Boiling point elevation osmotic pressure measurement

Activity data for electrolytes usually are obtained by one or more of three independent experimental methods measurement of the potentials of electrochemical cells, measurement of the solubility, and measurement of the properties of the solvent, such as vapor pressure, freezing point depression, boiling point elevation, and osmotic pressure. All these solvent properties may be subsumed under the rubric colligative properties. 

X V iution), the determination of the molar mass of a solute requires a measurement of mass, volume, temperature, and osmotic pressure. Osmotic pressures are generally large and can be determined quite accurately, thus yielding accurate molar masses. Boiling-point elevations and freezing-point depressions are usually small and not very accurate, so molar mass determinations based on those measures often are not accurate. 

The final colligative property, osmotic pressure,24-29 is different from the others and is illustrated in Figure 2.2. In the case of vapor-pressure lowering and boiling-point elevation, a natural boundary separates the liquid and gas phases that are in equilibrium. A similar boundary exists between the solid and liquid phases in equilibrium with each other in melting-point-depression measurements. However, to establish a similar equilibrium between a solution and the pure solvent requires their separation by a semi-permeable membrane, as illustrated in the figure. Such membranes, typically cellulosic, permit transport of solvent but not solute. Furthermore, the flow of solvent is from the solvent compartment into the solution compartment. The simplest explanation of this is the increased entropy or disorder that accompanies the mixing of the transported solvent molecules with the polymer on the solution side of the membrane. Flow of liquid up the capillary on the left causes the solution to be at a hydrostatic pressure... 

Just as we discussed in Chapter 9, we can use measured activities of solvents (determined from vapor pressure, freezing-point depression, boiling-point elevation, or osmotic pressure) to determine activity coefficients of electrolytes in solution. For an ionic substance, the Gibbs-Duhem equation is... 

For the determination of very high molar masses, freezing-point depressions, boiling-point elevations, and vapor-pressure lowerings are too small for accurate measurement. Osmotic pressures are of a convenient order of magnitude, but measurements are time-consuming. The technique to be used in this experiment depends on the determination of the intrinsic viscosity of the polymer. However, molar-mass determinations from osmotic pressures are valuable in calibrating the viscosity method. 

Which measure of concentration is used with (a) vapwr-pressure lowering, (b) freezing-point depression, (c) boiling-point elevation, and (d) osmotic pressure ... 

Barton [41] has assembled a well-referenced source book for the derivation and use of x and cohesion parameters for various polymer solvent pairs. There are many ways to measure solvent activity, the simplest being boiling point elevation, freezing point depression, and osmotic pressure discussed in Section 11.5, Solution and Suspension Colligative Properties. ... 

The difference (G — mi) is measured by the difference [(F -l- r)— Pi]/V, in the osmotic pressure experiment. Other colligative properties are similarly measured in terms of the difference between a property of the pure solvent and that of the solvent in solution, at a particular concentration and common temperature. Specifically, boiling point elevation (ebulliometry) measurements result in... 

An alternative approach is based on the theoretical foundation described earlier for the colligative properties. If the solution is isotonic with blood, its osmotic pressure, vapor pressure, boiling-point elevation, and freezing-point depression should also be identical to those of blood. Thus, to measure isotonicity, one has to measure the osmotic pressure of the solution and compare it with the known value for blood. However, the accurate measurement of osmotic pressure is difficult and cumbersome. If a solution is separated from blood by a true semipermeable membrane, the resulting pressure due to solvent flow (the head) is accurately measurable, but the solvent flow dilutes the solution, thus not allowing one to know the concentration of the dissolved solute. An alternative is to apply pressure to the solution side of the membrane to prevent osmotic solvent flow. In 1877, Pfeffer used this method to measure osmotic pressure of sugar solutions. With the advances in the technology, sensitive pressure transducers, and synthetic polymer membranes, this method can be improved. However, results of the search for a true semipermeable membrane are still... 

Boiling-point elevation can also be used to measure osmotic pressure and tonicity of a solution using just a reflux condenser and a thermometer. The commercially available instrument is the Cottrell boiling-point apparatus. However, this method is affected by the ambient barometric pressure and the presence of volatile solvents in the solution. 

Boiling point elevation AT = mK[, (the constants have been tabulated) Freezing point depression AT = —mK (the constants have been tabulated) A solution in contact with its pure solvent across a semi-permeable membrane experiences an increase in pressure as pure solvent flows through the membrane into the solution. This osmotic pressure can be measured quite accurately, and through the equation ttV = nRT permits determination of the molecular weight of the solute. 

A solution containing 4.22 g of a nonelectrolyte polymer per hter of benzene solution has an osmotic pressure of 0.646 torr at 20.0°C. (a) Calculate the molecular weight of the polymer, (b) Assume that the density of the dilute solution is the same as that of benzene, 0.879 g/mL. What would be the freezing point depression for this solution (c) Why are boiling point elevations and freezing point depressions difficult to use to measure molecular weights of polymers ... 

Describe how you would use freezing-point depression and osmotic pressure measurements to determine the molar mass of a compound. Why are boiling-point elevation and vapor-pressure lowering normally not used for this purpose ... 

In a similar fashion, solubility measurements (of a gas in a liquid, a liquid in a liquid, or a solid in a liquid) can be used to determine the activity coefficient of a solute in a solvent at saturation. Also, measurements of the solubility of a solid solute in two liquid phases can be used to relate the activity coefficient of the solute in one liquid to a known activity coefficient in another liquid, and freezing-point depression or boiling-point elevation measurements are frequently used to determine the activity of the solvent in a solute-solvent mixture. We have also showed that osmotic-pressure measurements can be used to determine solvent activity coefficients, or to determine the molecular weight of a large polymer or protein. 

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. 

Osmotic pressure. Dissolution of a solute reduces the chemical potential of the solvent and results in a number of observable phenomena known as coUigative properties, such as boiling point elevation, freezing point depression and osmotic pressure. The determination of the extent of change in these properties provides a measure for the molecular weight of the dissolved solute. Thus the molecular weight of a triomacromolecule can be... 

The quantity is obtained by end-group analysis or by measuring a colligative property such as elevation of boiling point, depression of freezing point, or osmotic pressure [1,2]. 

Osmotic pressure, equilibrium centrifugation, end group analysis, freezing point depression (cryoscopy), boiling point elevation (ebulliometry) and vapor pressure lowering measure the colligative... 

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