1774
ANALYTICAL CHEMISTRY
spectrograms is corrected for the “sum of squares’’ for operators according to the arithmetic example given below. n
0.005
=
8; Z.C = -0.2;
ZX2 = 0.62
2(X-Z)’
= 0.62 - 0.005 = 0.615
zc2
Z(C-(?)2
= --
202
=
=
0.04
1 30
Z(0-6)’
(ZZ)’/~=
0.04 4
1 30 4
= ---
- 0.005
-0
005
Degrees of Freedoin 1 2 1
3
Sum of Squares
= 0.005
=
0 320
Mean Square 0.0050 0.1325 0.3200
0.005 0.265 0 320 0 028
0 0083
The mean square tabulated values are estimates of the following variances, where the variance is designated s*: Mean Square Calculators Spectrograms Operators Residual
Variance Estimated (calculator) 8 2 (resid ) 2s2 (spec ) s2 (resid.) 492 (opr.) 292 (spec.) 8 2 (resid ) s 2 (resid ) 452
+ ++
(calculator) = 0 , 0 0 0 0 a* (spec.) = 0.0621 s2 (opr.) = 0.0469 s2 (resid.) = 0,0083 0.1173 52
The analysis of variance is most simply given by another table as follows: Source of Variance Calculators Spectrograms Operators Residual
From these relations the estimated variances are now coniputed:
+
Before carrying out the calculation of the estimated variances, the mean squares are tested for significant difference from the residual mean square by the standard F test for variance ratios. In the illustrat.ive example both the spectrogram and operators mean squares are significantly larger than the residual mean square and therefore j ust’ify calculation of the individual variances. LITERATURE CITED
(1) Blears, J., and Waldron, J. D., J . Inst. Petroleum, 40, 1-6 (19543. (2) Brownlee, K. -4., “Industrial Experimentation,” 4th ed., London, H.M. Stationery Office, 1949. (3) Shepherd, Martin, AKAL.CHEM., 22, 885 (1950). (4) Shepherd, Martin, Natl. Bur. Standards (TJ.S.),Research P a p e r
R P 1740 (March 1946). (5) Ibid., R P 2098 (May 1950). (6) Snedecor, G. W., “Statistical Methods,” 4th ed., arnes, Iowa, Iowa State College Press, 1946.
RECEIVED for review August 3, 1953. Accepted August 12, 1951
Determination of Trace Amounts of Iron, Nickel, and Vanadium on Catalysts by Fluorescent X-Ray Spectrography G. V. DYROFF and PAUL SKIBA Esso Laboratories, Research Division, Standard O i l Development Co., Linden,
The deleterious effect of minute amounts of metallic elements, notably nickel and vanadium, on cracking catalysts has been known in the petroleum industry for some time; accordingly, much time and effort have been expended in developing suitable methods of analyzing catalysts for these contaminating metals. Both chemical and spectrochemical methods have been used with varying degrees of success. The former method is time-consuming and both methods appear to lack the desired degree of precision for distinguishing between a good and a poor catalyst. A method for determining trace amounts of iron, nickel, and vanadium on a given cracking catalyst by means of x-ray fluorescence is described. The method is rapid, requiring about 15 minutes for a complete analysis. As little as 2 grams of sample can be used for the analysis, and because the method is nondestructive, the sample can be recovered completely after the analysis.
I
NASMUCH as both chemical and spectrochemical methods
of determining trace elements on cracking catalysts appeared to lack the desired degree of precision for distinguishing between a good and a poor catalyst, a more precise method of analysis was desirable. The introduction of the x-ray spectrograph as a tool for quantitative analysis seemed t o offer a method of determining trace elements rapidly and precisely. Consequently, after making a thorough study of the different variables, a method for determining trace amounts of iron, nickel, and vanadium on cracking catalysts by x-ray fluorescence has been developed. The method is rapid, requiring only 15 minutes for a complete analysis.
