Identification of Halides in the Presence of Thiocvanates J
G. B.
AND
L. K. HEISIG, School of
Chemistry, University of M i n n e s o t a , Minneapolis, M i n n .
D
trated nitric acid. It is important in testing for small quantities of either iodide or bromide ions t h a t the carbon tetrachloride be added before the free halogen is liberated. An orange-brown color possibly due t o perthiocyanogen always appears in both the aqueous solution and the carbon tetrachloride, but disappears within a minute if the mixture in the casserole or evaporating dish is agitated. The color due t o 0.5 mg. of bromine under the same conditions lasted about 5 minutes. Thus 0.5 mg. of bromide ion can be detected in the presence of 100 mg. of thiocyanate ion. The complete removal of the bromide ion is then accomplished by the addition of a slight excess of potassium permanganate and boiling one minute. To avoid any precipitation of silver sulfate which might be mistaken for silver chloride, the sulfate ion is removed by addition of a slight excess of barium nitrate.
IFFICULTY in detecting the bromide ion in the presence of thiocyanates was reported by Curtman and Wikoff (2) and noted independently by the writers in student analyses. Somewhat later, Sneed and Duschak ( 5 ) reported the same diflZculty and offered a method of destroying the thiocyanate ion by fusion of the mixture of silver halides and thiocyanate with a mixture of sodium and potassium carbonates in a platinum crucible. The hot-water solution of the melt was acidified with nitric acid, heated to boiling, and the insoluble residue of silver halides separated by filtering. The precipitate of silver halides was converted to silver sulfide by digestion with concentrated ammonium hydroxide and colorless ammonium sulfide. The presence of the halides was then determined by the successive use of ferric nitrate and nitric acid, potassium permanganate, and finally silver nitrate. A nurnber of difficulties have been experienced when using this method. (1) At least a partial conversion to free silver occurs during the fusion of the silver salts and the metal is not redissolved by heating the dilute nitric acid solution; consequently some of the halides are lost in the filtrate. (2) I n large classes porcelain crucibles are of necessity used instead of platinum thus contaminating the sample with silicates and in some cases with chlorides. The present u7ork was undertaken to find a more satisfactory method of analyzing for the bromide ion in the presence of thiocyanates. Since the thiocyanate ion is oxidized to sulfuric acid and hydrocyanic acid by persulfate in acid solution (S), the persulfate method of determining the halides in the presence of thiocyanates as described by McAlpine and Soule ( 4 ) was investigated. It was soon found that no test for iodides or bromides could be obtained when as much as 5 mg. of iodide ion or 25 mg. of bromide ion was present in a sample containing 25 mg. of thiocyanate ion. The sensitiveness of the test was greatly increased by extracting a portion of the solution after the persulfate had been added but before the solution was warmed. However, the method was still regarded as inadequate. Tests showed that the sensitivity of the test for iodides could be greatly increased by adding persulfate to a cold 10cc. sample containing 1 t o 2 cc. of carbon tetrachloride and shaking. As little as 0.25 mg. of iodide ion could be detected. That the thiocyanate ion was not destroyed during the oxidation of the iodide ion was repeatedly shown by testing a portion of the solution. However, if a sodium carbonate solution is added and the mixture is evaporated to dryness, nearly all of the thiocyanate ion will be destroyed. The small quantity remaining will be removed if the sample is slightly acidified and a small amount of persulfate is added. This last operation also serves to complete the removal of the iodide ion. Efforts made to eliminate the evaporation to dryness of the solution were not uniformly successful. I n most cases where higher concentrations of bromide ion were present satisfactory results were obtained, but the detection of small amounts of bromides in the presence of a large concentration of iodide proved very difficult, since the bromine was frequently removed with the iodine. After the removal of the iodide ion the test for the bromide ion is made by adding carbon tetrachloride and then concen-
Procedure TESTFOR IODIDE ION. To 10 ml. of neutral solution, add 1 ml. of carbon tetrachloride and 0.5 gram of powdered potassium persulfate in the order named and shake. A violet color indicates the iodide ion. (A minimum of 0.25 mg. of iodide ion may be detected by this method.) Shake thoroughly in a separatory funnel if available, to remove as much of the iodine from the a ueous solution as possible, and after removing the carbon tetraaloride destroy the remainder of the iodide and the thiocyanate ions, if present, before testing for the bromide ion. This is accomplished by neutralizing the solution with 2 N sodium carbonate and adding an additional 15 ml. of the sodium carbonate. Evaporate to dryness. A test to determine the completeness of the removal of the iodide ion should be made by dissolving the residue in 10 ml. of water, adding 1 ml. of carbon tetrachloride, acidifying with dilute nitric acid, and adding a few small crystals of potassium persulfate. Warm gently until the potassium persulfate decomposes. Unless a very large amount of iodide was present the solution will become colorless. If iodine is liberated, repeat the addition of a few crystals of potassium persulfate and the extraction until the carbon tetrachloride is no longer colored. This last step will be necessary only if a very large concentration of iodide ions was present in the original solution. TESTFOR BROMIDEION.When a test for an iodide is no longer obtained, add 1 ml. of carbon tetrachloride and 4 ml. of concentrated nitric acid to the cool iodide-free solution contained in a casserole or evaporating dish. The solution and the carbon tetrachloride will become orange. The mixture must be allowed to stand about a minute with occasional agitation in order to allow the orange color of the oxidation products of the thiocyanate to disappear. If as little as 0.5 mg. of bromide ion is present a yellowish color due to bromine will remain. The color of the carbon tetrachloride solution should be determined in a white casserole by daylight if possible. If desired, the solution may be transferred to a test tube and observed before a white background. If less than 0.5 mg. of bromide ion is present in 10 ml. of solution, it will be necessary to run a blank and compare the color of the carbon tetrachloride used to make the extractions. REMOVAL OF BROMIDE ION AND TEST FOR CHLORIDE ION. The bromide ion, if present, is removed before testing for the chloride ion by adding a slight excess of potassium permanganate and boiling 1 minute. The sulfate ion formed by the decomposition of potassium persulfate must be removed by the addition of a slight excess of barium nitrate; otherwise a turbidity due to silver sulfate may form and be mistaken for a small amount of silver chloride. Filter and add silver nitrate. A curdy white precipitate proves the presence of the chloride ion.
