INDUSTRIAL A N D ENGINEERING CHEJlISTRY
Alay, 1928
54 5
Unification of Bromination Methods of Analysis as Applied to Phenols and Aromatic Amines’.” Allan R. Day and Walter T. Taggart THETOWNS SCIENTIFICSCHOOL, UNIVERSITY OF
P E N N S Y L V A N I A , PHILADELPHIA, P A .
The excess bromination method or Koppeschaar’s subject are sufficiently voluUANTITATIVE bromethod has been used for t h e analysis of a number of minous to indicate its immination as a method phenols and aromatic amines. A general analytical portance in organic analysis, for the determination procedure for this method has been worked out which is but they are surprisingly deof phenol was described by applicable, with few modifications, to a great many ficient in two respects: (1) Koppeschaar in 1876.3 The compounds. Greater care was taken in t h e purification There are relatively few inmethod is founded upon the of t h e substances t o be analyzed t h a n in most of t h e stances in which the investifact that in phenol part of previous work, thus making possible a more accurate gations were thorough and the n u c l e a r h y d r o g e n is interpretation of t h e value of t h e method. critical; and (2) there is often r e a d i l y and definitely reGood results were obtained for phenol, 0 - , rn-, and p a lack of convincing evidence placed by bromine. The way nitrophenols, 2,4-dinitrophenol, p-chlorophenol, salithat the compounds analyzed is thus open for analysis, cylic acid, m-hydroxybenzoic acid, methylsalicylate, were pure, a lack of definiteeither by weighing the broacetylsalicylic acid, phenylsalicylate, m - and o-cresols, ness regarding the analytic m i n a t e d p r o d u c t or by a resorcinol, P-naphthol, thymol, aniline, p-chloroaniwork itself-e. g., as to the volumetric determination of line, 0 - and m-nitroanilines, acetanilide, sulfanilic accuracy of the measuring apthe amount of bromine conacid, metanilic acid, anthranilic acid, m-aminobenzoic paratus, standardization of sumed in the bromination. acid, and 0 - , rn-, and p-toluidines. solutions, introduction of the The volumetric method, the I n all cases with t h e exception of phenol, p-nitrop r e c a u t i o n s e s s e n t i a l to one employed by Koppesphenol, 0 - , r n - , and p-cresols, resorcinol, p-nitroaniline, precise iodometric analysischaar, is properly called by and t h e toluidines, better results have been obtained and a lack of analytical data his name. It is much the t h a n in any previous published work. The method to indicate properly the acmore generally useful of the has been extended to include p-chlorophenol, methylcuracy of the results obtained. two and is the subject of this salicylate, phenylsalicylate, acetylsalicylic acid, and p One of the few exceptions to paper. chloroaniline. these statements was the critAs rapid and definite broThe excess method, as compared with t h e direct ical and conclusive work of mination is more or less chartitration method, is t o be preferred when applicable. Redman, Weith, and Brock4 a c t e r i s t i c of phenols as a The direct method is tedious and desirable only when on the analysis of phenol. class, a n d a l s o of many t h e excess method cannot be used. The fact that a really adea r o m a t i c amines, Koppesa u a t e i n v e s t i g a t i o n of chaar’s methods should be widely applicable to compounds of these types. This was Koppeschaar’s method as applied to phenol wai not made quickly recognized and led to the extension of the method until thirty-seven years after the method was first proposed, to numerous compounds other than phenol, and to the de- gives perhaps a correct idea as to the state of research in this field hitherto. The extensions of the bromination method to velopment of its numerous modifications. The bromine for the analysis is best supplied by the inter- compounds other than phenol will be indicated in appropriate action of potassium bromate and potassium bromide in the places later. The work to be presented in this paper deals primarily presence of sufficient hydrochloric acid to make quantitative with the excess bromination method and was undertaken in the reaction: the hope of approaching the following objectives: (1) a KBrOs 5 KBr 6 HCl = 6 Br 3 H20 6 KC1 simple general procedure applicable, with the fewest possible The bromate is the carefully weighed constituent and deter- individual modifications, to the largest number of phenols mines the amount of bromine liberated. The bromide is and aromatic amines; (2) determination of the special desimply taken in excess of five equivalents, the excess serving viation from the general procedure necessary to secure accuboth to insure complete reduction of the bromate and to rate analysis of each individual compound; and (3) careful increase the solvent power of the solution for free bromine. and critical trials of the selected procedures, using carefully This device for the liberation of definite quantities of purified compounds, accurate measuring apparatus, and nascent bromine is applied