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Derivative spectrophotometric determination of mercury(II) with pan in the aqueous phase
~39-9~~/89
Tahma, Vol. 36, No. 4, pp. 457-461, 1989
$3.00 + 0.00
Pergamon Press plc
Printed in Great Britain
DERIVATIVE SPE~ROPHOTOMETRIC DETERMINATION OF MERCURY(I1) WITH PAN IN THE AQUEOUS PHASE RATTAN LAL SHARMAand WAR BHAJAN SINGH* Department of Chemistry, University of Delhi, Delhi-110007, India (Received 25 July 1985. Revised 10 Aprii 1988. Accepted 14 November 1988) Stmunary-The Hg-PAN complex can be ma& soluble in water by addition of surfactant, and this can be made the basis of a spectrophotometric determination of Hg at ppm level. The selectivity and sensitivity can be improved by use of derivative spectrometry. The method has been applied to mercury-containing pesticides.
PAN [l-(2-p~dylazo)-2-naphthol] forms coloured water-insoluble complexes with a large number of metal ions,’ and these are suitable for extractive spectrophotometric determination. Mercury(H) forms an orange-red complex with PAN which has been made the basis of a number of titrimetric2*3 and spectrophotometric4-7 determinations of mercury. The composition of the complex formed is variously reported as ML and ML,, and the existence of a mixture has been suggested.s In the present work the PAN-Hg system has been reinvestigated, with PAN and the PAN-Hg complex made water-soluble by use of a cationic surfactant. Derivative spectrophotometry was used to improve the sensitivity and
solution of the set containing the metal ion. The pH was adjusted in the range 2.0-12.5 with sodium hydroxide or hydrochloric acid, then the solutions were made up accurately to 10.0 ml with water and the spectra were recorded. Beer’s law study. Two sets of solutions were prepared with increasing amounts of mercury(H) added to solutions containing either 5.0 ml of 2 x 10v4M PAN, 1.0 ml of 0.01.84 CTMAB and 0.5 ml of 10% ammonium acetate solution, or 5.0 ml of 5.0 x 10W4MPAN. 2.0 ml of O.OlM CTMAB and 0.5 ml of 10% ammonium’acetate solution. The pH and volume of each solution were adjusted to the optimum values @H 9.0 and 10.0 ml respectively) and the spectra recorded. Study o~compositio~, interference etc. Sets of test solutions were prepared with metal #n~ntrations in the linear response range, except when lower or higher ligand concentration was required to complete the set. Other variables such as pH and volume were kept constant.
EXPERIMENTAL RESULTS AND DISCUSSION
intents Absorption spectra were recorded witb a Sack UV-260 recording spectrophotometer witb a 1 nm bandwidth and lO-mm matched silica cells. First and second order derivatives were recorded with AI = 2 and 4 nm respectively. Sohctions A stock O.OfM solution of mercuric chloride was prepared in O.lM hydrochloric acid and diluted as required. A 3.0 x 10m4M solution of PAN was prepared daily by diss&inn 0.01246 a of the reagent in 15.0 ml of concentrated hydro&loric acid and diluthg with water to 100 ml. A O.OlM solution of cetyltrimethylammonium bromide (CTMAB) was prepared by dissolving the required quantity in the appropriate volmne. of hot water. Since the solution bezomes hazy at temperatures below 20”, it was heated to 30” before use. Solutions of metal salts and auxiliary reagents were prepared from suitable analytical grade chemicals.
E$ecr of pH
The absorption a pH-independent the pH-dependent
spectra of the complexes showed peak at 555 nm in addition to ligand
peak at around
400 nm.
