Analyst, September, 1973, Vol. 98, $$. 655-658 655 The Determination of Trace Amounts of Barium in Calcium Carbonate by Atomic-absorption Spectrophotometry" BY F. J. BANOf (John & E. Sturge Limited, Liffoord Chemical Works, Kings Norton, Birmingham, B30 3 J W ) Barium present at the 1 to 20 p.p.m. level in calcium carbonate is separated from the calcium matrix by co-precipitation on a lead sulphate carrier. The lead sulphate - barium sulphate precipitate is dissolved in an ammoniacal solution of ethylenedianiinetetraacetic acid disodium salt and the barium in the resulting solution is determined by atomic-absorption spectrophotometry by using a nitrous oxide - acetylene flame and the atomic line at 553.6 nm. The method overcomes interferences that occur in the atomic-absorption spectrophotometric determination of barium in the presence of calcium. THE determination of barium by atomic-absorption spectrophotometry in materials that contain large amounts of calcium has been reported by Bowman and Willis.1 Ingamells, Suhr, Tan and Anderson2 have reported a comprehensive study of the determination of strontium and barium in various rocks and minerals that contain calcium, and while they stressed the usefulness of atomic-absorption spectrophotometry for the determination of strontium, they were unable to obtain satisfactory results for barium by using this technique.Koirtyohann and Pickett3 reported the existence of a molecular spectral interference when barium was determined in the presence of calcium by atomic-absorption spectrophotometry. This interference was ascribed to absorption by the CaOH band.Capacho-Delgado and Sprague* claim that the interference can be overcome by using the nitrous oxide - acetylene flame. However, in these laboratories, it has been found impossible to overcome the inter- ference when trace amounts of barium are to be determined in calcium carbonate. As the interference could not be overcome by the use of the nitrous oxide - acetylene flame, a means of separating trace amounts of barium from a calcium matrix was investigated. Separation of the analyte barium from the calcium matrix, as well as overcoming the inter- ference effect, would also lead to an over-all increase in the sensitivity of the method. Feig16 has described a test for barium and other alkaline earth cations, which is based on the induced precipitation of lead sulphate when barium is added to a solution of lead sulphate in acetic acid.Feigl suggests that the precipitate formed in this test is a mixed crystal or addition compound of lead sulphate and barium sulphate. As lead sulphate is less soluble than calcium sulphate in aqueous media, it was thought that it might be possible to precipitate lead sulphate from a solution containing calcium and to scavenge the barium from the solution as the mixed lead sulphate - barium sulphate compound. In order to test this hypothesis, the following experiments were carried out. EXPERIMENTAL AND RESULTS REAGENTS- All reagents used were of analytical-reagent grade. Sulphuric acid, 2.5 M-Mix 136 ml of concentrated sulphuric acid (sp. gr.1-84> with Nitric acid, 5 M-Mix 320 ml of concentrated nitric acid (sp. gr. 1.42) with about 800 ml Ammonia solution, 5 M-Mix 280 ml of concentrated ammonia solution (sp. gr. 0.88) Lead nitrate. * Presented a t the Elwell Award Presentation Meeting of the Midlands Region of the SAC, held in t Present address : London and Scandinavian Metallurgical Co. Ltd., Fullerton Road, Rotherham, @ SAC and the author. about 800ml of water, allow the mixture to cool and make the volume up to 1 litre. of water and make the volume up to 1 litre. with about 800ml of water and make the volume up to 1 litre. Birmingham on January 16th, 1973. Yorkshire, S60 IDL. The author was adjudged the winner.656 BANO : THE DETERMINATION OF TRACE AMOUNTS OF BARIUM IN [Analyst, Vol.98 Ethylenediaminetetraacetic acid disodium salt. Calcium carbonate-Specpure (Johnson, Matthey & Co. Ltd.). Barium chloride stock solution-Dissolve 1.78 g of BaC1,.2H20 in about 100 ml of water and make the volume up to 1 litre. 