Silica gel functionalized with resacetophenone: synthesis of a new chelating matrix and its...

Analytica Chimica Acta 454 (2002) 229–240

Silica gel functionalized with resacetophenone: synthesisof a new chelating matrix and its application as metal

ion collector for their flame atomic absorptionspectrometric determination

Anupama Goswami, Ajai K. Singh∗Department of Chemistry, Indian Institute of Technology, Hauz Khas, New Delhi 110016, India

Received 9 July 2001; received in revised form 4 September 2001; accepted 21 November 2001

Abstract

Silica gel modified with 3-aminopropyltriethoxysilane was anchored with resacetophenone to prepare a new chelatingsurface (or matrix). It was characterized with cross polarization magic angle spinning13C NMR spectroscopy and diffusereflectance infrared Fourier transform (DRIFT) spectroscopy and used for the separation and preconcentration of Cu(II),Pb(II), Ni(II), Fe(III), Cd(II), Zn(II) and Co(II) prior to their determination by flame atomic absorption spectrometry (FAAS).The optimum pH ranges for quantitative sorption are 6.0–7.5, 5.5–7.5, 5.0–7.0, 6.5–7.5, 6.0–7.5, 6.0–7.0 and 6.0–7.5 for Cu,Pb, Fe, Zn, Co, Ni and Cd, respectively. All the metals can be desorbed with 3 mol dm−3 HCl or HNO3. The sorption capacityfor these metal ions is in range of 57.8–365.0�mol g−1 of chelating matrix. Tolerance limits for electrolytes NaNO3, NaCl,NaBr, Na2SO4 and Na3PO4 and cations Ca(II) and Mg(II) in the sorption of these metal ions are reported. Preconcentrationfactors of 200, 300, 150, 250, 250, 200 and 200 for Cd, Co, Zn, Cu, Pb, Fe and Ni, respectively, are obtained andt1/2 valuesare<1 min except for Cd. 95% extraction by a batch method takes≤25 min. The simultaneous enrichment and determinationof all the metals are possible if the total load of the metal ions is less than the sorption capacity. In river water samples allthese metal ions were enriched with the present matrix and determined with a flame atomic absorption spectrometer (R.S.D.≤7.7%). Copper contents of pharmaceutical samples (vitamin capsule) were preconcentrated with the present chelating matrixand estimated with flame AAS, with R.S.D.∼ 2.2%. The results are in the good agreement with the certified value, 0.4 mg/gof capsule. © 2002 Elsevier Science B.V. All rights reserved.

Keywords:Resacetophenone; Silica gel; Preconcentration; Chelating matrix; Metal ions; Atomic absorption spectrometry

1. Introduction

The preconcentration and separation are oftenrequired when flame atomic absorption spectrom-etry (FAAS) or inductively coupled plasma-atomic

∗ Corresponding author. Fax:+91-11-686-2037.E-mail address:[email protected] (A.K. Singh).

emission spectrometry (ICP-AES) is employed to de-termine the metal ions at low concentrations. Solidphase and liquid–liquid extraction are widely usedfor metal enrichment and separation among a va-riety of methods. The former is advantageous overthe other due to minimal waste generation, betterreduction of sample matrix effect, ecofriendliness,high preconcentration factor, reusability and com-pactness for an on-line system [1]. Ion-exchangers

0003-2670/02/$ – see front matter © 2002 Elsevier Science B.V. All rights reserved.PII: S0003-2670(01)01552-5

230 A. Goswami, A.K. Singh / Analytica Chimica Acta 454 (2002) 229–240

and chelating matrices are two of the most importantclasses of solid phase extractors. However, the latterare attractive because of their selectivity. Among thechelating matrices, the current interest in chelating or-ganic polymers also stems from the fact that polymersupported ultrafiltration (PSUF) based on such poly-mers is emerging as a promising process for thetreatment of water contamination by toxic metal ions.At present attention is focussed largely on designingstable chelating matrices with high binding constants,selectivity and capacity for metal ions [2–5]. We arealso interested in designing such chelating matrices.For such designs a variety of organic and inorganicsupports have been exploited. They include cellulose[6], copolymers, e.g. resins of the Amberlite XADseries [7–15] and silica gel [16–24].

The immobilization of chelating ligands on poly-meric resins results in matrices of moderate sorptioncapacity. The silica gel is a good solid support dueto its thermal, chemical and mechanical stability. Itsfunctionalization also results in chelating matrices ofhigh sorption capacity as the number of reactive siteson its surfaces is large. A variety of chelating ligandshas been used to modify the silica gel surface. Silicagel modified with formylsalicylic acid [25] has beenused for extraction of iron(III). Thiourea-modified sil-ica gel [26] is used for the extraction and separationof silver(I), gold(III) and platinum(II). Silica gel loa-ded with 2-mercaptobenzothiazole [27] is used for pre-concentration and extraction of silver(I). Mercury(II)is enriched with silica gel modified with dithiocarba-mate [28] or dithizone [23]. The sorption of copper(II),iron(III), cobalt(II), zinc(II) and lead(II) has been rep-orted on 8-hydroxyquinoline [20], salicyldehyde [29],o-vanillin [30] and 2-hydroxy-5-nonylacetophenoneo-xime [16] modified silica gel. It was therefore thoughtworthwhile to use a multifunctional organic molecule,resacetophenone, to modify the silica gel surface andexplore the metal sorption behavior of the resultingchelating matrix. Such a chelating matrix is thoughtto exhibit good sorption capacity. For anchoring res-acetophenone, silica gel is reacted with 3-aminopropy-ltriethoxysilane and the resulting material is furthertreated with resacetophenone. The resulting chelat-ing matrix is studied for enrichment of Cu(II), Pb(II),Ni(II), Fe(III), Cd(II), Zn(II) and Co(II) from naturalwater samples. The results of these investigations arethe subject of this paper.

2. Experimental

2.1. Instruments

The cross polarization magic angle spinning (CP-MAS) 13C NMR spectrum was recorded at 75.3 MHzon a Bruker (Fallenden, Switzerland) 300 spectrome-ter. The CPMAS parameters used for this purpose are:contact time 2.5 ms, recycle time 2.5 ms, spinning rate4 kHz and the number of scans 5000. A flame atomicabsorption spectrometer of the Electronic Corporationof India Limited (Hyderabad, India), Model 4139,equipped with air-acetylene flame (air and acetyleneflow rates employed are 10 and 2 dm3 min−1, respec-tively) was used for metal ion determination. Thewavelengths used for monitoring Cd, Co, Cu, Fe,Ni, Zn and Pb are 228.8, 240.0, 324.8, 248.3, 232.0,213.9 and 212.0 nm, respectively. The diffuse re-flectance infrared Fourier transform (DRIFT) spectra(4000–400 cm−1) were recorded on a Nicolet (Madi-son, USA) FT-IR spectrometer (Model protégé 460).Autosorb-6B (Quantachrome Corporation, USA)analyzer was used for BET surface area analysis.Thermogravimetric analysis (TGA) was carried outon a Perkin-Elmer (Norwalk, CT) thermogravimetricanalyzer TGA-7. The metal sorption studies on thechelating resin were carried out on a column of 1 cmdiameter (Pharmacia, Bromma, Sweden). The flowof liquid into the column was controlled using aperistaltic pump (Watson-Marlow Model 101/U/R,Falmouth, UK). A mechanical shaker equipped withan incubator (Hindustan Scientific, New Delhi, India)with a speed of 200 strokes min−1 was used for batchequilibration.

2.2. Chemical and reagents

The metal salts and common chemicals used wereof analytical reagent grade. Silica gel (60–120 mesh)was procured from E. Merck Ltd., Mumbai, India.3-Aminopropyltriethoxysilane and resacetophenone(RATP) were obtained from Aldrich (Milwaukee,WI) and CDH (Mumbai), respectively, and were usedas received. The stock solutions of metal ions (1000mg dm−3) were prepared from analytical reagent gradecadmium(II) iodide, cobalt(II) chloride hexahydrate,copper(II) sulphate pentahydrate, nickel(II) sul-phate hexahydrate, lead(II) nitrate, zinc(II) sulphate

A. Goswami, A.K. Singh / Analytica Chimica Acta 454 (2002) 229–240 231

heptahydrate and ammonium iron(II) sulphate hex-ahydrate (followed by aerial oxidation) by dissolvingthe appropriate amounts in 10 cm3 of concentratedHCl or HNO3 and making up the volume to exactly.The working solutions of the metal ions were madeby suitable dilution of the stock solutions with dou-bly distilled water. HCl (pH 1 and 2), 0.5 mol dm−3

acetate–acetic acid buffer (pH 3–5), 0.5 mol dm−3

phosphate buffer (pH 6 and 7), 0.5 mol dm−3

NH3–NH4Cl buffer (pH 8–10) were used to adjust thepH of the solutions, wherever suitable. The water sam-ples from Yamuna river (New Delhi), Narmada river(Hoshangabad, India) and Berchaa Lake (Indore, In-dia) were collected, acidified with 2% HNO3, filteredand stored in glass bottles. The glassware was washedwith chromic acid and soaked in 5% HNO3 overnightand cleaned with doubly distilled water before use.

