Indian Journal of Chemistry Vol. 41A, August 2002, pp. 1646-1651

Differential pulse polarographic determination of lead (II) in complex

materials after adsorption of its trifluoroethylxanthate-cetyl­

trimethylammonium ion-associated complex on microcrystalline naphthalene or on

tri fluoroethy lxan thate-cetyltri me thy lammoni urn- naphthalene

adsorbent

B K Puri "·* Atamjyota, Keemti Lalb & Himanshu Bansalc

E-mail: balkpuri @hotmail.com

•o epartment of Chemistry, Indian Institute of Technology, Hauz Khas, New Delhi I 10 016, India

bDepartment of Chemistry, D N College, Meerut (UP), India

cDepartment of Chemistry, Vishveshwarya Institute of Engineering & Technology, G T Road, Dadri (U P), India

Received 16 July2001; revised 13 March 2002

Lead(II) is adsorbed as lead trifluoroethylxanthate (TFEX)­cetyltrimethylammonium (CTMA) ion pair complex on microcrystalline naphthalene quantitati vely in the pH range 4.50-10.50. In the absence of cetyltrimethylammonium as the counter ion, the adsorption is only 80%. Lead(II) is desorbed with HCl and determined with a differential pulse polarograph. Dissolved oxygen is removed by add ing 3 ml of 4% NaBH4 solution. Alternatively, lead (II) can be quantitatively adsorbed on TFEX­CTMA-naphthalene adsorbent packed in a column at a fl ow rate of 1-2 ml min·1

• Well defined peak has been obtained in thi s medium at -0.40 V versus S.C.E. Cyc li c voltammetric, differential polarographic and D.C. polarographic studies indicate th at lead(II) has been reduced reversibly in a two-electron reduction under these conditi ons. The detection li mit is 0.20 J..lg ml" 1 at the minimum instrumen tal settings (signal-to-noise ratio = 2). Linearity is maintained in the concentration range 0.5-16.40 J..lg mr 1 with correlat ion factor of 0.9997 and a relative standard dev iation of±l.07%. Various parameters such as the effect of pH , volume of aqueous phase, flow rate and the interference of a iarge number of metal ions and ani ons on the determinati on of lead(ll ) ha ve been studied in detail to optimize the condi tions for its trace determination in various standard alloys, standard biological materi als and environmental samples.

Lead is highly toxic to man and animals and causes envi ronmental di sease when released in the atmosphere. Very low concentration of lead is normally present in various matrices such as plants, so il , food and water. Therefore, it is very important from the analytical point of view to develop sensitive,

selective, rapid and economical methods for its quantitative determination when present in trace amounts. Lead(II) may be determined by anodic stripping voltammetry 1

-4

, but these methods are not sufficiently selective and reproducible. Metal s may be determined polarographically after extraction of their metal complexes into organic solvents, however, these methods have limited sensitivity and reproducibilit/-6.

Solvent extraction is a simple technique for the separation and preconcentration of metal ions, however, it suffers from several drawbacks especially in the case of organic solvents used for extraction which are expensive and toxic due to their low boiling points7

. Column methods using various adsorbents are effective for the preconcentration of metal ions, their main disadvantages being that the methods of preparation are lengthy and require rigorous control of experimental conditions. Also, desorption is carried out by the slow process of elution 7.

In the present note, we report a differential pulse polarographic method for the direct determination of lead(II) after quantitative adsorption of its trifluoroethylxanthate (TFEX)-cetyltrimethylammo­nium (CTMA) ion pair complex on microcrystalline naphthalene. Preconcentration of lead(Il) is also possible by pass ing its aqueous solution through tri fl uoroethy lxan thate (TFEX)-cety 1-trimethy !ammo­nium (CTMA)-naphthalene adsorbent packed in a column. After the preconcentration step, the metal can be eas ily desorbed with HCI, which is much cheaper than many of the organic solvents and is also easy to handle. NaBH4 is preferred as a source of hydrogen for removi ng dissolved oxygen quickly from the solu tion on mixing. Additional supporting electrolyte is not required as the NaBH4-HCl medium provides sufficient NaCl which acts as a supporting e lectro lyte and also maintains a pH range appropriate for getting well defined polarograms. At the same time, the reagent does not interfere in the polarographic determination of lead8

. Various parameters have been evaluated and the method developed has been successfully employed for the determination of lead in various complex materials like standard alloys, standard biological materials and environmental sampl es.

