294 Gooch and Moseley — Detection and Approximative 



liberated in the process has the effect of reoxidizing the arsenic 

 in the receiver and so making that element non-volatile under 

 the conditions until the reducing agent is again introduced. 

 The free bromine in the final distillate must be re-converted 

 to hydrobromic acid before the contents of the receiver may be 

 introduced into the reduction-flask, and we find that this effect 

 may be most easily and unobjectionably accomplished by the 

 addition of a little stannous chloride dissolved in hydrochloric 

 acid of half-strength and purified from arsenic by prolonged 

 boiling. Incidentally and simultaneously the arsenic is reduced 

 to the arsenious form, and, though Sanger has shown that 

 minute amounts of arsenic are completely eliminated from the 

 solution in the reduction flask when that element is intro- 

 duced in the higher form of oxidation, it is our experience that 

 the rapidity of elimination of the arsenic is so increased by the 

 introduction of the small amount of stannous chloride needed 

 to bleach the bromine that the mirror appears in from five to 

 ten minutes and is practically complete in half an hour, espe- 

 cially if the precaution is taken to add a little more stannous chlo- 

 ride, according to Schmidt's suggestion* after the operation 

 has been in progress about twenty minutes. 



It will be remembered that Schmidt has shown that the 

 addition of stannous chloride to the Marsh apparatus in action 

 not only does not effect the retention of arsenic, as many other 

 metallic salts do, but actually brings about the final evolution 

 in the form of the hydride of that portion of the arsenic which 

 may have been deposited during the process in elementary form 

 upon the zinc. 



We have used the Sanger apparatus in form essentially un- 

 changed ; but we find it advantageous to fill the reserve genera- 

 tor with zinc coated with copper by the action of a solution of 

 copper sulphate, instead of with pure zinc, since in this way 

 the zinc is made more sensitive to the action of the dilute sul- 

 phuric acid, while the presence of copper (which is of course 

 out of the question in the reduction-flask) can be of no disad- 

 vantage in the reserve generator and might even serve a useful 

 end in fixing traces of arsenic if the zinc and acid employed 

 were not absolutely free from that element. In the formation 

 of the mirror too, it has proved to be an advantage to enclose 

 the portion of the glass tube to be heated in a short thin tube 

 of iron or nickel slightly larger than the glass tube and kept 

 from contact with it except at the ends, which are notched 

 and bent inward. By keeping the outer tube of metal at a 

 low red heat it is possible to diminish the tendency, which 

 shows more particularly when the amounts of arsenic are 

 fairly large, toward the formation of a double mirror corres- 



*Zeit. fur Anorgan. Chern., i, 353. 