N. 1.
The present procedure uses approyimately 20 grams of sample, but a 2-gram sample could be used. I n either case, however, the sample is not destroyed and can be recovered completely after the analysis. Finally, the method is precise. The principle, upon which the analysis of elements by their characteristic x-ray spectra is based, is fundamental and is dependent upon the atomic properties of the element. When a given element is irradiated with x-rays, it will fluoresce other longer x-rays, and each element has a characteristic x-ray fluorescence spectrum. The emitted rays are passed to an analyzing crystal where they are diffracted and reflected into a Geiger tube which measures the intensity of the radiation. This intensity is proportional to the concentration of the element being counted ( 2 , 4-41. APPARATUS
The apparatus used for the development of this procedure is a modified North American Philips Geiger counter spectrometer. Many modifications of the basic equipment as well as specialized techniques are re uired when trying to determine trace elements on catalysts. ?‘he following modifications of the standard equipment have been made. Collimation. The spectrograph, as it was received from the factory, had four sets of collimators, a pair each of 8- and 4-inch aluminum 3/8-inch-square tubes, containing either l / 3 r or I/la-inch nickel tubes. (The newer models do not have this type of collimator and therefore, the following modification may not be necessary with the present collimators.) When using the l / l ~ inch tube of 4-inch collimator a desirable counting rate as not obtained. By removing the nickel tubes from the lower collimator (the one that sees the sample), higher intensities were obtained without loss of a significant amount of resolution. The half-height breadth of the peaks is of the order of 1.3” to 1.5”, more than enough resolution for the present requirements ( 1 ) Helium Atmosphere. Increased intensities can also be ob-
1775
V O L U M E 26, N O . 1 1 , N O V E M B E R 1 9 5 4 tained by permitting the x-rays to pass through a medium less dense than air. A vacuum, hydrogen, or helium atmosphere would be suitable for obtaining this increased intensity. Mechanical difficulties of construction precluded the first method, the safety hazard involved in the second method precluded its use, leaving helium as the choice of operating medium. The system consists of a plastic laboratory sleeve attached by rubber bands to two Lucite rings which were machined to fit over the Geiger tube and the lower (sample) collimator. Similar connections are made on the side of the bag to attach it to the collimator and crystal rocking shafts. Helium is continuously bled through the system through two inlet tubes, one located a t the Geiger tube, the other a t the side of the x-ray tube housing adjacent to the sample cell. The helium is exhausted through the natural openings in the system ( 5 ) . I
I
I
sseur. /MINUTE
the highest concentration sample in the sample holder and scanning through the spectrum. The various peak positions and the angular location used to obtain the background counts are shown in Table I. Interferences. The secondary x-rays emitted from the metals in the sample are partially absorbed by the sample itself. The excited metal atoms are located a t varying distances below the sample surface and the radiation from these atoms in reaching the surface is subject to absorption by other constituents of the sample. Each element has an absorption power nhich varies with the x-ray wave length This introduces the possibility of interference from the sample itself. For example, the presence of vanadium in a sample might result in the lowering of the intensity of the iron radiation, thereby giving an erroneously low value for the iron concentration.
To study this effect, more synthetic samples ere prepared. Samples covering the range of iron concentrations and each containing 0.1% of vanadium were prepared. Similarly, samples of nickel containing 0.1% of vanadium and samples of vanadium containing 0.5% of iron were prepared and counted.
310-
/
0
0
I
I * "INAOIY"
e* FRESH CIII\L"SI
I
I
The effect produced by using helium is shown in Figure 1. Here, a minimum flow rate of about 550 ml. per minute is required for the optimum effect of the helium. Crystals. Both sodium chloride and lithium fluoride analyzing crystals are available for this instrument. Inasmuch as the lithium fluoride crystal causes a slight increase in intensity, it, was used in the present v,-ork.
In the case of iron and nickel, the presence of nickel would be expected to enhance the iron intensity. However, because the nickel is a t a much lower concentration level than the iron and only nickel fluorescent radiation is involved, this effect should be slight. Various concentrations of iron would give rise to some self-absorption of the nickel radiation. The degree of this selfabsorption is small and usually cannot be measured precisely. Because no large discrepancies in iron or nickel values were obtained when analyzing standard samples, these specific effects were not studied extensively. Although other elements are present on catalysts, their concentrations generally are lower than those presently being determined; hence their effect should be negligible. For this reason, the effects of elements other than those studied were not investigated at this time.
