Vapour pressure osmometer

Machrolab vapour pressure osmometer (VPO) model 30/A is composed of a sample-chamber assembly and a control unit which houses a wheatstone bridge, a null indicator, and a heater-control circuit. 

Vapour pressure osmometer is a variation of the isopiestic or of the isothermal distillation techniques by which a solvent and a solution in that solvent are placed side by side in a closed container. It measures the difference in temperature created by the condensation of solvent on a sensitive thermistor containing a solution of the solute whose Molecular weight is to be determined. 

The vapour pressure osmometer method is more acceptable of all the methods involving measurement of colligative properties because of the sensitivity of the detector. For ideal solvent-solvents with a low heat of vaporisation, the differential thermistors of the VPO can detect differences in temperature of the order of 0.001°C this sensitivity determines the Molecular weight of the samples upto 20,000. 

Owing to the very low degree of polymerisation of the polymers obtained, molecular weights (Mn) were determined with a MECROLAB Vapour Pressure Osmometer, model 301 A with benzene as solvent. For values of Mn ranging between 400 and 2000 the reproducibility was 30 units. 

The reaction products were precipitated in methanol containing 0.880 ammonia. The precipitated polymer was filtered off and dried at 50 °C in a vacuum oven. Evaporation of the filtrate yielded the oligomers. For molecular weight determinations, the two fractions were dissolved together in CC14. Molecular weights were determined with a vapour-pressure osmometer. 

As noted above block copolymers had a poly(ethyleneoxide) head group Y and tails X of poly(12-hydroxystearic) acid. The polymers are formed in a one-step polymerisation from 12-hydroxystearic acid and poly(ethyleneoxide) polymers of various molar masses. The values of the number and mass average molar masses shown in Table 1 were obtained by GPC. Vapour pressure osmometer measurements gave the... 

An osmometer is an instrument which measures the osmolality of a solution, usually by determining the freezing point depression of the solution in relation to pure water, a technique known as cryoscopic osmometry. A small amount of sample is cooled rapidly and then brought to the freezing point (Fig. 6.1), which is measured by a temperature-sensitive thermistor probe calibrated in mosmol kg . An alternative method is used in vapour pressure osmometry, which measures the relative decrease in the vapour pressure produced in the gas phase when a small sample of the solution is equilibrated within a chamber. 

Using Mechrolab vapour pressure osmometer. b From experiments in all-glass apparatus. 

Figure 2.19 Schematic diagram of a vapour pressure osmometer.

The number average molecular weight (MJ was measured by vapour pressure osmometer weight average molecular weight (M ) was measured by gel permeation chromatography ... 

The vapour pressure osmometer (Dr. Knauer, Berlin) was provided with a universal-two-thermistor-probe for all kinds of liquids. The probe is one part of a Wheatstone-bridge, which is connected with an external amplifier, an analog meter and a recorder. The membrane osmometer is from the same producer. 

Vapour pressure depression and membrane osmometry are the most common methods to determine the polyer-solvent interaction parameter. The latter method will be described briefly. In a membrane osmometer a dilute polymer solution has been separated from pure solvent by means of a membrane. The membrane is penneable for solvent molecules but not for polymer molecules. Due to a chemical potential difference solvent molecules will diffuse from the diluted phase to the concentrated phase and this results in a pressure increase which is called the osmotic pressure ti (see also section VI - 2 for a more detaUed description of osmosis). The osmotic pressure is given by... 

Vapour pressure osmometry VPO). At present commercially available apparatus for VPO is the Corona molecular-weight apparatus models 114 and 117 manufactured by the Corona Co., which have been produced in the past by Hitachi Co. Ltd Knauer Vapour Pressure Osmometer No. 731.110000 (Dr. H. Knauer Wissenschaftliche Cerate, AG) Jupiter Model 233 Vapour Pressure Osmometer (Jupiter Instrument Co. Inc.) and Gonotec model 070 (Gonotec Gesellschaft fiir MeB- und Regeltechnik m6H). A typical VPO apparatus consists of (1) two thermistors covered with glass, to which solution and solvent drops are attached (2) a cell, the interior of which is saturated with solvent vapour (3) a solvent vessel, placed at the lower part of the cell and (4) an electronic circuit. The instrument is usually thermostatted. 

Figure 5.4 Plot of the ratio of steady-state temperature difference (T- 7, ) and concentration, c, against c for solutions of atactic polystyrene standards in benzene. The plot indicates data obtained in duplicate runs by the use of the prototype vapour-pressure osmometer of Kamide et al. at 40°C. Polymer codes are shown on the lines (see Table 5.2). Redrawn and adapted from ref. 14.

Fig. 3.12 Schematic illustration of a vapour pressure osmometer showing the solution and solvent drops on matched thermistor beads. The temperature difference between the solution and solvent is measured as the resistance difference, AR, between the thermistor beads.

Vapour pressure osmometers measure the resistance difference between the two thermistor beads and this is assumed to be proportional to the temperature difference, i.e. ARs = knATs. Thus, in order to be of practical use. Equation (3.109) must be modified to the form... 

Measurements of the equilibrium resistance difference (AT ) between the solution and solvent thermistor beads in a vapour pressure osmometer were made... 

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