2.3. Synthesis of silica gel functionalizedwith resacetophenone

Commercially available silica gel was first activatedby refluxing it with 6 mol dm−3 HCl for 8 h. It wasfiltered off, washed with doubly distilled water untilfree from acid, dried in a furnace at 393 K for 12 hand finally degassed at 373 K under vacuum for 8 h.The resacetophenone was immobilized in the two stepsgiven below.

(a) The activated silica gel (30 g) was suspended in100 cm3 solution of 3-aminopropyltriethoxysilanein dry toluene (10% v/v). The mixture was refluxedwith stirring for 12 h in a nitrogen atmosphere.

Table 1Optimum experimental conditions for the sorption and desorption of metal ions

Experimental parameter Metal ion

Cu(II) Pb(II) Fe(III) Zn(II) Co(II) Ni(II) Cd(II)

pH range 6.0–7.5 5.5–7.5 5.0–7.0 6.5–7.5 6.0–7.5 6.0–7.0 6.0–7.5Flow rate (cm3 min−1) 1.0–3.5 1.5–3.0 2.0–3.5 1.0–3.0 2.0–4.0 2.0–3.0 1.5–3.5HCl/HNO3 concentration for

desorption (mol dm−3)0.5–3.0/1.0–3.0 –/2.0–3.5 1.0–3.0/1.0–3.0 1.5–3.0 1.0–4.0 2.0–3.5/2.0–3.0 1.0–2.0/1.0–3.0

Average sorption capacity(�mol g−1)

186.4 66.6 272.4 191.4 365.0 253.8 57.8

Average recovery (%) 99.7 98.6 98.0 99.2 99.2 97.5 97.5Standard deviation (�g cm−3)a 0.032 0.028 0.020 0.043 0.052 0.024 0.038Relative standard deviation (%) 3.20 2.84 2.04 4.33 5.24 2.46 3.89

a For five determination of 1�g cm−3.

The slurry was filtered and the resulting solidaminopropyl silica gel (APSG) was washed suc-cessively with toluene, ethanol and diethyl ether. Itwas dried and degassed at 373 K under high vac-uum for 8 h. The nitrogen content of the aminatedsilica gel (APSG) was found to be 0.632±0.007%(w/w) by the Kjeldahl method [31].

(b) The APSG (10 g) was treated with RATP (15 g)dissolved in 30 cm3 of anhydrous diethyl ether withconstant stirring for 12 h. The mixture was filtered.The residue was washed with ethanol, heated at350 K in a vacuum for 8 h and used as such formetal ion enrichment.

2.4. Recommended procedure for preconcentrationand determination of metal ions

Column and batch methods were employed for thepreconcentration of metal ions.

2.4.1. Column methodA glass column C10/10 (Pharmacia, size 10 cm×

10 mm) was packed with 1.0 g of resacetophenone-loaded silica gel using the method provided by themanufacturer in the manual “Gel Filtration—Theoryand Practice” [32]. The column was treated with2.0 mol dm−3 HCl or HNO3 (50 cm3) and washedwith doubly distilled water until free from acid. Asuitable aliquot of the sample solution containingCu(II), Pb(II), Ni(II), Fe(III), Cd(II), Zn(II) or Co(II)in the concentration range 0.0033–1.0�g cm−3 waspassed through the column after adjusting its pHto an optimum value (Table 1), at a flow rate of


1.0–3.5 cm3 min−1 controlled by a peristaltic pump.The column was washed with distilled water to re-move free metal ions. The bound metal ions werestripped from the silica gel column with HCl orHNO3 (10–25 cm3) of optimum concentration givenin Table 1. The concentration of the metal ions in theeluate was determined with a flame atomic absorptionspectrometer, previously standardized. Dilution withdistilled water was made before aspiration in the caseof concentrated eluates.

