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Spectrophotometric determination of traces of lead with bromopyrogallol red and cetyltrimethylammonium or cetylpyridinium bromide
0039-Y 140mx0501_0429M2,00)/O
Jalonrs. Vol 27. pp 439 10 441 0 Pcrgamon Pres Ltd 1980. Prmled m Great Bwain
SPECTROPHOTOMETRIC DETERMINATION OF TRACES OF LEAD WITH BROMOPYROGALLOL RED AND CETYLTRIMETHYLAMMONIUM OR CETYLPYRIDINIUM BROMIDE
T. PRASADA
RAO and T. V. RAMAKRISHNA
Department of Chemistry. Indian Institute of Technology. Madras 600 036. India
(Received 25 May 1979. Recked
.
16 Ocroher 1979. Accepted 26 October 1979)
Summary-A method is described based on the sensitizing etrcct of cetyltrimethylammonium or cetylpyridinium bromide on the lead-Bromopyrogallol Red colour reaction. The reaction is instantaneous and the colour remains stable for over 120 hr in the presence of Triton X-100. Both colour systems obey Beer’s law up lo 5.5 ppm of lead. Methods are described’for dealing with interferences.
Dithizone’ is often recommended for the spectrophotometric determination of lead, but has wellknown drawbacks. Metallochromic reagents, which allow the direct determination of lead in aqueous media. have been proposed. but none except 4-(2-pyridylazo)resorcinol’ ,is sensitive enough to be useful for low concentrations of lead. The sensitizing effect of surfactants on colour reactions is now well known. In the present work it was found that addition of cetyltrimethylammonium bromide (CTAB) or cetylpyridinium bromide (CPB) to. the lead-Bromopyrogallol Red (BPR) complex causes a large shift in the waielength of maximum absorption, providing the basis for a sensitive spectrophotometric method.
RESULTS AND DISCUSSION
The colour reaction of lead with BPR at pH 5 was already known.3 We found in the present investigation that in the presence of 1 ml of 0.05% CTAB or CPB solution, the lead-BPR reaction proceeds instantaneously but gives a blue precipitate. Though large amounts of CTAB solubilize the complex, excess of CPB is without effect. However, the neutral surfactant Triton X-100, which by itself does not sensitize the lead-BPR colour reaction, effectively solubilizes both the CTAB and CPB &our systems. The best results are obtained when 1 ml of 0.5% Triton X-100 solution is added before the addition of CTAB or CPB to lead-BPR system at pH 5 to give a final volume of 25 ml. Under these conditions the colour development is instantaneous and the colour stable for over 120 hr.
EXPERIMENTAL Absorption
Reagents Lead solution. Dissolve O.lC00 g of lead nitrate in water and dilute to 250 ml. Dilute aoorooriate volumes of this 250-ppm stock solution with water io provide a 20.0 ppm solution. Eromopyrogallol Red solution, O.OI”/ Dissolve 0.1 g in hot water or 1% sodium acetate solution and dilute lo 1 litre. Triton X-100 solution, 0.5%. CTAB or CPB solutions, 0.05%. Acetate buffer, pH 5.0, 0.1M. Procedure Transfer a suitable volume (up lo 15 ml) of sample solution (containing not more than 140 fig of lead) to a 25-ml standard flask.-Add, with mixing, 2.ml of buffer, 5 ml of BPR solution. 1 ml of Triton X-100 solution and 1 ml of CTAB or 0.5 ml of CPB solution. Dilute to the mark with water and measure the absorbance in IO-mm cells at 630 nm against a reagent blank. Prepare a calibration graph for 10-140 pg of lead by the same procedure. 439
spectra
The
spectra of BPR, Pb-BPR, Pb-BPR-CTAB and Pb-BPR-CPB in the presence of Triton X-100 are shown in Fig. I. No new absorption band is produced by the CTAB or CPB and their presence merely strongly enhances the 630-nm band for the Pb-BPR complex. The absorbance at 630 nm. under the conditions of the procedure is constant over the pH range 4-7. Hence pH 5 (acetate buffer) was chosen. E@ct
of reagent
concentrations
With other variables held constant, for 75 gg of lead, the absorbance increased with increasing’ amount of 0.01% BPR solution up to 4.5 ml and then remained constant with amounts up to IOml. Analogous studies with the other reagents showed that at least 3 ml of O.Olo/,CTAB solution or 1.5 ml of 0.01%
SHORT
COMMUNICATIONS
560
EC0 Hwlmgth , nm
550
nm
3 1‘5
Fig 1. Absorption spectra (pH 5; total volume 25 ml; IO-mm cells): A, 1.0 ml of 4.8 x 10-‘M BPR and 1 ml of 0.5% T&on X-100; 8. as in A. with 2OOpg of lead; C, as in A. with 5Opg of lead and 1 ml of 0.05% CTAB; D. as in A, with 100 pg of lead and 1 ml of 0.025?/,CPB.
