ANALYTICAL EDITION
20
Vol. 5, No. 1
the presence of aluminum does not interfere with the colorimet- acid crystallizes with one molecule of water of crystallizaric determination of tartaric acid in a tartrate baking powder. tion, while d- and I-tartaric acids crystallize in the anhydrous As a matter of interest the application of the colorimetric form. This might suggest some difference between the method to the determination of other forms of tartaric acid chemical properties of racemic acid and the d- and Z-forms was studied. Using d-tartaric acid as a standard, it was of tartaric acid. Meso-tartaric acid also crystallizes with found that I-tartaric acid, I-ammonium tartrate, and meso- one molecule of water, but it gives the same color as the dtartaric acid produce a color equivalent to that of the stand- and Lforms. Hence it appears that the water of crystallizaard. With racemic acid the color intensity was approxi- tion is not a factor in color production. mately one-half that of the standard. With regard to the use of this method, the authors feel that This reaction of racemic acid was surprising. It was certain points should be emphasized. The pH of the standard thought that possibly there might be some union of the d- and of the unknown should be approximately the same at and Z-forms in racemic acid which was causing an inter- about 6.2. The sample taken should be of such a size that ference in the reaction, but molecular weight determinations the color of the standard and of the unknown is approxiby the freezing-point method indicates no such union. With mately the same. The amount of ferrous sulfate used should regard to the purity of the racemic acid used (obtained from be exactly 0.2 cc. The sodium hydroxide solution should the Eastman Kodak Company) it may be said that it was be added as soon as the lavender color appears. The method optically inactive and that it required the theoretical amount is not applicable in the presence of calcium or phosphates. of sodium hydroxide for neutralization. The melting point LITERATURE CITED was 202' C., whereas the accepted value is 205-206" C. A mechanical mixture of equal parts of d- and l-tartaric (1) -4ssoc. Official Agr. Chem., Official and Tentative Methods, p. 307 (19%). acids did not react like racemic acid but gave the proper color intensity. Two different samples of racemic acid were (2) Fenton, H. J. H., Chem. News, 33, 190 (1876). analyzed with identical results. No satisfactory explana- (3) Ibid., 43, 110-11 (1881). tion can be made for this behavior of racemic acid. Racemic RBCBIVBD July 18, 1930. Resubmitted June 17, 1932.
Determination of Organic Sulfur in Gas CHANNING W. WILSON,Research Department, Consolidated Gas Electric Light and Power Co., Baltimore, Md.
N
0 METHOD for the determination of organic sulfur compounds in gas has so far been described which combines the speed and convenience of the method by which it is customary to determine sulfur in motor fuels (1). The apparatus required for this determination is simple, conveniently handled and transported, and a determination requires about 2.5 hours. I n contrast, the well-known Referee method for ,determining organic sulfur in gas requires from 5 to 10 hours for a determination, and the apparatus required is cumbersome to operate and unwieldy to transport. A review (2, 6, 8) of the methods in general use for determining organic sulfur in gas will show that all have these difficulties in common. Huff's platinum spiral method (4) is very convenient when used on gas containing a large amount of hydrogen and rather small quantities of hydrocarbons. However, when the hydrocarbon concentration is high enough to give appreciable quantities of unsaturated compounds on passing over the platinum spiral, the sulfur found will be too great. This method cannot be used a t all on pure hydrocarbon gas, such as refinery gas. With the exception of the platinum spiral method, all procedures are similar in that a measured volume of gas is burned with air or oxygen. Subsequently, the sulfur dioxide formed by the combustion of the sulfur compounds is removed from the products of combustion in a suitable absorber, and is most frequently determined by precipitation as barium sulfate. The barium sulfate determination requires by far the greatest portion of the operator's attention, and while its accuracy is great, the time spent on this step of the determination may not be justified in view of the errors which may enter a t other points. Although volumetric procedures have been proposed (S), they are seldom used. Lieber and Rosen have developed a new modification of this general method ( 6 ) ,and the accuracy of the procedure as a whole was tested by burning a gas, initially sulfur-free, into which a known amount of pure sulfur compound has
