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Camphor-3-thioxo-2-oxime as an analytical reagent for extractive spectrophotometric determination and separation of lead
Spectrochimica Acta Part A 55 (1999) 825 – 831
Camphor-3-thioxo-2-oxime as an analytical reagent for extractive spectrophotometric determination and separation of lead S. Ninan, A. Varadarajan, S.B. Jadhav, A.J. Kulkarni, S.P. Malve * Department of Chemistry, The Institute of Science, 15, Madam Cama Road, Mumbai 400 032, India Received 15 January 1998; received in revised form 9 July 1998; accepted 9 July 1998
Abstract Camphor-3-thioxo-2-oxime (HCTO) is proposed as a new sensitive analytical reagent for the extractive spectrophotometric determination of trace amounts of lead. The method is based on the instantaneous formation of a stable yellow-orange colored 1:2 chelate with lead at room temperature in the pH range 9.3 – 9.6 selectively extracted in carbon tetrachloride. The extracted species exhibits an absorption maximum at 400 nm with a molar absorptivity of 4.14×104 mol − 1 cm − 1, complying with Beer’s law over the concentration range 0.1 – 0.5 mg ml-1 of lead with an optimum concentration range 0.18–0.37 mg ml − 1. The effects of pH, concentration of reagent and salting-out agents, time of equilibration, order of addition of diluents and the tolerance limit of the method towards various cations and anions usually associated with lead are reported. The developed method is successfully used for the determination of traces of lead in synthetic mixtures, alloys and ore samples. © 1999 Elsevier Science S.A. All rights reserved. Keywords: Lead determination; Camphor-3-thioxo-2-oxime; Extractive spectrophotometry; Sensitive; Separation
1. Introduction The significance of lead as an essential trace element lies in its toxicity as a harmful heavy metal and the wide spectrum of industrial applications covering many frontier areas of study [1,2]. Similarly, spectroscopic and structural aspects of metal complexes derived from nitrogen and sulfur containing ligands have been the focal points of vigorous research activities in recent years. Thus * Corresponding author. Tel.: +91-22-516-5566; Fax: + 91-22-542-5866.; e-mail: [email protected].
owing to the increased interest amongst research scholars worldwide due to the significant factors outlined above, it was considered worthwhile to develop a very sensitive method for the extractive spectrophotometric determination of microamounts of lead. Even though various chelating agents such as dithizone [3], 4-(2-pyridylazo)resorcinol (PAR) [4], 1-(2-pyridylazo)-2naphthol (PAN) [5], 8-hydroxy-7-[a-(2-methoxycarbonylanilino)benzyl]quinoline [6], thio-2-thenoyltrifluoroacetone [7], N-(4-chlorophenyl)benzohydroxamic acid [8], N-benzoyl-O-amine caproic acid [9], 2-(salicylideneamino)benzenethiol [10],
1386-1425/99/$ - see front matter © 1999 Elsevier Science S.A. All rights reserved. PII: S 1 3 8 6 - 1 4 2 5 ( 9 8 ) 0 0 2 2 7 - 3
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S. Ninan et al. / Spectrochimica Acta Part A 55 (1999) 825–831
malachite green [11], morphine-4-carbodithioate [12], cyclohexane-1,3-dione-bis(4-methylthiosemicarbazone) [13], N, N-bis(2-hydroxy-5-nitrobenzyl) cryptand-22 [14], 1-(4-methylthiazol-2-ylazo)2-naphthol [15], butyl rhodamine B [16], nitrochrome pyrazole [17], monooctyl-a-(4-carboxyanilino)benzylphosphine acid [18], sodium-N, Ndiethyldithiocarbamate [19] and pyridine-2-acetaldehyde salicyloyl hydrazone [20] etc., have been reported for the extractive spectrophotometric determination of lead, nevertheless, all of them are unsatisfactory for various reasons. These methods are insufficiently sensitive for lead determination at trace levels and suffer from limitations such as critical pH [16,17], long extraction time [7,10,13,14], strict temperature control [10] and interference from large number of ions [3,6,7,10,12,13,20] usually associated with lead etc. Recent literature survey indicates that camphor-3-thioxo-2-oxime (HCTO or isonitrosothiocamphor) has been used for extraction and spectrophotometric determination of Ni2 + [21], Se4 + [22], Cu2 + [23], Ca2 + [24], Cd2 + [25], Rh2 + [26], Pd2 + [27], Pt4 + [28] and Hg2 + [22]. HCTO reacts with lead to give a yellow – orange colored species which is extractable in carbon tetrachloride. The method is free from limitations.