Results Positive tests were obtained in analyzing solutions containing the ions shown in Table I. 249
INDUSTRIAL AND ENGINEERING CHEMISTRY
250
TABLE I. SOLUTIONS GIVINGPOSITIVE TESTS Iodide Mg. 0.25 0.5 0.5 0.5 50.0 50.0
100.0 100 100 100 20
Bromide Mg. 0.25 0.5 2.5
100 0.5 0.5
100,o 100 100 1.0 0
ChIoride
Thiocyanate Mg. 100 100 100 100
Mg. 0.25 0 0 0 0.5 0.5 15.0 0.25
however, they found it necessary to destroy the thiocyanate ion by ignition of the silver salts. The smallest quantity of bromide detected in mixtures was 5 mg. (page 161), which would make the method much less sensitive than that proposed by the authors.
50 0.5
100 100 250
100 0 0
0 100
The test for a bromide was much less sensitive in the absence of a thiocyanate. Less than 2 mg. of bromide ion could not be detected with certainty. If no bromide was present, the orange color due to the oxidation of the thiocyanate appeared but upon agitation disappeared in the course of about a minute. Since this work was done, Curtman and Schneiderman (1) have published a paper, which describes a method of detecting bromides in the presence of thiocyanates, using concentrated nitric acid to oxidize the bromide ion to bromine;
VOL. 7, NO. 4
Summary
A method of detecting halides in a mixture containing thiocyanates has been devised, by which as little as 0.25 mg. of iodide ion, 0.5 mg. of bromide ion, and 0.25 mg. of chloride ion can be detected in the presence of 100 mg. of thiocyanate ion. Literature Cited (1) Curtman and Schneiderman, Rec. trav. chim., 54, 158-61 (1935). (2) Curtman and Wikoff, J. Am. Chem. Soc., 37, 298 (1915). (3) McAlpine and Soule, “Qualitative Chemical Analysis,” p. 466, New York, D. Van Nostrand Co., 1933. (4) Ibid., p. 572. (5) Sneed and Duschak, J. Chem. Education, 8, 1388 (1931).
R ~ C E I VMay E D 13, 1936.
Determination of Diphenvlamine in Smokeless Powders STANLEY G. COOK, U. S. Naval Powder Factory, Indian Head, Md.
D
IPHEKYLAMINE is treat the p o w d e r w i t h about The purpose of this investigation was to eight times its weight of concena stabilizer introduced develop simpler and more accurate and trated nitric acid on the steam into smokeless powders practical methods for the determination of bath until the sample is all disfor the purpose of r e t a r d i n g diphenylamine in smokeless powders. solved, pour the solution into t h e i r decomposition. In old Four methods-nitration, soda distillawater, filter off the precipitated powders t h e r e i s c o n s i d e r a b 1e 1o s s of diphenylamine, nitro compound formed, dry, tion, extraction, and volumetric-are given cool, and weigh. The factors for some due to volatility of the in detail. All results are based upon analyconverting the weight of nitro diphenylamine as such, but more ses of prepared standard samples of known c o m p o u n d to diphenylamine accounted for by nitration of composition and verified by tests on manuv a r y f r o m 0.3981 to 0.4400, the diphenylamine by the defactured powders of known diphenylamine principally because of varying composition p r o d u c t s of the content nitrocellulose into n i t r o c o m conditions which produce higher or lower yields. pounds which have no value as Each method has its special application, The compounds formed by the stabilizers ( 5 ) . The need for depending on the age or condition of the nitration of pure diphenylamine a c c u r a t e methods to deterpowder to be tested. Excellent results were mine the a c t u a l a m o u n t of by concentrated nitric acid (sp. obtained with all methods. unchanged diphenylamine left gr. 1.42) were found to vary considerably in comDosition as well in a Dowder a s w e l l a s t h e amouit which was originally added to the powder led to this as yield when equal weights of diphenylamink were nitrated with equal weights of nitric acid, and any variation in the investigation. Practically all of this author’s work is based following conditions made it impossible to produce the same upon Dreger’s (6) first efforts along this line, Much work has been done by others (1-13). weight of nitro compound: concentration of acid, duration Four methods for the determination of diphenylamine are of heating, temperature of heating, and quantity of water given: (1) the nitration method, which gives the total diused to precipitate the nitro compound. The use of fuming nitric acid improved the yield conphenylamine originally added; (2) the soda distillation method, which gives only the active or available diphenylsiderably but it was not until pure, dry nitrocellulose was introduced that much higher yields were obtained. It was amine left in the powder; (3) the extraction method, which gives both the active and the inactive diphenylamine; and (4) while working with reground powders in which the nitrocellulose is very finely divided that difficulty was experienced. the volumetric method, which gives the active or available diphenylamine in new powders. Table I shows results found when treating 0.05 gram of pure diphenylamine with nitric acid and mixtures of nitric Nitration for Total Diphenylamine acid and glacial acetic acid, the nitric acid varying from 10 The literature on determining diphenylamine in smokeless to 100 per cent by volume and the acetic acid varying from 0 to 45 per cent by volume, no nitrocellulose present. All powder gives the well-known nitric acid method, which is to
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