Dead space determination

The gas to be used in the dead space determination must be carefully selected. In the procedure described, Step I can be carried out with any permanent gas (e.g. helium or nitrogen), whereas for Step 2 it is advisable to use a gas with the same virial coefficient Bm as the adsorptive, since Bm and the subsequent correction can vaty considerably from one gas to another (see Section 3.4.8). Since the measured value of VA depends on the virial coefficient Bm of the gas used, the simplest procedure is to use the adsorptive itself. Step 3 is also preferably carried out with a gas whose accessibility to the sample is comparable to that of the adsorptive here again the adsorptive itself, at a temperature at which it is known not to adsorb, is the best. 

Gravimetric methods have the great advantage over volumetric ones in that the volume of the adsoiption system is immaterial and the amount of gas adsorbed is observed directly by measuring the increase in the weight of the solid sample upon exposure to a gas or a vapor. The tedious volume calibration and dead-space determinations are thus eliminated. 

The Nova 1000 operates without the need for dead space determination, thus obviating Ae need for helium. It can generate a single BET analysis, a multiple BET analysis, and a 25 point adsorption and desorption isotherm together with total pore volume and sample density. The user places the sample in a calibrated sample cell and after outgassing transfers it to one of the two measurement ports. An optional five port degassing station is also available. The nitrogen adsorbate may be taken from a gas cylinder or from a Dewar flask. 

Following the pioneer work of Beebe in 1945, the adsorption of krypton at 77 K has come into widespread use for the determination of relatively small surface areas because its saturation vapour pressure is rather low (p° 2Torr). Consequently the dead space correction for unadsorbed gas is small enough to permit the measurement of quite small adsorption with reasonable precision. Estimates of specific surface as low as 10 cm g" have been reported. Unfortunately, however, there are some complications in the interpretation of the adsorption isotherm. 

Dead space. Anatomical dead space is equal to the volume of the conducting airways. This is determined by the physical characteristics of the lungs because, by definition, these airways do not contain alveoli to participate in gas exchange. Alveolar dead space is the volume of air that enters unperfused alveoli. In other words, these alveoli receive airflow but no blood flow with no blood flow to the alveoli, gas exchange cannot take place. Therefore, alveolar dead space is based on functional considerations rather than anatomical factors. Healthy lungs have little or no alveolar dead space. Various pathological conditions, such as low cardiac output, may result in alveolar dead space. The anatomical dead space combined with the alveolar dead space is referred to as physiological dead space ... 

Physiological dead space is determined by measuring the amount of carbon dioxide in the expired air. Therefore, it is based on the functional characteristics of the lungs because only perfused alveoli can participate in gas exchange and eliminate carbon dioxide. 

Stability of the test chemical in drinking water under study conditions should be determined prior to study initiation. Consideration should be given to conducting stability tests on test chemical-drinking water admixtures presented to some test animals. Besides difficulties of inherent stability, changes in chemical concentrations may result from other influences. Chemicals with low vapor pressure can volatilize from the water into the air space located above the water of an inverted water bottle thus, a majority of the chemical may be found in the dead space, not in the water. 

The three-way stopcock is opened to connect the gas burette with the sample tube. The new pressure and volume are read. From this, the volume of the dead space — the volume beyond the three-way stopcock that is not occupied by sample —can be determined (see Example 9.1). 

If the adsorption isotherm is to be determined at some temperature other than room temperature-liquid nitrogen temperature, for example—the sample tube is placed in a suitable thermostat. This is indicated by the dotted line in Figure 9.3. In this case two sets of readings are made with the nonadsorbed gas, one at room temperature and one with the thermostat in place. In this way the partitioning of the dead space between the two temperature regions can be determined. Several additional considerations should be cited that are important in actual practice ... 

The classical method involves admitting a known quantity of gas to the sample chamber, which is usually maintained near the condensation point of the gas. Adsorption of the gas on the surface of the solid occurs, decreasing the pressure in the chamber until the adsorbed gas is in equilibrium with the free gas phase. The volume of gas adsorbed is determined by subtracting the volume of gas required to fill the free space (dead space) at equilibrium pressure from the volume of gas admitted. The dead space is obtained by precalibration of the chamber volume or by repeating the determination with a sample of negligible adsorption. The specific surface area (S), in m2/g, is given by the following... 

The total accessible pore volume may be measured by the amount of adsorbate at the saturation pressure of the adsorptive, calculated as liquid volume, provided the adsorption on the external surface can be neglected or can be evaluated. The accessible pore volume may be different for molecules of different sizes. A method which is not subject to the effect of the external surface is the determination of the dead space by means of a non-sorbable gas (normally helium) in conjunction with the determination of the bulk volume of the adsorbent by means of a non-wetting liquid or by geometrical measurements. 

In volumetric measurements the volume of an adsorbed gas at constant pressure and temperature is determined. Therefore, we first determine the dead space or volume of the bulb by admitting some nonadsorbing (or weakly adsorbing) gas such as Helium. Then, after evacuating the bulb, the gas of interest is admitted into the bulb. This is done at constant pressure and temperature. The volume admitted into the bulb minus the dead space is the amount adsorbed. 

