Cold Finger Experiments

The simplest approach to investigate the potential of melt crystallization is the so-called bottle test. The feed mixture (molten or has to be molten) is filled in a glass bottle or flask. Thereafter, the flask is cooled slowly in a temperature-controlled bath or is allowed to cool at ambient conditions or, if necessary, in a freezer. Only a part not all has to be solidified Quite often, already a color shift clearly indicates a separation. However, to determine the quality of the purification, for instance by distribution coefficient, the residual melt has to be separated from the crystallized portion by decanting. Finally, the solid, the residual melt, and the feed material can be analyzed to decide whether melt crystallization is suitable as purification technique or not. The advantage of the bottle test is that it is quite simple. Only lab equipment is required to obtain the desired answer concerning the feasibility of melt crystallization. In case the result is positive, everything is clear. If the result, however, is not positive, a more detailed examination technique is required, for example, the cold finger experiment. 

The dynamic operating mode of the solid layer crystallization (see Chapter 16) can be tested quite realistically with the shown cold finger equipment, too. However, several modifications of the setup are required, for instance, a circulating loop of the melt has to be installed in order to create a falling film from the top of the cold finger (see, for example. Refs [4,14]). 

Left-right intermittent rotation glass tube 

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A further simple, but very efficient and powerful technique to obtain important information especially on the purification by melt crystallization is the so-called cold finger experiment. Compared to the bottle test, more but simple lab equipment is required for this setup. Figure 15.7 shows the essential parts of a cold finger apparatus allowing a static operating mode. 

More detailed information on the introduced design examples of solid layer crystallization plants is, in general, industrial secret and hence not published. The authors will provide here design examples based on cold finger experiments (see Section 15.4.2) by Neumann [3]. These experiments are representative for solid layer crystallization techniques and are helpful to understand the topic concerning... 

LiCl-free BuNTe(n-N Bu)2TeN Bu is obtained via vacuum sublimation. In a typical experiment a sublimation apparatus is charged with 1.770 g of the crude product. Sublimation is conducted at 90-95°C/10 mbar with the cold finger at 18°C. Over 24 h pure BuNTe(p-N Bu)2TeN Bu sublimes onto the cold finger as a yellow-orange solid (1.239 g). 

The competition between elimination and substitution channels when an alkyl halide is allowed to react with a nucleophile in the gas phase is a difficult problem to tackle, since in most gas-phase experiments only the ionic products of reaction are monitored (a few exceptions are reported below). Thus, for example, when w-propyl bromide is allowed to react with methoxide ion in the gas phase, the bromide ion produced can arise either by elimination (a) or by substitution (b) and the two pathways cannot be distinguished from the ions alone (Scheme 34). In this specific case it was possible to establish that the reaction follows exclusively the elimination channel through collection and analysis of the neutral products246. The experiments were performed on a FA apparatus configured with a novel cold finger trap coupled to a GC/MS system. Material collected by the trap was separated by capillary gas chromatography and the individual components identified by their retention times and El mass spectra246. 

Sublimation Experiments. The sublimation experiments were performed with a conventional cold-finger vacuum sublimation apparatus with a removable cold finger. The apparatus was evacuated by an oil pump to about 5 microns, lowered into a thermostat, and the timer started. 

At the conclusion of the experiment the sublimate was carefully removed from the cold finger and weighed. The weight of the residue was found by weighing the outer tube, washing out the residue, and reweighing the tube. The analyses were performed by iodometry. 

Place 1.5 g of , -1,4-diphenyl-1,3-butadiene (from the Wittig reaction. Chapter 34) and 1.0 mL (1.1 g) of dimethyl acetylenedicarboxylate caution, skin irritant ) in a 25 x 150-mm test tube and rinse down the walls with 5 mL of triethylene glycol dimethyl ether (triglyme) (bp 222°C). Clamp the test tube in a vertical position, introduce a cold finger condenser, and reflux the mixture gently for 30 min. Alternatively, carry out the experiment in a 25-mL round-bottomed flask equipped with a reflux condenser. Cool the yellowish solution under the tap, pour into a separatory funnel, and rinse out the reaction vessel with a total of about 50 mL of ether. Extract twice with water (50-75 mL portions) to remove the high-boiling solvent, shake the ethereal solution with saturated sodium chloride solution, and dry the ether layer over anhydrous sodium sulfate. Filter or decant the ether solution into a tared 125-mL Erlenmeyer flask and evaporate the... 