2.4.2. Batch methodA sample solution (50 cm3) containing 0.0033–

15.0�g cm−3 Cu(II), Pb(II), Ni(II), Fe(III), Cd(II),Zn(II) or Co(II) was taken in a glass stoppered bot-tle (250 cm3), after adjusting its pH to an optimumvalue (Table 1). The RATP modified silica gel (0.1 g)was added to it, the bottle was stoppered and shakenfor 30 min. The modified silica gel was filtered off,washed with distilled water, shaken with 10–25 cm3 ofHNO3 of optimum concentration (Table 1) for 30 minand again filtered. The filtrate was aspirated into theflame of a prestandardized flame atomic absorptionspectrometer after suitable dilution if required.

2.5. Calibration for various metal ions

For the determination of metal ions using flameatomic absorption spectrometer (FAAS), various pa-rameters (viz. wavelength, slit width, lamp current)were set at the optimum level. The linear ranges formeasurement under optimum conditions were found

Scheme 1.

to be 0.2–5.0, 1.0–10.0, 0.5–2.0, 1.0–15.0, 0.1–1.0,0.5–10.0 and 1.0–10.0�g cm−3 for Cu, Co, Cd, Pb,Zn, Fe and Ni, respectively. The linear equations alongwith the regression coefficient (R2) for each metal ionare as follows.

Element C R2

Fe 0.0519A + 0.0075 0.9995Pb 0.0320A − 0.0046 0.9998Cd 0.1782A − 0.0075 0.9972Zn 0.2571A − 0.0165 0.9991Cu 0.0790A − 0.0175 0.9996Ni 0.0451A − 0.0010 0.9997Co 0.1075A − 0.0270 0.9965

whereA is the peak height absorbance andC is theconcentration in�g cm−3. All the statistical calcula-tions are based on the average of four readings foreach standard solution in the given range, i.e.n = 4.

3. Results and discussion

3.1. Synthesis and characterizationof the resacetophenone-anchored silica gel

The resacetophenone is immobilized on silica gelvia the reactions given in Scheme 1. Anhydrous condi-tions were used for the silation reaction so that hydro-lytic condensation of the aminopropylsiloxane isprevented and maximum substitution of the silanolgroups takes place. The13C CPMAS NMR spectrum


Fig. 1. 13C CPMAS NMR spectrum of aminopropylsilica gel(APSG).

of silica gel modified with 3-aminopropyltriethoxy-silane is shown in Fig. 1. Its resonance signals areconsistent with the13C{1H} NMR spectrum (recordedin CDCl3) of 3-aminopropyltriethoxysilane. The sig-nals for –OCH2– and –OCH2CH3– were found to beabsent in the13C CPMAS NMR spectrum suggestinga complete loss of ethoxy groups. The three signalsobserved atδ = 12.5, 24.6 and 45.6 ppm may beassigned to Si–CH2, –CH2 and N–CH2, respectively.The DRIFT spectrum of resacetophenone-loaded sil-ica gel exhibits a band at 1615 cm−1 which appearsto be contributed to by C=N stretching. All theseobservations support the immobilization of re-sacetophenone onto the silica surface through a–O3Si–(CH2)3– spacer. The phenyl vibrations appearat 1595, 1535, 1500, 1460, 1415 and 1345 cm−1 andfurther support the immobilization of resacetophe-none. The nitrogen analysis of APSG suggests that0.450± 0.005 mmol organo-silicon moiety has beengrafted/g of silica gel. The specific surface area ofresacetophenone-modified silica gel was found tobe 180.01 m2 g−1. The thermogravimetric analysis(TGA) of APSG further supports the results of ni-trogen analysis, as it shows ca. 3% mass loss up to873 K, which concurs with the loading of 0.450 mmol

of an organo-silicon species on 1 g of silica gel. Thereduction (with common reagents) of Schiff base inScheme 1, expected to give a sorbent surface, doesnot show good reproducible results. However, fur-ther investigations to standardize the reduction to areproducible level are being made.