The difference in absorbance at 555 nm for the two sets of solutions was plotted against PH. The pH of maximum complex formation was thus found to be 9.0. Subsequent studies were, therefore, made at this pH. E$Tect of surfactant concentration
This was determined by measuring the absorbance, at 555 nm, of a set of solutions containing increasing amounts of CTMAB (1 x 10ms-2 x 10m3M), and fixed amounts of metal ion, PAN and ammonium Procedures acetate. The precipitate appearing in solutions constudy of pH effect. Two sets of solutions, one of which taining an inadequate amount of CTMAB was discontained the metal ion, were prepared. Each solution in a carded and the spectra of the su~matant liquids set contained 1.0 ml of 5.0 x IO-‘M PAN, LO ml of O.OIM were recorded. The absorbance increased sharply CIMAB and 0.25 ml of 10% ammonium acetate solution; with increasing [CTMAB] up to 0.8 x 10W3M and 0.1 ml of 1 x 10-3Mmercury(II) solution was added to each decreased slightly for [CTMAB] > 1.4 x lo-“M. In subsequent studies the concentration of CTMAB was kept close to the CMC (critical micelle con*Author for correspondence. 457
RATTAN LAL Cont.
= K * ABS. t B
K
= 9.9159
B
= 1.0420
and HAR BIWAN
SW
SINGH
2.0
R M2 = 0.9993
250
I
I
I
300
400
500
0 600
Wavelength
Fig. 1. Spectra of Hg(II)-PAN-CTMAB
centration,
i.e., 1.3 x 10-3M)9
to
obtain
system: 1, reagent blank, 2-10, increasing Hg(I1) concentration.
maximal
absorbance.
shown in Fig. 3. The various heights were found to be linearly related to metal ion concentration.
Eflect of PAN concentration
Composition of the complex
In the set of solutions containing 2.5 x 10m4M The mole ratio and Job methods both indicated a 1: 1 metal: PAN ratio in the complex. The following PAN and 2.0 x IO-‘A4 CTMAB, a fine precipitate appeared about 45 min after sample preparation but observations regarding the complex were made. (a) when 9 x 10T5M mercury(I1) was also present no The complex is stabilized only by a cationic surfacprecipitation occurred even after 3 hr. This suggests tant and not by a neutral one [such as Triton X-100 that the complex is more soluble than PAN in the or poly(ethylene glycol) 40001, indicating that it is anionic in nature. (b) The complex is resistant to micelles. At lower PAN (1.0 x 10w4M) and CTMAB addition of a second PAN ion even at high PAN (1 x lo-“M) concentration, the solutions were stable concentration, which suggests either that the free for longer and no precipitation took place even after 24 hr. The PAN concentration was kept at ligand is not able to react with the 1: 1 complex or that any higher complex formed is unstable relative < 2.5 x lo-*M in further investigations. to the 1: 1 complex. (c) An increase in the CTMAB Eflect of metal ion concentration concentration above 1.4 x 10m3M results in a slow decrease in the absorbance of the system. In the Beer’s law study, there was significant On the basis of these observations we suggest curvature of the calibration plot at the higher that a negatively charged mixed-ligand complex metal ion concentrations when the overall PAN of 6-co-ordinate mercury(H) is formed, of the type concentration was 1 x 10p4M. Increasing the PAN [HgPANX,12-. A calculation of [HgX,]/[Hg*+] concentration to 2.5 x 10m4M gave linear response for mercury(I1) in the range 2 x 10e6-9 x 10e5M, but ratios for the existing complexing anions in the there was a positive deviation in absorbance at solution, with due consideration of their concentration, showed X to be chloride. We also consider mercury levels < 2 x 10e6M. The molar absorptivity at 555 nm was found to be 2.03 x lo4 l.mole-i. cm-i. that the naphthalene moiety of the complex is held in The spectra are shown in Fig. 1. the micelle (Fig. 4) and that formation of a 1:2 First- and second-order derivative spectra of the metal: PAN complex is prevented by electrostatic test solutions were recorded (Fig. 2), and measured as repulsion between micelles containing PAN and
Derivative spectrophotometric (A)
determination
459
of mercury(H)
(B)
10 9 6 7 6 5 4 z 1
104 Wavelength
Wavelength
in nm
in nm
Fig. 2. First (A) and second (B) order derivatives of spectra in Fig. 1.
