Slush bath

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. 

Table 8.2 Working temperatures of cryogenic slush baths ...

After leaving the reactant zone, the product stream enters a 0.5 in diameter FEP tube cooled by either a salt-ice bath or acetone-carbon dioxide slush bath [16]. The gas mixture was scrubbed in a soda-Hme tower. Hydrogen fluoride was trapped by adding sodium fluoride to the reaction mixture or simply adding water. Then, the product solution was extracted with dichloromethane, washed with aqueous... 

With the reaction flask closed off from the vacuum line, the bath at -196° is replaced by a bath at -111° (CS2 slush bath), which results in a gradual increase in pressure to approximately 50 torr as observed on the blow-out manometer. As the reaction proceeds, the pressure slowly diminishes and stabilizes after about 2 hours. When no further pressure change is observed, the reaction mixture is frozen again at -196°. The reaction vessel is then warmed by removing the cold bath, while the contents are subjected to pumping of the volatile products through successive cold traps at -126 (methylcyclohexane slush bath), -140, (Skelly F) and-196°. 

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. 

The HC1 dissolves quickly in the water but the PF3 hydrolysis is slow. The product is then given a final purification from water and other materials by a vacuum distillation from a methylcyclohexane slush bath (—128 °C). However, the molecular sieve trap works better and avoids the 10 to 20% loss that occurs from PF3 hydrolysis. The product should give a negative chloride test7 indicating absence of HC1 and/or by-products PC1F2 or PC12 F. 

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. 

In a glove box several grams of benzophenone and freshly cut sodium metal were added to a glass tube (40 cm long, 2 cm ID) topped with a Kontes valve and vacuum line adapter. This glass tube was attached to the vaccum line and evacuated. The tube was immersed in a — 78°C isopropanol/COz slush bath. The lecture bottle valve on the (CH3)20 tank was opened slowly and (CH3)20 was condensed into the glass tube at — 78°C. The pressure of... 

CH3)20 was monitored via a manometer and never allowed to exceed 400 mm of pressure. Following condensation of 20 mL of (CH3)20, the lecture bottle valve was closed and the tube, still immersed in the — 78°C bath, the contents were periodically swirled, and the tube quickly reinserted into the slush bath. After 30 min the solution appeared dark blue in color, indicating that the solvent was dry. The solvent may be stored as a liquid infinitely at — 78°C behind a safety shield. 

A 1-L, 3-necked, round-bottomed flask was equipped with magnetic stirrer, pressure-equalizing addition funnel with N2 inlet, low temperature thermometer, and a Friedrich condenser with N, outlet. The outlet was attached to two traps in series. The first was cooled in a Dewar of salt/ice water (- 15 C) and the second in a Dewar of dry ice/i-PrOH (— 78 C). 2-Bromo-3,3,3-trifluoropropene (50 g, 0.286 mol) and hexane (250 mL) were added to the flask. 1.6 M BuLi in hexane (190 mL, 0.304 mol, commercial) was added to the addition funnel. The flask was cooled with a hexane slush bath by addition of liquid N2 until the temperature of the solution inside the flask was - 85 °C. Then the BuLi soln was added over a period of 25 min at such a rate that the temperature remained below - 80 C. The slightly cloudy, yellowish solution was allowed to stir for an additional 10 min. Then, the hexane slush was removed, Upon reaching - 30 C, a gelatinous precipitate formed and the temperature rapidly rose to 28 "C. The volatile product was removed from solution by heating the mixture at reflux for 30 min with a slow flow of N2 through the system. The product was obtained from the dry ice trap yield 21 g (97%). 

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]. 

A. /3-Isovalerolactam-N-sulfonyl chloride. A 200-ml. fournecked flask fitted with a mechanical stirrer, a dry ice-jacketed dropping funnel (Note 1), and a thermometer is cooled with a dry ice-methylene chloride slush bath while 67 ml. of sulfur dioxide (Note 2) is condensed into the flask Both the dry ice condenser and the dropping funnel are protected with drying... 

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... 

C. Dry Ice Baths. In contrast to the fixed temperature of slush baths, a Dry Ice bath is somewhat less reliable because the equilibrium temperature is a strong function of the partial pressure of C02 above the bath. For example, one test in which Dry Ice was freshly powdered in air had a temperature 8°C below the normal sublimation temperature of —78.5°C. In another test, 40-h standing was required for a Dewar of Dry Ice to attain —78.5°C.1 However, a small electric heater buried in the Dry Ice produced enough C02 to expel the air and attain the normal sublimation temperature in a matter of minutes. The temperature of an equilibrated Dry Ice bath at any barometric pressure may be found using the expression for sublimation pressure ... 

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-... 

Fig. 9.20. Apparatus for the generation and sampling of anion radicals. The sample is added to the purged apparatus, S, is sealed off, and the appartus is evacuated. Chunks of sodium are melted through the capillary and then sublimed into the reaction tube. Solvent is distilled into the sidearm from the vacuum line, and the apparatus is tipped so that the resulting solution is poured onto the sodium mirror in the reaction zone. The resulting solution of radical is poured into Ihe S-mm HSR tube, frozen down, and sealed off at S2. The radical generation and collection can be carried out at reduced temperature by immersing the tube in an appropriate slush bath.

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