Analyst, February, 1968, Vol. 93, p p . 79-82 79 The Suppression of Some Interferences in the Determination of Molybdenum by Atomic-absorption Spectroscopy in an Air -Acetylene Flame BY D. J. DAVID (Division of Plaizt Industry, C.S.I.R.O., Cnnbewa, A .C.T., Austvah) Earlier published work on the determination of molybdenum with an incandescent air - acetylene flame showed that aluminium could be used effectively to suppress the interferences of such elements as strontium, calcium, manganese and iron, but this has not been confirmed for manganese and iron by other workers. Indications are that this is caused by instru- mental characteristics. Ammonium chloride, subsequently shown to be effective in the suppression of manganese and iron interference in metallurgical analysis, was found to suppress the interferences of alkaline earth chlorides, but t o have no effect on a severe interference by calcium ion, sulphate and phosphate in combination.As aluminium ion was found to suppress this interference, a mixture of aluminium and ammonium chlorides is suggested for general application. DAVID,~ when using a laboratory-assembled instrument to determine molybdenum, showed that aluminium eliminated the interferences of strontium, calcium, manganese and iron. However, Mostyn and Cunningham,z by using a Perkin-Elmer model 303, found that the combined interference of iron, manganese, chromium and nickel was not completely suppressed by the addition of aluminium ion, but was by the addition of 2 per cent. of ammonium chloride. This has been confirmed here with a Perkin-Elmer instrument.As the gas supplies and designs of the sampling systems were identical, the different interference-suppressing effects of aluminium between the two instruments must be ascribed to differences in optics. The Perkin-Elmer model 303 has a beam convergent before, and divergent after, a focus in the flame, whereas the laboratory-constructed instrument1 has a 4 x 1-mni parallel beam. This work was intended to extend the study of ammonium chloride as an interference suppressor beyond the metallurgical field. Severe interference, unaffected by ammonium chloride, was encountered, and further investigation involved identification and suppression of it. Wide validity of results prompted the use of the commercial, rather than the labora- tory-assembled, instrument.EXPERIMENTAL The equipment used in this work was the Perkin-Elmer model 303 atomic-absorption spectrophotometer with the following conditions : burner, barrel type, 10-cm path length; air, pressure 30 lb per sq. inch and flow, 7.5 (10.3 litres per minute); acetylene, pressure, 8 lb per sq. inch and flow, 13-5 (5.9 litres per minute); slits, position 4 (1.0 mm); wavelength, 3132.6 A; burner height, 1.5 (1.8 cm from the burner top to the centre of the beam); solution uptake, 5.8 ml per minute; scale expansion, x 1 ; lamp, molybdenum hollow-cathode discharge tube supplied by Atomic Spectral Lamps Pty. Ltd., Melbourne; lamp current, 25mA. Settings with units unstated refer to the manufacturer’s arbitrary scales on the Perkin-Elmer model 303 instrument. The visible flame, resulting from the stated air and acetylene flow-rates, was about 8 inches in height, with about seven eighths of this height showing incandescence.As 2 per cent. of ammonium chloride was found to suppress the interference of alkaline earth chlorides almost completely, as seen in Table I, an attempt was made, by using am- monium ion as interference suppressor, to determine molybdenum in “molybdenised” super- phosphate. A 5-g sample of superphosphate was shaken intermittently for 1 hour with 3.1 ml of 12 N ammonia solution and 5 ml of water. The resulting slurry was filtered and the residue washed with water. The filtrate and washings were then acidified by adding 5 ml of 6 N hydrochloric acid and made up to volume with water in a 100-ml calibrated flask, giving a solution containing 2 per cent.of ammonium chloride or its equivalent in other ammonium salts. This solution gave an atomic-absorption reading amounting to about only 12 per cent. of that expected from the stated molybdenum content of the superphosphate, and 0 SAC and the author.80 DAVID SUPPRESSION OF SOME INTERFERENCES [AutabSt, VOl. 93 TABLE I THE EFFECT OF 2 PER CENT. OF AMMONIUM CHLORIDE IN THE INTERFERENCE OF ALKALINE EARTH CHLORIDES WITH THE ATOMIC ABSORPTION OF MOLYBDENUM AT 3132.6 A The results are means of duplicate atomic-absorption readings Solution Absorption, per cent. w i t h Without 2 per cent. of ammonium ammonium chloride chloride 20 p.p.m. of molybdenum . . .. .. .. .. .. . . 23.0 22.6 20 p.p.m.of molybdenum in the presence of 600 p.p.m. of strontium. . 