EXPERIMENTAL
Preparation of Calibration Standards. A series of standard samples containing the required constituent and covering the desired ranges was prepared as follows: Standard solutions of iron and nickel were prepared by dissolving the metals in nitric acid and making up to volume with distilled water; the vanadium standard solution was prepared by dissolving ammonium metavanadate in distilled water and making up to volume. As a check on their preparation, these solutions were then analyzed by standard chemical methods of analysis.
Table I. Angular Location of Peaks and Background (Csing lithium fluoride crystal) Degrees, 28 Element Peak Background Nickel, K a (1.66 A.) 48 48 54 Iron, K a (1.94 4 . ) 57.20 54 Vanadium, K a (2.50 A.) 76.70 74
A synthetic alumina-silica cracking catalyst \vas then contaminated with the desired metals as follows: The required volume of solution-Le., iron, nickel, or vanadium-was added to a weighed amount of catalyst. This slurry was then laced in a jar mill, thoroughly mixed and ground for 2 hours, &en placed overnight in an oven a t 110' C. The dry samples were then reground in the jar mill for 1 hour, and then calcined for 1 hour in a muffle furnace a t 500" C. Three sets of standard samples, each containing iron, nickel, or vanadium, were prepared by this method. Construction of Calibration Curves. A calibration curve for each element was prepared by plotting counts per second above background us. weight per cent of element. To obtain these counts, it is necessary to determine the location of the peaks for each of the elements and to decide upon an angular position for obtaining the background count. This can be done by placing
Figure 2. Calibration Curve for Iron on Catalj-sts 35 Kv., 20 m a . , LiF cqstal. a n d helium atmosphere
Another type of interference uncovered during this study is the result of impurities in the x-ray tube (target or filament). This is particularly true of the nickel determination. Because of the presence of nickel in the tube, high count rates for nickel are obtained on blank samples, and it is accordingly difficult to obtain good counts for small incremental changes in nickel concentration. (Graphically, Figure 3 shows that extrapolation of the nickel calibration curve to zero nickel concentration gives a count rate of about 900 counts per second. This high count rate is the result of nickel impurities in the w a y tube itself.) PROCEDURE
Calibration. Prepare or obtain standard samples containing the desired iron, nickel, and vanadium concentrations. Determine the location of the iron, nickel, and vanadium peaks by scanning the samples having high concentrations of these elements. Using the tungsten target tube at, 50 kv. and 50 ma.
1776
ANALYTICAL CHEMISTRY
with a lithium fluoride analyzing crystal, the K alpha line for vanadium came a t 76.6" (20). For optimum results, the excitation conditions for iron and nickel are 35 kv. and 20 ma. Thz iron-peak position is a t 57.25" (20), and the nickel peak a t 48.5 (20). The background count is taken a t the respective angular locations as shown in Table I. After locating the peak positions, take a fixed number of counts and record the time interval required to obtain these counts. Calculate the counts per second by dividing the total number of counts by the time. Repeat this procedure a t the appropriate background position a t the two respective excitation conditions. Plot the peak counts minus the refipective background counts against weight per cent concentration of the respective elements. Typical calibration curves are shown in Figures 2, 3, and 4. COUN:S/YCMID
1
1
i
i
I
I
I
1
I
ABWE BACXGRWMI
Figure 4. Calibration Curve for Vanadium on Catalysts
1
50 Kv., 50 ma., LiF crystal, and helium atmosphere
peak counts. This count above background is plotted against per cent weight concentration of each element. For better results it is desirable to obtain analyses on three aliquots of the same sample and report the average of these determinationfi. DISCUSSION
Figure 3.
Calibration Curve for Nickel on Catalysts
35 Kv., 20 ma., LiF crystal, and helium atmosphere
Care must be taken in establishing the calibration curves for each element. The count rates obtained should be within the linearity of the Geiger tube and count register. For that reason, the nickel and iron determinations are obtained a t a lower x-ray tube KVP. The calibration curves are strictly empirical, no corrections being made for dead time of the Geiger tube.
