I N D U S T R I A L A N D ENGINEERING CHEMISTRY
February, 1928
20 1
Determination of Sulfur in Volatile Fuels's' H. T. Kennedy BUREAUOF S T A N D A R D S . WASHINGTON. D. C.
EVERAL methods have been developed for the determination of sulfur in fuels. These include the A. s. T. hI. lamp method (D-90-26T), the -4.S. T. M. bomb
S
method (D-129-22T) and the Lomax3 method. The first of these usually gives fair results, is reasonably rapid, and has therefore been adopted as a tentative standard by the American Society for Testing Materials. On certain samples, however, especially those containing carbon disulfide, large deviations are obtained when tests on the same material are made by different operators. The following data,4 obtained on samples 1, 2, and 3, tested by members of SubCommittee VI1 of Committee D-2 on Sulfur Determination and Differentiation of this society, illustrate this point:
2 3 4 5
0.190 0.192 0.15
SAMPLE 2 0.157 0.150 0.152 0.040
6
0 : i94
o:i45
0.192
0.120
LABORATORK 1
7
SAMPLE 1 0.192
SAMPLE 3 0.201 0.203 0.179 0.300
0:201 0.223
As will be observed, the general agreement on sample 1 is excellent. On samples 2 and 3, which are known to contain carbon disulfide, the deviations are too large when it is considered that these figures are the average of two or more determinations from each laboratory. Since the sulfur compounds that occur in motor benzene usually include carbon disulfide, the failure of Method D-90-26T to determine correctly sulfur present as carbon disulfide is a serious defect. The bomb method, although probably accurate when adequate precautions are taken, has not proved so in practice when applied to motor fuels. It is also time-consuming and not suitable for routine work. However, it is one of the few methods available for products of low volatility. The Lomax method, similar in principle to Method D90-26T, appears to correct the tendency of the latter to give low results on carbon disulfide mixtures, but is extremely tedious. Further, as barium sulfate is precipitated from fairly concentrated (5 per cent) solutions of sodium chloride, it is not impossible that the higher results are due in part to adsorption of sodium chloride by the barium sulfate. In the method described in this paper, the fuel is vaporized in a carbureting device and burned, and the sulfur oxides so produced are absorbed by a sodium carbonate solution of known concentration.
90 degrees, this device affords a delicate and convenient means of adjusting the small flow necessary to maintain a flame. The fuel then flows on a small plug of glass wool, C, from which it is vaporized by a current of hot air passing through the air regulator, D, and the mixture is burned a t the tip, E. The air regulator D consists of a cock, the lower edge of which is ground off spirally, and whose position relative t o its seat determines the length of a narrow slit which is arailable for the passage of air. As an angle of nearly 360 degrees can be utilized, this device also affords a convenient means of regulation. The vaporization chamber is surrounded by a nichrome heating coil of 65 ohms resistance, designed to vaporize the fuel completely and insure that no unvaporized fuel is left on the glass wool. The proper temperature is secured when two sulfur lamps are connected in series across a 110volt line. The vaporization chamber and heater are mounted in an aluminum casting for convenience in handling. Procedure
To determine sulfur by means of this lamp, the heater is switched on a few minutes before the run is to be started, the reservoir is filled conveniently by means of a pipet somewhat above the upper graduation with the fuel to be tested, the fuel and air regulators are adjusted, and the flame is lighted, preferably with an alcohol flame since matches contain a large amount of sulfur. The flame should be approximately
-
J?-OPPE,,
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---- --
Apparatus
The accompanying drawing shows the essential parts of this apparatus. The fuel to be examined is contained in and measured by the reservoir A , making the weighing of the lamp before and after burning unnecessary. From the reservoir the fuel flows through the special regulator, R, which consists of an unbored stopcock, on the cylindrical surface of which is etched a tapering channel. By turning this cock, a channel of varying average width is available to conduct the fuel from the reservoir t o the vaporizing chamber and, as it can be utilized through an angle of nearly 1 Received September 27, 1927. Published by permission of the Director of the National Bureau of Standards. J . Ins!. Pefroleum Tech., 10, 914 (1924). 4 Proc. A m . SOC.Tesfing Maferials, 95, 269 (1925).
