INDUSTRIAL AND ENGINEERING CHEMISTRY
304
Table II. Sulfosalicylic A c i d and Ammonium Salt of Sulfosalicylic A c i d VI. Thorn Smith and Bureau of Standards Iron Ores
Bample 15%
T.S. 72 10%
B.S. 27 B.S. 29
Titrant
Gram
lM1.
Iron Found
2.44 2.83 2.85
0.0927 0.1075 0.1083
Gram 0.0000
+0.0027
$0.001 1
Ammonium Salt, Iron Titer 0.0247 Gram per M1. 0,0813 0.1126 0.1126 0.1356 0.1356 0.0905
3.36 4.57 4.56 5.47 5.46 3.66
0.0828 0.1128 0.1127 0.1351 0.1349 0.0902
precipitation and colorimetric reaction of organic reagents t o volumetric analysis. LITERATURE CITED
Difference
Grdm
Sulfosalicylic Acid, Iron Titer 0.0380 Gram per M1. 0.0927 0.1048 0.1072
T.S. 7 1 B.S. 26
Iron Taken
+0.0015 +0.0002 +0.0001 0.0005 0.0007 -0.0003
-
varies from 0.2 to 0.8 mg. of copper per 100 mg. of copper taken. The method is fast and may be used without separation of certain constituents. The conditions of the titration allow it to be fitted into the usual gravimetric procedure for brass or bronze. Methods for determining iron in iron ores are described which use both sulfosalicylic acid and its ammonium salt as titrating agents. The accuracy varies from 0.1 to 1.5 mg. per 100 mg. of iron taken. The conductometric method may be used in applying the
Vol. 18, No. 5
Alten, Weiland, and Hille, 2. anorg. allgem. Chem., 215, 81 (1933). Bauer and Eisen, Angew. Chem., 52, 459 (1939). Boulad, J. H., Bull. S O C . chim. [5], 3, 408-12 (1936). Germuth, F. G., Chemist-Analyst, 17, 3 (1928). Gutzeit, Helu. Chim. Acta, 12,713, 829 (1929). Kolthoff, I. M.,and Langer, A., J . Am. Chem. Soc., 62, 211 (1940). Ibid., 62, 3172 (1940). and Lingane, J.. “Polarography”, New York. Kolthoff. I. IM., - . . Interscience Publishers; 1941. Korenman, Mikrochemie, 15, 315 (1934). Lingane, J., J . Am. Chem. Soc., 65, 866-72 (1943). Lorber, Biochem. Ztg., 181,391 (1927). Moser, L., Monatsh., 44, 91 (1923). Moser, L., Brukl, A, and Ven, I., Ber., 58B,380 (1925). Moser, L., and Iranyi, E., Monatsh., 43, 673 (1922). Moser, L., Neumayer, K . , and Winter, K., Ibid.,55, 85 (1930). Neuberger, A4., Arch. Eisenhiittenw., 13, 171 (1939). Remsen, I., Ann.: 179, 107 (1875). Theil and Peter, 2. anal. Chem., 103, 161 (1935). Theil, and Van Hengel, Ber., 70B, 2491 (1937). Urech, Helv. Chim. Acta, 22, 322 (1939). Van Suchtelen and Itano, A., J . Am. Chem. SOC.,36,1793-1807 (1914).
Determination OF Tetraethyllead in Gasoline by X-Ray Absorption MILES V. SULLIVAN AND HERBERT FRIEDMAN United States Naval Research Laboratory, Washington,
D. C.
R
IGID control of the antiknock rating of gasoline and other motor fuels for use by the armed forces is of utmost importance in maintaining high standards of performance. The use of tetraethyllead to bring various base stocks up to a set standard of quality requires the addition of varying amounts of lead to each lot of fuel. Continuous control of this process by the manufacturer and continuous checking by the user may involve the determination of the lead content of hundreds of samples per day in one laboratory. As a result, chemical methods of analysis for lead in gasoline have received a great deal of attention. No single chemical method has been proposed, however, which is generally applicable to the analysis of lead in chemically different base stocks, and which does not require an excessive amount of time for reasonably accurate results (3). Aborn and Brown (1) employed an x-ray absorption technique for the quantitative analysis of lead in gasoline, but their method was limited in sensitivity by the use of an ionization chamber to detect the radiation. In the present work a Geiger counter was employed which had a quantum efficiency of roughly SOY0 for the radiation used. The result is a method for the quantitative determination of lead which is equivalefit t o the chemical analysis in accuracy, but may be completed by an unskilled worker in 5 minutes. X-RAY ABSORPTION
Under the conditions necessary for the production of x-rays one observes a spectrum of continuous radiation upon which is superimposed a series of monochromatic characteristic radiations. The continuous spectrum (Figure 1) is composed of all wave lengths down to a certain minimum wave length given by AO = 12395/V Angstroms, where V is the peak voltage applied. ”he intensity of wave lengths longer than this minimum passes
Figure
1. X-Ray
Spectrum of M o l y b d e n u m Bombarded with 35-Kv. Electrons
through a maximum a t about X = 1.3 XO and then gradually approaches zero. The intensity of hard radiation in the continuous spectrum increases with atomic number of the target material. The characteristic K series radiations appear only when the peak voltage applied to the tube exceeds the K shell ionization potential of the target atoms. By the proper choice of target, voltage, filters, and possibly a crystal monochromator, almost any range of x-ray wave lengths or monochromatic radiation may be obtained. When x-rays impinge on matter, the manner in which they are absorbed may be expressed by the relation:
I
= Iec-rP2
(1)
May, 1946
ANALYTICAL EDITION
305
where
I
= intensity after psssing through sample I , = intensity of monochromatic x-rays incident on sample p = mass absorption coefficient p = density of homogeneous sample z = thielincss of homogeneous samplr
The mass absorption coefficient, p, is a function of the wave length of x-rays and the atomic nature of the sample. It is possible to express the mass absorption Coefficient as a function of atomic number and wave length in the approximate relation: =
N/A
(2)
( ~ 2 4 x 3 )
where
C
Z A
N A
=
= = = =
a constant in the range between adjaoent edges atomienumber wave length of x-rays Avogadro's number atomic weight
.
