Preconcentration by Using Microcrystalline Phenolphthalein for Determining Trace Molybdenum(Ⅵ) in Wa

CHEM. RES. CHINESE U.2005 ,21(5),622- -625Preconcentration by Using Microcrystalline Phenolphthalein forDetermining Trace Molybdenum( VI ) in Water by GFAASLI Quan-min””, OUYANG Rui-zhuo ,ZHU Gui-fen and LIU Guo-guangThe Key Laboratory of Enwironmental Pollution Control Technology of Henan Province ,College of Chemistry and Environmental Science , Henan Normal University ,Xinxiang 453007 ,P. R. ChinaReceived Nov. 10 ,2004Keywords Molybdenum , Salicyl fluorone , Preconcentration , Microcrystalline phenolphthaleinArticle ID 1005 _9040( 2005 )-05-622-04Introductionphenolphthalein and SAF due to a close match of theirMolybdenum( VI ) is a biologically essential tracestructures , leading to a dramatic enhancement in theelement and its role in an extremely wide variety of sys-preconcentration of Md( VI ).tems has been reported. Most common methods oftenAs phenolphthalein is a derivative of triphenyl-fail to determine trace Mo( VI ) in the analysis of mo-methane , complex agents like SAF derived from tri-lybdenum-containing samples , due to limitations suchphenylmethane are deduced to be absorbed easily onas inadequate detection limits and matrix interference,microcrystalline phenolphthalein. The proposed methodwhich make the direct determinations impossible. Tooffers a high preconcentration factor ranging from 50 tosolve this problem , various approaches are employed to100. A successful application of such a method to theconcentrate and separate Md VI ) to detectable levels.determination of trace Md VI ) in tap water by GFAASConcentration and separation methods play a main rolehas been achieved after preconcentration by using mi-in the analysis of trace Md( VI ). Therefore , many secrocrystalline phenolphthalein. The recovery of 0. 5paration and preconcentration prcedures have been de-μg/L Md( VI )is above 85% with the relative standardveloped for the determination of Mo( VI )-9]. Wdeviation of about 2. 0%. The analytical results arehave reported the utility of phenolphthalein as an adsor-very satisfactory.bent to separate and concentrate trace Cd( II ) fromExperimentalaqueous solutionsThe method shows several ad-1 Materialsvantages over the other commonly used methods , suchA 1.0 x 10-3 mol/L salicyl fluorone( A. Ras high sensitivity and preconcentration factors , excel-grade , Shanghai Reagents Plant , Shanghai , China ) so-lent effectivity , freedom from matrix interference andlution was prepared by dissolving 0. 1681 g of salicylnontoxicitiy. In the present work , microcrystalline phe-fluorone in ethanol. A 5.0x 10 -3 mol/L cetyl-trimeth-nolphthalein was utilized to preconcentrate and separateylammonium bromide( CTMAB ) solution was obtainedtrace Md( VI ) for its determination by GFAAS in waterby dissolving 1. 8220 g of cetyl-trimethy lammoniumsamples.bromide in distilled water. A 15% ( mass fraction )It seems that there has been no reports describingphenolphthalein( A. R. grade , the British Drug Housesthe utility of phenolphthalein in the preconcentration ofLtd. B. D. H. Laboratory Chemicals Group Poole ,Md( VI ) so far. The analysis based on complexing in-England )solution was prepared in ethanol and used asteractions is more selective due to the specific interac-the adsorbent. A standard Mo( VI ) solution was pre-tions between specially-designed ligands and metalpared by dissolving 1.50 g of Mo( VI ) trioxide with aions. In an acidic system( pH = 1.0- -2.0 ) , salicylsmall amount of aqueous ammonia and diluted to 1000fluorone( SAF ) as a ligand can form a complex withmL ~中国煤化工solutions of pH 1.0-Md( VI ) , which provides a basis for the concentration6.0YHC N M H Gaccording to the methodof Mo( VI ) onto micocrystalline phenolphthalein.in ref.