OSMOMETRY

Members:Andohoyan, Brian

Cagadas, Carl

Manzano, Novie

Orozco, Xander

Tibe, Charisse

Osmometry- a laboratory technique to find the molecular weight of an unknown compound by determining the osmotic strength of a solution.

Osmometer- an instrument that can find the osmotic strength of a solution or colloid using data from a semipermeable membrane, freezing point depression or vapor pressure.

FREEZING POINT OSMOMETRY

Determines the osmotic strength of a solution, as osmotically active compounds depress the freezing point of a solution.

1. The sample in a small tube is lowered into a chamber with cold refrigerant circulating from a cooling point.

2. A thermistor is immersed in the sample.

3. To measure temperature, a wire is used to gently stir the sample until it is cooled to several degrees below its freezing point. It is possible to cool water to as low as -40°C and still have liquid water, provided no crystals or particulate matter is present—supercooled solution.

4. Freezing can also be started by “seeding” a supercooled solution with crystals.

5. When the supercooled solution start to freeze as a result of the rapid stirring, a slush is formed and the solution actually warms to its freezing point temperature.

6. The slush will remain at the freezing point temperature until the sample freezes solid and drops below its freezing point.

VAPOR PRESSURE OSMOMETRY

Determines the concentration of osmotically active particles that reduce the vapor pressure of a solution.

1. The vapor pressure osmometer operates on the principle of differential vapor pressure between a pure solvent and a solution.

2. Two carefully matched thermistors are placed in a chamber saturated with solvent vapor. When solvent is placed on both thermistors, they assume the same temperature.

3. If a solution is placed on one of the thermistors, condensation heats the thermistor until the vapor pressure is raised to that of the pure solvent.

4. The change in temperature causes a resistance change in the thermistor. This change in resistance is measured by a sensitive bridge circuit and displayed on a panel meter, strip chart recorder and/or computer terminal. 

5. A calibration curve relates the change in resistance to the molal concentration of the solution.

MEMBRANE OSMOMETRY

Measures the osmotic pressure of a solution separated from pure solvent by a semipermeable membrane.

1.The solvent is separated from the polymer solution by a semipermeable membrane which is tightly held between the two chambers.

2.The solvent chamber is filled with solvent and closed to the atmosphere except for the solvent passage through the membrane while the solute chamber is left open to the atmosphere.

3.The chemical potential of the solvent is much higher than that of the solute, therefore there is a tendency for flow to occur from the solvent through the membrane to the polymer solution.

4. Because the solvent is permitted through the membrane a change in concentration causes the solvent to diffuse to the solute side of the chamber. As this occurs, the pressure of the solvent decreases until the pressure difference across the membrane just counteracts the chemical potential difference caused by the solute.

5.This pressure reduction required to equilibrate the chemical potential on both sides of the membrane is regarded as the osmotic pressure.

Osmotic pressure is the minimum pressure which needs to be applied to a solution to prevent the inward flow of water across a semipermeable membrane.

Refrigerant- a compound that can readily absorb heat at one temperature.

Thermistor- a device for determining temperature; also may be used for control of temperature.

Slush- an equilibrium of liquid and ice crystals

Freezing point- a temperature at which a liquid solidifies.

Freezing-point depression- describes the process in which adding a solute to a solvent decreases the freezing point of the solvent.

Definition of Terms

Vapor pressure- defined as the pressure exerted by a vapor in thermodynamic equilibrium with its condensed phases (solid or liquid) at a given temperature in a closed system.

Condensation- the change of water from its gaseous form (water vapor) into liquid water

Pure solvent- a substance or liquid that is able to dissolve another substance within itself.

Molal concentration- also called as molality—defined as the number of moles of solute dissolved per kilogram of solvent (mol/kg = m).

Definition of Terms