Concentrations are determined in t h e following sequence: The iron concentration is read directly from the applicable calibration curve. For example, if the sample is chiefly tantalum, a s indicated b y the tantalum counting rate, the iron figure is read from the curve for iron in a tantalum matrix. The titanium concentration is read directly from the applicable calibration curve unless the iron concentration is greater than 5%. I n this case the titanium counting rate in counts per second is first corrected b y dividing i t by the factor indicated by its correction factor curl-e. The tantalum and niobium concentrations are read directly from their calibration curves unless iron or titanium is greater than 1%. I n this case, the tantalum and niobium counting rates are first corrected. Corrections are read from the factor curves according to the concentrations of iron and titanium, and the approximate ratio of tantalum to niobium. Siobium intensity is corrected by dividing b y the sum of the factors; tantalum intensity is corrected by division or by multiplication b y the factor sum if reciprocal factors are employed The determination of trace and small quantities in high-purity materials requires no intensity correction other than the choice of the applicable calibration curve. The accuracy and reproducibility of the method are indicated in Tables V
and T‘II shoiving typical results on n group of combined oxides. The accuracy of trace determinations in typical high-purity oxides prepared from metal and solution samples is shown in Table
VI. CONCLUSION
Accurate x-ray spectrographic analysis of tantalum, niobium, iron, and titanium in any combination of their oxides has been found practicable using arithmetic correction factors based on empirical calibration curves to compensate for interelement effects. A4fter chemical preparation of the oxides, materials such as ores, metals, and liquids fall within the scope of the method. The constancy of the correction factors with variations in instrumentation such as detectors, crystals, and x-ray tubes leads to their general adaptation. The possibility of applying the correction factor method to the x-ray spectrographic analysis of other combinations of elements suggests its widespread applicability. Thus, a start has been made toward the construction of tables of factors for the correction of interelement effects. ACKNOWLEDGMENT
The author wishes to thank the staff of the Analytical Laboratory for its unlimited cooperation in the preparation of standards, samples, and coni-
parative results, and the Electro Metallurgical Co. for its permission to publish this work. LITERATURE CITED
(I) Xdler, I., Axelrod, J. AI., Spectrochina.
Acta 7, 91 (1955). (2) Birks, L. S Brooks, E. J., ANAL. CHEhl. 22, loyy (1950). (3) Brisse R. X, Zbzd., 24, 1034 (1952). (4) C a m p h l , W. J., Carl, H. F., Zbid., 26, 800 (1954).
(5) Ibid., 28, 9GO (1956). (6) Carl, H. F., Campbell, IF’. J., Am. SOC. Testing J l a t e t i d s Spec. Tech. Publ. 157, 63 (1954). (7) Davis, E. X., Hoeck, B. C., h r a ~ . CHEM.27, 1880 (1955). (8) Hasler, 11. F., Kemp, J. W.,Ander-
mann, G., Fourth Annual Symposium on Industrial Applications of X-Ray Analyses, Denver, Colo., August 1955. (9) Kemp, J. IT’., Anderniann, G., Pittsburgh Conference on Analytical Chemistry and Applied Spectroscopy, Pittsburgh, Pa., Februarv 1956. (10) Mortimore, D. JI.,”Romans, P. A., Tews, J. L., Appl. Spectroscopy 8 , 24 (1954\. (11) ShiA’an, J., A t i ! . SOC. Testing ~ a terials Spec. Tech. Publ. 157, 27 (1954). (12) Sherman, J., Spectrochitta. Scta 7, 283 (1955). RECEIVED for reviex November 2, 1957. Accepted July 18, 1958. Presented in part, Sixth Annual S-Ray Symposium, Denver Research Institute, Cniversity of Denver, Denver, Colo., August 1957, and 12th -1nnual Symposium of the Society for hpplied Spectioscopy, Xe\y T o r k , S . T.,November 1057.
