Development of an X-Ray Emission Spectrography Method for the Determination of Molybdenum in Oils Dobrinka Jovanovit Institute f o r Technology of Nuclear and Other Mineral Raw Materials, Beograd-Fran3e D'Epere-a
A NUMBER OF METHODS for determining various metals in oils have been described in the literature. Reviews of these reports have been presented (1-7). The purpose of this work was to investigate the quantitative determination of molybdenum in oils using the X-ray spectrograph with the hope of reducing the time and quantity of sample necessary for an analysis. One method developed requires ashing of the oil sample, solution of the ash, and addition of an internal standard. An aliquot of the ash solution is analyzed with the X-ray spectrograph for molybdenum. The method is equal in speed and accuracy to emission spectrographic methods. Another method for direct determination of molybdenum in oils is faster than the method using an internal standard since the preliminary ashing and dissolution are not required.
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cooled. Next 15 ml of sulfuric acid, 3 ml of nitric acid, and 10 ml of Perchloric acid were added, and the sample was boiled to effect solution. Then it was transferred quantitatively to a 50-m1 DETERMINATION OF MOLYBDENUM BY INTERNAL STANDARD METHOD
INSTRUMENTATION
A method for the determination of molybdenum in oils should be simple, reasonably rapid, and applicable to a wide variety of oils. A compensation for matrix effects, for absorption effects, for enhancement effects, and for some instrument variables can be obtained by using an internal standard. Although strontium is a convenient internal standard for solid samples of molybdenum, it is not suitable for solutions which contain sulfate ions. An alternative standard, yttrium, has been found to be satisfactory for use with sulfate solutions
The instrument used was a Phillips Norelco Universal Vacuum X-Ray Spectrograph with a tungsten target tube, type FA-60. The X-ray tube was operated at 50 kV and 30 mA to excite to a suitable intensity the Kal line of molybdenum and Kaulline of yttrium. A flat crystal of lithium fluoride was used to diffract the desired wavelengths of fluorescent radiation. The detector was a scintillation counter.
The molybdenum KC^ analytical line intensity is measured at a fixed position of 20.40' 2 8. The yttrium K a I analytical line intensity is measured at a position of 23.75' 2 8. The background count at the position of the peak is interpolated from the background counts on either side of peak. The calibration curve obtained by plotting intensity ratios of the molybdenum to the internal standard us. the concentration of molybdenum is a straight line.
EXPERIMENTAL Preparation of Standards and Oils. A master standard was prepared by dissolving weighed amounts of high purity molybdenum trioxide in sulfuric acid. A series of standards then was made covering the concentration range from 0.1 to 1.2 mg of molybdenum per ml by accurate successivedilutions of the master standard. An appropriate amount of oil (about 10-20 g) was weighed into a 500-ml Vycor beaker. The beaker containing the sample was heated on a high temperature hot plate until only completely dry coke remained. The beaker containing dry coke was placed in a muffle furnace, and the coke was reduced to an inorganic ash at 450 "C. It should not be heated above 500 "C since molybdenum trioxide is volatile. After the sample had been reduced to an inorganic ash, it was removed very carefully from the muffle furnace, and ( 1 ) R. Louis, Erdoel Kohle, Vol. 17, 360 (1964), Chem. Abstr. 61,
4122a. ( 2 ) N. Okamoto, Kagaku No Ryoiki, 18, 547 (1964); Chem. Abstr. 63, 11201f. ( 3 ) G . Baron, and C. Fagot, Reu. Uniu.Min., 17, 275 (1961). ( 4 ) C. W. D. Dwiggins and H. N. Dunning, ANAL.CHEM.,31, 1040 (1959). ( 5 ) E. L. Gunn, Aduan. X-Ray Anal., 6,403 (1963). 27, 1880 (1955). ( 6 ) E. N. Davis and B. C. Hoeck, ANAL.CHEM., (7) G. V. Dyroff, and P. Skiba, ibid., 26, 1774 (1954).
(8).
DIRECT DETERMINATION OF MOLYBDENUM
A method has been developed for determining molybdenum directly in oils without prior ashing of the sample. Because the method requires neither ashing nor precipitation steps, it is extremely rapid. An analysis can be completed in about ten minutes. To make an analysis, we use two samples of oils with known concentration of molybdenum, one whose molybdenum content is near the high end of the specification range and one whose molybdenum content is near the low end. These standard samples of oils were obtained by analyzing a large number of samples that had previously been checked using the internal standard method. We selected two which were in our range of interest and had them reanalyzed very accurately. Properly chosen wavelengths of scattered radiation from the sample will also compensate for instrumental variables and absorption, acting in the same way as an internal standard. The scattered radiation is measured at a position of 19.30'2 9. We find the counting rate of the two samples-standards and plot a straight line graph of the intensity ratios of the (8) D. Jovanovid, Hemijska industrija, Tehnika, XXIII, No. I (1969). ANALYTICAL CHEMISTRY, VOL. 42, NO. 7, JUNE 1970
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Table I. Comparison of X-ray, Spectrophotometric and Spectrographic Results Mo uum
Sample No 1 2 3 4 5
6 7 8
9
X-ray with inter. Spectrostandard photometric 473 480 1250 1200 540 450 140 127 1200 1100 825 900 740 700 250 230 ... 690
Spectrographic 400 1040 360 120 970 645 492 180
...
