Gowda, H. Sanke

Shakunthala, R.

English

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ladian JOllrnal of ChemistryVol. 14A. June 1976. pp, 431-434Prochlorperazine Maleate as a New Redox Indicator In VanadametryH. SAXKE GCJWDA & R. SHAKUNTHALADepartment of Postgraduate Studies & Research in Chemistry, :\I?_lBS", G;\ngotri, University of Mysorc, MysoreReceived 28 JUII~ 1975; accepted 15 September 1975Formal and transition potentials of prochlorperaztne maleate, a potential indicator invanadametry have been determined. Titrations of Fe(II), Mo(V), U(IV), Sb(III) and hydro-quinone of 0·05 and 0'002N have been carried out with sodium vanadate using the indicator inthe presence of phosphoric and oxalic acids. Its chief advantages are: (i) it can be used at loweracidities, (if) the colour change is sharp, (Hi) it can be used in mlcrotitrations, and (iT) it hasnegtlglble indicator correction.THERE are only a limited number of redoxindicators introduced in vanadarnetry. Theseare Nsphenylanthranilic acid- (NPA), di-phenylamincs (D), diphenylbenzidines (DB). di-phenylaminesulphonic acid! CDS), ferroins, copper-phthalocyaninetetrasulphonic acid' (Cu- PTS), neutralred, phenosafranine and safranine", dimethylnaph-thidinedisulphonic acids, iodine monochloride withan extractive end-point", chlorpromazine hydro-chloride-? (CPH), promethazine hydrochloride" (PH)and 3,4,7,8-tetramethyl-l, 10-phenanthroline iron (II)sulphate'>. We have now investigated the efficacyof 2-chloro-lO[3-(4-methyl-1-piperazin yl)propyIJphe-nothiazine (prochlorperazine maleate, PPM), inthe vanadametric determination of Fe(II), Mo(V),U(IV), Sb(III) and hydroquinone. The resultsare presented in this paper.Materials and MethodsPpM (0·1 g) was dissolved in hot water, cooled,diluted to 100 ml and stored in an amber colouredbottle.Approximately 0'05N and 0-OO2N solutions ofMO(V)13, U(IV)1<l, Sb(III)15, hydroqninonest andsodium vanadate'? were prepared. MO(V)lS andU(IV)17.19 were standardized against eerie sulphate.Sb(III) was standardized against potassium bro-mate20• Hydroquinone was standardized againsteerie sulphate with a potentiometric end-points!and sodium vanadate was standardized againstMohr's salt solution'P.Solutions of eerie sulphate, ferrous ammoniumsulphate, ferroin and NPA were prepared by theusual procedures.The potentiometric assembly consisted of a directreading potentiometer, a mirror galvanometer, abright platinum gauze indicator electrode, a satu-rated calomel reference electrode and an agar saltbridge. The titration mixture was stirred by amagnetic stirrer.Determination. of the formal redox and transitionpotentials of PPM -- The formal redox potentialof P PM was determined in various concentrationsof sulphuric acid by the method of Schilt22• Thepotentials (extrapolated to zero time) in 0-25, 0·50,I-a and 1·5M sulphuric acid were found to be 795,766, 748 and 732 ± 5 mV respectively. The tran-sition potentials of PPM in the titration of Fe(II)with vanadate were determined by the methoddescribed by the authors-". In 0·25, 0-5, 1-0 and1·5M sulphuric acid the values of transition potentialsare 754, 740, 725 and 720 ± 5 mV respectively.Determination. of the sensitivity of PPM towardsvarious oxidizing agents - The sensitivity of PPMtowards various oxidising agents were determinedby mixing a drop of the test solution containingthe oxidant, a drop of 2N hydrochloric acid anda drop of 1% PPM solution in a spot plate. Eachtest was allowed to stand for 30 see with each oxidantexcept pei manganate where nearly 5 min wererequired for full development of colour. Thesensitivities of the oxidants are in the order ofCe4"'> V5+>CrIO,->IOa>Br03>Mn04>Fe~fN):-.V'anadametric determination of Fe (II) , MpW),U,(IV), bydroquinoeu: and Sb(III)- 20 ml of 0-05N(or 10 ml of 0-002N) Fe(II) , Mo(V), U(IV) or hydro-quinone, syrupy phosphoric acid, l'ON oxalic acidand 0·1% indicator solution were taken in a conicalflask and diluted with sulphuric, hydrochloric oracetic acid to 40 ml (or to 25 ml). The solutionwas titrated with 0'05N (or 0·002N) sodium vanadatesolution to the appearance of violet (or pink) colour.In the determination of Sb(III), a known amountof SbCls was heated with 100% excess of O·lNvanadate in 40 ml of 6N sulphuric acid solutionat 95-100° for 2-3 min. The solution was cooledand titrated with standardized Mohr's salt solutionin the presence of 10 ml