3.2. Optimum conditions for sorptionand desorption

The glass column was packed with resacetophen-one-loaded silica gel and cleaned as described in therecommended procedure given in Section 2. The sorp-tion and desorption (under optimum conditions) ofCu(II), Pb(II), Ni(II), Fe(III), Cd(II), Zn(II) and Co(II)on this column were quantitative. The optimum condi-tions for their sorption (pH and flow rate) and desorp-tion (acid concentration) were established by varyingone of them and following the recommended proce-dure. The results are summarized in Table 1. A typ-ical optimization process (for pH) is as follow. A setof solutions (volume: 50 cm3), containing one of theseven metal ions at a concentration upto 1.0�g cm−3

(0.2�g cm−3 for Pb and Zn; 0.5�g cm−3 for Cu andCd, and 1.0�g cm−3 for Ni, Fe and Co) was taken.The pH of the solutions of the set was adjusted to dif-ferent values in the range 2.0–9.0. The enrichment ofthe metal ion from these solutions was studied by therecommended procedure. The optimum pH range formaximum recovery of each metal ion is reported inTable 1 (see Fig. 2 for pH profile). The effect of pHon sorption was also studied using the recommendedbatch method and the results are found to be consis-tent with those of the column method. pH >9.0 wasnot studied, as silica gel degenerates in strong alkali.The use of 4–5 cm3 of acetate, phosphate and ammo-nia buffer to adjust the pH does not affect the sorptionbehavior. The metal ions are not significantly desorbed(<3%) by distilled water; therefore chelation con-tributes predominantly to the retention of metal ions.

3.3. Kinetics of metal sorption

The effect of shaking time on the extraction of theseven metal ions was studied. The modified silica gel(0.1 g) was equilibrated by shaking with 50 cm3 of asolution containing 0.005 g of a metal ion (78, 20, 80,76, 80, 80 and 40�mol for Cu, Pb, Fe, Zn, Co, Ni


Fig. 2. Effect of pH on the sorption of Cu(II), Pb(II), Ni(II), Fe(III), Cd(II), Zn(II) and Co(II).

and Cd, respectively) for different time intervals (2, 5,10, 15, 20, 25, 30, 35, 40 min and 5 h) and the recom-mended batch procedure was applied. The metal ionswere 2–6 times in excess over the sorption capacity.The concentration of metal ions in the supernatant so-lution was determined as well as calculated by massbalance (in mg g−1 resin). The profile of sorption as afunction of time for all the seven metal ions is shownin Fig. 3. The equilibration time of 2 min was foundto be sufficient to saturate ca. 70% of the matrix withCu(II), Ni(II), Fe(III) and Co(II) when comparedto the total metal loaded after 5 h, whereas 15 min

Fig. 3. Kinetics of metal ion sorption on resacetophenone-loadedsilica gel.

equilibration was enough for 90% saturation of thematrix. 45, 50 and 60% saturation was observed after2 min for Cd(II), Zn(II) and Pb(II), respectively. Thegraph suggests that the active donor atoms (N and O)on the modified silica gel surface are so oriented thattheir accessibility is not difficult and consequentlyfast interaction with the free metal ions present insolution is feasible.

3.4. Effect of eluent and flow rate

The degree of metal ion sorption on a resaceto-phenone-modified silica gel (1 g) packed column wasstudied at various flow rates of the metal ion solu-tion controlled with a peristaltic pump. The optimumflow rate for loading all these metal ions is between1.0 and 3.5 cm3 min−1. A flow rate <1.0 cm3 min−1

was not employed to avoid the long extraction time.However, at flow rates >4.0 cm3 min−1, there was adecrease in the % sorption, probably because the metalions do not equilibrate sufficiently with the resin. HCland HNO3 were found to be the most suitable forstripping off the bound metal ions. The use of EDTAand thiourea does not give reproducible results. Forstripping off the metal ions from the matrix a flowrate of 1.0 and 3.5 cm3 min−1 was found effective.To study the effect of concentration of stripping so-lution on desorption of metal ion from the modifiedsilica gel, the metal ions were eluted out from thecolumn with 10 cm3 of HCl/HNO3 of varying con-centration (1–4 mol dm−3) at the optimum flow rate.


An acid concentration >4 mol dm−3 was not used fordesorption as the necessity for its partial neutraliza-tion in the eluate would reduce the preconcentration.The optimum acid concentration for maximum andinstant recovery (≥97.5%) of each individual metalion is given in Table 1. The efficiency of the eluent(3 mol dm−3 HCl/HNO3) was studied by taking differ-ent volumes (1–10 cm3). It was found that 6 cm3 wassufficient for 98.0–99.7% recovery of Cu, Co, Fe andPb whereas 7 cm3 was required for Zn, Ni and Cd toachieve 97.5–99.2% recovery level.