Tables 11, 111, and I V present the data obtained when the effect of possible interferences was being studied. Statistical tests of significance of these data indicated that there is no significant difference between the counts obtained on a sample containing only one constituent and one containing more than one element. Therefore] it was concluded that over the concentration ranges studied, the interferences are insignificant and will not interfere with the accuracy of the determinations.
.Table 11.
Operating conditions are tabulated. Helium Atmosphere, LiF Crystal, Tungsten Radiation Determination Vanadium Sackground Background Iron Nickel
Excitation K v . Ma. 50 50 50 50 35 20 35 20 35 20
Bragg Angle, 76.70 74.00 54.00 57.20 48.48
O
28
Fixed Counts 256,000 12,800 6,400 512,000 128,000
Analysis of Samples. By means of a mechanical grinder, thoroughly grind the usual plant or laboratory fluidizable cracking catalysts and then place in the sample holder. Fill the sample holder to overflowing and tap lightly while in a level position. Make the surface of the sample level with the surface of the holder by drawing the edge of a spatula across the top of the holder. Place the sample in position in the fluorescence unit. After 2 minutes, so as to ensure displacement of air by helium, set the spectrometer on the correct angular position for measuring the desired element or background. The correct excitation conditions and fixed count factors are pre-set for a desired analysis hleasure the time for the given fixed count for a given determination. Repeat this procedure on two other separate aliquots of the sample, reporting the average value obtained. In order to check the daily operation of the instrument, one of the standard samples used for calibrating is run before any samples are run. If the count rate obtained is within the expected precision, then the samples are analyzed. If there is a significant change in the count rate, which frequently hap ens to the nickel calibration] new calibration curves are preparecfbefore analyzing any samples. Calculations. Convert the fixed counts to counts per second after measuring the time elapsed to obtain a given count and determine the Der cent of iron, nickel, or vanadium from resDective calibration e k e s . Counts per second =
total counts time in seconds
The counts per second for background are subtracted from the
(1
Effect of Vanadium on Iron Count Rate
Average Counts/Second above Background at Fe Peak" Fe Concn., % 0% V 0.05%V O.lO%V 860.7 863.3 0 30 868 1 315 2 317.5 0.10 317 0 123 2 125 2 124 5 0 03 Average of 5 determinations made on 5 successive days. ~
Table 111.
a
Effect of Iron on Vanadium Count Rate
Average Counts/Second above Background at V Peak= V Concn., % 0 % Fe 0.5% Fe 302.7 0.10 296 7 173.4 172 9 0.05 Average of 5 determinations made on 5 successive days. ~
Table IV.
a
Effect of Iron on Nickel Count Rate
Average-Counts/Second above Background at Ni 0 . 5Ni %Peak" Fe Concn., 7, 0% Fe 1100.3 1107.2 0.1 1056.4 1067.9 0.05 Average of 5 determinations made on 5 successive days.
After the calibration curves had been prepared and the effect of other elements had been determined, it was only necessary to run some actual catalyst samples. Precision of Determination. The sources of error in a determination of this sort may be the result of sampling errors, counting errors, and other inherent errors of the spectrograph. Barring gross inadequacies in current and voltage stability of the spectrograph, and assuming a perfectly homogeneous sample, the precision of the determination should be equivalent to the counting error. However, the standard total counting error, S, = dN may not be taken as a measure of the precision of the analytical result. When making an analytical determination as
1777
V O L U M E 26, NO. 1 1 , N O V E M B E R 1 9 5 4 described, two counting errors are present as the standard background counting error, &, and the standard peak counting error, S,. The standard analytical counting error, Sa, which can be related to the error of determination, can be expressed by this formula (3). ..
where iV, and lyb t, and
tb
total counts collected a t the peak and background, respectively = time to collect .v, and S b , respectively =
If S. does not differ significantly from the calculated standard deviation of the final result, S, then a change in counting procedure gives promise of improving S. If Sa and S are significantly different, then other errors are present and exceed the counting error. To obtain an estimate of the precision of the determination, two production catalysts were chosen for study. Quintuplicate determinations were made on each sample. These replications were made as random as possible by running them on different t i actions of the samples on different days. Table V presents the data obtained on this experiment. Ex:iinination of these data show that although the precisions calrulated for the vanadium and nickel determinations agree sen-il)ly with the estimated precision from counting, #, the stand,iddeviation of the iron determination is five times higher than that predicted from the counting error.