Hrlght mrosurrd from thin point
(A)
Flrlt
ordw d.rivatlv.
rp.etra
(8)
Secondorderdrrlvotivr
IpOCtrO
Fig. 3. Measurement of (A) first and (B) second order derivative spectra. In (A) the vertical distance from the crossover point to the trough is measured. In (B) a line is drawn between the crossover points and the vertical height from this line to the trough is measured.
460
RATTAN LAL SHARMAand
HAR BHNAN SINGH
those containing the complex [HgPANXJ’-. This suggested orientation of PAN in the micelle allows interaction with mercury(I1) ions in the bulk aqueous phase but inhibits its interaction with the 1: 1 PAN:Hg complex. To obtain better insight into observation (c) above, cetyltrimethylammonium nitrate was used and the effect of adding increasing concentrations of Cl-, Br- and acetate was noted. These anions show a similar effect, indicating this to be a consequence of competing equilibria between the principal ligand (PAN) and the auxiliary ligand(s) (Cl-, Br- and/or acetate). Stability constant of the complex The conditional stability constant was calculated on the basis of the following equilibrium, paying due attention to the existence of the metal ion in various forms with auxiliary ligands and of the ligand in different protonated forms: [HgClJ2- + PAN- +[HgCl,PAN]*-
+ Cl-
The average of eight values (20”; Z = 2) was found to be 3.5 x 10’ with s.d. 1.2 x 10’. MICELLE 1 containing
1 :l
I complex
Interference by foreign ions
1
The effect of cations and anions was investigated. Serious interference was caused by Zn2+, Cd2+, In”+, Ga3+, T13+, Ni2+, Mn2+, Cu*+, Fe’+, Fe2+ and others forming stable PAN complexes. The interference due to Zn2+, Cd2+ and Ni2+ could be deduced from the
Fig. 4. Proposed reaction mechanism.
600 Spectra
500
6
6 First
derivative
Fig. 5. Ordinary and first and second order derivative spectra of the Hg (-), Ni (. . .) PAN-CTMAB systems.
Second
derivative
Zn (---),
Cd (-.-)
and
Derivative spectrophotometric determination of mercury(H) first- and second-order derivative spectra for these three metal ions (Fig. 5). Mercury could be determined in the presence of these metal ions by means of the derivative spectra, but the error was rather high. Of the anions investigated, cyanide, EDTA and iodide interfered seriously. Alkali and alkaline-earth metal ions and lead did not interfere. The cobalt complex is precipitated quite soon on standing. This precipitate can be removed by centrifugation, and the absorbance of the supematant liquid measured for determining mercury in presence of cobalt. Application The mercury content of a mercury-based pesticide (EMISAN 6), containing 6% mercury was determined by this method. The relative deviation was < 1% for 10 determinations.
Acknowledgement-The authors are thankful to BP R & D
461
for financial assistance to RLS during this work, and also to the USIC staff for their co-operation.
REFERENCES 1. R. G. Anderson and G. Nickless, Analyst, 1967, -207. 2. Ming-Lien Lu, Ta-Chun Liu and Chih-Ling Ching, Acta Pharm. Sinica, 1963, 10, 436; Chem. Abstr., 1963,
59, 1333511. 3. H. Plasohka and H. Abdine, Chemist-Analyst, 1956,4!( 58. 4. Li-Shuh Ho, Ching-Nan Kuo, Chih-Sheng Shih and Wu Chiang, Chem. Bull. Peking, 1965, 250; Chem. Abstr., 1965, 63, 17135f. 5. S. Shibata, Anal. Chim. Acta, 1960, 23, 367. 6. I&m, ibid., 1961, 25, 348, 7. Li-Shuh Ho, Chih-Sheng Shih and Wu Chiang, Chem. Bull. Peking, 1965,253; Chem. Abstr., 1965,63, 15547a. 8. J. Ciba, M. Langova and L. KubiEkovl, Collection Czech. Gem. Commun., 1973, 38, 3405. 9. L. J. Cline Love, J. G. Habarta and J. G. Dorsey, Anal. Chem., 1984, Sa, 1132A.
Tahma, Vol. 36, No. 4, pp. 457-461, 1989
$3.00 + 0.00
Pergamon Press plc
Printed in Great Britain
DERIVATIVE SPE~ROPHOTOMETRIC DETERMINATION OF MERCURY(I1) WITH PAN IN THE AQUEOUS PHASE RATTAN LAL SHARMAand WAR BHAJAN SINGH* Department of Chemistry, University of Delhi, Delhi-110007, India (Received 25 July 1985. Revised 10 Aprii 1988. Accepted 14 November 1988) Stmunary-The Hg-PAN complex can be ma& soluble in water by addition of surfactant, and this can be made the basis of a spectrophotometric determination of Hg at ppm level. The selectivity and sensitivity can be improved by use of derivative spectrometry. The method has been applied to mercury-containing pesticides.
PAN [l-(2-p~dylazo)-2-naphthol] forms coloured water-insoluble complexes with a large number of metal ions,’ and these are suitable for extractive spectrophotometric determination. Mercury(H) forms an orange-red complex with PAN which has been made the basis of a number of titrimetric2*3 and spectrophotometric4-7 determinations of mercury. The composition of the complex formed is variously reported as ML and ML,, and the existence of a mixture has been suggested.s In the present work the PAN-Hg system has been reinvestigated, with PAN and the PAN-Hg complex made water-soluble by use of a cationic surfactant. Derivative spectrophotometry was used to improve the sensitivity and
solution of the set containing the metal ion. The pH was adjusted in the range 2.0-12.5 with sodium hydroxide or hydrochloric acid, then the solutions were made up accurately to 10.0 ml with water and the spectra were recorded. Beer’s law study. Two sets of solutions were prepared with increasing amounts of mercury(H) added to solutions containing either 5.0 ml of 2 x 10v4M PAN, 1.0 ml of 0.01.84 CTMAB and 0.5 ml of 10% ammonium acetate solution, or 5.0 ml of 5.0 x 10W4MPAN. 2.0 ml of O.OlM CTMAB and 0.5 ml of 10% ammonium’acetate solution. The pH and volume of each solution were adjusted to the optimum values @H 9.0 and 10.0 ml respectively) and the spectra recorded. Study o~compositio~, interference etc. Sets of test solutions were prepared with metal #n~ntrations in the linear response range, except when lower or higher ligand concentration was required to complete the set. Other variables such as pH and volume were kept constant.
EXPERIMENTAL RESULTS AND DISCUSSION
intents Absorption spectra were recorded witb a Sack UV-260 recording spectrophotometer witb a 1 nm bandwidth and lO-mm matched silica cells. First and second order derivatives were recorded with AI = 2 and 4 nm respectively. Sohctions A stock O.OfM solution of mercuric chloride was prepared in O.lM hydrochloric acid and diluted as required. A 3.0 x 10m4M solution of PAN was prepared daily by diss&inn 0.01246 a of the reagent in 15.0 ml of concentrated hydro&loric acid and diluthg with water to 100 ml. A O.OlM solution of cetyltrimethylammonium bromide (CTMAB) was prepared by dissolving the required quantity in the appropriate volmne. of hot water. Since the solution bezomes hazy at temperatures below 20”, it was heated to 30” before use. Solutions of metal salts and auxiliary reagents were prepared from suitable analytical grade chemicals.
E$ecr of pH
The absorption a pH-independent the pH-dependent
spectra of the complexes showed peak at 555 nm in addition to ligand
peak at around
400 nm.
