Determination of Trace Impurities in Magnesium by Activation Analysis G. J. ATCHISON AND W. H. BEAMER The Dow Chemical Co., Midland, Mich. For many elements the technique of neutron activation analysis is more sensitive than conventional methods of analysis. Activation analysis has been investigated as a means of determining concentrations of certain trace impurities in highly purified experimental magnesium metal. For samples irradiated in the Oak Ridge pile at a flux of approximately 5 X 10” neutrons/sq. cm./sec. for 4 weeks the following sensitivity values at *lo% accuracy have been found: arsenic 1 X gram, phosphorus 3 X gram, copper 3 X 10-8 gram, potassium 1 X 10-7 gram, sodium 1 X lO-’gram, strontium 2 X 10-6 gram, calcium 2 X 10-6 gram, chromium 5 X gram, and sulfur 1 X 10-5 gram. The sensitivity for gram when the samples were iron at *lo% accuracy was found to be 5 X irradiated in the Chalk River pile at a flux of approximately 7 X 10l2neutrons/ sq. cm./sec. for a 4-week period. Chemical procedures are given for the separation of the above elements from irradiated magnesium metal.
T
HE need for methods of determining concentrations of certain
impurities in highly purified experimental magnesium metal has prompted the investigation of activation analysis for this purpose. The principles of activation analysis have been aptly reviewed by Boyd ( 2 ) , Leddicotte and Reynolds ( 1 2 ) , and Taylor and Havens ( 2 1 ) . Numerous other publications dealing with the application of activation analysis to specific analytical problems have appeared recently. Kotable among these are: the determination of gallium and palladium in meteorites by Brown and Goldberg (3), the analysis of biological tissue by means of inthe determination duced radioactivity by Tobias and Dunn (E?), of gold and rhenium in meteorites by Goldberg and Brown ( 6 ) , the determination of submicrogram amounts of arsenic by Smales and Pate ( 2 0 ) ,the determination of antimony in zirconium oxide samples by Hudgens and Cali ( 9 ) , and the determination of zirconium in zirconium-hafnium mixtures by Hudgens and Dabagian (IO). Keutron irradiation services are offered by both the United States and Canadian &4tomicEnergy Commissions; thus it is possible for laboratories that do not have direct access to a suitable neutron source to utilize this potent analytical tool. The work described in this paper was carried out viith samples some of which were irradiated a t Oak Ridge and Pome at Chalk River for periods of 4 weeks. The samples after irradiation were returned to this laboratory for chemical separations and radioactivity determinations. NUCLEAR DATA FOR ELEMENTS
This investigation was limited to elements which upon irradiation by thermal neutrons give rise to radioactive isotopes with half-lives greater than 12 hours. Nuclear data for the elements considered in this work are listed in Table I. SAMPLE PREPARATION
Magnesium metal samples were turned on a lathe, using a clean high-speed steel cutting tool, to a diameter that would easily fit the standard Oak Ridge aluminum irradiation tubes. Disks approximately 3 mm. in thickness were prepared from these rods. Each disk had a mass of approximately 1.2 grams. Care was exercised in preparing and handling the samples so as to avoid contamination by materials containing the elements to be determined. For the determination of iron a similar procedure was followed, with the exception that a Carboloy cutting tool was
used and the samples were turned to fit the Chalk River irradiation containers. NEUTRON FLUX MONITORING
The comparative method of neutron flux monitoring was used for this work. Milligram quantities of pure compounds were sealed in quartz ampoules and irradiated with the sample disks in the irradiation containers. tlfter irradiation the quartz ampoules were wrapped in small pieces of platinum foil and the ampoules viere broken. The broken ampoules and contents, protected from loss by the platinum foil, were then immersed in appropriate reagents for dissolving the standards. The platinum foils were opened with glass rods and the standards were completely dissolved and diluted to proper volumes in volumetric flasks. Suitable aliquots of the standard solutions were used for obtaining the relationship of activity to mass of the various elements determined. Equal quantities of inactive carrier element were added to the standard and the sample and both were carried through identical chemical separations. Compounds used for comparison standards are listed in Table 11. Duplicate standard samples were irradiated and carried through all steps of the procedure independently. The neutron intensity a t the center of the sainple being irradiated uill be less than the intensity a t the surface. A correction for the decrease of the neutron beam intensity can be calculated from the expression (18): f
