Determination of Nuclear Halogens in Organic Compounds A Critical Study and Standardization of the Stepanow Method WALTERA. COOKAND KATHRYN HARTKOFF COOK,University of Akron, Akron, Ohio
D
The Stepanow procedure for the determination A sample of the organic comURING a study of the of nuclear halogen has been subjected to a critical pound with a halogen content h a l o g e n a t i o n of m-diequivalent to 30 t o 35 cc. of 0.1 N phenylbenzene, one of study because o f the controversial opinions silver nitrateis weighed into a small the problems under investigation glass vial and transferred to a dry in this l a b o r a t o r y , it became regarding its accuracy. Through a study and 500-cc. Kjeldahl flask fitted with a change of cedain factors inJluencing the preLiebig reflux condenser with the necessary to analyze a number of cision of the results, a standardization of the hitherto unreported compounds ~ ~ ~o f $ $ ~ ~ for halogen. Early in this work, procedure has been eflected. The application of ordinary ab s o 1u t e a1co h o l and it was found that the C a r i u s this method to the analysis of halogenated nitro metallic sodium (Merck blue label rade) to be added were calculated method was unsatisfactory, befrom the empirical equations given cause of consistent and incorncompounds is described in detail for the 3 r d byDrogin and Rosanoff. In plete d e c o m p o s i t i o n of the time. several cases it was found necessary sample. The well-known sodium to increase the quantities of alcohol peroxide fusion method of Lemp and Broderson (6)also proved and sodium required, and reference to these particular instances are made elsewhere in this article. The alcohol is then introduced unsatisfactory. other investigators (4) experienced into the flask and heat applied cautiously to dissolve the sample difficulties in the application of each of the above-mentioned (if possible). The weighed quantity of $odium, from which the methods in the analysis of the chlorinated biphenyls. The oxide had been previously removed, was cut in rodlike lengths of authors then considered the possible application of the Stepa- 2.5 Cm.9 kept Under kerosene, and thorou@;hlydried with filter paper ~before introducing into thef alcohol mixture, through the top method (8) as modified by ~~~~i~ and ~ ~ (9) for~ ~ ~ of the condenser. The reaction mixture isfoccasionallyshaken and halogen analyses, and again the results obtained were quite a proximately 0.5 hour is s a c i e n t to complete the addition of unsatisfactory. However, in view of the many possible advan- t i e sodium. The reaction mixture is heated gently for at least tages of the Stepanow method (8)Over the peroxide fusion and one hour after the last portion of the sodium has been added (in cases heating for a longer period of time is necessary). the Carius methods, it was decided to subjectthe former method exceptional A darkening of the reaction mixture at this point is of no imto a thorough and critical study withtheobjectofdeveloPinga portance. The flask and contents are then cooled until lukewarm procedure which would be of general application for the deter- and carefully diluted with 200 cc. of water, adding the first portion mination of nuclear linked halogen in organic compounds. cautiously 60 as t o avoid a violent reaction due to traces of unsodium. A stri of litmus paper is added to the diluted procedure (8) has been made the reacted The original stepanow reaction mixture and, iPthe halogen is chlorine or bromine, the subject of investigations by Bacon (11, walker and &Rae solution i s carefully acidified with 6 N nitric acid, adding the (IO), Maryott (6),and Drogin and Rosanoff (d), and, more latter in 10-cc. portions with shaking and occasional cooling of recently, by Proner (7), Van Duin (Q), and Favrel and the reaction mixture with tap water. If the halogen is iodine, the flask and its contents are cooled in a mixture of crushed ice Bucher Favrel and Bucher proposed a substitution of and water and a normal solution of nitric acid, previously chilled, isoamyl ~lcoholin Place of ethyl ~lcoholin the original added in small portions until the solution is just faintly acid to Stepanow method. They contend that successful application the litmus paper. In no case should more than 10 cc. of excess of the lattermethod in the case of nuclear substituted halogen nitric acid rea ent be