Products of Inorganic Substances. 11. Inorganic Liquids With Solubility Products of Inorganic Substances,” Chem. SOC. (London), Spec. Publ. KO. 7 (1958). ( 2 ) Chu, B., Diamond, R. M., J . Phys. Chem. 6 3 , 2021 (1959). (3) Cimerman, C., Alon, .4.,Mashall, J., A n d . Chim. Acta. 19,461 (1958). (4) Guerrin, G.. Sheldon, M. V., Reilley, C. N., Chemist Analyst 49,36 (1960). (5) Haar, K. ter Bazen, J.. Anal. Chim. Acta 10, 23 (1954)
(6) Hillebrand, D’. F., Lundell, G. E. F., Bright, H. A., Hoffman, J. I., “8pplied Inorganic .4nalysis,” p. 494, Wiley, Xew York, 1953. (7) Horne, R. 8., Holm, R. H., Meyers, M. D., J . Phys. Chem. 61, 1655 (1957). (8) Kraus, K. A., Nelson, F., “Anion Exchange Studies of the Fission Products,” Vol. VII, Proc. Intern. Conference on Peaceful Uses of Atomic Energy, Geneva, 1955, United Nations, New York, N. Y., 1956.
(9) Lewis, L. L., Melnick, L. M., ANAL. CHEW32 , 38 (1960). (10) Welcher, F., “The Analytical Uses of Ethylenediaminetetraacetic Acid,” D. Van Nostrand, New York, 1958. (11) Wilkins, D. H., Anal. Chim. Acta 18, 374 (1958). RECEIVED for review March 24, 1961. Accepted June 9, 1961. Pittsburgh Conference on Analytical Chemistry and Applied Spectroscopy, Pittsburgh, Pa., February 1961.
The Rapid Determination of Sulfur in Petroleum Fractions by X-Ray Absorption of Tritium Bremsstrahlung A. F. PYRAH, R. S. ROBERTON, and JEFFREY WISEMAN Refinery Technical Division, Mobil Oil Co., ltd., Coryfon, Essex, England
b The bremsstrahlung emission from a tritium-titanium source is utilized in a rapid method for the estimation of sulfur in petroleum products.. The calibration procedures described were developed to compensate for the polychromatic nature of the x-rays. Products ranging from gas oils to heavy fuel oils, containing up to 570 sulfur, can be tested conveniently using relatively inexpensive equipment. Time per test is about 6 minutes, including calculation. The method commends itself to laboratories carrying out frequent sulfur determinations by semiskilled staff. Accuracy and repeatability are of the order of 0.05 sulfur.
70
of measuring the sulfur content by x-ray absorption depend mainly on the high absorptivity of sulfur compared with that of carbon and hydrogen and on the fact that other elements, such as oxygen, nitrogen, and various metals, are present only in trace quantities. Up to a few years ago x-ray tubes were the only source of radiation used, but the cost of the equipment was very high. More recently, in a method described by Hughes and Wilczewski (3), a radioactive isotope of iron (iron55) has been used with some success, offering advantages of cheapness, compactness, and stability. In 1957, Kannuna (6) suggested the use of tritium as an alternative to iron-55. The x-rays (bremsstrahlung) are emitted when the soft beta radiation from tritium strikes a titanium or zirconium target in which the tritium is absorbed. The radiation has a wave length of 0.68 A. upwards, and the source, obtainable from the Atomic Energy Research Establishment, HarEmODs
well, England, has a half life of 12.5 years against 2.94 years for iron-55. In addition, it presents a relatively small radiation hazard, is cheaper than iron-55, and is entirely free from the effects of interfering isotopes. The effective wave length of the x-rays is somewhat shorter than the iron-55 emission, being therefore more penetrating and more easily adaptable to the determination of sulfur contents above 3.0% wt. The characteristics of different sources are practically identical, a distinct advantage over iron-55. The main disadvantage of the tritium source is that the emission is not monochromatic, whereas that from iron-55 is. The polychromatic nature of the x-rays makes calibration a more difficult problem, since on passing through any medium the transmitted radiation has a shorter wave length than the incident beam, and therefore the effective wave length decreases with increasing absorption. The way in which this latter difficulty has been overcome forms the basis of this paper. The calibration procedures adopted are designed to achieve the maximum possible accuracy of the method compatible with simplicity cf operation. BASIC THEORY
The basic equations used in deriving the formula for calculation of sulfur percentage are the following: p = p , 1o-a’Pbc a’CH8
=
a’CCC
+
a‘HcH
(1)
f
a‘8c8
(2)
Applying Equation 1 to a hydrocarbon with or without sulfur [designated as subscript CH(S) ]
To allow for changes in density, a new symbol, A’, is introduced.
