385
J U N E 1947 Table I . Titration of Ferrous Iron in Sulfuric Acid Solution Using Bipyridine Ferroin as Indicator (25.00 ml. of approximately 0 . 1 ferrous sulfate per titration. Titration volume 150 ml. Sulfuric acid concentration, 1 formal, with sulfato-ceric acid as oxidant) 0.1 N Sample C e + - + + No. Required Test of Indicator Stability
M1. 1
21.50 Indicator reversibly oxidized a n d reduced 12 consecutive times a t equivalence point without diminution of color intensity
2
21.50 One drop excess oxidant added a t equivalence point. After 10 minutes stirring indicator color was restored using one-drop excws of ferrous iron solution
3
21.50 0.5-ml. excess of cerate oxidant added. 7 minutes’ stirring did not destroy indicator, as shown b y addition of minute excess of ferrous iron
4
21.51 1 .O-ml. excess of cerate oxidant added. 5 minutes’ stirring did not destroy indicator, a s shown b y same test with excess ferrous ion
5
21.49
One-drop excess of oxidant followed by stirring for 2 hours was required t o destroy indicator property of reversal in color upon reduction
hydrogen atoms bonded to carbon. the nitrogen is likewise not likely.
Direct oxygen bonding to
IYDICATOR CHARACTERISTICS Ih P R E S E b C E OF HYDROCHLORIC ACID
The data given for sulfuric acid solutions provide conditions suitable to the use of the Jones (amalgamated zinc) reductor for the preparation of ferrous sulfate solutions for oxidimetric determination. In order to use the Walden silver reductor for similar purposes it is necessary to xork in the presence of hydrochloric acid. Titrations of the same nature as those given in Table I, except for an approximately 0.1 S solution of ferric chloride in 1 formal hydrochloric acid, were taken and reduced in a Walden silver reductor and titrated, using the same cerate solution as previously employed. The results were of equal accuracy. In this case, however, the color change is from orange to yellow because of the color of complex ferric chlorides formed. It is not satisfactory t o add phosphoric acid to form a complex ferric iron if quadrivalent cerium is to be used as oxidant because of the formation of insoluble cerium phosphates. LITERATURE CITED
The instability of the indicator in its oxidized form may be due to a combination of effects. If the bipyridine ferrous ion, upon oxidation to the ferric form, dissociates to give 2,2’-bipyridine and ferric ions, these may fail t o recombine when the ferric ion is again reduced because of the low pH of the solution. Another possible explanation consists in the plausible oxidation of the 2,2’-bipyridine to form addition products which no longer form complexes with the ferrous ion. The former explanation is the more acceptable, since the pyridine ring is known to be extremely stable in resisting both oxidation and substitution for any of the
Blau, Monatsh., 19, 647 (1898). Bode, Wochschr. Brau., 50, 321 (1921). (3) Gerber, Claassen, and Boruff, ISD.EKG.CHEM., .ISAL. ED., 14, (1) (2)
364 (1942).
Gray and Stone, Ibid., 10, 415 (1938). Harrison, J . Assoc. Oficial Agr. Chem., 24, 215 (1941). Hill, Proc. Roy. Soc. London, B107,208 (1930). (7) Moss and Mellon, IND. ERG.CHEY.,ANAL.ED., 14, 862 (1942). (8) Moss, Mellon, and Smith, I b i d . , 14, 931 (1942). (9) Smith and Richter, “Phenanthroline and Substituted Phenanthroline Indicators,” Columbus, Ohio, G. Frederick Smith Chemical Co., 1944. (4) (5) (6)
Determination of Carbon and Hydrogen in l tes Petroleum Distila A Lamp Technique M. c. sImIonTs* P e t r o l e u m Experiment S t a t i o n , Bureau of Mines, Bartlesville, Okla. A method determining simultaneously the carbon and hydrogen content of organic materials, such as petroleum distillates, is based on their combustion in a special lamp suitable for aromatics as well as other hydrocarbon types. The lamp combustion procedure simplifies difficulties normally encoun-
C
HARACTERIZATIOK of petroleum distillates often requires accurate determinations of carbon and hydrogen. However, customary methods of analysis for carbon and hydrogen, which involve burning the sample from a boat or other container within a combustion tube, become tedious and time-consuming a h e n volatile materials must be analyzed. Recognizing the inadequacy of these Liebig-type determinations for petroleum distillates, Hindin and Grosse (1) recently reported an analytical method for determining the hydrogen content of such materials which simplifies their combustion by using an -4.S.T.M. method D90-41T sulfur lamp. The procedure mas intended primarily for rapid, routine analyses, and relatively large samples \\-ere 1 Present address, Houston Refinery Resrarch Laboratnries, Shell Oil Company, I n c , Houston. Tex.