CPB solution should be added and at least 1 ml of 0.25% Triton X-100 solution. At lower concentration of Triton X-100, both colour systems are unstable and a precipitate forms. The order of addition of reagents is not critical provided the Triton X-100 is added before the CTAE or CPB. Beer’s law and prerision
Both systems obey Beer’s law over the range IO-14Opg of kad in a final volume of 25ml. The apparent molar absorptivities at 63Onm are 2.6 x IO4 and 2.0 x lo* I.mole-l.cm’i for the CTAB and CPB systems respectively. Ten determinations on standard solutions that contained 75 fig of lead showed a mean recovery of 100.40/, by the CTAB procedure and lOLO? by the CPB procedure, with relative standard deviations of 0.6% and 0.87; respectively. interferences
At the S-mg level, Li’, Mg2+, Ca*‘, Sr2+, Ba’+, NH:. AsO;. NO;. SO:-, S,O:-, SO:-. CT’+. SeO$-, F-. Cl-, I-, ClO;. SCN-, tartrate, thiourea ascorbic acid, hydroxylamine hydrochloride, hydrazinc sulphate and sulphamic acid do not interfere in the determination of 5Opg of lead, but WO:-, MOO:-, VO;, Sn2+, Ti4’, Bi3+ and Zr*+ precipitate on addition of CT’AB or CPB or as hydroxides, AsO:- and IO; cause a decrease in the absorbance by oxidizing the reagent, and Cu2+, Zn’+. Cd’+. Co’+. Ni2+, Pd”, La3’, UO:+, Ce4’, Pt4+, Sb’+, Mn’+ and Fe’+ enhance the absorbance. Sb3+ interferes by being precipitated. The interference of Cd2+, Co2+, Nit* and AsO:- can be overcome by addition of tartrate and that of Bi3+, Pdz+, Pt*+ and Cu2+ (in presence qf sulphite) by adding thiourea. Addition of fluoride eliminates the interference due to Be2+, La” and Ce4+ (after reduction with ascorbic acid). Sb’+
and Sb3+ interference is eliminated by adding mannito1 and that of Mn2* by adding triethanolamine. Other interferences can only be overcome by selectively holding Pb2+ on a cation-exchanger (Dowex 50 x 8 Na’-form) from neutral solutions containing fluoride (to exclude Ti4+, UO$+. Sn2+. Sn*+. Ce’+ and Fe”) and thiocyanate (to exclude Zn2+) and then eluting with IN nitric acid. Such an approach. even in the absence of compkxing ligands, also permitted the selective determination of lead in the presence of MoOi-. WO:- and VO;. Stoichiometry
of the cwnplex
Dhupar et al.’ reported formation of the 1:I Pb-BPR compkx. With a constant volume of Triton X-100 present, the composition of the complexes was determined by the mole-ratio and continuous variation methods. Both methods show a 1:l : 1 Pb:BPR:CPB complex; the continuous variation method shows a 1:l :l Pb:BPR:CTAB complex but the mole-ratio method indicates the existence of both 1:l :1 and 1 :1:2 complexes. The equilibrium shift method also gives evidence for existence of the 1: 1:2 complex, indicating that CI’AB not only forms an ion-pair system with the sulphonate group but also displaces the proton of the phenolic group of the BPR molecule, which otherwise is held by hydrogen bonding. Triton X-100. being a neutral surfactant having a much higher aggregation number in its micelks’ (> 10)) than cationic surfactants (10-100) evidently stabilizes the CTAB and CPB systems. by inclusion of the complex species in the interior of the micelks. Determination CJ~ lead in brass
The method was tested for the analysis of brass samples after separation of the lead on calcium carbonate as described elsewhere.5 Systematic studies revealed that lead can be selectively sep?rated from iron, zinc and copper, if the precipitation is done in
SHORT
Table 1. Analysis of brass samples
Sample
I
Lead found. “A Atomic Proposed method* absorption 2.80
Nil
2 2 with added 2 with added
O.lZS’& lead 0.629; lead
* Volume taken
441
COMMUNICATIONS
2.85 (0.2 ml) 2.77 (0.4 ml) 2.82 (1 ml) Nil O.IZI (5 ml) 0.12(10.0ml) O-60 (2.5 ml) 0.61 (5.0 ml)
a standard flask. treating 0.2-10ml of the solution with 2 ml of calcium solution (prepared by dissolving 2g of calcium carbonate in the minimum amount of hydrochloric acid and diluting to 50ml with water) followed by 2 ml of 1004 triethanolamine solution, sufficient 2M ammonia to raise the pH to c 9, and 2 ml of loo/, sodium carbonate solution (with stirring). the suspension obtained then being centrifuged and the supernatant liquid discarded, the precipitate washed twice with water and finally dissolved in dilute nitric acid. The results agree with those obtained by atomic absorption spectrometry.