For any medium containing carbon, hydrogen, and sulfur only, Equation 2 can be rewritten: ~ ‘ C H S= U’CHCCH
Since C C H
+ cs = 1 A‘
Thena’c~s= P
= U’CH
+
~’SCE
+ (a’s -
U’CH)C~
(4)
This is the working equation. Equation 3 requires an accurate knowledge of Po, the intensity of the incident beam. Since this is too high to be measured accurately, the transmittance of the sample can be referred to the transmittance of a standard aluminum reference absorber. As the alumi. num absorbs a constant fraction of Po, the count rate with the absorber in place, represented by PA1, is always proportional to Po. Use of this relationship greatly simplifies the calculations. If the source is monochromatic, the plot of log,, of count rate against thickness of aluminum will be a straight line, the slope of which is equal to the mass absorptivity of aluminum. With a polychromatic beam, this plot will be a curve, the mass absorptivity of alumi, can be obtained from num, ~ ’ ~ 1which the slope a t any point, decreasing with increasing aluminum thickness. The VOL 33, NO. 10, SEPTEMBER 1961
1355
Figure 2,
I
CARBON/ W YDROGE N
1.2
1.4
1.3
I5
WAVE LENGTH,
1.7
16
1.11
A.
Figure 1. Variation of mass absorptivities of sulfur, aluminum, carbon, and hydrogen with change of wave length
effective wave length is related to the observed mass absorptivity (Figure 1) (2) The relationship between the intensity of the incident and transmitted radiation through a standard absorber is given by : +
log PA’
- a’Al(Pb)ai
PO
log P A l + a ’ A l ( P b ) A l substituting in Equation 3
p’
=
log P A 1 ’
- log P C H ( 8 ) + U’Al(Pb)Al
s =
(6)
(d’)CH(S,
This equation can be used to determine the mass absorptivity of a hydrocarbon or a hydrocarbon containing sulfur. The mass absorptivity of sulfur is obtained by measuring the count rate on blends or on oil samples containing 8’ known amount of sulfur. For a monochromatic source the mass absorptivity of sulfur is independent of concentration and may be calculated from the following equation, deduced from Equation 4. a‘s =
U’CHS
- U’CH
(1
- CE)
CS
--P“
1
(7)
- P”t
Statistical Fluctuations. Radioactive disintegrations are entirely random in nature. For a large number of quanta arriving a t a Geiger tube, Poisson’s law is obeyed and the standard deviation is given by :
log P e
U’CH(B)
I
.\/N
The most probable error is defined as 0.67s. To achieve the desired accuracy in the calibration procedures, total counts of a t least 200,000 should be taken for each observed count rate. The standard deviation is therefore: 8
=
4200,000 = 447
= 0.23%
The most probable error is 0.67 X 0.23 = 0.15%. This is considered satisfactory for calibration purposes. Background Radiation. A Geiger counter always shows some background count due to cosmic radiation, natural radioactive substances, etc. This must be subtracted from all measurements of count rate after the correction for resolving time has been made.
