May 15, 1934
INDUSTRIAL AND ENGINEERING CHEMISTRY
SUMMARY The steam distillation method for removing coumarin from rJlant tissues has certain decided advantages over the ether extraction method. It gives excellent results with sweet clover and seems to apply SUCCeSSfUlly to other species also. For comparative purposes where a high degree of accuracy is unnecessary, the number of distillations may be reduced and the distillates titrated directly without further treatment.
213
For more accurate results the number of distillations is increased, and the noncoumarin reducing substances are removed by precipitation with lead acetate. LITERATURE CITED (1) Dean, J. R., J. IND. ENG.CHEX., 7, 519 (1915). (2) Obemayer, E., 2. anal. Chem., 52, 172-91 (1913). RECEIVED September 21,1833. Published with the approvsl of the Director of the West Virginia Agricultural Experiment Station as Scientific Paper 130.
Determination of Bromide in the Presence of Large Excess of Chloride ROYF. NEWTONAND EDITHR. NEWTON,Purdue University, Lafayette, Ind.
D
URING a study of the metabolism of brominated
compounds, a number of the methods previously reported for the determination of bromides in the presence of large excess of chlorides proved unsatisfactory. Most of them depend upon the oxidation of the bromide to bromine under conditions so regulated as to avoid the simultaneous production of chlorine. When the concentration of bromide is small and that of chloride is high, this is impossible; to overcome this difficulty the bromine first obtained is frequently converted to bromide and is then reoxidized in a similar manner. I n some processes even a third oxidation is used. Usually the bromine finally obtained is determined iodometrically. Each oxidation introduces the possibility of loss of bromine, and if any chlorine is carried over in the last oxidation it will be reported as bromine. The procedure of Behr, Palmer, and Clark (1) gave the most satisfactory results, but even with very careful manipulation it was accurate to only about 2 per cent. Since this was not sufficient for the authors’ purposes, the following procedure was worked out: The bromine is liberated by means of moderate excess of chlorine water, collected in sodium sulfite solution, and titrated potentiometrically in sulfuric acid solution with potassium bromate. This procedure has been satisfactorily used by one of the authors in collaboration with R. C. Corley for the analysis of about fifty urine samples in a study of the excretion of bromides following the ingestion of brominated compounds. MATERIALS AND APPARATUS Potassium bromide for use in making known solutions had to be synthesized, since the best obtainable potassium bromide contained “not more than 0.75 per cent potassium chloride.” Bromine was freed of chlorine by shaking with three successive portions of aqueous sodium sulfite and separating. The purified bromine was then reduced by sulfur dioxide and water, and the resulting hydrobromic acid solution was redistilled until free of sulfuric acid, and neutralized with c. P. potassium hydroxide. The potassium bromide obtained in this manner was recrystallized twice and dried a t about 110’ C. Chlorine water was made by absorbing in water the chlorine obtained by electrolysis of hydrochloric acid. To remove any traces of bromine from the hydrochloric acid, it was diluted to about constant boiling strength, treated with chlorine, and boiled. The strength of the chlorine water was checked iodometrically from two to three times a week. The potassium bromate was a e. P. product recrystallized and dried a t about 110” C. Sulfuric acid, potassium hydroxide, copper sulfate, and
sodium chloride of c. P. or reagent quality grade were used without further purification. The sodium chloride was found to contain 0.001 per cent of,bromide. The chloride in the other chemicals was reported from 0.001 to 0.005 per cent, and undoubtedly the bromide was only a small fraction of this. With the exception of the chlorine water, all solutions were made by weighing the appropriate substance and making up to volume in calibrated volumetric flasks. The apparatus for liberating the bromine from the solution of halides consisted of a 50-ml. flask with an inlet tube extending from top to b o t t o m , a n d a delivery tube leading to an absorption train of two test tubes, illust r a t e d i n Figure 1. Suction is applied a t F. A , C, and D are F u n g r e a sed groundglass joints. A rubber stopper was used a t E, s i n c e t h e air passing it no longer contains bromine vapor. This apparatus was designed for samples not exceeding 25 ml- A larger flask FIGURE 1. APPARATUSFOR LIBERATwould be desirable for ING BROMINE larger samples, and if the bromine content is high, the absorption system should also be enlarged. A b e d s & Northrup student potentiometer with a portable, enclosed lamp and scale galvanometer was used to measure the potentials.
