V O L U M E 23, NO. 9, S E P T E M B E R 1 9 5 1 in tin, repeat with a third addition of hydrochloric acid. Finally, heat on a hot plate a t a temperature of about 150" C. t o expel absorbed water and hydrochloric acid or any remaining stannic chloride. Add to the dry salts 50 ml. of the 2% hydrogen chloride reagent and continue with the determination of lead as in Method I. Method 111. Determination of Lead in Alloys Insoluble in Hydrochloric Acid and Hydrogen Peroxide. I n the analysis of low-melting alloys of the Wood alloy type and other material high in lead, such as type metal or antimonial lead, which do not dissolve rapidly in hydrochloric acid and hydrogen peroxide, it is best to dissolve 1 gram or more of the alloy in a 150-ml. beaker in 15 to 20 ml. of 1 to 3 nitric acid. When solution is complete, evaporate to complete dryness. Expel tin, antimony, and arsenic by three evaporations with 15-ml. portions of hydrochloric acid, take the solution up with 50 ml. of the 2% hydrogen chloride reagent, and determine the lead as the chloride as in Method I. If required, determine bismuth in the filtrate of the lead chloride in the manner described above. In the case of low-melting alloys determination of cadmium may also be required. The filtrate from the bismuth oxychloride is perfectly suited for the direct precipitation with benzotriazole. Other elements can be recovered by standard methods.
1293 Interfering elements are sodium, potassium, barium, strontium, and silver, which form insoluble chlorides in butyl alcohol. These elements, however, are not encountered in the metals and alloys discussed in this paper. If present, silver, sodium, and potassium can be removed by a prior ammonium carbonate separation. EXPERIMENTAL
To determine the solubility of lead chloride, weighed portions of C.P. lead metal were dissolved in nitric acid and converted t o chloride by evaporation with hydrochloric acid. The lead chloride was treated with varying concentrations of the Willard and Smith reagent in n-butyl alcohol. The solution was filtered through a tared Gooch crucible and washed with the same concentration of hydrogen chloride in butyl alcohol used in the particular experiment. The results are presented in Table 11. Verification. The method described in this paper was applied to the determination of lead in a number of metals, alloys, and artificial mixtures (Table 111).
DISCUSSION
In addition to the elements indicated above, lead is quantitatively separated from all elements which form soluble chlorides in the 2% hydrogen chloride reagent. I n this category belong bismuth, copper, zinc, cadmium, iron, aluminum, chromium, manganese, calcium, magnesium, cobalt, nickel, and tin, antimony, and arsenic left after the evaporations with hydrochloric acid.
LITERATURE ClTED
(1) Kallmann, ANAL.CHEM.,20, 449-51 (1948). (2) Ibid., 21, 1145-6 (1949).
(3) Kallmann, IND.ENG.CHEX,ANAL.ED., 16, 712-17 (1944) (4) Ibid., 18, 678-80 (1946). (5) Willard and Smith, J. Am. Chem. Soc., 44, 2816 (1922).
RECEIVED December 13, 1950.
Determination of Tetraethyllead in Gasoline by X-Ray Absorption SAMUEL W. LEVINE' AND A. H. OKAMOTO The Atlantic Refining Co., Philadelphia, Pa.
A n x-ray absorption method for the determination of tetraethyllead in gasoline has been developed to provide a faster method for plant control and other analytical needs than the existing chemical methods. Several fundamental factors not mentioned by other workers have been found to have a significant effect on accuracy, particularly when analyzing gasolines
Q
UANTITATIVE chemical determination of tetraethyllead in gasoline is a slow and tedious process in the petroleum refinery laboratory. X-ray absorption analytical methods, which are much faster and more economical, may be applied to this problem because lead is a high x-ray absorber compared to the pure gasoline. Much work has been done by other workers (3% 4, 6, 8) in the field because of the many advantages inherent in x-ray absorption methods. Several complicating factors must be taken into consideration when applying the x-ray absorption technique to the determination of tetraethyllead in gasoline. The x-rays are absorbed to a certain degree by contaminants in the gasoline and by components of the tetraethyllead mixture. Sulfur is always found as a contaminant in gasoline and it is a good absorber. The tetraethyllead mixture for automotive gasoline contains halo1 Present
address, Fisher Scientific Co., Philadelphia, Pa.
of different base stock having different ratios of carbon to hydrogen and different density. The method developed permits the analysis of a sample in a total elapsed time of 10 to 15 minutes with an accuracy of f0.02 ml. of tetraethyllead per gallon. This accuracy and speed make it well suited for control work and other analytical needs.
