Osmotic pressure equilibrium time

At the beginning of an osmotic experiment, the difference in heights A/z observed after filling both chambers of the osmometer does not correspond to the osmotic pressure at equilibrium. The equilibrium pressure is only observed after solvent molecules permeate the membrane. If A/z is greater than the equilibrium osmotic pressure, the solvent molecules permeate from the solution chamber into the solvent chamber, and in the reverse direction if A/z is smaller than the equilibrium osmotic pressure. The time taken to reach equilibrium increases with the amount of solvent that must be displaced, i.e., increases with the diameter of the capillaries. Since, experimentally, problems such as dirt in the capillaries, etc., limit the size of capillary that one can go down to, and since the membranes must be tight (semipermeability), the establishment of osmotic equilibrium can take days or weeks. Other problems such as poor solvent drainage in the capillaries, adsorption of solute on the membrane, partial permeation of solute through the membrane, etc., can interfere with the attainment of a true osmotic equilibrium. The absence or presence and allowance for these complications must be individually established. 

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. 

Namely, the osmotic pressure on the side boundary vanishes in equilibrium. This condition was realized in the experiment after long times of swelling equilibration. The counterpart of Eq. (2.33) is written as... 

Osmotic pressure, another of the colligative properties, can be understood from Fig. 17.14. A solution and pure solvent are separated by a semipermeable membrane, which allows solvent but not solute molecules to pass through. As time passes, the volume of the solution increases while that of the solvent decreases. This flow of solvent into the solution through the semipermeable membrane is called osmosis. Eventually the liquid levels stop changing, indicating that the system has reached equilibrium. Because the liquid levels are different at this point, there is a greater hydrostatic pressure on the solution than on the pure solvent. This excess pressure is called the osmotic pressure. We can take another view of this phenomenon, as illustrated in Fig. 17.15. Osmosis can be prevented by applying a pressure to the solution. The pressure that just stops the osmosis is equal to the osmotic pressure of the solution. 

Over time water diffuses from side A (more dilute) to side B (more concentrated). Equilibrium between the solutions on both sides of a semipermeable membrane is attained when there is no net movement of water molecules from side A to side B. Osmotic pressure stops the net flow of water across the membrane. 

Practical Aspects of Osmometry In static osmometers, the heights of liquid in capillary tubes attached to the solvent and solution compartments (Fig. 4.3) are measured. At equilibrium, the hydrostatic pressure corresponding to the difference in liquid heights is the osmotic pressure. The main disadvantage of this static procedure is the length of time required for attainment of equilibrium. 

Figure 13.13 The development of osmotic pressure. A, In the process of osmosis, a solution and a solvent (or solutions of different concentrations) are separated by a semipermeabie membrane, which allows only solvent molecules to pass through. The molecular-scale view below) shows that more solvent molecules enter the solution than leave it in a given time. B, As a result, the solution volume increases, so its concentration decreases. At equilibrium, the difference in heights in the two compartments reflects the osmotic pressure (11). The greater height in the solution compartment exerts a backward pressure that eventually equalizes the flow of solvent in both directions. C, Osmotic pressure is defined as the applied pressure required to prevent this volume change.

The measurement of an osmotic pressure can also be carried out more accurately than can the measurement of a boiling-point elevation or a freezing-point depression. One difficulty in measuring very small osmotic pressures is the long time required for the system to reach equilibrium. This difficulty is sometimes overcome by imposing a pressure on the solution side of the membrane and observing how the rate of flow of liquid varies over time. The osmotic pressure can be calculated from this variation. Molecular weights of up to 3 000000 have been measured by the use of such techniques. 

A simple apparatus to determine the osmotic pressure of macromolecular substances has been described by Dobry and by Schulz To meet the disadvantage, frequently encountered in macromolecular substances, that the establishment of the osmotic equilibrium is slow and may even take several days, a special time saving arrangement with a stirring device has been proposed by Albert and Kratky . 

Figure 2 A diagrammatic representation of the way in which the osmotic pressure is developed with time in instruments which allow migration of solvent through the semi-permeable membrane. The time to achieve equilibrium might be minutes or hours depending on the quality of the membrane. The longer the time to reach equilibrium, the more likely that small solute molecules will migrate in the opposite direction to solvent molecules giving rise to error. The impact of solute diffusion on the observed osmotic head is similar to that of physical leaks through or around the...

Figure 2.7 In this instrument, the osmotic pressure is measured by observing the pressure developed on a diaphragm attached to a capacitance measurement device. The solvent side of the membrane is constrained in volume. Pressure is used to prevent solvent diffusion, a rapid process as it is not necessary to allow time for the solvent to migrate through the membrane to establish a pressure equilibrium. The applied pressure which is just sufficient to achieve equilibrium is the...

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