analytically in two familiar using in general a uniform analytical technic, in order to ascerprocedures: (1) the excess of Koppeschaar’s method, and tain the merits or demerits of each analysis. (2) the direct titration method. Many of the procedures suggested since 1876 are apparGeneral Procedure for Excess Method ently hasty adaptations of the direct or excess method to individual compounds or to mixtures. Often they were A quantity of the substance sufficient for ten analyses is developed for technical use and are of doubtful accuracy. weighed out, dissolved in water, dilute sodium hydroxide It seems very probable that considerable unpublished work (for phenols and sulfonic acids), or dilute hydrochloric on the bromination method must have been done in technical acid (for amines), and the solution diluted to 250 cc. A 25laboratories. The published records of research on the cc. aliquot is pipetted into a 500-cc. iodine flask, followed by 25 cc. of 0.2 N “bromine” solution (75 grams of potassium 1 Received December 6, 1927. bromide and 5.6 grams potassium bromate per liter) and then 2 Abstract of thesis submitted by Mr. Day in partial fulfilment of requirements for the degree of doctor of philosophy at the University of Penndiluted with 50 cc. of water. Five cubic centimeters of
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sylvania.
:Z. anal. Chem., 15, 233
(1876).
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J. IND.ENC.C H ~ X 0. , 389 (1913).
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concentrated hydrochloric acid are added and the flask is stoppered at once. It is shaken for 1 minute and then allowed to stand, with occasional agitation, for a definite interval, which varies with the ease of bromination of the compound analyzed. The flask is well cooled under the tap or in ice water and 5 cc. of 40 per cent potassium iodide solution are poured into the trough. The stopper is partly dislodged, whereupon the iodide solution is drawn into the flask with no loss of bromine. The flask is thoroughly shaken, the stopper removed, and the neck of the flask and the stopper washed with water. The free iodine, equivalent to the excess of bromine taken, is titrated with 0.1 N thiosulfate, using 5 cc. of 0.5 per cent starch solution near the end of the titration. For the blank analyses there are taken 25 cc. of 0.2 N “bromine” solution and 50 cc. of water, the procedure being otherwise identical with that of the analysis proper. The expression “general procedure,” when used in later pages, refers to the method just described. Any deviations from the conditions specified are noted in the discussion of the individual compounds. I n all cases when the substance is not subject to oxidation, the weight of the sample should be so chosen that a large excess (in general about 100 per cent) of bromine will be present. This increases both the rate and the completeness of bromination. General Procedure for Direct Titration Method
This is a general procedure as described by Callin and Henderson.6 The sample is dissolved as before. A 25-cc. aliquot is transferred to a beaker, diluted to 200 cc., and 10 cc. of a 20 per cent potassium bromide solution are added. It is now acidified with 5 to 10 cc. concentrated hydrochloric acid and titrated with a standard potassium bromate solution (0.2 N ) until a drop of the solution produces a blue spot on starch-iodide paper, which persists for 2 to 4 minutes after the addition of the last drop of the bromate solution. The strength of the bromate solution is determined by titrating the iodine liberated when 25 cc. of the potassium bromate solution are acidified with hydrochloric acid in the presence of 10 cc. of 20 per cent potassium bromide solution and 2 grams of iodate-free potassium iodide. Comparison of Two Methods
The apparent advantages of the direct titration seem to be largely illusory. The use of an outside indicator and the requirement that the final small excess of bromine persist for 2 to 4 minutes combine to make the titration unduly tedious. There is, furthermore, some possibility that bromine may be lost, especially near the end of the titration. Whenever the excess method is applicable, it is preferable. At its best, this method leaves little to be desired as regards accuracy, rapidity, and convenience. Difficulties in Bromination Methods
I n any bromination methods certain difficulties are introduced by reactions other than those expected. One cause for failure, in a number of cases-. g., 0- and p-toluidinesis the ease with which certain compounds are oxidized by aqueous bromine. This leads to abnormal and indefinite results. It constitutes a limitation upon the applicability of the method, unless the oxidation can be either prevented, as by operating at lower temperatures, or regulated so that the total bromine consumed will still be stoichiometrically related to the compound present. It is hoped in later work to investigate the last-mentioned possibility. Cooling to 8
J . SOC.Chem I n d . . 41, 161 (1922).
