Microdetermination of Chlorine, Bromine, and Iodine in Organic Compounds Simplified G ra v irnet ric Met hod H. W. SAFFORD A N D G. L. STRAGAND, University of Pittsburgh, Pittsburgh, P a .
A modified gravimetric method has been devised by which the chlorine, bromine, or iodine content of organic compounds may be determined quantitatively on a micro scale. Conventional microanalytical equipment is used. The presence of nitrogen, sulfur, and certain other elements causes no interference. The sample is burned in an atmosphere of oxygen using a platinum catalyst, and the halogen gases formed are absorbed by a silver gauze with the quantitative formation of the corresponding silver halides; the resulting gain in weight of the gauze represents directly the halogen content of the sample. I n the presence of sulfur, the percentage of the halogens is calculated after silver sulfate has been removed from the absorbing silver gauze by water extraction. Samples with halogen contents ranging from approximately 10 to 977' have been analyzed sucoessfully by this method.
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HE quantitative microdetermination of chlorine, bromine, and iodine in organic compounds may be accomplished in a variety of ways. Reviem of the earlier literature and outlines of gravimetric and volumetric procedures for the estimation of these elements can be found in classical textbooks dealing with quantitative organic microanalysis (14, 15). Excellent discussions of later advances in organic microchemistry were published not long ago (2, 6, 19, 20). Because of certain attendant difficulties in many of the earlier analytical schemes, there has been a relatively recent increase in the number of publications devoted to new methods and modifications of existing procedures. Metallic silver has been used for many years as an analytical reagent in quantitative organic microanalysis. Until recently, perhaps its major role has been that of a scavenger for removing interfering elements during the dry combustion of an organic compound. Lately, increased attention has been given to the use of silver as a primary reagent in the microdetermination of certain elements. As early as 1897, however, a procedure was developed by Dennstedt in which, during a carbon and hydrogen determination, tiny silver boats filled with molecular silver rested in the combustion tube to react with halogens and sulfur that might be in the sample (5). In the presence of oxygen gas, sulfur was presumably fixed by the silver as silver sulfate, while silver halides were formed directly. Subsequently, the silver boats were removed from the tube and weighed, and the silver compounds that had formed were dissolved in dilute potassium cyanide solution. Sulfate ions in this solution were determined gravimetrically with barium chloride. The difference between the weight of the silver boats following combustion and the weight of the sulfate gave the halogen content of the sample. Following the original Dennstedt combustion procedure, Lacourt and co-workers (11-13) caught the single halogens (except fluorine) on silver gauze heated to 350" C. The resulting silver halides were put into solution, and precipitated as silver iodide with excess standard potassium iodide solution, and the potassium iodide not consumed was titrated iodometrically. These authors claimed that of the halogens, chlorine, bromine, and iodine, only chlorine could be determined directly from the gain in weight of the silver gauze through halide formation.
In an adaptation of the ordinary carbon and hydrogen microcombustion train, Balis et al. (1)carried out a simultaneous determination of carbon, hydrogen, and chlorine in gaseous organic compounds. An absorption tube which contained fine silver wire and electrolytic silver crystals a t a temperature of 600" C. was attached to the exit end of the combustion tube. The chlorine content of a sample was estimated from the direct gain in weight of this absorption tube during combustion. The tube was said to require special handling in protecting the formed silver chloride from the light during cooling and weighing. In certain cases quantitative results were not obtained and the authors suggested that the fixing of chlorine by silver is not a simple combination reaction. A somewhat similar procedure was used by Teston and N c Kenna in their simultaneous determination of carbon, fluorine, and chlorine in halocarbons (18). Chlorine (and bromine, if present) reacted with silver wire in an absorption tube maintained a t 295" C. No interference from fluorine was reported and the chlorine and bromine analyses seemed accurate within *0.9%. However, fluorine compounds containing hydrogen could not be analyzed since hydrogen fluoride apparently was formed; it etched the capillary tips of the tube containing the silver, thereby changing its weight. In addition, the relatively small surface area of the silver wire required that the absorption tube be repacked after every four determinations.
