Dilution refrigeration still

In most applications, 3He refrigerators are now replaced by dilution refrigerators (see Chapter 6). They are still used in the range 0.3-1.3 K, when a very compact vibration-free system is required. 

This system has two gas lines for mixture He-" e gas line (line 1) and " He gas line (line 2). The " He is supplied from liquid He tank by the vacuum power of the Pump2. This mixture He-" He gas is cycled in the dilution line by the Pumpl. The " He gas is used to cool the IK pot in the dilution refrigerator below 2K because the " He gas is cooled by the reduction in pressure in the line2. The mixture gas is cooled 1 K pot, still, and the mixing in that order. 

In a 500 cc. round-bottomed flask attached to a reflux condenser are placed 60 g. (58.5 cc., 0.5 mole) of acetophenone (Note 1), 52.7 g. (0.65 mole) of dimethylamine hydrochloride, and 19.8 g. (0.22 mole) of paraformaldehyde. After the addition of 1 cc. of concentrated hydrochloric acid (sp. gr. 1.19) in 80 cc. of 95 per cent ethyl alcohol, the mixture is refluxed on a steam bath for two hours (Note 2). The yellowish solution is filtered, if it is not clear (Notes 3 and 4), and is transferred to a 1-1. wide-mouthed Erlenmeyer flask. While still warm, it i6 diluted by the addition of 400 cc. of acetone (Note 5), allowed to cool slowly to room temperature, and then chilled overnight in the refrigerator. The large crystals are filtered and washed with 25 cc. of acetone. After it has been dried for two and one-half hours at 40-50°, this crude product weighs 72-77 g. (68-72 per cent of the theoretical amount) and melts at 138-141° (Notes 6 and 7) it is suitable for many reactions. 

Dimethylamino)propiophenone hydrochloride 715 Acetophenone (60 g, 58.5 ml, 0.5mole), dimethylammonium chloride (52.7 g, 0.65 mole), and paraformaldehyde (19.8 g, 0.66 mole) are placed in a round-bottomed flask (capacity 500 ml). A mixture of concentrated hydrochloric acid (d 1.19 1 ml) and 95% ethanol (80 ml) is added and the whole is boiled under reflux for 2 h. If the yellowish solution produced is turbid it is filtered through a heated filter and, whilst still hot, diluted with acetone (400 ml). The solution is allowed to cool slowly to room temperature and placed in a refrigerator overnight. The crystals formed are filtered off, washed with acetone (25 ml), and dried for 2.5 h at 40-50° this crude product is obtained in 68-72% yield (72-77 g) and melts at 138-141°. It is somewhat hygroscopic and if dried for a further 4 h melts at 152-153°. It can be recrystallized by dissolution in 95% ethanol (85-90 ml) and slow addition of acetone (450 ml) recovery is then about 90%, the m.p. being 155-156°. 

The 4 + 1 part dilution of supernatant and water/TEA causes additional lipids to precipitate. Refrigerated centrifugation causes that precipitate to stick to the walls and bottom of the centrifuge tube/bottle. Eiltration (see Section 3 1.2, step 7) should be performed immediately after centrifugation while the supernatant is still cold. 

---