X-ray direct ... 1230 470 90 1290
...
720 670
molybdenum peak to the scattered radiation as the ordinate us. the concentration of the molybdenum as the abscissa. From the counting rate of the unknown samples we read the molybdenum concentration from the graph. COMPARISON WITH OTHER ANALYTICAL METHODS
molybdenum in lubricating oils, the procedure described by Meeker and Pomatti (10) was adopted in our laboratory. Agreement is in the range of the precision of the X-ray spectroscopy. The comparison does not indicate that any method is less accurate than the other three. The four methods are all reasonably accurate, and the choice among them will depend on convenience, cost, and required sensitivity. CONCLUSION
X-Ray fluorescence analysis greatly reduces the time required for the analysis of molybdenum in lubricating oils. It is possible to do molybdenum analysis directly on lubricating oils samples. If reliable and more accurate results are required, the addition of an internal standard such as yttrium is necessary. The simplicity of the method is an important feature. Sample preparation and operation of‘the X-ray equipment are both simple procedures that require a minimum of technical skill. The chief limitation of the direct method is that a calibration curve must be prepared for each type of oil.
The molybdenum contents of lubricating oils obtained by the internal standard and direct methods are compared with those obtained by spectrophotometric (9) and spectrographic methods in Table I. For the spectrographic determination of
RECEIVED for review October 17, 1969. Accepted February 10, 1970. This report was presented at XV Colloquium Spectroscopicum Internationale, Madrid, May 1969.
(9) E. B. Sandell, “Colorimetric Determination of Traces of Metals,” 3rd ed., Interscience, New York, p 644, 1959.
(10) R. F. Meeker and R. C. Pomatti, ANAL. CHEM.,25, 151 (1953).
Mass Spectrometric Determination of Zirconium Hafnium and Titanium in Mixtures Using the Integrated Ion Current Method M. G . Allcock,l Ronald Belcher, J. R. Majer, and Roger PerryZ Chemistry Department, University of Birmingham, P.O.Box 363, Edgbaston Birmingham, 15, England BECAUSE of the closely similar size of the ionic and atomic radii of zirconium and hafnium, the properties of the compounds of these two metals are very similar. As a result the separation of the two metals is difficult and analytical procedures for the determination of one metal in the presence of the other have been difficult to devise. Methods involving differences in molecular weight have been suggested in which a mixture of the pure metal salts is weighed and converted quantitatively to a mixture of a second type of compound [e.g. oxide (1-3)]. The second mixture is then weighed and the relative amounts of the two metals calculated knowing their atomic weights and the appropriate conversion factors. A variation Present address, Dudley College of Education, Dudley, Worcestershire. Present address, Department of Civil Engineering, Imperial College of Science and Technology, London. (1) R. B. Hahn, ANAL.CHEM., 23, 1259 (1931). (2) S.Fujiwara, Chem. Absfr.,45, 4171 (1951). (3) Ibid.,46, 10047 (1952). 776
ANALYTICAL CHEMISTRY, VOL. 42, NO. 7 , JUNE 1970
of this technique uses the molar extinction coefficients of complexes of the metals with dyes such as Alizarin Red S and Xylenol Orange ( 4 , 5) rather than their molecular weights. An alternative approach is to utilize differences in stability of metal complexes in solution by measuring absorbances at different pH values (6, 7). Among the physical methods which have been recommended are X-ray emission and fluorescence (8,9), emission spectroscopy (IO),and neutron activation (11).
(4) H. Freund and W. F. Holbrook, ANAL.CHEM.,30, 462 (1958). ( 5 ) R. Z. Bachmann, ibid.,37, 103R (1965). (6) A. Mayer and G. Bradshaw, Analyst, 77, 476 (1952). (7) L. J. Kononenko, R. S. Lauer, and N. S. Pluektov, Chem. Abstr., 54, 9603 (1960). (8) G. deHevesey, “Chemical Analysis by X-rays and its Applications,” McGraw-Hill, New York, 1932. (9) L. Birks and E. Brooks, ANAL.CHEM., 22,1017 (1950). (10) G. W. Kingsbury and R. B. Temple, J. Appl. Chem., 25, 296
(1953). (11) W. D. Mackintosh and R. E. Jervis, ANAL.CHEM.,30, 1180 (1958).