Table V.
iron on the catalyst is contributing more to the standard deviation than is the precision of counting. The variance due to packing (methods) is also significant, but it is not significantly different from the sample variance and is probably result of the same source of variance-Le., iron distribution on catalyst. There is no interaction variance. A similar experiment was performed for the determination of vanadium and nickel, but no significant effects were disclosed. The results of these experiments indicate that it may be possible to improve the precision of the iron determination by homogenizing the sample. Accordingly, all of sample 21715 was thoroughly ground with a mechanical grinder and then further ground and mixed in a jar mill. Replicate determinations were than made on different portions of this sample. These results are shown in Table VIII. Obviously, improvement in the precision has been obtained by thoroughly grinding and mixing the sample. Statistical tests of significance show that, for the ground sample, there is no significant difference between Saand S.
Table YI.
2
Iron Found, % 3 4
5
6
0.2450 0.2425 0.2600 0.2526 0.2525 0.2525 0,2450 0 2486
0.2625 0.2700 0.2725 0.2700 0.2713 0.2700 0.2750 0.2702
0 2700
Av.
0.2675 0.2750 0.2650 0.2675 0.2650 0.2750 0.2675 0 2689
Undisturbed Sample 0 2900 0.2575 0.2950 0.2560 0.2950 0.2550 0.2925 0.2600 0.2925 0.2825 0.2900 0.2600 0.2950 0.2600 0,2929 0 2579
0 2700 0 2375 0 2550 0 2325 0 2250 0 2475 0 2488
AV.
0 2625 0 2250 0 2625 0 2950 0 2300 0 2600 0 2326 0 2525
Repacking Sample 0 2525 0 2488 0 2450 0 2600 0 2375 0 2488 0 2325 0 2732 0 2464
0 2700 0 2625 0 2975 0 2725 0 2525 0 2500 0 2950 0 2714
0 2700 0 2625 0 2650 0 2700 0 2988 0 2900 0 2975 0 2787
1
Determination of Precision
Sample No. 21715
S
Iron 0.2225 0.2525 0.2400 0.2325 0.2475 0.2390 0.011 0.003 0.2750 0.2925 0 2550 0.2780 0.2610 0 2740 0 015
S.
0 003
Av.
S S.
21717
Av.
Element, % Nickel Vanadium 0.0075 0,0090 0.0120 0.0100 0.0180 0.0110 0,0110 0.0085 0.0130 0.0125 0.0123 0.0102 0.003 0.002 0,002 0.002 0,00525 0.0055 0.00525 0.0040 0.00750 0.0040 0.00975 0.0055 0.00700 0.0065 0.00690 0 0051 0.002 0 001 0 002 0 002
-in explanation for this phenomenon is that a major portion of the iron on a cracking catalyst is contributed by corrosion and erosion of various size particles from various metal parts of the cracking unit, resulting in an uneven distribution of iron on or in the catalyst; whereas the nickel and vanadium are primarily deposited from the feed stock, resulting in more uniform distribution of these components. This hypothesis was tested by the following experiment. One particular sample was chosen for study, and the effect of element distribution on the catalyst was determined on six separate aliquots taken from the sample for counting. Each aliquot was counted in two ways: by replicate counts on each aliquot without disturbing the sample; and by replicate counts on each aliquot-after each count, the sample was poured out of the sample holder and then returned to the holder for another count. By this method, it was possible to separate the variance due to the distribution of the element on the catalyst (as shown by the various aliquots), the variance due to packing the sample, and the replicate variance. Table VI presents the data obtained for the determination of iron in this fashion, and Table VI1 shows the analysis of variance for these data. From this analysis of variance, it can be seen that the variance due to aliquots is significant (at the 99% level); therefore, the distribution of the
Replicate Iron Determinations on Various Aliquots of Sample 21716
0 2452
0 3125 0 2475 0 2925 0 2525 0 2825 0 2700 0 2550
0.2726 0.2725 0.2750 0.2725 0 2750 0.2700 0 2725
RESULTS
At the conclusion of the experimental program, several samples were analyzed by the procedure described. These samples consisted of artificially contaminated synthetic and natural catalysts and regular production catalysts. The results are shown in Table IrU, CONCLUSIONS
A method for determining small amounts of iron, nickel, and vanadium on a silica-alumina cracking catalyst by means of x-ray
Table VII.
Analysis of Variance from Data in Table VI
Degrees of Source of Corrected Sum Freedom Variance of Squares illiquots 0.01396 5 Packing 0.00105 1 Interaction 0.00184 5 Reulicates 0.01307 72 0,02992 83 Total a Significant at 99% confidence level. b Significant a t 9570 confidence level.
Variance 0 002792 0.001050 0.000368 0.000181
F 15.42" 5.80b 2.03
Table VIII. Determination of Iron in Sample 21715
Av.
S S.
Iron Found, % Original sample Sample after grinding 0.2225 0.2401 0.2525 0.2444 0.2400 0.2440 0.2325 0.2522 0.2475 0.2426 0.2447 0,2390 0.0046 0.0110 0,0030 0.0030
ANALYTICAL CHEMISTRY
1778
Table IX. X-Ray Spectrographic Results for Analysis of Catalysts Sample No.
19137a
Description
A P I , Fresh Natural Catalyst VanaIronb Nickel dium
Constituent
33966“
9234@
~
Iron
A P I , Artificially Contaminated Synthetic Catalyst VanaSickel dium Iron
Kickel
21261- __
21715
Vanadium
Iron
0,0670 0,0670 0 0700 0.0710 0,0743 0.0726 0 0700 0.0725 0.0690 0.0700 0.0704 0.0048
0 2444 0 2444 0 2422 0 2422 0 2467 0 2450 0 2444 0 2450 0 2430 0 2460 0 2443 0 0028
Regular Production Catalyst VanaNickel dium Iron Sickel
~T’ann-
dirini
% Found
AV. Precision,2#
0 . 8 9 5 0 0050 0.920 0 0050 0.885 0 0060 0.900 0 0045 0 . 9 2 0 0 0055 0.895 0 0050 0.895 0 0045 0 . 9 0 0 0 0060 0 . 9 1 5 0 0055 0.885 0 0050 0,901 0 0052 0 . 0 2 6 0.0010
0 0 0 0
0080 0113 0110 0098
0 0085 0 0110 0 0095 0 0115 0 0100 0 0100 0 0101 0.0023
0.4550 0.4725 0.4726 0.4600 0.4555 0.4550 0.4600
0 1009 0.1012 0 1018 0 1027 0 1008 0 1015 0 1018 0.4550 0 1020 0.4555 0 1000 0 . 4 6 0 0 0 1010 0.4601 0.1014 0.014 0.0015
1012 I005 1015 1008 0 1006 0 1006 0 1015 0 I005 0 I010 0 1010 0.1009 0.0008 0 0 0 0
0.2500 0.2550 0.2525 0.2498 0.2560 0.2500 0.2525 0.2450
0835 0890 0825 0825 0835 0825 0830 0880 0.2550 0 0835 0 . 2 5 0 0 0 0835 0.2514 0 0842 0.0063 0.0047 0 0 0 0 0 0 0 0
0 0123 0 0180 0 0110 0 0130 0 0120 0 0125 0 0130 0 0110 0 0115 0 0120 0.0126 0 0044
0112 010.5 0124 0115 0 0123 0 0115 0 0120 0 0115 0 0112 0 0120 0 0116 0.0011 0 0 0 0
0125 0950 1025 0950 0925 0925 0950 0 0960 0 0920 0 0950 0 0948 0.0061
0 0 0 0 0 0 0
0.0113 0.0120 0 0115 0.0145 0.0130 0 0115 0.0120 0.0115 0.0110 0.0115 0.0120 0.0022
Chemical< 0.947 0.0026 0.0081 0.416 0.119 0 101 0 251 0.086 0.070 0.239 0.0109 0.0072 ... . a These samples and chemical analyses were received through courtesy of Committee on .inalytical Research of American Petroleum Institrite. I, Obtained by extrapolation of calibration curve. C Average of replicate determinations in several laboratories.
fluorescence has been developed. The method has shown to be rapid, about 15 minutes being required for a complete analysis. When proper sampling techniques are employed, excellent precision is obtained, and the precision, measured by the standard deviation a t the 95% confidence level, can be expected to be of the following orders of magnitude: Element Iron Nickel Vanadium
Range, W t . 0 . 1 -1.0 0.002-0.10 0.002-0.10
Precision, 2 0 3% (relative) 0,002 0.002
With recognition of rapidity, the accuracy of the x-ray method aa determined by comparison with analyzed synthetic samples
is reasonably good.