for analysis is shown in parentheses.
the presence of triethanolamine. Such an approach is simpler in that it does not require subsequent extractive separation of lead from interfering elements that accompany it during the co-precipitation step. However, a slight decrease in the recovery of lead in presence of excess of calcium necessitates the preparation of a calibration graph with the collection procedure applied to lead standards. Table 1 gives the results for two solutions prepared by dissolving 1 g of sample in 10 ml of nitric acid, filtering off the metastannic acid, diluting to 250 ml in
Acknowledgement--One of us (TPR) is grateful to CSIR. New Delhi. for financial assistance. REFERENCES I. E. B. Sandell, Colorimerric Determination
of
Trace
3rd Ed., p. 568. Interscience. New York, 1959. 2. F. H. Pollard, P. Hanson and W. J. Geary. Anal. Chim. Metals,
Acta. 1959.20.26. 3. S. C. Dhupar, K. C. Srivastava and S. K. Banerji. J. Chim. Chem. Sot. Taipei, 1973.20, 145. 4. L. R. Fisher and D. G. Oakenfull. C/tern. Sot. Rev.,
5.
1977,6. No. 1. R. M. Dagnall. T. S. West and P. Young, 12. 583. 589.
Talanta,
1965,
Jalonrs. Vol 27. pp 439 10 441 0 Pcrgamon Pres Ltd 1980. Prmled m Great Bwain
SPECTROPHOTOMETRIC DETERMINATION OF TRACES OF LEAD WITH BROMOPYROGALLOL RED AND CETYLTRIMETHYLAMMONIUM OR CETYLPYRIDINIUM BROMIDE
T. PRASADA
RAO and T. V. RAMAKRISHNA
Department of Chemistry. Indian Institute of Technology. Madras 600 036. India
(Received 25 May 1979. Recked
.
16 Ocroher 1979. Accepted 26 October 1979)
Summary-A method is described based on the sensitizing etrcct of cetyltrimethylammonium or cetylpyridinium bromide on the lead-Bromopyrogallol Red colour reaction. The reaction is instantaneous and the colour remains stable for over 120 hr in the presence of Triton X-100. Both colour systems obey Beer’s law up lo 5.5 ppm of lead. Methods are described’for dealing with interferences.
Dithizone’ is often recommended for the spectrophotometric determination of lead, but has wellknown drawbacks. Metallochromic reagents, which allow the direct determination of lead in aqueous media. have been proposed. but none except 4-(2-pyridylazo)resorcinol’ ,is sensitive enough to be useful for low concentrations of lead. The sensitizing effect of surfactants on colour reactions is now well known. In the present work it was found that addition of cetyltrimethylammonium bromide (CTAB) or cetylpyridinium bromide (CPB) to. the lead-Bromopyrogallol Red (BPR) complex causes a large shift in the waielength of maximum absorption, providing the basis for a sensitive spectrophotometric method.
RESULTS AND DISCUSSION
The colour reaction of lead with BPR at pH 5 was already known.3 We found in the present investigation that in the presence of 1 ml of 0.05% CTAB or CPB solution, the lead-BPR reaction proceeds instantaneously but gives a blue precipitate. Though large amounts of CTAB solubilize the complex, excess of CPB is without effect. However, the neutral surfactant Triton X-100, which by itself does not sensitize the lead-BPR colour reaction, effectively solubilizes both the CTAB and CPB &our systems. The best results are obtained when 1 ml of 0.5% Triton X-100 solution is added before the addition of CTAB or CPB to lead-BPR system at pH 5 to give a final volume of 25 ml. Under these conditions the colour development is instantaneous and the colour stable for over 120 hr.