The difference in absorbance at 555 nm for the two sets of solutions was plotted against PH. The pH of maximum complex formation was thus found to be 9.0. Subsequent studies were, therefore, made at this pH. E$Tect of surfactant concentration
This was determined by measuring the absorbance, at 555 nm, of a set of solutions containing increasing amounts of CTMAB (1 x 10ms-2 x 10m3M), and fixed amounts of metal ion, PAN and ammonium Procedures acetate. The precipitate appearing in solutions constudy of pH effect. Two sets of solutions, one of which taining an inadequate amount of CTMAB was discontained the metal ion, were prepared. Each solution in a carded and the spectra of the su~matant liquids set contained 1.0 ml of 5.0 x IO-‘M PAN, LO ml of O.OIM were recorded. The absorbance increased sharply CIMAB and 0.25 ml of 10% ammonium acetate solution; with increasing [CTMAB] up to 0.8 x 10W3M and 0.1 ml of 1 x 10-3Mmercury(II) solution was added to each decreased slightly for [CTMAB] > 1.4 x lo-“M. In subsequent studies the concentration of CTMAB was kept close to the CMC (critical micelle con*Author for correspondence. 457
RATTAN LAL Cont.
= K * ABS. t B
K
= 9.9159
B
= 1.0420
and HAR BIWAN
SW
SINGH
2.0
R M2 = 0.9993
250
I
I
I
300
400
500
0 600
Wavelength
Fig. 1. Spectra of Hg(II)-PAN-CTMAB
centration,
i.e., 1.3 x 10-3M)9
to
obtain
system: 1, reagent blank, 2-10, increasing Hg(I1) concentration.
maximal
absorbance.
shown in Fig. 3. The various heights were found to be linearly related to metal ion concentration.
Eflect of PAN concentration
Composition of the complex
In the set of solutions containing 2.5 x 10m4M The mole ratio and Job methods both indicated a 1: 1 metal: PAN ratio in the complex. The following PAN and 2.0 x IO-‘A4 CTMAB, a fine precipitate appeared about 45 min after sample preparation but observations regarding the complex were made. (a) when 9 x 10T5M mercury(I1) was also present no The complex is stabilized only by a cationic surfacprecipitation occurred even after 3 hr. This suggests tant and not by a neutral one [such as Triton X-100 that the complex is more soluble than PAN in the or poly(ethylene glycol) 40001, indicating that it is anionic in nature. (b) The complex is resistant to micelles. At lower PAN (1.0 x 10w4M) and CTMAB addition of a second PAN ion even at high PAN (1 x lo-“M) concentration, the solutions were stable concentration, which suggests either that the free for longer and no precipitation took place even after 24 hr. The PAN concentration was kept at ligand is not able to react with the 1: 1 complex or that any higher complex formed is unstable relative < 2.5 x lo-*M in further investigations. to the 1: 1 complex. (c) An increase in the CTMAB Eflect of metal ion concentration concentration above 1.4 x 10m3M results in a slow decrease in the absorbance of the system. In the Beer’s law study, there was significant On the basis of these observations we suggest curvature of the calibration plot at the higher that a negatively charged mixed-ligand complex metal ion concentrations when the overall PAN of 6-co-ordinate mercury(H) is formed, of the type concentration was 1 x 10p4M. Increasing the PAN [HgPANX,12-. A calculation of [HgX,]/[Hg*+] concentration to 2.5 x 10m4M gave linear response for mercury(I1) in the range 2 x 10e6-9 x 10e5M, but ratios for the existing complexing anions in the there was a positive deviation in absorbance at solution, with due consideration of their concentration, showed X to be chloride. We also consider mercury levels < 2 x 10e6M. The molar absorptivity at 555 nm was found to be 2.03 x lo4 l.mole-i. cm-i. that the naphthalene moiety of the complex is held in The spectra are shown in Fig. 1. the micelle (Fig. 4) and that formation of a 1:2 First- and second-order derivative spectra of the metal: PAN complex is prevented by electrostatic test solutions were recorded (Fig. 2), and measured as repulsion between micelles containing PAN and
Derivative spectrophotometric (A)
determination
459
of mercury(H)
(B)
10 9 6 7 6 5 4 z 1
104 Wavelength
Wavelength
in nm
in nm
Fig. 2. First (A) and second (B) order derivatives of spectra in Fig. 1.
