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CHAPTER – III SYNTHESIS, SPECTRAL DATA AND SPECTROPHOTOMETRIC
STUDIES ON o-VANILLIDINE-2-AMINOBENZOTHIAZOLE, o-VANILLIDINE-2-AMINO-4-METHYL BENZOTHIAZOLE,
o-VANILLIDINE-2-AMINO-6-CHLORO BENZOTHIAZOLE AND o-VANILLIDINE-2-AMINO-6-BROMO BENZOTHIAZOLE FOR THE
DETERMINATION OF COPPER
3.1 INTRODUCTION Organic reagents have many applications in chemical analysis. They are
useful in gravimetric, volumetric, spectrophotometric and other instrumental
analysis. Unquestionably scientists constantly look for new, better, simpler,
sensitive and selective reagents. An effort towards developing simpler and more
selective organic reagents was made by preparing several Schiff’s bases by the
condensation reaction between 2-amino benzothiazole and its 4-methyl, 6-chloro,
6-bromo derivatives with o-Vanillin.
The present work was aimed to develop a sensitive and efficient
spectrophotometric method for the determination of Copper, based on
formation of coloured complexes, which are formed by the reaction of the Copper
ion with o-Vanillidine-2-amino-benzothiazole (VBT), o-Vanillidine-2-amino-4-
methyl benzothiazole (VMBT), o-Vanillidine-2-amino-6-chloro benzothiazole
(VCBT) and o-Vanillidine-2-amino-6-bromo benzothiazole (VBBT).
3.2 MATERIALS AND METHODS 3.2.1 Instruments used Infrared Spectra were recorded on KBr disc on a Perkin Elmer FT-IR. 1H NMR spectra were recorded on a Bruker Advanced 600 MHz spectrometer in
CDCl3 and thermo DFS double focusing mass spectrophotometer for mass
spectral measurements. The pH was determined with a model 7007 Digisun
electronic pH meter with combined glass calomel electrode. Graphite furnace
Atomic Absorption Spectrometer was used for the analysis of cations in water
56
samples. Shimadzu UV double beam spectrophotometer was used for
spectrophotometric measurements. 3.2.2 Reagents required All the reagents were of analytical grade and the solutions were prepared
with doubly distilled water. Schiff’s base solutions (0.05% (w/v) were prepared by
dissolving appropriate amounts of VBT, VMBT, VCBT and VBBT in methanol.
o-Vanillin was purchased from Sigma Aldrich (Germany), 0.2 M acetic acid and
0.2 M sodium acetates were used for pH adjustments. Acetone and ethanol were
dried over anhydrous calcium sulphate and calcium oxide respectively and
distilled for getting pure solvents. 2-amino benzothiazole and its 4-methyl,
6-chloro, 6-bromo derivatives were synthesized starting from respective
α-phenylthiourea.
3.3 EXPERIMENTAL PROCEDURES
3.3.1 Synthesis of α-phenylthiourea
In a 500 mL three necked flask fitted with a reflux condenser, a
mechanical stirrer and 100 mL dropping funnel were placed 17 g (0.22 mol) of
ammonium thiocyanate and 100 mL of dry acetone through the dropping funnel
along with stirring, 28.2 g (0.2 mol) of benzoyl chloride. After the complete
addition, the mixture was refluxed for 5 minutes and then a solution of 18.6 g
(0.2 mol) of aniline in 50 mL dry acetone was added at such a rate that, reaction
mixture starts refluxing gently. The mixture was poured carefully with stirring into
1.5 L of water. The yellow precipitate (α- benzoyl-β-phenyl thiourea) obtained
was separated by filtration. The precipitate was dissolved in NaOH (30 g of
NaOH in 270 mL of water) and heated for 5 minutes. After the removal of a small
amount of insoluble material by filtration, the solution was acidified with
Conc. HCl and then made slightly basic with ammonium hydroxide. On allowing
the solution to stand for some time, the crystalline product of α-phenylthiourea
obtained was filtered off. The yield of oven-dried material (m.p 153°C) was 25.8 g
57
(85%). Recrystallization from ethanol yielded 23.2 g of white crystalline solid,
melting at 152-153°C. [Snyder H.R., 1948]
NH4SCN+ NH4Cl
NaOH
Ammonium thiocyanate
+
N
S
H2N
CO N
CS
H
HN
O
NCSbenzoyl isothiocyanate
O
ClBenzoyl chloride
NH2
Aniline
Hα-phenylthiourea
C6H5COOH +Benzoic acid
3.3.2 Synthesis of 2-amino benzothiazole
In a 250 mL R.B flask, 27.6 g (0.2 mol) of α-phenylthiourea was added
with stirring to 72 mL of thionyl chloride (0.56 mol). Refluxed the reaction mixture
for 2-4 hours at 50-55°C. After 4 hours, the reaction mixture, which was viscous,
poured into 2 L of water and stirred for 30 minutes at 60-80°C and filtered.
The filtrate was adjusted to pH 8 with 25% ammonia and the crystals of
2-aminobenzothiazole precipitated out was filtered by suction, washed with water
and dried (m.p 127-128°C). Recrystallization from hot water yielded 16 g of white
flakes melting at 130°C [Papenfuhs Von Theodor, 1982].
(i) SOCl2 50-550C(ii) Hydrolysis
S
N
NH2
2-aminobenzothiazole
NH
S
NH2
α-phenylthiourea
58
3.3.3 Synthesis of 1-(2-methylphenyl) thiourea Similar procedure was adopted as described earlier for the synthesis of
α-phenylthiourea except that, instead of 18.6 g (0.2 mol) of aniline in 50 mL dry
acetone, solution of 21.5 mL (0.2 mol) o-toluidine in 50 mL dry acetone was
used. The crystalline product of 1-(2-methylphenyl) thiourea obtained was filtered
and dried. The yield of oven-dried material was 18.5 g. