A simple colorimetric method is presented for the determination of submicrogram traces of molybdenum in soils and rocks. The procedure is developed on the extractable green complex formed between molybdenum and dithiol. Interference by ferric iron is eliminated by reduction with potassium iodide and sodium thiosulfate. Isoamyl acetate extracts of the molybdenum-dithiol complex are color-stable. Photometric measurements permit accurate determinations over the range of 0.02 to 10 y of molybdenum in mineral materials.OLYBDESUM frequently occurs in soils and rocks in concentrations less than 1 y per gram. The detection and estimation of these minute traces require sensitive and precise analytical procedures. Methods described in the literature either lack sensitivity or appear to yield test solutions generally susceptible to color fading and consequent uncertain precision (1 6 ) . This investigation was undertaken therefore to develop a simple colorimetric procedure of requisite color stability for the determination of molybdenum in mineral materials Substituted dimercaptobenxenes were studied by Clark ( 4 ) as precipitants for metals of the hydrogen sulfide group. He gave to the compound 4-methyl-l,2-dimercaptobenzene (toluene-3,4-dithiol) the shortened name "dithiol" and recommended its use in the determination of tin. Later Hamence (6) discovered that molybdenum formed an extractable green precipitate with dithiol, and applied this reaction in a qualitative test for trace amounts of molybdenum. IlIiller (IO, 11) studied the reactions of dithiol with molybdenum, tungsten, and rhenium. Wells and Pemberton (169, Bagshawe and Truman (f), and Short (f7) used dithiol in microchemical determinations of molybdenum in metals, Bickford, Jones, and Keene ( 2 ) employed the delicate dithiol complexes for the simultaneous estimation of molybdenum and tungsten in organic materials. After systematic evamination of reaction conditions, Piper and Becknith ( I S ) presented a dithiol method for the determination of small amounts of molybdenum in plants. The foregoing applications suggested the suitability of this reagent for the determination of molybdenum in mineral materials.Soils and rocks, however, were found to possess constituents in concentrations sufficient to prevent the normal reaction between molybdenum and dithiol. Procedures were introduced to control interferences from silica, platinum, iron, tungsten, and copper. The modified method was then applied successfully to the determination of microgram quantities of molybdenuni in a wide variety of inorganic materials. REAGENTS Water. Dithiol, 0.2%. Redistill in an all-borosilicate glass apparatus Dissolve a 1 gram v d of dithiol in 500 ml. of 1 yo sodium hydroxide solution. Stir the mixture occasionally during a period of 1 hour. Add thioglycolic acid (about 8 ml.) dropwise until a faint permanent opalescent turbidity begins to form. Transfer to 4-ounce narrow-mouthed bottles fitted with plastic screw caps. Store the reagent solution in a refrigerator a t 5" C. a t all ti...

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