[ 11 ] All the other reagents were of analyticalThere is a stable absorption between microcrystallinereagent grade.。 Supported by _the Natural Science Foundation of Henan Province( No. 0511020500 ).* *石熬据rrespondence should be addressed. E-mail : mercury6068 @ hotmail. comNo. 5LI Quan-min et al.6232 A pparatusescentration of Mo( VI ) was examined by varying pHA Z-5000 atom absorption spectrometer( Japan )from 1.0 to 6. 0 under optimum conditions. As shownequipped with an A-type graphite furnace was used forin Fig. 1 , acidity exerts a notable effect on Mo( VI )the determination of Mo( VI ). All the absorption mea-concentration. In a pH range of 1.0一-2. 0 , the concen-surements were performed under the following operatingtration yield was found to be 100% and decreased dra-conditions : wavelength ，313.3 nm ; slit setting ,matically with the decrease in acidity. It is possible0.4 nm ;lamp current , 17 mA. A Model 722 spectro-that at a high acidity ,Md( VI ) exists in an aqueous so-photometer( Xiamen Analytical Instrument Plant , Xia-lution in the form of MoO2 +[12] , which can form a com-men , China ) was employed for the photometric mea-plex of MoO2 SAF by chelating with SAFI31. Then thesurements. A Shanghai Model pHs-2 pH meter wascomplex is adsorbed onto microcrystalline phenolphthal-used to measure all the pH values.ein via intermolecular interactions. However , various3 Procedurespolymerized species , such as Mo,O24 and MoO2- are3.1Test of Separation Conditionssuccessively formed with increasing the pH valuAn aliquot of metal ion solution was placed into awhich causes a decrease in the MoO2 + concentration.50 mL beaker. A given amount of the 1.0 x 10The concentration yield of Mo( VI ) decreases due tomol/L SAF solution was added to it. The pH was ad-the decreasing ability of these species to form comp-justed and the solution was diluted to 10 mL with dis-lexes with SAF. Hence , the rest of the study was car-tilled water. The resulted solution was shaken well andried out at pH=1.0.allowed to stand for a few seconds. Then 0. 30 mL of a15% solution of phenolphthalein in ethanol was added100F■to it under continuous stirring. The supernatant liquid95(1.0 mL) was taken and added to a 25 mL gradedflask. Then ,2.5 mLof 5.0 mol/L HCI ,2. 00 mL of1 x10-3 SAF and 3.0 mL of 5.0 x 10-3 mol/L CT-82MAB were added to it. The absorbance was measuredat 530 nm against the reagent blank prepared in the8(same way. Then the concentration yield( % ) of Mo256( VI ) was calculated. .pH value3.2 Test of Molybdenum( VI ) Pre-concentrationFig.1 Effeet of pH value on the concentration yield ofSAF( 5.0 mL, 1.0 x 10-3 mol/L ) and CTMABMo( VI).(5.0 mL,5.0x10-3 mol/L )were added to 1.0 L ofMo(V): 50 ug; SAF(1.0x10-3 mol/L): 1.0 mL;tolal volume: 10 mL; phenolphthalein (15%): 0. 30the sample solution , and the pH was adjusted to 1. 0mL; stimring time: 10 min.with 6. 0 mol/L hydrochloric acid. Then 3. 0 mL of aFrom the aforementioned discussion ,it can be seen thatto the mixture under continuous stirring. After beingMo( VI ) is adsorbed on microcrystalline phenolphthale-stirred for 1 h at room temperature , the supernatant liq-in in the form of molybdenum complexes. The possibleid was filtrated out. During the process , the metalreaction mechanism is considered as follows.complex [ MoO2( SAF )2( CTMAB ) ] was formed in( 1 ) According to the procedure of separationthe reaction system and then well adsorbed onto the mi-10 mL of a solution of SAF was employed to thecrocrystalline phenolphthalein due to intermolecular in-Md( VI ) concentration experiment. W ithout CTMABteractions. As a result ，Mo( VI ) was quantitativelyMoO2 + combines with SAF into complex MoO2SAF thatconcentrated on microcrystalline phenolphthalein. Aftercan be adsorbed on microcrystalline phenolphthalein.filtration , the residue was dissolved in 10 mL of ethanolMoO2+ +SAFMoO2SAF +2H 'followed by the determination of GFAAS at 313.3 nm.