3.5. Effects of electrolytes and cations

Chloride, nitrate, sulphate and phosphate ions arepresent in natural waters and have the capability tocomplex with many metal ions. Consequently theymay compete for metal ion with the immobilizedligand on the silica gel and reduce the extraction.Thus, the effect of NaCl, NaBr, NaNO3, Na2SO4 andNa3PO4 on the sorption efficiency of Cu(II), Pb(II),Ni(II), Fe(III), Cd(II), Zn(II) and Co(II) (0.1 mg dm−3

each) on the modified silica gel was studied usingthe recommended column method under the optimumconditions (Table 2). A species is considered to inter-fere when it lowers the recovery of metal ions morethan 3% in comparison to the value observed in itsabsence. Each reported tolerance/interference is inthe preconcentration and not in the determination byAAS, as checked with the help of reagent-matchedstandard solutions. The tolerance limits of the elec-trolytes in the sorption of Cd(II) are somewhat lowwhile, for other metal ions, the limits are reasonablygood. Ca(II) and Mg(II) are tolerable (0.2 mg dm−3

Table 2Sorption capacities of the metal ions for different batches ofresacetophenone-loaded silica gel

Metal ion Sorption capacity (�mol g−1)

Batch 1 Batch 2 Batch 3 Batch 4

Cu(II) 186.8 190.4 194.0 174.8Pb(II) 62.0 70.2 68.7 66.2Fe(III) 264.0 278.6 277.0 270.0Zn(II) 188.1 199.0 184.1 194.6Co(II) 358.0 360.8 370.0 372.2Ni(II) 248.6 259.8 254.3 252.8Cd(II) 52.8 62.0 54.0 62.4

of each analyte ion) up to a concentration level of0.008–0.10 mol dm−3 and 0.008–0.12 mol dm−3, re-spectively. These results demonstrate that the presentsolid matrix may be used to enrich these metal ionsfrom natural water samples, as common constituentsat normal level do not adversely affect the sorptionefficiency of the matrix for these metal ions.

3.6. Sorption capacity

The sorption capacity (maximum amount of metalsorbed per g of the matrix) for each metal was deter-mined by shaking 0.1 g of resacetophenone-modifiedsilica gel with a metal ion solution of concentration100�g cm−3 (total amount of metal available forsorption is much greater than the sorption capacity)for 1 h and monitoring the metal ions after strippingthem from the matrix as described in the recom-mended batch method. The results are given in Ta-ble 1. The variation in sorption capacities of variousmetal ions probably arises due to their sizes, degreeof hydration and binding constants of their complexeswith the matrix. The capacity of the modified silicagel was found to be almost same for the differentbatches of RATP-functionalized silica gel (Table 2).

3.7. Resin stability and reusability

The stability of resacetophenone-loaded silica gelwas studied in acid (1–5.0 mol dm−3 HNO3/HCl) bythe recommended batch method. The modified silicagel was shaken with acid solutions of varying con-centrations for 5 h and filtered. The solid was washedwith distilled water until free from acid, dried at363 K under vacuum and its sorption capacity wasdetermined using the procedure mentioned above.The sorption capacity of the acid treated resin wasfound to be similar (variation≤2.5%) to that of theuntreated one. When the modified silica gel wastreated with acid of concentration >5.0 mol dm−3, thesupernatant acid turned brown. Thus the modifiedsilica gel matrix can easily withstand 5.0 mol dm−3

HCl/HNO3, but showed signs of degradation in theacids of concentration >5.0 mol dm−3. The exchangecapacity of the matrix for the metal ions was deter-mined after several loading and elution cycles. Thereduction in the capacity was≤1.2% even after 15repeated uses. Consequently, the chelating matrix can


Table 3Tolerance limits of electrolytes, Ca(II) and Mg(II)

Electrolytes/cation(mol dm−3)

Metal ion

Cu(II) Pb(II) Fe(III) Zn(II) Co(II) Ni(II) Cd(II)

NaNO3 0.10 0.15 0.10 0.10 0.09 0.08 0.09NaCl 0.10 0.004 0.15 0.009 0.10 0.06 0.08NaBr 0.005 0.03 0.03 0.007 0.01 0.05 0.001Na2SO4 0.10 0.08 0.09 0.09 0.07 0.10 0.004Na3PO4 0.008 0.006 0.004 0.06 0.02 0.01 0.007Ca(II) 0.08 0.05 0.10 0.009 0.05 0.04 0.008Mg(II) 0.12 0.04 0.09 0.009 0.05 0.02 0.008

be reused many times. On storing for a year the sorp-tion capacity of resacetophenone-modified silica geldoes not change significantly.