FACTORS AFFECTING ACCURATE MEASUREMENT O F RADIOACTIVITY
APPARATUS
Resolving Time of Geiger Tube. An error in counting is introduced by the fact that the tube is not perfectly efficient. The true count rate, in counts per second, can be obtained by the formula:
Tritium-Titanium X-Ray Source. (Normally called a tritium-titanium target.) This is in the form of a copper disk 1.125 inches in diameter covered with a thin layer of titanium in which tritium is absorbed. There is an inactive edge to the disk for han-
1356
ANALYTICAL CHEMISTRY
X-ray absorption assembly
dling purposes (approx. 0.25 inch) Nominal tritium content is 10 curies. Counting Equipment. This consists of a Geiger counter, scaler with automatic timer, power pack, and probe unit. The scaler, power pack, and probe unit were supplied by Bendix Ericsson U.K., Ltd., Nottingham, England. The tube used is the Mullard MX118 halogenquenched tube, with an operating voltage of 1000 to 1400 volts. The use of a probe unit is not essential with this tube, but is strongly recommended aa the quench time of the tube is replaced by the known imposed paralysis time. A small modification to the circuit of the probe unit is necessary to adapt it for use with the MX118 tube. Absorption Assembly. This consists of a precision-made assembly for mounting the radioactive source, cell holder, and Geiger tube. It can be conveniently fabricated in brass. The geometry of the apparatus is important, and when assembled the centers of the source, cell window, and Geiger tube must be coaxial. The positions of the latter are adjustable along the common axis. A diagram of the assembly is shown in Figure 2. An aluminum screen consisting of a disk 1.15 inches in diameter and having 25 to 35 mg. per sq. cm. areal density is placed in front of and in contact with the tritium source. A brass iris, of approximately the same internal diameter as the cell window, is fitted in front of the screen. This acts as a collimator. The source, screen, and iris are held in place in the source holder by a spring clip. I t is convenient to use the following dimensions between source, cell, and Geiger tube: From source to center of re11 From center of cell to Geiger window
Mm. ca. 22 ca. 25
Cells with Beryllium Windows. The cells have the following dimensions: over-all size, 1 1 / 2 x 11/* X inches; diameter of window, 0.625
inch; window material,. pure beryllium of 0.015-inch thickness; internal distance between windows, 0.5 i 0.003 cm. It is convenient to make the cell in two parts. The beryllium windows are sealed t o the brass with an epoxy resin cement. The two parts are finally screwed together with a thin film of epoxy resin between the faces to ensure a leakproof seal. (Cells made in this way have been in constant use for nearly 2 years without leaks.) A lid of brass or Plexiglas is desirable, particularly when testing more volatile samples. Set of Aluminum Absorbers. The range of areal densities required is from 25 to 75 mg. per sq. cm. The Panax range, which can be obtained from Panax Equipment, Ltd., Redhill, Surrey, England, was used. Absorber Holder. This can be made from Plexiglas and is used to hold an absorber in place directly in front of the cell on the Geiger tube side (Figure 2).
.
o
IHICIKSS
x
)
m
OF ALUMINUM,
o
o
w
~
o
MC/SQCH
Figure 3. Aluminum absorber curve EXPERIMENTAL
The apparatus used initially was the same as that described except that an unscreened weak source containing 2 curies of tritium was used. Preliminary work was designed to obtain data on the effect of increasing absorption on the effxtive wave length of the x-rays emitted by the unscreened tritium source. This would provide information for comparison with Kannuna’s work (6) and give some measure of the energy distribution of the incident radi‘A t’ion. Aluminum Absorption Curve. With the empty absorption cell in place, the count rate was determined on a number of aluminum absorbers. Kannuna’s work covered only the range of areal densities from 25 up to 55 mg. per sq. cm., but for expected sulfur contents up to 5% w t . S, it was necessary to extend the range to 75 mg. per sq. cm. A plot of log of count rate against aluminum thickness is shown in Figure 3. For a monochromatic source this should be a straight line. The experimental plot is a curve, the change in slope representing the change in the mass absorptivity of aluminum with increasing absorption. The calculated change in aluminum mass absorptivity is from around 21 sq. cm. per gram a t 30 mg. per sq. cm. thickness to 15 sq. cm. per gram a t 60 mg. per sq. cm. thickness, which corresponds to a change in effective wave length from 1.53 to 1.39 A. (Figure 1). This observed change in aluminum mass absorptivity is relatively large and is indicative of the polychromatic nature of the bremsstrahlung emission. In the development of any satisfactory method for the determination of sulfur, due regard must be given to this change in effective wave length with increasing absorption. Mass Absorptivities. A study of the aluminum absorption curve (Figure 3) shows that reasonably straight lines can be drawn through two
