragmed compartment. 

Although osmotic pressure determinations depend upon the same thermodynamic principles, the disadvantages of the methods outlined above do not occur the pressure concerned is much more easily measured, and impurities exert no osmotic effect if they penetrate the membrane that is employed. In fact, the osmotic method is probably the most convenient and reliable of all and the one most capable of being developed to the highest degree of precision. 

There are two methods of determining the degree of dissociation a. First, the equation = where A is the equivalent conductivity at the dilution v and A is the equivalent conductivity at infinite dilution, and, secondly, any of the osmotic methods described in Chapter VIII. (change in freezing point, boiling... 

The calculation of the degree of dissociation by the two methods mentioned above leads to different values, so that at least one of them must be false, or rather based on false assumptions. The discrepancy may be accounted for by assuming that the simple laws of van t Hoff are not applicable to the free ions. The law of mass action would then no longer be valid for equilibria between ions and undissociated molecules, and apart from this the osmotic methods of calculating the ionic concentrations would no longer be strictly accurate. 

Thermodynamic parameters are calculated from intrinsic viscosities and their temperature coelBcients. The values are significantly lower than those obtained by other methods, such as osmotic method (cf. Exercise 3.10) and phase separation studies (see Problem 3.16)... 

Ultracentrifugation experiments yield Mz values. They are used primarily for biological polymers. The random coil conformation of most synthetic polymers in solutions makes it difficult to interpret these data and this method is little used with such materials. Among the absolute methods of molecular weight determination, only the osmotic method and the light scattering method are given primary consideration in this chapter. 

Colloid science has also long ago recognised the validity of this principle for the kinetic units in which it has a special interest and has based its various osmotic methods of determining the molecular weight of colloids on it, using the gas laws. 

A further discussion of some experiments is given in the chapter on gels (p. 558). Recent investigations have also made use of light scattering to obtain thermodynamic data. This method is referred to in the chapter on molecular weight determinations (p. 146) in which we also consider the osmotic method (p. 133). The experiments on coacervation are discussed in great detail in Chapter VIII, p. 232. 

Moreover, the application of indirect osmotic methods in macromolecular substances sometimes gives rise to anomalies which have not yet been cleared up. This particularly refers to the method of isothermic distillation recommended by Ulmann (see footnote 1). For a discussion of these anomalies see e.g. O. Kratky and H. Mark, Fortschritte der Chemie organischer Naturstoffe 1, 255 (1938) F. Klages, Kolloid-Z, 93 (1940) 19. 

The osmotic method has proved to be a very useful tool in the molecular weight determinations of a great many macromolecular substances In addition it is a relatively simple method. 

Schulz made his investigations with nitrocellulose containing 12.5— 12.7% nitrogen. (About 2.5 NO2 per Q) and with various preparations of molecular weight 50.000 — 443,000, determined with the osmotic method by extrapolation to infinite dilution. Macromolecular substances generally do not follow Van t Hoff s law, which... 

It may be emphasized that, in employing the Staudinger viscosity method and, particularly, in comparing it with osmotic methods, the above line of thought must be borne in mind although this is not always sufficiently done. 

Ultrafiltration provides a suitable alternative to conventional drying techniques, such as spray drying, in some applications and is ideally suited to materials with high solids contents (Bhave et al., 1992). Applications in the food industry include the concentration of pasteurised skimmed milk and whole milk. The advantages of osmotic methods when compared with conventional drying techniques are that there is no flavour loss or heat deterioration as with heating methods (Ishikawa and Nara, 1992). 

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