present at the end of the neutralization. Furthermore, t i e solution should be kept cold until the solution of depends not Only On the proportion Of 'Odium but standard silver nitrate has been added. If the litmus paper turns also on the temperature of the reaction mixture during the blue after an iodide solution has been acidified, the results are reduction, and that the higher the temperature, the more likely to be low due to oxidation of h driodic acid and consequent the experimental values approach the theoretical loss of halogen. The contents of d e flask are then transferred to a 600- to 800-cc. beaker and approximately 8 to halogen content. Their procedure is similar in general to quantitatively 10 cc. excess over the calculated quantity of 0.1 N silver nitrate methods previously cited with the exception that the reaction solution added dropwise from a buret with continuous stirring of mixture, after reduction is complete, is diluted with water and the mixture during the addition. To facilitate rapid filtration of extracted with additional portions in a separatory funnel. the solution, the silver halide precipitate is allowed t o stand for 5 minutes t o complete coagulation and settling of the insoluble The combined water extract after acidification with nitric material and then filtered into an BqCm. funnel fitted preferably acid is treated with an excess of silver nitrate and the quantity with a grade of filter paper especially prepared for filtration of of the latter equivalent to the halogen determined either by a agar in the reparation of culture media. (Filter paper 5250, s catalog.) gravimetric precipitation of excess silver nitrate as silver *rthur H. ~ L m a company Regardless of whether the oom ound analyzed contained chloride or by a Volhard titration of excess silver with stand- chlorine, bromine, or iodine, it was Pound necessary to filter off ard thiocyanate. These investigators admit that their the precipitated silver halide which was quite discolored after method cannot be used for volatile substances, even though a coagulation and settling. The precipitate is thoroughl washed reflux condenser was employed during the reduction, and with distilled water and the filtrate and washin s are cosected in liter beaker and tjhentransferred to a large porfelain evaporating they also state that attempts to an alcohol of even adish (2-liter capacity) and 10 to 12 cc. of saturated ferric alumhigher boiling Point than isoamyl alcohol--benzYl dCOhO1nitric acid indicator solution added. In a few analyses, particuyielded unsatisfactory results, the nascent hydrogen being larl those of nitro derivatives, the solution gives evidence of a consumed largely in the reduction of the alcohol itself. dig& darkening after the indicator solution is added. With the further addition of 10 t o 20 cc. of 6 N nitric acid, the solution becomes much clearer and lighter in color. A 0.1 N ammonium thioPROCEDURE cyanate Rolution is added slowly with stirring. In the immediate The procedure employed and described herein is a modifica- vicinity of the end point the thiocyanate solution is added in portions of 2 to 3 drops until the end-point color is definitely observed. tion of that by Drogin and Rosanoff (8): 186
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May 15, 1933
INDUSTRIAL AND ENGINEERING CHEMISTRY
187
Contrary to the observations of Drogin and Rosanoff (2), a blank correction must be established for the titration, even
are dissolved in proportionate quantities of absolute alcohol (as calculated from the equations proposed by Drogin and though the reagents employed in the procedure are found to Rosanoff, 8), refluxed one hour, cooled, and then diluted. be halogen-free. This blank correction has been found to be The diluted solution after acidification with 6 N nitric acid is independent of the quantities of alcohol and metallic sodium then treated with a measured volume (20 to 30 cc.) of standemployed in the analysis. As will be noted also, the blank ard silver solution, filtered, and the filtrate titrated with value is a personal factor and is dependent upon the color standard ammonium thiocyanate (ferric alum as indicator). intensity of the end point to which the individual titrates. The blank value (cubic centimeters of silver nitrate) repreTherefore, it is necessary that the determination of the blank sents the difference between the volume of silver nitrate value and the volume ratio in which the silver nitrate and originally added and the volume of silver nitrate equivalent ammonium thiocyanate solutions react, the standardization to the thiocyanate required in the titration. For the authors’ of the solution, and the analysis proper, all be carried out solutions, this correction was 0.1 cc. (an average of ten blank under identical conditions with respect to final volume of determinations, with the maximum deviation of any single solution titrated, acidity of solution, quantity of indicator value not exceeding 0.03 cc. from the average value). In the employed, and titration in the large porcelain dish. actual calculations of an analysis, this blank value was added The results of the analysis of a variety of halogen-sub- to the volume of excess silver nitrate solution as determined stituted aromatic derivatives as obtained by following the from the back titration with ammonium thiocyanate and the above described procedure appear in Table I. resulting sum subtracted from the volume of standard silver solution originally added. The difference is equivalent to TABLEI. HALOQEN CONTENT OF COMPOUNDS ANALYZED~ the halogen in the sample taken. HALOQEN HALOQEN It should be pointed out that very little reference is made COMPOUND ANALYZED THEORETICPLL FOUND to the analysis of halogenated nitro aromatic compounds. % % 31.52 1. Chlorobenzene 31.28 Stepanow (8) reports the analysis of one such compound and 48.26 48.39 2. o-Dichlorobenzene 48.26 Favrel and Bucher (S),another. The authors have analyzed 48.39 3. p-Dichlorobenzene 65.71 65.69 4. 1.2,4,5-Tetrachlorobenzene four such compounds and the results are given in Table I. 57.63 5. Chloranil 57.65 74.74 6. Hexachlorobenzene 74.71 It was found that the quantities of sodium and alcohol pro7. Benzene hexachloride 73.16 72.77 posed by Drogin and Rosanoff ($) were insufficient for com8. Bromobenennn . .. .~ ____ 50.92 51.11 Dibromobeneene 68.11 9. 67.77 plete reduction of such compounds. Illustration of this 50.44 10. 5etrabromophenolpht halein 50.45 47.32 47.58 11. 9.10-Dibromoanthracene point is given in Table 11. 43.83 43.78 12. 2,5-Diohlorosniline 13. 14. 15. 16. 17. 18. 19. 20. 21. 22. 23. 24. 25. 26. 27. 28. 29. 30. 31. 32. 33. 34. 35. 36. 37. 38. 39. 40. 41. 42.
a
p-Bromoaniline
46.47 46.30 37.34 37.31 96.70 96.49 62.22 62.01 -1odoaoetanilide 48.63 48.89 Iodonaphthalene 49.97 49.86 P-Chlorotoluene 28.03 27.90 6-Chloro-3-sminotoluene 25.06 24.89 5-Bromo-2-aminotoluene 42.96 43.06 6-Bromo-3-aminotoluene 42.96 43.19 4-Bromo-3-aminotoluene 42.61 42.96 4-Bromo-3-benzoylaminotoluene 27.84 27.55 4-Bromo-3-acetaminotoluene 35 .06 35.43 3-Bromo-4-aoetaminotoluene 36.06 35.04 rn-Nitrochlorobenzene 22.52 22.45 p-Nitrobromobenzene 39.57 39.93 p-Nitroiodobenzene 50.98 51.03 4-Nitro-2-chlorotoluene 20.67 20.77 p-Chlorobenzoic poid 22.66 22.68 o-Iodobenzoic acid 51.18 50.93 p-Chlorobiphenyl 18.81 18.75 3-Methyl-6-chlorobiphenyl 17.51 17.61 3-Methyl-4-bromobiphenyl 32.35 32.27 3-Methyl-6-bromobiphenyl 32.35 32.45 4-Chloro-1,3-diphenylbenzene 13.40 13.62 4-Bromo-1 3-diphenylbenzene 25.88 26.98 3-Phenyl-d-chlorobenzoic acid 15.25 15.18 Methyl ester of 3-phenyl-4-chlorobenzoio acid 14.38 14.47 Benzyl ester of 3-phenyl-4-ohlorobenzoic acid 10.99 11.13 3-Phenyl-4-bromoben~oicacid 28.85 28.81 Duplicate analyses checked within O.l%, in nearly a11 instances. Bromoacetanilide i-odoform IodXnzene
%-
I n the original Stepanow procedure (8) no need of a blank correction existed, because of purity of the reagents, removal of solvent after reduction with sodium, and carrying out of the titration in a limited volume of approximately 150 cc. Other methods deviated in various ways from the original Stepanow procedure and therefore a blank correction is necessary. Drogin and Rosanoff (8) pointed out that a blank correction was required because of the presence of traces of halogen in the metallic sodium employed, and therefore their value was dependent on the weight of sodium. However, because of a limitation of the sample weight to 0.2 to 0.3 gram, their blank value did not undergo any appreciable variation. The authors found that the blank correction is independent of the quantity of sodium and alcohol used, and furthermore it represents a correction for the sensitivity of the end point in the volume of the solution as titrated.