Relationship between log count rate of tranrmitted beam through aluminum (log PA1) and thickness of aluminum
sections of the curve covering the 25 to 40 and the 45 to 70 mg. per sq. cm. ranges. In a previous method of calibration, nom superseded, two sets of data relating to these two ranges were used for low and high sulfur, respectively. However, for the purpose of comparison with Kannuna’s figures (6) the average mass absorptivity of aluminum was calculated over the lowel range. One absorber, approximately near the middle of the range, was chosen as the standard reference absorber. The corrected count rate of this absorber was used in all subsequent calculations, that is as PA,in Equation 6. Blends of methylcyclohexane and toluene were prepared covering a range of carbon-hydrogen mass ratios between 6 and 9. 1he count rate on each blend was obtained and the corresponding mass absorptivities calculated using Equation 6 and the average value for the mass absorptivity of aluminum previously obtained. These values were used to obtain the mass absorptivitirs of carbon and hydrogen using a plot of atCH(1 r ) against T , the slope giving a value for and the intercept on the alCH (1 T ) axis a value for a’=. Comparisons with Kannuna’s figures are given below. Kannuna Authors (6) Aluminum, U’AI 21.1 21 .o Carbon, a’c 1.8! 1.83 Hydrogen, a ’ ~ ... 0.39 The values obtained for aluminum and carbon are close to Kannuna’s, but no satisfactory value could be obtained for a’= since no points could be plotted for low r values. In certain cases it was possible, depending on the hydrocarbons used, to obtain a negative a’= value. Using n-dibutylsulfide and a methylcyclohexane/toluene mixture of 6.4
+ +
carbon-hydrogen mass ratio, sulfur blends were prepared containing 0.1 to 1.0% sulfur (6.4 carbon-hydrogen ratio base stock was used since this is equivalent to gas oil composition). From the count rate and the previously determined for the base stock, the mass absorptivity of sulfur was calculated for each sulfur concentration using Equations 6 and 7. A study of the results (Table I) shows that the calculated mass absorptivity of sulfur varies with the amount of sulfur present, the value decreasing with increasing sulfur concentration. A logical explanation of this phenomenon, which is in agreement with the observed change in aluminum mass absorptivity, is that the softer radiation component of the x-ray beam is preferentially absorbed resulting in a drop in the effective wave length with increasing absorption. In calculating the mass absorptivities of sulfur certain assumptions have, of necessity, been made in that a’c and a ’ ~ remain constant. It can reasonably be assumed that will not change-significantly, although there will be a small change in a’c and a relatively large change in u ’ ~ . Since it would be extremely difficult to measure the variation of both a’c and u ’ ~it, is convenient and basically sound to identify all changes of mass absorptivities with the observed variation of sulfur.
Table I. Variation of Sulfur Absorptivity with Increasing Sulfur Concentration, Unscreened Source Sulfur Content, % Wt. u’8 Calcd. 0.13
43.4
0.21. 0.45
40.2
0.84
1.00
41.1
37.6 36.6
From the practical standpoint, it is difficult to decide what value of a’8 to use. It is possible to insert an average value for in the working equation, accepting consequent errors. The results obtained on the prepared blends using this value are given in Table 11. The correlation is not particularly good, although the results might be considered acceptable. As would be expected, the results are slightly high a t low sulfur cuncentrations and low a t high sulfur concentrations. Above 1.3% sulfur the discrepancy between the actual and determined sulfur contents would increase still further if a mean a’8 were used. It is concluded from this series of experiments that the relatively large variation of sulfur absorptivity over a comparatively short range of sulfur concentrations, taken in conjunction with the rather indefinite values for hydrogen obtained, indicates that the method of calibration suggested by Kannuna, based on monochromatic xrayB, is not sufficiently precise for accurate sulfur determinations. VOL 33, NO. 10, SEPTEMBER 1961
1357
Table 11.