"._ _-._ ._-.AAt eash critical absorption fir.."~"_.. _Ir tion'potential of the atom) the absorption coefficient reaches B m i m u m value on the short wave-length side of the limit. Where it is desired to identify one component of a mixture by absorption, the optimum contrast will be obtained by working with a wave Length slightly shorter than the absorption limit far the particular component being measured. This is not always practical, since i t is generally necessary t o sacrifice intensity enomously to monochromatiee the radiation. Because lead has a high absorption coefficient for practically all x-ray wave lengths compared t o any organic material, no attempt was made t o utilize an abBorption edge in this particular analysis. According to Equation 2 the mass absorption Coefficient increases as the cube of the wave length. I n a single-component ~
...
.
Figure 3. A.
E.
%Ray Absorption Equipment
Powerru~dy X-ray 1-*-
C. Abrorr
D. Gdwr
system, therefore, one employs the longest wave length transmitted by the sample, thus obtaining a maximum change in intensity for a given change in sample thickness. However, leaded gasoline is a two-component system in which the mass absorption coefficientschange a t different rates 8.8 the wave length is varied. A s a result an optimum wave length exists above which the advantage of using longer wave lengths disappears because the absorption coefficient for gasoline is increasing more rapidly than that of lead. With an absorption cell length of 15 to 25 cm. the optimum conditions were obtained with the x-ray tube operated a t 17 kv. AWARATUS
The power supply (see Figures 2 and 3) consisted of a 25-kilovolt, Zmilliampere transformer, the output of which was halfwave rectified by m 8013 tube. A 0.05-microfarad condenser reduced the ripple helow 10%. The x-rsy and rectifier 6laments were supplied by a single transformer wLth two secondaries. A molybdenum tm@t Machlett x-ray tube of the line focus, wstercooled type was used. The entire supply including the x-ray tube was oil-immersed and oooupied less than 28.32 liters ( I cubic foot). The power supply was stabilized by a line voltage reeulator.
tt the Geiger tubes employed in this work were &&en{ in the detection of soft x-rays. In its simplest form
~~
1
-
-
~
~
-
~
a.
.~ .~
acrom the electrodes in the manner illustrated in Figure 4. Under these conditions the passage of a n ionizing radiation through the tube triggers a momentary discharge which may he detected m a voltage pulse a t the capacitor, C. Following the discharge the counter quickly recovers to its initial condition and is ready to detect the next ionizing event. The high sensitivity of the counter is based upon the fact that B primary ionhatianof only a single ion pair is sufficient to trigger a momentary discharge of the order of 10,000,000,000electrons. I n the ranee of wave lenntbs emuloved in these ahsorotion meamnement: x-ray want; are atkoibed almost entireiy by . .
~~~
~
~
~
~
~, ~~
a counter tube is filled witE 40 cm.-of krypton, 80% of the x-ray
306
INDUSTRIAL AND ENGINEERING CHEMISTRY
beam a t 17 kv. is absorbed photoelectrically in the active part of the counter. I n other words, 80 out of every 100 quanta entering the counter produce counts. The construction and operation of these tubes have been advanced to the point where their life is comparable to other tube elements in the equipment, and they are capable of from 10,000 to 100,000 counts per second, depending on the type of amplifier employed. A more detailed description of these tubes may be found in a previous publication (2). The counting circuits used in the present equipment consisted of an amplifier capable of passing 25,000 random counts per
Vol. 18, No. 5
should be applicable to virtually all gasoline stocks using the same type of ethyl fluid. I n obtaining the calibration curve (a straight line), ten alternate 32-second counts were taken for each sample and averaged. Since the values plotted are ratios of counts obtained with clear gasoline to that of leaded gasoline, a total of 640 seconds or 10 minutes was required to locate each point. These results were obtained with an argon-filled counter. A counter filled with 40 cm. of mercury pressure of krypton was 6 times as efficient a t 17 kv. and reduced the counting time correspondingly.