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Figure 1. Combustion and Absorption Apparatus
For organic compounds containing iodine, Jurecek (8) decomposed the sample by catalytic oxidation and passed the combustion gases over a boat containing silver dispersed on mapnesium oxide. The contents of the boat were then dissolved in dilute nitric acid and the silver iodide that had formed was filtered and weighed. Following the Belcher and Spooner empty-tube combustion technique ( 3 ) almost exactly, Korshun and Sheveleva (10) h a w reported the analyses of samples containing carbon, hydrogen, sulfur, and the halogens (except fluorine). Silver ribbon placed in an absorption tube following the combustion tube, and heated to 450" C., retained the halogens. The limit of accuracy for the halogen analyses based on the weight gain of the silver was approximately 0.5%. Divided opinions have been expressed (3, 4,7 , 9, 1 7 ) concerning the general applicability of the unpacked- or empty-tube method of combustion. Undoubtedly further studies should be made to eliminate some of the difficulties that exist a t present. In view of the researches cited and results of earlier studies in this laboratory ( l 7 ) , it was believed that a simple, direct gravimetric method could be devised for the determination of chlorine 520
V O L U M E 23, NO. 3, M A R C H 1 9 5 1 bromine, or iodine through the reaction of these elements with silver to form weighable compounds. It seemed necessary to ascertain an appropriate temperature for the fixing of all three of these halogens, since for chlorine alone recommended temperatures ranging from 295" to 600" C. had been reported. Also, it was hoped that any procedure developed would not be too greatly restricted by the nature of the elements, other than the halogens, that might be present. A final desirable feature was to be the use of conventional microanalytical equipment. The method that evolved from these considerations is described herein. APPARATUS
The essential features of the combustion and absorption apparatus are shown in Figure 1. Commercial tank oxygen is passed directly into a standard ACS pressure regulator (16) three fourths filled with saturated sodium carbonate solution, through a (trying tube containing anhydrous magnesium perchlorate, and finally into a bubble counter containing 5% sodium hydroxide solution (these items of equipment are not shown) W o r e entering the combustion tube. The latter is a 3-foot (90cm.) length of standard wall Vycor tubing with an outside diametvr of 13 mm. and a wall thickness of 1.2 mm. Side arm J is optional. The exit end of the combustion tube normally remains open to the atmosphere. In this investigation, Fisher and Sargent microcombustion furnaces were used interchangeahly for furnaces L and H . Hoivever, furnace L might well be of the standard ACS niicropwheater t pe, in which case a standard Vycor microcombustion tube could { e used after cutting off the conventional reduced end. The distance which the combustion tube protrudes from furnace I, is not critical, except that its shortness will minimize possible loss from abrasion when the silver gauze roll at M A is introduced and removed. This metallic silver absorbent is a compact roll formed from an approximately 5-cm. square of 30-mesh wire gauze. I n its preparation all protruding tips of silver wire are
521 folded in, and the completed roll fits the combustion tube with moderate snugness but not so t,ightly that it cannot be removed easily with a platinum wire hook. A typical roll used in this work was 4.8 cm. long, had 3.25 turns, and weighed 4.7 grams. Before use, each gauze roll is cleaned and conditioned as described in an earlier publication (17). The three platinum contacts a t DEFG are the familiar ACS micro stars and are placed end to end so that contact FG extends about 1 cm. from the entrance end of furnace H . Normally a temperature of 650" to 700" C. is maintained a t E, the hottest zone of combustion furnace H . During an analysis, the silver gauze roll is placed with one end a t A , the center of absorption furnace L, while the maximum temperature of this furnace is held between 400" and 425" C. in order that the melting points of the formed silver halides will not be exceeded. PROCEDURE