1 ml of solution = 1000 pg of Ba. 1 ml of solution = 10 pg of Ba. Barium chloride working solution-Dilute 10 ml of barium chloride stock solution to 1 litre immediately before use. APPARATUS- Atomic-absorption measurements were made with an EEL 240 Mark I1 atomic-absorp- tion spectrophotometer (Evans Electroselenium Limited, Halstead, Essex) , fitted with a 50-mm slot nitrous oxide - acetylene burner and a barium hollow-cathode lamp. The instrumental conditions used were as follows- Nitrous oxide . . .. .. . . 5-51 min-l Acetylene .. . . .. . * 3.5 1 min-l Wavelength . . .. . . . . 553.6nm Slit width . . .. . . . . 0.14 mm (position 3) Lamp current . . .. .. .. 7.5mh Burner height , . .. .. . . Position 6 Scale expansion . . .. .. x10 RECOVERY OF BARIUM BY THE CO-PRECIPITATION PROCEDURE- A 100-pg amount of barium (in the form of 10 ml of barium chloride working solution) and 160mg of lead nitrate were dissolved in 100ml of 0.1 M nitric acid. The solution was heated to boiling and 5 ml of 2.5 M sulphuric acid were added. The solution was allowed to stand for 2 hours on a steam-bath, after which period a dense crystalline precipitate had settled out. The supernatant solution was decanted off and the precipitate was washed with two 10-ml portions of water. The supernatant liquid and washings were combined and evaporated to a volume of approximately 20 ml.To the precipitate were added 200 mg of ethylenedia- minetetraacetic acid disodium salt and 10 ml of 5 M ammonia solution. The mixture was gently warmed until all the solids had dissolved and the solution was transferred into a 25-ml calibrated flask and the volume made up to the calibration mark with water. To the supernatant solution were added 200 mg of ethylenediaminetetraacetic acid di- sodium salt, and the solution was transferred into a 25-ml calibrated flask and the volume made up to the calibration mark with water. The barium in each of the two solutions was determined by atomic-absorption spectro- photometry by using the instrumental conditions described above. The sodium derived from the ethylenediaminetetraacetic acid disodium salt functioned as an ionisation buffer.A large absorption signal was obtained for the solution that contained the dissolved lead sulphate, while no signal was obtained for the solution that contained the supernatant liquid. The absorption signal obtained from the solution that contained the dissolved precipitate was identical with that obtained from the solution to which 100 pg of barium (0.1 ml of barium chloride stock solution) had been added. It was therefore concluded that lead sulphate would function as a suitable carrier for barium. CONCENTRATION OF SULPHURIC ACID REQUIRED FOR THE PRECIPITATION OF LEAD SULPHATE- The concentration of sulphuric acid used for the precipitation of the lead sulphate carrier is critical, because too low a concentration of sulphuric acid would lead to incomplete precipitation of lead sulphate while too high a concentration would cause precipitation of calcium sulphate.Five 5 & 0.01-g samples of Specpure calcium carbonate were weighed into 150-ml beakers, 25 ml of 5 M nitric acid were added to each beaker and the solutions were warmed so as to effect complete dissolution of the calcium carbonate. Nitric acid was used because hydrochloric acid was found to inhibit the precipitation of lead sulphate. To each beaker were added 100 pg of barium (10 ml of barium chloride working solution) and 160 mg of lead nitrate, and the solutions were heated to incipient boiling. To the separate solutions wereSeptember, 19731 CALCIUM CARBONATE BY ATOMIC-ABSORPTION SPECTROPHOTOMETRY 657 added 1, 2, 3, 4 and 5 ml of 2.5 M sulphuric acid, with stirring, and the solutions were allowed to stand on a steam-bath for 2 hours.After this period of time, no precipitate was observed to have settled out of the solution to which 1 ml of sulphuric acid had been added, while the solutions to which 3, 4 and 5 ml of sulphuric acid had been added were