tered in the ultimate analysis of volatile samples, and refinements improve accuracy and permit use of relatively small samples. Routine determinations can be made, and maximum deviation for repeated analyses from mean is about 0.01% when 0.5- to 1.0-gram samples are used.
employed to obtain good precision and agreement with theoretical values-for example, the average deviation of repeated analyses from the mean was about 0.027, when 3- t o 6-gram samples were used. The lamp employed by Hindin and Grosse was not suitable for burning compounds such as aromatics, because a smoky flame was obtained, nor for the simultaneous determination of carbon, apparently because of incomplete combustion. The authors pointed out these difficulties with their procedure and indicated means whereby it might be improved. The present paper describes an improved lamp technique for the determination of both carbon and hydrogen, based largely on the suggestions made by Hindin and Grosse. The technique comprises: (1) burning the sample in a lamp suitable for all volatile petroleum oils, including aromatics; ( 2 ) completing the
-
V O L U M E 19, NO. 6
386 oxidation of combustion products from the lamp by passage through a hot, packed combustion tube; (3) absorbing water and carbon dioxide in the customary manner; and (4)making corrections and blank determinations to increase precision. A desirable balance of sample size, burning rate, and precision is achieved which furnishes carbon and hydrogen analyses of suitable reproducibility for most requirements with a reasonable time consumption per analysis. Once the apparatus is assembled and the analyst becomes familiar with the technique, determinations can be made in a routine fashion, requiring only 2 hours for each analysis completed. The average deviation for repeated analyses from the mean is about 0.01% carbon, or less, when 0.5- to 1.0-gram samples are used, and it is believed that increased precision might be obtained, when required, by using larger amounts of sample. The lamp used in the present work is similar to the one described by Javes (2) for estimating the sulfur content of aromatic fuels. Its principal feature is a provision for introducing primary air into the PRIMARY wick a t a point below the flame level, and this procedure gives a smoke-free flame, even with samples containing only aromatic-type compounds. Tests by Javes in developing thelamp indicate that it can be used to burn such materials as gasoline, kerosene, and light gas oil without introducing errors caused by fractionation of the sample within the lamp wick; therefore, use of the lamg for carbon-hydrogen analyses of light gas oils should be possible, although tests on such materials are not available a t this writing. The lamp burns at a higher rate than the ordinary sulfur Lmp, and the required amount of sample (about 1.0 gram) can be consumed easily in a 30-minute combustion period. Other features and advantages of the lamp technique are given in the sections which follow. APP.4RATUS
A detailed diagram of the apparatus used in making carbonhydrogen determinations by the lamp technique is given in Figure 1. The apparatus consists of a purification system for primary and secondary air; the lamp and its chimney; a two-element combustion furnace containing a quartz tube packed with sections of copper oxide and lead chromate; an absorption train consisting of four bulbs; a rotameter for measuring the flow rate; and a regulator for maintainin2 a constant pressure in the lamp chimney. Additional requirements include a source of compressed air, a supply of pure helium, a vacuum pump, and a precision analytical balance supplied with calibrated weights.