Errors Introduced Using Mean a’s (a’s = 39.6 sq. cm. per gram)
Sulfur, yo Ca1cd.a using mean Actual a’s Blend No.
msonpmrn
OF
BLEND
-
1 2 3 4 5
roculcaru
Figure 4. Change of sulfur mass absorptivity, a’s, with mass absorptivity of n-butyldisulfide-hydrocarbon blends, U’CHS, showing reduction in rate of change effected by screening Using 2-curie source
SOLVING THE CALIBRATION PROBLEM
In addition to the change in sulfur absorptivity, the rate of chan e also alters, becoming smaller a t higfer absorption. This is probably due to the fact that as a softer radiation component of the x-ray beam is absorbed in its passage through the sample, the energy distribution of the remaining x-rays is more uniform, resulting in a more monochromatic beam. Therefore, if some of the longer wave lengths can be removed before passing through the cell, the characteristics of the x-rays may be modified sufficiently to produce very much smaller changes in effective wave length, and hence of sulfur mass absorptivity with increasing absorption. Thought was therefore given to evolving a method of calibration which would substantially reduce the spread of xrays and also compensate to a large extent for the polychromatic nature of the remaining radiation. Use of Aluminum Screen. To remove a high proportion of the softer x-rays-Le., longer wave length -the effect of placing a thin aluminum screen permanently in front of and in contact with the tritium source was examined. With this screen !ca. 10 mg. per sq. cm. thickness) in place, the aluminum absorption curve was drawn and the various mass absorp &’CHI a’s, were calculated tivities, as before. The aluminum absorption curve between the limits 25 to 40 mg. per sq. om. was reasonably linear and a’*, was obtained with little difficulty. Sulfur mass absorptivities showed a very striking improvement in constancy a t different sulfur concentrations, indicating a more monochromatic x-ray beam (shown graphically in Figure 4). The improvement was good enough, assuming the method is being used solely for oils containing up to about 1.3% sulfur, to justify the use of an average a’B in the working equation. It was evident from these results that the reduction in polychromaticity brought about by the aluminum screen would be of considerable value in 1358
ANALYTICAL CHEMISTRY
0.13
o.Zi
0.45 0.84 1.00
Error,
%
0.15 0.23 0.43 0.78
+0.02
0.44 0.62 0.81 1.21
-0.02 -0.04
0.90
+O:Oi -0.02 -0.06 -0.10
Figure 5. Mass qbsorptivity of hydrocarbon, U’CH, related to carbon-hydrogen mass ratio, r, of sample
Oil Sample N0.b
1 2
3
0.46 0.66 0.88 1.30
-0.07 4 -0.09 Each calculated result is average of five determinations. * Actual results on oil samples are averages of five bomb determinations, Method IP.61/59 (4).
extending the use of the tritium method to high sulfur fuel oils. Experiments showed that although the variation of u’g over a wide range of sulfur concentrations (0 to 5% weight S) was still too large to permit an average u ’ ~to be used, a satisfactory calibration procedure was possible on the basis of relating the observed u ’ ~to the corresponding a’CHs for the sample. This is a somewhat empirical relationship, but it has a practical value since it is not feasible to obtain the true mass absorptivities of carbon, hydrogen, and sulfur for all variations in carbon-hydrogen mass ratio and sulfur content. The following calibration procedure, covering the entire range of sulfur contents 0 to 5.0% weight 5, was found to be convenient and accurate. In order to achieve more rapid analyses, a much stronger tritium source was used. The characteristics of this source were found to be identical t o those of the weaker source. This strong source is equivalent to 10 curies of tritium absorbed on an area of 0.44 sq. inch of titanium compared with the standard source of 2 curies on 0.78 sq. inch. With an aluminum screen (33 mg. per sq. cm. thickness) in place, the aluminum absorption curve was determined over the range 25 to 75 mg. per sq. cm. thickness of aluminum. A value for the was aluminum mass absorptivity (dAJ calculated from the slope of the curve a t the mid-point (that is at 50 mg. sq. cm.). The aluminum absorgz: closest to this mid-point was chosen as the standard reference absorber. In our experiments the average aluminum absorptivity was 11.66 sq. cm. per gram and the chosen reference absorber was 48 mg. per sq. cm. The mass absorptivities of hydrocarbon blends of varying carbonhydrogen mass ratios were determined and a graph was prepared of mass absorptivity against carbon-hydrogen ratio (Figure 5 ) . The mass absorp-
