LOT 
CHLORINATION OF MIXED SILVER HALIDES IN GOOCH CRUCIBLES. 
M. G. MELLON and J. C. SIEGESMUND. 
Any two of the three halogens, chlorine, bromine, and iodine, in the 
form of their salts, may be determined in a mixture of these salts by 
the familiar indirect, gravimetric method. This procedure consists of 
the following steps: precipitating the two halogens together in the 
form of their silver salts; drying and weighing as such in a Gooch 
crucible; chlorinating the weighed residue in order to replace the 
bromine or iodine, or both, and thus converting this residue to silver 
chloride; and the weighing of the latter salt. Then from the weights 
of sample, mixed silver halides, and the resulting silver chloride, it 
is possible to calculate the percentage of the halogens in the sample. 
As an example of an indirect analysis, the determination of one 
pair of the three halogens is included in the course on general quantita- 
tive chemical analysis as given in this laboratory. The scheme em- 
ployed* for the chlorination of the mixed silver halides has been to 
transfer the weighed residue, along with the asbestos in the Gooch 
crucible, to a boat. The boat was then placed in a hard glass tube, 
where it could be heated with a burner when necessary, and chlorine 
passed through the tube until a constant weight was obtained, indicat- 
ing a complete conversion to silver chloride. This procedure necessitates 
at least four weighings: the sample, the mixed silver halides, the boat 
and contents before chlorination, and the same after chlorination. 
One distinct liability for error in the process just described lies 
in the transfer of the asbestos and mixed silver halides from the Gooch 
crucible to the boat. It is often a difficult matter to obtain a quantita- 
tive transfer of the halides, especially when a film clings to the inside 
surface of the crucible. Small pieces of asbestos in the holes in the 
crucible may be neglected, since it is necessary to transfer only those 
portions upon which are liable to be found the particles of silver halide. 
With the aim of avoiding the troublesome transfer of the contents 
of the crucible to the boat, the above procedure has been modified so 
as to chlorinate the mixed halides directly in the crucible. The scheme 
adopted for class work consists in placing the Gooch crucible inside a 
larger crucible, covering the latter with a watch glass having a hole 
in the center and with the convex side up, and bringing chlorine in 
contact with the residue by means of a glass tube extending down 
through the hole in the watch glass to within about 1 cm. of the bot- 
tom of the crucible. To hasten the chlorination the outer crucible may 
be heated. The bromine or iodine replaced are easily driven out of 
the inside crucible. They often condense at first on the under side of 
the watch glass but soon disappear with later heating of the crucibles. 
It is sometimes well to break up a hard residue by means of a glass 
rod before chlorination, since it is not as well exposed to the action 
of the chlorine as when spread out in a boat. The weighings neces- 
1 Mahin—Quantitative Analysis, p. 115 (1919). 