NOTES 1647

Experimental Polarograms were recorded with a three electrode

Elico (Model CL-90, India) polarographic analyzer outfitted with a X-Y recorder (Model LR-108 New Delhi, India). Cyclic voltammograms were recorded with a cyclic voltammeter (BAS Model CY -27, U.S.A.) outfitted with a X-Y-T recorder using mercury drop as working and Ag/ AgCI as the reference electrodes, respectively. All atomic absorption measurements were made using an atomic absorption spectrometer (Model 4129) supplied by Electronic Corporation of India Ltd. An Elico pH meter Model LI-612, India was employed for pH measurements . For preconcentration, a glass column [60 mm x 7 mm (i.d.)] was plugged with polypropylene fibre and filled with the adsorbent up to a height of 1.0-1.2 em after pressing lightly with a flat glass rod.

All reagents used were of analytical grade. Lead nitrate solution was prepared by dissolving 1.6000 g of the salt in the presence of a few milliliters of cone. nitric acid and diluted to 1000 ml with water in a calibrated flask. It was standardized by literature method9

. Working solution of the metal ion (10 Jlg mr 1

) was prepared by diluting the stock solution. Sodium trifluoroethylxanthate (TFEX) was prepared by the method of Dewitt and Roper 10

• Reagent solution was prepared by dissolving 0.2 g of TFEX in 100 ml of distilled water. A 4% solution of NaBH4

was prepared in 0.2 M NaOH solution. A 10% solution of potassium sodium tartrate in distilled water was used to prevent the hydrolysis of lead. A 1% solution of cetyltrimethylammonium bromide (CTMAB) was prepared in water. A 20% solution of naphthalene was prepared in acetone. Buffer solution of pH - 8.5 was prepared by mixing appropriate volume of 0.5 M ammonia and ammonium acetate solutions. Solutions of alkali salts (1%) and various metal salts (0.1%) in distilled water were used to study the interference of anions and cations, respectively. Doubly distilled water was used whenever required.

The solution of the standard alloys was prepared as described in the Iiterature 11

• A 0.1 g sample of each of the standard alloys was dissolved in 50-60 ml of 6 M HCI by heating on a hot plate, and then 3-5 ml of 30% hydrogen peroxide was added to it. Excess of peroxide was decomposed by heating the above solution on a water bath . This solution was cooled, filtered if needed and diluted to 100 ml with distilled water in a standard flask.

The solutions of the standard biological materials were prepared by dissolving 1.0-5.0 g of each of the biological samples (National Institute for Environmental Studies, Japan CRM: No. 5 Human hair, CRM: No. 7 Tea leaves, CRM: No.3 Chlorella, and CRM No.I Pepperbush) in 25 ml of cone. nitric acid and 1 ml of cone. perchloric acid by heating. The solution was cooled and diluted up to a volume of 100 ml with distilled water in a standard flask 11

•

Fly ash samples were collected directly from the storing tanks of the "lndraprastha Electric Thermal Power Station", New Delhi, India and stored in the polythene bottles. A 100-500 mg sample of fly ash was weighed in a platinum crucible and 500-1000 mg of lithium borate was added to it. The crucible was heated to 900°C for 20-25 min and then cooled. The resulting solid was dissolved in hydrochloric acid (l + 1 0) at 60°C and diluted to 250 ml in a calibrated flask 11

.