EXPERIMENTAL Absorption
Reagents Lead solution. Dissolve O.lC00 g of lead nitrate in water and dilute to 250 ml. Dilute aoorooriate volumes of this 250-ppm stock solution with water io provide a 20.0 ppm solution. Eromopyrogallol Red solution, O.OI”/ Dissolve 0.1 g in hot water or 1% sodium acetate solution and dilute lo 1 litre. Triton X-100 solution, 0.5%. CTAB or CPB solutions, 0.05%. Acetate buffer, pH 5.0, 0.1M. Procedure Transfer a suitable volume (up lo 15 ml) of sample solution (containing not more than 140 fig of lead) to a 25-ml standard flask.-Add, with mixing, 2.ml of buffer, 5 ml of BPR solution. 1 ml of Triton X-100 solution and 1 ml of CTAB or 0.5 ml of CPB solution. Dilute to the mark with water and measure the absorbance in IO-mm cells at 630 nm against a reagent blank. Prepare a calibration graph for 10-140 pg of lead by the same procedure. 439
spectra
The
spectra of BPR, Pb-BPR, Pb-BPR-CTAB and Pb-BPR-CPB in the presence of Triton X-100 are shown in Fig. I. No new absorption band is produced by the CTAB or CPB and their presence merely strongly enhances the 630-nm band for the Pb-BPR complex. The absorbance at 630 nm. under the conditions of the procedure is constant over the pH range 4-7. Hence pH 5 (acetate buffer) was chosen. E@ct
of reagent
concentrations
With other variables held constant, for 75 gg of lead, the absorbance increased with increasing’ amount of 0.01% BPR solution up to 4.5 ml and then remained constant with amounts up to IOml. Analogous studies with the other reagents showed that at least 3 ml of O.Olo/,CTAB solution or 1.5 ml of 0.01%
SHORT
COMMUNICATIONS
560
EC0 Hwlmgth , nm
550
nm
3 1‘5
Fig 1. Absorption spectra (pH 5; total volume 25 ml; IO-mm cells): A, 1.0 ml of 4.8 x 10-‘M BPR and 1 ml of 0.5% T&on X-100; 8. as in A. with 2OOpg of lead; C, as in A. with 5Opg of lead and 1 ml of 0.05% CTAB; D. as in A, with 100 pg of lead and 1 ml of 0.025?/,CPB.
CPB solution should be added and at least 1 ml of 0.25% Triton X-100 solution. At lower concentration of Triton X-100, both colour systems are unstable and a precipitate forms. The order of addition of reagents is not critical provided the Triton X-100 is added before the CTAE or CPB. Beer’s law and prerision
Both systems obey Beer’s law over the range IO-14Opg of kad in a final volume of 25ml. The apparent molar absorptivities at 63Onm are 2.6 x IO4 and 2.0 x lo* I.mole-l.cm’i for the CTAB and CPB systems respectively. Ten determinations on standard solutions that contained 75 fig of lead showed a mean recovery of 100.40/, by the CTAB procedure and lOLO? by the CPB procedure, with relative standard deviations of 0.6% and 0.87; respectively. interferences
At the S-mg level, Li’, Mg2+, Ca*‘, Sr2+, Ba’+, NH:. AsO;. NO;. SO:-, S,O:-, SO:-. CT’+. SeO$-, F-. Cl-, I-, ClO;. SCN-, tartrate, thiourea ascorbic acid, hydroxylamine hydrochloride, hydrazinc sulphate and sulphamic acid do not interfere in the determination of 5Opg of lead, but WO:-, MOO:-, VO;, Sn2+, Ti4’, Bi3+ and Zr*+ precipitate on addition of CT’AB or CPB or as hydroxides, AsO:- and IO; cause a decrease in the absorbance by oxidizing the reagent, and Cu2+, Zn’+. Cd’+. Co’+. Ni2+, Pd”, La3’, UO:+, Ce4’, Pt4+, Sb’+, Mn’+ and Fe’+ enhance the absorbance. Sb3+ interferes by being precipitated. The interference of Cd2+, Co2+, Nit* and AsO:- can be overcome by addition of tartrate and that of Bi3+, Pdz+, Pt*+ and Cu2+ (in presence qf sulphite) by adding thiourea. Addition of fluoride eliminates the interference due to Be2+, La” and Ce4+ (after reduction with ascorbic acid). Sb’+