Primary and Secondary Air Systems. During combustion the lamp burns a t substantially atmospheric pressure in a stream of secondary air. Primary air mixes with the sample in the wick prior to ignition and must be available a t a slightly higher pressure. The purification systems for the primary and secondary air streams are identical. Compressed air passes through a needle valve into a quartz tube packed with wire-form copper oxide in the section heated by an electric furnace. The furnace temperature is controlled by a variable-voltage transformer, and the temperature of the copper oxide section is measured by a Chromel-Alumel thermocouple placed in the middle of the furnace in the annular space between the tube and furnace element. The tube extends 15 cm. (6 inches) beyond each end of the furnace, and the exit is connected to the first of the series of absorption tubes A , B, and C, using spring-clamped standardtaper glass joints for all connections where possible. The absorption tubes measure approximately 3.78 cm. (1.5 inches) in diameter and 38 cm. (14 inches) in length. Tube A is charged with anhydrous magnesium perchlorate, tube B is charged with 20- to 30-mesh Ascarite (a commercial mixture of sodium hpdroxide and asbestos), and tube C is charged with anhydrous magnesium perchlorate with a small top layer of phosphorus pentoxide, separated from the perchlorate with dry, acid-washed asbestos. The charge of each tube is supported by a layer of glass wool in the
PRESSURE
*,,
1 AIR
REGULATOR
4
SYSTEM
Figure 1.
Diagram of Apparatus
bottom, over the inlet, and a thin mat of asbestos followed by a layer of glass wool is placed in the top of each tube over the absorbent to prevent loss of solid material. Tube C of the primary air system is connected by means of gum-rubbp. tubing to a rotameter for measuring the flow of primary air. The rotameter, in turn, is connected by gum-rubber tubing to the primary inlet of the lamp. Tube C of the secondary air system is connected to the side arm of the lamp chimney by a standard-taper glass joint. A tee and stopcock in this line provide a manometer connection for measuring and controlling the pressure within the lamp chimney. Lamp and Chimney. Figure 2 shows details of the lamp and chimney. The lamp is constructed of glass and is provided with standard-taper fittings for connecting or closing all outlets. The wick-stem portion of the lamp is detachable from the sample bulb. This arrangement facilitates cleaning the lamp, and the wick stem is made accessible for threading and adjusting the wick. Connection of the lamp bulb to the wick stem is made with a 12/5 spherical joint. Primary air is introduced into the wick through the 0.5- to 1-mm. separation in the wick tube. The annular space surrounding the wick tube provides a convenient means of transmitting the primary air from the 5/20 standard-taper joint to the proper point of entry into the wick. The modified 7/25 male joint a t the top of the lamp, upon which the flame rests during combustion, is produced by cutting off about 8 mm. of a standard joint on the large end before sealing it to the wick tube. This serves to shorten the distance b e h e e n the point of entry of primary air and the flame. The seal is made uniform, so that the internal diameter of the wick tube is unchanged, and the standard-taper cap for closing the end of the wick tube is cut to fit the shortened male joint. All glass joints are given an additional lapping with fine Carborundum or Turkish emery. This ordinarily permits leak-free use of the lamp, but for very volatile samples a fine film of lorn-vapor-pressure stopcock grease around the outer edge of the spherical joint is necessary to eliminate loss of sample. When grease is used, care should be taken not to contaminate the sample. Twelve-strand cotton wicking with a braided outside sheath is used; and as in the case of sulfur lamp practice, the number of strands which should be left in the Kick, when threaded through the wick tube, depends t o a large extent on the volatility of the sample. I t is usually necessary to pull all but a fex of the strands from the original twelve-strand wicking, and in some instances the braided wick sheath alone is used. The wick fits loosely and is pulled back into the wick tube approximately 10 mm. below the lamp tip. On very volatile samples, such as isopentane, an ice bath may be placed around the bulb of the lamp to reduce the oombustion rate. The lamp is attached to the chimney at its bottom opening by means of a S o . 7 rubber stopper. During flushing and when the lamp is not in the chimney, the opening is closed with a solid rubber stopper. To produce smooth burning, the top of the lamp, when in the chimney for a determination, is placed approximately 40 mm. above the secondary air inlet6 The outlet of the
JUNE 1947
387