Sediment samples were collected in polythene bags from the center of the river when it was dry near Wazirpur Industrial Area. Soil samples were collected from Wazirpur Industrial Area near to· the place where solid waste is dumped. These environmental samples were oven dried at 200°C for 2h. A 0.01-0.2 g sample of each was decomposed with 40-60 ml of 6 M HCI and then 3-5 ml of 30% hydrogen peroxide was added to it. The mixture was heated on a hot plate almost to dryness. The residue was dissolved in 10-15 ml of 1 M HCI, diluted with 10 ml of water and filtered if needed. Finally, the solution was diluted to 100 ml with distilled water in a calibrated flask 11

•

Liquid samples (industrial effluents) were collected from a drain coming out from the main building of electroplating units (chrome and nickel plating baths) and ceramic industries and stored in polythene bottles. Water samples from the Okhla canal and the Yamuna river were collected in polythene bottles from the center of the canal/river with the help of a boat while tap water samples were taken directly from the taps of the various chemical laboratories of our Institute. The environmental samples ( 100 ml each) were taken separately and to each was added 5 ml of cone. nitric acid. The solution was heated to almost dryness on a hot plate. The residue was dissolved in 5 ml of 1 M HCl and diluted to 100 ml in a calibrated flasY. with distilled water 11

•

Trifluoroethylxanthate (TFEX)-cetyltrimethylam­monium bromide (CTMAB)-naphthalene adsorbent was prepared by dissolving 10 g of naphthalene in I 00 ml of acetone and mixing with 500 ml of distilled

1648 INDIAN J CHEM, SEC A, AUGUST 2002

water containing 0.05 g of cetyltrimethylammonium bromide (CTMAB). The mixture was stirred at 25-300C on a hot plate stirrer, followed by the addition of 100 ml of 0.2% TFEX solution in water. The mixture was stirred for 3 h and kept for another 5 h at room temperature. The supernatant liquid was drained off by decantation and the remaining mixture was washed twice with distilled water. The final adsorbent was in the form of slurry of TFEX-CTMAB-naphthalene adsorbent and stored in a bottle of subsequent use.

General procedure An aliquot of a solution containing 7.5-246.0 jlg of

lead(II) was taken in a I 00 ml conical flask and diluted it to - 40 ml with distilled water. To this solution, 2 ml of 10% sodium potass ium tartrate, 1 ml of 1% CTMAB and 2 ml of 0 .2% reagent solutions were added. The pH of the solutions was adjusted to-8.5 with the addition of 2 ml of buffer. The solution was allowed to stand for 3-5 min at room temperature to ensure complete complex formation. To the above solutions, 2 ml of 20% naphthalene solution in acetone was added with vigorous and continuous shaking for 2-3 min. The solid mass containing the metal complex adsorbed on naphthalene was filtered on a sintered glass filter. The metal ion was desorbed from the solid mass with 12 ml of - I M hydrochloric acid and the solution was transferred to the polarographic ·cell. In the column method, the adsorbent was conditioned to pH - 8.5 by passing 5 ml of buffer solution of pH - 8.5 through the column at a flow rate of 1-2 ml!min before lead(II) solution was passed. The column was finally washed with water. The desorption of metal was carried out by passing 12 ml of 1 M of HCI at a flow rate of 1-2 ml!min . The differential pulse polarogram(DPP) was recorded after the addition of 3 ml of 4% NaBH4 soluti on .

Results and discussion Preliminary observations indicated that NaBH4 is

quite effective for the removal of dissol ved oxygen from the solution in a few seconds. Highly purified N2 in contrast takes 5-l 0 min for the complete removal of 0 2. Another advantage of using NaBH4 is that it provides oH· ions and brings the pH in the desired range for obtaining well defined polarograms. There is no need to add extra NaCI as a supporting electrolyte because sufficient quantity of it is formed in the reaction. Many o ther supporting electro lytes like HN01. H1POA. KCIOA and NH. CI were also tried

The differential pulse polarogram of lead(II) in L ml of I M HCI and 3 ml of 4% NaBH4 (Fig. I )showec half peak width of 48 ± I mY; in both normal anc reverse scan modes. Plots of the applied potentia versus log[il(ict-i)] from D.C. polarogram had a slop( of -28 ± 2 mY. The cyclic voltammogram of lead(Ir using stationary mercury drop or hanging dro~

mercury as the working electrode, recorded unde1 these conditions, showed cathodic and anodic peak~ with a difference (flEp) of -32 ± 2 mY and ratio ol cathodic peak current Urc) anodic peak current Ura) ol - 1.0. All these studies clearly indicate that lead(II) i ~

reduced reversibly involving a two-electron proces~ under these conditions 12·13

.

