Spectrographic Determination of Certain Rare Earths in Stainless Steels ELLEN W. SPITZ, JOSEPH R. SIMMLER, BYRON D. FIELD, KARL H. ROBERTS, and SAMUEL M. TUTHILL Department
o f Chemical Control, M a l l i n c k r o d t Chemical Works, St. Louis 7, M o .
The addition of a mixture of rare earth oxides or metals to molten steel has created an interest in the determination of the residual rare earths in the steel. A spectrographic method using a direct current arc, following electrochemical and chemical separations, has been developed which permits the determination of as little as 50y of total rare earths in a 1-gram sample of steel. Electrolytic separation with a mercury cathode is used to remove the major interfering components from the sample solution. The less than milligram amounts of cerium, lanthanum, neodymium, and praseodymium which remain in solution are concentrated by precipitation with ammonium hydroxide, using ferric iron as a carrier. The combined hydroxides are dissolved in a minimum of hydrochloric acid, and uranium and sodium chloride are added to serve as the internal standard and as the spectroscopic buffer, respectively.
cerium and lanthanum in stainless steels, as described in the working manual (3) distributed by the Eberbach Corp., .4nn Arbor, Mich., with its commercial mercury cathode apparatus, the DynaCath, was brought to the authors’ attention. The Dyna-Cath and the bulletin referred to were the result of research conducted a t the Battelle Memorial Institute, Columbus, Ohio (2, 3), under the sponsorship of the Eberbach Corp. In the procedure given in the Dyna-Cath bulletin, chromium is removed by volatilization from solution as chromyl chloride, and the sample is then electrolyzed to the absence of iron. Cerium and lanthanum are determined spectrographically in the residual solution by means of a low voltage spark discharge. Although few details of the procedure are given and there is no provision for eliminating interferences resulting from elements which are not removed by the mercury cathode, the existence of this method made it appear most probable that this general approach would be applicable to the problem of determining small quantities of rare earths in stainless steels. The present paper, therefore, describes the development of a procedure based upon the electrolytic removal a t the mercury cathode of the. major steel components, including most of the chromium, followed by chemical treatment to remove as much as possible of impurities, such as calcium and aluminum, which remain in the rare earth concentrate. Cerium, lanthanum, neodymium, and praseodymium are then determined spectrographically by means of a direct current arc under conditions which eliminate the interference of small amounts of elements not removed by either the electrolytic or chemical separation steps.
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URING World War I1 it was reported that the addition of an alloy of cerium and other rare earth metals to steel improved certain mechanical properties such as hardness and resisb ance to breaking and bending (fa). The direct spectrographic determination of residual rare earths in steels has been generally regarded as impossible because the complex spectra of the major components have prevented the identification and measurement of the weak rare earth lines. Thus, it has not been possible to correlate by a direct spectrographic analysis the change in the properties of the steel with the amounts of rare earth elements incorporated into the steel. Difficulties arising from the high neutron absorption of certain rare earth elements stimulated research in the development of procedures capable of determining trace amounts of the rare earths in uranium metal and in uranium compounds. Although a similar problem of spectral interference by the major component existed in this case, Scribner and Mullin (fs),Short and Dutton (IC), and Hirt and Nachtrieb (6) succeeded in developing spectrographic procedures suitable for determining microgram quantities of individual rare earths in uranium compounds. In each caae ether extraction served as a means of separating uranyl nitrate, and the residual rare earths were concentrated chemically prior to the spectrographic analysis. Since stainless steels contain several components, it would be necessary, in carrying out a similar separation, to remove them individually, or, preferably, by some generally applicable method. Westwood and Mayer (16)have described a colorimetric method for determining 0.02 to 0.2% cerium in plain and alloy cast iron. I n the course of this work, i t was found that certain types of alloy cast irons required a preliminary mercury cathode separation, which indicated that this technique could be adapted to the problem of determining residual rare earths in steel. Further indication that such a separation could be effected was found in the quantitative separation of iron from as much as 0.2 gram of cerium, lanthanum, neodymium, praseodymium. and yttrium oxides by Myers (11); the separation of iron from 0 25 gram each of cerium, lanthanum, neodymium, and praseodymium by Kollock and Smith (7); and in the determination by Benner and Hartmann (1) of iron and nickel, respectively, in the presence of 0.2 gram of rare earth oxides obtained by extraction from monazite. During the course of this investigation into methods for the determination of rare earths in steels a procedure for determining
APPARATUS AKD REAGENTS
