Analyst, June, 1967, Vol. 92, $9. 387-390 387 The Colorimetric Determination of Molybdenum in Soils and Sediments by Zinc Dithiol BY R. E. STANTON AND MRS. A. J. HARDWICK (Departme& of Geology, Impevial College, London, S. W.7) A method is presented for determining molybdenum in soils, sediments and rocks. The sample is decomposed by fusion with potassium hydrogen sulphate and the molybdenum is taken into solution with hydrochloric acid. Interference by iron is prevented by reduction to the iron(I1) state and the reaction with copper by adding potassium iodide. Tungsten interference is suppressed by careful control of the time allowed for complex formation. The molybdenum - dithiol complex is extracted into light petroleum and determined either by visual comparison or spectrophotometry.NORTH'S^ procedure for the determination of molybdenum has been in use in geochemical exploration and research studies for some years, but it is slow and imprecise. The methods of Marshall2 and Baker,3 although more rapid, involve the use of dilute hydrochloric acid digestion for the sample attack and have a poor tolerance towards copper, which is particularly undesirable as molybdenum and copper are often associated in soils and sediments. The procedure described below is a modification of these two methods that overcomes such defects. METHOD REAGEKTS- Potassium hydrogen sulphate-Fused and powdered. Hydrochloric acid, sp.gr. 1 18-Analytical-reagent grade. Hjldrochloric acid, 6 M. Reducing solution-Dissolve 75 g of citric acid and 150 g of ascorbic acid in water and dilute to 1 litre.Potassium iodide solidion, 50 per cent. wlw. Zim dithiol. Thio&collic acid, sp.gr. 1-33. Ethanol, absolute. Sodium hydroxide-Pellets, analytical-reagent grade. Dithiol solution-Add 2 ml of ethanol to 0.3 g of zinc dithiol, followed by 4 ml of water, 2 g of sodium hydroxide and 1 ml of thioglycollic acid. Mix well, and when clear dilute the solution to 50ml with water, when it will become cloudy again. Mix with 50ml of potassium iodide solution (50 per cent. w/w), and store in a refrigerator when not in use. Petroleum spirit, sp.gr. 0.72-Boiling range 80" to 100" C, analytical-reagent grade. Iron sohtion-Dissolve 5 g of ammonium iron(II1) sulphate crystals in 500 ml of 6 M Sodium moly bdnte-Na,MoO,. 2H20, analytical-reagent grade. Standard moZybdenum solutions-Dissolve 0.1261 g of sodium molybdate in 6 M hydro- chloric acid and dilute to 500 ml with this acid in a calibrated flask to give a solution containing 100 pg of molybdenum per ml.From this solution prepare dilute solutions containing 1 and 10 pg of molybdenum per ml in 6 M hydrochloric acid. hydrochloric acid. PROCEDURE- Weigh 0.25 g of sample into a borosilicate test-tube and fuse it with 1 g of potassium hydrogen sulphate until a quiescent melt is obtained; continue heating for a further 2 minutes. Leach on a sand-tray with 5 ml of 6 M hydrochloric acid, then add another 5 ml of 6 M hydro- chloric acid, mix and leave to settle. Transfer by pipette 5 ml of the clear sample solution into a test-tube (16 x 150 mm), add 2 ml of reducing solution, mix and leave to stand for388 STANTON AND MRS.A. J. HARDWICK: COLORIMETRIC DETERMINATION [A.tzaZyst, Vol. 92 2 minutes. Add 2 ml of potassium iodide solution and, mixing after each addition, 1 ml of dithiol solution and leave to stand for 2 minutes. Add 0.5 ml of petroleum spirit, stopper the tube with a silicone rubber bung, shake it vigorously for 90 seconds and then compare the intensity of colour in the solvent phase with that of a standard series. PREPARATION OF THE STANDARD SERIES- To each of thirteen test-tubes (16 x 150 nim) add 2 ml of iron solution, followed in order by 0, 0.2, 0.4, 0-6, 0-8, 1.0, 1.5, 2.0, 3.0, 4.0, 5-0, 7.0 and 10.Opg of