required pH with 4 N hydrochloric acid using a model LI 120 digital ELICO pH meter consisting of a combined glass electrode assembly. Spectroscopic grade solvents (Merck ] 99.5%) were used for extraction and absorbance measurements, recorded on Shimadzu 140-02 UV-visible and Spectronic-20 spectrophotometers with 10 mm matched quartz and glass cells respectively. The solvents do not absorb appreciably at the wavelengths of interest. The solutions of other metal ions are prepared by dissolving their commonly available chemically pure salts in distilled water or dilute hydrochloric or sulfuric acid to give 510 mg ml − 1 concentration of the ions. Synthetic samples are prepared by mixing lead and other metal ion solutions to get the desired composition.
2. Experimental Analytical grade purity chemicals and solvents are used unless otherwise stated. Deionised, double distilled water is used throughout the experiment. HCTO is freshly prepared as described in the literature [29]. A 1% (w/v) stock solution of HCTO is prepared by dissolving an accurately weighed amount (1.000 g) of reagent in 100 ml absolute ethanol (Merck ]99.5%). An accurately weighed amount of lead nitrate salt (3.990 g, Merck 99%) is dissolved in 3 ml of concentrated nitric acid. The solution is diluted to 250 ml with double distilled water to prepare a 10 000 ppm solution and standardised by the known salicyloyl–aldoxime method [30]. Aliquots are suitably diluted to give solutions of Pb at mg ml − 1 level. Buffer solution of pH 9.5 is prepared by adjusting the pH of aqueous ammonia solution (1:1) to the
Fig. 1. Absorption spectra of HCTO and its Pb2 + complex extracted in carbon tetrachloride.
S. Ninan et al. / Spectrochimica Acta Part A 55 (1999) 825–831
Fig. 2. Effect of pH on the extraction of Pb2 + with HCTO in carbon tetrachloride.
3. Determination of traces of lead in synthetic/real samples To an aliquot of solution containing up to 100 mg Pb2 + , 2 ml 4 M sodium chloride, 1 ml 1% sodium citrate solution and 1 ml 0.6% HCTO are mixed in a separatory funnel. The pH of the
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Fig. 3. Logarithmic plot of the distribution of Pb2 + versus concentration of HCTO in the organic phase.
solution mixture is adjusted to 9.5 by the addition of 0.5 ml ammonia buffer solution and diluted to 10 ml with distilled water. The mixture is equilibrated with 10 ml carbon tetrachloride for 1 min with vigorous shaking. The two phases are allowed to separate and the absorbance of organic extract is measured at 400 nm against a reagent blank.
Table 1 Precision and accuracy in the determination of Pb2+ Concentration of Pb2+/ ppm
Taken
Founda
0.50 0.75 1.00 1.25 1.50
0.506 0.742 1.010 1.249 1.500
a b
Average of five determinations. Relative standard deviation.
Standard deviation (S/ppm)
R.S.D.b (Sr (%))
0.007 0.009 0.013 0.013 0.017
1.38 1.26 1.31 1.09 1.17
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4. Results and discussion Lead reacts with HCTO forming a yellow–orange colored species in alkaline medium at pH 9.3–9.6 which is extractable in carbon tetrachloride. Under optimum conditions the yellow–orange colored complex exhibits an absorption maximum at 355 – 365 nm [Fig. 1, Curve (a)–(c)] against reagent blank in carbon tetrachloride. Under similar conditions the spectrum of reagent blank against pure solvent carbon tetrachloride indicates that the reagent does not absorb appreciably at this wavelength [Fig. 1, Curve (d)]. Hence all absorption measurements are carried out at 400 nm against a reagent blank in carbon tetrachloride. Fig. 2 shows the extraction of lead as a function of pH. The extent of extraction is established by determining the amount of lead remaining in the aqueous layer spectrophotometrically using the sensitive PAN method [5]. As much as 100 mg lead is quantitatively extracted over the pH range 9.3–
Table 2 Interference of various ions in the determination of Pb2+ with HCTO in carbon tetrachloridea Intefering ion
Anions Bromate, bromide chloride, citrate, fluoride, iodide, nitrate, nitrite, sulfate, sulfite, tartrate, thiosulfate Acetate, iodate, oxalate, persulfate, thiourea, urea Cyanide Chlorate, phosphate, thiocyanate, thiocyanide EDTA
20 000
10 000 2000 1000 100
Cations NH4+, Na+, Mg2+, K+, Ca2+, 10 000 As3+, Te4+, Se4+, Sr2+, Sb3+, Bi3+ Li+, Sn4+ 5000 Cr3+, Zr2+ 2000 Al3+, Zn2+, Ce4+, W6+, Th4+ 1000 Hg2+ 700 Mn2+, Ag+ 500 Cu2+, Rh2+ 200 Co2+, Ni2+, Cd2+, Pb2+ 100 Fe2+, Fe3+ 5 a
Fig. 4. Mole ratio method for determination of the composition of Pb2 + complex of HCTO extracted in carbon tetrachloride.
