Osmometer osmotic pressure measurement

Finally, several attempts have been made to develop an absolute molar mass detector based on osmotic pressure measurements. Commercially available membrane osmometers are designed for static measurements, and the cell design with a flat membrane is not suited for continuous flow operation. Different from the conventional design, Yau developed a detector which measures the flow resistance of a column caused by osmotic swelling and deswelling of soft gel particles used for the packing (see Fig. 12) [65,78]. With a microbore gel column, a... 

Number average molecular weight (M ) by osmotic pressure measurements has been described previously (12). The osmometer cell was thermostatted at 35 0.1 C and the solvent was 0.047 M in NaCl and... 

Since pressure measurements are relatively simple, Eq. 11.5-4 can be the basis for determining solvent activity coefficients in a solvent-solute system, provided a suitable leakproof membrane can be found. Osmotic pressure measurements are, however, more commonly used to determine the molecular weights of proteins and other macromolecules (for which impermeable membranes are easily found). In such cases an osmometer, such as the one shown in schematic form in Fig. 11.5-1, is used to measure the equilibrium pressure difference between the pure solvent and the solvent containing the macromolecules (which are too large to pass through the membrane) the pressure difference AF, which is the osmotic pressure 77, is equal to pg/t, where p is the solution density and h is the difference in liquid heights. If the solute concentration is small, we have... 

The initial number average molecular weights (Mn) for the poly-(amic-acids) were between 10,000 and 20,000, and no attempt was made to optimize them. Dimethylacetamide solvent was distilled from calcium hydride under vacuum and stored under nitrogen. The resin solutions were stored also under nitrogen. The water content of the solvent was measured with a Porapak Q column in a Hewlett Packard 5750 gas chromatograph. Unless otherwise stated, the water concentration in the solvent remained below 0.05 vol %. Mn was determined from osmotic pressure measurements on a Hewlett Packard-Mechrolab 502 osmometer using ArRo Lab gel cellophane 600D membranes. 

Number average molecular weights of the samples were measured by the osmotic pressure measurements. The Knauer high-speed membrane osmometer was used with Sartorius Ultra-cellafliter sd as a semipermeable membrane. 

The membrane osmometer is a device for measuring the osmotic pressure, n, of a biopolymer solution separated from the pure solvent by a semi-permeable membrane (Tanford, 1961 Edmond and Ogston, 1968 Tombs and Peacocke, 1974 Edsman and Sundelof, 1987 Amur et al.,... 

Osmometry. Measurements were made in tetrahydronaphthalene at 130 °C by using a membrane osmometer (Mechrolab model 502). The plots of osmotic pressures at different concentrations are shown in Figure 5. Comparing the osmotic pressure results with those of GPC (interpretation 1) it appears that molecules larger than A = 50 may have permeated the membrane. If this is the case, the MN by osmometry is too high. 

Unfortunately, the use of the terms osmotic pressure and osmotic potential, as well as their algebraic sign, varies in the literature. Osmotic pressures have been measured using an osmometer (Fig. 2-8), a device having a membrane that ideally is permeable to water but not to the solutes present. When pure water is placed on one side of the membrane and some solution on the other, a net diffusion of water occurs toward the side with the solutes. To counteract this tendency and establish equilibrium, a hydrostatic pressure is necessary on the solution side. This pressure is often called the osmotic... 

Modern osmometers reach equilibrium pressure in 10-30 min and indicate the osmotic pressure automatically. Several types are available. Some commonly used models employ sensors to measure. solvent flow through the membrane and adjust a counteracting pressure to maintain zero net flow. Other devices use strain gauges on flexible diaphragms to measure the osmotic pressure directly. 

Suppose 0.125 g of a protein is dissolved in 10.0 cm of ethyl alcohol (C2H5OH), whose density at 20°C is 0.789 g cm . The solution rises to a height of 26.3 cm in an osmometer (an apparatus for measuring osmotic pressure). What is the approximate molar mass of the protein ... 

Because of their large molecular weight, proteins contribute only about 1 mOsmol/kg H2O to the total serum osmolality measured by freezing point depression. Occasionally, one may be asked to determine the contribution of macromolecules to the serum osmolality. Colloid osmotic pressure (COP) is a direct measure of the contribution of macromolecules (primarily proteins) to the serum osmolality. It is used primarily in the assessment of pulmonary edema or other abnormahties of water balance and serum protein concentrations. However, its utility has been questioned and the method is seldom used. Previous editions of this textbook describe the principles of a COP osmometer. 

Dynamic osmometers reach equilibrium pressures in 10 to 30 minutes and indicate osmotic pressure automatically. Several types are available. Some commonly used models employ sensors to measure solvent flow through the membrane and adjust a counteracting pressure to maintain zero net flow. A commercially available automatic osmometer operates on the null-point principle. In this high-speed membrane osmometer schematically represented in Fig. 4.4, the movement of an air bubble inside the capillary immediately below the solvent cell indicates the solvent flow to the solution cell. Such movement is immediately detected by a photocell, which in turn is coupled to a servomechanism. If any movement of the air bubble is detected by a photocell, the servomechanism is stimulated to move the solvent reservoir upward or downward in order to adjust the hydrostatic pressure such that the solvent flow is completely arrested. The pressure head of the reservoir gives the osmotic head. Some osmometers also use strain gauges on flexible diaphragms to measure the osmotic pressure directly. 

Osmotic pressure depends on solute concentration. A device called an osmometer (Figure 3.13) measures osmotic pressure. Osmotic pressure can also be calculated using the following equation, keeping in mind that the final osmotic pressure reflects the contribution of all solutes present. 

In the dilute case, the osmotic pressure was measured at membrane equilibrium, with the help of an osmometer (Chapter 5, Section 1.9). 

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. 

Relatively small concentrations of macromolecules produce easily measurable osmotic-pressure differences. For example, suppose 1 g of a protein or polymer of molecular weight 60 000 is dissolved in 100 mL of water and placed in the osmometer of Fig. 11.5-1. At a temperature of 25°C, the osmotic-pressure difference would be... 

In the simplest case, the osmotic pressure II is measured in a single-cell osmometer with a horizontally arranged membrane (Figure 9-2). II is then identified as the manometrically measured difference in pressure Apeq at equilibrium. 

A device used to measure osmotic pressure is called an osmometer. 

Figure 4.4.16. Principle scheme of a membrane osmometer 1 - solvent, 2 - polymer, 7C - osmotic pressure. Ah - hydrostatic height difference, - ordinary pressure or measuring pressure, Vj- partial molar volume of the solvent in the polymer solution.

Measurements became easier with rapidly equilibrating membrane osmometers with servo pressure control. The instrament senses differences in osmotic pressure based on the mass transport through the membrane. The pressure differential between solvent and solution is then controlled by increasing the solution level to hydrostatically counterbalance the osmotic pressure 71. The success of the osmometry... 

In Figure 2.6, the movement of solvent is detected by the movement of a bubble in a very narrow tube attached to the solvent compartment of the osmometer. When the photocell detects movement of the bubble, an external head of solvent is established by the raising of the solvent reservoir. In Figure 2.7, a capacitance device is used to detect solvent movement across the membrane. The measured signal is used to generate an appropriate external solvent head equivalent to the osmotic pressure. Modern instrumentation, once set up properly, can reduce the measurement time to a matter of minutes thus reducing the errors which are found if the membranes are anything less than truly semi-permeable. 

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