molybdenum. Dilute to 5 ml with 6 M hydrochloric acid and treat the solutions as described in the “Procedure” for a sample solution.DISCUSSION OF THE METHOD The use of a different sample weight was checked over the range 0.05 to 0.50 g, there being no significant difference in results. The only alteration necessary in the procedure is the use of 1.5 g of potassium hydrogen sulphate for a sample weight greater than 0-25 g. If a sample aliquot of less than 5 ml is used, 2 nil of iron solution must be added and the volume made up to 5 ml with 6 M hydrochloric acid. As alternative solvents, petroleum spirit with a boiling range of 120” to 160” C and white spirit (B.S. 245) were similar to the one recommended, except that there was a tendency to give a slight turbidity in the solvent phase and at the interface. Variation in acidity of the final aqueous phase over the range 0-5 to 5 N hydrochloric acid had no effect upon the intensity of colour of the molybdenum complex, which was fully developed during the 2 minutes’ standing period before solvent extraction, but tolerance towards copper improves with increasing acidity.The standard series shows an increasing intensity of green colour in the solvent phase. There was complete recovery of molybdenum when standard solutions were incorporated in the sample leach solution. The inclusion of potassium iodide in the dithiol solution serves to increase the density of this solution, thus promoting rapid mixing when it is added to the sample solution. In its absence, when dithiol solution was added slowly enough to remain at the top of the sample solution, a heavy grey precipitate was formed that inhibited the formation of the molybdenum complex and obscured the colour of the subsequent solvent phase.This precipitate is believed to be a dithiol complex of iron. A spectrophotometric finish could be adopted, the molybdenum - dithiol complex exhibiting an absorption maximum at 670 mp. INTERFERENCE FROM OTHER ELEMENTS- The elements found to interfere were iron, copper, tungsten, arsenic, antimony and selenium, the effects of elements other than the first named being shown in Table 1. Interference from iron(II1) is prevented by reducing it to the iron(l1) state, and the procedure is applicable even to the analysis of iron(II1) oxide. However, in the presence of iron(I1) there is a decrease of about 10 per cent. in the intensity of the molybdenum - dithiol complex, and consequently, when the iron content of the sample aliquot is less than 2 mg, some modification is necessary to ensure that standards and samples are comparable. If all of the samples are known to be virtually iron-free, it is convenient to omit the iron solution when preparing the standard series.Otherwise, a 2-ml addition of iron solution should be made to the sample aliquot. A minimum standing period of 2 minutes is necessary for complete reduction to the iron(I1) state, but it is not harmful to exceed this time. Copper is suppressed by the addition of iodide, the order of addition of reagents being critical. When iodide is added after the iron(II1) ions have been reduced, 1000 pg of copper in the sample aliquot can just be tolerated.Above this level, precipitation of the grey copper complex occurs at the solvent interface and formation of the molybdenum complex is suppressed. Tolerance towards copper can be improved further by adding more potassium iodide. Tungsten is masked by citric acid, and up to 2000pg in the sample aliquot may be tolerated, but it is important not to exceed the period of 2 minutes allowed for complex formation after the addition of the dithiol solution. After a standing period of 10 minutes, even 50 pg of tungsten showed a trace of its blue complex in the solvent phase.June, 19671 OF MOLYBDENUM IN SOILS