Sublimates closed tube test

Table 12.11. Sublimates during the closed tube test ...

Small quantities of selenium in solution may be detected by the test described by Meunier,1 which is similar to Marsh s test for arsenic. If there is a relatively large amount of selenium present it is readily detected by its red colour, but if the amount of the element be small the deposit closely resembles that of arsenic, and in this case the following process may be adopted to detect the selenium. A current of hydrogen sulphide is passed into the hot solution containing a little sulphurous acid. The precipitate of finely divided sulphur carries down with it any selenium present and arsenic as arsenious sulphide, and it is clotted by stirring. In the presence of selenium the clots are brown they may be dried and the sulphur and selenium separated by careful sublimation in a closed tube. 

Aluminium Chloride.—For the success of a Friedel-Crafts synthesis it is essential that the quality of the aluminium chloride used as catalyst should be beyond reproach. Commercial preparations are often partially decomposed and unfit for use in consequence of the entrance of moisture into insufficiently closed containers. In order to make sure, a small sample should be heated over a flame in a test tube held obliquely the whole, or at least the bulk, of the chloride should sublime. Preparations which are not too seriously decomposed can be made fit for use by resublimation. 

In the case of a simple sublimation, it is sufficient to slide the corundum rodlet into a quartz test tube the heating is by means of a small flame kept at a fixed distance below the tube. The temperature can be determined directly by placing the thermoelement feeler into the quartz test tube in place of the corundum rodlet. If gaseous decomposition products are formed, the quartz test tube can be closed with a well-fitting cork stopper and a delivery tube. 

---