Cooling dewars

Two 500-mL glass traps with gas inlet and outlet, magnetic stirrer, liquid nitrogen cooling Dewar safety glasses, laboratory coat, protective shield, protective gloves, and well-ventilated hood. 

Pentyn-l-ol. Prepare a solution of sodamide in liquid ammonia as detailed for n-Butylacetylene. Use a 3-htre three-necked flask, equipped with a Dewar type of reflux condenser (Fig. II, 1, 4, h) cooled with Dry Ice... 

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. 

A Neslab LILT-80 refrigerated circulating bath was used. Alternatively, a Dewar condenser cooled with acetone-dry ice can be used. 

If the pump is a filter pump off a high-pressure water supply, its performance will be limited by the temperature of the water because the vapour pressure of water at 10°, 15°, 20° and 25° is 9.2, 12.8, 17.5 and 23.8 mm Hg respectively. The pressure can be measured with an ordinary manometer. For vacuums in the range lO" mm Hg to 10 mm Hg, rotary mechanical pumps (oil pumps) are used and the pressure can be measured with a Vacustat McLeod type gauge. If still higher vacuums are required, for example for high vacuum sublimations, a mercury diffusion pump is suitable. Such a pump can provide a vacuum up to 10" mm Hg. For better efficiencies, the pump can be backed up by a mechanical pump. In all cases, the mercury pump is connected to the distillation apparatus through several traps to remove mercury vapours. These traps may operate by chemical action, for example the use of sodium hydroxide pellets to react with acids, or by condensation, in which case empty tubes cooled in solid carbon dioxide-ethanol or liquid nitrogen (contained in wide-mouthed Dewar flasks) are used. 

Diketene polymerizes violently in the presence of alkali. Distd at reduced pressure, then fractionally crystd by partial freezing (using as a cooling bath a 1 1 soln of Na2S203 in water, cooled with Dry-ice until slushy, and stored in a Dewar flask). Freezing proceeds slowly, and takes about a day for half completion. The crystals are separated and stored in a refrigerator under N2. See ketene on p. 276. 

Figure 1. A, Dewar flask B, sintered glass filter C, metal cooling coil D, water inlet E, water outlet F, reaction vessel < , quartz immersion well /f, pyrex filter /, lamp ...

The submitters filled the Dewar flask with ice and water. After 2 hours the ice had melted and water was left in the flask for cooling. 

Immersion well (quartz or pyrex), fitted with inlet (a) and outlet (6) for cooling liquid e.g. methanol), and with ground joint (c) to Reaction flask, fitted with outlet d) to vacuum system ( 10 mmHg) Liquid nitrogen Dewar flask... 

We have seen that hydrogen becomes slightly warmed in this process, so that its liquefaction by free expansion would be impossible under ordinary conditions. Dewar in 1900 showed, however, that if the hydrogen was previously cooled, it suffered a further cooling on free expansion, and in this way he obtained liquid hydrogen. Olszewski (1902) found that the inversion point of hydrogen is situated at — 80 6° C. This effect of temperature is general, and implies that the ratio of the potential to the kinetic... 

We have seen in Chapter 2 that the frequency of an EPR spectrum is not a choice for the operator (once the spectrometer has been built or bought) as it is determined by the combined fixed dimensions of the resonator, the dewar cooling system, and the sample. Even if standardized sample tubes are used and all the samples have the same dielectric constant (e.g., frozen dilute aqueous solutions of metalloproteins), the frequency will still slightly vary over time over a series of consecutive measurements, due to thermal instabilities of the setup. By consequence, two spectra generally do not have the same frequency value, which means that we have to renormalize before we can compare them. This also applies to difference spectra and to spectra... 

A minimum amount of liquid nitrogen contained in a 1 -L Dewar bowl was used to cool the solution to -105°C. 

There are two phases in the use of a dewar or a cryostat the cool-down period and the constant temperature period. 

From eq. (5.2) we see that the total power emitted by 1 cm2 with e = 1 at 300 K is 45 mW corresponding to an evaporation of 70cm3/h of 4He. At 77 K, a surface of 1 cm2 emits about 0.2 mW, with a 4He consumption of 0.3 cm3/h. Hence the part of the dewar cooled at helium temperature is surrounded by radiation shields or baffles at intermediate temperatures. The latter are either gas cooled or thermally anchored to a LN2 reservoir. 

Some of them use two reservoirs (liquid N2 and 4He, see below), but most metallic dewars do no longer use LN2 (which produces vibrations in boiling) to cool radiative shields. Instead superinsulation is used. 

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