= f( -U.Y*X
oe
1
where j” is the flux of neutrons (neutrons/sq. cm./second) after passing through a thickness X of the sample having a cross section U . N z is the density of the sample in atoms per cubic centimeter and jo is the neutron flux a t the surface of the sample. This correction amounted to less than 1% of the neutron flux for the magnesium samples and standard compounds used in this work and was not considered in calculating the results. DETERMINATION OF RADIOACTIVITY OF SAMPLES AND STANDARDS
All radioactivity determinations were made using a shielded Geiger-Muller counter with a mica end window of a thickness less than 2 mg. per sq. cm. and a commercial Higinbotham scaler. The activity of the samples was compared with the activity of known amounts of pure compounds irradiated under identical conditions; thus it was unnecessary to measure absolute disintegration rates. By adding identical amounts of carrier to the standards and samples and counting both in the same chemical form and physical dimensions, back-scattering and self-absorption corrections were avoided. The sodium chloride and potassium 1812
V O L U M E 24, NO. 11, N O V E M B E R 1 9 5 2
1813
phomolybdate, since thermal neutron irradiation of orthophosphate compounds yields a radioactive product, the P3* content of which is only partly Isotopic in the orthophosphate condition (13, 14). Cross Radioactive Section Isotopes Determination of Chromium. Add 25 mg. of Stable Natural Abundance Element Isotopes Barns' Produced Half-Life Radiation chromium(V1) carrier to the beaker containing the weighed, irradiated sample. Dissolve the sam26.8b As75 4.2 As 100 8-. Y ple in hydrochloric acid and separate the chro14.3d P Pal 100 0.2 8mium as the benzoate salt (11). Dissolve the pre12.8h 69.09 2.8 cu cuss 8 - , 8+, Y cipitate in a sodium hydroxide-hydrogen peroxide 2.1 4.3m CU85 30.91 8-, Y mixture and oxidize the chromium to chromate. 4.41 26.5d Cr50 11 Cr Y, K Make the solution acid with acetic ecid and add 83.46 Cr62 15 mg. of iron(II1) and 5 mg. of phosphorus as 9.54 Cr63 1 . 3 h 0.006 2.61 Cr64 phosphate(II1). Precipitate the iron and phosphorus with ammonium hydroxide. Acidify the Ca40 96.92 Ca 0.64 Ca42 filtrate with hydrochloric acid, add more iron and 0.13 Ca45 phosphorus carriers, and repeat the precipitation. 152d 0.6 2.13 Ca44 8ddjust the acidity of the filtrate with acetic acid 5.8d 0.003 Ca46 8-. Y 0.179 Ca48 and precipitate the chromium XTith barium acetate ( 1 7 ) . Dry the precipitate a t 110' C. and 16 X lO8y 8 - , Y -3 K59 93.08 K. 12.4h 1 . 0 K41 6.91 8-3 Y then ignite a t 500" C. for 5 minutes, Weigh as BaCrOl to determine the chemical yield. Srs4 0.56 Sr 2.7h Sr85 9.86 1.3 Y, K Determination of Calcium. Add 30 mg. of calSra7 7.02 cium( 11) carrier t o the beaker containing thc 53d 0.005 Sr' 82.56 8weighed, irradiated sample. Dissolve the sample 2.91~ K Fe54 5.90 Fe with hydrochloric acid. Alternatively the filtrate Fe65 91.52 from the hydrogen sulfide separation of copper can Fe57 2.24 46.3d 0.36 Fe58 0.33 8-, Y be used, provided the calcium carrier was added prior to dissolving the sample. Adjust the acidity 96.06 s3a S 0.74 533 of the solution and separate the calcium as cal87. I d 0.26 4.18 s34 8cium oxalate from the bulk of the magnesium (8, 0.14 5.04m 0.013 s35 8-, Y pp. 497-9). Dissolve the precipitate in nitric acid, 14.9h 0.6 Nag' 100 8-. Y Na add 10 me. each of bariumfI1) and strontiumfI1). and precicitate the nitrates oi these elements (h: p. 491; 15, p. 25). Again add 10 mg. of barium and strontium carriers and precipitate the nitrates as before. Evaporate the filtrate to remove most of the acid. perchlorate precipitates were prepared for counting by spreading Dilut,e with water and precipitate calcium oxalate (15, p. 25). the granular material