Osmometer

Our primary objective in this section is the discussion of practical osmometry, particularly with the goal of determining the molecular weight of a polymeric solute. We shall be concerned, therefore, with the design and operation of osmometers, with the question of units, and with circumventing the problem of nonideality. The key to these points is contained in the last section, but the details deserve additional comment. 

Figure 8.8 Two osmometer designs (a) solution (inner chamber) separated from solvent by clamped membrane. [Reprinted with permission from D, M. French and R. H. Ewert, Anal. Chem. 19 165 (1947), copyright 1947 by the American Chemical Society.] (b) Solution and solvent in grooved faces shown in detail in (c). [Reprinted with permission from R. M. Fuossand D. J. Mead,/. Phys. Chem. 47 59 (1943), copyright 1943 by the American Chemical Society.]...

In both of these pieces of apparatus, isothermal operation and optimum membrane area are obtained. Good temperature control is essential not only to provide a value for T in the equations, but also because the capillary attached to a larger reservoir behaves like a thermometer, with the column height varying with temperature fluctuations. The contact area must be maximized to speed up an otherwise slow equilibration process. Various practical strategies for presetting the osmometer to an approximate n value have been developed, and these also accelerate the equilibration process. 

We consider this system in an osmotic pressure experiment based on a membrane which is permeable to all components except the polymeric ion P that is, solvent molecules, M" , and X can pass through the membrane freely to establish the osmotic equilibrium, and only the polymer is restrained. It does not matter whether pure solvent or a salt solution is introduced across the membrane from the polymer solution or whether the latter initially contains salt or not. At equilibrium both sides of the osmometer contain solvent, M , and X in such proportions as to satisfy the constaints imposed by electroneutrality and equilibrium conditions. 

Aero Hydrolysis. A solution of kasugamycin hydrochloride (1.5 grams, 3.46 mmoles) dissolved in 15 ml. of 6N hydrochloric acid was heated at 105°C. for five hours in a sealed tube. The solution was condensed to 5 ml. under a reduced pressure and the addition of 50 ml. of ethyl alcohol afforded a crude solid overnight. It was recrystallized from aqueous ethyl alcohol, showing m.p. 246°-247°C. (dec.). It showed no depression in the mixed-melting point and completely identical infrared spectrum with d-inositol which was supplied by L. Anderson of the University of Wisconsin. The yield was 81% (503 mg., 2.79 mmoles). Anal Calcd. for CgH12Og C, 40.00 H, 6.71 O, 53.29 mol. wt., 180.16. Found C, 40.11 H, 6.67 O, 53.33 mol. wt., 180 (vapor pressure osmometer). 

Attempts were made to determine number average molecular weights (Mn) by osmometry (Mechrolab Model 502, high speed membrane osmometer, 1 to 10 g/1 toluene solution at 37 °C), however, in many instances irreproducible data were obtained, probably due to the diffusion of low molecular weight polymer through the membrane. This technique was abandoned in favor of gel permeation chromatography (GPC). 

Figure 14 shows the computed osmotic volume changes the red cells undergo when suspended in the same four test solutions at room temperature (left side) and when slowly frozen (right side). They are responses characteristic of any cell that behaves as an ideal osmometer. Initially the cells are at normal isotonic volume (relative volume 1.0). After transfer to the test solution, however, they shrink in all... 

Figure 6.1 Diagram to show the essential components of a vapour phase osmometer...

C12-0105. Poiystyrene is a piastic that dissoives in benzene. When a benzene soiution (density = 0.88 g /mL) containing 8.5 g/L of poiystyrene is piaced in an osmometer at 25 °C, the soiution reaches equiiibrium when the height of iiquid inside the osmometer is 12.9 cm higher than outside. Caicuiate the moiar mass of the poiystyrene. 

Fig. 35.—Assembled osmometer of Weissberg and Hanks." (1) Solution and reference capillary (2) solution cell (3) osmometer base (4) pressure ring (5) perforated plate (perforations not shown) (6) semipermeable membrane (7) mercury seal (8) solvent container (9) solvent level (10) cover plate ...

