Extract io n ci nd S pe ctro phot o met ric Dete rmina ti o n of MoIy bd e n um and Tungsten with 2-ThenoyltrifIuoroacetone. Application to Alloy Steels SIR: Colorless chelates extractable by acetophenone-butyl alcohol are formed by the reaction of 2-thenoyltrifluoroacetone (TTA) ( I ) with molybdenum(V1) in acidic medium and with tungsten(V1) in strongly acid medium. Treatment of the organic phases (after extraction) with stannous chloride and thiocyanate gives the colored thiocyanate complexes of molybdenum and tungsten, respectively, which can be measured spectrophotometrically. This forms the basis of the method reported in this paper for rapid extraction and determinatior. of molybdenum(V1) and tungsten(V1) a t milligram levels. The method has been successfully applied to alloy steels. The thiocyanate methods for molybdenum and tungsten are well known (7, 8). These metal:: in their reduced states form colored complexes with thiocyanate which are extractable by organic solvents like i:;oamyl alcohol (6), and methyl isobutyl ketone ( 3 ) . Dithiol has been reported as a good extractant for both molybdenurr (2) and tungsten (6). EXPERIMENTAL
Apparatus and Reagents. T h e apparatus has been described previously (1).
TTA (Columbia Organic Chemicals, Columbia, S. C.) solutions (-0.15X) in butyl alcohol were used for molybdenum and tungsten. A stock solution of ammonium molybdatcl was prepared by dissolving about 7.5 grams of molybdic acid in dilute ammonia and making up to 1 liter with water. The solution, standardized by the oxine method, contained 4.91 mg. per ml. Test solutions for extraction purposes were prepared by proper dilution of the stock solution so as l o contain 49 pg. of molybdenum per ml. A stock solution of sodium tungstate was obtained by dissolving ,%bout9 grams of sodium tungstate in 1 liter of water containing 4 grams of sodium hydroxide and 8 grams of tartaric acid. The solution, also standardized as oxinate, contained 4.42 mg. of tungsten per ml. Test solutions, prepar1.d by appropriate dilution of the stock solution, had a tungsten content of 188 pg. per ml. Buffer solutions (1M) were prepared by standard methods; acetic acidammonium acetate (pH 4-6) and ammonia-ammonium chloride (pH 8). Preliminary Experiments. Because of t h e very complex extraction behavior of molybdenum a n d tungsten, a large number of trial runs were undertaken. A seriei, of solvents were examined : butyl alcohol, methyl
HCL CONCENTRATION. N
[FOR
Mq
10
6 HCI
CONCENTRATION.
N
(FOR
1
W)
Figure 1. Extraction of (A) molybdenum(VI) and (B) tungsten(V1) with TTA as a function of hydrochloric acid concentration
the solution containing the foreign ion was added t o the aqueous phase just before extraction. At the end of extraction the layers were allowed t o settle and separated. The aqueous phase was rinsed once with 2 ml. of acetophenone and preserved for measurement of acidity as well as of molybdenum (duplicate runs). To the combined colorless organic extracts was added 2.5 ml. of 6% potassium thiocyanate and 10 ml. of 7% stannous chloride in 1 to 3 5 hydrochloric acid. The mixtures were shaken for 5 minutes in a separatory funnel. The color developed almost immediately. The orange-red orgmic layer was then diluted to 25 ml. with n-butyl alcohol in a 25-1111. volumetric flask and measured a t 470 mp against a reagent blank. Calibration curves were prepared with known concenxations of molybdenum using identical extraction conditions. TUNGSTEN. -1 suitable aliquot (0.5 t o 2.5 ml.) of test solution (188 pg. per ml.) was extracted as above. The color of the organic extracts was developed by shaking for 10 minutes with 2 ml. of 10% potasqium thiocyanate and 10 ml. of 7% stannous chloride in 10Llr hydrochloric acid. Finally the yellowcolored organic layer was separated, and diluted with butyl alcohol as before. Measurements were done a t 420 mp within 2 t o 4 hours when full color development had occurred. Calibration curves were prepared under identical conditions.
isobutyl alcohol, ethyl acetate, methyl isobutyl ketone, acetophenone, cyclohexanol, cyclohexane, xylene, benzene, a n d chloroform. T h e best solvent was butyl alcohol-acetophenone mixture from 3 t o 9 N hydrochloric acid in case of molybdenum and from 8 N hydrochloric acid in case of tungsten. For both molybdenum and tungsten, sulfuric acid medium was undesirable in view of poor extraction. Again, it was necessary to maintain a strongly acidic medium for optimum extraction of RESULTS AND DISCUSSION tungsten and to avoid the precipitation of tungstic acid. I n moderately strong Effect of Acidity. MOLYBDENIJM. hydrochloric acid (
>
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acidity varies from 7.5N onwards). Stronger acids (>ION) are avoided because of troublesome manipulation, although quantitative extraction occurs even from concentrated hydrochloric acid. For acidity
