The Gravimetric Technique

The main advantage of the gravimetric technique is that it requires a much smaller sample than the stoichiometric technique. In many cases, samples as small as 70 mg are sufHcient. Accurate temperature and pressure control and measurement are still required, but gas adsorption on the metal walls of the equipment is no longer a concern because it is only the weight gain of the sample that is measured. 

There are two main disadvantages to this technique. First, the sample is placed in a static sample bucket i.e. there is no possibility of stirring. Thus, equilibrium is reached solely by diffusion of the gas into the IL sample. For the more viscous samples this can require equilibration times of as much as several hours. Second, the weight gain must be corrected for the buoyancy of the sample in order to determine the actual gas solubility. While the mass is measured accurately, the density of the sample must also be known accurately for the buoyancy correction. 

Quartz crystal microbalances have also been used to measure gas solubilities in ILs [15]. In these experiments a thin layer of IL is spread on a quartz crystal and the mass uptake is measured by the change in vibration frequency of the crystal. An important consideration in these experiments is that a sufficiendy large frequency change results from the mass uptake. As a result, this technique is best suited to high solubility gases. 

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Styrene conversion calculated by this equation and styrene conversion obtained for runs 12-15 by gravimetric methods were in good agreement. In general, the gravimetric technique was 1 to 5% points greater than conversions calculated using the GPC data. 

We have also compared the determination of the adsorption isotherms with those from a standard gravimetric technique. The precision of the data obtained by the gravimetric technique was found to be lower than that obtained by the FT-IR technique, particularly at very low silane concentrations (about 5%, and 3% at 0.4 g/100 ml concentration for the gravimetric and the FT-IR techniques, respectively). This is because very small weights of the silanes cannot be measured accurately and the silane is volatile. The effects of suspended silica particles in the solution were also examined by centrifuging methanol solutions... 

Subsequent work on catalyst characterization via reduction studies has involved gravimetric or volumetric techniques. The gravimetric method measures weight loss (due to O loss), while the volumetric method measures amount of H2 consumed [and amount of water formed (O loss)]. In the latter case, the amount of H retained by the catalyst can also be determined (20). The gravimetric technique has also been applied to sulfided catalysts (27). From these measurements, the stoichiometric state of the catalyst can be determined. 

Electrogravimetry and coulometry are moderately. sensitive and among the most accurate and precise techniques available to the chemist. Like the gravimetric techniques discussed in Chapter 12, electmgravimetiy requires no preliminary calibration against chemical standards becau.se the functional relationship between... 

The Sieverts techmque is sensitive to the density of the sample because the volume occupied by the sample must be subtracted from the volume of the empty sample cell in order to calculate H/X. The gravimetric technique is intrinsically sensitive to the volume or density of the sample through the buoyancy force on it. Knowledge of the sample volume is also required to... 

An alternative technique to the stoichiometric method for measuring gas solubilities has evolved as a result of the development of extremely accurate microbalances. The gravimetric technique involves the measurement of the weight gain of an IL sample... 

A simple nondestructive capacitance method is proposed (Adamyan et al, 2006) for the determination of basic PSi parameters such as layer thickness, porosity and dielectric permittivity. The method is based on two comparative measurements of the capacitance of the metal/PSi/single crystalline silicon/metal structure one measurement is taken when there are air-filled pores, while the other measurement involves pores filled by an organic compound with a high value of dielectric permittivity. Comparison of results obtained in Adamyan et al. (2006) by the ball lap and the gravimetric techniques before and after anodization, with the data of capacitance measurements carried out with the same samples prior to their destruction, shows sufficiently good agreement. 

With binary Th-N alloys having no interferring cation, the spectrophotometric methods lend themselves to rapid routine analyses especially useful for limited sample size, but the gravimetric techniques are ordinarily most accurate. 

Calorimetric measurements are less common than the measurements mentioned above and yield a different physical quantity. To be effective, the calorimetric method needs to be combined with either the volumetric technique, which is normal, or with the gravimetric technique which is a little more difficult for high-quality work. Both methods are used. Calorimetry measures the temperature change as the adsorption occurs. This along with a heat capacity measurements of the resultant adsorbate-adsorbent combination yields the heat of adsorption as a function of pressure. Less precise calorimetric measurements measure only the heat evolved which gives... 

Cost. Usually, the gravimetric technique is costlier than the volumetric technique. The volumetric technique requires only high precision pressure transducers and high precision volume measurements. The gravimetric, however, requires a high precision vacuum balance and, perhaps, considerable set-up effort. 

Capability. Usually, the gravimetric technique is more precise and accurate. It is a better research method than the volumetric technique. The volumetric technique is incapable of some measurements needed in research, but for most routine work, given some important caveats, it is sufficient. 

The primary disadvantage of the technique is that it is not as suited for careful research work as the gravimetric technique. To use it in this mode, the cost advantage begins to disappear and the amount of effort required to do careful work becomes quite large, with many potential pitfalls. This is especially true for the low-pressure range of the isotherm, but it can also be true for die upper range where porosity measurements are extracted. 

With the buoyancy correction and the molecular flow correction, the data obtained from the gravimetric technique should be very accurate. The limit of... 

The maximum pressure to which the gravimetric technique has been applied is about 15 MPa. The volumetric method is very seldom applied above 100 MPa [29]. Bose and co-workers [30] developed a precision dielectric method for the determination of gas-solid adsorption. This method is particularly suitable for adsorption measurements up to 200 MPa actually, adsorption data up to 650 MPa were reported [31]. The great advantage of the dielectric method is that it is self-sufficient up to the highest pressures and does not depend on the availability of compressibility factor values, as in volumetric or gravimetric measurements [29]. Other new, yet less widespread, adsorption measurement techniques include oscillometry [32,33], calorimetry [34,35], and electromagnetic measurements [36,37]. 

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