AN IMPROVED METHOD OF HALOGEN-ION SEPARATION AND DETECTION BY MEANS OF CHLORAMINE-T EDWARD M. GE~STENZANG, 1560 EAST 'RURTY-SECONoSTREET, BROOKLYN, NEWYORK
Although the basic principles of a previous paper (1) by the writer are retained, the improved method eliminates disagreeable factors such as the use of bromine solution, and materially hastens and defines the procedure. (1) A 10-cc. portion of the solution containing the fluoride-free alkali halides is diluted to 20 cc. The solution must be neutral to litmus. If alkaline, acidify it with a little dilute acetic acid, and then add a slight excess of ammonium carbonate solution to neutralize the acid. Boil the resulting solution until it is neutral to litmus, then cool. (2) Detection of Iodides. To half of the neutral solution prepared in (I), add one gram of solid ammonium chloride. Now add slowly drop by drop a 5% solution of chloramine-T with constant stirring. In the presence of traces of iodides the solution will become pale to deep tea-colored, which becomes colorless again as more chloramine-T is added. In the presence of fairly large amounts of iodides, the solution becomes black due to the precipitation of free iodine. If no coloration takes place by the time the third drop is added, proceed to the test for bromides. (3) Detection of Bromides. If iodides are absent, the presence of bromides is ascertained by merely acidifying the portion of the solution which failed to respond to the test for iodides with dilute hydrochloric acid, when the solution will be colored very pale @ deep yellow depending upon the concentration of bromine liberated. If there is no response to the bromide test, the only other anion present is, of course, chloride ion. (4) Separation of Iodide from Bromide and Chloride. If iodides have been shown to be present in (2), they must be removed before bromide can be identified. Dilute the remaining 10 cc. of the solution prepared in (1) to 25 cc. and add an excess of ferric chloride solution which oxidizes the iodide ion to elementary iodine.
+ 21- 42Fet+ + I,
2Fe+++
If iodine is present in sufficient quantities to more than saturate the solution, i t will precipitate. This will rarely occur. Filter off the precipitate and discard it. Boil the filtrate until it is certain that the iodine left in solution has been completely volatilized. This readily takes place. During the boiling the iron chloride will probably hydrolyze and precipitate. This will not interfere with the process. Add a slight excess of ammonium carbonate solution to precipitate any unhydrolyzed iron salts, and filter. Discard the precipitate. Cool and acidify the filtrate with dilute hydrochloric acid and add several drops of chloramine-T, when the color change described above will show the presence of bromides. 318
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HALOGEN-ION SEPARATION
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(5) Separation of Bromides and Iodides from Chlorides and Detection of the Latter. A new 10-cc. portion of the original fluoride-free solution of the alkali halides is diluted to 25 cc. ; 10 cc. of concentrated nitric acid is added. If iodine precipitates, filter i t off and discard it. Boil the solution or filtrate until i t becomes absolutely colorless. This accomplishes the oxidation and expulsion of iodides and bromides and leaves behind only the alkali chlorides. A portion of the resulting solution is diluted with an equal part of water and some silver nitrate is added. The characteristic precipitate of silver chloride shows the presence of the chloride ion.
Notes
( A ) Chloramine-T can be used to show the presence of less than 0.14 gram per liter of iodine in the form of iodide as follows: Place a 50-cc. beaker containing 20 cc. of the solution containing the trace of iodide upon a white surface. Add several drops of fresh starch solution. Now add 1 drop of 5% chloramine-T solution and stir. The characteristic blue starch iodide color will appear. An excess of chloramine-T solution will destroy this color due to the oxidation of the iodime to iodate, etc. ( B ) A very faint precipitate or opalescence in the silver nitrate test for chloride should not be reported unless it is certain that all reagents used are free from chlorides, as this ion is a common impurity. (C) The use of a dropper bottle for the chloramiue-T solution will facilitate the work. ( D ) As chloramine-T is not readily available in most laboratories, a free sample can be obtained by addressing thz Abbott Laboratories, North Chicago, Illinois, who will be glad to furnish a trial amount of their "chlorazene" brand of chloramine-T. Literature Cited (1)
GERSTENZAN~, "A New Method of Separation and Detection of the Halogen Ions Involving the Use of Chloramine-T," J. CAEM.EDUC.,8, 1187-9 (June, 1931).