in thin layers in aluminum sample pans with Dissolve the precipitate with nitric acid, destroy the oxalate with a spatula. Iron samples were counted in the form of thin elecpotassium chlorate, add 10 mg. of iron(III), and make the solutrolytic deposits of metallic iron on brass counting plates. The tion alkaline with ammonium hydroxide. Filter the solution and remainder of the precipitates m r e prepared for counting in the acidify the filtrate with hydrochloric acid. Precipitate the calcium form of compact deposits on thin aluminum plates by the evaporawith ammonium oxalate (8, p. 501). Dry the precipitate at tion technique of Dauben et al. ( 5 ) . 110" C. and weigh as C:aCZ04.H20to det3rmine the chemical yield. Decay curves and aluminum absorption curves of the standards and samples were prepared and used to check the identities of the isotopes counted. T a b l e 11. C o m p o u n d s Used for Standards
Table I. Nuclear D a t a for E l e m e n t s D e t e r m i n e d (23)
~
CHEMICAL PROCEDURES
Determination of Arsenic. Add 25 mg. of arsenic( 111) carrier to the beaker containing the weighed, irradiated sample and dissolve the sample with dilute sulfuric acid. Keep a small quantity of liquid bromine present in the mixture to avoid reduction and loss of arsenic. After the sample is in solution boil out the excess bromine and add 10 mg. each of copper(II), antimony(III), and tin(I1). Adjust the acidity with hydrochloric acid and saturate with hydrogen sulfide (8, p. 214). Dissolve the precipitate in ammonium hydroxide and transfer it to a glass still. .4dd 10 mg. each of antimony and tin, reduce the arsenic(V) with a saturated solution of cuprous chloride, and distill the arsenic as AsC13 (24, pp. 341-3). Precipitate the separated arsenic as AsZ& (8, p. 214), drv a t 110" C., and weigh to determine the chemical yield. Determination of Copper. Add 50 mg. of copper(I1) carrier to the beaker containing the weighed, irradiated sample and dissolve the samDle with hvdrochloric acid. Lyse nitric acid, if necessary. t o dissofve any insoluble residue. Adjust the acidity of the solition and saturate with hydrogen sulfide (8, p. 202). Dissolve the precipitate in nitric acid, add sulfuric acid, and fume off the nitric acid. Add 5 mg. each of arsenic(III), antimony(III), and tin(I1) and separate the copper as CuCSS (8, pp. 200-1; 16, pp. 72-6). Dry the precipitate a t 110" C. and weigh to determine the chemical yield. Determination of Phosphorus. -4dd 10 mg. of phosphorus carrier as phosphate(II1) to the beaker containing the weighed, irradiated sample. Dissolve the sample with nitric acid. Maintain a small quantity of li uid bromine in the mixture until the sample is dissolved. Boil %e solution to remove excess bromine. Separate the phosphorus as ammonium phosphomolybdate (15, p. 21; 24, pp. 202-5). Dry the precipitate a t 110" C. and weigh as ("#%.12.1?IoO3 to determine the chemical yield. Use the empirical factor 0.0164 to calculate the phosphorus content of the precipitate. I t is important to subject the comparison standards to a strong oxidizing agent before separating the ammonium phos-
Element Determined Arsenic Phosphorus Copper Iron Potassium Sodium Strontium Calcium Chromium Sulfur
Compound Irradiated AS208
.41P04 CUO Fe KzCOa NazCz04 SrCOa CaCOs
ES01.H20
Determination of Strontium. Add 40 mg. of strontium(I1j carrier to the beaker containing the weighed, irradiated sample. Dissolve the sample with nitric acid and add 5 mg. each of calcium(I1) and barium(I1). Precipitate the strontium as strontium nitrate from a strong nitric acid solution (25). Dissolve the precipitate in water, add 10 mg. of iron(II1) and precipitate the iron with carbonate-free ammonium hydroxide (15, pp. 103-7). Adjust the acidity of the filtrate and precipitate the barium with ammonium dichromate (8, p. 492). Make the filtrate slightly ammoniacal and precipitate the strontium with ammonium oxalate (8, p. 504). Dry the precipitate a t 110" C. and weigh as SrC204.H20to determine the chemical yield. Determination of Sulfur. Add 15 mg. of sulfur carrier as sulfate to t,he beaker containing the weighed, irradiated sample. Dissolve the sample with hydrochloric acid. Maintain an excess of liquid bromine in the solution until the sample is dissolved. Expel the bromine by boiling the solution. Adjust the acidity and precipitate the sulfur with barium chloride (8, p. 580). Collect the precipitate on filter paper and place the folded paper in a platinum crucible. Add 10 mg. each of barium, strontium, and calcium ion. Dry the material in the crucible and burn off the paper. Fuse the residue with sodium carbonate (8, p: 573). Extract the melt with water and filter the mixture, Acidify the filtrate with hydrochloric acid and boil out the carbon dioxide. Add 5 mg. of phoe-