Greater speed of attainment of equilibrium as well as greater precision are possible with a block-type osmometer like the one shown schematically in Fig. 36. Osmometers of this type usually consist of a pair of matched, stainless-steel or brass blocks, in each of which is cut a shallow circular cell cavity. The membrane fits between the two blocks, preferably with a lead gasket on one side of the membrane. The blocks are firmly bolted together. Each cell may be emptied and refilled through a metal tube connected with the bottom of the cell and closed with a needle valve during operation. Various schemes have... 

Fig. 37.—Photograph of the block osmometer diagramed in Fig. 36. This design affords access of solution and solvent to maximum membrane area and rigid support of the membrane. - ...

The centrifugal field in the sedimentation equilibrium experiment is the analog of the membrane in an osmometer. 

If 0.6 N lithium bromide is added to the solution of the polyelectrolyte and also to the solvent on the opposite side of the osmometer membrane, the lowermost set of points in Fig. 145 (lower and left scales) is observed. The anion concentration inside and outside the coil is now so similar that there is little tendency for the bromide ions belonging to the polymer to migrate outside the coil. Hence the osmotic pressure behaves normally in the sense that each poly electrolyte molecule contributes essentially only one osmotic unit. The izjc intercept is lower than that for the parent poly-(vinylpyridine) owing to the increase in molecular weight through addition of a molecule of butyl bromide to each unit. 

By DVP measurement at 37° C in a Mechrolab Model 301A osmometer, calibrated for pyridine with benzil. Here... 

Osmosis is the passage of a pure solvent into a solution separated from it by a semipermeable membrane, which is permeable to the solvent but not to the polymeric solute. The osmotic pressure n is the pressure that must be applied to the solution in order to stop the flow. Equilibrium is reached when the chemical potential of the solvent is identical on either side of the membrane. The principle of a membrane osmometer is sketched in Figure 2. 

For a typical experiment, a series of solutions with concentrations between 1 and 10 g/L is prepared. Solutions and pure solvent, which defines the baseline, are injected alternately into the solution chamber of the osmometer and enough time is given for the pressure to equilibrate. Equilibrium is reached after seconds to days, depending on the instrument, the membrane, and the sample. 

Figure 3 Membrane osmometry. Top concentration series with multiple injection of polystyrene (PS, 5,250 g/mol) in toluene. Concentrations 1.07,1.91, 2.94, 4.11, and 5.03 g/L. Multiple solvent injections establish the baseline (subtracted). Inset data evaluation for concentration series of PS (5,250 g/mol) and PS (47,400 g/mol) in toluene. Bottom design of osmometer equipped with flow cell. Reproduced with permission from Lehmann et al. [16]. Copyright 1996 American Chemical Society.

Membrane osmometry measurements were carried out with the capillary osmometer shown in Figure 3. Owing to the short equilibration time of the instrument and the low cut-off molar mass of the membrane, solute permeation through the membrane, which would show up as a drift of the baseline, did not cause problems even for the lowest molar mass fraction. M was obtained from... 

Material required Air-dried (27-30°C) aerial parts of S. deppei to prepare aqueous leachate, Seeds of tomato (Lycopersicon esculentum Mill. cv. Rio Grande), Osmometer, Growth Chamber Laminar flow hood. 

Procedure Allelopathic aqueous leachate is prepared by soaking dried leaves (lg/100 mL or 1% w/v) in distilled water for 3 h. This leachate is filtered through Whatman paper (No. 4) and then through a sterile Millipore membrane (0.45 mm). Then, it is poured in Petri dishes and mixed with agar (2%) for a final aqueous leachate concentration of 0.5%. The volume will depend on the size of the Petri dish, 3 mL of leachate plus 3 mL of agar are enough for a 6 cm Petri dish. Osmotic potential of the leachate is measured with a freezing-point osmometer (Osmette A, Precision System Inc.). 

For example, the correction is often negligible in a Zimm-Myerson osmometer, but considerable in a Fuoss-Mead instrument. 

In the past few years various types of osmometers have been developed and used. They differ generally in the details of their technical construction. These osmometers are based on two types of cells ... 

The simplest types of osmometer contains an osmotic cell with a horizontal. This type of osmometer was first proposed by Schulz. Several modification of this type of osmometer have subsequently been made. 

The membrane area in osmometers having a horizontal membrane is generally very small and the time for equilibration is very long. Nevertheless, osmometers of this type are now widely used because of their simple construction. 

---