Chem. Anal. (Warsaw), 38, 639 (1993) Gravimetric Determination of Vanadium as V(IV)-Oxinate by S. Kaur, A. K. Chhakkarand L. R. Kakkar* Department ofChemistry, Kurukshetra University, Kurukshetra -132119, Haryana; India Key words: vanadium(V), oxine, 8-hydroxyquinoline, gravimetry A very simple gravimetric method for the determination of vanadium has been worked out. In acid medium vanadium is reduced to tetravalent state which forms greenish-black precipitate with 8-hydroxyquinoline (2 % in 2 mol 1-1 CH3COOH) in the presence of ammonium acetate. The precipitate is dried and weighedas VO(C9H60 N)2. The con­ version factor for vanadium is 0.1437. The method is free from the interference of molybdenum(VI), chromium(III,VI), uranium(VI), selenium(IV), arsenic(I1I), bis­ muth(III), lead(U), calcium(II), manganese(I1) and maguesium(II). Opracowanoprosta wagow'\ metode oznaczania wanadu. Wauad(V) redukuje silt w kwasnym roztworze do wanadu(IV), ktory w obecnosci octanuamonu tworzy z 8-hydro­ ksychinolina (2 % roztwor w 2 mol 1-1CH3COOH) zielonoczaruy osad. Po wysuszeniu osad jestwazonyjako VO(C9H60 Nh Mnoznik analityczny wynosi dla wanadu 0,1437. W oznaczeniu nie przeszkadzaja: molibden(VI), chrom(III,VI), uran(VI), selen(IV), arsen(III), bizmut(III), o16w(II), wapriffl), mangan(II} i magnez(II), Many inorganic [1-4a] and organic [5-8, 11] precipitants employed for the estimation of vanadium in milligram amounts are. unsuitable for routine analysis, either because they are not quantitative or because other elements are coprecipitated withvanadium, and also in some cases, the precipitate formed does not have a definite composition. Cup ferron [4b] is generally recommended for the precipitation of vanadium, but several precautions are necessary because ofthe instability of cupfer­ ron and many other elements are precipitated by the reagent under conditions ofthe * Senior author for correspondence. 640 s. Kaur, A. K. Chhakkar andL. R. Kakkar procedure. Though 8-hydroxyquinoline [3] and its derivative [9] have been used as precipitants, a large number of other ions also form oxinates. As vanadium is known to exist in lower valence states, which are not as well established as V(V), it is considered worth to make an attempt to study the. various factors which affect the precipitation of tetravalent vanadium with oxine in order to finally work out a new and better method for the determination of vanadium. EXPERIMENTAL Reagents and materials Standard stock solution of vanadium(V) containing 5 mg ml"! of the metal ion is prepared by dissolving weighed amount of ammonium metavanadate, NH4V03 (B.D.H.) in distilled water containing a few drops of dilute NaOH solution and standardized by oxine method [3]. Lower concentrations were prepared by appropriate dilutions. 8-Hydroxyquinoline (oxine) reagent, 2 % solution in 2 mol I-I aceti~ acid. Ammonium acetate, 20 %. Hydrogen peroxide, 3 %, and hydrazine sulphate (solid) were used. Hydrochloric acid, 1 mol l". Samples Synthetic samples were prepared by mixing vanadium solution with solutions of other ions in a suitable proportion as shown in Table 1. Table 1. Analysis of samples by the proposed method Sr.No. Composition of the sample" V added, mg V found, mg 1 MO(l.5) 5.0 5.02 2 Mo(2) 8.0 8.08 3 Mo(l), Cr(8) 7.0 6.8 4 Mo(2), Cr(10) 10.0 10.1 5 Mo(O.5),Cr(5), U(2) 9.0 9.05 6 Mo(1.5), Cr(2), U(l.5) 6.0 6.09 7 Mn(20), Fe(O.l), Re(O.I) 8.0 8.04 8 Mn(10), Fe(0.05), Re(0.05) 9.0 9.06 9 Mn(O.5),Cr(2), U(1.5), Fe(0.05) 7.5 7.47 10 Mo(1.2), Cr(3.5), U(O.5), Fe(O.l) 8.5 8.56 ·11 Reverberatory flue dust (500 mg) 8.0 8.10 (300 mg) 5.0 5.08 12 Ferrovanadium (100 mg) 35% ** 35.3 % * The figure in bracket indicates the amount in mg. ** Reported value. Gravimetric determination ofvanadium... 641 Ferrovanadium: Dissolve [12] 100 mg of the sample in minimum volume of cone. HN03 by slow heating. The solution is evaporated down to - 2 mI. Excess of nitric acid is completely destroyed by heating it to dryness with cone. HCI. The residue is cooled and treated with distilled water and the final volume is made to 50 ml ina volumetric flask for determination of vanadium. Reverberatoryflue dust: Flue dust containing no vanadium is mixed with a known amount of metal ion and dried in an oven. After fusion ofthe sample with sodium peroxide (8 times the weight of the sample), the leach is neutralized with cone. H2S04 and made slightly alkaline. Itis boiled and hydroxide precipitate