中国煤化Ienolphthalein phase )Then the concentration yield( % )of Md( VI ) was cal-cedure of preconcentra-culated.MHCNMH(ion ，Ju0-1vwU mu 0I une solution was employed toResults and Discussionthe Md( VI ) preconcentration experiment. In the pres-Effect of Acidity on the Concentration Yield ofence of CTMAB , the concentration yield of Md( VI ) isMo( VI )higher than that without it. A report 14] showed thatThe布布熬据of acidity on the quantitative con-MoO2+ ,SAF and CTMAB can react and form an ion-624CHEM. RES. CHINESE U.Vol. 21associated complex MoO2( SAF )( CTMAB ) whichaddition of phenolphthalein to the reaction system,can be adsorbed on microcrystalline phenolphthalein.which makes Md( VI ) not to be effectively adsorbed on-MoO2+ + 2SAF + 2CTMAB*-to microcrystalline phenolphthalein. To avoid reagent( Water phase )consuming ,0. 30 mL of phenolphthalein was chosen asMoO2( SAF ).( CTMAB ) +4H+the optimum volume.( Phenolphthalein phase )The same trend was found in the influence of pre-2Effect of SAF Amount on the Concentrationconcentration time upon the concentration yield ofYield of Mo( VI )Md( VI ). When the stirring time is 10 min , the ad-In order to investigate the effect of the SAF amountsorption equilibrium and a complete quantitative con-on the concentration yield of Md VI ) ,an SAF amountcentration of Md( VI ) can be achieved. The reason mayranging from0 to 1. 2 mL was submitted to the proposedbe that the amount of SAF adsorbed onto microcrystal-procedure in the microcrystalline system. Fig. 2 showsline phenolphthalein increases with the increase of thethat the concentration yield increases substantially withstirring time. Consequently , in the proposed procedurethe rise of the SAF amount. A small amount ofa time of 10 min was suggested to ensure a completeMd( VI ) was found to be concentrated in the absence ofconcentration.SAF. The best result was obtained when 1.0 mL ofAdditionally , when the preconcentration time isSAF was used. It is probably because the amount of thechanged from 2 to 7 min , there is a good linear rela-Md( VI ) complex increases with increasing SAF used,tionship between the concentration yield of Mo( VI )and therefore , causing an increase in the Md( VI ) con-and the prenconcentration time. The linear equation iscentration yield. The concentration yield keeps constantE=2.072t + 84. 60 with R = 0. 9997. Its differentialwith further increasing SAF. Hence ,the use of 1. 0 mLequation is dE/dt = h = 2.072 , where dE/dt is theof SAF was chosen for the subsequent work.concentration yield of Mo( VI ) at time t , so k is de-duced to be the concentration velocity constant of105 [Md( VI) and its unit is min-'. This shows that theMd( VI ) concentration can be considered as a kinetic75pseudo-first-order reaction , and that the concentrationvelocity of Md( VI ) is only controlled by the concentra-tion of the Mo( VI ) complex adsorbed onto microcrys-4talline phenolphthalein.4 Separation ExperimentsAt pH 1.0 , the separations of 50 μg/mL of0.0 0.2 0.4 0.6 0.8 1.0 1.2Md( VI ) from its binary mixtures with 50 and 100 μg/V/mLmL Mn( II ),Fe( II ),Pb( II ),Co II),Cr( II ),Fig.2 Effect of the SAF amount on the concentrationyield of Mo( VI).Ni( I ),Al( II ),Hg( I ),Zn( II )and Cd( II )Mo( V): 50 ug; SAF: 1.0x10-3 moVL; pH=1. 0;were performed , respectively , according to the separa-total volume: 10 mL; phenolphthalein ( 15%): 0. 