AUGUST 15, 1938
ANALYTICAL EDITION
I n connection n-ith studies which resulted in the development of this method, tests were also carried out on the applicability of other iron reagents and included 1,lO-phenanthroline. This is a n oxidation-reduction indicator, forming a colored iron complex, and is structurally similar to 2.2'bipyridine. The results obtainable were similar to those found for 2,2'-bipyridine, though 2,2'-bipyridine is to be preferred. The color with 1,lO-phenanthroline tends toward the orange rather than the red, making visual comparisons much less sensitive than with the 2,2'-bipyridine. Moreover, this reagent was more susceptible to interferences by traces of certain metals, particularly cobalt, nickel, and copper. Over a period of more than 3 years, many hundreds of samples of beer, ales, and the like have been subjected to the above iron test in the authors' laboratories, n-ith entirely satisfactory results. It has the special advantage, by reason of the rapidity with which results may be secured, that the iron content of beer in process, during storage, and in the
417
finished form, may be regulariy and carefully controlled. Such regular control is impracticable with the mare time-consuming methods. B y means of such regularity o f control, it has been possible to discern danger signals far ahead of actual trouble and t'o take necessary remedial steps.
Literature Cited (1) Bode, Wochschr. Brau., 50, 321 (1933). (2) Hill, R., Proc. Rog. SOC.London, B107, 205 (1'330).
(3) Kohler, Elvehjem, and Hart, J . Bid. Chem., 113, 49 (1936). (4) Scott, W.W., "Standard Methods of Chemical Analysis," K'ew Tork, D. Van Nostrand Co., 1925. (5) Siebenberg and Hubbard, J . Assoc. Oficiai -407. Chem., 19, 489-93 (1936). (6) Snell, F. D., and Snell, C. T., "Colorimetric M e ~ h o d sof Anslysis," S e w ITork, D. Van Nostrand Co., 1936. (7) Toe. J. H., "Photometric Chemical Analysis," New York, John W l e y Br Sons, 1928. RECEIVED May 2 , 1938.
A Precise Method for the Determination of Carotene in Forage DONALD W. BOLIN AND ASS;ID M. KHbL-iPUR University of Idaho, los scow, Idaho
REPORT of recent methods for the determination of carotene in forage is giren by Xunsey (2) and his referees. These methods give consistent results only when special precaution is taken. The Guilbert method ( 1 ) has been used in this laboratory for some time, but the use of ethyl ether in the extraction of the carotene froin the saponified mixture is objectionable. Peterson and Hughes' (3) modification of Guilbert's method eliminates the use of ethyl ether and extracts tlie carotene directly from the saponified mixture Jyith petroleum ether, forming emulsions which result in a loss of carotene. During the past year a niodification of this method ( I , 3) has been used in the authors' laboratory; it permits the use of definite quantities of reagents, avoids the formation of emulsions. and gives more precise results.
Procedure Reflux a 5- to 10-gram sample SO minutes with 200 cc. of ethyl alcohol (approximately 95 to 97 per cent). Filter the hot alcoholic solution through a S o . 31 Whatman paper placed in a Bdchner funnel, and wash the residue with hot ethyl alcohol unt,il tlie alcoholic filtrate comes through clear (150 cc. of hot alcohol are usually sufficient). Make the alcoholic filtrate up to 400-cc. volume, transfer one-half to a 250-cc. volumetric flask, and add 25 cc. of 10 per cent alcoholic potash. Shake and let the alkaline alcohol solution stand for 2 hours at room temperature to ensure complete saponification, or place the flask containing the alkaline alcohol solution in hot m t e r for 0.5 hour at 80" C. to hasten saponification; cool and make up t3 volume. Transfer 25 cc. of the saponified alcoholic carotene solution t o a 100-cc. separatory funnel. Add 15 cc. of petroleum ether (b. p. 4G60') and shake the alcohol and petroleum ether vigorously. Add 7 cc. of water t o the contents in the separator)- funnel and again shake vigorously. Drain off the alcoholic so!ution into a similar separatory funnel and ext,ract twice more with 10 cc. of petroleum ether. The last extraction of the petroleum ether will be colored. Tests have shoivn that the carotene is almost completely removed 11-ith the first extraction of petroleum et.her. Combine the petrdleuni ether extracts and wash gently ]Tit11 25-cc. portions of distilled water until the wash TI-ater no longer gives a color with phenolphthalein. Extract the xanthophyll from the