DETERMINATION OF BLANK VALUE In order to determine the value of the blank correction, varying quantities of sodium from 0 to 14 grams, inclusive,
TABLE 11. EFFECT OF INCREASED QUANTITIESOF SODIUM AND ALCOHOLON THE ANALYSISOF 2-CHLORO-4-NITROTOLUENE RUN 1
2 3 , 4 5
SODIUM AND ALCOHOL HALOGEN Excess THEORETICAL
HALOQEN
FOUND
%
%
%
None
20.67
50 100 150
...
17.75 18.07 19.65 20.43 20.77
20
... ... ...
The data in column 2 of Table I1 refer to the excess over the quantities of sodium and alcohol proposed by Drogin and Rosanoff (8). I n each of the first four analyses, evidence for incomplete reduction of the sample appeared in the form of a reddish azo or azoxy precipitate, the quantity of the latter diminishing with increasing quantities of sodium and alcohol taken. No such red precipitate was detected in the final analysis of this series. Therefore, it should be noted that in the analysis of the nitro compounds reported in Table I not less than 150 per cent excess of the quantities of sodium and alcohol proposed by Drogin and Rosanoff (8) were employed. These observations are in accord with the proposed mechanism of the reduction of aromatic compounds as suggested by Favrel and Bucher (3). They state that reduction of the benzene nucleus to a cyclohexane derivative occurs before cleavage of the carbon-halogen linkage. Consequently, an excess of reducing agent would be necessary for halogenated aromatic nitro compounds, both to reduce the nitro group and to produce the halogenated cyclohexane derivative from which the halogen is readily removed, Inasmuch as Drogin and Rosanoff (2) also proposed the introduction of not more than a 3 to 4 cc. excess of the standard silver nitrate solution in the precipitation of halide in the sample, it was considered advisable to study the effect of the addition of an increasingly larger excess of silver solution on the accuracy of the method, with the hope that no appreciable adsorption or occlusion of precipitant would occur, and that consequently the end-point sensitivity of the indicator would be increased. The results obtained in this phase of the study appear in Table 111.
ANALYTICAL EDITION
188
Vol. 5, No. 3
TABLE111. EFFECTOF INCREASED EXCESSOF PRECIPITANT gravimetrically (as silver chloride). I n all analyses reported UPON PRECISION OF RESULTS herein, the Volhard procedure was employed. EXCESSOF AgNOs ADDED (Ow: HALOQEN HALOGEN COMPOUND ANALYZED theoretical) THEORETICAL FOUND
1. p-Chlorobenzoic acid
%
%
4
22.66
4
37.34
22.78 22.60 22.68 37.45 37.31 37.31 51.17 51.15 51.11 65.58 65.59 65.69 61.79 61.91 62.01
8 12
2. p-Bromoacetanilide 3. Bromobenzene 4. 1,2,4,5-Tetrachlorobenzene 5. Iodobenzene
cc.
8 12 4 8 12 4 8 12 4
8 12
50.92 65.71 62.22
It is apparent that the larger excess of standard silver nitrate accounts for smaller differences between the theoretical and experimental halogen percentages. But the primary reason for the use of a larger excess of the standard solution is that the end-point transition seemed much sharper than ordinary, because of the increased quantity of silver thiocyanate appearing in the back-titration.
LITERATURE CITED (1) Bacon,
49 (1909).
(2) Drogin and Rosanoff, Ibid., 38, 711 (1916). (3) Favrel and Bucher, Ann. chim. anal. chim. appl., 32, 2(e), 321-324 (1927).
(4) Jenkins, McCullough, and Booth, IND. ENG. CHEM., 22, 32 (1930).