3.8. Preconcentration and recovery of metal ions

The column technique is a common procedure forextraction and separation of metal ions from large sam-ple volumes. The ability of the RATP-modified silicagel to preconcentrate metal ions was studied and thelimit of preconcentration was determined using therecommended procedure while increasing the dilutionof metal ion solution (total amount of metal ion fixedat 10�g) used for sorption. The feed volume for load-ing and preconcentration factors are given in Tables 3and 4. The limit of detection (LOD) values (defined as(blank+3σ ) whereσ is standard deviation of the blankdetermination) are 0.70, 0.45, 1.80, 0.86, 0.92, 1.10and 0.90�g dm−3 for Cd(II), Co(II), Zn(II), Cu(II),Pb(II), Fe(III) and Ni(II), respectively. The limit ofquantification (blank+ 10σ ) values are 0.92, 0.67,1.92, 1.23, 1.42, 1.38 and 1.24�g dm−3, respectively.

Table 4Preconcentration factors and preconcentration limits for metal ions

Metal ion Volume of solutionpassed (cm3)

Concentration (ng cm−3) Average recovery (%)a Preconcentration factor

Cd(II) 2500 4.0 97.5 250Co(II) 3000 3.3 99.2 300Zn(II) 1500 6.6 99.2 150Cu(II) 2500 4.0 99.7 250Pb(II) 2500 4.0 98.6 250Fe(III) 2000 5.0 98.0 200Ni(II) 2000 5.0 97.5 200

a For three determination, in a volume of 10 cm3 of 3 mol dm−3 HNO3.

3.8.1. Determination of copper in pharmaceuticalsample

Solid phase extraction with the resacetophenone-anchored silica gel coupled with a FAAS methodof determination was applied to determine copperin pharmaceutical samples. Multivitamin capsules(Revital, Ranbaxy Research Labs, New Delhi) weredigested in a beaker containing 10 cm3 of concen-trated HNO3 by slowly increasing the temperatureof the mixture to 400 K, and the mixture was fur-ther heated until a solid residue was obtained. It wasallowed to cool and treated with 50 cm3 of distilledwater. The solution was filtered and its volume madeup to 100 cm3. A 6 cm3 aliquot of this solution wasdiluted to 500 cm3 and its pH was adjusted to 7 us-ing 10 cm3 of NH3–NH4Cl buffer. The solution waspassed through the column and the copper content wasestimated by the recommended procedure using flameAAS. The average (four determinations) amount ofcopper was found to be 0.407 mg/g of capsule withan R.S.D.(n = 4) = ca. 2.2%. The certified value ofcopper in the capsule is 0.40 mg g−1.

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3.8.2. Determination of Cd(II), Co(II), Zn(II), Cu(II),Pb(II), Fe(III) and Ni(II) in natural water samples

The present method was applied to preconcentrateand determine Cd(II), Co(II), Zn(II), Cu(II), Pb(II),Fe(III) and Ni(II) ion in the samples of natural water.1500 cm3 of samples collected from Berchaa lake, andYamuna and Narmada rivers, after adjusting the pHto the optimum value, were subjected to the recom-mended procedure (column) for the preconcentrationand determination of the metal ions directly and afterstandard addition. In the standard addition method, thesamples (1500 cm3) were spiked with 100�g of eachof the seven metal ions before loading onto the col-umn containing resacetophenone-modified silica gel.The results reported in Table 5, with R.S.D., indi-cate that the matrix effects of the dissolved inorganicand organic matter are insignificant on the enrichment

Table 6Comparison of sorption capacities

Matrix Metal ion (�mol g−1)