chimney is connected to the quartz combustion tube by means of a No. 3 rubber stopper. Combustion Furnace and Tube. A translucent quartz combustion tube is used which is approximately 2.5 cm. (1 inch) in diameter by 63.5 cm. (25 inches) in length and is drawn down a t the outlet end to facilitate connection to the absorption bulbs. I t is packed with a 20-cm. (8-inch) section of wire-form copper oxide and a 10-cm. (4-inch) section of wire-form copper oxide impregnated with lead chromate, separated by a 2.5-cm. (1-inch) plug of copper gauze. Similarly 2.5-cm. (1-inch) copper gauze plugs are placed a t the entrance and exit ends of the charge to hold it in place and reduce any oxides of nitrogen, should they be formed during combustion of samples containing this element. Provision for sulfur and halides in the sample is made by the lead chromate section. The wire-form copper oxide is impregnated by heating to a dull redness in a stainless steel crucible and sprinkling powdered lead chromate over the hot mass. Separate furnaces are used to heat the copper oxide and lead chromate sections, and each is supplied with a Chromel-Alumel thermocouple for temperature measurement. The temperature of the lead chromate section should not be allowed to exceed 850" F., because lead chromate begins to attack the quartz combustion tube a t this temperature. Variable-voltage transformers are used to control the temperatures of the furnaces. Absorption Bulbs. Four absorption bulbs of the Piesbitt type are connected in series with the exit end of the combustion tube. The bulbs are of approximately the same shape and volume displacement, and in use the first two bulbs absorb the water and carbon dioxide, respectively. The third bulb is used as a guard in the event of incomplete absorption of carbon dioxide by the second bulb, and the fourth bulb serves as a tare. The bulbs are coated n i t h polystyrene enamel to minimize humidity effects during weighings ( 3 ) . Chemical absorbents identical to those used in the purification tubes are used in the absorption bulbs. A layer of glass wool is placed in the bottom of each bulb to support the charge. Then the water bulb is charged with anhydrous magnesium perchlorate, and the carbon dioxide, guard, and tare bulbs are charged with 20- to 30-mesh Ascarite, A small layer of phosphorus pentoxide, separated by layers of dry, acid-washed asbestos, is placed in the top of each of the bulbs to remove last traces of water. An additional layer of asbestos between two small pieces of glass wool is placed in the hollow, ground-glass plug of each bulb to prevent loss of solid material (phosphorus pentoxide). The plugs are lubricated with lon--vapor-pressure stopcock grease, and any excess caught in the exit tubes is removed with a pipe cleaner. When samples of approximately 1 gram are burned, the waterabsorption bulb may be used about six times in succession and the carbon dioxide bulbs about twice. R h e n the carbon dioxide bulb nears saturation it is exchanged with a bulb in the guard or tare position. In this manner, about six complete determinations can be obtained with the original set of four freshly charged DETAIL OF LAMP TIP
bulbs, provided the carbon dioxide absorbers are not allowed to become completely saturated before being replaced by fresh bulbs. The absorption bulbs are connected to a rotameter which is used to measure the secondary air flox a t the beginning of an analysis and the effluent gas from t,he absorption bulbs during combustion. Pressure Regulator. To eliminate variations in the lamp flame during combustion, it is desirable to maintain the chimney a t constant, and substantially atmospheric, pressure. Provision is made for the slight resistance to ROT t,hrough the combustion tube and absorption bulbs by application of reduced pressure to the end of the absorption train. The regulation system used to control t,his pressure, shon-n in Figure 1, is act'uated by an adjustable-contact, control manometer connected to the secondary air inlet of the chimney. The manometer is filled with mercury and changes in the chimney pressure cause the mercury t o make or break the relay circuit, which in turn actuates the normally closed flutter valve on the surge vessel. Crude adjustment of the reduced pressure is made with the needle valve between the surge vessel and continuously operating vacuum pump. Fine adjustment is achieved by the action of the flutter valve. As the absorption bulbs near saturat'ion a larger pressure difference is required; however, to prevent the loss of solid material from the absorption bulbs the pressure should not be reduced more than 20 cm. (8 inches) of mercury below atmospheric, as measured by the surge vessel manometer. Helium Purification. The absorption bulbs are flushed with purified helium before each weighing. Helium is recommended, because its low density reduces the effect of temperature and pressure variations on absorption bulb weights, and it is safer to use than hydrogen, the gas frequently employed. Helium is commercially obtainable in a high state of purity; however, water or carbon dioxide may be present. These impurities are removed by filtration through a series of charged absorption tubes identical to tubes A , B , and C of the primary or secondary air system. The helium cylinder and purification tubes are assembled in a convenient location, and air present, in the system is displaced by purging for about 30 minutes with helium a t 300 cc. per minute. The absorption bulbs are flushed by connecting them in series to the helium system with gum-rubber tubing. Their plugs are not opened until after they have been connected. This reduces the chance of laboratory air being sucked into the bulbs. At the end of the flushing period the helium supply is disconnected and the bulbs are closed so that they are a t approximately atmospheric pressure. EXPERIMENTAL