Vol. 20, No. 5
prevent oxidation has been suggested by Francis.6 It must be remembered, however, that if the temperature is too low the velocity of reaction is retarded, making low results possible. Another difficulty is the precipitation of incompletely brominated products, or the occlusion of unbrominated or partly brominated material by the precipitate. The use of alcohol to prevent precipitation of the brominated product was suggested by Francis for certain compounds, This procedure eliminates the difficulty mentioned, but introduces the complication that alcohol is somewhat attacked by nascent bromine in acid solution. Tests showed that 25 cc. of a “bromine” solution, requiring normally 35.50 cc. of 0.1 N thiosulfate, required 35.30 cc. after 1 minute in the presence of 25 cc. of alcohol (at room temperature), 35.15 cc. after 2 minutes, and only 34.84 cc. after 5 minutes. Furthermore, duplicate trials for equal periods of time gave inconsistent results, which seem to exclude the possibility for application of a “correction” for the alcohol. Experimental
APPARATUS AND MATERIALS Usm-Each piece of volumetric apparatus was carefully calibrated. The weights used were calibrated by Richards’ method. The thiosulfate solution was standardized against purified iodine.’ The weighing bottles were put in a desiccator, after adding water to the potassium iodide, and allowed to stand for one hour. They were then transferred to the balance and allowed to stand 15 to 20 minutes before the iodine was weighed out.& The bromate-bromide solution was standardized as described in the general procedure. The normality of the two most important solutions was thus based directly and indirectly upon pure iodine. The phenols and amines used in this work were carefully purified, starting with the best materials obtainable on the market. I n a few cases the product was made in this laboratory. Melting points were determined in an apparatus similar to that of Friedrichs.g Boiling points were determined in an ordinary side-arm distillation flask. PROCEDURE AND REsuLTs-The general procedure for the excess method was used with the fewest possible modifications. The samples, as given in Table I, were sufficient for ten analyses, so the amounts actually used were only onetenth of those given. The specimens of methylsalicylate, phenylsalicylate, and acetylsalicylic acid were hydrolyzed before analysis. The methylsalicylate was hydrolyzed by treating with a few cubic centimeters of 5 per cent sodium hydroxide. The sodium salt formed was dissolved in a small amount of water and the solution heated just to boiling for 10 minutes. The solution was cooled, acidified with hydrochloric acid, made alkaline with sodium hydroxide, and finally diluted to 250 CC. The phenylsalicylate was readily hydrolyzed by heating just to boiling for 25 to 30 minutes with 0.1 N sodium hydroxide. The cooled solution was then diluted to 250 CC. Acetylsalicylic acid undergoes hydrolysis very readily in cold dilute acid solution, so no preliminary treatment was necessary before analysis. I n the case of m-cresol it was found necessary to allow the mixture to stand for 60 minutes after the addition of the potassium iodide. Fox and Barkerlo 6
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p.
Analysis,”
646 (1924). 8 9
p.
J . A m . Chem. SOC.,46, 2498 (1924). Treadwell, “Analytical Chemistry. Vol. 11-Quantitative
Wagner, IND. END.CHEM.,16, 617 (1924). Houben, “Die Methoden der organischen Chemie,” 3rd ed
799. 10 J . SOC. Chem. I n d . , 39, 169T (1920).
