Osmotic correction pressure

Since capillary tubing is involved in osmotic experiments, there are several points pertaining to this feature that should be noted. First, tubes that are carefully matched in diameter should be used so that no correction for surface tension effects need be considered. Next it should be appreciated that an equilibrium osmotic pressure can develop in a capillary tube with a minimum flow of solvent, and therefore the measured value of II applies to the solution as prepared. The pressure, of course, is independent of the cross-sectional area of the liquid column, but if too much solvent transfer were involved, then the effects of dilution would also have to be considered. Now let us examine the practical units that are used to express the concentration of solutions in these experiments. 

What makes the latter items particularly important is the fact that the charge and electrolyte content of an unknown polymer may not be known hence it is important to design an osmotic pressure experiment correctly for such a system. It is often easier to add swamping amounts of electrolyte than to totally eliminate all traces of electrolyte. Under the former conditions a true molecular weight is obtained. Trouble arises only when the experimenter is indifferent toward indifferent electrolyte this sort of carelessness can be the source of much confusion. 

The similarity between the plots of c/r vs. c shown in Figs. 47 and 48 and those for tc/c vs. c shown in Figs. 38 and 39 is apparent. Deviations from ideality (i.e., the changes in iz/c and in c/r with c) have the same origin for both types of measurements. As with the osmotic pressure-concentration ratio, the change of c/r with c may be reduced by choosing a poor solvent. A further advantage of a poor solvent enters because of the smaller size assumed by the polymer molecule in a poor solvent environment, which reduces the dissymmetry correction. 

Van t Hoff introduced the correction factor i for electrolyte solutions the measured quantity (e.g. the osmotic pressure, Jt) must be divided by this factor to obtain agreement with the theory of dilute solutions of nonelectrolytes (jt/i = RTc). For the dilute solutions of some electrolytes (now called strong), this factor approaches small integers. Thus, for a dilute sodium chloride solution with concentration c, an osmotic pressure of 2RTc was always measured, which could readily be explained by the fact that the solution, in fact, actually contains twice the number of species corresponding to concentration c calculated in the usual manner from the weighed amount of substance dissolved in the solution. Small deviations from integral numbers were attributed to experimental errors (they are now attributed to the effect of the activity coefficient). 

The density correction is extremely important in such low osmotic-pressure measurements, but has been infrequently applied. 

It is fortunate that theory has been extended to take into account selective interactions in multicomponent systems, and it is seen from Eq. (91) (which is the expression used for the plots in Fig. 42 b) that the intercept at infinite dilution of protein or other solute does give the reciprocal of its correct molecular weight M2. This procedure is a straightforward one whereby one specifies within the constant K [Eq. (24)] a specific refractive index increment (9n7dc2)TiM. The subscript (i (a shorter way of writing subscripts jUj and ju3) signifies that the increments are to be taken at constant chemical potential of all diffusible solutes, that is, the components other than the polymer. This constitutes the osmotic pressure condition whereby only the macromolecule (component-2) is non-diffusible through a semi-permeable membrane. The quantity... 

The easiest way to extend these considerations to the osmotic pressure of nonideal solutions is to return to Equation (22), which relates ir to a power series in mole fraction. This equation applies to ideal solutions, however, since ideality is assumed in replacing activity by mole fraction in the first place. To retain the form and yet extend its applicability to nonideal solutions, we formally include in each of the concentration terms a correction factor defined to permit the series to be applied to nonideal solutions as well ... 

It is well known that the association-dissociation reaction goes in both directions and that there is always some finite fraction of ion pairs [28]. Counter ions that form ion pairs do not participate in creating osmotic pressure accounting for such counter ions is thus important for the correct determination of the dimensions of the network. 

As in the analogous case of gases (Section 2.4), corrections for nonideality can be obtained by measurements of osmotic pressure at different solute concentrations, with extrapolation toward the infinite-dilution limit. For electrolytes, the correction for ionic dissociation is important. 

Acute angle closure glaucoma has been reported as rare complication of rapid insulin therapy for hyperglycemic non-ketotic coma (20). It was postulated that raised glucose concentrations in the lens leads to increased sorbitol and water influx. The osmotic changes in the lens are not immediately corrected when the glucose concentration in aqueous humor is lowered, and this can lead to obstruction of the canal of Schlemm and increased intraocular pressure. 

Clearly, the results of osmotic pressure measurements on solutions of charged colloidal particles, such as proteins, will be invalid unless precautions are taken either to eliminate or to correct for this Donnan effect. Working at the isoelectric pH of the protein will eliminate the Donnan effect but will probably introduce new errors due to coagulation of the protein. Working with a moderately large salt concentration and a small protein concentration will make the... 

An excellent study of DMSP in maintaining osmotic pressure in algal cells is that of Reed (2 ). (It should be noted that the term osmoregulation is probably not correct as used in some of the recent literature concerning DMSP... 

The osmotic pressure of a solute is the hydrostatic pressure that must be applied to a solution in order to increase the activity, a. (or fugacity, designated f, introduced by G. N. Lewis as a measure of thermodynamic escaping tendency . It is an effective gas pressure corrected for deviations from the perfect gas laws) of the solvent sufficiently to balance its decrease caused by the presence of the solute. Equilibrium is established through a membrane permeable only to the solvent. This pressure is, by integrating... 

The plasma membrane envelops the cell, separating it from the external environment and maintaining the correct ionic composition and osmotic pressure... 

Equation 11.1 is shows how temperature, in terms of the temperature correction factor, TCF pressure, P and concentration, as osmotic pressure,, are used to normalize product flow rate.3... 

All n = difference between the osmotic pressure on the membrane feed and permeate sides TFC = temperature correction factor (membrane and manufacturer dependent)... 

In [84] the corrections are expressed by the gas volume fraction and the osmotic pressure in the foam (see below). 

There are various ways to calculate nf/. The first expression for IL/ has been derived by Frumkin [20] in 1938 who calculated it as osmotic pressure. Derjaguin and Landau [1] in 1941 have calculated disjoining pressure as a change in film pressure. Some years later (1948) Verway and Overbeek [2] evaluated n<./ by the change in the energy of the diffuse electric layer. Scheludko [134] has determined Iin a very simple way as a deformation of the two opposite diffuse electric layers at the film surfaces. Later various correction to Y ei have been introduced [e.g. 135-143]. 

Because a° depends on the external solutes present, Equation 3.42 (a corrected version of Eq. 2.20) indicates that the external osmotic pressure n° at incipient plasmolysis can vary with the particular solute in the solution surrounding the plant cells. Suppose that solute i cannot penetrate the membrane, so o/ equals 1, a situation often true for sucrose. Suppose that... 

---