Tolerence limit (mg ml−1)
Lead concentration, 4 mg ml−1.
9.6 by a single equilibration of 60 s in the presence of 0.8 M sodium chloride solution with 1 ml 0.6% HCTO in 10 ml carbon tetrachloride. The presence of excess HCTO and sodium chloride solution do not have any significant effect on the recovery of lead. The system has a stability of more than 6 h in carbon tetrachloride. Although several extraction solvents like carbon tetrachloride, chloroform, monochlorobenzene, nitrobenzene, benzene, toluene, xylene, MIBK, ethyl acetate and diethyl ether are found to be useful, carbon tetrachloride is chosen because of convenience and sensitivity as very few ions are coextracted in this solvent under the given conditions. The absorbance of the metal complex is not affected by the increasing temperature and remains constant in the range 27–65°C. Under these conditions the extraction remained unaffected up to an aqueous phase volume of 110 ml indicating that the preconcentration of lead can be achieved up to 11× . The absorbance of the colored species
S. Ninan et al. / Spectrochimica Acta Part A 55 (1999) 825–831
in carbon tetrachloride shows a linear response obeying Beer’s law in the concentration range 0.1–0.5 mg ml − 1 of lead with an optimum concentration range 0.18 – 0.37 mg ml − 1 Pb2 + , as obtained from Ringbom’s plot for effective recovery of lead. Sandell sensitivity of the method is 5.0× 10 − 3 mg Pb2 + cm − 2. The precision and accuracy of the developed method is studied by analysing a series of solutions containing known amounts of Pb2 + . Table 1 gives the statistical analysis of the results obtained, with a relative standard deviation of 1.2%. It is necessary to evaluate the distribution coefficient D, while varying extractant concentration, to ascertain the nature of the extracted species. Fig. 3 shows a graph of log D versus log [HCTO]org at fixed pH 9.5, a straight line with a slope # 2 indicates the probable composition of the extracted species to be 1:2, i.e. Pb(CTO)2. The metal-to-ligand ratio is also confirmed by the mole ratio method given in Fig. 4. Under optimum conditions of the procedure, tolerance level of different metal ions are studied by carrying out determination of 40 mg lead in presence of each of the investigated ions added to 10 ml aqueous phase. Table 2 lists the observed tolerance levels, defined as the highest concentration of the interfering ion causing an error of 9 2% in the absorbance of a suitable lead standard. Interference of Fe2 + and Fe3 + is eliminated by adding an appropriate amount of sodium citrate, forming non-extractable complexes in carbon tetrachloride. All other ions commonly associated with lead in nature do not interfere.
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5. Applications The wide applicability of the method is tested by the successful satisfactory analysis of a variety of synthetic samples containing lead up to 40 mg in the aliquot (Table 3). This method is quite selective for lead determination in the presence of large number of elements, especially cadmium, mercury, arsenic, manganese, antimony, bismuth and tin which normally interfere in most of the existing methods of lead determination. The developed method is well in agreement with the results obtained using the universal extractant PAN. The other applications of the developed method include lead analysis of solder, lead brass, leaded gun metal, tin base white metal and galena samples. The results of alloy and ore sample analysis are shown in Table 4. The alloy samples are treated with 5 ml concentrated nitric or 47% hydrobromic acid. The residue obtained are dissolved in hot 5 M nitric or 4 M hydrochloric acid and made up to a known volume with distilled water. The ore samples are treated with 15 ml aqua-regia and filtered. The filtrate is made up to a known volume using distilled water. Suitable aliquots are taken and analysed by the procedure given in Section 2.