petrcleum ether solution xith 25-i:c. portions of 85 per cent methyl alcohol until the alcohol is colorless. For the first extractitin n.ith 85 per cent methyl alcohol, pour the alcohol gently don-n the sides of the separatorp funnel 53 as not to disturb the small amount of water left in the bsttom of the #separatoryfunnel. Drain off approximately 5 cc. of the alcohol and water and then shake the remaining alcohol arid petroleum ether gently. Subsequent extractions with 25 cc. of 85 per cent methyl alcohol can be shaken more vigorously without danger of forming emulsions, Finally, extract once or twice more with 25 re. of 90 per cent methyl alcohol. Filter the petroleum ether-carotene solution through anhydrous sodium sulfate, Tvhich is placed over a cotton plug in the stem of a funnel, into a 50-cc. volumetric flask, and make up to volume with petroleum ether. Compare the carotene solution against tlie standard Idye solution as described by Guilbert ( I ) or determine the carotene spectrophotometrically. Smaller quantities of alcohol can be used for the extraction of carotene from the forage if tlie size of the sample is decreased. Reflux a 1- to 2-gram sample with 50 cc. of itlcohol, filter, and wish tlie residue ivith small portions of hot alcoliol through a S o . 3 fritted-glass crucible directly into a 100-cc. volumetric flask containing 5 cc. of 20 per cent potassium hydroxide-alcohol scilution under a bell jar. Proceed as described above.
Results and Discussion Determinations of carotene on commei~ialdehydrated alfalfa meal by Guiliiert's method and the modified method are shown in Table I. More carotene is i.ecovered by the modified method. The difference of the two methods may be due to the loss of carotene during the evaporation of the ethyl ether from the carotene. Results of duplicate analyses of TABLE I.
DETER\fIT %TION O F CAROTENE IX D E H Y D R ~ T E D BY THE GCILBERT AKD MODIFIED METHODS
-ALFALFA
Sample h-0.
Guilbert hl et hod M Q . / l O O 8.
1
2 3
4
7.6 7 8 16 0 6 7
7 1
Modified Vethod
-viu / l o o g. 8.3 8.6 16.5 8.1 8.3
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INDUSTRIAL AND ENGINEERING CHEMISTRY
carotene in dehydrated alfalfa with a carotene content of 10 to 20 mg. per 100 grams are reproducible within 0.1 mg. per 100 grams when the carotene content is determined photoelectric-spectrophotometrically, using Peterson and Hughes’ ( 3 ) extinction coefficient for /?-carotene. T o determine whether the extraction of carotene was complete with 95 per cent alcohol, the residue from the alcoholic extract was analyzed for carotene by the Guilbert and Peterson-Hughes methods. S o measurable amount of carotene was found in the residue. The use of 95 per cent alcohol permits rapid filtering, washing of the alcohol-carotene solution from the forage residue, and the use of definite quantities of reagents in the extraction of carotene. T o prevent the formation of emulsions and to obtain a quantitative extraction of the carotene
VOL. 10, NO. 8
from the alkaline alcohol solution with petroleum ether, it is important that the concentration of the alcohol be between 70 and 75 per cent. At this concentration the separation of the petroleum ether from the water and alcohol phase is clear cut.
Literature Cited (1) Guilbert, H . R., ISD. Esc. CHmf., Anal. Ed., 6, 452 (1934) ( 2 ) Rlunsey, V. E.. J . Assoc. Oficial z l g r . Chem., 20, 459-68 (1937). ( 3 ) Peterson, IT. G., Hughes, J. S., and Freeman, H. F., IND.E s c . CHEW,Anal. Ed., 9, 71 (1937). RECEIVED April 19, 1938. Constructed from a portion of thesis submitted by .issad 52. Khalapur in partial fulfillment of t h e requirements for a master’s degree i n science. Published with approval of t h e Director of t h e I d a h o Agricultural Experiment Station as Research Paper Xo. 167.