, Vol. I,
I N D U S T R I A L A N D ENGINEERING CHEMISTRY
May, 1928
first suggested this as a means of improving the end point. The excess method has not proved satisfactory for the oand p-cresols. The results were too much affected by the analytical conditions. Pence," who studied the quantitative bromination for resorcinol, allowed the bromination mixture to stand for 5 minutes after the addition of the iodide. His method has been used, with litble modification, in this investigation and good results were obtained. T a b l e I-Results
b y Excess M e t h o d PERIOD BRO- SAM-
OF MINE PLES M E A N DILU- BROMI-EQUIV-ANA- RESAMPLE SOLVENT TION NATION ALENT LYZED SULT Grams Cc. H20 M i n . % 0.5-0.6 h-aOH Phenol 50 5-30 3 Brz 5 99.89 0.5-0.7 NaOH 100 0-Chlorophenol 30 2 Br2 4 99.87 o-Nitrophenol 0.5-0.7 NaOH 50 30 2 Brz 4 99.77 m-Nitrophenol 0.5-0.6 NaOH 50 5-30 3 Brz 4 99.88 $-Nitrophenol 0.7-0.9 NaOH 50 5-30 2 Brz 4 99.90 2,4-Dinitropnenol 1.5-1.8 h-aOH 50 30 1 Br? 4 99.92 Salicylic acid 0.5-0.6 NaOH 50 5 99.86 30 3 BIZ m-Hydroxy benzoic acid 0.5-0.6 NaOH 15 3 Brz 4 50 99.85 Methylsalicylate NaOH 0.6 60 99.82 7 30 3 Brz Phenylsalicylate 0.4-0.5 NaOH 50 99.87 5 30 6 Brz Acetylsalicylic acid 0.6-0.7 h-aOH 50 30 3 Brz 7 99.84 m-Cresol 0.5-0.6 NaOH 1 3 BIZ 50 99.75 6 Resorcinol 0.5-0.6 Hz0 1 3 BIZ 4 100 99.95 @-Naphthol 1.2-1.5 NaOH None 15-20 1 Brz 99.89 5 Thymol 0.4-0.8 h-aOH None 15-20 2 Br? 99.77 7 Aniline 50 99.92 0.5-0.6 HC1 5-10 3 Br2 5 P-Chloroaniline 1 -1.2 HCI 99.93 4 50 10 2 Br? o-Nitroaniline 0.5-0.6 HCI 30 2 Brz 100 99.86 4 m-Nitroaniline 0.5-0.6 HCI 30 3 BIZ 50 99.89 5 Acetanilide 99.84 0.5-0.7 HCI 5-10 3 Brz 50 5 Sulfanilic 30 3 Brz 5 50 0.5-0.6 NaOH 99.98 Metanilic acid 0.5-0.6 NaOH 99.80 5-15 3 BIZ 5 50 Anthranilic 0.5-0.6 NaOH 99,89 30 3 BIZ 5 50 m-Aminobenzoic 10-15 3 Brz NaOH 0.5-0.6 50 4 99.86 m-Toluidine 5-10 3 BIZ 0.5-0.6 HCI 50 99.87 6
SUBSTANCE
The excess method, in the absence of a special so!vent, was found not be applicable to p-naphthol and thymol. Chloroform was adopted as the best solvent for these two, being not only a suitable solvent but a t the same time fairly stable toward bromine. Five cubic centimeters were used for each analysis. Blank determinations were made under exactly the same conditions. Thus, even though the chloroform was slightly attacked by the bromine, an accurate correction was made. It was found best to cool the mixture slightly before acidifying because of the vapor pressure of the chloroform and to shake the flask gently, after acidifying, until the brominated precipitate dissolved in the solvent. I n the titration of the liberated iodine vigorous shaking was necessary just before the end point was reached, as the chloroform retains the last of the iodine rather tenaciously. 11
