Various attempts have been made to modify the methods for determining vanadium in micro amounts spectrophotometrically 1,2 so as to reduce the interferences of various elements and to enhance the sensitivity of systems in order to widen their scope. From a survey of the recent literature, it appears that the highest sensitivity (ε on the order of 10 5 or 10 6 ) can generally be achieved only in the case of mixed-ligand complexes of vanadium.3-5 However, these methods suffer from a lack of selectivity, since the colored species is not extracted into organic solvents, thus restricting their scope of applicability.In our attempt to develop a highly sensitive and selective binary system for an extractive spectrophotometric determination of vanadium, we have introduced a new reagent, 6-chloro-3-hydroxy-7-methyl-2-(2-thienyl)-4H-chromen-4-one (CHMTC), which not only enhances the sensitivity (ε=8.26×10 4 ) to a much higher value as compared to the existing binary complexes of vandium(V), but also makes the method free from almost all of the analytically important interfering elements. The procedure is simple, rapid and can be applied to the analysis of various kinds of natural and industrial samples with satisfactory accuracy. The details of the method are given below.
Experimental
Reagents and solutionsVanadium(V) solution: A stock solution of vanadium(V) (1 mg ml -1 ) is prepared by dissolving an accurately weighed amount of sodium metavanadate (s.d. fine-chem) in deionized water and standardized by the iron(II) method 6a volumetrically. Lower concentrations of the metal ion at the µg ml -1 level are obtained by suitable dilutions therefrom.Solutions of other metal ions of appropriate concentration are prepared by dissolving their commonly available salts (A.R.) in water or dilute acid.Hydrochloric acid (1 M) is prepared by suitable dilution of 11.3 M HCl (RANBAXY).Carbon tetrachloride (RANBAXY) is used as such. 6-Chloro-3-hydroxy-7-methyl-2-(2-thienyl)-4H-chromen-4-one (CHMTC), (m.p. 212˚C) is synthesized by a literature method 7 and its 0.2% (w/v) solution in acetone is used.
Preparation and characterization of CHMTCTo a solution of powdered NaOH (9 g) in methanol (20 ml) kept at 0˚C are added 5-chloro-2-hydroxy-4-methylacetophenone (4.8 g) and 2-thiophenecarboxaldehyde (3.6 g); the mixture is then stirred for 2 h at 0˚C. Thereafter, the reaction mixture is poured onto crushed ice, neutralized with 4 M HCl and the precipitated solid is filtered, dried and crystallized from ethanol to give yellow needles of 5′-chloro-2′-hydroxy-4′-methyl-3-(2″-thienyl)acrylophenone (I).To a well-stirred solution of NaOH (3.3 g) in methanol (30 ml) kept at 0˚C is added solid (I) (6.9 g). To the thus-obtained dark-red solution is then added 30% H 2 O 2 (8 ml) dropwise with constant stirring at 0˚C for an additional 3 h. The resulting yellow reaction mixture is neutralized with ice-cold 4 M HCl. The thus-obtained light-yellow precipitate is filtered, A simple, rapid, highly sensitive and selective method for the extractive spectrophoto...
A simple, rapid, highly sensitive and selective spectrophotometric method for the determination of tungsten(VI) in trace amounts is developed using 3-hydroxy-2-(2'-thienyl)-4-oxo-4H-1-benzopyran (HTB) as a reagent for the complexation of metal ion and extracting the 1:2 (metal:ligand) complex into dichloromethane from 0.2 M HCl solution. It obeys Beer's law in the range 0-2.8 microg Wml(-1) with molar absorptivity and Sandell's sensitivity at 415 nm as 6.45 x 10(4) L mol(-1) cm(-1) and 0.0029 microg W(VI) cm(-2), respectively. The method is free from the interference of a large number (39) of elements and handles satisfactorily the analysis of various samples of varying complexity.
3-Hydroxy-2-[1'-phenyl-3'-(p-chlorophenyl)-4'-pyrazolyl]-4-oxo-4H-1benzopyran (HPCPB) is used as an analytical reagent for the spectrophotometric determination of niobium in trace amounts with which it forms a yellow coloured complex (4:1) in perchloric acid medium. The complex is extractable into chloroform and shows absorption maximum at 407-418 nm with a molar absorptivity of 2.79 x 10(4) L mol(-1) cm(-1) and Sandell's sensitivity equal to 0.0033 microg Nb(V) cm(-2), respectively. Beer's law holds good in the range 0-1.2 microg Nb ml(-1), with a standard deviation of +/- 0.0015 absorbance units. The method is free from the interference of a large number of elements and handles satisfactorily the analysis of various samples of varying complexity.
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