PROCEDURE The bromine in the sample to be analyzed should be present as bromide ion. For materials in which the bromine is in organic combination, or when organic material is mixed with the bromide, the ashing method of Behr, Palmer, and Clark (1) was found to be very satisfactory. Sodium sulfite amounting to about 20 mg. plus three to five times the estimated weight of bromine in the sample is weighed out, dissolved in water, and put into the absorption tubes, with about 80 per cent of the sulfite in the first tube. The rapid oxidation of sulfite by air makes the large excess necessary. While a slow stream of air is sucked through the apparatus, the sample is introduced through the inlet tube, rinsed down, made acid with sulfuric acid, and chlorine water estimated to be in slight excess
214
ANALYTICAL EDITION
of the bromine content is introduced. The flask is heated to about 100” C. for 10 t o 15 minutes and cooled. Two or three further additions of chlorine water are made, heating the flask after each addition. Unless the bromine content is very small it is desirable to use a total of not more than three times the theoretical amount of chlorine water, and the later additions are made smaller than the first one unless the development of color on the second addition shows that the first was too small. The air stream is then stopped, and the contents of the absorption tubes are rinsed into a small beaker suitable for the titration, made alkaline, and evaporated on a steam bath. A drop of 1 per cent copper sulfate is added before evaporation to catalyze the oxidation of the excess sulfite by air. After evaporation the sides of the beaker are rinsed down and the sample is again evaporated to insure the oxidation of all of the sulfite, made up to suitable volume with about 7.5 N sulfuric acid, and titrated otentiometrically with standard potassium bromate, using a ppatinum wire electrode as indicator, and a 0.1 N calomel electrode as reference. The solution i s stirred by a small mechanically driven glass stirrer throughout the course of the titration. The titration vessel is surrounded by ice to minimize the loss of bromine.
Before addition of any bromate the potential is erratic and of no significance. The potential after the addition of a small quantity of potassium bromate (0.02 ml. was usually used) is a very useful indicator of sulfite; if sulfite is still present, the potential remains lo?, below 0.3 volt, but if it has been completely oxidized, the potential goes to 0.57 to 0.61 volt. The end point is taken as the point of maximum rate of change of potential with added potassium bromate. The authors have used Fenwick’s (3) formula for calculating the end point. The end-point potential varies from case to case, from 0.78 to 0.83 volt, probably because of the variation in the concentration of dissolved bromine. When no bromide is initially present in the solution to be titrated, the end-point potential is exceeded upon the first addition of bromate. DISCUSSION I n order to test the reliability of the potentiometric titration of bromide in the presence of chloride, a series of titrations was carried out under various conditions, with the results shown in Table I. The ratio of chloride to bromide was kept small and the preliminary oxidation with chlorine water was omitted. Known quantities of chloride and of bromide were mixed, made alkaline and evaporated, made to suitable volume with sulfuric acid and titrated. Series 1 shows that if the concentration of chloride does not exceed two to three times the equivalent concentration of bromide present, the amount of bromate required is independent of the chloride content. With larger relative amounts of chloride the end point is not sharp and the results are high. For this reason a controlled amount of chlorine water is employed in the first oxidation. Series 4 and 5 show that an increase in final volume, with corresponding decrease in acid coqcentration, decreases the amount of bromate used toward the stoichiometric value. With appropriate adjustment of acid concentration and of volume a t the end point, the stoichiometric ratio of bromate to bromide may be obtained, but when the concentration of bromide is very small, such adjustment makes the titration very tedious, for under these conditions the potentials are slow to reach steady values. For this reason it i s recommended that in the determination of small quantities of bromides, the volume of solution a t the end point be kept small, the final acid concentration be from 6 N to 7 N , and that known samples be run under the same conditions and the corresponding correction factor be applied. I n effect the bromate is standardized by the known bromide solution. Table I1 gives some results of the complete procedure. From 0.3 to 1.0 gram of sodium chloride was added to the samples containing more than 1 mg. of bromide, and approximately 30 mg. of sodium chloride to those containing less than 1 mg. Very great excesses of sodium chloride were