and Sb3+ interference is eliminated by adding mannito1 and that of Mn2* by adding triethanolamine. Other interferences can only be overcome by selectively holding Pb2+ on a cation-exchanger (Dowex 50 x 8 Na’-form) from neutral solutions containing fluoride (to exclude Ti4+, UO$+. Sn2+. Sn*+. Ce’+ and Fe”) and thiocyanate (to exclude Zn2+) and then eluting with IN nitric acid. Such an approach. even in the absence of compkxing ligands, also permitted the selective determination of lead in the presence of MoOi-. WO:- and VO;. Stoichiometry
of the cwnplex
Dhupar et al.’ reported formation of the 1:I Pb-BPR compkx. With a constant volume of Triton X-100 present, the composition of the complexes was determined by the mole-ratio and continuous variation methods. Both methods show a 1:l : 1 Pb:BPR:CPB complex; the continuous variation method shows a 1:l :l Pb:BPR:CTAB complex but the mole-ratio method indicates the existence of both 1:l :1 and 1 :1:2 complexes. The equilibrium shift method also gives evidence for existence of the 1: 1:2 complex, indicating that CI’AB not only forms an ion-pair system with the sulphonate group but also displaces the proton of the phenolic group of the BPR molecule, which otherwise is held by hydrogen bonding. Triton X-100. being a neutral surfactant having a much higher aggregation number in its micelks’ (> 10)) than cationic surfactants (10-100) evidently stabilizes the CTAB and CPB systems. by inclusion of the complex species in the interior of the micelks. Determination CJ~ lead in brass
The method was tested for the analysis of brass samples after separation of the lead on calcium carbonate as described elsewhere.5 Systematic studies revealed that lead can be selectively sep?rated from iron, zinc and copper, if the precipitation is done in
SHORT
Table 1. Analysis of brass samples
Sample
I
Lead found. “A Atomic Proposed method* absorption 2.80
Nil
2 2 with added 2 with added
O.lZS’& lead 0.629; lead
* Volume taken
441
COMMUNICATIONS
2.85 (0.2 ml) 2.77 (0.4 ml) 2.82 (1 ml) Nil O.IZI (5 ml) 0.12(10.0ml) O-60 (2.5 ml) 0.61 (5.0 ml)
a standard flask. treating 0.2-10ml of the solution with 2 ml of calcium solution (prepared by dissolving 2g of calcium carbonate in the minimum amount of hydrochloric acid and diluting to 50ml with water) followed by 2 ml of 1004 triethanolamine solution, sufficient 2M ammonia to raise the pH to c 9, and 2 ml of loo/, sodium carbonate solution (with stirring). the suspension obtained then being centrifuged and the supernatant liquid discarded, the precipitate washed twice with water and finally dissolved in dilute nitric acid. The results agree with those obtained by atomic absorption spectrometry.
for analysis is shown in parentheses.
the presence of triethanolamine. Such an approach is simpler in that it does not require subsequent extractive separation of lead from interfering elements that accompany it during the co-precipitation step. However, a slight decrease in the recovery of lead in presence of excess of calcium necessitates the preparation of a calibration graph with the collection procedure applied to lead standards. Table 1 gives the results for two solutions prepared by dissolving 1 g of sample in 10 ml of nitric acid, filtering off the metastannic acid, diluting to 250 ml in
Acknowledgement--One of us (TPR) is grateful to CSIR. New Delhi. for financial assistance. REFERENCES I. E. B. Sandell, Colorimerric Determination
of
Trace
3rd Ed., p. 568. Interscience. New York, 1959. 2. F. H. Pollard, P. Hanson and W. J. Geary. Anal. Chim. Metals,
Acta. 1959.20.26. 3. S. C. Dhupar, K. C. Srivastava and S. K. Banerji. J. Chim. Chem. Sot. Taipei, 1973.20, 145. 4. L. R. Fisher and D. G. Oakenfull. C/tern. Sot. Rev.,
5.
1977,6. No. 1. R. M. Dagnall. T. S. West and P. Young, 12. 583. 589.
Talanta,
1965,