Effect of pH on the DPP of lead (II)

The effect of pH was studied wi th the addition o1 4% NaBH4 in 0.2 M NaOH solution, while keepin~ other conditions constant. The shape of the differential pulse polarograms and peak heights were found to be almost constant over the pH range 1.0-6.( (the pH may also be adjusted with dil. solution o1 NaOH instead of alkaline NaBH4). The peak potential (Ep) shifted towards more negative values as the pH was increased. However, a plot of Er vs. pH was no1 linear, indicating that protonation is not taking part in the overall electrode process 14. The peak height was considerably decreased above pH 8, probably due to hydroxide formation . Therefore, all studies were carried out at pH-2 since it was achieved simply by mixing the solution after the preconcentration process with HCI and alkaline solution of NaBH4.

0.24

4 0.20

"I 2- 0.16 .... c

"' ... 0.12 ....

:::J u

-"'- 0-08 0 _) "' Pb Q.

0.04

o.o -0.4 -0.8 -1 .2 - 1.6

Applied voltage,V(versus S. C.E.)

Fi g. 1- Typica l differential pulse polarogram of lead(! I). [Pb(ll ): 120 Jlg in 15 ml of the fina l so lution ( 12 ml 1.0 M HCl + 3 ml of 4% solut ion of NaBH4 in 0.2 M NaOH, pH - 2); reference: reagent blank ; adsorpti on pl-1:8.5; in strumental setting: scan rate: 12 tn\1/c • ,,.,n,,-llll 'l ti r m 'lt,.,nl i tllri P · C\f'l .-n\1· Y-Y--"0 r-n\1/rrn

NOTES 1649

Retention capacity of the adsorbent The retention capaci ty of the adsorbent was

determined by batch method. The experiment was performed by taking 5 mg of lead(II), 2 ml of buffer solution (pH- 8.5) and 40 ml of water in a beaker. This solution was transferred into a separating funnel and then a suitable amount of the adsorbent (TFEX­CTMAB-naphthalene) was added to it. The separating funnel was shaken vigorously on a mechanical shaker for 15 min. The solid mass was separated by filtration and lead(JI) was determined from the filtrate by AAS. The solid mass on the filter paper was dried in an oven, kept in a desiccator and then weighed to determine the mass of the adsorbent. The maximum amount of lead(Il) retained was 3.7 mg g- 1 of the adsorbent. It was also noted that the retention capacity depends on the amount of CTMA and TFEX supported on naphthalene.

The reaction conditions were established using 120 J..l.g of lead(Il). The lead(II)-TFEX-CTMA ion pair complex was quantitative ly adsorbed on micro­crystalline naphthalene in the pH range 4.5-1 0 .5. It was observed that 0.3 - 5.0 ml of I% CTMAB and 0.5-7.0 ml of 0.2% TFEX were sufficient for the quantitative adsorption of Pb(II)-TFEX-CTMA complex on microcrystalline naphthalene. Therefore, I ml of CTMAB and 2.0 ml of TFEX have been used in the present work. The lead(II ) complex was desorbed completely with 12 ml of 0.1-5.0 M hydrochloric acid. The peak height and shape of the