30tion procedure. The results show that Mo( VI ) in themL; strring time: 10 min.binary mixtures can be completely retained by the mo-3 Effects of Phenolphthalein Amount and Precon-dified microcrystalline phenolphthalein. Meanwhilecentration Time on the Concentration Yield ofthe concentration yields of other cations are very low.Mo( VI )Thus，they could be satisfactorily separated fromTo evaluate the effects of these two factors on theMd VI ). Moreover , the concentration behavior of eachconcentration yield of Mo( VI ) ,50 μg of Mo( VI )，metal ion existing individually is almost the same as1.0 mL of a KCl-HCl buffer solution and 1.0 mL ofthat of metal ions in multiple ions mixtures ，which1. 0x 10 - mol/L SAF were submitted to the separationmear中国煤化工havior of each metal ionprocedure. The results show that by increasing the vol-in mHC N M H G estimated according to .ume of phenolphthalein up to 0. 30 mL , the concentra-that ot the corresponding Ind1vidual ion. Thus , it istion yield rises progressively and is constant above it.very important and significant to develop such a methodWhen the volume is less than 0.30 mL , it slightly defor the separation and concentration of Md( VI ).creases. It may. be because just a small amount of mi-5 Determination of Mo( VI ) in Synthetic Samplescrocrystalknepttholphthalein will be formed after theand Tap WaterNo. 5LI Quan-min et al. .625To check the applicability of the developed methodadded Mo( VI ) was preconcentrated perfectly and re-to real samples , with different matrices containing var-covered selectively and quantitatively from the sampleying amounts of a variety of diverse ions by keeping thesolution with a high recovery and excellent precision.relative deviation less than士5% ,a study was carriedThe results in Table 1 show that the presence of a ma-out on various amounts( milligram level ) of matrixestrix in a high concentration did not cause any error inand tap water for the Mo( VI ) separation , preconcen-the described separation and preconcentration proce-tration and determination( Tables 1 and 2 ). To estimatedure. Table 2 shows that the analytical results forthe accuracy of the procedure , an adequate amount ofMo( VI ) obtained by the proposed method are in satis-Mo( VI ) was added to each synthetic sample and tapfactory agreement with that by using activated car-water and the resulting solutions were submitted to thebont 15].preconcentration procedure. As seen from the data , theTable 1 Determination results of molybdenum( VI ) in multiple ions mixtures( pH=1. 0 )Initial concentation of each co-ion[ Fd II ),N( I ),C( II ),AK I ),The recovery ofRSD( % )Mr( I ),Cd I ),C( I),Hg( I ),PL( I ),Z( I )]( mg" mL-1 )Md( VIX% )*(n=7)0.4101. 02.0.099. 51.92. 098.71.The initial concentrationof Md( VI )is 0.04 ug /mL-1 ;the volume of multiple ions mistures is 250 mL. The values are the mean for seven deter-minations. .Table 2 Preconcentration and determination of molybdenum( VI )in tap water( pH =1.0 )Amount of Amount ofAmount ofI( Sample )Md VI ) found Md( VI ) foundTotal amount ofRSD*(% )Recovern( % )MdVI)mLbefore concen- after concentra-M( VI)*/ugμgf15]tration/ μtion/ ug500一1.721.02.7199. 01.755.06. 702. 399. 610.011. 6899.6.10003.460.53. 8985. 03.483.46.4.422. 196.03. 468. 3798.213.442.199.8* The values are the mean for seven deterninations.trom. ,1987 ,2 ,189[ 9 ] Monte V. L. ,CurtiusA. J.小Anal. at Spectrom. ,1990 ,5 ,References211 ] Giacomelli M. B. 0. ,SilvaJ. B. B. , Curtius A. J. , Talan-[10] LiQ. M. ,OuyangR. Zh. ,Liu G. G. , Talanta , 2004 ,64 ,ta ,1998 ,47 ,877906. [ 2 ] ShahN. ，Menon S. K. ,Desai M. N. ,et al. ,Anal. Lell..[11 ] Chang W. B. ,Li K. A. , The Chinese Handbook of Analytical1989 ,22 ,1807Chemistry , Beijing University Press , Beijing , 1981 ,241[ 3 ] Sanchez M. , Gazquez D. , Garcia P. , Talanta ,1991 ,38 ,747[ 4 ] JiangZ. 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