Furnaces L and H are brought to operating temperatures and a clean, dry, silver-gauze roll is weighed and positioned inside the combustion tube as indicated previously. A sample of approximately 3 mg. is introduced in platinum boat K and an oxygen flow rate of from 6 to 8 ml. per minute is established by attaching a Mariotte bottle a t the exit end of the combustion tube and adjusting the pressure regulator as required. After the desired rate is attained, the Mariotte bottle is permanently disconnected and the rate of flow is checked periodically by observing the bubble counter. Combustion is carried out with a movable burner, the rate of vaporization and burning of the sample depending upon the type of compound analyzed. The usual reburning operation is recommended, followed by a 10-minute flushing-out period to permit all products of pyrolysis to pass the silver gauze. When combustion is complete, the gauze roll is withdrawn, trnnsferred to a copper cooling block for 10 minutes, and then weighed. In the absence of sulfur, the gain in weight noted a t this Doint (caused bv halide formatcon) represents directly Table I. Analysis of Organic H a l o g e n C o m p o u n d s the chlorine, bromine, or iodine Deviation of content of the sample. (AnalyAverage from Elements % Halogen, % Halogen, ses bawd on subsequent exType Compounda Present Theory Found Theory traction of the silver halides Hydrocarban C , H, C1 13.00 13.08, 12.85 0.12 in various solvents have not Acid chloride C , H , 0, C1 11.78 12.01 0.23 proved satisfactory.) 8-Chloroanthraquinone C, H , 0, C1 14.61 14.62 0.01 Ester C , H , 0, CI 17.50 17.27, 17.29 0.22 When both sulfur and one o-Chlorobenaoic acid 22.44 C , H , 0, CI 22.65 0.21 of these halogens are present, Ester C, H, 0, CI 39.20 38,96 0.24 Chlorohydrin the gain in weight of the eilver C , H , 0. CI 55.03 55.09 0.06 T a r base 71.30 71.62 C, H , N , CI 0.32 gauze represents sulfate plus Oipanosilicon C H Si CI 14,58 14.40 0.18 halide. For this case, following Amino acid deriv. C' H ' O ' S CI 12.12 11.83 0.29 conihstion and weighing, the Peptide c' H' 0' h-*CI 13.65 0.08 13.73 Quinoline hydrochloride C : H: 0 : S : CI 16.13 15.91 0.22 gauze roll is placed in a beaker Butyryl chloride C , H , 0, S , CI 16.22 16.27 0.05 and covered with boiling disPeptide C, H, 0 , pi, C1 21.16 21.26 0.10 Peptide tilled water for approximately C, H , 0, N , CI 25.68 0.03 25.65 Aromatic sulfonyl chloride c H 0 S CI 16.21 0.02 16.23 5 minutes to dissolve and reAromatic sulfonyl chloride c: H: 0: 5:c1 17.16 16.89, 16.89 0.27 move the silver sulfate formed Organosilicon C , H, 0, Si, C1 20.00 20.20 0.20 Organosilicon during combustion. The silver C , H , 0, Si, Cl 30.10 29.84, 29.88 0.24 g a u z e is r e m o v e d , washed Acetylenic bromide C , H , Br 42 30 42,18 0.12 thoroughly with distilled water, Olefin C, H , Br 43.19 43.42 0.23 Olefin C , H , Br 74.71 74.84 alcohol, and ether, dried di0.13 Steroid ketone C , H , 0, Br 17.24 17.15 0.09 rectly in a standard microreEster C H O B r 21.64 21.48 0.16 generating or drying block, Ester C: H: 0 : Br 27.48 27.54 0.06 Eater C H O B r 28.13 and weighed after c o o l i n g . 28.14 0.01 Lactone deriv. C: H: 0 : Br 28.13 27.91 0.22 The loss in weight a t this Ester 28.14 C , H, 0, Br 28.09 0.05 stage represents the weight of Ester C H O B r 29.60 29.47 0.13 l3stt-r 31.24 C: H: 0 : Br silver sulfate formed during 31.06 0.18 32.60 32.61 C. H. 0, Br 0.01 the analysis; multiplication of 39.77 39.61 C , H , 0. Br 0.16 this weight loss by the factor C , H , 0, Br 73.33 73.49 0.16 C, H , S, Br 49.02 48.89, 4L9.08 0.3081 (SOa/Ag,SOd) gives the 0.03 22.05 21.75 C, H, 0, N, Br 0.30 weight of sulfate formed originally; this latter quantity p-Iododiphenyl c, H, I 45.31 45.16, 45.00 0.23 p-Iodotohiene C, H , I 58.21 57.99, 58.20 0.11 subtracted from the total gain 96.69 96.55, 96.63 0.10 Iodofornih c , H, I in weight of the gauze yields o-Iodobenzoic acid * C, H. 0. I 51.17 51.16, 51.09 0.04 the halogen content of the subSteroid quaternary salt C H O N 1 21.00 20.92 0.08 Quaternaryammoniumcompound C: H: 0 : N: I 32.40 32.11, 32.19 0.25 stance analyzed. @-Diketone C, H , 0 , S, F, 1 36.46 36.62, 36.73 0.22 After each analysis it is adHydrocarbon C H C I I 53.70 53.41, 53.52 0.24 visable to remove accumulated Hydrocarbon 58.10 58.42, 58.31 C: H: C1: I 0.27 silver wits from the silver S o t known C , H , 0 . C1, I 57.02 57.20, 57.01 0.09 gauze. For silver chloride, ima All the compounds ara research samples except those marked with a sup'erscript b which are standard commersion in 10% ammonia pounds. Standard compounds. water for 5 minutes is effective, but for silver bromide
ANALYTICAL CHEMISTRY
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and silver iodide the respective solvents are 10% sodium thiosulfate and 2 % potassium cyanide solutions. Potassium cyanide is the best solvent for all the silver halides. Excess solvent and reaction products are removed by thorough washing with hot distilled water, alcohol, and ether. Before being used for another analysis, the gauze roll is placed in the combustion tube a t operating temperature while oxygen is passed over it. After 10 minutes the roll is withdrawn, cooled, and weighed. This procedure of heating and cooling is continued until constant weight ( * 5 micrograms) of the roll is established. DISCUSSION
