Vapor pressure apparatus

In our laboratories, extensive use has been made of vapor pressure (14-18) and membrane methods ( 2, 3, 19, 20) to Infer thermodynamic results for ternary aqueous systems containing an ionic or a nonionic surfactant and an organic solute. The most precise solubilization measurements ever reported have been obtained with an automated vapor pressure apparatus for volatile hydrocarbon solutes such as cyclohexane and benzene, dissolved In aqueous solutions of sodium octylsulfate and other Ionic surfactants (15, 16). A manual vapor pressure apparatus has been employed to obtain somewhat less precise results for solutes of lower volatility (17, 18). Recently, semi-equilibrium dialysis (19, 20) and MEUF (2) methods have been used to investigate solute-surfactant systems in which the organic solubilizates are too involatile to study by ordinary vapor pressure methods. 

The solubilization results reported here were obtained with an automated vapor pressure apparatus described previously (21). Benzene samples, from an external reservoir at 50 C, were added incrementally to the main measuring system by means of a 6-port HPLC valve. Successive increments of benzene are allowed to flash from the valve into the main solution reservoir these samples contain 2.907 x 10" moles of benzene, with a reproducibility better than 1 part in 6,000 or 7,000 (22). 

Table I and Figures 1-4 contain a wealth of information about the solubilization of benzene in aqueous surfactant micelles. Plots of K vs. Xg exhibit shallow minima in the case of the SDS solutions, and rather more pronounced minima for the CPC solutions. The plots of Tg vs. Xg show corresponding maxima, reflecting the fact that K and Yg are related reciprocally by K l/(TBcB ), where cB° is the monomer concentration of benzene in the aqueous phase at saturation. (The minimum in K and the maximum in Yg for the CPC solutions, shown in Figure 1, are not quite reached at the benzene concentrations attainable with the automated vapor pressure apparatus. The automated apparatus is restricted to operating at partial pressures less than about 70% of the vapor pressure of pure liquid benzene. However, the manual apparatus can be used for measurements almost to saturation, and results obtained with this apparatus show extrema in K and Yg at approximately X = 0.55.)...

Low-temperature vapor-pressure apparatus. (Metal parts to be well tinned and soft soldered, except where otherwise indicated.)... 

Average molecular weight may be determined by osmometry by utilizing either membrane or vapor pressure apparatus. Most gums are found to lie in the molecular weight range from about 200,000 to over 1,000,000 (22). This in turn implies that 1100 to over 5500 monomeric units make up each assemblage. 

Conceptually, this is the simplest type of vapor pressure apparatus. The sample is placed in a closed container and all air and other volatile impurities are removed as completely as possible. The container is placed in a thermostat kept at constant temperature until phase equilibrium occurs. The temperature and pressure are measured. The pressure gauge can be connected to the system directly or through a pressure transducer. 

Attention must also be paid to the distance between the condenser and the evaporator when subliming substances that have low vapor pressures. Apparatus that has a variable condenser position is thus extremely useful because this position can then be varied to suit the substance and the pressure to be used. The distance from the substance to the condenser is usually 7-25 mm in micro apparatus. 

Absolute vapor pressure measurement may be considered to be the classical technique for our purposes, because one measures directly the vapor pressure above a solution of known polymer concentration. Refs. 56-65 provide a view of the variety of absolute vapor pressure apparatuses developed and used by different authors. The common principle of an absolute vapor pressure apparatus is shown in Figure 4.4.2. 

Figure 4.4.2. Schematic of the common principle of an absolute vapor pressure apparatus 1 - polymer solution, 2 - connection to the manometer, 3 - Hg-manometer, 4 -heating coils. The whole construction is thermostatedat the measuring temperature, the connection to the manometer is kept slightly above the measuring temperature to avoid condensation.

Figure 4.4.3. Schematic diagram of a modem absolute vapor pressure apparatus T - temperature meter, P - vapor-pressure meter, V - vacuum meter, Z - measuring cell, M - magnetic stirrer, GI and Gil - degassing units for the solvent and for the polymer. ptq)rinted with permission from Ref 58, Copyright 1990,Wiley-VCH].

Figure 4.4.5. DifFerential vapor-pressure apparatus. 100 ml Pyrex flasks connected (a) to a differential pressure transducer (c) with digital readout (d) and (b) to vacuum pump (e) and absolute pressure vacuum thermocouple gauge (f). The constant temperature in the water bath is maintained by a temperature controller (g). The transducer and cormecting glassware are housed in an insulated box (i) and kept at constant temperature shghtly above the measuring temperature by controller (j). Polymer solution and pure solvent (here water) are stirred by underwater magnetic stirrers (h). [Reprinted with permission from Ref. 66, Copyright 1989, American Chemical Society].

The liquid chamber of the vapor pressure apparatus is filled with the chilled sample and connected to the vapor chamber that has been heated to 37.8 C (100 F) in a bath. The assembled apparatus is immersed in a bath at 37.8 C (100 F) until a constant pressure is observed. The reading, suitably corrected, is reported as the Reid vapor pressure. 

Vapor pressure apparatus meetiiig the requirements of Procedure B are available from UIC Inc., PO Box 863, Joliet, IL 60434 or Walter Herzog GmbH, D-6970 Lauda-Konigshofen Postfach 320, West Germany. 

Al.l Reid Vapor Pressure Apparatus, consisting of two chambers, a vapor chamber (upper section) and a liquid chamber (lower section), shall conform to the following requirements ... 

A1.4 Water Bath— The water bath shall be of such dimensions that the vapor pressure apparatus can be immersed to at least 25.4 mm (1 in.) above the top of the vapor chamber. Means for maintaining the bath at a constant temperature of 37.8 0.1 C (100 0.2 F) shall be provided. In order to check this temperature the bath thermometer shall be immersed to the 37 C (98 F) mark throughout the vapor pressure determination. 

A2.2 Pressure Gage—The pressure measuring system shall be a Bourdon type spring gage as described in A 1.2 or a suitable pressure transducer and digital readout. The pressure measuring system shall be remotely mounted from the vapor pressure apparatus and terminations provided for use of a quick connection type fitting. 

A2.7 Flexible Coupler—A suitable flexible coupling shall be provided for connection of the rotating vapor pressure apparatus to the pressure measuring device. 

The liquid chamber of the vapor pressure apparatus is filled with the chilled sample and connected to the vapor chamber at 37.8 C (lOO F). The apparatus is immersed in a... 

Note S—For some Test Method D 323 apparatus, a Number 6.3 rubber stopper has been found satisfactory. For the horizontal or Herzog apparatus, a Number 3 rubber stopper and a Number 000 cork in the vent hole is satisfactory. Another procedure is to attach a spare liquid chamber to the vapor chamber during the conditioning period. A third alternative is to utilize a cap threaded to match the threads of the vapor chamber. Several apparatus manufacturers have indicated the intention to supply such caps for equipment. In any procedure used, the interior surfaces of the vapor pressure apparatus and the sample must be kept completely free of water. 

A2.1 Vapor Pressure Apparatus—Refer to Annexes A1.1 through A 1.1.4. 

Vapor Pressure Apparatus—The type of apparatus suitable for use in this test method employs a small volume test chamber incorporating a transducer for pressure measurements and associated equipment for thermostatically controlling the chamber temperature and for evacuating the test chamber prior to sample introduction. 

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