syrupy phosphoric acidusing 2 ml of PPM indicator near the end-pointto the disappearance of violet colour. The detailsof the titration medium and results were presentedin Table 1.Results and DiscussionThe PPM indicator is soluble in water grvmgcolourless solution which is stable for a long timeat low temperature (8°) in dark. It slowly under-goes atmospheric and photochemical oxidation atroom temperature (25°) after two days which doesnot in any way interfere in its indicator action.The oxidation of PPM by strong oxidants such aseerie sulphate takes place in two steps. The indi-431INDIAN J. CHEM., VOL. 14A, JUNE 1976TABLE1- VANADAKETRICDETERMINATIONWITHPPM INDICATORReductant taken Medium Normality Reductant Colour changemg of determinedvanadate mgr. 40·8 of Fe (II) 4 ml of syrupy H.PO.+O·1 ml of IN HaC.O. 0·05 40·8 Greenish blue+ IN H.SO./HCI/HAC (parrot green in HCI)through blue to violet2. 1·78 of Fe(I1} 1-4 ml of syrupy HsPO.+O·l ml of IN 0'002 1'79 Colourless to pinkH.C.O.+O·2-O·5N H.SOa/HCl/O·4-2·0NHAC3. 65·3 of Mo(V) 0'5-2'0 ml of syrupy H.PO./H.C.O. + 1·8- 0-05 6504 Greenish blue through2'SN H.SOa/l·S-2·SN HCI blue to violet4. 1·48 of Mo(V) 0'5-2'0 ml of syrupy H.PO./IN H.C.o. 0'002 1·48 Faint green to light+1-3N H.SO./HCI violet5. 54'0 of U(IV} 2-4 of syrupy H.PO./1-6 ml of IN H.C.O. 0-05 54-0 Parrot green to pinkish+1-3N H SO.IHCI green6. 1-62 of U(IV} 0'5-2 ml 01 syrupy H.PO./IN H.C.O.+1- 0'002 1-64 F2.int green to lightI'SN H.SO.IHCI violet7. 20'5 of HQ 2-5 ml of IN H.C.O.+1-3N H.SO.IO-S-2·ON 0-05 20'5 Parrot green to pinkHCI8. 1'24 of HQ 1-3 ml of IN H.C.O.+0·S-2·ON H.SO.IHCI 0-002 1-24 Light parrot green topink9. 27'5 of Sb(I1I} 100% excess vanadate + 6N H.SO., heat to [0'05N 27·6 Violet through blue to95-1000 for 2-3 min. Cool, dilute to give iron (II)] greenish blue2N H.SO.+I0 ml syrupy H3PO.+2 mlPPM near the end-point[O'OSN 16'9 do10. 16·9 of Sb(III) doiron (II)]cator first undergoes a one-electron oxidation to ared intermediate2S,u which is believed to be a freeradical with semiquinonoid structure=. The radicalis oxidized to a colourless sulphoxide= with theloss of one more electron. The probable mechanismof oxidation is shown in Chart 1.The oxidation of PPM from colourless to redspecies by vanadate is a reversible process. Thespeed of oxidation of the indicator increases withincreasing acidity and the red colour develops muchmore quickly in hydrochloric acid medium thanin sulphuric acid medium of the same normality.The reduction of the red species of the indicatorto a colourless one with reductants is also reversible.The stability of the red colour increases with in-creasing concentration of acid and with decreasingconcentration of vanadate in the solution. Whena large excess of vanadate is added, the red colour(appearing violet in the presence of blue vanadylions) is gradually destroyed leaving pale blue solu-tion in which it is impossible to get the characteristicviolet colour again by the addition of reductantshowing that the indicator is irreversibly oxidizedto the sulphoxide by a large excess of vanadate.The indicator does not give red colour with V(V)in IN acetic acid even in 30 min. However, a fewdrops of oxalic or phosphoric acid catalyse theO:::OCI < ~:~ 'I r=»;(CH2>,-~ N - CH3' DIMAlEATE"PM (COLOURLESS)oxidation of PPM to red intermediate in aceticacid medium.Experiments canied out to determine the con-centration of PPM in all actual titration revealedthat at least 0·8 ml of 0·1% indicator solution isrequired in a total volume of 60 ml for proper indi-cator action. The average indicator correctionneeded for I ml of 0'1% PPM indicator solution is0·02 ml of O'OlN vanadate.Determination of the formal redox and tra1tsitiofJpotentials of PPM - The change in colour andcomposition of a redox indicator takes place. at adefinite potential. It is therefore, necessary whenchoosing a redox indicator for a particular titrationto ensure that its standard redox potential (Eo)lies within those of the redox systems. Thus Eovalues of Fe(III)-Fe(II)26, U(VI)-U(IV)26, Mo(VI)-MO(V)27, quinone-hydroquinonets and vanadate-vanadyl couplesw are 0'771, 0'334, 0·48, 0·7 and0·9994 V respectively. The formal redox potentialof the PPM indicator in IN H2S04 is 766 mV whichlies between the titrate and the titrant systems.The indicator so selected should have a colourchange as close as possible to the equivalence pointand the PPM has suitable redox potential which isclose to the equivalence potential, (801 mY) ofV(V)-V(IV)jFe(III)-Fe(II) systemw, Ideally theoQ~DaI ;--'\(CH2's-N'\.