Muralidhar, J.

Sant, B. R.

Thakur, R. S.

English

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TABLE1- RESULTSOFESTIMATIONOFMn(I1) WITHSODIUMBENZILATEAmount of Mn(I1) (mg) Error(%)Volumetricmethod usingbismuthate10·0013·5521·9532·5348·3355·8366·0874·5279·10Presentmethod10·1013·4921·9532·4348·5955·4266·2474·3679·45+1'00-0,44-0·00-0,30+0'53-0,73+0·24-0'21+0·44gravimetric method for the estimation of Mn (II)using sodium benzilate as precipitating reager t is'being reported in this note.Manganous sulphate (GR, E. Merck) and benzilicacid, (CSHS)2C{OH)COOH (E. Merck) were used.Thermogravimetric analysis was carried out on a-derivatograph (MOM).Sodium benzilate when mixed with Mn(II) gavean instantaneous pale-pink precipitate of Mn (II)-benzilate. It was washed and dried at 120° andanalysed [Found: C, 61'45; H, 4·78; Mn, 9·92.Required for MuBz2.2H20 [where Bz = (C6Hs)2C(OH) COO-]: C, 61'65; H, 4·88 and Mn 10'10%].On heating the sample (100 mg) in the derivatographat a rate of 5°/min decomposition started at 156°,the maximum decomposition occurring around 280°.The calculated and observed metal oxide (MnO)were 13·0 and 13,9 respectively for the compositionMnBz2.2H20. The magnetic moment (Gouy method,.,....,30°) of the solid complex has been found to be5·88 BM which is only slightly lower than thevalue expected theoretically- for a dS system (5'92BM) on the basis of spin-only formula. The in-frared spectrum (Perkin-Elmer model 337) of thesolid complex in nujol shows a peak around 815cm-1 which is absent in free benzilic acid. Thisband may be due to the presence of coordinatedwater in the solid complex. This finds support fromthe earlier studies+" on aquo coordinated com-plexes which show bands between 795 and 910 crrr+for the coordinated water. It is possible thatMnBz2.2H20 has octahedral symmetry.The insolubility of MnBz2.2H20 in water and theconstant composition when dried at 110-20° arethe main advantages of sodium benzilate cs theprecipitating agent.Procedurefor the determination of Mn(II)-Aqu~oussodium benzilate (0'2M) was added gradually withstirring to a solution of Mn(II) (10-80 mg) till theprecipitation was complete. The optimum pHrange was 3,2-5,4. The precipitate was filtered,washed thoroughly with water, 50% ethanol, driedat 110°_20° and weighed. The manganese contentwas calculated on the basis of formula Mn[(C6Hs)2C.(OH)C02]20. The results are given in Table 1 whichshow an accuracy of about 1% as compared to thevalue obtained using volumetric method. .Effect of foreign ions - Interference due to Fe2-f,C02+, Ni2+, Cu2+, U02+, Cr3+, Ti4-f Ce4~, ci-. NO;, NOs,NOTESSOi- and CrO-~ was studied. Ti4+, Cr2+, cr-and SO:- do not interfere. The estimations wnichwere carried out in the presence of varying amountsof foreign ions indicate that for each 100 mg ofmanganese solution, the tolerance limit is 10 mg forall the divalent cations (except UOi+) whereas only5 mg of Ce4+ and UO~+ are tolerated. The anionsNOi, NOg and CrOf- interfere seriously since theseanions oxidize benzilate and should be avoided underthe condition of the experiment.Estimation of Mn(II) in pyrolusite - The mineralwas first dissolved in hydrochloric acid and conta-minated Fe(III) removed by complexing the lattewith ammonium bifluoride. After adjusting thepH, between 3·2 and 5'4 sodium benzilate solutionwas added. The precipitate was then washed,filtered, dried and weighed to constant weight.The accuracy of the method was found to be ± 1%as compared to the value obtained by volumetricmethod using bismuth ate (Table 1).References1. VOGEL,A. I., A text book of quantitative inorganic analysis(Macmillan & Co., London), 