Liquid nitrogen slush baths

The samples were irradiated with 60Co gamma rays in an AECL Gammacell or in the pond facility of the AAEC, at dose rates from 0.1 to 2 Mrad h-1, to total doses from 0.1 to several hundred Mrad. Constant temperatures from 77° to 448°K (-196° to 175°C) were maintained using liquid nitrogen, slush baths, solid C02 and aluminum block heaters. 

Operating conditions 460 C, He flow 20 cc/min liquid feed rate 3 ml/min. Collected in two traps cooled with air and dry ice/acetone, analyses as per Table Collected in trap cooled with 1-propanol/liquid nitrogen slush bath, analyses gby glc. 

A quartz Dewar (Fig. 14.2) as described in Sect. 14.3.3 is a relatively cheap alternative for low temperatme work. 77 K is the most convenient temperature to work at, but thermostatting at various other temperatures down to 113 K is possible using solid CO2 or liquid nitrogen slush baths [1, 38]. If a thermocouple or thermistor can be placed in the Dewar the temperature can be monitored, and measurements made, as the whole assembly warms to ambient temperature. 

In the laboratory, a range of slush baths may be used for speciality work. These are prepared by cooling organic liquids to their melting points by the addition of liquid nitrogen. Common examples are given in Table 8.2. Unless strict handling precautions are instituted, it is advisable to replace the more toxic and flammable solvents by safer alternatives. 

For other organic materials used in low temperature slush-baths with liquid nitrogen see R.E.Rondeau [J.Chem.Eng.Data 11 124 7966]. NOTE that the liquid nitrogen should be oxygen-free. Liquid nitrogen that has been in contact with air will contain oxygen (see Table 8 for boiling points) and should not be used. 

A solution of tricyclohexylphosphine (4.65 g, 16.58 mmol) dissolved in diethyl ether (120 mL) is added to a freshly prepared solution of Li[GaH4]16 (16.30 mmol) in diethyl ether (100 mL), cooled in a liquid nitrogen-chloroform slush bath. The mixture is stirred for 30 min and warmed to ambient temperature as it stirs overnight. 

As already mentioned, there are two general approaches to cooling the cell, immersion in the coolant and pumping coolant through the cell jacket. The simplest approach [21,27] for immersion is to use standard slush baths or salt-ice mixtures that are available for temperatures down to -160°C [28]. Crude but effective control of temperature can be achieved by cooling the cell in liquid nitrogen followed by slow warm-up in the vapor above the boiling liquid [5]. 

E. Example Separation of BF3 and CH2CI2. Boron trifluoride (bp —110.7°C) is readily separated from methylene chloride (bp 40.7°C) as illustrated in Fig. 5.6. Inspection of the vapor pressure data in Appendix V reveals that BF3 exerts 75 torr at — 126°C, whereas extrapolation of the vapor pressure data for CH2CI2 to this temperature (log P vs. 1/T plot) indicates a vapor pressure of less than 10 3 torr for this component. Therefore, the reaction mixture is slowly passed through a trap cooled to — 126°C (methylcyclohexane slush bath, see below), which retains the methylene chloride, and into another trap at — 196°C (liquid nitrogen), which retains the boron trifluoride. The rate of trans-... 

B. Slush Baths. Liquid nitrogen and Dry Ice are convenient and inexpensive refrigerants. But, as shown in the examples in this chapter, a wider range of low-temperature baths is necessary for trap-to-trap fractionation of gases and for the characterization of a substance by vapor pressure measurements. A convenient constant-temperature slush bath consists of a mixture of a frozen compound in equilibrium with its liquid. The bath is made in a clean Dewar no more than... 

Dry Ice, 112-113 liquid nitrogen, 109-110 liquid nitrogen boil-off, 193, 195 slush baths, 110-112 Residual gas analyzers, 142-143 Ring seal, 253-255 Rough vacuum apparatus, 118-119 definition of, 118 Rubber ... 

The sequence of operations (assuming the initial solid is not air sensitive) would be to load the sample tube with a weighed amount of reactive compound and the stirrer, to attach this tube to the tensimeter, and to pump out the air in the tensimeter. The sample tube is cooled to liquid nitrogen temperature and solvent is then condensed into the sample tube from a storage container on the vacuum line. The main valve on the tensimeter is then closed and the sample container allowed to warm so the solid may dissolve, perhaps with the aid of the stirrer. A constant temperature slush bath is next placed around the sample tube as illustrated in Fig. 9.5 and an initial pressure measurement is taken on the manometer. Next, the first alloquot of the reactive gas is transferred from a storage bulb elsewhere on the vacuum system into the calibrated bulb using the techniques outlined in Section 5.3.G (the bubbler manometer shown in Fig. 9.5 is used for the pressure determination required for this process). This gas is con-... 

A more common method for preparing a low temperature bath is to mix an organic substance with either dry ice or liquid nitrogen. Dry ice (C02, —78 °C) can be added in small lumps to the solvent until a slight excess of dry ice remains. Alternately, liquid nitrogen (N2, —196 °C) can be poured into the solvent until a slush is formed that consists of the solid-liquid mixture at its melting point. 

A slush bath can be described as a low-melting-point liquid (typically a hydrocarbon solvent) that is being kept in a partially frozen state by either liquid nitrogen or dry ice. The temperature will remain constant as long as you continue to add liquid nitrogen, or dry ice, to the bath to maintain its slushy state. Table 6-3 is a comprehensive list of slush baths made of dry ice (C02) and liquid nitrogen (N2). Duplicate temperatures indicate a choice of solvent or coolant. 

An alternative to the slush bath is the coolant, or cooling bath. These baths are handy when you may not have (or wish to use) the required low-melting-tempera-ture liquid for a particular temperature. However, they require more work to maintain their specific temperatures. Liquid nitrogen is recommended for cooling baths because dry ice can be difficult to introduce to the bath in sufficiently small amounts. Like the slush bath, the cooling bath should be mixed in a fume hood. 

Cold traps use either water, dry ice slush baths, or liquid nitrogen as a coolant. The low temperatures of a cold trap cause condensable vapors to change their state of matter into a liquid or a solid depending on the temperature and the vapor. As temperature decreases, there is an increase in price and an increase in performance. 

Improper selection of coolant for a cold trap may artificially limit the potential vacuum of your system. For instance, the vapor pressure of water (which is often the primary condensable vapor in many vacuum systems) is quite high without any cold trapping, moderate at dry-ice temperatures, and negligible at liquid nitrogen temperatures (see Table 7.11). If your vacuum needs are satisfied within a vacuum of 5 x 10"4 torr, you can safely use dry ice (and save money because dry ice is less expensive than liquid nitrogen). Another temperature option for a coolant is the slush bath (for more information on coolants see Sec. 6.2). 

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