WIRE GLASS ENVELOPE-,
PERFORMANCE
4k
Y METAL CYLINDER.”
I1,000 v o C
+I+
Figure 4.
~
-
Fundamental Circuit of Geiger Counter
second coupled to a scale of 32, the output of which is registered by a mechanical impulse counter. In order to register 1000 random counts per second without missing more than 4%, the recording system must be able to count a t least 25,000 uniform pulses per second. This is true because about 4y0 of the 1000 random counts arrive a t an average rate greater than 25,000 per second. Since all measurements were made a t a rate of well under 1000 counts per second, the recorded intensities were very nearly a linear function of the true intensity of radiation reaching the Geiger tube. Recently the measurement of x-rays by means of a fluorescent screen and a multiplier phototube has been successfully applied in photofluorographic equipment by Morgan (4), and to the rapid examination of fuses by Smith ( 5 ) . A comparison test performed a t this laboratory showed that a t x-ray intensities just sufficient to be observed above the background noise of the photomultiplier tube-fluorescent screen combination, the Geiger counter regis-’ tered over 1000 counts per second. EXPERIMENTAL
The unleaded gasoline’used was standard aviation-base stock, 62-octane rating, and the fully leaded sample contained 4.6 ml. of tetraethyllead per gallon according to the accompanying chemical analysis. For the purpose of this work it was assumed to be correct and by careful dilution with the unleaded gasoline a series of samples was obtained ranging from 0 to 4.6 ml. per gallon in steps of 10%. The absorption cells were constructed of 15-cm. lengths of brass tubing (inside diameter, 0.5 inch, outside diameter, 0.76 inch) with 0.025-mm. (0.001-inch) aluminum windows. I n the optimum voltage range of 16 to 19 kv., and using 4 cell length of 15 to 25 cm., a ratio of intensities through leaded and unleaded gasoline of 140 to 1 was obtained. Counting rates 01 75 to 1500 per second were registered when the tube current was about 5 milliamperes. The ethylene bromide present in commercial ethyl fluid accounts for about one fourth of the absorptipn observed. Since a stoichiometric ratio of lead to bromide (or bromide-chloride mixture) is usually maintained in the ethyl fluid, this need not affect the accuracy of the measurements unless they are made on gasoline stocks whose history suggests that this ratio may h a w been altered. For special applications it may be desirable to work near an absorption edge of one of the component- to minimize such interference. The present work covered only one base stock used with a particular ethyl fluid. Since the x-ray absorption coefficients for most base stocks are very nearly the same and much smaller than that of lead, a single calibration
By the method of least squares a calibration line was obtained which was more accurate tha? any one of the points, and by calculating the standard deviation of the points from the line (the square root of the mean of the squares of the delta y deviations from the line), an estimate was obtained of the precision of the determination. It was assumed in these calculations’ that all points have equal weight in the determination of the calibration line. This treatment is not rigorous, but a more exact treatment is not warranted. The standard deviation was 0.05 ml. of tetraethyllead per gallon of gasoline, or .1%. Similarly a precision of 1% was obtained with samples containing a maximum of 0.46 ml. of tetraethyllead per gallon. This corresponded to the detection of 0.005 ml. of tetraethyllead per gallon, or 0.0002% lead. The precision attained here in the determination of lead in gasoline is comparable to that obtained by chemical methods. The x-ray absorption method is superior to the chemical method in most respects. It can be carried out in a routine manner by an unskilled operator in a small fraction of the time required for chemical methods. If conditions warrant it, the whole process can be made automatic for a number of simultaneous runs. The simultaneous exposure of the standard gasoline and the unknown will avoid the necessity for alternate exposures, eliminate errom due to fluctuations in x-ray intensity between successive exposures, and materially increase the precision. OTHER APPLICATIONS
Although all measurements were made on leaded and unleaded gasoline, the method of analysis is applicable to lead in any other media of low atomic number. It will be possible to attain the same sensitivity for lead in any of the oils or fuels in common usage. The method is not limited to heavy metals, however, because all elements have discontinuities in their absorption curves which may be utilized by careful control of the wave length employed t o give maximum absorption for the particular metal desired. Further development of the technique may open up promising new fields for this method of analysis, many of which may be of great value in oil and fuel research. The testing of a good oil or fuel involves service runs in large mechanical units over extremely long periods of time. If a continuous method of analysis for metal particles could be employed, it is possible that “break in” and “break down” could be followed step by step. Wear rates of different metallic sections such as shafts and bearings could be selectively observed by the proper choice of radiation. LITERATURE CITED
(1) Aborn, R. H., and Brown, R. H., IND.Eso. CHEM.,AKAL.ED., 1, 26-7 (1929). (2) Friedman, Herbert, Electronics, 18, 132-7 (1945). (3) Lykken, Louis, Treseder, R. S., Tuemmler, F. D., and Zahn, Victor, IND.ENO.CHEM.,ANAL.ED., 17, 353-60 (1945). (4) Morgan, R. H., Am. J. Roentgenol. Radium Therapy, 48, 88 (1942). (5) Smith, H. M . , Gen. Elec. Rev., 48, No. 3, 13-17 (1945).