4 . i
Establishing Equilibrium. ilfter the apparatus is assembled as indicated above and before analyses are made, the furnaces are brought to operating temperature. The primary and secondary air furnaces are maintained a t 1250" to 1300" F., and the copper oxide and lead chromate sections of the combustion tube are maintained a t 1200' to 1250" F. and 750" to 800" F., respectively. Then, the entire system is flushed for not less than 10 hours. During the flushing, the lamp chimney is closed with a solid rubber stopper and the primary air is vented after passing through the rotameter. The extended flushing e s t a b 1i s h e s equilibrium conditions in the absorption tubes, combustion tube, and absorption bulbs, and it is LAMP CHIMNEY necessary to repeat this operation each time the absorbers are Figure 2. Diagram of Lamp and Chimney recharged. Continued equilibA l l dimensions in millimeters
V O L U M E 19, NO. 6
388 rium from day to day is assured by leaving the furnaces turned on overnight with a small amount of air flushing through the apparatus (about 150 cc. per minute in the secondary system and about 75 cc. per minute in the primary system). Blank Determinations. Before freshly charged absorbers are used in an analysis they are subjected to blank determinations: (1) to make sure that the absorbers are in a state of equilibrium, (2) t o check the purity of the primary and secondary air, and (3) to check the purity of the helium. The absorption bulbs are made ready for initial weighing by the 10-hour flushing with secondary air a t 600 cc. per minute. After flushing, the bulbs are allowed t o remain in a desiccator overnight (or for a t least 4 hours), flushed 15 minutes with helium a t 300 cc. per minute, gently wiped with a cle'an dry chamois, set in the dmiccator for 30 minutes, and xeighed. Then the bulbs are connected to the combustion tube and flushed for 1 hour with secondary air at 600 cc. per minute. The bulbs are closed and allowed to stand overnight, and the helium flushing, wiping, setting, and weighing operation is repeated. Allowing the bulbs to stand a minimum of 4 hours before weighings are made is deemed necessary to ensure that temperature equilibrium is reestablished, especially after a sample combustion, because the fast burning rate used causes the water and carbon dioxide bulbs to attain temperatures appreciably above atmospheric. -klthough the temperature rise is not so high during the blank determinations, it is advisable to follow the same practice here also. The change in weight of .the absorption bulb5 caused by the 1hour air flushing is used to check the equilibrium condition of the bulbs and the purity of the secsondary air. Satisfactory equilibrium is assumed when the change in weight of the tare bulb is within 0.3 mg. of the change in weight of the guard bulb. A check on the purity of the primary air is made by connecting it through the lamp chimney, cutting off the secondary air, and flushing the bulbs with a measured quantity of primary air. A similar check is made on the purity of the helium. The water and carbon dioxide content o f ' the primary air, secondary air, and helium found in these blank experiments is used to make corrections on the apparent masses of water and carbon dioxide obtained in an analysis. Results obtained for a large number of these blank determinations have shon-n that, n-ith the chemical absorbents indicated, helium is made substantially free of water and carbon dioxide. The primary and secondary air contain impurities which pass through the purification systems amounting to approximately 0.5 mg. of water for each 1-hour flushing a t 600 cc. per minute but are substantially free from carbon dioxide. Weighing Procedure. The absorption bulbs are weighed to 1 0 . 