Cd Co Cu Fe Ni Pb Zn

Support: silica gelResacetophenone 57.8 365.0 185.8 272.4 253.8 66.6 191.12-Hydroxy-5-nonylacetophenone oxime [16] 162.0Salicyldoxime [17] 60.0 80.0 50.0 50.0 40.0 40.03-Methyl-1-phenyl-4-stearoyl-5-pyrazolone [18] 45.0 42.9 48.91-(2-Thiazolyazo)-2-naphthol [38] 2600.0 24.9Dithizone [23] 35.0 35.0 95.6 60.0 40.0 40.0 35.5Acid red 88 [24] 11.7 15.1 12.1 3.9 11.3 16.2 12.1Xanthurenic acid [19] 50.08-Hydroxyquinoline [20] 65.0Didecylaminoethyl-�-tridecylammonium [21] 120.0 120.0 120.4Dithiocarbamate [28] 150.0 160.0 300.0 67.0 233.0 167.0

Support: Amberlite XAD-21-(2-Thiazolyazo)-p-cresol [14] 3.71-(2-Pyridylazo)-2-napthol [12] 1.87Salicylic acid [9] 17.5Tiron [10] 84.5 110.2 220.0 100.2 214.0 60.3 169.9o-Aminophenol [15] 30.4 58.0 53.0 55.2 44.9Chromotopic acid [7] 83.4 65.2 134.0 58.0 103.4 147.5

Support: Amberlite XAD-78-Benzenesulphonoamide quinoline [34] 13.0 8.0

Support: Amberlite XAD-47-Dodecenyl-8-quinoline [39] 520.0 550.0 521.0

Support: polyacrylonitrileAminophosphonic and dithiacarbamate [35] 93.0 69.2 31.9 27.6Chelex-100 [37] 47.2 2150.0

of all the seven metal ions with the present matrix.Moreover, the Cd, Co, Zn, Cu, Pb, Fe and Ni con-tents of Yamuna river water samples found in thepresent study are similar to those reported earlier usingAmberlite XAD-2-based chelating resins [7,10].

4. Comparison with other preconcentratingmatrices

The sorption capacity of resacetophenone-modifiedsilica gel is compared with those of other matri-ces in Table 6. This matrix shows better or com-parable capacity values than most other matrices.However, the resacetophenone-modified silica gelexhibits a lower capacity than those of silica gelmodified with dithiocarbamate [28] (Co, Cd, Fe and


Table 7Comparison of preconcentration factors

Matrix Metal ion

Cu Pb Fe Zn Co Ni Cd

Support: silica gelResacetophenone 250 250 200 150 300 200 250Salicylaldoxime [17] 40 40 40 40 40 403-Methyl-1-phenyl-4-stearoyl-5-pyrazolone [18] 40 40 401-Nitroso-2-naphthol [40] 1003-Hydroxy-2-methyl-1,4-naphthoquinone [33] 10 10 10 10Xanthurenic acid [19] 100 100

Support: Amberlite XAD-21-(2-Thiazolyazo)-p-cresol [14] 1001-(2-Pyridylazo)-2-naphthol [12] 50Salicylic acid [9] 140 180Tiron [10] 200 25 80 180 56 150 48o-Aminophenol [15] 50 40 100 65 50Chromotropic acid [7] 100 200 120 200 150 200 100

Support: Amberlite XAD-78-Benzenesulphonoamide quinoline [34] 10 10

Support: polyacrylonitrileAminophosphonic and dithiocarbamate [35] 200 200 200 200

Support: activated carbon8-Hydroxyquinoline [36] 100 100Cupferron [36] 100 100

Support: polyvinyl8-Hydroxyquinoline [20] 50 50 50 50 50 50Chelex-100 [37] 100 1000 100 100 1000 100 50

Pb), didecylaminoethyl-�-tridecylammonium [21](Co and Cd) and 1-(2-thiazolyazo)-2-naphthol [38](Cu). The preconcentration factors achieved with thepresent modified silica gel for the seven metal ionsare better or comparable to all the important chelat-ing matrices (Table 7). The short loading time ofthe present matrix makes the analytical procedurereasonably fast. The recovery of all the seven metalions is nearly quantitative (97.5–99.7%). The matrixeffects with the present collector are low, as shownfrom the pharmaceutical and natural water sampleanalyses. The low acid concentration required fordesorption of metal ions avoids any further dilutionstep for FAAS measurement. The repeated use ofthe resin is feasible and may be considered as anadditional advantage of resacetophenone-modifiedsilica gel.

Acknowledgements

AG thanks AICTE (India) for awarding Junior Re-search Fellowship. AKS thanks DAE for financial sup-port to this project.

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