Vol. 6 , No. 3
avoided to obviate large corrections for the bromide in the sodium chloride. There is no reason to believe that larger amounts of chloride would in any way interfere with the liberation of bromine. The “Bromine Present” of Table I1 is the sum of the bromine added as potassium bromide and that present as impurity in the sodium chloride. When the conditions of titration were such that a stoichiometric ratio was not expected, the appropriate factor obtained from Table I was applied. The calculations for experiment 7 of Table I1 are given for illustration. Bromine added: as potassium bromide, 0.8035 mg.; as impurity in 117 mg. of sodium chloride, 0.0012 mg.; total, 0.8047 mg. Approximately 0.75 ml. of 0.04 N chlorine water was used to liberate the bromine, which was absorbed in about 25 mg. of sodium sulfite. Near the end point the potassium bromate was added in 0.04-ml. portions. Fenwick’s (3) formula indicated the end point at 1.688ml. of 0.0598 N potassium bromate, corresponding to 0.8068 mg. of bromine. However, an examination of series 6 of Table I shows that the titration under these conditions gives results which average 0.8 per cent too high; subtracting 0.8 per cent or 0.0064 mg., a final corrected value of 0.8004 mg. is obtained. TABLEI. TITRATION OF BROMIDE IN THE PRESENCE OF MODERATE QUANTITIES OF CHLORIDE MOLAJ
RATIO S E R I ~C1 S TO Br
1
0.0 1.0 2.0 3.0 4.0 2.0 2.0 2.0 0.5 0.5 1.0 1.0 0.5 1.0 1.0 2.0
2 3
4
e
1.0 1.0 1.0 2.0 2.0 1.0 2.0 2.0 2.0
7
VOLUME AT END BROMINE POINT ACIDITY PRISENT M1. N M g 121 6.2 199.3 121 6.2 199.3 121 6.2 199.3 121 6.2 199.3 121 6.2 199.3 166 6.3 199.3 166 6.3 199.3 6.3 199.3 166 24 6.3 39.94 39.94 24 6.3 24 6.3 39.94 24 6.3 39.94 9 6.7 3.991 3.991 9 6.7 9 6.7 3.991 9 6.7 3.991
.
4.7 4.7 4.7 4.7 4.7 3.3 3.3 3.3 3.3
6.4 6.4 6.4 6.4 6.4 6.8 6.8 6.8 6.8
BROMINB FOUND RECOVERY
Mo .
%
200.5 200.7 200.8 200.6 200.5 199.7 199.7 199.8 39.91 39.99 39.99 39.89 4.022 4.009 4.015 4.015
100.6 100.7 100.7 100.6 100.6 100.2 100.2 100.2 99.9 100.1 100.1 99.9 100.8 100.5 100.7 100.7
0.8035 0.8035 0.8035 0.8035 0.8035 0.4006 0.4006 0.4006 0.4006
0.8079 0.8114 0.8066 0.8119 0.8119 0.4031 0.4059 0.4035 0.4050
100.6 101.0 100.5 101.0 101.0 100.6 101.3 100.7 101.1
TABLE11. DETERMINATION OF BROMIDE IN LARGE EXCESS OF CHLORIDE EXPT. 1 2
9 10 11 12
BROMINE PRESENT
BROMIN~C FOUND
Mo.
Mo.
39.94 39.94 3.994 3 * 994 3.994 0,8064 0.8047 0.8038 . .... 0.4009 0.4009 0.4009 0.4009
39.96 39.88 3.985 3.997 3,995 0.8088 0.8004 0.8018 0.3998 0.4026 0.4017 0.4012
REICOVERY % 100.I 99.8 99.8 100.1 100.0 100.3 99.5 99.8 09.7 100.4 100.2 100.1
EFFECTS OF IODIDES When the iodide content is small relative to the bromide content, no care is necessary to insure its removal. Some experiments were performed in which iodide equivalent to the bromide present was added. By adding a t least six equivalents of chlorine water for each equivalent of iodide present, the iodide was completely oxidized to iodic acid and retained in the flask. The subsequent titration showed no evidence of iodide, and gave normal results for the bromide.
INDUSTRIAL AND ENGINEERING CHEMISTRY
May 15, 1934
Data upon the application of this method to the analysis of urine samples appear in Table 111. TABLE111. DETERMINATION OF BROMIDES IN NORMAL URINE( 2 ) SUBJECT
2 4 - H o u ~URINE BROMINE FOUND
cc.
Ma.
A
1300
13.18
A
1300
9.32
B
1313
5.50
B
1010
6.60
C
1800
14.70
REMARKB Usual diet Table salt Usual diet c. P. salt Usual diet c. P. salt Usual diet c. P. salt Usual diet Table salt
SUMMARY A method for the determination of bromides in the presence of large excess of chlorides has been developed. The bromide
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is liberated by chlorine water in moderate excess, collected in sodium sulfite solution, and titrated potentiometrically after air oxidation of the excess sulfite. For 4 mg. or more of bromine present as bromide the maximum error is about 0.2 per cent, and for a bromine content of from 0.5 to 1 mg. the error is about 0.5 per cent. Moderate quantities of iodides do not interfere.