6. Conclusion The method described above allows precise determination of lead at nanogram levels. The molar
Table 3 Analysis of synthetic mixtures Sample
Synthetic mixture compositiona
Amount foundb
Recovery (%)
Sc
C.V.d
1 2 3 4 5
Pb(40), Pb(40), Pb(40), Pb(40), Pb(40),
40.00 40.00 39.66 39.83 40.00
100.00 100.00 98.90 99.57 100.00
0.326 0.295 0.420 0.050 0.120
0.80 0.62 1.06 0.12 0.30
a
Bi(200), Sn(100) Cd(50), Hg(100) Bi(100), Sb(300) As(200), Sb(200) Sn(200), Mn(300)
Values in paranthesis are in microgram amounts of the metals. Average of five determinations. c Standard deviation. d Coefficient of variation. b
S. Ninan et al. / Spectrochimica Acta Part A 55 (1999) 825–831
830 Table 4 Analysis of alloys and ores Sample
Sample and its compositiona
1
Leaded gun metal (BCS 184/4) Cu, 84.06; Sn, 7.27; Zn, 3.47; Pb, 3.15; Ni, 1.30; Sb, 0.23; As, 0.13; Fe, 0.056; Bi, 0.005, P, 0.09
2
Tin base white metal (BCS 178/2) Sn, 82.20; Sb, 9.45; Pb, 3.18; Ni, 0.17; Cd, 0.14; Bi, 0.11; Zn, 0.04; Fe, 0.02; Cu, 4.48; As, 0.15
3
Leaded brass (Italab no. 4014) Cu, 60.04; Zn 33.02; Pb, 2.25; Al, 0.37; Sn, 0.30; Ni, 0.30; Fe, 0.18, Sb, 0.02; P, 0.02
4 5
Solder metal Galena
Certified value
Amount foundb
Sc
C.Vd
3.15% Pb
3.07%
0.038
0.124
3.18% Pb
3.18%
0.029
0.091
2.25% Pb
2.25%
0.023
0.040
0.562 0.294
0.950 0.640
60.0 mg Pbb 46.5 mg Pb
59.20 46.30
a
Average of five determinations. 60 mg of lead was taken. c Standard deviation. d Coefficient of variation. b
absorption coefficient of the extracted species is far superior to the existing methods and is virtually specific because of high selectivity of extraction. Also, with increasing importance of thio compounds, especially HCTO as a superior chelating agent/extractant with potential medicinal applications and the recent upsurge in use of coordination compounds in industry and medicine makes the method absolutely interesting as it can be employed for efficient lead determination at trace levels, while HCTO can be used as an antidote for lead poisoning. The developed method offers several distinct advantages of excellent sensitivity, high selectivity, simplicity, rapidity, good stability and satisfactory accuracy for effective spectrophotometric determination and recovery of nanogram amounts of lead in various synthetic and real samples.
Acknowledgements The authors wish to sincerely express their deep sense of gratitude to the Director, The Institute of Science, Mumbai, India for extending facilities to carry out this work and Mr S. Varadarajan for typing this manuscript.
References [1] T.W. Gilbert Jr, I.M. Kolthoff, P.J. Elving, Treatise on Analytical Chemistry, Part II, vol. 6, Interscience, New York, 1964. [2] M. Sittig, Toxic Metals-Pollution Control and Worker Protection, Noyes Data, New Jersey, 1976. [3] E.B. Sandell, Colorimetric Determination of Trace Metals, 3, Interscience, New York, 1959.
S. Ninan et al. / Spectrochimica Acta Part A 55 (1999) 825–831 [4] D. Negoiu, Anal. Univ. Buccan. Ser. Stiint. Nat. Chim. 13 (1) (1964) 165. [5] R.M. Dagnall, Talanta 12 (6) (1965) 583. [6] G. Roebisch, Anal. Chim. Acta 47 (1969) 539. [7] S.B. Akki, S.M. Khopkar, Bull. Chem. Soc. Jpn. 45 (1972) 167. [8] Y.K. Agarwal, S.A. Patel, Bull. Soc. Chim. Belg. 88 (1979) 1027. [9] R. Pietsch, Anal. Chim. Acta 115 (1980) 379. [10] G.N. Rao, A. Varadarajulu, J. Indian Chem. Soc. 59 (1982) 1009. [11] P.P. Kish, Zh. Anal. Khim. 39 (7) (1984) 1226. [12] C.L. Sethi, B.K. Puri, M. Satake, Microchem. J. 32 (1985) 272. [13] F. Salinas, S.J.C. Jimenez, D.T. Galeano, Quim. Anal. 5 (1986) 197. [14] Y. Sakai, N. Kawano, Talanta 33 (1986) 407. [15] A. Pilipenko, Ukr. Khim. Zh. 54 (5) (1988) 509. [16] P.P. Kish, Zh. Anal. Khim. 39 (6) (1989) 1052. [17] V.K. Akimov, Zav. Lab. 55 (7) (1989) 27. [18] V. Ziviek, J. Radio Anal. Nucl. Chem. 140 (2) (1990) 409. [19] Z. Zhou, Y. Liang, Yejin Fenxi 11 (1991) 13.