found to have precipitated substantial amounts of needle-shaped crystals, which are characteristic of calcium sulphate. The recovery of barium from the solution to which 2 ml of sulphuric acid had been added was checked in the above manner and found to be satisfactory. CALIBRATION GRAPH- Weigh 160 mg of lead nitrate and 200 mg of ethylenediaminetetraacetic acid disodium salt into each of six 50-ml beakers.Dissolve the solids in approximately 10 ml of 5 M am- monia solution. Add to the separate beakers 0,2,4,6,8 and 10 ml of barium chloride working solution (containing 10 pg ml-l of barium), transfer the solutions into 25-ml calibrated flasks and make the volumes up to the calibration mark with water. Spray the solutions into the flame of the atomic-absorption spectrophotometer, obtain the absorbance readings and plot them against barium concentration. PROCEDURE- Weigh 5 & 0.01 g of sample and dissolve it in 25 ml of 5 M nitric acid in a 150-ml beaker. Dilute the solution to 100 ml with water and add 160 mg of lead nitrate, then heat the mixture to incipient boiling and add 2 ml of 2.5 M sulphuric acid by means of a pipette.Allow the solution to stand on a steam-bath for 2 hours, then decant off the supernatant liquid, wash the precipitate with two 10-ml portions of water, taking care not to lose a significant amount of precipitate, and add 200 mg of ethylenediaminetetraacetic acid disodium salt. Add 10 ml of 5 M ammonia solution and warm the mixture gently so as to effect complete dissolution of the solids. Transfer the solution into a 25-ml calibrated flask and make tlie volume up to the calibration mark with water. Spray the solution into the flame of the atomic-absorption spectrophotometer, obtain the absorbance reading, and read off the amount of barium in micrograms from the calibration graph. PRECISION- The precision of the method at the 5, 10 and 20 p.p.m. levels of barium was established by determining the barium contents of solutions of Specpure calcium carbonate to which 25, 50 and 100 pg of barium, respectively, had been added.The determination at each level was repeated six times. The Specpure calcium carbonate was known to have a nominal barium content of 1 p.p.m. The results are shown in Table I. TABLE I PRECISION OF THE METHOD METHOD Barium level, Mean barium content, Standard deviation, Coefficient of variation, p.p.m. p.p.m. p.p.m, per cent. 5 6.9 0.86 12.5 10 10.9 0.65 6-0 20 21.1 0.42 2.0 ACCURACY- The accuracy of the method was established by comparing the mean value of barium found in samples of calcium carbonate to which barium had been added with those in synthetic standard solutions containing known amounts of barium. It should be noted that the calcium carbonate used for determining the accuracy of the method was known to contain approxi- mately 1 p.p.m.of barium. The results are shown in Table 11. TABLE I1 ACCURACY OF THE METHOD Barium found less the 5 pg Barium added to of barium in the sample Recovery, 5-g samplelpg Barium found1p.g (equivalent to 1 p.p.m.)/pg per cent. 25 34.5 29.5 118 50 54.5 49.5 99 100 105-5 100.5 101658 BAN0 CONCLUSION The method described should provide a rapid and accurate method for the determination of barium in calcium carbonate. The method should be applicable -to the determination Qf barium in limestone and other materials that contain calcium, such as the rocks and minerals discussed by Ingamells, Suhr, Tan and Anderson.2 The author thanks the Board of Directors of John & E. Sturge Limited for permission to publish this paper. REFERENCES 1. Bowman, J. A., and Willis, J. B., Analyt. Chem., 1967, 39, 1210. 2. Ingamells, C. O., Suhr, N. H., Tan, F. C., and Anderson, D. H., Analytica Chim. Acta, 1971, 53, 345. 3. Koirtyohann, S. R., and Pickett, E. E., Analyt. Chem., 1966, 38, 585. 4. Capacho-Delgado, L., and Sprague, S., Atom. Absorfition Newsl., 1965, 4, 363. 5. Feigl, F., “Spot Tests in Inorganic Analysis,” Fifth Edition, Elsevier Publishing Company, Received March 2nd, 1973 Accepted Mads 29th, 1973 Amsterdam, 1958, p. 218.