tivity of sulfur was determined a t various sulfur concentrations between 0.2 to 5.0% and a graph was prepared of mass absorptivity of sulfur against mass absorptivity of the blend (Figure 6). [The value of a’s for a given a’CHg also varies slightly with the carbon-hydrogen mass ratio of the base stock. Possible errors within the range of carbonhydrogen ratios to be expected in petroleum products (that is between 6 and 9) would only be of significance in the higher sulfur concentration (for example on residual fuel oils). For this reason a 7.8 carbon-hydrogen ratio base stock was used for calibration purposes.] Using the count rate obtained on the sulfur blends a calibration graph was prepared showing the relationship b e tween absorbance per cm. of cell length (A’ = p c ~ ’ and ~ ~ ~lo) transmittance relative to aluminum log - over
r: 2:)
the whole range 0.2 to 5.0% S. This is used in actual determinations to obtain A’, the absorbance per cm. of the sample. In applying the method to actual samples, the basic working equation (Equation 5) is used in the calculation of sulfur contents. In this equation thefe are three unknowns, a’cm or A a f C Hand , a’s. The values - and a’g
(
P
3
are obtained from the count rate and the density specific gravity is sufficiently accurate with reference to the calibration curves. The value U’CH is obtained from the known carbon-hydrogen mass ratio and the appropriate graph. Before putting the method into use on a routine basis, it was necessary to determine the accuracy of the method as applied to actual oil samples such as gas oils and residual fuel oils. The results of a correlation program are discussed elsewhere in this paper. Accuracy and precision were excellent, although care was required in determining the standard absorber count. In general, to reduce statistical and mechanical timing errors, it was advantageous to take a long count (250,000) on the standard absorber once daily. The plotted points for log PA1 against time then fell fairly close to a straight line showing the decay characteristics of tritium. A count well out of line will show up an instrument fault and this has proved to be valuable in practice.
24.
5
;23. e
p
22.
8
1
0.9
1.0
1.1
1.2
1.3
ABSORPTIVITY
OF
1.4
1.5
BLEND,
1.6
1.7
1.B
G9
SO.CU./GRAU
figure 6. Change of sulfur mass absorptivity, a’s, with total mass absorptivity, a’cns, Using screened 10-curie source. 0 n-Butyldirulflde-hydrocarbon blends X Petroleum products
DISCUSSION
The basic concept of the above method is the recognition that U’S is not a constant and that the value is dependent on, and can easily be obtained from, the absorptivity of the sample. This is essentially a practical device, which cannot be proved easily by mathematical reasoning owing to the complex relationship arising from the absorption of polychromatic radiation. This approach can be justified only by the results obtained. There is much to be said for the use of petroleum oils to calibrate an x-ray instrument, since variations due to differences in density, composition, presence of trace impurities, etc., tend to cancel out. This technique has given the most satisfactory results on older methods using x-ray tubes as the source of radiation. In the development of the method described in this paper, blends of pure compounds were used throughout for calibration purposes, but in practice, for the determination of a’s, petroleum oils of known sulfur content may be used in place of the standard sulfur blends. As a means of compar-
ing the agreement between these two calibration procedures, separate experimental graphs of a’s against U’CHE were prepared using synthetic blends in one case and petroleum oils in the other; the two graphs are practically identical except for a slight divergence at low sulfur concentrations (Figure 6). The excellent correlation over such a wide sulfur range between curves obtained from totally different products is indicative of the soundness of the calibration technique. Although a knowledge of the carbonhydrogen mass ratio is egsential, in practice frequent carbon-hydrogen ratio determinations are quite unnecessary. Similar products, even from different crudes, usually have similar carbonhydrogen ratios and one determination on each product normally handled should provide sufficient information for accurate sulfur analyses. For example, in the authors’ laboratory, light, medium, and heavy fuel oils were found to have carbon-hydrogen ratios of 7.6, 7.8, and 8.0, respectively. The average figure of 7.8 is sufficiently precise for use with all three oils, different batches being unlikely to vary more than 0.1
