Vacuum volumetric method

The errors in the vacuum volumetric method arise principally from two sources. The error in measured doses is cumulative, requiring very accurate calibration. Also, the void-volume correction becomes more significant at higher pressures when measuring low surface areas because more molecules are retained in the void volume than are adsorbed. 

Because of the dosing technique associated with the vacuum volumetric method there exists a potential source of error which in principle cannot be avoided on samples that are slow to equilibrate. Figure 14.2 represents a plot of the pressure in the sample cell versus time. The pressure up to time tj represents the equilibrium pressure in the sample cell before a new quantity of adsorbate is admitted. At time adsorbate is admitted into the sample cell and is accompanied by a rapid pressure rise. The pressure then gradually decreases to a new equilibrium value at time t2- If the decay to the new equilibrium pressure is slow, it is possible for open or accessible parts of the surface, such as the interior of wide pores, to contain more adsorbate before equilibrium is attained than after equilibrium is established. Less accessible regions, such as the interior of long narrow pores, will adsorb slowly and as the pressure falls because of adsorption in these pores, desorption must occur from the more accessible pores which tend to equilibrate more rapidly. If a porous sample is subjected to... 

The consequences of this type of activated physical adsorption is not only that the quantity adsorbed can lie off the isotherm but also that the measured quantity of adsorption is far less than the equilibrium value. No experiments have been conducted to illustrate whether or not the quantity adsorbed lies within the hysteresis loop. The occasional failure of the vacuum volumetric method to agree with the dynamic method, which is not subject to any pressure overshoot, may in part be attributed to this phenomenon. 

Both the static and continuous flow (quasi equilibrated) techniques may be described as vacuum volumetric methods, whereas DYNAMIC technique (known by some workers as continuous flow) does not use vacuum technology,but instead employs a non-adsorbing carrier gas/adsorptive mixture. 

This method is smiple but experimentally more cumbersome than the volumetric method and involves the use of a vacuum microbalance or beam balance [22], The solid is suspended from one ann of a balance and its increase in weight when adsorption occurs is measured directly. The dead space calculation is thereby avoided entirely but a buoyancy correction is required to obtain accurate data. Nowadays this method is rarely used. 

A known amount of zeolite was loaded into a 10 mm NMR tube with an attached vacuum valve. The sample was evacuated to about 2xl0 < torr for 3 days at room temperature, then it was heated to 350 C with a heating rate of 0.2 C/min, the sample was allowed to maintain at this temperature for about 30 hours (2x10 torr). After cooling to room temperature, a known amount of xenon gas was introduced into the sample tube and was sealed by the vacuum valve. All the xenon adsorption isotherms were measured by volumetric method at room temperature. 

The home-made heat-flow calorimeter used consisted of a high vacuum line for adsorption measurements applying the volumetric method. This equipment comprised of a Pyrex glass, vacuum system including a sample holder, a dead volume, a dose volume, a U-tube manometer, and a thermostat (Figure 6.3). In the sample holder, the adsorbent (thermostated with 0.1% of temperature fluctuation) is in contact with a chromel-alumel thermocouple included in an amplifier circuit (amplification factor 10), and connected with an x-y plotter [3,31,34,49], The calibration of the calorimeter, that is, the determination of the constant, k, was performed using the data reported in the literature for the adsorption of NH3 at 300 K in a Na-X zeolite [51]. 

The static methods arc volumetric or gravimetric. The volumetric method involves the use of a vacuum system comprising two sections, a dosing section which allows the introduction of accurately measured quantities of the adsorbate, and a sample section which contains the catalyst. The precision of the volumetric method depends on accurate calibrations of volumes. 

As described above, the main purpose of the chemisorption methods is to evaluate the number of active sites that can be reached or that can interact with a fluid phase. These techniques are based on a chemical reaction between a suitable reactive gas and the surface reactive site. There are different methods to perform the above operation, the static volumetric, the static gravimetric or the flow methods. In the volumetric method, the sample is kept under high vacuum before the analysis. The analytical instrument then introduces known doses of reactive gas into the sample holder, measuring afterwards the equilibrium... 