'
*
inch (13 mm.) high and colorless or slightly yellow. When the fuel meniscus reaches the upper graduation, the flame is placed under an absorber, as in Method D-90-26T, the suction on which is so adjusted as to accommodate all the products of combustion. No further attention is needed until the meniscus is approaching the lower graduation, when the fuel regulator is so adjusted that the flame is approximately the same height as a t the start. This step is necessary to eliminate the effect of a slight lag in vaporization from the glass wool. When the lower mark is reached, the
Vol. 20, No. 2
I.VDUSTRIAL AND ENGINEERING CHEMISTRY
202
flame is removed from under the absorber, the solution in which is then titrated as it1 Method D-90-26T. For more accurate work it is titrated until its color matches that in a similar absorber in which 10 cc. of standard sodium carbonate, 10 cc. of standard hydrochloric acid, and sufficient methyl orange have been mixed. One cubic centimeter of each solution is equivalent to 1 mg. of sulfur, and the same amount of methyl orange is present in each absorber. Since the volume of sample burned is always 5 cc., the weight burned may be calculated from the specific gravity. For most unblended fuels only a small error is introduced if the figure 3.75 is assumed as the weight burned. The sulfur content may be expressed on a volume basis-i. e., as milligrams per liter, or grains per gallon-without measuring or assuming a value for the specific gravity. As soon as the fuel has completely run out of the reservoir, another run may be started. It is unnecessary to rinse or clean the apparatus. When working with very volatile fuels, it is sometimes advisable to decrease the heating current by connecting an auxiliary resistance in series. This is necessary, however, only in the rare case that a fuel boils in the reservoir when the full heat is maintained. It is also sometimes necessary, with such fuel or fuels containing dissolved gases, to place a few threads of glass wool in the tube leading from the fuel regulator to the vaporization chamber, so that bubbles of vapor forming there will not interfere with the smooth operation of the burner. As these threads cause no inconvenience when working with other fuels, they may be left in place permanently.
Variation of the size of flame has little, if any, effect on tho results. It is convenient, however, to adjust the air and fuel regulators so that the flame is initially about l / Z inch (13 mm.) high and colorless or slightly yellow a t the tip of the inner cone. This flame is steady and does not overheat the glass chimney. As the fuel is burned nearly four times as fast in this method as in Method D-90-26TI it is necessary to increase the suction on the absorber to accommodate the larger amount of products of combustion, Advantages of Method (1) Accurate determination of sulfur, either elementary or in any form of combination which occurs in motor fuels, naphthas, and similar products. (2) Precision. The average deviation, based on two runs on each of fifty samples containing from 0.04 to 0.17 per cent by the same operator was 0.0013 per cent of sulfur. The average deviation for different operators, based on available data of Sub-committee VII, is 0.0034 per cent of sulfur, as compared with 0.0142 per cent when the same samples were tested by the same operators using Method D-90-26T. Further data are being obtained. (3) Rapidity. The average time consumed in making one run by the new method is 45 minutes. This is less than half the time consumed by any other method. As practically no attention need be paid to the apparatus between the times of starting and titrating, a large number of tests may be run simultaneously, or other work may be attended to while the test is proceeding.
Vitamins in Canned Foods’ VI-S trawberries E. F. Kohman, W. H. Eddy, and Nellie Halliday NATIONAL CANNERS .%SSOCIATION,
I
WASHINGTON,
D. c., AND TEACHERS
h- T H E previous papers of this scries2 it has been amply
demonstrated that the major factor in destruction of vitamin C in cooking and canning of foods is oxidation. I n fact, it has been shown that in canning, after a certain amount of preliminary destruction, subsequent heating has very little effect in the way of further destruction. I n the canning of tomatoes there is either no preliminary destruction or it is of such small magnitude that it has not yet been demonstrated. With apples and peaches it was shown that after a preliminary treatment of these fruits, in which their respiratory process was utilized to deplete their oxygen supply and any intermediary respiratory oxygen, if such exists, then canning could proceed with no demonstrable loss of vitamin C. These results indicate that if vitamin C were heated in a suitable medium entirely free from oxidative factors, it would be unaffected by temperatures met with by canning or cooking. Zilva3 subjected the highly potent concentration of vitamin C obtained from lemon juice to a temperature of 140’ C. for several hours in an oxygen-free atmosphere, and under those conditions could not demonstrate any destruction in potency. Little is known about the detailed mechanism of the respiratory process in vegetable tissue whereby oxygen is consumed and carbon dioxide evolved. Analogy, honTever, strongly 1 Received
November 11, 1927. Ind. Eng. Chcm., 16, 52, 1261 (1924); 17, 69 (1925); 18, 85, 302 (1926). 1 Private communication to one of the authors. 9
COLLEGE, C O L U M B I A UNIVERSITY,
NEW
YORK,
N. Y.
suggests that there must be an intermediary form of respiratory oxygen and that this might exert an oxidative effect on vitamin C. This experiment with strawberries was planned with the hope that some such effect might manifest itself unless the process of “exhausting” in commercial canning was adequate to nullify any such effect. It is known that when fruits are held a t a low temperature and then transferred to a warmer temperature the evolution of carbon dioxide is more rapid for some time than it would be if held continuously a t that particular temperature. This may be explained in part by the fact that the oxygen content of the gas in fruits is higher in cold storage owing to the inhibition of oxygen consumption. It is held by some to mean that during this period in cold storage the fruits accumulate intermediary respiratory oxygen, which when transferred to a warmer temperature gives rise t o a more rapid formation of the carbon dioxide. I n the exhaust box used in canning, fruits, with their oxygen supply cut off by the sirup surrounding them, are gradually warmed to above the temperature a t which enzymes cease to function. Before this is reached the respiratory processes are greatly accelerated, and if the time before this is reached is adequate, all available oxygen is consumed. It would be desirable to know the approximate vitamin content of all food products and how this is affected by the conditions that each meets with during distribution and preparation for consumption. The laborious methods of