All of the mercury cathode separations connected with the present paper were performed using the Dyna-Cath apparatus (8, 3). Although commercial equipment was used in this work, a mercury cathode apparatus constructed as described by Maxwell and Graham (10) and by Lingane (8) should also be suitable. STANDARD RAREEARTH SOLUTION, 1.00 mg. of total rare earths per ml. The standard rare earth metal solution was made up to contain 0.50 mg. of cerium, 0.25 mg. of lanthanum, 0.15 mg. of neodymium, and 0.10 mg. of praseodymium per ml. Cerium was added as ammonium hexanitratocerate and the other elements were added as oxides. The cerium and lanthanum standard compounds were ectrographically examined in this laboratory and found to be%ee of other lanthanide rare earths. The neodymium and praseodymium compounds showed spectrographically only a trace of yttrium. Dissolve the calculated quantities of the rare earth oxides, dried a t 110’ C. for 1hour in a mixture of 50 ml. of distilled water and 2 to 5 ml. of hydrochioric acid. Add the calculated amount of ammonium hexanitratocerate to the mixture and boil gently until solution of the compounds is complete. Add 2 to 3 drops of hydrogen peroxide to reduce the cerium to the trivalent state. Boil the solution to remove the excess of hydrogen peroxide, cool to room temperature, transfer to a 100-ml. volumetric flask, and dilute to volume. DILUTERAREEARTHSOLUTION, 0.10 rng. of total rare earths per ml. IRON CARRIERSOLUTION, 6.0 mg. of iron per ml. Dissolve 14.6 grams of ferric chloride hexahydrate in 100 ml. of distilled water. Add 5 ml. of hydrochloric acid, transfer to a 500-ml. volumetric flask, and dilute.to volume. DILUTEIRON CARRIERSOLUTI.ON, 0.6 mg. of iron per ml. SYNTHETIC STEELSOLUTION.The synthetic steel solution was prepared to simulate a steel containing 54% iron, 24% chromium, 18% nickel, 2.5% molybdenum! and 1.5y0 manganese. A 20-ml. aliquot of this solution was equivalent to a 1-gram sample of steel. Dissolve individually in mixtures of 100 ml. of water and 10 ml. of hydrochloric acid, 73 grams of nickel chloride hexahydrate, 123 grams of chromium chloride hexahydrate, and 5.40 grams of manganese chloride tetrahydrate. Dissolve 261 grams of ferric 304
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V O L U M E 26, N O . 2, F E B R U A R Y 1 9 5 4 chloride hexah drate in a mixture of 400 ml. of distilled water and 50 ml. of gydrochloric acid. Dissolve 3.8 g r a m of molybdenum trioxide in a mixture of 50 ml. of distilled water and 15 ml. of ammonium hydroxide. Combine all of the solutions in a 2-liter volumetric flask, add 100 ml. of hydrochloric acid, and dilute to volume. INTERNAL STANDARD-BUFFER SOLUTION.Dissolve in a minimum of nitric acid 0.3773 gram of uranium oxide, prepared by igniting uranyl nitrate hexahydrate. Add 50 ml. of distilled water and 25 grams of sodium chloride. Transfer to a 100-ml. volumetric flask and dilute to volume. HYDROCHLORIC ACID,37% by weight. NITRICACID,70% by weight. PERCHLORIC ACID,70% by weight. SULFUROUS ACID,6% by weight. AMMONIUM HYDROXIDE, 58% by weight. HYDROGEN PEROXIDE, 30% by weight. AMMONIUM HEXANITRATOCERATE, standard of reference purity, G. F. Smith Chemical Co., Columbus, Ohio. LANTHANUV, NEODYMIUM, and PRASEODYMIUX OXIDES, Research Chemicals Laboratories, Burbank, Calif. With the exception of the chemicals used as a source of cerium and the other rare earths, all reagents and chemicals used were Mallinckrodt analytical reagents. PROCEDURE
Weigh accurately 1 gram of stainless steel turnings and transfer to a 150-ml. beaker. To the metal turnings add 20 ml. of 1 to 1 hydrochloric acid, 5.0 ml. of nitric acid, and 10 ml. of perchloric acid. Cover the beaker with a watch glass and heat on a hot plate at moderate heat until the decomposition of the sample is effected and the formation of chromic acid is definitely indicated by the presence of a red-orange precipitate. Cool the mixture, add 40 ml. of water, and heat to boiling. Add 10 mlc of sulfurous acid and continue heating for 5 minutes more. Filter the solution while still hot through a Whatman No. 40 filter paper to remove any insoluble residue. Wash the precipitate and paper several times with hot 1% perchloric acid solution and collect the washings with the filtrate. Transfer the filtrate to the Dyna-Cath cell containing 35 ml. of mercury and adjust the volume of the solution in the cell to 100 ml. T"wn on the cooling water and place the split plastic cover glass over the cell. Pass a current of 15 amperes through the solution for 1 hour. With the current still on, transfer the solution from the cell into a 400-ml. beaker. Add a small amount of filter pulp to the solution and filter through a Whatman No. 40 filter paper into another 400-ml. beaker. Wash the filter paper and pulp four times with hot 1% perchloric acid. Add 2.0 ml. of the dilute iron carrier solution to the filtrate, place a cover glass on the beaker, and evaporate on the hot plate to dense fumes of perchloric acid, Continue fuming until the volume of the acid has been reduced to 1 to 2 ml. Cool the solution and transfer quantitatively to a 15-m]. centrifuge tube with a conical tip, using no more than 10 to 12 ml. of water. Precipitate the iron and the rare earths by adding an excess of ammonium hydroxide. Centrifuge and pour off the supernatant liquid. Dissolve the precipitate with 5 drops of hydrochloric acid. Tilt the centrifuge tube until the acid is almost to the lip of the tube and rotate it to make certain that the entire inside of the centrifuge tube is wetted by the acid. Wash down the inside of the tube with a fine stream of water until a volume of about
Table I.
Spectrographic Conditions for the Analysis
Spectrograph Upper electrode (cathode) Lower electrode (anode)
Analytical gap Excitation source Length of exposure Emulsion Wave-length region Filter Development Emulsion calibration
Baird 3-meter grating spectrograph, Eagle mounting, 25-micron slit '/s-inch diameter high purity graphite, 1 inch long, pointed a t end; in water-cooled eleotrode elamp l/einch diameter high 'purity .graphite, '/a inch long. crater a/~s-inch diameter, 1/ninch wall deDth. SUDDOrted bv l/a-in. eraDhite pedestal in wate'rkooled eiectrode
