Phosphorus pentoxide traps

Bromine (7 ml) is added dropwise to a mixture of white sand (14 g) and red phosphorus (3 g, dried at 165° under vacuum) moistened with 5 ml of deuterium oxide. The apparatus is fitted with an exit tube to allow the liberated deuteriobromic acid to pass through two U-tubes and into a receiving flask. The first trap contains glass beads and is cooled in an ice-salt slurry. The second contains glass beads and red phosphorus moistened with deuterium oxide. The deuterium bromide gas is collected in the appropriate solvent at ice bath temperature. A small amount of phosphorus pentoxide should be added to remove any deuterium oxide if anhydrous reagent is required. 

BTF has unique properties that encourage its consideration as a solvent for synthetic chemistry. BTF (bp 102°C) forms an azeotrope (bp 80 °C, 90% BTF) with water which can be used to drive dehydration reactions [30]. Since BTF is heavier than water, a reverse Dean-Stark trap must be used. Commercial BTF (distilled) contains less than 50 ppm water, and this low water content suggests that commercial BTF could be used directly for many applications. The use of phosphorus pentoxide as drying agent is also recommended [31], but we found that over time, decomposition of BTF took place, resulting in black precipitation. We instead typically dried BTF over anhydrous potassium carbonate followed by distillation however, we have not measured the water content of the BTF before and after drying so the effectiveness of this procedure is not known. 

Anhydrous HI can be obtained without much demand on apparatus as follows 149 Sodium iodide (1 mole) is added to a solution of phosphorus pentoxide (0.35 mole) in 85 % phosphoric acid (226 g, 2 moles) placed in a 500-ml round-bottomed flask connected via a reflux condenser to three cold traps (1, 0° 2, —40° 3, —80°). The flask is then warmed, whereupon about 50% of the calculated amount of HI collects in traps 2 and 3. When evolution of HI ceases, the connexion of trap 1 to trap 2 is sealed and the HI is distilled from trap 2 into trap 3 by allowing trap 2 to come to room temperature. In these operations care must be taken that tubes do not become choked with solid HI. The HI can be distilled from trap 3 into reaction solutions. 

Thermal conductivity detectors are very sensitive to residual water vapour in the carrier gas stream. A gas chromatographic separation unit consisting of a short silver column and a longer copper colunm, both with silica gel as the stationary phase, separates the water vapour plus carbon dioxide from N2. Residual traces of water vapour are trapped in a column filled with phosphorus pentoxide (Siccapent). Ihe N2 passes through all three columns undelayed. It is detected as the first peak with the TCD. After detection of the N2 peak, heating of the silica gel to 85 C releases the CO2- Finally, the first silica gel column (silver) is heated to 230 °C to release the bulk of the water which bypasses the copper reduction column and the water absorbent (Siccapent) before it is vented to the outside atmosphere. 

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