been introduced. The average error of seven tests, in which several different sulfur compounds were burned, was slightly more than 1 grain per 100 cu. ft. of gas. The Bureau of Standards states (2) that with the Referee method " . . . . . the uncertainty of the experiment may be as g e a t as 1 grain [per 100 cu. ft. of gas] on the average of two tests." This investigation was undertaken to ascertain whether or not the A. 8. T. M. procedure for the determination of sulfur in motor fuels could be modified to give a rapid and convenient method for the determination of organic sulfur in gas: It was believed that no accuracy need be sacrificed, while the time required for a determination would be substantially decreased. Further, if additional time were available for the determination, it would be used in burning additional gas and would result in increased accuracy. The accuracy of the procedure as a whole has been checked by burning a gas of known sulfur content. DESCRIPTION OF METHOD The apparatus used is substantially that recommended in A. S. T. M. Designation D 90-30T ( 1 ) for the determination of sulfur in motor fuels. The liquid fuel lamp is replaced, however, by a gas burner, preceded by a regulator and meter. A schematic diagram of the apparatus assembly is shown in Figure 1. I n order that the method may be applicable to gas of any density and calorific value, the burner is constructed with a mixing chamber in which primary air from a compressed air line is mixed with the incoming gas. The supply of air is regulated by a needle valve so that a steady Bunsen flame is obtained, with well-defined blue inner cone. Secondary air for the flame is supplied by gentle suction on the absorber, as in the method used on gasoline. The products of combustion are drawn through standard sodium carbonate solution contained in the absorption tube recommended by the A. S. T. M. The sulfur dioxide formed by the combustion of the sulfur compounds in the gas is
January 15, 1933
INDUSTRIAL AND E N G I N E E R I N G CHEMISTRY
absorbed by the carbonate solution, and is oxidized to sulfate by the excess air, according to the equations SO2 Na~C08+'NazSOs CO, 2NazSOa 0 2 +2NasSOd
+ +
+
At the end of a determination, the excess sodium carbonate is titrated with standard hydrochloric acid solution using methyl orange as an indicator. The specifications of A. S. T. M. D 90-30T with respect to titration of the solution are followed. The standard solutions used have the following concentrations: 1. Na2COs: a solution containing 3.306 grams of the anhydrous salt per liter. One cubic centimeter of this solution is equivalent t o 0.001 gram of sulfur. 2. HC1: a solution containing 2.275 grams of hydrochloric acid per liter. Ten cubic centimeters of this solution neutralize exactly 10 cc. of the sodium carbonate solution.
21
To carry out a determination of the accuracy of the method about 2 cc. of one of the solutions were introduced into a clean bubbler tube, shown in Figure la. The exact quantity of the solution was determined by weighing the bulb before and after filling, and from this weight the weight of sulfur introduced into the gas was calculated. The bubbler was then inserted in the gas train in the position x-x, Figure 1. The stopcocks were opened, the gas turned on a t a rate of 0.1 cu. ft. per hour, and the burner lighted. The chimney of the absorber was immediately placed over the flame, and the products of combustion drawn through the absorption
Using solutions of this strength, the sulfur content of the gas burned is equal to (cc. Na2C03 taken - cc. HC1 used) x 10-3 grams, The concentration of sulfur in the gas in grains per 100 cu. ft. is given by the expression: cc. NazCOItaken - cc. HCl used Sulfur concentration = 1.543 cu. ft. of gas burned The quantity of sodium carbonate solution introduced into the absorber a t the beginning of an experiment is dictated by the estimated concentration of sulfur in the gas to be burned. If the concentration is no greater than 75 grains per 100 cu. ft., and 0.2 cu. ft. of gas is to be burned, 10 cc. of the sodium carbonate solution are sufficient. The volume of solution in the absorber is always made up to 20 cc. with distilled water. Since the determination of sulfur by this method depends on an acidimetric titration, the gas to be burned must be ammonia-free. If ammonia were present, an indeterminate quantity of acidic oxides of nitrogen would be formed, which would react with the sodium carbonate in the absorber and give a high result for sulfur.
DETERMINATION OF ACCURACY The accuracy of the method was checked in a manner similar to that used by Lieber and Rosen (6). A sulfur-free hydrocarbon gas was burned a t a rate of 0.1 cu. ft. per hour. This gas had a calorific value of about 2400 B. t. u. per cu. It., so that about 240 B. t. u. per hour were dissipated in the absorber. If the gas is burned a t a greater rate, the glass chimnex and absorber become unduly hot. The rate of flow of the gas was measured by a calibrated capillary flowmeter. A known amount of a pure sulfur compound was introduced into the gas stream and burned in the apparatus. The quantity of sulfur determined by titration of the absorption solution was compared to the amount introduced. The pure sulfur compounds selected to check the determinations were obtained from the Eastman Kodak Company, with the exception of carbon disulfide which was Baker analyzed c. P. grade, and were used without further purification. The sulfur compounds were dissolved in acetone or benzene to give solutions of the concentrations shown in Table I. The dilutions were made by weighing the sulfur compound into a weighed quantity of solvent. Blank tests on the solvents showed them to be entirely sulfur-free.