Table I presents the results of the analyses of various known compounds for chlorine, bromine, and iodine, together a i t h determinations on research materials. The values reported are based on the gain in weight of the silver gauze roll through silver halide formation. Exceptions are the three samples containing sulfur, for which the halogen contents were calculated by difference after silver sulfate had been removed by water extraction. Few duplicate values could be given for the chlorine and bromine analyses because, for the research compounds analyzed, only single determinations had been requested of the service laboratory. However, for the large number of compounds investigated the data indicate that the accuracy of the method is satisfactory for the determination of chlorine, bromine, and iodine in a variety of substances without interference from the other elements shown. When a simultaneous determination of sulfur ( 1 7 ) and chlorine or bromine is attempted, one of these elements usually is found quantitatively a t the expense of the other. The explanation lies in the fact that the lowest temperature, 450' C., recommended for the quantitative reaction of sulfur trioxide with the silver gauze to form silver sulfate is too high for complete retention of either silver chloride or silver bromide. At this temperature these compounds will volatilize in part from the roll, since silver chloride and silver bromide melt a t 455" and 434' C., respectively. On the other hand, if one wishes to determine chlorine or bromine, the silver gauze must not be heated beyond, say, 425" C., and this temperature appears to be too low for quantitative absorption of sulfur trioxide. There should be no difficulty in determining sulfur and iodine simultaneously, because silver iodide decomposes a t a temperature considerably in excess of 450' C. Nevertheless, sulfur and these halogens can be estimated accurately in the presence of one another by carrying out two separate analyses. For the first determination, the silver roll is maintained a t a temperature of 450' to 550' C. to favor quantitative formation of silver sulfate; this compound is then extracted in boiling water and the sulfur percentage is calculated. The
second analysis is carried out with the silver gauze a t 400' to 425" C. Any silver sulfate formed is extracted and the weight loss is converted to a weight of sulfate; the latter quantity when subtracted from the initial gain in weight of the gauze roll yields the halogen content quantitatively. A further possibility for a simultaneous determination would involve the use of two silver rolls, the first being heated to 400" to 425' C. to react with all of the halogen and a portion of the sulfur (sulfur trioxide), and the second roll being maintained at approximately 500" C. to retain the rest of the sulfur. Therm* static sleeves similar to those used to heat the lead dioxide reactant in a carbon and hydrogen determination would be suitable as furnaces to develop the various temperatures which are required. LITERATURE CITED
(1) Balis, E. W., Liebhafsky, H. A., and Bronk, L. B., IND.ENG. CHEY.,ANAL.ED., 17, 56 (1945). Belcher, R., and Phillips, D. F., British Intelligence Objective Survey, BIOS Rept. 1606, Item 22 (1948). Belcher, R., and Spooner, C. E., J . Chem. SOC.,1943, 313. Colson, A. F., Analyst, 73, 541 (1948). Dennstedt, M., Ber., 30, 1590 (1897). Gouverneur, P., A n a l . Chim. Acta, 2, 510 (1948). Ingram, G., Analyst, 73, 548 (1948). Jurecek, M., Collection Crechoslou. Chem. Conmuns., 12, 455 (1947). Korshun, hl. O., and Klimova, V. A., Zhur. Anal. Khim., 2, 274 (1947). Korshun, M. O., and Sheveleva, N. S., Doklady Akad. Nauk S.S.S.R., 60, 63 (1948). Lacourt, A., and Chang, C. T., BUZZ.SOC. chim. Belg., 50, 135 (1941). Lacourt, A., Chang, C. T., and Vervoort, R., Ibid., 50, 67 (1941). Lacourt, A., and Timmermans, A. hl., Anal. Chim. Acta, 1, 140 (1947 ). Kiederl, J. B., and Niederl, V., "hlicromethods of Quantitative Organic Analysis," pp. 151-81, New York, John Wiley & Sons, 1942. Pregl, F., and Grant, J., "Quantitative Organic Microanalysis," pp. 85-101, Philadelphia, Blakiston Co., 1946. Royer, G. L., Alber, H. K., Hallett, L. T., Spikes, W. F., and Kuch, J. A., IND.ENG.CHEM.,ANAL.ED., 13, 574 (1941). Stragand, G. L., and Safford, H. W., ANAL.CHEX., 21, 625 (1949). Teston, R. O., and McKenna, F. E., Ibid., 19, 193 (1947). Willits, C. O., Ibid., 21, 132 (1949). Willits, C. O., and Ogg, C. L., Ibid., 22, 268 (1950). RECEIVED June 29, 1950. Presented before the Division of Analytical CHEMICAL SOCIETY, Chemistry at the 118th Meeting of the AMERICAN Chicago, Ill. Contribution 779, Department of Chemistry, University of Pitteburgh.