LITBRATURE CITED (1) Behr, Palmer, and Clark, J. Biol. Chem., 90, 133 (1930). (2) Corley, R.C., and Newton, E. R., unpublished results. (3) Fenwick, F., IND.ENQ.CHEM., Anal. Ed., 4, 144 (1932). R ~ C E I V EOctober D 12, 1933. Presented before the Division of Physical and Inorganic Chemistry a t the 86th Meeting of the American Chemical IIL September lo to 151 1933. Society~
Improved Apparatus for Quantitative Estimation of Helium in Gases FRANK E. E. GERMANN, K. A. GAGOS,AND C. A. NEILSON Department of Chemistry, University of Colorado, Boulder, Colo.
T
HERE are two general methods in use for determining the rare gas content of natural gases. Cady and NIcFarland (S) determined only the helium content, whereas Moureu and Lepape (IS) estimated the total rare gas content and then further separated it into helium and neon, argon with small amounts of krypton and xenon, and krypton and xenon with small amounts of argon. In both methods use is made of the ability of charcoal, discovered by Dewar (7), t o adsorb all gases except helium, neon, and hydrogen when cooled in liquid air. It was the experience of Cady and McFarland that hydrogen was freely adsorbed under these conditions, neon much less than hydrogen, and helium very slightly. Their apparatus consisted of a preliminary tube for condensing methane and other hydrocarbons, two tubes filled with charcoal, and two U-dips to condense moisture and mercury vapor, all immersed in liquid air. I n addition there were two Plucker tubes, an automatic Sprengel pump, and a tube in which the helium was collected and measured. The purity of the gas was tested by observing its spectrum, and if any nitrogen was found present, the gas was again passed through the charcoal. This is essentially the apparatus which has been used by the U. S. Bureau of Mines and described by Seibel (16). Other workers who have used essentially the method of Cady and McFarland are Czako (6),McLennan ( l a ) , Yamada ( I @ , Yamaguti and Kano (19), Chlopin and Lukasuk (6),and Butescu and Atanasiu ( 2 ) . Erdmann (8) mixed oxygen with the hydrocarbons and hydrogen and exploded the mixture. Moureu and Lepape burned hydrogen, carbon monoxide, and the hydrocarbons to carbon dioxide and water over heated copper oxide. Carbon dioxide was then absorbed by potassium hydroxide, water by phosphorus pentoxide, and the nitrogen and oxygen were combined with heated calcium or magnesium. When no further pressure drop could be noticed and only the noble gases were distinguishable spectroscopically, the volume of gas was measured. Argon, krypton, and xenon were adsorbed by charcoal cooled with liquid air and the volume of the remaining helium and neon was measured. Through partial vaporization, argon with slight amounts of krypton and xenon is sometimes estimated, or xenon and krypton with slight amounts of argon.
Many authors have used at least the principle of the method of Moureu and Lepape. Bamberger ( I ) as well as Ewers (9) removed the last traces of nitrogen from the residual gas mixture with oxygen by sparking over potassium hydroxide. Other workers removed the total nitrogen content, after adding oxygen, by sparking over potassium hydroxide and removing the excess oxygen with pyrogallol. Paneth (14, 16) and his students have developed a micromethod by means of which they claim to be able to estimate cc. of helium with an accuracy of 2 per cent. Sokolov (17) has constructed a balance for measuring the density of the unadsorbed helium and neon for use with gases containing a t least a helium and neon concentration totaling 0.1 to 0.2 per cent. A method for the continuous separation of helium from a gas mixture by the adsorption of the other gases by charcoal a t liquid air temperatures has been described by Cherepennikov (4). He also claims to have accomplished the removal of all gases except helium by circulating the gas through charcoal cooled with solid carbon dioxide, regenerating the charcoal after each cycle. The volume of gas sample used by different workers varies widely. Cady and McFarland used 13to 15liters, McLennan about 6 liters, Moureu and Lepape 200 cc., and Paneth, Gehlen, and Peters only 10 cc. With the exception of Chlopin and Lukasuk, who used a modification of an apparatus described by Guye and Germann (IO), all workers cited have used LE complex apparatus. The essential part of the simplification introduced by the latter authors lay in the combination of LE vacuum pump, McLeod gage, and storage reservoir. The apparatus described in the present communication is a further adaptation of the apparatus of Guye and Germann to the analysis of helium. DESCRIPTION OF APPARATUS A diagram of the apparatus is given in Figure 1. The tube C containing 30 grams of charcoal serves, when cooled with liquid air, to adsorb all the gases contained in the sample except helium (and some neon). The combination mercury pump and modified McLeod gage E serve t o create a high vacuum, to measure the volume and pressure of the original sample, to cir-