PRESSURE
METER,
P:Y4:,Aley7
BURNER
EEGULATOR
ASTM. SULFUR LAMP
FIGURE1. APPARATUS ASSEMBLY FOR SULFUR DETERMINATION
solution by suction of a water aspirator pump. The test was continued until all the solution had evaporated from the bubbler, and then the gas was burned for about 15 minutes longer to clear the lines of the sulfur compound. From the flowmeter reading and elapsed time, and the calculated volume of the vaporized solution, the total amount of gas burned was obtained. The sulfur content of the gas burned was found by titration of the solution in the absorber, and the concentration of sulfur in the gas in grains per 100 cu. ft. was calculated. The sulfur concentration in the gas varied from about 10 to 75 grains per 100 cu. ft.
TABLErI.
RESULTS OF
ACCURACYDETERMINATIONS
SULFUR COMGas SULFURSULFUR SULFUR SULFUR POUND BURNEDSOUQHTFOUNDSOUQHT FOUND Cu. j t .
Mg.
Mg.
CSn
csn csz csz
0.318 0.213 0.318 0.256
11.89 8.93 11.89 9.59
11.56 8.94 11.87 9.63
CrHiiSH CsHiiSH
0.172 0.172
8.12 8.31
8.05 8.36
C4H4S C~HIS
0.317 0.318
11.24 11.66
(CzHs)zS 0.36s (CzHs)zS 0.210 (CzHs)zSz 0.172 (CnH6)zsz 0.172
(CzHs)zSz0.172 (CzHs)zSz 0.172 CzHs)zSz 0.222 CzHs)zSz 0.244
I
ERROR G r a i n d l 00
cu. j t .
Mg.
cu. j t .
56.2 64.6 57.8 58.0
0.33 0.01 0.02 0.04
1.5 0.0 0.1 0.2
72.8 74.5
72.2 75.0
0.07 0.05
0.5
11.24 11.82
54.7 56.6
54.7 57.3
0.0 0.16
0.0 0.7
7.59 4.35
7.53 4.30
31.9 31.7
31.6 31.6
0.06 0.05
0.3 0.1
1.15 1.15 1.15 1.15 6.02 8.22
1.25 1.15 1.10 1.19 5.94 8.36
10.3 10.3 10.3 10.3 41.8 52.0
11.2 10.3 9.9 10.7 41.2 52.9
0.10 0.0 0.05 0.04 0.08 0.14
0.9 0.0 0.4 0.4
Grains/l00 57.7 64.6 57.7 57.8
0.6
0.6
0.9
The results of the accuracy determinations are given in Table 11. The first column gives the type of sulfur compound TABLEI. COMPOUNDS USEDTO CHECK DETERMINATIONS introduced into the gas for the test. The second shows the SULFUR SULFUR IN SULFURCOMPOUND SULFUR total volume of gas burned-that is, the measured volume PURECOMPOUND SOLVENT IN SOLN. IN SOLN. COMPOUND of hydrocarbon gas plus the calculated volume of vapor % % % obtained from the sample of solution of the sulfur com84.2 Benzene 0.81 0,682 0.515 30.8 Acetone 1.67 pound. The remainder of the table requires no explanation. 52.4 0.696 Acetone 0.365 0.139 0.073 The average of column 7 is 0.075 mg., and of column 8 is 0.45 1.23 0.437 35.6 Benzene grain per 100 cu. ft. With one exception the error was 38.1 1.78 0.678 Benzene
ANALYTICAL EDITION
22
never as great as 1 grain per 100 cu. ft. Thus it, is apparent that this method is as accurate as any in general use, regardless of whether volumetric or gravimetric methods are used.
