A quartz test tube about 30 mm in diameter and at least 200 mm long is placed inside a sealed tube oven or a circular opening in a box oven, and
inclined upwards at 5°–8° to the horizontal. About 3–4 g of sodium metal, which has been shaken under ether to remove the last traces of
paraffin, is placed inside a stainless steel tube, closed at one end, and inserted into the quartz reactor. A measured amount of steel wool at the
bottom of the quartz test tube is used to adjust the location of the steel tube opening so it lies just inside the oven at the point where a
temperature gradient is expected to commence. A side arm on the quartz tube leads to a U-tube containing several sections filled with P2O5 and
separated by glass wool. The other end of the U-tube is connected to a hydrogen cylinder through a flow-rate adjusting valve. A long-arm stainless
steel spatula (which can be fashioned out of a thin- diameter S/S pipe) is inserted through a seal so that its end lies just outside the zone where
NaH is expected to be formed. The spatula runs through a fairly long section of straight tube prior to reaching the active zone in order to minimize
the angular movement of the spatula and disturbance to the seal when removing the product. The outlet gases from the reactor are passed through an
empty washbottle and an oil bubbler, which both isolates the reactor and serves as an indicator of the pressure inside.
The air inside the reactor is purged by opening the hydrogen valve until no oxygen is evident in the outlet gas, and the oven temperature is raised in
the range 610°C– 640°C (corresponding to a tube temperature of about 550°C–580°C). Hydrogen absorption commences at about 570°C as evidenced
by a slow rise of oil inside the washbottle capillary, and the hydrogen valve is opened so that the level in the capil- lary remains about constant.
There is some nonuniformity in hydrogen absorption with time, and the hydrogen feed rate should be adjusted on the high-side, which leads to some loss
of hydrogen. Alternatively, a hydrogen balloon can be used.
The sodium hydride starts forming immediately outside the tube opening where the local temperature is below its decomposition temperature at 1 atm
hydrogen pressure. Figure 6.2 shows the result after about 20 min of operation. After about 2 h, a wool-like plug of sodium hydride needles completely
occupies the temperature region suitable for hydride formation and hydrogen absorption slows. The hydrogen flow rate can be increased at this point to
produce positive pressure inside the reac- tor, and the spatula can be inserted into the active region and rotated to remove the plug into the
low-temperature region of the reactor where the hydride is stable. This operation is repeated every few hours, resulting in an NaH formation rate of
about 0.2–0.3 g/h. When sufficient NaH has formed, the quartz tube is withdrawn from the reactor and allowed to cool to near room temperature. At
that point, the hermi- ticity of the apparatus can be broken, and the sodium hydride removed in the open atmosphere. The author has found that no
spontaneously ignitable sublimates form in the reaction.
Raising the reactor temperature above about 640°C does not lead to an increased rate of hydride formation; rather, a gray color appears in the
product corresponding to condensed unreacted sodium. Higher temperatures still lead to decomposition of the hydride already formed and its reformation
in the section of the reactor, which now has the appropriate temperature. However, the higher evaporation rate also leads to sodium globules forming
in the NaH matrix as well as condensation of liquid sodium on the walls of the quartz reactor. This is deleterious to the quartz tube because of the
danger of liquid sodium flowing into the high temperature region and reducing the quartz in depth. Gaseous sodium on the other hand does not seriously
attack quartz, producing just a superficial discoloration, which disappears (due to the silicon being oxidized back to silica) on exposure to air.
|