Determination of Organic Sulfur in Gas Titration of Sulfate in the Sulfur Lamp with Barium Chloride Using Tetrahydroxyquinone as an Indicator CHANNIIVG W. WILSON A K D WILLIAM A. KEJIPER Research Department, Consolidated Gas, Electric Light & Power Company of Baltimore, Baltimore, Md.
A
X ADAPTATION of the A. S. T. AI. (1) method for the
determination of sulfur in motor fuels, with which it is possible to determine the concentration of organic sulfur in gas, has been described (3). The speed and con\-enience of this method, as ne11 as its accuracy, have prored of great value, and it is now extensively used. I n this procedure, the gas is burned, and the sulfur dioxide formed by the combustion of sulfur compounds in the gas is absorbed in standard sodium carbonate solution. The excess sodium carbonate is titrated with standard hydrochloric acid, and the sulfur concentration is calculated from the amount of carbonate used and the volume of gas burned. Thus, the sulfur is determined through the acidic character of the sulfur dioxide formed. I n some cases this has been found objectionable, since any other constituent of an acid character absorbed by the carbonate will be determined as sulfur, and the result will be too high. The sulfur can be determined gravitrically as barium sulfate, but a t a sacrifice in time and convenience. The present paper describes an adaptation of a procedure for the direct titration of sulfates with hariuni chloride which may be used in conjunction with the sulfur lamp method ( 3 ) . Titration of sulfates with standard barium chloride solution using tetrahydroxyquinone as an indicator has been discussed by Sheen and Kahler ( 2 ) . Such a procedure will overcome the objection noted to the acidimetric titration which has been used heretofore with the sulfur lamp, yet retain the speed and convenience of the sulfur lanip and rolumetric determinations. The accuracy obtainable by the barium chloride titration has been tested, and the results of the experiments are presented here. While the discussion is confined to the determination of organic sulfur in gas, obvious changes in apparatus will make the procedure applicable to the determination of sulfur in motor fuels (1) with an accuracy comparable to that shown for gas
LMaterialsand Reagents Standard barium chloride solution, 1 ml. = 1 mg. of sulfur (7.634 grams of BaCl~2Hr0 per liter). Standardize gravimetrically by precipitation as barium sulfate. Sodium carbonate solution, containing 3.306 grams of the anhydrous salt per liter. Hydrochloric acid, containing 2.275 grams of hydrochloric acid per liter. This and the sodium carbonate solution are the same as used in the acidimetric determination of organic sulfur. I t is convenient to use the strength noted, although exact standardization is not necessary. Methyl orange indicator solution. Tetrahydroxyquinone indicator (THQ, obtained from the W. H. & L. D. Betz Laboratories, Philadelphia, Pa.). Ethyl alcohol, ethyl alcohol denatured by formula 30 or 3-A, or isopropyl alcohol.
Procedure For the determination of organic sulfur in gas the following procedure is now proposed. The metered gas is burned at a rate from 14 to 28 liters (0.5 to 1 cubic foot) per hour, and the products of combustion are absorbed in Bodium carbonate solution in the A. S. T. hl. sulfur lamp (1, 3 ) . At the conclusion of the test, the lamp is washed down Tvith the smallest possible quantity of distilled water, and 3 drops of methyl orange indicator are added. The solution is neutralized with dilute hydrochloric acid which, if standardized, will give an estimate of the sulfur present. A t this point the determination was concluded according t o the earlier procedure. The tan color of the acid methyl orange is discharged with a few drops of the sodium carbonate solution, and 30 ml. of ethyl or isopropyl alcohol are added (2). About 0.22 gram of tetrahydroxyquinone indicator is added, and the solution is mixed ne11 and titrated with standard barium chloride solution. The end point is reached when the color of the solution changes from yelloiv to red, which is permanent with good mixing.
Calculation of Results From the total volume of barium chloride used, 0.05 ml. is subtracted for a blank. Concentration of organic sulfur in the gas is then calculated using the following expression :