differential pulse polarograms were found to be constant in the concentration range 0.25-4.0 M HCI. Therefore, 12 ml of 1.0 M HCl was used for subsequent studies. The amount of naphthalene (20% in acetone) was varied from 0.5 to 5.0 ml and adsorption was carried out by the general procedure. The adsorption was constant and maximum irrespective of the amount of the naphthalene used and hence in all the experiments, 2 ml of the 20% naphthalene solution was used . It was observed that peak current was maximum and constant when the volume of the aqueous phase did not exceed 350 ml (preconcentration factor -23) in case of the microcrystalline method and 750 ml for the column method (preconcentration factor = 50). Hence, in all experiments, - 40 ml of aqueous phase was maintained for convenience. The flow rate was varied from 0.2-10.0 ml min-1 in the column method. It was found that up to a flow rate of 6.0 ml min-1 did not affect the retention of the metal complex and hence a flow rate of 1-2 ml min-1 was used in the present work. However, in the case of large volumes of the aqueous phase, a flow rate of 6.0 ml min-1 may be used.

A calibration plot for the determination of lead(II ) was prepared according to the general procedure under the optimum conditions developed above from its differential pulse polarograms with different concentrations. The detection _limit was 0.20 Jlglml (signal-to-noise ratio =2) and this was linear over the

Table 1- Effect of fore ign salts and metal ions. [Pb(ll ): 120 Jl g; reagen t:2.0 ml(0.2% TFEX; CTMAB: 1.0 ml ( I%); naphthalene; 2.0 ml (20%)]

Salt or ion added

CH3C00Na.3H20, NaCI, NaBr, KSCN, NaF, KN03, NaCN

Sodium potassium tartrate

K2C03, Na2HP04, trisodium citrate, Na2S20 3, thiourea

Sodium oxalate

Kl, K2S04

Na2EDTA

Ca(II) , Mg(II) , Ba(ll)

Zr(IV). Ti(IV), Cr(lll), Mn(ll)

U(VI), W(VI), Mo(VI), Zn(ll), Rh(lll), Ni(ll), Co(ll)", Cu( ll )", Pd(II)", Cd(II)", Hg( IJ )"·b

Os(VIII), Pt(IV), In( III), Co(! I), lr(III ), As( III), Sn(ll), Ru(ll I)

V(V), Te(IV), Bi(III), Cd(ll ), Pd(ll), Cu(II), Cr(VI)

Fe(lll) c. Sb(IIJ)d

TI(I)"

•Masked with 10 ml of 5% NaCN solution

Tolerance limit, mg

100

80

50

42

18

0.01

85

70

50

25

10

8

5

bHg(II) was masked with the addit ion of 10 ml of 5% KCI /KSCN solution during adsorption step cFe(III) was masked with the addition of 5 ml of 10% triethanolamine solution during adsorption step d Adsorption carried out at pH - I 0.0 e Adsorption carried out at pH - 4.7

1650 INDIAN J CHEM, SEC A, AUGUST 2002

concentration range 0.5-16.4 J..lg/ml (7 .5-246.0 J..lg in 15 ml of the final solution after the preconcentration step) with correlation factor 0.9997 and a relative standard deviation of± 1.07%.

Effect of foreign ions The effect of various salts of anions and cations on

the adsorption and subsequently on the differential pulse polarographic determination of 120 J..lg of lead(II) was studied individually by the general

procedure (Table I) . Among the anions studied many could be tolerated up to mglg level, except EDT A. Adsorption is almost nil in the presence of EDT A. Probably the formation constant of lead-EDT A complex is higher than that of lead xanthate complex. Among the cations studied, many could be tolerated up to mg levels except Hg(II), Fe(III) and TI(I). The interference of Hg(II) was eliminated with the addition of 10 ml of 5% sodium cyanide/KCVKSCN solution as masking agent. Fe(III) was masked with 5

Table 2-Determination of lead(II) in standard alloys and biological sample

Sample Alloys

NIST SRM-94C, Zinc base die casting alloy

N.B.S. SRM-85 B Aluminum alloy

NIST SRM-629, Zinc alloy

NKK 920, Aluminum alloy

NKK 916 Aluminum alloy

Biological samples

(NIES) CRM No. 5 Human hair

(NIES) CRM No. 7 Tea leaves

(NIES) CRM No. 3 Chiarella

NIES No. I Paperbush

•column method was used bMicrocrystalline method was used 1Method of standard addition was applied

Lead(II)* Cert. Found

0.006 0.0058 ±0.000 15"

0.21 0.2IS ± 0.003"

0.0135 0.0137 ± 0.00025"

0. 10 0.102 ± 0. 0018b

0.04 0.0415 ± 0. 0006.

