Osmometers, types

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... 

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. 

Such an osmometer was first designed by Herzog and was further modified by Fuoss and Mead. This type of osmometers is widely used for the determination of Molecular weight. 

In such an osmometer the lateral walls on the depth of the half cells are made up of perforated brass plates the width and the depth of each groove in the plate are 1.5 mm and the distance between two adjacent groove is also 1.5 mm. The diameter of osmometer cell is 11.5 cm. The semi-permeable membrane is clamped between the two half-cells. The solution is placed in the glass-tube having a needle-type stopcock and is fitted with pure solvent. The volume of the osmometer cell is about 7 ml. The assembled osmometer is put in a double-walled air thermometer. The temperature fluctuations in the thermostate are 0.05°C. 

The Osmotic pressure equilibrium in this type of osmometers is attained quick and very small quantities of solvent and solution are needed in this case. Sometimes mixed solvents may be used in this type of osmometer. The disadvantages of the osmometer Organisation and Qualities... 

Simple osmometers have also been developed by Adair particularly for aqueous colloidal solutions. A thimblc-typc collodion membrane is attached to a capillary tube and contains the solution, When equilibrium is established the difference in level inside and outside the capillary is measured, Capillary corrections are made. For organic solvents a dynamic type osmometer may be used. A membrane of large surface area is clamped between two half cells and attached to each half cell is a fine capillary observation tube. With such an apparatus, equilibrium is rapidly established between solution and solvent contained in the half cells, The volume of the half cell may be small (about 20 cubic centimeters), The level of the solvent is usually aiiangecl to be a little below the equilibrium position, and the height of the solvent in the capillary as a function of time is measured, This procedure is repeated with the level of the solvent just above the equilibrium position. A plot is then made of the half sum of these readings. 

Number average molecular weights were determined in toluene using a Mechrolab high speed membrane osmometer with Schleicher and Schuell, type U.O. very dense cellophane membranes. Previous work has established that under these conditions diffusion of this type of polymer through the membrane is not detectable at molecular weights down to about 6000 (18). 

The collected urine is centrifuged to remove solid debris and analyzed by standard methods for sodium, potassium and chloride (Durst and Siggard-Andersen, 1999, Scott et al. 1999). Osmolality is also measured with an osmometer (the freezing point depression type of instrument is recommended (Scott et al. 1999). 

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. 

Another type of osmometer is the vapor pressure osmometer. In reality, osmolality measurement in these instruments is not related directly to a change in vapor pressure (in millimeters of mercury), but to the decrease in the dew point temperature of tlie pure solvent (water) caused by the decrease in vapor pressure of the solvent by the solutes. In this instrument, temperature is measured by means of a thermocouple, which is a device consisting of two dissimilar metals joined so that a voltage difference generated between the points of contact (junctions) is a measure of the temperature difference between the points. 

An important clinical difference between the vapor pressure technique and the freezing point depression osmometer is the failure of the former to include in its measurement of total osmolality any volatile solutes present in the serum. Substances such as ethanol, methanol, and iso-propanol are volatile, and thus escape from the solution and increase the vapor pressure instead of lowering the vapor pressure of the solvent (water). This makes use of vapor pressure osmometers impractical for identifying osmolal gaps in acid-base disturbances (see Chapter 46). Thus use of this type of osmometer cannot be recommended for most clinical laboratories. 

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. 

A full description of various osmometers and the necessary experimental technique can be found in the book edited by Allen. More-recent types of osmometer have been described, but the outstanding problem in osmometry is still the preparation of suitable semi-permeable membranes. Methods of preparing membranes claimed to be suitable for materials of low molecular weight have been described, and there are several reports of comparisons of the behavior of different types of membranes in osmometryThe problem of correcting observed osmotic pressures for any solute diffusion which may occur has been considered theoretically, - and a suitable technique established. ... 

Figure 4.4.14. Principle scheme of a vapor-pressure osmometer of the hanging drop type. T - measuring temperature, AT - obtained temperature difference (time dependent), measurements are made at atmospheric pressure where T determines the partial vapor pressure of the solvent P, in air.

A static capillary osmometer is illustrated in Fig. 2.18. Rather than rely on the liquid to rise in the capillary on the side of the solution in response to osmotic pressure, as is done in the static method, a dynamie equilibrium method can be used. Here a counter pressure is applied to maintain equal levels of the liquid in both capillaries and prevents flow of the solvent. Different types of dynamic membrane osmometers are available commercially. The principle is illustrated in Fig. 2.19. 

There are now several types of automatic osmometers that operate with essentially zero flow and that reach equilibrium very rapidly, usually within minutes. Osmotic equilibrium depends on an equal and opposite pressure being developed. The critical part of their design relates to the method of automatic adjustment of the osmotic pressure of the solution side so that the activity of the two sides is equal. Since several concentrations usually need to be run, the time required to determine a molecular weight by osmometry has been reduced from a week to a few hours by these automatic instruments. 

There are various types of osmometers. In Figure 9.1 we show two examples. Some commonly used membranes are cellulose acetate, cellulose hydrate, cellulose nitrate, polyurethanes, gel cellophane membranes, and poly(chlorotrilluoroethy-lene). 

FIGURE 9.1 Osmometers (a) Helfritz type b) Schultz type. 

In MO a dilute polymer solution is separated from pure solvent (of the same type used to prepare the solution) by a semipermeable membrane, which allows transport of solvent but not solute. Solvent molecules will diffuse from the solvent side into the solution side due to the lower chemical potential of the solvent in the solution. This solvent diffusion ceases when the potential difference is zero. In practice, with conventional osmometers this occurs when the hydrostatie pressure of a column of liquid equals the osmotic pressure, 7T, caused by the difference in chemical potential. An osmometer is depicted in Fig. 2. 

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