carbon-hydrogen ratio. Similarly, all gas oil blends have carbon-hydrogen ratios around 6.4. Separation of the oils into groups in this way also allows the use of simplified forms of the working equation. Accuracy and speed combined with the low cost (approx. $1000) of the equipment make this an invaluable method for any laboratory carrying out regular sulfur analyses on petroleum products. I n the authord laboratory where 15 to 20 sulfur determinations are carried out per week, the calculated payout was less than 6 months, based on laboratory savings alone. Reproducibility and Accuracy. For general acceptance b y petroleum testing laboratories as a routine quality control test it is necessary that results obtained should agree closely with those obtained by standard American Society for Testing Materials or Institute of Petroleum procedures. The bomb method (1, 4 ) has for many years been accepted as the reference method for nonvolatile petroleum products. A number of gas oils and fuel oils obtained from Kuwait crude oil covering the entire range up to 5% wt. of S were tested by the x-ray and bomb methods by independent operators. To reduce random error pertaining to the bomb method, five tests were carried out on each sample. All samples were retained and later tested by x-ray a t intervals of 3 to 4 months. In addition, a number of samples from unknown crude sources were also tested by the bomb and x-ray methods. The results are given ‘in Tables I11 and IV. The agreement between the bomb method and the x-ray method is very good and well within precision limits of the A.S.T.M. bomb method (1) over the whole range, except for a slight deviation a t the low end-i.e., below 0.8%-where the x-ray results are slightly higher. This is not serious and may be corrected for easily if necessary
Table 111.
Sample Gas oil A B C
Marine diesel D
Resid. fuel oil E F G H
Correlation Tests on Oil Samples Showing Comparison with Bomb Sulfurs (Method of calibration, standardization with prepared blends containing n-dibutylsulfide) Sulfur, %, by X-Rayb Sulfur, yo, by Bomb. 10/7/59 11/26/59 1/20/60 5/27/60 9/1/60
11/17/60
0.88 0.66 0.46
0.92(0.89) 0 . 7 1(0.68) 0.52(0.48)
0 . 9 1(0.88) 0 . 7 1(0.68) 0.51(0.47)
0.89(0.86) 0.72(0.69) 0.52(0.48)
0.88(0.84) 0.71(0.68) 0.53(0.49)
0.85(0.82) 0.66(0 62) 0.50(0.46)
0.85(0.82) 0.67(0.63) 0.47(0.43)
1.30
1.32
1.32
1.33
1.31
1.28
1.33
3.44 2.70 4.04 4.56
3.47 2.75 4.08 4.59
3.42 2.71 3.97 4.58
3.39 2.70 3.95 4.53
3.44 2.74 4.04 4.60
3.40 2.68 3.98 4.54
3.42 2.70 3.95 4.50
I
Average of 5 determinations ( 4 ) . Figures in parentheses show results using standardization with petroleum oils and bomb sulfurs; above 1.0% sulfur the results are same by both methods.
VOL. 33, NO. 10, SEPTEMBER 1961
1359
by a modification of the calibration procedure using hydrocarbon oils. The repeatability of the x-ray method over periods up to one year is also extremely good and considerably better than the precision limits of the bomb method. A number of skilled and semiskilled laboratory personnel have been concerned in these long-term correlation programs and the accuracy and reproducibility obtained is indicative that personal bias and the likelihood of human error have been reduced considerably. The various factors, statistical and otherwise, which may influence the accuracy of any one determination have been discussed fully by Hughes and Wilczewski (9). and their findings are in general applicable to this method. It is of interest to point out the significance of the carbon-hydrogen mass ratio: in this respect a n error of 0.5 in the carbon-hydrogen mass ratio will result in an error only of the order of 0.05% S. No apparent errors from metallic impurities present in oils have been encountered. Trace analyses on residual fuel oils for nickel, vanadium, iron, sodium, etc., have shown total metals contents of less than 200 p.p.m, in all cases. Concentrations of this order are unlikely to have any fiignificant effect on x-ray results. The method is, however, not suitable for products contain-
.