21.4 20.7 20 p.p.m. of molybdenum in the presence of 600 p.pm. of barium . . 19.3 20 p.p.m. of molybdenum in the presence of 600 p.p.m. each of stron- tium, calcium and barium . . .. .. .. .. .. 6.2 31-4 2.1 5.8 8.6 20 p.p.m. of molybdenum in the presence of 600 p.p.m. of calcium . . measurement of a 2 per cent. ammonium chloride dilution showed that this depression was not lessened by increasing the proportion of ammonium chloride to extract solution. Confirmation of the assumption that this depression was caused by interference rather than poor extraction of molybdenum was obtained: by adding molybdenum to the extract solution, which increased the reading by only about 11 per cent. of that to be expected in an inter- ference-free situation ; from colorimetric analysis of the extract solution by using the dithiol method3; and by adding aluminium chloride to the extract solution, which enhanced the molybdenum reading about 7-fold.Data from the atomic-absorption tests are given in Table 11. The colorimetric result appears in Table 111. TABLE I1 MEANS OF DUPLICATE ATOMIC-ABSORPTION READINGS FOR MOLYBDENUM AT 3132.6 A ON AN ACIDIFIED AMMONIA SOLUTION EXTRACT OF “MOLYBDENISED” SUPERPHOSPHATE UNDER VARIOUS CONDITIONS COMPARED WITH THE INCREMENT TRATION IN 2 PER CENT. AMMONIUM CHLORIDE IN ABSORPTION PRODUCED BY A 20 P.P.M. INCREASE IN MOLYBDENUM CONCEN- Absorption, Solution per cent. Acidified ammonia solution extract of molybdenum superphosphate . . .. 3.1 2.2 9 ml of acidified extract + 1 ml of 200 p.p.m.molybdenum in 2 per cent. ammonium chloride . . .. .. .. .. .. . . .. 4.6 20 p.p.m. of molybdenum increment (10 to 30 p.p.m.) in 2 per cent. ammonium chloride . . .. .. .. .. .. .. .. .. 16.0 10 ml of acidified extract + 16 ml of 3333 p.p.m. of aluminium in 2 per ceni. ammonium chloride . . .. .. . . .. .. .. .. 16.5 10 ml of acidified extract + 16 ml of 2 per cent. ammonium chloride . . .. TABLE I11 RECOVERIES OF ADDED MOLYBDENUM BY ATOMIC-ABSORPTION ANALYSIS AND COMPARISON OF ATOMIC ABSORPTION WITH COLORIMETRIC RESULTS IN THE DETERMINATION OF MOLYB- DENUM IN ACIDIFIED AMMONIA SOLUTION EXTRACTS OF SUPERPHOSPHATE The results are derived from the means of duplicate atomic-absorption readings 16 ml of extract solution + 10 ml of additive to give Additives in standards Molyb- A r -i P denum, Recovery Estimate Am- Added Am- p.p.m., in of added monium Alu- Phos- molvb- monium Alu- Phos- extract 4- molvb- chloride, per cent.2 2 2 2 2 9 Y minium, p.p.m. 1000 1000 1000 1000 1000 1000 phorus, denum, chloride, p.p.m. p.p.m. per cent. - 2 - 20 2 2 600 600 20 s 2000 - 2 2000 20 2 - - minium, p.p.m. 1000 1000 1000 1000 1000 1000 phorus, additions den;m, p.p.m. solution per cent. - 17.0 - 39-0 110.0 2000 16.4 2000 34.1 94.1 200@ 16.2 2000 36.3 103.5 DithioP colorimetric result of molyb- denum in solid, p.p.m. 567 613 640 52 1February, 19681 I N THE DETERMINATION O F MOLYBDENUM 81 Arc-emission analysis showed, in addition to molybdenum, the presence of phosphorus, calcium, magnesium and silicon, with traces of copper, aluminium, vanadium and sodium. Sulphur and zinc would also have been present.In 2 per cent. ammonium chloride, these elements, individually, had little effect, but calcium with sulphate or phosphate, or both, severely depressed molybdenum absorption ; aluminium lessened the depression (see Table IV). Concentrations of the interfering ions different from those given in Table IV produced less interference. TABLE IV THE EFFECTS OF ALUMINIUM AND 2 PER CENT. OF AMMONIUM CHLORIDE ON THE INTERFERENCE OF CALCIUM ION, SULPHATE AND PHOSPHATE WITH THE ATOMIC ABSORPTION OF MOLYBDENUM AT 3132.6 The results are means of duplicate atomic-absorption readings Solution composition* r Molybdenum, p.p.m. 20 20 20 20 90 20 20 20 Calcium, Sulphur, Phosphorus, p.p.m.p.p.m. p.p.m. - - - - - 600 - 400 - - - 400 600 400 - 600 - 400 600 400 400 600 400 400 Absorption, per cent. 1 -7 Without In 2 per .cent. Aluminium, ammonium ammonium p.p.rn. chloride chloride - 23-3 22.6 - 5-6 21.4 - 21-9 21.3 - 24-1 23.0 - 2.3 18.0 - 4.9 10.4 - 2.6 3.6 1000 24.0 22.6 * Compounds used : molybdenum trioxide, calcium chloride, sulphuric acid, phosphoric acid and aluminium chloride. 30 U 2c L aJ n S Y .