genated compounds and these are good x-ray absorbers. In this method total x-ray absorption is used to give a measure of the tetraethyllead content of the gasoline and therefore appropriate corrections must be made for sulfur absorption. In the analytical procedure a correction is made for sulfur by use of a calibration curve showing the contribution of sulfur to x-ray absorption for different percentages of sulfur in the gasoline. The sulfur conten't of the gasoline must be found by an independent method. Because in most refinery laboratories sulfur is determined as a routine matter, no additional work is involved. For halogenated compounds found in thc tetraethyllead mixture it is assumed that the ratio of tetraethyllead compound to the halogenated compounds remains constant. The tetraethyllead is then measured in terms of the mixture. The ratio of tetraethyllead to halogenated compounds in commercial tetraethyllead fluid is held so nearly constant that no significant error due to variation in this ratio has been found. If an attempt is
ANALYTICAL CYEMISTRY
1294
made, however, to analyze experimental mixes containing a different ratio of lead to halide, errors xi11 obviously appear. I n cases where the sulfur content of the gasoline is unknown, it may be estimated and this value used for a correction. From a survey of the many gasoline samples handled in these laboratories, it has been found that the sulfur content varies from 0.03 to 0.08%. An average value of 0.05% is used when the sulfur content is unknown. An error of O . O l ~ oin this estimate will contribute an error of 0.015 ml. per gallon in the tetraethyllead determination. DEVELOPME\T OF METHOD
The factors that need be considered in developing an x-ray absorption method involving polychromatic radiation ale: \.ariable ratio of carbon to hydrogen of the gasoline Instability in the x-ray beam intensity Instability in the a-ray beam effective wave length Instability in the w a y detecting equipment Errors due to impurities, surh as sulfur Possible uncertainties due to the variable carbon to hydrogen ratio of the base stock are minimized by using an effective wave length of 0.53 A . At this wave length the mass absorption coefficients ( 7 ) of carbon and hydrogen are equal. Error due to instability in the x-ray beam intensity and Geiger counting equipment is minimized by making all measurements comparative ones. A gasoline sample containing tetraethyllead is compared in the x-ray beam to a pure hydrocarbon. By so doing, variation in absorption due to change of effective wave length is minimized
pTEL
=
density of leaded sample
X = sample cell length, centimeters FTEL= mass fraction of tetraethyllead fluid in leaded gasoline
The derivation of this equation is very similar to that in the work on sulfur analysis ( 6 ) . A graphic representation of the equation as given in Figure 1 shows that, for constant density samples, straight lines are obtained when milliliters per gallon of tetraethyllead are plotted against the ratio ITELIICH on a semilog scale. For samples having intermediate densities, interpolation between calibration curves must be used. A condition for the derivation of Equation 1 is that the density of the pure hydrocarbon standard be the same as that of the sample. To obviate the necessity of preparing a pure standard for each gasoline sample, the technique resorted to in the sulfur work is used. Pure hydrocarbon mixtures having density increments of 0.01 and covering the density range of gasoline are prepared. A series of polystyrene standards is then made t o have absorption approximately equivalent to these pure liquid hydrocarbon standards. These polystyrene standards are then calibrated against the pure liquid hydrocarbon standards as described in the previous paper ( 5 ) . The results are plotted as in Figure 2. By the derivation used in the previous paper ( 5 )the folloning equation for density interpolation is obtained:
ICE= (1
+ mAP)I’cE
(2)
where
ICH
= count obtained on a pure hydrocarbon standard
nearest in density to a gasoline sample being analyzed Z’CH = count that would have been obtained on a pure hvdrocarbon of exactly the same densitv as the gisoline sample = difference in densit7 between gasoline sample and Ap pure hydrocarbon; may be positive or negative and always 0.005 or less = slope of curve of Figure 2 and for this work is m 4.80
PERCENT SULFUR
1.00 90 .80 .70
The x-ray absorption method of analysis then resolves itself into taking the density of a gasoline sample, comparing it in the x-ray beam to a pure hydrocarbon standard nearest it in density, interpolating the small density difference by use of Equation 2, and reading the result from the calibration curve of Figure 1.