6.00 6.10 ± 0.08

0.80 o.803 ± o.o1t

0.60 o.62 ± o.oo9t

5.5 5.60 ±0.07

NIST SRM: National institute of standards and technology, standard reference materials NNK: Nippon Keikinzoku Kogyo 'Average of 5 determinations ±Standard deviation. For alloys, values in % and for biological samples, values in Jlg g - I

5.0 ml of 10% triethanolamine solution and 10.0 ml of 5% sodium cyanide solution were added during the preconcentration step for masking Fe(Ill) and Hg(II)

Table 3-Determination of lead(II) in environmental liquid and solid samples

Sample Lead (II)* Present method AAS method

Liquid samples

Waste water from electroplating and ceramic industries, Wazirpur Industrial Area, New Delhi, India 75.65 ± 0.85 75.80 ± 0.92

Waste water from Mayapuri Industrial Area, New Delhi, India

Water from Okhla canal near Okhla Industrial Area, New Delhi, India1

Water from Yamuna river near Wazirpur Industrial Area, New Delhi , India1

Tap water, Research Laboratory liT, New Delhi, India1

Solid samples

Fly ash near IP Power House, New Delhi, India

Soil, near Wazirpur Industrial Area, New Delhi, India

Soil, near Indraprastha Power Station, New Delhi, India

Sediment of Yamuna river near Wazirpur waste disposal point, New Delhi, India

56.2 ± 0.8 55.5 ± I

0.045 ± 0.00062 0.048 ± 0.0007

0.051 ± 0.0006 0.055 ± 0.00064

0.0955 ± 0.0022 0.0962 ± 0.0024

62.75 ± 0.75

24.71 ± 0.36

28.55 ± 0.52

50.65 ± 0.72

63.24 ± 0.82

24.95 ± 0.42

28.95 ± 0.55

51.25 ± 0.85

Sediment of Yamuna River near lndraprastha Power Station, New Delhi , India 44.58 ± 0.68 45.78 ±0.70

*Average of five determinations, ± standard deviation. For liquid samples, values in Jlg ml- 1 and for solid samples, values in Jlg g - I. 1Method of standard addition was applied Hg(II) was masked with 10.0 ml of 5% NaCN solution Fe(lll) was masked with 5.0 ml of 10% triethanolamine solution

NOTES 1651

ml of 10% triethanolamine solution during the adsorption step. When Tl(I) was present along with lead(II), the preconcentration step was carried out at pH- 4.7 Thus, the method is fairly selective and may be used safely for the determination of lead(Il) m various complex materials.

Application to the analysis of standard alloys, standard biological materials and environmental samples

The accuracy and applicability of the method proposed was evaluated by its application to analysis of various standard alloys, standard biological materials and environmental samples. An aliquot of the pretreated standard alloys, standard biological materials and environmental samples (50-100 ml) was analyzed by the general procedure after Fe(III) and Hg(II) were masked with 5 ml of 10% triethanol­amine and 10 ml of 5% sodium cyanide solution, respectively. Tap water from the Department of Chemistry, Indian Institute of Technology, New Delhi, India, was analyzed directly by the general procedure after masking Fe(III) with triethanolamine solution. With samples, having a low concentration of lead(ll), the method of standard addition was used. Recovery was more than 95% in each case. For samples of unknown composition, lead(II) was determined by the AAS method after preconcentrating it by the proposed method in order to verify the

results obtained by the proposed method. In this case, naphthalene had to be removed by filtration before aspirating the sample in the flame. The results are given in Tables 2 and 3.

Acknowledgement Sincere thanks of the authors are due to Council of

Scientific and Industrial Research, India for providing the financial assistance to one of them (Atamjyot).

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