1814
ANALYTICAL CHEMISTRY
phorus as phosphate and 15 mg. of iron(II1). Precipitate these elements with ammonium hydroxide. Filter the solution and adjust the acidity of the filtrate with hydrochloric acid. Precipitate the sulfur with barium chloride (8, p. 580). Dry the precipitate a t 110" C. and weigh as Bas04 to determine the chemical yield. Determination of Sodium and Potassium. Add 60 mg. of potassium and 80 mg of sodium ion carriers to 10 ml. of water in a 250-ml. beaker containing the weighed, irradiated sample. Dissolve the metal by adding 8 ml. of concentrated hydrochloric acid to the sam le in 2-ml. portions. Heat the beaker and contents and allow t i e solution to evaporate to a small volume to remove excess acid. Transfer the solution to a 100-ml. mixing cylinder. Dilute to approximately 75 ml. and add 13 ml. of n-butylamine. Dilute to 100 ml. and mix well. Centrifuge the slurry and decant the clear liquid through a dry filter paper. Place a 60-ml. aliquot in a 150-ml. beaker and evaporate to dryness on a hot plate. Sublime the amine hydrochloride away, using a Bunsen flame while passing a stream of dry nitrogen into the beaker. After the beaker cools add 2 ml. of concentrated nitric acid and evaporate to dryness. Repeat this treatment until all organic matter is destroyed. Four or five evaporations are usually required. Add 5 ml. of 70% perchloric acid and evaporate to dense fumes. Separate the potassium by treatment with ethyl acetate (8, p. 523). Reserve the filtrate for the determination of sodium. Dissolve the precipitate in water, add 5 mg. of sodium carrier, and precipitate the potassium as the perchlorate. Filter and dissolve the precipitate in water. ildd 5 mg. of sodium and 1 drop of perchloric acid. Evaporate the solution to dryness. Then carry out a final precipitation of potassium from ethyl acetate. Dry a t 110" C. for 20 minutes and then ignite a t 300" C. for 20 minutes. Weigh as KC104 to determine the chemical yield. Determine sodium using the filtrate and washings from the first potassium perchlorate extraction. Add n-butyl alcohol and evaporate to expel the ethyl acetate. Precipitate the sodium by adding a solution of hydrogen chloride gas in n-butyl alcohol (8, D. 524). Collect the precipitate and dry a t 110" C. for 10 minutes and then ignite a t 600" C. for 5 minutes. Weigh as KaCI to determine the chemical yield. Determination of Iron. Add 15 mg. of iron(II1) carrier to the beaker containing the weighed, irradiated sample. Dissolve the sample n i t h hydrochloric acid, add nitric acid, and boil to oxidize the iron. Adjust the acidity to 7 with hydrochloric acid and extract the iron with isopropyl ether (8, p. 106; 24, p. 53). Extract the iron from the ether phase using water. Evaporate the dissolved ether and precipitate the iron with ammonium hydroxide. Separate the precipitate, dissolve it with hydrochloric acid, and reprecipitate with ammonium hydroxide. Separate the precipitate and dissolve it with hydrochloric acid. Evaporate the excess acid and electroplate the iron onto a brass counting plate from a solution of ascorbic arid, ammonium hydroxide, andsodium citrate ( 7 ) . Wash the plate first in ethyl alcohol and then in ether. Allow to stand in a desiccator for 10 minutes and weigh as Fe to determine the chemical yield. RESULTS
A sample of commercial electrolytic magnesium metal was irradiated in the Oak Ridge pile for 4 weeks a t a neutron flux of approximately 5 X 1011 neutrons/sq. cm./sec. The results obtained for this sample by activation analysis and by conventional methods of analysis are given in Table 111.
Table 111. Analysis of Commercial Electrolytic Magnesium Element Arsenic Calcium Chromium Copper Phosphorus Potassium Strontium
(Parts per million) Activation Analysis Other Methods of Analysis , . . . . .. 0.132,0.120 8 (Spec.) 1.7, 1.3 1-b(Spec.1 0.4, 0.1
