# Osmometry

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## The membrane osmometry measuring cell

The membrane osmometry measuring cell has a semipermeable (semi-permeable) membrane, which ideally is only permeable to solvents. It should hold back all polymers from degree of polymerization 2 on. In reality, however, all membranes allow molecules with a low degree of polymerization to permeate, especially after a long measurement period. The shape of the polymer molecules also plays a role: compact spherical particles diffuse more easily through the pores than loose thread structures.

A strict semi-permeability cannot therefore be achieved. Fast-measuring osmometers are therefore preferred, since the permeability of the membrane for low-molecular-weight fractions becomes more noticeable with longer measuring times. As a test, one and the same solution can be measured with two membranes of different pore sizes. If the same osmotic pressure is established at both, then an error due to the diffusion of smaller polymer molecules can be safely ruled out.

The membrane osmometer can be used for measurements in both aqueous and organic solvents. The concentration of the solutions to be examined should be between 0.2 and 2 g per 100 mL of solution. The semipermeable membrane determines both the lower measurement limit and the setting speed of the osmometer through the number of continuous pores and through its pore size distribution.

A common measurement curve is characterized by a steep rise at the beginning, which gradually changes to a constant value. The data acquisition of a modern device is set so that it responds to this type of curve. The sensitivity can be manually adapted to the system to be examined.

Proper membrane preparation is also very important. The membrane material (mostly regenerated cellulose) is often delivered in water and is protected against bacterial attack with formaldehyde or similar. It makes sense to store the membrane in a 60% acetone-water mixture in the refrigerator. From this environment, the membranes can be "accustomed" to both water and organic solvents. The membrane material has to be conditioned for certain times in solutions with graduated concentrations. It should also be noted that shrinkage can occur when "getting used to". This is why a larger diaphragm diameter is initially assumed than will be required later. Finally, a slow adaptation of the membrane to the measurement temperature is necessary.

The following procedure is used to ensure meaningful values:

Measurement

• at concentrations below 1% by weight
• in various solvents
• at constant temperature
• with a good membrane
• with absolutely tight measuring cell.

This method is for molar masses of 2 $104$ until $106$ g / mol suitable. The lower limit results from the pore size of the membrane, which allows smaller particles to permeate.