.60
.50
.40
EQUIPMENT
.30
.2
.6
1.0 L4 1.8 22 2.6 TE.L. IN MILLILITERS PER GALLON
3.0
Figure 1. X-Ray Absorption of Tetraethyllead and Sulfur in Gasoline
Using the comparative method, the following equation may be derived: In
ITELIICR = -(F’TEL
- II’CH)PTEL
XFTEL
(1)
where ITEL = intensity of beam after passing through leaded sample I C H = intensity of beam after passing through a pure unleaded hydrocarbon sample of same density as leaded sample ~ ’ T E L = mass absorption coefficient of tetraethyllead fluid a t 0.53 A. wave length U’CH = mass absorption coefficient of pure hydrocarbon at 0.53 A. wave length
The x-ray equipment used is a North American Philips 90” Geiger counter x-ray spectrometer. Electrical alterations ( 5 , Figure 3) were made to give a variable high voltage control for the x-ray tube. This was accomplished by introducing a Type 116 Superior Electric Co. Powerstat into the primary side of the high voltage transformer. A double-pole double-throw switch was added, so that the equipment could be changed from absorption to diffraction work without resetting the Powerstat. The filament voltage control for the x-ray tube was brought out on front of the control panel, so that the x-ray tube current could be adjusted without shutting the equipment down. An electronic line voltage stabilizer, the Sorensen hlodel 20005 of 2-kva. capacity, was installed and the Sola line voltage stabilizer furnished with the original equipment was removed. The increased accuracy resulting from this obviously will depend upon the stability of the original line voltage. No appreciable improvement in accuracy WLLBfound in the authors laboratories. To facilitate comparison of standard and sample in the x-ray beam, a sliding dual cradle (Figure 4, 5 ) makes possible rapid and reproducible placement of the standard cell and u n k n o m sample in the x-ray beam. The cradle is mounted on a steel post and centering jig that duplicates the one used in x-ray diffraction work. It is then a simple matter to change the equipment from absorption work to diffraction measurements. Sample cells are made of I5-mm. borosilicate glass tubing and are 15 cm. long. Mica windows 0.02 mm. thick are attached to
V O L U M E 23, NO. 9, S E P T E M B E R 1 9 5 1
1295
the ends by use of deKhotinsky cement. The technique of roughing the mica slightly with crocus cloth and working the cement around the outer edges of the window is used. Polystyrene ( 3 )rods having the same effective absorption as the pure hydrocarbons are used to replace the pure hydrocarbon standards. The collimating slits on the x-ray tube and Geiger counter are adjusted to give a beam intensity of 400 counts per second with a 0.73 density hydrocarbon absorber in the x-ray beam. An iron target tube was used in this work, but any tube giving sufficient x-ray beam intensity may be used. A setting of 95 volts on the variable transformer will give approximately 30 kv. peak on the x-ray tube. The tube current was 5.8 ma.
determine this effective wave length. The value obtained with the sulfur standard, honever, has been found to be the same as that obtained using a leaded hydrocarbon standard within the limits of accuracy of the method. The use of a sulfur standard instead of a leaded standard is suggested because of simpler calculations.
Table I. Sample
Standard Gasoline Samples TEL, MI./Gal. 3.0 3.0
Density
CALIBRATION
3.0
Calibration is carried out in the following order: 1. High Voltage Setting for Effective X-Ray Wave Length of 0.53 A. Effective wave length of polychromatic radiation is defined by comparing the absorption properties of the beam to the absorption properties of monochromatic radiation under identical conditions of measurement. "hen both types of radiation are absorbed equally, the effective wave length of the polychromatic radiation is the same as the wave length of the monochromatic beam.
I In order to set the high voltage of the x-ray tube to give an effective wave length of 0.53 A. "at ivhich wave length carbon and hydrogen have the same mass absorption coefficient" as required for Equation 1, the mass absorption coefficients as given by T'ictoreen ( 7 ) are used. A standard is made up to contain approximately 1.0% sulfur by adding diphenyl disulfide to a inkture of pure hydrocarbons having a density near 0.72. The pure hydrocarbons used in this xork were 2,2,4trimethylpentane and toluene. This sample is placed in a Ij-cm. cell and is compared in the x-ray beam to a pure hydrocarbon standard having the same final density as the sulfur standard. By use of the equation In ( Z S / ~ C E ) = -
- P'CE)
( ~ ' 8
PSXFS
(3)
where the symbols have definitions similar to those of Equation 1. the term ( p ' s - ~ ' c E is ) solved for. Reference to the tables of mass absorption coefficients versus wave length as given by Victoreen ( 7 ) will give an effective wave length for the radiation used. The Powerstat on the primary side of the high voltage transformer is readjusted and measurements and calculations are repeated until an effective wave length of 0.53.4. is obtained. Theoretically a leaded hydrocarbon standard should be used to
2.0 0.5
2. Sample Cells. As it is impractical to construct sample crlls of exactly the same window thickness and length, a correction factor is assigned to each cell to give it the equivalent absorption of a selected standard cell. By simple derivation, it can be shown that Zstd = KZ' (4)when the correction factor,
K = l f p p A X(WAX)' + 2
+
,
.