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[20] M.N. Bale, D.P. Dave, A.D. Sawant, Talanta 42 (1995) 1291. [21] G.S. Katiyar, B.C. Haldhar, Indian J. Chem. 22A (12) (1983) 1084. [22] J.P.N. Trakru, Ph.D. Thesis, University of Bombay, India, 1982. [23] G.S. Katiyar, B.C. Haldhar, Indian J. Chem. Soc. 61 (4) (1984) 353. [24] G.S. Katiyar, B.C. Haldhar, Indian J. Chem. Soc. 63 (10) (1986) 937. [25] J.P.N. Trakru, B.C. Haldhar, J. Radio Anal. Nucl. Chem. 84 (1) (1984) 137. [26] P.K. Paria, P.K. Thokdar, S.K. Majumdhar, J. Indian Chem. Soc. 66 (12) (1989) 918. [27] S.K. Majumdhar, P. Chattopadhyay, P.K. Paria, J. Indian Chem. Soc. 62 (7) (1985) 544. [28] P.K. Paria, T.K. Thokdar, S.K. Majumdhar, Curr. Sci. 58 (12) (1989) 739. [29] D.C. Sen, J. Chem. Soc. 12 (1935) 751. [30] A.I. Vogel, Textbook of Quantitative Chemical Analysis, 5, English Language Book Society, 1991, p. 465.
Camphor-3-thioxo-2-oxime as an analytical reagent for extractive spectrophotometric determination and separation of lead S. Ninan, A. Varadarajan, S.B. Jadhav, A.J. Kulkarni, S.P. Malve * Department of Chemistry, The Institute of Science, 15, Madam Cama Road, Mumbai 400 032, India Received 15 January 1998; received in revised form 9 July 1998; accepted 9 July 1998
Abstract Camphor-3-thioxo-2-oxime (HCTO) is proposed as a new sensitive analytical reagent for the extractive spectrophotometric determination of trace amounts of lead. The method is based on the instantaneous formation of a stable yellow-orange colored 1:2 chelate with lead at room temperature in the pH range 9.3 – 9.6 selectively extracted in carbon tetrachloride. The extracted species exhibits an absorption maximum at 400 nm with a molar absorptivity of 4.14×104 mol − 1 cm − 1, complying with Beer’s law over the concentration range 0.1 – 0.5 mg ml-1 of lead with an optimum concentration range 0.18–0.37 mg ml − 1. The effects of pH, concentration of reagent and salting-out agents, time of equilibration, order of addition of diluents and the tolerance limit of the method towards various cations and anions usually associated with lead are reported. The developed method is successfully used for the determination of traces of lead in synthetic mixtures, alloys and ore samples. © 1999 Elsevier Science S.A. All rights reserved. Keywords: Lead determination; Camphor-3-thioxo-2-oxime; Extractive spectrophotometry; Sensitive; Separation
1. Introduction The significance of lead as an essential trace element lies in its toxicity as a harmful heavy metal and the wide spectrum of industrial applications covering many frontier areas of study [1,2]. Similarly, spectroscopic and structural aspects of metal complexes derived from nitrogen and sulfur containing ligands have been the focal points of vigorous research activities in recent years. Thus * Corresponding author. Tel.: +91-22-516-5566; Fax: + 91-22-542-5866.; e-mail: [email protected].