1966, 536; 19.2. KATO,M., JOSASSEN,H. B. & FANNING,J. C., Chem,Reu., 64 (1964), 99.3. FIZITA, J., NAKAMOTO,K. & KOBAYASHI,M., J. Am.chem. Soc., 78 (1956), 3963.4. SENGUPTA,K. K., CHATTERJEE,A. K., MOULIK,S. P. &DEY, K., J. inorg. nucl, Chem., 33 (1971), 4368.5. SEN GUPTA,K. K., MOULIK,S. P. & DEY, K., J. inorg.nucl. Chem., 32 (1970), 1052.Determination of V(V) & Cr(VI) in Presenceof ChlorideJ. MURALIDHAR,. S. THAKUR& B. R. SANTRegional Research Laboratory, Bhubaneswar 751004Received 10 November 1975; accepted 24 May 1976Two methods are described for the titrimetric esti-mation of V(V) and Cr(VI) in the presence of chloride.They di ffer only in the manner in which preferentialoxidation of V(IV) by potassium perrnanganate iscarried out. In the first method two titrations are re-commended at room temperature in order to providerequisite time for V(IV) oxidation. The second methodis useful when simultaneous estimation is to be donein the same aliquot. Here the solution is cooled to 5°and then V(IV) is oxidized selectively with excess potas-sium perrnanganate, Oxidation of Cr(IlI) which be-comes prominent in the presence of chloride is madenegligible in these methods.AMONGST the trtrimetric methods for theestimation of vanadium(V) and chromium(VI),the one generally employed involves titration withstandard iron(II) sulphate in sulphuric acid mediumin the presence of phosphoric acid, using diphenyl-amine sulphonate as the indicator-A, This givesthe total titre value for both V(V) and Cr(VI).The vanadium is then selectively oxidized with excesspotassium permanganate and -after destroying thelatter, V(V) is retitrated with iron(II); Cr(III)remains unaffected and its value is obtained bydifference". During our work on the recovery of71INDIAN J. CHEM., VOL. 15A, JANUARY 1977TABLE 1 - INFLUENCE OF CHLORIDE[Vanadium and chromium in each experiment, 10·12 and3·47 mg respectively; time for V(IV) oxidation 3 min;temp. 28°]NaCl added Vanadium NaCl added Vanadiummg found mg foundmg mg10 10·12 100 10·1925 10·16 250 10·2950 10·19 500 10·40vanadium from bauxite residues (which also containchromium) we found that conventional method ofvanadium-chromium analysis in a mixture is subjectto inaccuracies when chloride is present in thesystem. The present note reports two simplemodified procedures for the estimation of V (V) andCr(VI) in the presence of chloride.The oxidation of vanadiumlf.V) by permanganateis a sluggish reaction necessitating addition of excessoxidant to ensure complete conversion to V(V).Chromium (II I) can also be oxidized by perman-ganate in acid solution but the reaction can bemade negligible when oxidizing V(IV)-Cr(III) mix-tures by controlling the excess permanganate andtime. The sluggish reaction of V(IV) with per-manganate can be made to proceed rapidly at anelevated temperature but then Cr(III) gets in-creasingly oxidized. Presence of chloride furthercomplicates the selective oxidation of V(IV), givinghigher values of V(V) (Table 1) in vanadium-chromium analysis. We have observed that at roomtemperature (25-30°) oxidation of Cr(III) is negli-gible in 2 min but becomes significant as the time isincreased. If sodium chloride is present con-currently, Cr(III) oxidation even in the short periodof 2 min is not negligible and increases with in-creasing time. Oxidation study at different tem-peratures revealed that Cr(III) oxidation in thepresence or absence of chloride decreases withdecrease in temperature and becomes negligible at5° or below without affecting the complete oxidationof V(IV). Even when excess permanganate is keptfor 10 min at 5°, Cr(III) is not oxidized, therebyaffording