Spectrographic Determination of Boron and Silicon in Low-Alloy Steel by Fluoride EvoIutio n JAMES E. PATERSON and WILLIAM F. GRIMES Graham Research Iaborafory, Jones & laughlin Steel Corp., Pittsburgh 30, Pa.
b Copper fluoride decomposes when heated by relatively low amperage arcs to provide labile fluorine which will combine with boron and silicon to form volatile fluorides. The fluorides can be distilled from a deep cratered supporting electrode preferentially to most other compounds, giving a spectrum essentially free of lines other than those of boron and silicon, This system has been applied to the determination of boron and silicon in steels in the range of 0.001 to 0.020% with a coefficient of variafor boron and +3.270 tion of 2~3.070 for silicon. This system may also be applied to other materials.
T
spectrographic determination of boron and silicon in steel is not new; many excellent procedures are HE
1900
ANALYTICAL CHEMISTRY
available (1-6). Fluoride evolution was applied to the determination of these elements in steel to provide a check method on the standard procedures used in this laboratory. The method has since found application in the determination of boron and silicon in samples which do not fit more normal metallurgical spectroscopic practices. Because the bulk of the sample is not excited, the fluoride evolution can be used to determine boron and silicon in samples for which standards for direct excitation are not available. The procedure is applicable to the estimation of boron in low-alloy steel in concentrations below 0.00170, but no test of ita applicability to silicon in concentrations below 0.001% has been made. &o the procedure has been applied to the estimation of boron in low alloy steel in
concentrations above 0.020%. The major application of the fluoride evolution principle to the determination of boron and silicon in low alloy steel in this laboratory has been in the concentration range of 0.001 to 0.020%, Because of the electrode form and lack of matrix effect on the burning, a high precision of analysis can be obtained using direct current excitation. This procedure is not suitable for routine control analysis but the method is presented as a n example of the application of controlled evolution of boron and silicon by fluorine for their determination in poivdered samples. APPARATUS A N D REAGENTS
The instruments and chemicals used in this work are standard, except for the cupric fluoride. The cupric fluoride
is so simple t o prepare that no comniercia1 compounds have been tested. The reagents used for the preparation of cupric fluoride were of standard reagent grade. Preparation of Copper Fluoride. Dissolve 50 grams of cupric sulfate pentahydrate in 200 ml. of distilled n-ater. Filter this solution into a 500nil. plastic beaker. Add a filtered solution of ammonium fluoride (75 grams in 200 nil. of distilled water). Xdjust t h e pH of t h e solution t o 6 a n d allow t h e precipitate of copper fluoride t o settle. K a s h three times by decantation and filter using a plastic funnel. Kash the copper fluoride on the filter paper with distilled n-ater, then Kith a final wash of acetone to speed drying. .lir dry until the acetone evaporates and dry the precipitate a t 105' C. for 1 hour. Store the dry copper fluoride in a plastic bottle. It should be a fine, dense powder with relatively few hard lumps. The acetone rinse may be omitted, but the copper fluoride will tend to be lumpy. PROCEDURE
Basic inforniation for the determination of boron and silicon in steel using fluoride evolution is given in Table I. The sample is usually received as chips or the piece to be analyzed is made into chips. The chips are dissolved in nitric acid (1 to 1) and the resulting solution is dried, ignited a t 600' C. to convert t o oxide, and ground, using a boronand silicon-frce mortar and pestle or a suitable dental amalgamator. The sample charge is neighed into the supporting electrode in two steps, First, 20 nig. of copper fluoride are added to the electrode and the 20 mg. of sample oxide are then added. The nhole is pressed firmly into place with a tamper (flattened steel or carbon rod nhich fits the crater snugly) and capped. The actual excitation and exposure will vary according t o t h e spectrograph used. The conditions shown in Table I \\ere satisfactory in this laboratory. Currents to 10 amperes have been used. A t higher currents more iron is excited, but this does not interfere so long as adequate resolution of the iron line a t 2496.53 and 2496.91 -4.from the boron line a t 2496.8 A. is obtained. In matrices other than iron, less control of the amount of sample matrix excited can be tolerated. Boron and silicon concentrations are deterniined by plotting the relative transmittance of the boron line a t 2496.8 A. and the silicon line a t 2516.1 A. against their concentrations to form analytical curves. The relative transmittances of the sample lines are referred to the proper curve to determine concentrations. Samples are exposed in duplicate and a set of standards (usually in single exposure) is exposed on each film.