DISCUSSION The data in Table I1 show that the experimental errors observed are smaller than the maximum error possible due to errors in reading the burets. I n the entire procedure, four buret readings are required. If the burets can be read to *0.02 cc., and if the maximum error is made in all four readings in such a way that they all affect the result in the same direction, the result will be in error by *0.08 cc. This is equivalent to +lo8 mg. of sulfur. If 0.1 cu. ft. of gas is burned, this quantity of sulfur is equivalent to * 1.24 grains per 100 cu. ft., but if a greater volume of gas is burned for a determination, the absoIute error which may be caused by errors in buret readings is reduced proportionately. Another source of error in the determination is the methyl orange end point. This error is indeterminable, but it can be minimized by comparing the color of the solution being titrated with the color of a portion of the solution a t the end point found on standardization of the solutions. An operator with a little practice, however, usually finds no difficulty in determining the end point to within a drop of hydrochloric acid solution, as the experience of operators using the A. S. T. M. method for motor fuels indicates. This represents an error of about *0.7 grain of sulfur per 100 cu. ft., when 0.1 cu. ft. of gas is burned in a determination. Errors in metering the gas burned are common to all methods, and therefore no special mention is required here. These errors have been considered by the Bureau of Standards
Vol. 5, No. 1
Thus, from theoretical considerations, as wcll as from actual determinations, it is believed that this volumetric method is as accurate as any method heretofore used for the determination of organic sulfur in gas. The speed and convenience of the procedure described in this paper is unsurpassed by any of the methods in general use in the gas industry. A complete determination needs to extend over a period of less than 2.5 hours, and claims hardly 20 minutes of the operator’s attention, while a Referee’s determination usually requires from two to five times as long. The A. S. T. M. method for the determination of sulfur in motor fuels has been modified to make it applicable to the determination of organic sulfur in gas. The accuracy of the method has been checked by burning a gas of known sulfur content, and it has been found to be as great as that of other methods in general use. The average error of sixteen determinations, in which the sulfur concentration in the gas was varied from 10 to 75 grains per 100 cu. ft., was 0.45 grain per 100 cu. ft. The convenience of this method is greater than that of any other now in use, and the time required for a determination is about one-fourth that required by other methods. LITERATURE CITED Am. Soo. Testing Materials, Tentative Standards, p. 391 (1930). Bur. Standards, Cire. 48, 129 (1916). Hollinger, 2. anal. Chvm., 49,84 (1910). Huff, Proc. Intern. ConJ Bituminous Coal, 2nd Conf., 1928, Vol. 11, p. 814. ( 5 ) Lieber and Rosen, IND. ENQ.CHEM.,Anal. Ed., 4,90 (1932). (6) McBride and Weaver, Bur. Standards, Tech. Paper 20 (1913). RECEIVEID July 30, 1932.
(2)*
Rubber Beaker Rings for Accelerating Evaporation on Steam Bath J. A. SCHERRBR, U. S. Bureau of Standards, Washington, D. C.
THE
rate of evaporation of a liquid from a beaker placed on top of a steam bath is notoriously slow. If, however, the beaker is supported so that from two-thirds to threefourths of its length is immersed in the bath (Figure l),the rate of evaporation can be increased threefold and made fully as rapid as from a porcelain evaporating dish. A convenient support is a rubber ring which fits the beaker snugly enough to hold it in the desired position, and yet slips on and off easily when wetted.
or Berzelius form on a set of porcelain steam-bath cover rings having the specified openings. The rings must be made from rubber compounded to withstand the action of heat and moisture, because ordinary grades of rubber deteriorate in a few days on the steam bath. A satisfactory fomuIa, in parts by weight, is as follows: Crude rubber 100 Zinc oxide “Kadox” variety 100 Phenyl-p-naphthylamine 1 Stearic acid 2 Tetramethylthiuram disulfide 3
-
Total
FIGURE 1. BEAKERSUPPORTED BY RUBBERRING The size of the ring is determined by the size of the beaker and the difference between the outside diameter of the beaker and the next larger opening in the steam bath. The inside diameter of the ring should be 3 to 5 mm. smaller than the outside diameter of the beaker in order to hold it firmly, and the sectional diameter must be large enough to seat the beaker snugly. The schedule of sizes shown in Table I is suitable for rings to be used with Pyrex beakers of the usual
206
Rings of this composition made and vulcanized for 30 minutes a t 125’ C. showed no swelling, tackiness, or other evidences of deterioration after 500 hours’ use on a steam bath. TABLEI. SIZESOF SUITABLE SIX-RING SET BEAKER^
Capaoity
MZ. 150 250 400 000 800
1000
Outside diam. Mm. 67 68 77 88 99 107
RUBBERRINGS STEAM-BATH Inside Outside O P ~ N I N G diam. diam. Mm. Mm. Mm. 63 54 72 80 65 96 80
111 111 111
74 86 96
104
90
120 120 120
RECEIVEID August 19, 1932. Publication approved by the Acting Director of the Bureau of Standards of the U. S. Department of Commerce.
