A NEW METHOD OF SEPARATION AND DETECTION OF THE HALOGEN IONS INVOLVING THE USE OF CHLORAMINE-T EDWARD M. GERSTENZANG, 1560 EAST3
2 STREET, ~ ~ BROOKLYN, NEWYORK
Fluoride is separated in the usunl manner. Iodide i s separated from bromide and chloride by treating the neutral solution of their alkali salts vith Chloramine-T. Free bromine i s liberated from the solution free of fluoride and iodide by adding Chloramine-T and acidifying with hydrochloric acid. A n m sample of the neutral fluoride-free solution is treated with bromine water to precipitate iodine. The iodidefree solution i s boiled with nitric acid to liberatefree bromine from the bromides which i s driven 08as the boiling i s continued. Chloride is then detected in the solution freed from the other halogen ions by means of silver nitrate.
. . . . . .
If to a nentral solution of the alkali halides a solntion of Chloramine-T is added, elementary iodine is precipitated. If the precipitated iodine is separated from the rest of the solution by filtration and the filtrate boiled to expel elementary iodine left in the solution, the presence of a bromide can be detected by first adding a few drops of Chloramine-T solution, and then several drops of concentrated hydrochloric acid, which liberates free bromine, indicated by the colorless solution changing to the characteristic color of bromine water. The presence of chloride is detected in a new sample of the original nentral solution, free from fluoride, after expelling bromide and iodide as indicated in the procedure by means of silver nitrate. There are two interfering factors involved in this method. The first occurs when the iodine is being precipitated by the Chloramine-T. Due to the decomposition of the Chloramine-T, the solution becomes strongly alkaline, with the result that some iodine remains in solution as iodide, hypoiodite, or iodate. However, this effect is greatly minimized by the addition of ammonium acetate, which represses the concentration of hydroxyl ion due to the formation of slightly ionized ammoninm hydroxide. The second also occurs during the precipitation of the iodme, for if too great an excess of Chloramine-T is added, some free iodine is oxidized to iodic acid. This can also be controlled by adding the Chloramine-T to the solution drop by drop, with constant stirring until the mixture becomes black and then adding several additional drops. Procedure 1. Detection and Separation of Iodides. A 5 cc. portion of the neutral or uery faintly alkaline solution of the alkali halides, which has been freed from fluoride by the ordinary methods, is diluted to 25 cc. and 2-3 grams of ammonium acetate are added. A 5y0 solution of Chloramine-T 1187
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is slowly added, drop by drop, with constant stirring, until a permanent black precipitate is formed. The mixture is filtered and the filtrate is caught in a beaker containing 2 cc. of the Chloramine-T solution. If incomplete precipitation is indicated, by the filtrate turning black, repeat the process with the filtrate, adding Chloramine-T very cautiously. The color of the filtrate should be a pale lemon color, due to some elementary iodine in solution. The presence of iodide will be immediately shown, just as the first two or three drops of Chloramine-T comes in contact with the solution, by a dark coloration at the point of contact, which may disappear as the solution is stirred. If this effect does not occur, iodide is definitely absent. The precipitated iodine is filtered off and confirmed by treating it with sodium thiosulfate, in which it immediately dissolves. 2. Detection of Bromides. The filtrate from (1) is diluted to 40 cc. and boiled until the solution becomes absolutely colorless. Ten cc. of water are added and the solution reboiled for five minutes. Water may be added if the volume of solution becomes too small. The solution is allowed to cool to 70°, and then several drops of concentrated hydrochloric acid are added. If the solution becomes lemon-colored, bromide is present. If no color change takes place several drops of Chloramine-T should be added and the solution tested with litmus. If it is alkaline it should be acidified with dilute hydrochloric acid, when the color change will take place if bromides are present. If iodides have been shown to be absent in (I), the presence of bromides can be shown at once by acidifying the solution to which the Chloramine-T has been added, with concentrated hydrochloric acid. If both iodide and bromide are absent the only other anion present is, of course, chloride, which is proved by acidifying a portion of the original fluoride-freesolution with nitric acid and adding silver nitrate, thereby causing silver chloride to precipitate. 3. Separation of Iodide and Bromide from Chloride and Detection of the Latter. A new 5 cc. sample of the original neutral or very faintly alkaline, fluoride-free solution is diluted to 20 cc.; and bromine water is slowly added with constant stimng, (provided iodides have been shown to be present; otherwise the bromine treatment is unnecessary), until all the iodine has been completely precipitated. The iodine is filtered off and discarded and the filtrate is diluted to 40 cc. Ten cc. of concentrated nitric acid are added and the solution is boiled until it becomes absolutely colorless. 1tSis well to continue the boiling for about five minutes after complete decolorization, adding water if necessary. The solution is cooled, a small sample of it is diluted with water and some solution of silver nitrate is added, which shows the presence of chlorides by the formation of the characteristic precipitate of silver chloride. Sometimes due to imperfect
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expulsion of iodides and bromides, traces of these anions may be carried over to the chloride test and cause a turbidity. T o ascertain the cause of this turbidity, add 5 cc. of concentrated nitric acid to the main part of the solution and boil again for five minutes. Repeat the test for chlorides. If the turbidity reappears only traces of chloride are present.
Notes 1. The volume of solution should not be permitted to fall below 15 cc. during the boiling prior to the test for chlorides, as some chloride may be decomposed into free chlorine which will be expelled with the bromine. Add water if necessary. 2. In the absence of iodides, the presence of bromides is also confirmed, during the boiling prior to the chloride test, by the colorless solution becoming reddish and then colorless again as the boiling is continued. 3. As excess Chloramine-T oxidizes elementary iodine to colorless iodic acid, etc., wherever traces iodides are suspected use a 0.25% solution of the reagent adding it slowly drop by drop. 4. For the benefit of those who may be interested in the reactions of Chloramine-T with the alkali halides, the following equations, kindly fnrnished by Dr. E. H. Volwiler,* are furnished. a. Chloramine-T added to an alkaline or a neutral solution of alkali chloride causes no reaction. Chloramine-T added to an acid solution of an alkali chloride causes the following reaction; ZCHaCsHdSOnNaNCl
+ ZCHICOOH
--t CHsCsHSOpNCla 2CH.COONa
+ CHsCHSO&H* +
Dichloramine is precipitated by this reaction. b. Chloramine-T added to an alkaline or neutral solution of an alkali bromide causes no reaction. Chloramine-T added t o an acid solution of an alkali bromide causes the liberation of bromine and the precipitation of toluene sulfon dibromamide. c. Chloramine-T added to an alkaline solution of an alkali iodide causes no liberation of iodine, if the solution is su5dently alkaline. Chloramine-T added to a neutral solution of an alkali iodide causes the Chloramine-T to decompose, the solution to become strongly alkaline and t o liberate part of the iodine although i t is mostly combined as iodide, hypoiodite, or iodate. Chloramine-T added to an acid solution of an alkali iodide causes the following reaction; CH,Cs&SO1NaNC1 NaOCl
-
+
HzO
+ ZHI
+ C H & H ~ O ~ N H Ef
4IZ
+ H20 + NaCl
* Of the Abhott Laboratories. North Chicago, IUinois.
NaOCl