AND SEDIMENTS BY ZINC DITHIOL TABLE I EFFECTS OF VARIOUS ELEMENTS ON THE DETERMINATION OF Element Aluminium -4ntimony . . Arsenic . . Calcium .. Chromium . . Cobalt . . Copper . . Lead . . Magnesium . . Amount added, mg 30.0 30.0 1.25 1.25 0.2 0.2 0.1 0.1 1.25 1.25 0.2 0-2 0.1 0.1 30.0 30.0 1.25 1-25 1.25 1.25 2.0 2.0 1.0 0.5 1.25 1-25 30.0 30-0 Molybdenum, pg & Present 0 2.0 0 2.0 0 2.0 0 2.0 0 2.0 0 2.0 0 2.0 0 2.0 0 2.0 0 2.0 0 4.0 4.0 4.0 0 2.0 0 2.0 Found t0.05 2.0 -* -* < 0.05 2.0 < 0.05 2.0 -* - * < 0.05 2.0 < 0.05 2.0 < 0.05 2.0 < 0.05 2.0 < 0.06 2.0 < 0.05 3.0 4.0 4-0 < 0.05 2-0 < 0.05 2.0 Element Manganese . . Mercury . . Nickel . . Potassium . . Selenium . . Sodium . . Ti anium . . Tungsten . . Vanadium . . Zinc . . Amount added, mg 1.25 1.25 1.25 1.25 1-25 1-25 30.0 30.0 0-25 0.25 0.02 0.02 30.0 30.0 30.0 30.0 3.0 3.0 2.0 2.0 1.0 1.25 1.25 1.25 1-25 ?VIOLYBDENUM 389 Molybdenum, pg +- Present 0 2.0 0 2.0 0 2.0 0 2.0 0 2.0 0 2.0 0 2.0 0 2.0 0 1.0 0 1.0 1.0 0 1.0 0 1.0 Found < 0.05 2.0 < 0.05 2.0 < 0.05 2.0 < 0.05 2.0 < 0.05 2.0 < 0.05 2.0 < 0.05 2.0 < 0.05 2.0 --t --t < 0.05 1.0 1.0 < 0.05 1.0 < 0.05 1.0 * Solvent phase coloured yellow.Solvent phase coloured blue. Both in the presence and absence of molybdenum, 1-25mg of arsenic((II1) caused a yellow-coloured solvent phase. In the presence of 0.2 mg of arsenic there was a slight yellow colour, but it was possible to assess correctly the presence of 2 pg of molybdenum. The effect of antimony(II1) was similar, except that a turbidity also occurred. Selenium was troublesome because of its reduction to the red elemental stage, which occurred with amounts greater than 20 pg.However, although difficult, it was possible to determine correctly the amount of molybdenum present. RESULTS Many samples have been analysed by both North’s method and the proposed procedure, and the comparison of the results obtained is presented in Table 11. The two main defects of North’s method are tendencies towards the inadequate leaching of the sample fusion, and the incomplete formation and extraction of the molybdenum complex in samples and standard-s, both of which defects are avoided in the proposed procedure. Replicate aliquots were taken from each of two sample solutions and by the proposed method gave mean values of 79 and 53 p.p.m., with standard deviations of & 1 and & 5 p.p.m., respectively. Replicate analysis of one sample by both North’s and the proposed procedure gave mean values of 106 and 101 p.p.m., respectively, and their standard deviations were k35 and $ 3 p.p.m.390 STANTON AND MRS.A. J. HARDWICK TABLE I1 COMPARISON OF RESULTS BY DIFFERENT METHODS Sample No. 1 2 3 4 5 6 7 8 9 10 11 12 Molybdenum, p.p.m. w North’s Proposed method procedure 30 50 48 58 12 26 50 76 40 56 50 46 14 12 26 34 54 74 40 36 14 18 34 66 Sample NO. 13 14 15 16 17 18 19 20 21 22 23 24 Molybdenum, p.p.m. r North’s Proposed method procedure 80 74 10 12 2 <2 90 56 80 120 36 90 30 46 100 120 38 36 20 20 84 96 4 <2 The U.S. Geological Survey standard samples G-1 and W-1 were analysed by the proposed procedure, molybdenum values of 7.6 and 1.0 p.p.m. being obtained, respectively. These values are in satisfactory agreement with those obtained by Hamaguchi, Kuroda, Shimuzu, Tsukahara and Yamamoto,* with neutron activation, who reported 7.0 and 1.3 p.p.m., respectively. This work forms part of the programme of the Applied Geochemistry Research Group under the direction of Professor J. S. Webb. REFERENCES 1. 2. 3. Baker, W. E., Bull. Australas. Inst. Min. Metall., 1965, No. 214, 125. 4. North, A. A., Analyst, 1956, 81, 660. Marshall, N. J., Econ. Geol., 1964, 59, 142. Hamaguchi, H., Kuroda, R., Shimuzu, T., Tsukahara, I., and Yamamoto, R., Geochim. Cosvnochim. Received July 27th, 1966 Acta, 1962, 26, 503.