owing to the increased interest amongst research scholars worldwide due to the significant factors outlined above, it was considered worthwhile to develop a very sensitive method for the extractive spectrophotometric determination of microamounts of lead. Even though various chelating agents such as dithizone [3], 4-(2-pyridylazo)resorcinol (PAR) [4], 1-(2-pyridylazo)-2naphthol (PAN) [5], 8-hydroxy-7-[a-(2-methoxycarbonylanilino)benzyl]quinoline [6], thio-2-thenoyltrifluoroacetone [7], N-(4-chlorophenyl)benzohydroxamic acid [8], N-benzoyl-O-amine caproic acid [9], 2-(salicylideneamino)benzenethiol [10],
1386-1425/99/$ - see front matter © 1999 Elsevier Science S.A. All rights reserved. PII: S 1 3 8 6 - 1 4 2 5 ( 9 8 ) 0 0 2 2 7 - 3
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S. Ninan et al. / Spectrochimica Acta Part A 55 (1999) 825–831
malachite green [11], morphine-4-carbodithioate [12], cyclohexane-1,3-dione-bis(4-methylthiosemicarbazone) [13], N, N-bis(2-hydroxy-5-nitrobenzyl) cryptand-22 [14], 1-(4-methylthiazol-2-ylazo)2-naphthol [15], butyl rhodamine B [16], nitrochrome pyrazole [17], monooctyl-a-(4-carboxyanilino)benzylphosphine acid [18], sodium-N, Ndiethyldithiocarbamate [19] and pyridine-2-acetaldehyde salicyloyl hydrazone [20] etc., have been reported for the extractive spectrophotometric determination of lead, nevertheless, all of them are unsatisfactory for various reasons. These methods are insufficiently sensitive for lead determination at trace levels and suffer from limitations such as critical pH [16,17], long extraction time [7,10,13,14], strict temperature control [10] and interference from large number of ions [3,6,7,10,12,13,20] usually associated with lead etc. Recent literature survey indicates that camphor-3-thioxo-2-oxime (HCTO or isonitrosothiocamphor) has been used for extraction and spectrophotometric determination of Ni2 + [21], Se4 + [22], Cu2 + [23], Ca2 + [24], Cd2 + [25], Rh2 + [26], Pd2 + [27], Pt4 + [28] and Hg2 + [22]. HCTO reacts with lead to give a yellow – orange colored species which is extractable in carbon tetrachloride. The method is free from limitations.
required pH with 4 N hydrochloric acid using a model LI 120 digital ELICO pH meter consisting of a combined glass electrode assembly. Spectroscopic grade solvents (Merck ] 99.5%) were used for extraction and absorbance measurements, recorded on Shimadzu 140-02 UV-visible and Spectronic-20 spectrophotometers with 10 mm matched quartz and glass cells respectively. The solvents do not absorb appreciably at the wavelengths of interest. The solutions of other metal ions are prepared by dissolving their commonly available chemically pure salts in distilled water or dilute hydrochloric or sulfuric acid to give 510 mg ml − 1 concentration of the ions. Synthetic samples are prepared by mixing lead and other metal ion solutions to get the desired composition.
2. Experimental Analytical grade purity chemicals and solvents are used unless otherwise stated. Deionised, double distilled water is used throughout the experiment. HCTO is freshly prepared as described in the literature [29]. A 1% (w/v) stock solution of HCTO is prepared by dissolving an accurately weighed amount (1.000 g) of reagent in 100 ml absolute ethanol (Merck ]99.5%). An accurately weighed amount of lead nitrate salt (3.990 g, Merck 99%) is dissolved in 3 ml of concentrated nitric acid. The solution is diluted to 250 ml with double distilled water to prepare a 10 000 ppm solution and standardised by the known salicyloyl–aldoxime method [30]. Aliquots are suitably diluted to give solutions of Pb at mg ml − 1 level. Buffer solution of pH 9.5 is prepared by adjusting the pH of aqueous ammonia solution (1:1) to the
Fig. 1. Absorption spectra of HCTO and its Pb2 + complex extracted in carbon tetrachloride.
S. Ninan et al. / Spectrochimica Acta Part A 55 (1999) 825–831
Fig. 2. Effect of pH on the extraction of Pb2 + with HCTO in carbon tetrachloride.
3. Determination of traces of lead in synthetic/real samples To an aliquot of solution containing up to 100 mg Pb2 + , 2 ml 4 M sodium chloride, 1 ml 1% sodium citrate solution and 1 ml 0.6% HCTO are mixed in a separatory funnel. The pH of the
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Fig. 3. Logarithmic plot of the distribution of Pb2 + versus concentration of HCTO in the organic phase.
solution mixture is adjusted to 9.5 by the addition of 0.5 ml ammonia buffer solution and diluted to 10 ml with distilled water. The mixture is equilibrated with 10 ml carbon tetrachloride for 1 min with vigorous shaking. The two phases are allowed to separate and the absorbance of organic extract is measured at 400 nm against a reagent blank.
Table 1 Precision and accuracy in the determination of Pb2+ Concentration of Pb2+/ ppm
Taken
Founda
0.50 0.75 1.00 1.25 1.50
0.506 0.742 1.010 1.249 1.500
a b
Average of five determinations. Relative standard deviation.
