Gases liquid nitrogen

When condensing explosive gases, the temperature of the bath and the effect on the reactant gas of the condensing material selected must be determined experimentally (see Chapter 6, section 6.D). Very small quantities should be used because detonations may occur. A taped and shielded Dewar flask should always be used when condensing reactants. Maximum quantity limits should be observed. A dry ice solvent bath is not recommended for reactive gases liquid nitrogen is recommended. (See also Chapter 3, section 3.D.3.I.)... 

A better method is to cool the specimen with a stream of cold gas. Liquid nitrogen, for example, boils at 77°K ( = - I96°C) and can easily produce nitrogen gas at the specimen at about — 150°C. Gas produces much less background scattering than a liquid coolant. 

Oxides of nitrogen are thought to have been involved in several violent mcidents. See Hydrogen (Gas), Liquid nitrogen, 4453... 

Figure 8. Influence of the cooling speed on the same specimen as in Figure 7. Cooling speeds 1 °/mn ----, He gas -------, liquid nitrogen quenching.

FIGURE 8.6 Cold Gas Liquid Nitrogen Bubble Point as a Function of the Liquid Screen Side Temperature. 

Fig. 2. FIM image of tungsten tip prior to contact (18.0 kV, helium image gas, liquid nitrogen cooling).

Several instniments have been developed for measuring kinetics at temperatures below that of liquid nitrogen [81]. Liquid helium cooled drift tubes and ion traps have been employed, but this apparatus is of limited use since most gases freeze at temperatures below about 80 K. Molecules can be maintained in the gas phase at low temperatures in a free jet expansion. The CRESU apparatus (acronym for the French translation of reaction kinetics at supersonic conditions) uses a Laval nozzle expansion to obtain temperatures of 8-160 K. The merged ion beam and molecular beam apparatus are described above. These teclmiques have provided important infonnation on reactions pertinent to interstellar-cloud chemistry as well as the temperature dependence of reactions in a regime not otherwise accessible. In particular, infonnation on ion-molecule collision rates as a ftmction of temperature has proven valuable m refining theoretical calculations. 

Apparatus-. 1-1 three-necked, round-bottomed flask with a thermometer+gas outlet, a stirrer and a gas inlet. During the experiment a slow stream of nitrogen was passed through the flask. A Dewar flask containing liquid nitrogen (note 1) was used as a cooling bath. 

In the flask was placed a solution of 50 g of carbon dioxide in 250 ml of dry THF, made by introducing the gas from a cylinder into THF, cooled at about -90°C (liquid nitrogen bath). The gas inlet was removed and through the neck of the reaction flask was poured a cold solution (-70°C) of 0.40 mol of allenyl1ithiurn in 280 ml of hexane and 280 ml of THF (prepared as described in Chapter II,... 

A solution of 0.20 mol of butyl lithium in about 140 ml of hexane was cooled to -6Q°C and 140 ml of dry THF were added. The mixture was cooled to about -80 C (liquid nitrogen bath) and 0.23 mol of the allenic hydrocarbon (see Chapter VI, Exp. 1, 2, 44) was added in 5 min (methylal1ene was added as a 1 1 solution in THF). The solutions were kept for 1 h at -55°C. Into another 1-1 flask (see also Fig. 1, but without a dropping funnel), cooled at -90°C by immersion in liquid nitrogen, was poured a solution of dry carbon dioxide (from a cylinder) in 130 ml of dry THF. This solution was obtained by introducing about 40 g of carbon dioxide (note 1) into the THF at -90°C. The gas inlet was removed from the second flask and the solution of the lithiated allene (still cooled below -60 C) was poured... 

A number of potential sources of error must be taken into account. In the volumetric method the following items need attention (a) constancy of the level of liquid nitrogen (b) depth of immersion of the sample bulb ( S cm) (c) temperature of sample (monitoring with vapour pressure thermometer close to sample bulb) (d) purity of adsorptive (preferably 99-9 per cent) (e) temperature of gas volumes (doser, dead space), controlled to 01 C. 

Liquid nitrogen is used in cold traps to remove and recover solvents or volatile organic compounds from gas streams to reduce atmospheric emissions. Liquid nitrogen can be used to accelerate the cooldown time for process reactors (29). 

External cryogenic requirements None required 1. Helium hquefier 2. Helium compressor 3. Liquid helium storage tank 4. liquid nitrogen storage tank 5. Helium gas ballast tank Not apphcahle... 

Ga metal (cooled to solid phase using liquid nitrogen) Powders... 

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