Al, C, H, S
Table IV. Correlation Tests on Residual Fuel Oils from Unknown Crude Sources
Sulfur, yo I.P. bomba X-ray 3.14 3.27 3.23 3.20 3.22 3.21 3.62 3.62 3 60 3 16 3.27 3 25 3.95 3.97 4.00 a Average of 3 determinations.
ing such substances as tetraethyllead, lubricating oil additives, metal soaps, etc. ACKNOWLEDGMENT
The authors thank J. F. Cameron of Atomic Energy Research Establishment (Isotope Division), Harwell, England, particularly for procuring the 10-curie source, and the Technical Services Laboratory, Mobil Oil Co., Ltd., for assistance in setting up the equipment. The authors also thank the Chairman and Directors of Mobil Oil Po., Ltd., England, for permission to publish this paper. NOMENCLATURE
A = angstrom A’ = ahsorbance per centimeter a’ = mass absorptivity, sq. cm. per gram
= chemical symhols of el+ ments, used i n equations directly and as subscripts b = thickness of absorbing medium, or internal cell length, cm. c = mass fractional concentration N = number of Quantataken for a single reading P” = radiant Dower of x-rav beam leavine absorbing medium, uncorrected;
c.p.5.
P’ = radiant power of x-ray beam leaving absorbing medium, corrected for resolving error, c.p.3. P = radiant power of x-ray beam leaving absorbing medium, corrected for resolving error and background, c.p.m. Po = radiant power of x-rav heam leaving empty cell. corrected for resolving error and harkp.rorind, c.p.m. r = carbon to hydrogen mass ratio = CC/cH s = standard deviation t = resolving time or quench time, seconds = density of absorbing medium p LITERATURE CITED
(1) Am. Soc. Terting Materia’s, Philadelphia, Pa., Method D 129-5R, 1958. (2) Chemical Rubher Piihlishing Co., Cleveland, Ohio, “Handhonk of Chemistry and Physics,” 32nd ed., p. 2182,
1950. (3) HugheR, H. K., FT7ilcmwki, J. W., ANAL.CHEM.26, 1889 (1054). (4) Inrtitute of Petroleum, London, England, Method IP61-59, 1059. (5) Kannuna, N. M., J . Inst. Petrol. 43, 199 (1957). RECEIVEDfor review December 19, 1960. Accepted May 19,19G1.
.Fluorometric Determination of Aluminum in the Pa rt- pe r- B iIIio n Ra nge FRITZ WILL 111 Alcoa Research laboratories, Aluminum Co. o f America, New Kensington, Pa.
b Conditions were established for the fluorometric determination of aluminum with morin in the part-per-billion concentration range. A simple procedure has been developed where the fluorescence of the aluminum-morin complex is determined instrumentally at a p H of 3 with a minimum of sample dilution. The tolerance to diverse ions has been established. A sample can b e run in less than 25 minutes at room temperature, or less than 5 minutes with heating. For example, the method i s applicable to high-purity boiler water condensates.
S
reagents have been used recently for the fluorometric determination of aluminum. Collat and Rogers (2) have determined aluminum EVERAL
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ANALYTICAL CHEMISTRY
and gallium in mixtures of their oxinates. Simons, Monaghan, and Taggart (3) used Pontachrome Blue Black R for the determination of aluminum and iron in surface sea water. More comprehensive reviews of fluorescent reagents for aluminum have been written by White (4, 6). In addition, Bozhevol’nov and Yanishevskava (1) reported the use of o-(salicy1ideneamino)phenol for the fluorometric determination of aluminum. In 1940, White and Lowe (6) published a quantitative method for aluminum in the l1.1t o l.Z-mg.-per-liter range using morin (2 ’.3,4‘,5,7-pentahydroxyflavone). The need arose for the determination of microquantities of aluminum (parts per billion) in high-purity boiler water condensates in electric power station boiler condensers using aluminum con-
denser tubes. Morin appeared t o be the most promising reagent of those reported. This paper deals with the use of morin as a reagent for the fluorometric determination of aluminum in the partper-billion range in high-purity water with a minimum of sample dilution in the shortest length of time. These three advantages are not found simultaneously in any other method. EXPERIMENTAL
Apparatus. Fluorometric measurements were made with a Fisher Nefluoro-Photometer, equipped with a mercury arc source, a 440-mp excitation filter, and a 525-mp fluorescence filter. Reagents. MORINSOLUTION. A solution containing 0.1 gram of mofin