Krypton adsorption at 77 K is often used for the determination of relatively low solid-surface areas. At this temperature the vapour pressure of krypton (and so the dead-space correction) is small, and a reasonable precision is attainable. 

It is essential to take into account a number of potential sources of experimental error in the determination of an adsorption isotherm. In the application of a volumetric technique involving a dosing procedure it must be kept in mind that any errors in the measured doses of gas are cumulative and that the amount remaining unadsorbed in the dead space becomes increasingly important as the pressure increases. In particular, the accuracy of nitrogen adsorption measurements at temperatures of about 77 K will depend on the control of the following factors ... 

Dead space was determined with He and the surface area determined from a nitrogen isotherm at —195°. The catalyst had a surface area of 10.2 m.2/g. in agreement for this catalyst type with the activity measured. 

Steps 3 and 4 are not required if the dead space volume is determined in another way. 

Determination of dead space volume by gas expansion into adsoiption bulb and measurement of new pressure another method is to determine the modified slope of the p versus t curve. 

Thus in addition to the data required to determine the surface excess amount (cf. Section 3.3.1), one needs to know dQKV (the heat exchanged reversibly during each adsorption step) and Vc (the volume - dead space - of that part of the adsorption bulb which is located within the calorimetric detector (cf. Figure 3.15). Vc is evaluated by liquid weighing or by geometrical considerations and corrected for the sample volume. 

When thermal equilibrium reached (i.e. after c. 3-5 hours), the dead space volume is determined as in Steps 3 and 4 of the discontinuous manometric procedure in Section 3.3.1. In the case of low-temperature calorimetry this must be done between Steps 2 and 3, by simply connecting the adsorption bulb to the manometric equipment. 

Steps 1-5 as for the discontinuous experiment. Dead space volume determination (Step 4) also possible by the discontinuous sonic nozzle procedure (Section 3.3.2). 

As we have already seen, for the application of most manometric techniques for the determination of the amount adsorbed it is necessary to have an accurate knowledge of the volumes of two parts of the overall dead space. The first is the connecting volume located between the stopcock above the adsorbent bulb and the lowest valve of the dosing volume (see Figure 3.2). The second, and more important, volume is that of the dead space within the adsorbent bulb. Although the connecting volume does not need to be determined for each experiment, its value can be checked in the first stage of the gas expansion calibration procedure. 

The determination of the dead space volume of the adsorbent bulb is not quite as straightforward as one might think. It is necessary to consider the following three questions (1) How do we define the remaining gas volume in relation to the volume occupied by the adsorbent (2) What is the most suitable procedure (3) If gas expansion is to be used, then which gas (e.g. He or Nj) should be adopted, and at what temperature ... 

As can be seen, helium, in contrast to what was once assumed, is not necessarily the best gas to select for the determination of dead space. It is sometimes thought that helium allows dead space to be determined directly at 77 K in the presence of the sample, since it will not adsorb. However, since its virial coefficient is much smaller than that of most adsorptives (see Table 3.2), and because of the possibility of adsorption in micropores (see Chapter 9), its use cannot be recommended. This problem has been discussed recently by Neimark and Ravikovitch (1997). 

The indirect route for determining the dead space volume makes use of an estimated volume of the adsorbent sample. This volume can be obtained in two ways ... 

Helium is often used in adsorption manometry for the determination of the dead space volume (see Chapter 3), but this procedure is based on the presupposition that the gas is not adsorbed at ambient temperature and that it does not penetrate into regions of the adsorbent structure which are inaccessible to the adsorptive molecules. In fact, with some microporous adsorbents, significant amounts of helium adsorption can be detected at temperatures well above the normal boiling point (4.2 K). For this reason, the apparent density (or so-called true density ) determined by helium pycnometry (Rouquerol et al., 1994) is sometimes dependent on the operational temperature and pressure (Fulconis, 1996). 

The establishment of the International Temperature Scale has been accomplished largely with the aid of measurements made with the helium gas thermometer. The most precise gas thermometry method is the constant-volume method, in which a definite quantity of the gas is confined in a bulb of constant volume Eat the temperature T to be determined and the pressure p of the gas is measured. A problem is encountered however in measuring the pressure a way must be found to communicate between the bulb and the pressure gauge. This is usually accomplished by connecting the bulb to the room-temperature part of the system by a slender tube and allowing a portion of the gas to occupy a relatively small, constant dead-space volume at room temperature. Thus, it is important that the gas volume in the pressure manometer be as small as possible. 

For the determination of each isotherm, the sample cell was kept to within 0.1 °C of the experimental temperature by use of a liquid thermostat. Dead space correction was carried out using a helium calibration. A difference in the measured dead space of up to 0.4 % was observed over the experimental pressure range. When varying the temperature within the 20 -60°C range however, this variation in the measured dead space was far less. 

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