Standards are prepared on a metal basis. Boric acid in standard solution is added t o weighed samples of NBS 55d and t h e whole prepared as are samples, Silicon standards are prepared by diluting KBS 129a with NBS 55d to give the desired concentration of silicon and the whole prepared as are samples. Boron and silicon standards may be prepared in the same set by adding standard boric acid solution to the set weighed for silicon standards. Where boron and silicon standards are to be prepared together or deterniined in the same sample, all equipment must be boron and silicon free.
Table I. Determination of Boron and Silicon in Steel Using Fluoride Evolution
Sample preparation Sample converted to oxide Supporting UCP1990 with UCPelectrode 300 cap or equivalent Counter electrode UCP104U or equivalent Sample charge, mg. 20 CuF2, 20 sample 3 Analytical gap Tidth, mm. Samply polarity Positive Slit width, y 40 Excitation, amperes 5 direct current Exposure, sec. 20 Intensity control No filter or sector Film Kodak Spectrum Analvsis No. 1 Order 2 Spectrograph Applied Research Laboratories 2meter grating -
DISCUSSION
K h e n applied to lom-alloy steel, the fluoride evolution of boron and silicon from the matrix is most efficient from the oxide. Attempts to evolve boron and silicon from metal filings were unsuccessful. Oxides prepared after dissolving the metal in hydrochloric, perchloric, phosphoric, or sulfuric acid, or mixtures of these acids R ith nitric acid gave low and rariable recovery of boron. Thus, the procedure has been liniited to the low-alloy steels n-hich can be dissolved in nitric acid. The current used for exciting the sample can be varied over a wide range. A current of 5 amperes is adequate for the concentration range of 0.001 t o 0.020% of boron or silicon in steels. Moving film studies indicate a n evolution time of 20 seconds for boron and silicon using capped electrodes a t 5 or 10 amperes. At 15 amperes the evolution time is reduced to 15 seconds. Higher currents have not been investigated. The sensitivity of the burning rises as the current is increased, but the intensity of the iron spectrum increases more rapidly than boron because more iron is vaporized from the sample at higher electrode temperatures. The amount of copper fluoride used does not appear to be critical so long as enough fluorine is evolved t o remove the boron and silicon from the sample. Twenty milligrams of copper fluoride have been adopted as a standard, and this amount is adequate for the complete evolution of 60 y of boron and silicon from a 20-mg. sample of oxide. The boiler cap serves a dual purpose. It helps to keep the heat which is transferred to the sample to a minimum while a sufficiently intense arc to give satisfactory excitation of boron and silicon is maintained, and the pointed cap helps to stabilize the arc by reducing wandering. By controlling the temperature of the sample, the amount of sample spectrum obtained can be kept to a minimum, nhich is a definite advantage when determining boron in steel. A short test was made to determine
I
Table II. Evolution of Boron from Oxides of Several Elements
(20 Oxide
y
B/g. oxide) Relative Transmittance (B 2497.7)
Table 111. Repeatability of Fluoride Evolution Procedure for Determination of Boron and Silicon in Steel
Run
Boronb
1
Silicona 0.012 2 0.012 3 0.013 4 0.013 5 0.013 6 0.013 7 0.015 8 0 016 9 0.013 10 0.013 Av. 0.013 Std. concn. 0.013 Coefficient of variation, 3t3.2 a NBS 8g. Synthetic standard.
0.0051
0.0050 0.0054 0,0052 0.0051 0.0050
0.0053 0.0052 0,0051
0.0052 0.0050 3t3.0
Table IV. Comparison of Spectrographic and Chemical Determinations of Boron in Steel
Boron, % Point-toSample Evolution plane Chemical 0.0064 0,006" 0.0064 R H 1092