Standard deviation (S/ppm)
R.S.D.b (Sr (%))
0.007 0.009 0.013 0.013 0.017
1.38 1.26 1.31 1.09 1.17
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4. Results and discussion Lead reacts with HCTO forming a yellow–orange colored species in alkaline medium at pH 9.3–9.6 which is extractable in carbon tetrachloride. Under optimum conditions the yellow–orange colored complex exhibits an absorption maximum at 355 – 365 nm [Fig. 1, Curve (a)–(c)] against reagent blank in carbon tetrachloride. Under similar conditions the spectrum of reagent blank against pure solvent carbon tetrachloride indicates that the reagent does not absorb appreciably at this wavelength [Fig. 1, Curve (d)]. Hence all absorption measurements are carried out at 400 nm against a reagent blank in carbon tetrachloride. Fig. 2 shows the extraction of lead as a function of pH. The extent of extraction is established by determining the amount of lead remaining in the aqueous layer spectrophotometrically using the sensitive PAN method [5]. As much as 100 mg lead is quantitatively extracted over the pH range 9.3–
Table 2 Interference of various ions in the determination of Pb2+ with HCTO in carbon tetrachloridea Intefering ion
Anions Bromate, bromide chloride, citrate, fluoride, iodide, nitrate, nitrite, sulfate, sulfite, tartrate, thiosulfate Acetate, iodate, oxalate, persulfate, thiourea, urea Cyanide Chlorate, phosphate, thiocyanate, thiocyanide EDTA
20 000
10 000 2000 1000 100
Cations NH4+, Na+, Mg2+, K+, Ca2+, 10 000 As3+, Te4+, Se4+, Sr2+, Sb3+, Bi3+ Li+, Sn4+ 5000 Cr3+, Zr2+ 2000 Al3+, Zn2+, Ce4+, W6+, Th4+ 1000 Hg2+ 700 Mn2+, Ag+ 500 Cu2+, Rh2+ 200 Co2+, Ni2+, Cd2+, Pb2+ 100 Fe2+, Fe3+ 5 a
Fig. 4. Mole ratio method for determination of the composition of Pb2 + complex of HCTO extracted in carbon tetrachloride.
Tolerence limit (mg ml−1)
Lead concentration, 4 mg ml−1.
9.6 by a single equilibration of 60 s in the presence of 0.8 M sodium chloride solution with 1 ml 0.6% HCTO in 10 ml carbon tetrachloride. The presence of excess HCTO and sodium chloride solution do not have any significant effect on the recovery of lead. The system has a stability of more than 6 h in carbon tetrachloride. Although several extraction solvents like carbon tetrachloride, chloroform, monochlorobenzene, nitrobenzene, benzene, toluene, xylene, MIBK, ethyl acetate and diethyl ether are found to be useful, carbon tetrachloride is chosen because of convenience and sensitivity as very few ions are coextracted in this solvent under the given conditions. The absorbance of the metal complex is not affected by the increasing temperature and remains constant in the range 27–65°C. Under these conditions the extraction remained unaffected up to an aqueous phase volume of 110 ml indicating that the preconcentration of lead can be achieved up to 11× . The absorbance of the colored species
S. Ninan et al. / Spectrochimica Acta Part A 55 (1999) 825–831
in carbon tetrachloride shows a linear response obeying Beer’s law in the concentration range 0.1–0.5 mg ml − 1 of lead with an optimum concentration range 0.18 – 0.37 mg ml − 1 Pb2 + , as obtained from Ringbom’s plot for effective recovery of lead. Sandell sensitivity of the method is 5.0× 10 − 3 mg Pb2 + cm − 2. The precision and accuracy of the developed method is studied by analysing a series of solutions containing known amounts of Pb2 + . Table 1 gives the statistical analysis of the results obtained, with a relative standard deviation of 1.2%. It is necessary to evaluate the distribution coefficient D, while varying extractant concentration, to ascertain the nature of the extracted species. Fig. 3 shows a graph of log D versus log [HCTO]org at fixed pH 9.5, a straight line with a slope # 2 indicates the probable composition of the extracted species to be 1:2, i.e. Pb(CTO)2. The metal-to-ligand ratio is also confirmed by the mole ratio method given in Fig. 4. Under optimum conditions of the procedure, tolerance level of different metal ions are studied by carrying out determination of 40 mg lead in presence of each of the investigated ions added to 10 ml aqueous phase. Table 2 lists the observed tolerance levels, defined as the highest concentration of the interfering ion causing an error of 9 2% in the absorbance of a suitable lead standard. Interference of Fe2 + and Fe3 + is eliminated by adding an appropriate amount of sodium citrate, forming non-extractable complexes in carbon tetrachloride. All other ions commonly associated with lead in nature do not interfere.