Experimentally K can be
determined as that multiplication factor that will make the transmittance reading of the cell filled with a hydrocarbon numerically equal to the transmittance reading of the standard cell filled with the same hydrocarbon. Theoretically this correction factor is slightly different for samples of different density and different effective absorption coefficient. When the cells are made so as to have correction factors within 1 or 2% of unity, however, variations in density and absorption coefficient among samples cause no significant change in the correction factor. This factor is determined by filling all the cells with toluene and comparing each to a selected standard cell. The correction for each cell is that multiplication factor u-hich will make the transmittance reading of the cell numerically equal to the transmittance of the standard cell. 3. Polystyrene Standards. Polystyrene standards are made of rod 15 mm. in diameter. A 15-cm. sample cell is filled with a mixture of pure hydrocarbons such as 2,2,4-trimethylpentane and toluene and having a density of approximately 0.74. A length of polystyrene rod is cut and the ends are polished with jeweler's rouge until its x-ray transmittance is within five counts of the liquid hydrocarbon sample. The exact equivalent density is calculated by the use of the density interpolation Equation 2. This procedure is repeated for densities of 0.73 and 0.72 to cover the gasoline density range of 0.715 to 0.745. As the density of liquid and solid hydrocarbon is a function of temperature, correction must be made for change of density with temperatuie. The correction for polystyrene is small but significant. The linear coefficient of evpansion of polystyrene is 0.00004 per F. and from this value the change in effective density in a radial direction is ralculated to be 0.00008 per ' F. When a polystyrene cell is calibrated for effective density as given above, the density determined is applicable a t the temperature of measurement. A table is made up giving the effective densities of the three polystyrene standards a t all desired temperatures by correcting measured density with the factor 0.00008for example, if a cell were calibrated a t 74" F. with an effective density of 0.7300, at 84" F. the effective density would be 0.7292. 4. Determination of Slope of Density Curve. T o determine the slope of the density curve of Figure 2-that is, the value of m in Equation 2-the 0.72 and 0.74 polystyrene standards are compared t o the 0.73 polystyrene standard. The results are plotted as in Figure 2 and the slope of this curve is the value of m of Equation 2 used for density interpolation. 5. Tetraethyllead and Sulfur Calibration Curves. Five standard gasoline samples are made by blending 2,2,4-trimethylpentane, toluene, and automotive tetraethyllead fluid to give
1296
ANALYTICAL CHEMISTRY
samples with the densities and concentrations of tetraethyllead compound shown in Table I. A plot of the calibration curves using these standards is given in Figure 1. The fact that the lines are straight is shown by the data obtained on standards 2, 4, and 5. In order to correct for the absorption of x-rays by sulfur in the gasoline, one unleaded gasoline sample of 0.73 density is made up to contain 0.20% sulfur. Sulfur is added as diphenyl disulfide. Curve S of Figure 1 is plotted by comparing the absorption of this sample to that of the corresponding polystyrene standard and plotting the logarithm of the ratio against the known sulfur content. As may be seen from Figure 1, the absorption of 0.01% sulfur is equal to the absorption of 0.015 ml. per gallon of tetraethyllead. In making measurements with gasoline samples, extreme care must be taken to minimize errors introduced by vaporization. Samples are never poured into cells until immediately before running. When the density of a sample is taken, the portion used is always discarded and a fresh portion is poured into the sample cell for x-ray absorption measurements. PROCEDURE
Take the density of gasoline sample by hydrometer and then discard this portion. Select the polystyrene standard whose density is nearest that of the sample, note room temperature, and read exact effective density of standard from a prepared chart of effective density versus temperature. Rinse the sample cell with the sample to be analyzed. Pour a fresh portion of gasoline sample into the sample cell, stopper loosely, and compare the x-ray absorption of the sample to that of the polystyrene standard. A cycle of counting the sample for 64 seconds and then the standard for 64 seconds and then the standard for 64 seconds is repeated three times. The results for each absorber are averaged arithmetically to give the initial values of ZTEL and I C E .
