Traps volume calibration

After refluxing, disconnect the trapping tube, and transfer the yellow solution into a 25-mL volumetric flask. Rinse the mbe with ethanol, and adjust the solution to volume with ethanol. Measure the absorbance of the solution at 435 nm against a blank prepared by diluting 15 mL of color reagent to 25 mL with ethanol. Determine the carbon disulflde content from a calibration curve obtained by plotting carbon disulfide concentrations of different standard solutions on the abscissa versus the absorbance on the ordinate. 

A modification of this technique, applicable to many step polymerizations, involves the continuous monitoring of the small-molecule by-product. For example, for polyesterification between a diol and diacid above 100°C, water distills out of the reaction vessel and its volume can be measured by condensation and collection in a calibrated trap. 

H. Example Calibration of a Trap and Measurement of a Gas Sample. In contrast to the previous example, the objective here is to measure the amount of an entire gas sample, such as the BF3 recovered in the trap-to-trap distillation discussed in Section 5.3.E. in this type of measurement a manometer is included in the calibrated volume, and it is necessary to account for the change of volume as the mercury level changes with changes in pressure. As described in Chapter 7 a constant-volume gas buret can be used, but this is somewhat cumbersome. So the simpler procedure described here is more frequently used. 

Suppose that we wish to calibrate the volume of trap E connected to manometer D on the vacuum line in Fig. 5.2. A known quantity of a gas, such as CO, is condensed into trap E using the calibrated bulb and the techniques just outlined in Example 5.3.G. The stopcocks are then turned so that trap E communicates with the central manometer D but is isolated from the rest of the vacuum system. At this point, the cold trap is removed from E, the trap is allowed to come to room temperature, and the pressure and room temperature are measured. The volume of the manometer-trap combination is determined from the known moles of gas, the pressure, and the temperature, using the ideal gas law. The process is repeated with successively larger samples of CO2, and a plot of volume versus pressure is constructed from the data. Since the bore of the manometer is of constant diameter, this plot should be a straight line. It also is possible to... 

Fig. 7.7. The McLeod gauge. The principles of operation follow. Let the unknown pressure in a system be P when the Hg level is below point 1. Let the volume of the bulb and closed capillary above I be V, which is known. When the mercury is allowed to rise past point I, the gas is trapped and finally compressed into the capillary. Suppose that when the mercury in the reference capillary is at 0, the mercury in the dead-ended capillary is B mm below 0 (i.e., the pressure of the compressed gas is B mm). Since the initial pressure-volume product equals the final pressure-volume product, PV = pv, the volume in thecapillary v will be the height B times the area of the capillary bore A. Thus P = pv/V = B (A/V). Since A and V are known and B is measured, the original pressure (P) may be calculated. Most commercial gauges are provided with a calibrated scale which presents pressures directly. Alternatively, it is possible to devise a linear scale for the McLeod gauge, in one such method the mercury height in the closed capillary is always adjusted to the same point (B0), and then the difference in meniscus heights between the two capillaries is measured (AB). For this case the pressure being measured is P = pv0/V = (B0A/V)AB. As in the previous example, the quantity in parentheses represents the gauge calibration constant.

Fig. 1.17, Apparatus for dispensing known quantities of condensable gases. The trap may be calibrated by using water (for large volumes) or mercury (for small volumes). A mercury bubbler is included to prevent blowing the apparatus apart.

For accurate analysis, we must know the total volume of air passed through the adsorbent tube, the mass of the analyte trapped, and the desorption efficiency of the solvent. Before sampling, the pump must be calibrated using a bubblemeter, a rotameter, or a gasometer to determine the flow rate. Using the flow rate and the time sampled, the total volume of air sampled can be determined. 

Figure 8.13 Dynamic volumetric gas adsorption apparatus, (A) sample compartment, (B) calibrated volume bowls, (C) cryogenic trap, (D) manometer, (E) evacuation line, (F) circulation pump.

Positive-displacement (PD) flowmeters are used when the total quantity of the flowing process stream is of interest or when a recipe is being formulated in a batch process. These meters operate by trapping a fixed volume of fluid and transferring that volume from the inlet to the outlet side of the meter. The number of such calibrated "packages" of fluid is counted as a measure of total volumetric flow. These measuring devices are used in both gas and liquid services. 

Anhyd nitrosyl chloride is condensed into a trap cooled to — 12°C which has been previously calibrated to a known volume of 4.62 mL. corresponding to 0.1 mol of nitrosyl chloride. The contents are added under N2 to a mixture of 10.15 mL (0.1 mol) of cyclohcxcnc and 50 mL of SO, cooled to —40°C. The flask temperature is allowed to rise to — 30 °C and is vigorously stirred for 3 h at this temperature. The mixture is then filtered to yield 12.55 g (85%) of slightly green solid, which is washed immediately with cold CH3OH to give a white solid, which is rccrystallizcd from EtOH, mp 152-153 °C. This compound may be either the meso or the dl form. 

The vacuum line is isolated from the forepump, and a liquid-nitrogen bath is put around trap D. The fluorine metering system is partially evacuated with the water aspirator, as discussed above. Then the vacuum line is completely evacuated with the forepump. Fluorine is allowed to expand slowly into the metal system by means of the needle valve 3. As the pressure increases above atmospheric, some fluorine is allowed to bleed into the vacuum line and storage bulbs until a pressure of 650-700 mm. is reached. First needle valve 1 on the metal system and then the fluorine control valve 3 are quickly closed. (Some fluorine may escape from the blowout manometer during this operation.) Approximately a 10% excess of fluorine (0.023 mole in this case) is condensed into the metal pressure reactor containing the thionyl fluoride. The amount of fluorine used is measured by the pressure drop in a calibrated volume in the pressure range of approximately 700-400 mm., since fluorine has a vapor pressure of approximately 400 mm. at —196°. 

Separation of a noncondensible gas like CO or H2 from the solvent is effected simply by carefully opening the reactor (at 25°) to a cold (-196°) spiral trap (of approximate volume 100 mL). After completion of vacuum transfer of acetonitrile into the sprial trap, the volatile gas is Toepler pumped (10) into the calibrated region of the vacuum line until further strokes of the pump give no change in pressure. The number of mmoles of gas is determined from the calibration of the vacuum line. 

Figure 9.6 Results obtained for the purge-and-trap extraction of BTEX from water, showing (a) the calibration graphs, and (b) the influence of sample volume , benzene , toluene A, ethylbenzene x, m-, p-xylene T, o-xylene [1] (cf. DQ 9.4).

The first of the separation techniques to be used in process measurement was gas chromatography (GC) in 1954. The GC has always been a robust instrument and this aided its transfer to the process environment. The differences between laboratory GC and process GC instruments are important. With process GC, the sample is transferred directly from the process stream to the instrument. Instead of an inlet septum, process GC has a valve, which is critical for repetitively and reproducibly transferring a precise volume of sample into the volatiliser and thence into the carrier gas. This valve is also used to intermittently introduce a reference sample for calibration purposes. Instead of one column and a temperature ramp, the set up involves many columns under isothermal conditions. The more usual column types are open tubular, as these are efficient and analysis is more rapid than with packed columns. A pre-column is often used to trap unwanted contaminants, e.g. water, and it is backflushed while the rest of the sample is sent on to the analysis column. The universal detector - thermal conductivity detector (TCD)-is most often used in process GC but also popular are the FID, PID, ECD, FPD and of course MS. Process GC is used extensively in the petroleum industry, in environmental analysis of air and water samples" and in the chemical industry with the incorporation of sample extraction or preparation on-line. It is also applied for on-line monitoring of volatile products during fermentation processes" ... 

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