J. TND. ENO.CHEM.,3, 820 (1911).
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Owing to their ease of oxidation, aniline and those compounds which yield aniline by hydrolysis (acetanilide) or which yield tribromoaniline on bromination through the replacement of a carboxyl or sulfonic acid group (sulfanilic acid, anthranilic acid, etc.) must be brominated a t a lower temperature, usually 10-15" C. I n certain cases, however. (0- and p-toluidines) good results could not be obtained, even when working a t much lower temperatures. For such types the excess method is of little value and the direct method is more applicable. The acetanilide was hydrolyzed before analysis by heating just to boiling for 10-15 minutes with 6 N hydrochloric acid. The solution was neutralized with sodium hydroxide, then made slightly acid with hydrochloric acid, and finally diluted to 250 cc. Low results were always obtained for p-nitroaniline. The brominated precipitate was very voluminous and it is probable that some incompletely brominated product was occluded. The excess method was not applicable to p-aminobenzoic acid. Instead of obtaining tribromoaniline, as would be expected (similar to the action of bromine on the ortho isomer), an insoluble dibromo derivative was formed, but at the same time some tribromoaniline was also formed. The results were then high based on 2 Brz and low based on 3 Brz. Other compounds than those given in Table I have been tried (anisole, phenolphthalein, qvinol, dimethylaniline, benzidine, arsanilic acid, carbazole, etc.), but apparently good results cannot be obtained by the usual excess method. T a b l e 11-Results SUBSTANCE o-Cresol o-Toluidine $-Toluidine
by Direct T i t r a t i o n M e t h o d BROMINE SAMPLES MEAN SAMPLE SOLVENTEQUIVALENT ANALYZED RESULT Grams % 1 6-1.8 NaOH 2 Brz 3 100.04 1.5-1.8 HCl 2 Brz 3 100.08 1.5-1.8 HC1 2 BIZ 3 100.19
I n all cases where the excess method was not applicable the direct method was tried, but only in the cases given in Table I1 were fair results obtained. It would seem that this method as now carried out, seemingly simple though it may be, is not so accurate as the excess method. With the idea of improving the direct method, work has been started using an electrometric method as described by Fo-ilk and Bawden.'* N o t e A complete bibliography and a more detailed description of the work done is given in the original paper, including the melting and boiling points of the materials analyzed, suitably corrected, and compared with the best published values. 1%
J . A m . Chem. SOC.,48, 2045 (1926).
Transmissive Properties of New Glasses The number of inquiries which the Bureau of Standards received concerning the ultra-violet transmission of glasses and glass substitutes led them to issue Letter Circular 235 on this subject. This letter circular gives the results of ultra-violet transmission tests which have been made a t the Bureau of Standards upon a number of glasses transparent to the shortest wave lengths which the atmosphere permits the sun to furnish and common window glass. Using a filter method, direct measurements, with sunlight as source, of total transmission of various glasses for those ultraviolet solar rays to which common window glass is opaque were made during the noon hours of especially clear days from April to December, 1927. These measurements covered the solar spectral region to which common window glass is opaque (below about 310 mp). Table I gives for that region the total transmission found for several specimens. By means of spectral transmission curves an estimate of the relative transmissions of the various specimens for rays shut out by common window glass may be obtained by reading from
the curves, which are shown on a chart accompanying the letter, the values of the transmission a t 302 mp-the wave length of an intense mercury line of convenient value for making such tests. Transmission values for this wave length are also included in the circular. T a b l e I-Total T r a n s m i s s i o n s of Various Glasses w h e n New, f o r Ultra-Violet Solar R a y s t o W h i c h C o m m o n Window Glass Is O p a q u e TRADE NAME TRANSMISSION TRADENAME TRANSXISSION Per cent Per cent Fused quartz 92 Cel-o-glassb 20 Corex 92 Quartz-lite 5 Helioglass (Vioray)' 50 Flexoglassc 1 Vitaglass 50 Common window glass 0-5 a Vioray is the foreign trade name for Helioglass. b This consists of a fine wire screen whose interstices are covered with cellulose acetate. c This is a loosely woven fabric usually covered with paraffin.
A limited number of mimeographed copies of Letter Circular 235, with accompanying chart showing transmission of various window glasses when new, is available a t the Bureau of Standards.