1 mg. by the method of swings, using calibrated weights. Buoyancy effects are minimized by using a counterpoise of shape and volume equal t o the absorption bulbs. Lead shot is added to the counterpoise until it is approximately 20 grams heavier than the heaviest of the freshly charged absorption bulbs, and the counterpoise is covered with polystyrene enamel, as are the absorption bulbs. The ground-glass plug of the counterpoise is left closed, so that an equal mass of air remains in it during all weighings; when not in use, counterpoise is kept in a desiccator. iifter the bulbs have been flushed with helium, their plugs are closed and are not reopened until they are placed back on the absorption line. Each bulb is weighed by placing i t on the right balance pan with enough weights to balance the mass of the counterpoise placed on the left balance pan. In this manner the change in mass of the absorbers is determined by substitution. Weighings of the sample lamp before and after a determination are also made to d0.1 mg., but since these weighings are made within an hour of each other and buoyancy or humidity variations should be negligible, a counterpoise is not used. The change in weight of the lamp is designated the apparent weight of sample. The true sample weight is obtained by correcting the apparent weight by the fraction of the total combustion period that the lamp actually burns in the chimney. Analytical Procedure. When the system is in condition for use, the absorption bulbs are connected in order, and the carbon dioxide bulb is immersed in a beaker of crushed ice and water. The secondary air rate is set at 600 cc. per minute, and the pressure regulator is adjusted to maintain approximately 1.25 cm. (0.5 inch) of mercury pressure above atmospheric within the lamp chimney. The bulbs are flushed a few minutes t o make sure that the regulator is operating properly. Then, the lamp is lighted
and inserted in the chimney as rapidly as possible, and a notation is made of the t'ime elapsed after the lamp is lighted until i t is in the chimney. The sample is burned for a measured period of about 30 minutes, and a t the end of the burning period the lamp is removed, extinguished by disconnecting the primary air, plugged, and capped. The chimney is also quickly closed with a rubber stopper, and the time elapsed between removal of the lamp and flame extinction is noted. After the lamp is removed, the combustion tube and absorption bulbs are flushed for an additional 30 minutes with secondary air a t 600 CC. per minute. The absorption bulbs are then disconnected, closed, and placed in a desiccator overnight (or for a t least 4 hours). Finally, the bulbs are flushed 15 minutes with purified helium at 300 cc. per minute, gently wiped with a clean dry chamois, allowed t,o remain in a desiccator for 30 minutes, and x-eighed.
It is important to obtain an accurate estimation of the time lost while the lamp is being lighted and extinguished, because this is of utmost importance in estimating the total amount of sample burned in the chimney. Assuming that the total combustion period for a 1-gram sample is 30 minutes (or 1800 seconds), the error caused by a difference of 1 second in estimating the time lost would amount to an error in sample weight of 0.00055 gram. Wit,h practice, it is believed that an experienced operator can consistently estimate the time lost factor to within 1 second. However, this limitation of precision in obtaining a true sample weight makes weighing of the sample lamp to better than * 0.1 mg. unnecessary. True Mass of Water and Carbon Dioxide. According to the method of weighing, the absorption bulbs are filled with helium gas when weighed, a counterpoise of equal volume is used t o cancel air buoyancies, and brass weights are used to equalize the balance. To obtain the true mass of water and carbon dioxide in a determination, the apparent weight of water and carbon dioxide must be corrected to vacuum. These corrections include both the buoyancy effect of air on the brass weights and compensations for the decrease in mass of contained gas caused by an increase in volume of the solid phase in the absorption bulbs during the absorption process. The values, when calculated in similar manner to those given by Rossini (4, reveal that the true mass of water is m (H,O) = 1.00000 X apparent M (HZO) and the true mass of carbon dioxide is
m (GOz) = 0.99993 X apparent M (GO,) The correction for water is assumed to be exact to within 1 part in 100,000, whether the anhydrous magnesium perchlorate goes to the tri- or hexahydrate. Similar accuracy is assumed for the carbon dioxide factor. The nearness to unity of these factors
Table I. Calculation of Results of a ' Carbon-Hydrogen '4nalysis Itlade on Iso-octane (2,2,4-Trimethylpentane) H20 CO? Guard Tare Bulb
Adsorption bulb d a t a Original weight. grams Final weight, grams Difference T a r e correction
Bulb
Bulb
Bulb
12.8426 11.8611 -0.0001 0.9815
8.4003 11,9109 6,2736 11.9109 2.1267 -0:0001 0.0000 -0.0001 2.1266 - 0.0001 0.9814 0.0005 0.0000 0.9809 2.1266
12.699s 12.6994 0.0001
Blank correction B ~ u a r e n tweight, grams Sam-ple recovered Weight of carbon recovered = 0.27289 X 0.99993 X 2 . 1 2 6 6 = 0.58029 Weight of hydrogen recovered = 0.11 190 X 1,00000 X 0.9809 0.10976 Total Sam le recovered 0.69005 Sample weiggt data Original lamp weight, grams 81.3750 Burning time = 30 min. Final lamp weight, grama 80.6786 1800 seconds Apparent sample weight, grams 0.6964__Total time lost = 16 seconds Estimatedsample weight, grams = - X 0.6964 = 0.6902 Estimated Sample Sample Recovered Basis Weight Basis 84.08 Per cent carbon 84,094 15.90 Per cent hydrogen 15.906 99.98 Total 100.000 5.29 Carbon-hydrogen ratio 5.287