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5. Applications The wide applicability of the method is tested by the successful satisfactory analysis of a variety of synthetic samples containing lead up to 40 mg in the aliquot (Table 3). This method is quite selective for lead determination in the presence of large number of elements, especially cadmium, mercury, arsenic, manganese, antimony, bismuth and tin which normally interfere in most of the existing methods of lead determination. The developed method is well in agreement with the results obtained using the universal extractant PAN. The other applications of the developed method include lead analysis of solder, lead brass, leaded gun metal, tin base white metal and galena samples. The results of alloy and ore sample analysis are shown in Table 4. The alloy samples are treated with 5 ml concentrated nitric or 47% hydrobromic acid. The residue obtained are dissolved in hot 5 M nitric or 4 M hydrochloric acid and made up to a known volume with distilled water. The ore samples are treated with 15 ml aqua-regia and filtered. The filtrate is made up to a known volume using distilled water. Suitable aliquots are taken and analysed by the procedure given in Section 2.
6. Conclusion The method described above allows precise determination of lead at nanogram levels. The molar
Table 3 Analysis of synthetic mixtures Sample
Synthetic mixture compositiona
Amount foundb
Recovery (%)
Sc
C.V.d
1 2 3 4 5
Pb(40), Pb(40), Pb(40), Pb(40), Pb(40),
40.00 40.00 39.66 39.83 40.00
100.00 100.00 98.90 99.57 100.00
0.326 0.295 0.420 0.050 0.120
0.80 0.62 1.06 0.12 0.30
a
Bi(200), Sn(100) Cd(50), Hg(100) Bi(100), Sb(300) As(200), Sb(200) Sn(200), Mn(300)
Values in paranthesis are in microgram amounts of the metals. Average of five determinations. c Standard deviation. d Coefficient of variation. b
S. Ninan et al. / Spectrochimica Acta Part A 55 (1999) 825–831
830 Table 4 Analysis of alloys and ores Sample
Sample and its compositiona
1
Leaded gun metal (BCS 184/4) Cu, 84.06; Sn, 7.27; Zn, 3.47; Pb, 3.15; Ni, 1.30; Sb, 0.23; As, 0.13; Fe, 0.056; Bi, 0.005, P, 0.09
2
Tin base white metal (BCS 178/2) Sn, 82.20; Sb, 9.45; Pb, 3.18; Ni, 0.17; Cd, 0.14; Bi, 0.11; Zn, 0.04; Fe, 0.02; Cu, 4.48; As, 0.15
3
Leaded brass (Italab no. 4014) Cu, 60.04; Zn 33.02; Pb, 2.25; Al, 0.37; Sn, 0.30; Ni, 0.30; Fe, 0.18, Sb, 0.02; P, 0.02
4 5
Solder metal Galena
Certified value
Amount foundb
Sc
C.Vd
3.15% Pb
3.07%
0.038
0.124
3.18% Pb
3.18%
0.029
0.091
2.25% Pb
2.25%
0.023
0.040
0.562 0.294
0.950 0.640
60.0 mg Pbb 46.5 mg Pb
59.20 46.30
a
Average of five determinations. 60 mg of lead was taken. c Standard deviation. d Coefficient of variation. b
absorption coefficient of the extracted species is far superior to the existing methods and is virtually specific because of high selectivity of extraction. Also, with increasing importance of thio compounds, especially HCTO as a superior chelating agent/extractant with potential medicinal applications and the recent upsurge in use of coordination compounds in industry and medicine makes the method absolutely interesting as it can be employed for efficient lead determination at trace levels, while HCTO can be used as an antidote for lead poisoning. The developed method offers several distinct advantages of excellent sensitivity, high selectivity, simplicity, rapidity, good stability and satisfactory accuracy for effective spectrophotometric determination and recovery of nanogram amounts of lead in various synthetic and real samples.
Acknowledgements The authors wish to sincerely express their deep sense of gratitude to the Director, The Institute of Science, Mumbai, India for extending facilities to carry out this work and Mr S. Varadarajan for typing this manuscript.
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