(6) Lemp and Broderson, J. Am. Chem. SOC.,39, 2069 (1917). (6) Maryott, Am. J. Sci., 30, 378 (1910); Chem. News, 103, 1 (1911). (7) (8) (9) (10)
Proner, Ph.D. theais, Nancy, p. 79 (1925). Stepanow, Be?., 39, 4056 (1906). Van Duin, Rec. trav. chim.,45, 363 (1926). Walker and McRae, J. Am. Chem. Soc., 33, 598 (2911).
RECEIVED January 10, 1933.
Improved Laboratory Screw Clamp w. A. SPERRY 11th and Washington Sts., Wilmington, Del.
SUMMARY I n the determination of nuclear halogen in organic compounds, by the method described, the effects of a variation of several factors were studied and the following conclusions attained: 1. Drogin and Rosanoff (8limit the sample weight to 0.2 to 0.3 gram, regardless of halogen content of the material analyzed. Consequently a high percentage error occurs in the analyses of substances of low halogen content. The authors have found it advisable to adjust the sample weight in accordance with the halogen content, so that it is equivalgnt to 30 to 35 CC. of 0.1 N AgN03. 2. The blank determination, the volume ratio of silver‘ nitrate to ammonium thiocyanate and the standardization of the silver nitrate (against C.P. sodium chloride) must be carried out under parallel conditions of acidity, volume of solution titrated, and titration in the large porcelain dish. 3. The analysis of halogenated nitro compounds requires an excess over the quantities of sodium and alcohol specified by Drogin and Rosanoff. 4. The method is not suited to the analysis of volatile substances. Preliminary attempts to determine the percentage of chlorine in chloroprene yielded low results due to loss by volatilization, (The sample of chloroprene was supplied by Ira Williams of the Jackson Laboratory] E. I. du Pont de Nemours & Co.) 5. If the substance analyzed is crystalline, difficultly soluble in alcohol, and highly refractory chemically, the fineness of division is extremely important to the success of the analysis, I n the earlier literature divergent results for hexachlorobenzene and benzene hexachloride were reported. Similar difficulties were encountered in the authors’ analysis of tetrachlorobenzene. These discrepancies were probably due to the state of division of the sample and also to insufficient refluxing with the reducing media. In such cases it was found necessary to increase the quantities of sodium and alcohol employed to twenty per cent over those proposed by Drogin and Rosanoff, and to extend the time of heating to 2 hours after addition of the last portion of sodium. 6. I n all analyses, filtration of the precipitated silver halide is necessary. Should the filtrate produce interfering colors on the addition of indicator solution, the analysis must be repeated, with the excess of precipitant determined
J. Am. Chem. SOC.,31,
A
S THE ordinary laboratory screw clamp is equipped with a small, narrow screw head, the thumb and fingers of the operator often suffer injury when frequent tightening or adjusting is performed. It is often necessary to use pliers to open or close this type of clamp, especially if used on pressure tubing or when the screw has become corroded. This prompted the writer to provide the clamp with an enlarged knurled head, having a broader gripping surface and a greater diameter. This also simplifies fine adjustment when the clamp is used for regulating the flow of gases or liquids through rubber tubing. The device tried first consisted merely of a new screw, with enlarged head, turned from solid rod. This was soon replaced by s i m p l y brazing a k n u r l e d b r a s s cup of the a p p r o p r i a t e depth and diameter to the original screw head. The brazing was accomplished t h r o u g h a s m a l l hole in ,__ 1-1-2__ ’__ --b/ the center of the cup and ,-__ - -- _ , when properly executed I ; gives a strong union. / ,;_____ 1 i___: I 1 A more convenient deI\qj vice is the auxiliary head turned from stainless steel FIGURE1. METHODS FOR IMOF LABOand given a set of triangular PROVING EFFICIENCY HATORY SCREW CLAMPS “lands and grooves” as eleA stainless steel. B , screw head ments of the interior taper. griphed by lands and grooves. C, D , interior taper, 1 on 10 This steel head is d r i v e n knurled. with 24 lands and grooves 0.031 incd onto t h e screw head of deep. the customarv clamm the “lands” bitiig into “te brass and effecting a secure hold. Since screw clamps are of inexpensive construction and are ordinarily short-lived] this head may be removed and used again when the clamps are discarded. I
i
RIDCEIVBD February 11, 1933.
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