is filtered off and finally washed well with distilled water. The filtrate alongwith the washings is adjusted to pH-7 and vanadium is determined as in procedure given below. Procedure for precipitation and determination To 2 ml of vanadium solution (5 mg ml") add 0.5 ml of 10 mol I-I HCI and dilute it to 5 ml with water in a 100 ml beaker. Warm the solution and add 25-50 mg of hydrazine sulphate (solid), heat to boil for 3 minto reduce vanadium(V) to vanadium(IV) blue. The solution is cooled to 5-10°C. Add 10 ml of oxine reagent (2 %), 15 ml of water, 20 ml of ammonium acetate (20 %) slowly with constant stirring (all the reagents before addition are maintained at 5-10°C) till precipitation is complete. The precipitate is kept in ice for 40 min at 5-10°C. It is then filtered through sintered glass crucible No 4 and washed with cold water (5-10°C) 4-5 times, each time with 5 ml of water. The precipitate is dried at 110-130°C for an hour, cooled in a desiccator for about 45 min and weighed. The conversion factor for vanadium is 0.1437. The presence or absence of traces of vanadium in the filtrate alongwith the washings is tested [3] after oxidation of reduced vanadium, if any, to pentavalent with H20z in slightly alkaline solution and decomposing excess of it in acid solution. RESULTS AND DISCUSSION In acid solution, hydrazine sulphate reduces pentavalent vanadium to tetravalent which is blue in colour. V(IV) so obtained forms a greenish-black precipitate with 8-hydroxyquinoline in the presence of ammonium acetate. The completness of precipitation is influenced by several parameters, namely, acid, precipitant, ammoni­ um acetate, reductant, temperature (Fig. 1). The standing time for the precipitate in contact with solution at 5-10°C was investigated in the time range from 0 min to 60 min. Quantitative precipitation was found for the standingtime 40 or more minutes. When the vanadium solution was very diluted (s 0.05 mg ml-1) no precipitation occurred. Complete precipitation was found in the concentration range from 0.1 to 0.2 mg mr', whereas in more concen­ trated solutions (up to 0.8 mg mr') the efficiency slightly decreased to 94 %. From consideration of the data pertaining to various parameters it has been concluded that for up to 10 mg of vanadium the optimum conditions for the quanti­ tative precipitation of V(IV) as oxinate and its subsequent determination are as described under procedure. 642 s. Kaut; A. K. Chhakkar andL. R. Kakkar C B 80 0 0 0 0 A .~.. c 60 .2a Q. 'u- 4O CD a.'"' 20 0 0 10 20 30 40 A,B&C 100 0 ~o 0 0 0 80 ~.. c 60 0 ;; CJ '0.- 40 U QJ Q.'"' 20 0 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 E 0 20 40 60 80 100 120 140 160 0 Figure 1. Dependence of V(IV)-oxinate precipitation on various parameters. 10 mg vanadium, reduc­ tion at 1 mol 1-1 HCl, 100 mg hydrazine sulphate, 10 ml 2 % oxine, 5 ml ammonium acetate, total volume 50 ml, except variation of the parameter shown on the abscissa (curves and corresponding scales are shown by same letters). A- 8-hydroxyquinoline in 2 mol 1-1 CH3COOH [2 %(m/V)], ml; B - ammonium actate (20 %), ml; C - temperature, °C; D- hydrazine sulphate, mg; E - hydrochloric acid, mol 1-1 Effect ofdiverse ions In 50 ml aqueous volume, Mo(VI) 2 mg; Cr(VI), Se(IV), As(III), Pb(II), Ca(H), Mn(H), Mg(I1), 100 mg each; Bi(Ill), Cr(Ill) 10mg each; U(VI) 3 mg; do not form any precipitate with oxine under the conditions chosen for the precipitation of vanadium and hence, these metal ions are non-interfering. Similarly Re(VII), 100 ug; W(VI), AI(I1I), Ni(II), Co(I1), Cu(II), Zn(ll), 50 ug each; Fe(III, II), Pd(II), 200 I-lg each; are also without effect. Phosphate, fluoride, tartrate, 100 mg each, decrease the precipitation by 0.2-0.5 %. Chloride, 50 mg, in excess to the necessary amount Gravimetric determination ofvanadium... 643 decrease the precipitation by 10 %. Sulphate, 5-50 mg, lowers it by 5 %. Oxalate has considerable effect which brings down the precipitation to 80 % even when 2 mg of it are present in solution. Citric acid, EDTA, 100 mg each, inhibit precipitation. The reproducibility of the method is tested by performing ten determinations, each time with 10 mg of vanadium, under the optimum conditions laid down for the precipitation and determination ofthe element. The weight ofthe precipitate obtained hardly shows any change which indicates that the method has good reproducibility with a standard deviation±O.OOOl.