-/N - CHs' DlMALUTlFREE RADICAL (RED)Chari 1432I r=;(CH2'3-N,---", - CH3•DlMALUT£$ULPHOXIDE (COLOUItLESS)GOWDA & SHAKUXTHALA: PROCHLORPERAZINE A NEW REDOX INDICATORtransition potential of a redox indicator shouldcoincide with the equivalence potential in thetitration. The transition potential of PPM in INH2SO, is 740 mV which is close to the equivalencepotential of the V(V)-V(IV)/Fe(III)-Fe(II) system.It was found that both formal redox and transitionpotentials of PPM lie within the potential break;640-970 mY, in the potentiometric titration of0'05N iron (II) with 0·05N vanadate in IN sulphuricacid solution containing 0·5Jf phosphoric acid29•Similarly it has been observed that the formalredox and transition potentials of PPM lie withinthe potential break in the potentiometric titra-tions of Mo(V), U(IV) and hydroquinone with vana-date.Titration of Fe(II) ammoniHm s'ulphate - ThePPM indicator gives sharp end-points in sulphuricor hydrochloric acid medic" ranging from 0·2-2·0Nonly in the presence of 3-8 ml of syrupy phosphoricacid in a total volume of 60 ml and in acetic acidmedia ranging from 0'4-4'0N in the presence of5-8 ml of syrupy phosphoric acid, The presence ofphosphoric acid results in a very high potentialbreak at the equivalence point and lowers the formalpotential of the Ft(III)-Fe(II) couple by complexingwith Fe(III) more strongly than with Fe(II)lO,End-point becomes sharper in the presence of 2drops of IN oxalic acid which acts as an indicatorcatalyst. Premature end-points are obtained athigher concentrations of sulphuric, hydrochloric,acetic or phosphoric acid. The colour change atthe end-point is from greenish blue -+ blue ~ violetin sulphuric and acetic acid media and from parrotgreen ~ blue ~ violet in hydrochloric acid media.The violet colour obtained at the equivalence pointis stable for more them 30 min.Titration of 0·002N F e(II) ammonium sulphate- Contrary to Stiepin's observationsw, the authorsfound that NPA gave sluggish end-points consumingmore vanadate. The PPM indicator works satis-factorily in the titration of very dilute Fe(II) solu-tions with 0'002N vanadate. It gives sharp end-points under conditions given in Table 1. Thecolour at the end-point is stable for about 1-5 min,depending upon the concentration of oxalic acid.Higher is the concentration of oxalic acid, loweris the stability of the pink colour.The PPM indicator is far superior to NPA, themost widely used redox indicator in vanadametrybecause, (i) it works at lower acidities, (ii) it worksin acetic acid media, (iii) it is a more sensitive indi-cator for the microtitrations of Fe(II) with 0·002Nvanadate, (iv) the colour change is very muchsharper and (v) the colour at the end-point is morestable.Titration of Mo(V) - D31, DB1?, DS31, NPN7,32,33,Cu-PTS6 and CPH10 have been recommended asredox indicators for the titration of Mo(V) withvanadate in varying concentrations of sulphuricand hydrochloric acid. All these indicators exceptCPH function in high concentrations of acid.The PPM indicator forms white precipitate orturbidity with Mo(VI) and not with Mo(V) in sul-phuric acid media ranging from 0·1-1·5N and inhydrochloric acid media ranging from 0·1-1·0N.However, the precipitate is soluble in higher con-centrations of acid.Vanadiumfl'V) exerts a retarding effect on theoxidation of PPM with V(V), the magnitude ofwhich decreases with increasing concentration ofsulphuric or hydrochloric acid. Phosphoric oroxalic acid catalyses the oxidation of PPM withV(V). The reduction of the oxidized form of theindicator by Mo(V) is fast in solutions of loweracid concentrations and slow in solutions of higheracid concentrations. Molybdenum(V I) and V(IV)catalyse the reduction of the red radical by Mo(V),the combined catalytic effect being more than thatof either substance acting individually. In thistitration phosphoric or oxalic acid acts as an indi-cator catalyst. The stability of the colour at theend-point in the presence of phosphoric acid (0·5-2·0 ml) is the same as in the titration of Fe(II).But in the presence of oxalic acid (0·5-2·0 ml) thecolour is stable for about 3-5 min. The behaviourof the indicator is similar to that observed in thetitration of Fe(II) at acidities higher than the acidconcentration given in Table 1. It is found thatthe PPM indicator functions satisfactorily in thetitration of very dilute Mo(V) solutions also.Titration of U(IV)- D34.