Figure 7.3. Main features of the apparatus used for transpiration experiments. 1 - thermocouple, 2 - gas entrance, 3 - graphite tube, 4 - furnace, 5 - capillary openings, 6 - graphite shields, 7 - Kanthal heating, 8 - graphite boat, 9 - cold-finger, 10 - Inconel tube, 11 - gas exit.

Cold Finger Freezing Experiment Simulation Model... 

In preparation for the tga and td/ms experiments, acetic acid ( 20 Torr) was sorbed on to a previously evacuated H+ZSM-5 sample held at 150°C. Any excess acetic acid was removed by trapping into a liquid nitrogen cold finger. In one case this was followed by sorption of ammonia ( 100 Torr), with removal of the excess. 

Isobenzofuran can be isolated by trapping on a cold finger, following thermolysis of a suitable precursor such as l,4-epoxy-l,2,3,4-tetrahydronaphthalene, but although isolable, for trapping experiments it can be conveniently produced by either acid- or base-catalysed elimination of methanol from 1-methoxyphthalan in the presence of the intended dienophile. ... 

Conditions. NH3 gas is introduced into the source from a reservoir kept at —40°C. In most of these experiments the pressure in the ion source was 3 X 10 3 torr (1014 molecules/cc.). The pumping rate is 5 liters/sec. when the cold finger is filled with liquid nitrogen, the flowing rate being 6 X 1017 molecules/sec., and the transit time of molecules between the interaction zone and the cold finger — 2 X 10"s sec. 

In those experiments which incorporated a mesh electrode the mesh blocked access to the sample for gas chromatographic sampling. Therefore, the entire reaction mixture was distilled out of the reaction cell into a detachable cold finger on the vacuum line at —196 °C. The isobutylene was then evacuated at 0°C., and 1 ml. hexane was added to dissolve the C8 and C12 products. This procedure discriminated against complete C12 product recovery and invalidated C8/Ci2 ratios. However, the relative amounts of C8 products were still accurate. 

In a typical experiment all the S4N4 was consumed and provided a 57.8% yield of crystalline (SN)X and an 8% yield of blue (SN)X flakes on the cold finger. 

The glass liquid helium cold finger used to freeze out reaction products was about 2 inches in diameter and had a 1 liter capacity. To minimize heat leakage it was surrounded by a liquid nitrogen heat shield. All surfaces in the Dewar system were silvered. After an experiment the cryo-... 

Reaction Conditions. The ozone-oxygen mixture used in these reactions was obtained from either a Welsbach T-23 or T-408 Laboratory Ozonator. When pure oxygen was fed into the ozonator at the rate of 0.6 liter/min., the efiluent was ca. 5% ozone as determined by iodometric titration. A small fraction of this stream giving 0.5-1 mg. of ozone per minute was bubbled through 2-10 ml. of hydrocarbon solvent contained in a small flask fitted with a cold finger condenser (to minimize solvent evaporation). Most of our results were obtained from competition experiments either with known mixtures of solvents or with compounds which can react to give several products. In a few experiments the total yield of cyclohexanol and cyclohexanone obtained from the oxidation of cyclohexane at room temperature was estimated to be 0.2 to 0.3 mg./min. 

There is still little, if any, substantive data on the structure-reactivity profile of different heterocyclic -quinodimethanes, but the expected trends are supported by experience. The stability of the -quinodimethanes appears to be related to the loss of aromatic character o-quinodimethanes derived from the more stable aromatic heterocycles are the most reactive. Thus, when generated by flash vacuum pyrolysis and condensed at low temperatures, some heterocyclic rt-quinodimethanes can be isolated. It is clear that the furan derived. species 7 is considerably more stable than the thiophene analogue 2. The species 7 is sufficiently. stable for NMR spectra to be recorded at -60 °C and Diels-Alder adducts can be obtained by adding dienes to the cold finger <81JA669I>. In contrast, the species 2 can be observed directly only in an argon matrix <88CB791> and it readily polymerises even in the presence of dienophiles. Similar qualitative trends are observed with other heterocyclic o-quinodimethanes. 

Figure 19. A density gradient electrofocusing experiment run in the apparatus depicted in Fig. 17 with the electric field on during elution according to Svendsen (128). The apparatus was fitted with a cold finger. The gradient had a height of 21 cm and a cross section of 7.2 cm. It was a run for 48 hours at 17°C. The protein load was 6.5 mg human hemoglobine cyanide. Elution was done with an input of 2.25 ml/h and an output of 13.7 ml/h. Fractions were taken so that one fraction corresponded to 0.1 cm column height. The fractions were read on a spectrophotometer. However, pH determination cannot be done since every sample is diluted with phosphoric acid during the elution. (Svendsen, 56).

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