- P 0, 2 IC I I I I I I I 1 500 loo0 1500 2000 2m 3000 3500 Aluminium in solution, p.p.m. Fig. 1. The effect of aluminium ions, at two concen- trations of phosphate, in the suppression of the combined interference of calcium, phosphate and sulphate on the absorption of solutions containing 20 p.p.m. of molyb- denum. Measurements were carried out a t 3132.6 A.Curve A: 20 p.p.m. of molybdenum, 600 p.p.m. of calcium, 400 p.p.m. of sulphur, 400 p.p.m. of phosphorus and 2 per cent. of ammonium chloride. Curve B: 20 p.p.m. of molybdenum, 600 p.p.m. of calcium, 400 p.p.m. of sulphur, 2000 p.p.m. of phosphorus and 2 per cent. of ammonium chloride. Curve C: 20 p.p.m. of molybdenum and 2 per cent. of ammonium chloride82 DAVID SUPPRESSION OF INTERFERENCES- Although ammonium chloride was shown to have little effect on the interferences under study, consideration of its usefulness in the suppression of other interferences made its presence in test solutions desirable. The 2 per cent. concentration found most effective by Mostyn and Cunningham2 was used. Two series of solutions were prepared, in each of which the aluminium ion was varied over a wide range at constant concentrations of molybdenum, phosphate, sulphate and calcium ions.Two phosphate concentrations were used to examine its enhancing effect. Atomic-absorption results derived from these solutions are displayed in Fig. 1. Conclusions to be derived from Fig. 1 are that 600 p.p.m. or more of aluminium in solution effectively eliminates the combined interference of calcium ion, sulphate and phosphate ; at 1000 p.p.m. of added aluminium, 800 p.p.m. of aluminium in solution originating in the sample can be tolerated. STUDY OF SUPERPHOSPHATE EXTRACT- Acidified ammonia solution extract, prepared as outlined earlier, was used in the tests to be described. Three recovery tests for added molybdenum, the details of which are shown in Table 111, were carried out.Having found an enhancement of 10 per cent. in the first test, the subse- quent two tests were designed to test the assumption that phosphate was the cause of the enhancement. Colorimetric analysis of the extract solution by the molybdovanadophos- phate method4 indicated the presence of 30 p.p.m. of phosphorus. Variation of phosphate at constant molybdenum (20 p.p.m.), ammonium chloride (2 per cent.) and aluminium (lo00 p.p.m.) showed maximum enhancement at between 2000 and 3000 p.p.m. of phosphorus. DISCUSSION As this work has shown that interference can be overcome, it is suggested, for the following reasons, that ammonia solution, rather than the more conventional acid,5 extraction of superphosphate would be preferable : molybdenum trioxide is more soluble in alkaline than in acid solutions, ammonia solution dissolves less of other components and experience here indicates that an acid extract is less stable than an ammonia solution extract.Agreement between the extent of enhancement shown by the third recovery test and comparison of the colorimetric with the atomic-absorption result when 1000 p.p.m. of aluminium, 2000 p.p.m. of phosphorus and 2 per cent. of ammonium chloride were present in both sample and standard solutions (Table 111) indicates that the effect is caused by experimental error. Phosphate interacting with other components is probably the cause, but as it is small it can be ignored for most purposes. The effectiveness of ammonium chloride2 in some instances, and of aluminium ion in others, suggests that a mixture of the two might be suitable for general application.A test showed that, in the absence of nitrate, 1000 p.p.m. of aluminium did not impair the effective- ness of 2 per cent. of ammonium chloride in the suppression of iron and manganese inter- ference in the determination of molybdenum in a simulated stainless-steel solution. Nitrate could easily be removed as a routine measure during the preparation of stainless-steel solutions for analysis. Aluminate may exclude molybdate from solid solution in a relatively involatile calcium sulphate or calcium phosphate solid phase in the sample particle in the flame. Reduction of particle size in the flame arising from the low decomposition temperature of ammonium chloride may be the active principle where it is effective, and alkali salts’ may act similarly in their limited range of effectiveness. REFERENCES The mechanisms of suppression of these interferences are obscure. 1. David, D. J., Analyst, 1961, 86, 730. 2. Mostyn, R. A., and Cunningham, A. F., Analyt. Chem., 1966, 38, 121. 3. Sandell, E. B., “Colorimetric Determination of Trace Metals,” Second Edition, Interscience Publishers Inc., New York and London, 1960, p. 459. 4. Snell, F. D., and Snell, C . T., “Colorimetric Methods of Analysis,” Volume 2, Third Edition D. Van Nostrand Co. Ltd., London and New York, 1961, p. 672. 6. Horwitz, W., Editor, OfficiaI Methods of Analysis of the Association of Official Agricultural Chemists,” Eighth Edition, The Association of Official Agricultural Chemists, Washington, D.C., 1965. Received April 7th, 1967