.
0 0575 0 0600 0 0565 0 0560 0 0580 0 0570 0.0560 0 0575 0 0580 0 0565 0 0573 0 0024
...
deep interest and helpful criticisms which have made this work possible. LITERATURE CITED (1)
Brissey, R. JI.,ANAL.C H E M .24, , 1034 (1952).
(2) Ibid., 25, 190 (1953).
Brissey, R. M., Liebhafsky, H. A,, and Pfeiffer, H. G.. “Examination of Metallic Materials by X-Ray Emission Spectrograph,” Symposium on Fluorescent X-Ray Spectrographic Analysis -4STJl Meeting, Atlantic City, N. J.,June 1953. (4) Compton, A. H., and Allison, S. K., “X-Rays in Theory and Experiment,” 2nd ed.. Piew York, D. Van Nostrand Co.. 1935. (5) Davis, E. N., and Van Nordstrand, R. 8.. . ~ X A L . C H E X . ,26, (3)
973 (1954). ACKNOWLEDGMENT
(6) Friedman, H., and Birks, L. d.,Rev. Sci. Instr., 19, 343 ( 1 9 4 8 ) .
The authors wish to express their gratitude t o E. L. Baldeschwieler of the Esso Laboratories, Research Division, for his
RECEIYEn for reriew hfay 26, 1954. Accepted August 19, 1954. Presented before the meeting of the American Petroleum Institute, Houston, Tex., 1954.
Photometric Determination of Magnesium in Electronic Nickel C. L. LUKE and M A R Y E. CAMPBELL Bell Telephone Laboratories, Inc., M u r r a y
Hill, N. 1.
In the manufacture of vacuum tubes it is necessary to control the magnesium content of the nickel from which the cathodes are made. -4 new photometric 8-quinolinol-chloroform extraction method for the determination of 0.001 to 0.1% of magnesium in electronic nickel has been developed. After removal of interfering metals by hydroxide, oxalate, sulfide, and carbamate separations, butyl Cellosolve and ammonium hydroxide are added and the magnesium is extracted into a solution of 8-quinolinol in chloroform and determined photometrically.
B
ECBUSE one of the important factors which determine the
life expectancy of a vacuum tube is the magnesium content of the nickel used in the manufacture of its cathode, an accurate analytical method for the control of the magnesium content is required. As the amount of magnesium present is usually in the range of 0.001 t o O . l % , and the sample size is often limited, the method used must be very sensitive. The Titan Yellow photometric method has been used in this analysis, but it has not proved satisfactory because of its inade-
quate sensitivity and poor reproducibility. I n view of the fact that aluminum and several other metals have been determined by photometric 8-quinolinol (exine)-chloroform extraction methods, it seemed probable that magnesium might be determined in the same manner, following the removal of all interfering metals by suitable separations. Experiments showed, however, that whereas magnesium can be quantitatively precipitated in ammoniacal solution by 8-quinolinol, it is very incompletely extracted from ammoniacal solution by a solution of 8-quinolinol in chloroform. rin attempt was made, therefore, to find a suitable water-immiscible solvent for the magnesium quinolinate. Of all those investigated, only acetophenone was found to be a good solvent for the magnesium compound. The analytical results obtained with it, however, were not reproducible and its odor was objectionable. The possibility of improving the chloroform extraction was next considered. Experiments have shown that this can be done by the addition of a water-miscible organic solvent to the aqueous solution. It has been shown that butyl Cellosolve improves the extraction of antimony-rhodamine B compound by benzene (1), and that the addition of butyl Cellosolve to the aqueous solution