-36, NPN?, CuPTS8,and PHll have been proposed as redox indicatorsfor the titration of U(IV) with vanadate in varyingconcentrations of sulphuric or hydrochloric acid.All these indicators except PH work only at higheracidities.The PPM indicator gives sharp colour changeonly in the presence of phosphoric or oxalic acidbecause the reduction of the coloured oxidizedform of the indicator by U(IV) is induced by thereaction between U(IV) and V(V) only in the pre-sence of phosphoric or oxalic acid. The behaviourof the indicator is the same as that observed in thetitration of Fe(II) and Mo(V) at acidities higherthan the acid concentration given in Table 1.The PPM indicator functions satisfactorily inthe titration of very dilute U(IV) solutions.Determination of Sb(III)- Antimony(III) cannotbe titrated directly with vanadate either in thecold or in the hot (50°C) as the reaction is not rapid.Osmium tetroxide, copper sulphate and silversulphate fail to exhibit any catalytic effect on thereaction in the cold. The rate of oxidation of Sb(III)increases with increasing concentration of sulphuricacid and increasing time of reaction. Preliminaryexperiments showed that Sb(III) is quantitativelyoxidzed to Sb(V) if the sulphuric acid concentrationis 6-12N, the excess vanadate is 75-150% and thetime taken is 2-10 min at 95-100°. Hence excessvanadate is added and back titrated with Fe(II).The indicator should be added near the end-point(after 96% titration). It gives sharp colour changefrom violet ~ blue ~ greenish blue.Titration of hydroquinone - D28,38, NPA38, PHuand CPH1~ have been used as redox indicators inthe titration of hydro quinone with sodium vana-elate. DB and NPA are unsatisfactory becausethey give sluggish end-points and NPA functionsonly at higher acid concentrations. The redoxpotential difference between the hydroquinone-433INDIAN J. CHEM., VOL. 14A, JUNE 1976quinone and the vanadate-vanad~l systems indicatesthat the reaction between hydroquinone and vanadateis likely to be fast. Howeve~, the titr.ati?n ofhydroquinone with vanadate using PPM Indicatorhas not been found feasible in sulphuric or hydro-chloric acid media ranging from O·5-5·0N. Thisis due to the retardation of the reaction betweenthe indicator and vanadate by the quinone andV (IV) formed in the reaction. The. magnitude. ofretardation by quinone and V(IV) increases withincreasing concentration of quinone and V~IV),the combined effect being more than that of eitherquinone or V(IV) acting individually. Ox.ali~ acidcounteracts the retardation and acts as an indicatorcatalyst. The speed of. ox~dation. of the indica~orby vanadate increases with .1llCr~aslllg concentr.atlOnof oxalic acid so that the titration of hydroquinonewith vanadate has been carried out in sulphuric orhydrochloric acid media only in the presence ofoxalic acid. The colour change is from parrotgreen-s-violet in titrations of lower concentrations ofhydroquinone « 1 mgjml). The violet colour ismasked and seen as greenish pink in the tit rationsof haher concentrations of hydroquinone.The PPM indicator also functions Well in titrationsof nry dilute hydroquinone solutions.The PPM indicator has advantages over manyof the proposed redox indicators in the titrationof M)(V), U(IV) and hydroquinone because it canbe used at lower acidities, the colour change issharper, it can be used in microtitrations and it hasnegligible indicator correction.References1. SYROKOMSKII, V. S. & STIEPIN, v. V., Zav. Lab., 5(1936), 144; J. Am. chem, Soc., 58 (1936), 928.2. FURMAN, N. H., Ind. Engng cs-«, 17 (1925), 314.3. '"VILLARD, H. H. & YOUNG, P., Ind. E11gng cs-«; 20(1928), 764.4. KOLTHOFF, 1. M. & SARVER, L. A., J. Am. cheni. Soc.,53 (1931), 2902.S. WALDEN, G. H., HAMMETT, L. P. & EDMONDS, S. M.,J. Am. chem. Soc., 56 (1934), 57.6. RAO, G. G. & SASTRI, T. P., Z. analyt. cu«; 167 (1959). 1.7. RAO, G. G. & MURTHY, B. V. S. R., Talanta, 8 (1961), 438.4348. BELCHER, R, NUTTEN, A. 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Prochlorperazine Maleate as a New Redox Indicator In Vanadametry

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Prochlorperazine Maleate as a New Redox Indicator In Vanadametry

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