a simple method for the estimation. ofvanadium in the presence of both chloride andCr(III) (Procedure B).If it is preferred to oxidize V(IV) selectively inthe presence of Cr(III) and chloride at room tem-perature, permanganate is to be added dropwise tillit is in slight excess as indicated by its pink colour.In the presence of diphenylamine sulphonate indi-cator this may be difficult due to intense colour ofthe indicator. For the analysis of vanadium-chromium mixture in the presence of chloride there-fore two titrations are recommended (Procedure A).Both the procedures gave similar results and anyone could be adopted.Reagent grade chemicals were used. Solutions(0'02N) of potassium dichromate, sodium vanadate,and ammonium ferrous sulphate were prepared indistilled water. The solutions of other reagentswere potassium permanganate (O·lN), sulphamic72TABLE 2 - DETERMINATIONOF VANADIUMAKD CHROMIUM:IN PRESEKCE OF CHLORIDE(Sodium chloride added in each case 500 mg)Vanadium, mg Chromium, mgTaken Found Diff. Taken Found Diff.PROCEDUREA1·00 1·00 0·00 20·00 20·09 0·095·00 5·00 0·00 15·00 15·10 0·1010·00 10·06 0·06 10·00 9·95 0·0515·00 15·10 0·10 5·00 4·95 0·0520·00 20·01 0·01 1·00 1·00 0·00PROCEDUREB1·00 1·00 0·00 20·00 20·10 0·105·00 5·02 0·02 15·00 15·20 0·2010·00 10·00 0·00 10·00 10·08 0·0815·00 15·10 0·10 1·00 1·00 0·00acid (10%), sodium nitrite (0'5%) and bariumdiphenylamamine sulphonate indicator (0·2%).Procedure A - To an aliquot (of a mixture}containing dichromate, vanadate and sodium chIo--ride, sufficient 1: 1 sulphi.ric acid to maintain anoverall acidity of 2N and 5 ml syrupy phosphoricacid were added. The solution was diluted to100 mI· and the mixture titrated with standardiron(lI) solution after adding 2-3 drops of indicator'solution. The titre value correspcnds to the sumof vanadium(V) and chromium (VI) present in the-mixture. To another similar aliquot of a mixtureunder similar conditions calculated (correspondingto the titre value above) quantity or little excess ofiron(II) solution was added, followed by dropwiseaddition of permanganate with constant stirring-(preferably magnetic) till its colour persisted for2 min. Excess permanganate was reduced by adding-just sufficient sodium nitrite solution and excessof the latter was destroyed with 10 ml of sulphamicacid. The vanadate formed was titrated withiron(II) solution after adding 2-3 drops of indicatorsolution. The chromium value was obtained bydifference.Procedure B - The total vanadium and chromiumin an aliquot was estimated by iron(II) titration asin procedure A. After the end point had reachedthe solution was cooled to 5°. An excess of per-manganate solution was added with ccnstant stirring(preferably magnetic) for 3 min. Thereafter per-manganate was destroyed by sodium nitrite andsulphamic acid as before. The solution was nowbrought to room temperature (25-30°) and thevanadate formed titrated with iron (II) solution;The chromium value was obtained by difference.Representative data of a series of experiments are-given in Table 2.The authors thank Prof. P. K. jena, Director •.for his keen interest in the work.References1. KOLTHOFF, 1. M. & BELCHER, R., Volumetric analysis •.Vol. 3 (Interscience Publishers, New York), 1957, 608.2. KOLTHOFF, 1. M. & ELVING, P. J., Treatise on analytical'chemistry. Part II, Vol. 8 (Interscience Publishers, NewYork), 1963, 227.3. DUCRET, L., Analytica Cbim, Acta, 1 (1947). 135.

Determination of V(V) & Cr(VI) in Presence of Chloride

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Determination of V(V) & Cr(VI) in Presence of Chloride

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