The value of (ZTELIZCH) to be used in reading the tetraethyllead result from Figure 1 is calculated in the following manner: Resolving Time of Geiger Tube. The count of x-ray photons given by the Geiger counter is not a true measure of the x-ray beam intensity. It is in error by a factor known as the resolving time correction or “dead-time” correction and is given by the equation Ca
+ c:
7
+
+
ACCURACY AND REPRODUCIBILITY
The accuracy of this method has been evaluated by comparing x-ray analysis results with those obtained by chemical analysis ( 2 ) . For 54 chemical results available for comparison, the average difference between chemical and x-ray analysis has been found to be zk0.02 ml. per gallon. These samples were taken from the refinery over a period of 3 months, during which time the type of crude oil charged to the processing units changed appreciably. Sulfur content of the gasolines was always obtained by the ASTM lamp method (1). Typical results are given in Table 11.
Table 11. Determination of Tetraethyllead in Gasoline by X-Ray and Chemical Methods Sample No.
CALCULATIONS
CT =
Sample Calculation. Room temperature = 76 O F. Resolving time of Geiger tube = 150 microsecond Sample cell factor = 1.004 Slope of density curve = 4.80 Sample density = 0.7300 (at 76” F.) Polystyrene standard density = 0.7342 (at 76“ F.) Density difference = -0.0042 ZTEL = 180 counts per second ITEL (corr.) = CO CO% = 180 f (180)2(150 X = 185 counts per second ZCH = 420 counts per second ZCH (corr.) = 420 (420)2(150 X love) = 446 counts per second Sulfur content of gasoline = 0.06% Sulfur correction (from Figure 1) = 0.969 Density interpolation = 1 - (4.80) (0.0042) = 0.980 ITEL/ZCH = (185/446) (1.004) (0.980/0.969) = 0.421 From Figure 1, tetraethyllead content = 2.61 ml. per gallon
(5)
where CT is the true count, COis the observed count, and I is the resolving time. This equation does not give the exact corrected value, but is the most convenient to use. If it is used both in calibration and in sample determinations, the error will be negligible. Most single-chamber Geiger tubes have a resolving time of 100 to 300 microseconds. The tube furnished with the Norelco apparatus has a resolving time of approximately 150 microseconds. Density Interpolation Correction. An interpolation correction is made for the difference in density between sample and polystyrene standard. The sample density is taken a t room temperature and the effective density of the polystyrene standard a t room temperature is read from the correction chart mentioned in the discussion on calibration. The difference in density is the Ap used in Equation 2. This value may be positive or negative, depending on which density is the greater. Cell Factor Correction. A cell factor correction is made to the sample cell reading. This correction accounts for the difference in construction between the sample cell used and the selected standard sample cell. Sulfur Correction. The sulfur in the gasoline absorbs x-rays and will give a high tetraethyllead value unless a correction is made. From the chemically determined or estimated value of sulfur in the gasoline, the value (ICHS/ZCH) is read from Figure I, curve S. The measured ratio (ITEL/ICH) is divided by this factor to give a corrected value of (ZTELIZCH).
X-Ray hfl./Gai.
Chemical MI./Gal.’ 0.93 0.71 1.24 0.85 1.49 0.91 1.57 0.50 3.00
Difference, Ml./Gal. -0.02 +0.02 +o. 02 -0.02 -0.02 +O. 03 10.01 $0.05 0.00
Reproducibility was obtained by rerunning the same samples over a period of several weeks. This figure was found to be h0.012 ml. per gallon. Portions of samples used for reproducibility checks were discarded and fresh portions taken for each subsequent analysis. All samples were refrigerated, and no significant precipitation of tetraethyllead was noted. LITERATURE CITED
(1) Am. SOC.Testing Materials, “ASTM Standards on Petroleum Products and Lubricants,” p. 1330, Xovember 1949. (2) ASTM Standards, Petroleum, Part 111-A, ASTM Designation D 526-42, P.287,1946. (3) Calingaert, G., Lamb, F. W., Miller, H. L., and Noakes, G. E., ANAL.CHEM.,22, 1238 (1950). (4) Hughes, H. K., and Hochgesang, F. P., Ibid., 22, 1248 (1950). (5) Levine, S.W., and Okamoto, A. H., Ibid., 23,699 (1951). (6) Liebhafsky, H. A,, I b i d . , 21, 17 (1949). (7) Victoreen, J. A., J . A p p l i e d Phys., 20, 1141 (1949). (8) Vollmar, R. C., Petterson, E. E., and Petruzelli, P. A., ANAL. CHEX, 21, 1491 (1949). RECEIVED April 6, 1951. Presented st the Second Pittsburgh Conference on Analytical and Applied Spectroscopy, Pittsburgh, Pa., March 5 t o 7, 1951.
