Volumetric-gravimetric instrument

In this section we want to present data for binary coadsorption equilibria of gas mixtures on activated carbon (NORIT R 1) which have been taken by combined volumetric-gravimetric measurements. We start with a set of data which have been measured with the instrument depicted in Figs. 4.1,4.2. Then we describe in brief a new type of volumetric-gravimetric instrument including a magnetic suspension balance allowing also measurements with corrosive sorptive gases. Equilibria as well as kinetic data taken at this instrument will be presented. Finally we hint at a commerciahzed version of this instrument offered by BEE - Japan company. 

Gravimetric instruments can accommodate any adsorbate. Continuous flow instruments are limited to adsorbates which will not condense at room temperature. Volumetric apparatus require various internal volume and manometer alterations when adsorbates with low vapor pressures are used. 

Vapour and gas sorption measurements can be performed with static or dynamic methods, either of which can provide information on equilibrium behaviour. Furthermore, the measurements can be performed using gravimetric or volumetric based instrumentation. The most common flow methods are inverse gas chromatography (IGC) [1] for volumetric studies and dynamic gravimetric instrumentation [2]. 

In practice, activation of a sorbent material much more easily can be accomplished in gravimetric than in volumetric sorption instruments. The main reason for this is that in gravimetry the mass of the sample can be recorded even during the activation process whereas in volumetry / manometry it normally can not. Hence in gravimetry the initial state of a sorbens is fairly well known. Also presorption in a new sorbent sample and remnant sorption after a desorption experiment easily can be checked. 

Figure 4.1. Experimental setup of a multipurpose instrument for coadsorption measurements of multicomponent gas mixtures on porous solids. The instrument allows volumetrie-chromatographic, gravimetric-chromatographic and also combined volumetric-gravimetric measurements for binary coadsorption equUihria.

Figure 4.2. Instrument for volumetric-gravimetric-chromatographic measurements of coadsorption equilibria of gas mixtures on porous materials. The gas storage vessel can be recognized on the left hand, the microbalance in the middle above the thermostat and the gas chromatograph on the right hand side of the picture. IFT, University of Siegen, 1990.

Figure 4.11a. Schematic diagram of an instrument for automated binary coadsorption equilibria measurements using the volumetric-gravimetric method. The installation has been designed by BEL-Japan Corp. and includes a magnetic suspension balance, RUBOTHERM, Bochum. (Reprint by permission of BEL-Japan.)...

Instrument for automated combined volumetric-gravimetric measurements of binary coadsorption equilibria of gas mixtures with non-isometric components. The RUBOTHERM magnetic suspension balance is on the left side. The large white closet includes the volumetric / manometiic part of the instrument and is manufactured by BEL-Japan, Osaka. (Reprint by permission of BEL-Japan.)... 

In this section we would like to address in brief the problem of wall adsorption. By this we understand the phenomenon that any gas or Uquid contacting the inner surfaces of vessels, tubes, valves etc. of any device will be adsorbed to a certain, though normally only tiny, amount on them. This phenomenon first has observed in gas thermometers at very low temperatures i. e. low gas pressures [4.20]. Here it added considerably to the uncertainties of measurements. The same can be tme for volumetric or volumetric-gravimetric adsorption measurements at low pressures or if only very small amounts of sorbent material - say 100 mg or less - are available, cp. Chap. 2. Therefore it sometimes may be necessary in setting up mass balances for adsorption measurements to take this phenomenon into account. A possibihty to do this is to measure it. This on principle can be done by using the instrument sketched in Fig. 4.20 below. It mainly consists on three parts a gas storage vessel (SV) equipped with a piston to change its volume (Vsv) and... 

In practice an instrument for volumetric-gravimetric coadsorption measurement requires a considerable amount of equipment and expertise to operate it properly. Hence the instrument will be quite expensive, especially if it is designed and equipped for automatic measurements. 

Figure 5.14. Training instrument for oscUlometric, volumetric, gravimetric, and dielectric measurements of gas adsorption equilibria in rigid and swelling sorbent materials. The pendulum (left) is covered by a plexiglass vessel allowing direct optical observations of its rotational oscillations.

Gravimetric measurements require corrections to account for buoyancy effects while volumetric measurements must be corrected for the sample volume. It is often not realised that these corrections are equivalent. Helium is used to determine both corrections since it is assumed not to be adsorbed. The degas procedure for gravimetric instruments involves continuous monitoring of mass in relation to time, which allows outgas rate to be specified and the equilibrium state and any sample mass losses... 

Standard instrument control and pacing signals are generally acceptable for common feeder system operation. Volumetric and gravimetric feeders are usually adaptable to operation from any standard instrument signals. 

When solution must be pumped, consideration should be given to use of holding tanks between the dry feed system and feed pumps, and the solution water supply should be controlled to prevent excessive dilution. The dry feeders may be started and stopped by tank level probes. Variable-control metering pumps can then transfer the alum stock solution to the point of application without further dilution. Means should be provided for calibration of the chemical feeders. Volumetric feeders may be mounted on platform scales. Belt feeders should include a sample chute and box to catch samples for checking actual delivery with set delivery. Gravimetric feeders are usually furnished with totalizers only. Remote instrumentation is frequently used with gravimetric equipment, but seldom used with volumetric equipment. 

Oximes, hydroxamic acids and related species are often used as reagents in inorganic analytical chemistry for precipitation, gravimetric and volumetric determinations as well as for preconcentration, extraction, derivatizations and subsequent determination of analyte using instrumental techniques. A brief review of analytical chemistry in general and of these species in particular follows. 

By their nature, gravimetric apparatus are quite fragile and care must be exercised in their use. Volumetric equipment usually contains fragile glass components. Continuous flow instruments are all metal, except for the sample cell, and are the least fragile of the various apparatus. 

Four types of sorption measuring instruments ire on the market volumetric/manometric, gravimetric, carrier gas and calorimetric. 

In Table 1 manufacturers of carrier gas and volumetric adsorption measuring instruments are compiled. Most of these companies offer also mercury porosimeter and gas pycnometer. Mercury porosimetry extends the measuring range of the sorption method towards larger pores. Table 2 gives a survey on the commercial offer of gravimetric apparatus. 

The Karl Fischer test method ASTM D-6304) can be applied to the direct determination of water in lubricating oil. In this method, the sample injection in the titration vessel can be done volumetrically or gravimetrically. The instrument automatically titrates the sample and displays the result at the end of the titration. Viscous samples can be analyzed by using a water vaporizer accessory that heats the sample in the evaporation chamber, and the vaporized water is carried into the Karl Fischer titration cell by a dry, inert carrier gas. 

Traditional methods of elemental analysis depend on specific chemical reactions for given elements, either in solution using titrations (known as volumetric analysis) or precipitation of solids that can be weighed (gravimetric analysis). Although such methods are still used for specific and very accurate purposes, they have been replaced in routine work by automated instrumental methods. Combustion analysis is used to determine C, H, N, and sometimes S, by complete oxidation of the compound forming C02, H20, N2 and S02. The gases are separated and determined automatically... 

H2O, n-hexane and cyclohexane sorption capacities of SAPO-31 were determined gravimetrically using a vacuum microbalance (Cahn Instruments, USA). The size and the morphology of the crystals of SAPO-31 were examined using a JEOL (JSM-840 A) scanning electron microscope. The BET surface area was determined using a volumetric adsorption apparatus ( Model Omnisorb lOOCX, Coulter, USA). 

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