In all the volumetric methods the basic principles are the same. The adsorbate is degassed under vacuum to remove surface contamination. Helium is next admitted into a burette of known volume and its pressure and temperature measured so that the amount at STP can be calculated. The sample tube is immersed in liquid nitrogen and helium admitted. The residual amount in the burette is determined and the amount expanded into the sample tube determined. Since helium does not adsorb on to the solid, this volume is termed the dead space volume and it is found to be linearly dependent on pressure, l e helium is removed and the procedure repeated with nitrogen. When the nitrogen expands into the sample tube, it splits into thi parts, residual in the burette, dead space which can be calculated from the previously found dead space factor, and adsorbed. The process is repeated at increasing pressures and the amount adsorbed determined as a function of relative pressure. 

In all volumetric methods, the principle underlying the determination is the same. Hie powder is heated under vacuum to drive off any adsorbed vapors. The pressure, volume and temperature of a quantity of adsorbate [nitrogen gas usually although krypton is used for low surface areas (5 < 1 m2 g-1)], are measured and the amount of gas present is determined. Traditionally this is recorded as cm3 at standard temperature and pressure (STP) although some prefer moles. 

Nitrogen adsorption was performed at -196 °C in a Micromeritics ASAP 2010 volumetric instrument. The samples were outgassed at 80 °C prior to the adsorption measurement until a 3.10 3 Torr static vacuum was reached. The surface area was calculated by the Brunauer-Emmett-Teller (BET) method. Micropore volume and external surface area were evaluated by the alpha-S method using a standard isotherm measured on Aerosil 200 fumed silica [8]. Powder X-ray diffraction (XRD) patterns of samples dried at 80 °C were collected at room temperature on a Broker AXS D-8 diffractometer with Cu Ka radiation. Thermogravimetric analysis was carried out in air flow with heating rate 10 °C min"1 up to 900 °C in a Netzsch TG 209 C thermal balance. SEM micrographs were recorded on a Hitachi S4500 microscope. 

The gravimetric and volumetric apparatus both require vacuum systems. The continuous flow method does not. 

Accessible metal fractions were determined by hydrogen chemisorption. The volumetric adsorption experiments were performed at room temperature in a conventional vacuum apparatus. Hydrogen uptake was determined using the double isotherm method, as previously reported (3). The platinum dispersion (D ) was calculated by assuming a... 

Method. Three columns containing PL-Thiol MP SPE, PL-Urea MP SPE and PL-Thiourea MP SPE (available from Polymer Laboratories Ltd) are studied to test the efficiency of extraction of metals from ethylene glycol dimethacrylate dissolved in methanol. The spiked and non-spiked monomer are dissolved in ethanol at 5% concentration and a 10.0 ml solution of each monomer is passed through the columns using a vacuum suction water pump. The columns containing the metals are washed with 1.0 ml of 0.25 M HC1 and extracts made up to 25.0 ml in plastic volumetric flasks with deionised water. Extracts are analysed for metal content against standards of 0.0, 0.1, 0.25 and 0.5 ppm of Cu, Fe, Co and Ni prepared in 0.25 M HC1. 

BET surface area and microporous volume determination. Surface area measurements were performed by nitrogen physisorption at 77 K using the static volumetric apparatus (Micromeritics ASAP 2000 adsorption analyzer) with application of the BET method. All measurements were performed after treatment of solids under vacuum at 110 °C. 

An independent method uses a volumetric fill by a modified dosator method. The piston inside the dosator is narrower than those used for powder filling, and this allows air to flow between the piston and the dosator wall. The dosator is lowered into the pellet bed, but in this case, there is no compression applied. A vacuum source is applied from above the piston to retain the pellets as the dosator is moved above the capsule body. Once over the capsule body, the vacuum is removed, and the ejection of the pellets is aided by an air jet. 

Nitrogen adsorption-desorption isotherms at 77 K were determined with a volumetric devise equipped with an integral Barocel pressure transducer [22]. Prior to the measurements, the samples (100 mg) were outgassed at 300°C under vacuum (up to 2.10 Torr) for 10 h. The specific surface areas were calculated by using the BET method and the micropore volumes according to both Dubinin and Singh methods. 

Vacuum Extraction. Vacuum extraction requires the sample to be heated in an evacuated system with determination of the evolved gas, either volumetrically or by differential pressure measurement. This method is recommended whenever possible because of the speed and precision, particularly where the amount of hydrogen is below 0.01%. 

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