Hrlght mrosurrd from thin point
(A)
Flrlt
ordw d.rivatlv.
rp.etra
(8)
Secondorderdrrlvotivr
IpOCtrO
Fig. 3. Measurement of (A) first and (B) second order derivative spectra. In (A) the vertical distance from the crossover point to the trough is measured. In (B) a line is drawn between the crossover points and the vertical height from this line to the trough is measured.
460
RATTAN LAL SHARMAand
HAR BHNAN SINGH
those containing the complex [HgPANXJ’-. This suggested orientation of PAN in the micelle allows interaction with mercury(I1) ions in the bulk aqueous phase but inhibits its interaction with the 1: 1 PAN:Hg complex. To obtain better insight into observation (c) above, cetyltrimethylammonium nitrate was used and the effect of adding increasing concentrations of Cl-, Br- and acetate was noted. These anions show a similar effect, indicating this to be a consequence of competing equilibria between the principal ligand (PAN) and the auxiliary ligand(s) (Cl-, Br- and/or acetate). Stability constant of the complex The conditional stability constant was calculated on the basis of the following equilibrium, paying due attention to the existence of the metal ion in various forms with auxiliary ligands and of the ligand in different protonated forms: [HgClJ2- + PAN- +[HgCl,PAN]*-
+ Cl-
The average of eight values (20”; Z = 2) was found to be 3.5 x 10’ with s.d. 1.2 x 10’. MICELLE 1 containing
1 :l
I complex
Interference by foreign ions
1
The effect of cations and anions was investigated. Serious interference was caused by Zn2+, Cd2+, In”+, Ga3+, T13+, Ni2+, Mn2+, Cu*+, Fe’+, Fe2+ and others forming stable PAN complexes. The interference due to Zn2+, Cd2+ and Ni2+ could be deduced from the
Fig. 4. Proposed reaction mechanism.
600 Spectra
500
6
6 First
derivative
Fig. 5. Ordinary and first and second order derivative spectra of the Hg (-), Ni (. . .) PAN-CTMAB systems.
Second
derivative
Zn (---),
Cd (-.-)
and
Derivative spectrophotometric determination of mercury(H) first- and second-order derivative spectra for these three metal ions (Fig. 5). Mercury could be determined in the presence of these metal ions by means of the derivative spectra, but the error was rather high. Of the anions investigated, cyanide, EDTA and iodide interfered seriously. Alkali and alkaline-earth metal ions and lead did not interfere. The cobalt complex is precipitated quite soon on standing. This precipitate can be removed by centrifugation, and the absorbance of the supematant liquid measured for determining mercury in presence of cobalt. Application The mercury content of a mercury-based pesticide (EMISAN 6), containing 6% mercury was determined by this method. The relative deviation was < 1% for 10 determinations.
Acknowledgement-The authors are thankful to BP R & D
461
for financial assistance to RLS during this work, and also to the USIC staff for their co-operation.
REFERENCES 1. R. G. Anderson and G. Nickless, Analyst, 1967, -207. 2. Ming-Lien Lu, Ta-Chun Liu and Chih-Ling Ching, Acta Pharm. Sinica, 1963, 10, 436; Chem. Abstr., 1963,
59, 1333511. 3. H. Plasohka and H. Abdine, Chemist-Analyst, 1956,4!( 58. 4. Li-Shuh Ho, Ching-Nan Kuo, Chih-Sheng Shih and Wu Chiang, Chem. Bull. Peking, 1965, 250; Chem. Abstr., 1965, 63, 17135f. 5. S. Shibata, Anal. Chim. Acta, 1960, 23, 367. 6. I&m, ibid., 1961, 25, 348, 7. Li-Shuh Ho, Chih-Sheng Shih and Wu Chiang, Chem. Bull. Peking, 1965,253; Chem. Abstr., 1965,63, 15547a. 8. J. Ciba, M. Langova and L. KubiEkovl, Collection Czech. Gem. Commun., 1973, 38, 3405. 9. L. J. Cline Love, J. G. Habarta and J. G. Dorsey, Anal. Chem., 1984, Sa, 1132A.