-
-
J U N E 1947
389 Table 11.
Material Iso-octane
Results of Analyses &lade on Pure Compounds
Grams
Burning Time Sec.
Time Lost Sec.
Estimated Sample Weight Grams
0.6945 0.8963 0.8276 0.9169
1680 1800 1800 1800
20 16 7 6
0.6862 0.6901 0.8243 0.9138
Amount Burned
Amount Recovered Carbon Hydrogen Total Grams Gram Grams
Recovered Basis Carbon Hydrogen 76
76
0.57688 0.10903 0.58030 0.10975 0.69326 0.13106 0,76835 0,14520
84.104 84.095 84.101 84.106 84.102 10.005 84.115 -0.013 91.215 91.221 91.212 91.221 91.222 91.218 10.004 91.248 -0.030 83.261 83.235 83.248 1.0.013 83.234
15.896 15.905 15.899 15.894 15.898 *0.005 15.885 +0.013 8.785 8.779 8.788 8.779 8.778 8.782
0.68591 0.69005 0.82432 0,91355
Average Precision (average deviation from mean) Theoretical Accuracy. .(deviation f r o m theoretical) Toluene
0.6527 0.9004 0.9751 0.0758 1.3543
,
1200 1500 1800 1800 1680
10 8 15 6 5
0.6473 0,8956 0.9670 0.9725 1.3542
0.58029 0,81704 0.88169 0.88682 1.23545
0,05685 0,07863 0.08495 0.08535 0,11888
0,64714 0,89567 0.96664 0.97217 1.35433
Average Precision (average deviation from mean) Theoretical Accuracy (deviation from theoretical) Isopentane
..
..
.... ....
0,81217 0,75814
0 16328 0 15270
0.97545 0.91084
llverage Precision (average deviation from mean) Theoretical Accuracy (deviation from theoretical)
when helium is used reduces their significance. However, the importance of establishing this fact can be seen by calculating the factors for gases other than helium. For instance, when oxygen is$the contained gas the corrections for water and carbon dioxide become 1.00097 and 1.00045, respectively. the weights of the sample lamp and absorpCalculations. tion bulbs before and after an analytical combustion are obtained, per cent carbon, per cent hydrogen, and carbon-hydrogen ratio of the sample may be calculated. In Table I are given the calculations of results of an analysis made on a relatively pure sample of iso-octane for illustration. For samples of unknown composition it is necessary to determine the true sample weight by correcting the apparent weight of sample by the fraction of the total burning time that the lamp is actually in the chimney. However, results can also be _calculated on a sample recovered basis. Calculations on the recovered basis are especially valuable n-hen very volatile materials are analyzed and accurate sample weights are difficult to obtain. On the other hand, the need for an accurate sample weight is obviated in instances Lvhere it is desirable to evaluate only the carbon-hydrogen ratio, provided any material lost is representative of the original sample with respect to carbon and hydrogen. Furthermore, it is often true, especially in petroleum work, that hydrocarbon mixtures must be analyzed which contain only traces of elements other than carbon and hydrogen. In such cases, increised precision is obtained by calculating results on the recovered basis, because the experimental limitations to obtaining a true sample weight may be eliminated. However, where it is necessary t o evaluate the per cent carbon and hydrogen present and the sample contains sizable amounts (more than O.lOyoby weight) of additional elements, it is imperative that the true weight of sample be known. If required, increased precision in estimating the time lost factor would be made possible by increasing the combustion period by (1) reducing the combustion rate, or (2) retaining the suggested combustion rate (1.0 to 2.0 grams per hour) and increasing the amount of sample consumed. Either practice would increase precision but a t the same time the period required for analysis would be lengthened. In addition to permitting a more precise sample weight estimation, increasing the amount of sample would permit an improvement in precision of estimating carbon and hydrogen recovery. However, the capacity of the absorption bulbs for water and carbon dioxide limits the amount of sample used. The bulbs described in the present paper will permit a maximum sample weight of about 3.0 grams.
C/H Ratio
Sample Weight Basis Carbon Hydrogen Total
%
t0.004
8.752 +0.030 16.739 16.765 16.752 10.013 16.766 +0.014 -0 014
5.291 5.288 5.290 5.292 5.290 10.001 5.295 -0.005 10,383 10,391 10.379 10.391 10.392 10.387 *0.005 10.425 -0.038 4.974 4.965 4.970
10.005
%
84.07 84.09 84.10 84.08 84.08 ~ 0 . 0 1 84.115 -0.04 91.19 91.23 91.18 91.19 91.23 91.20 t0.02 91.248 -0.05
15.89 15.90 15.90 15.89 15.89
. *0.005
~0.015
15.885 100.00 f0.005 -0.02 8.783 99.97 8 , 7 8 0 100.01 8.785 99.97 8.777 99.97 8 779 100.01 8.781 99.99 *0.003 +=0.02 8.752 100.00 +0.03 -0.01
....
....
.... ....
....
....
4 964 +0.006
7 6
99.96 99.99 100.00 99.97 99.98
..
....
.... ..
RESULTS
Tabulated in Table I1 are the results of analyses made on relatively pure compounds. These analyses indicate the accuracy and precision obtainable --hen carbon and hydrogen are determined by the impSoved lamp technique. Data are given for four analyses made on iso-octane (2,2,4-trimethylpentane), five analyses made on toluene, and two analyses on isopentane. The total amount of sample and time data used t o calculate the estimated sample weight are given for each analysis of iso-octane and toluene. Results are given both on a samplerecovered basis and an estimated sample weight basis, except for the isopentane determinations where no sample vieights were obtained. The over-all precision, given in terms of average deviation, is 1 part in 5000, or better, for the iso-octane analyses. The precision for the toluene analyses is somevihat lower, but it is still better than 1 part in 2000, and for isopentane the precision is slightly less than 1 part in 1000. An index of the precision obtainable in the sample weight estimation is shown by the average deviation in the total percentage of carbon and hydrogen based on estimated sample weights. Accuracies are given in terms of differences between average values obtained by analysis and calculated theoretical values. The over-all accuracy as shown is better than 1 part in 1000 for the iso-octane determinations, 1 part in 1000 for the iso-octane determinations, 1 part in 300 for the toluene determinations, and 1 part in 800 for the isopentane determinations. These values for accuracy are based on 100 mole '%sample purity, andsince the samples analyzed were actually only 99 mole % pure, the accuracy is probably better than shown. However, the accuracy indicated is sufficiently good for most usage of the method, and the precision indicates the possibility of accuracy improvement.
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ACKNOWLEDGMEYT
The author wishes to make acknowledgment to J. H. Hale, E. A. Breault, and Catherine Svoboda, all of whom assisted in various phases of the investigative work, and to J. W. Knowlton for several helpful suggestions. LITERATURE CITED
(1) Hindin, S. G., and Grosse, A. V., ISD. ESG. CHEM.,ASAL.ED.,17, 767-9 (1945). (2) Javes, A. R., J . Inst. Petroleum Tech., 31, 129-53 (1945). (3) Knowlton, J. W., and Huffman,H. 11..unpublished data. (4) Rossini, F. D., J . Research S u t l . B u r . S t a n d u r d s , 6 , 1, 37 (1931).
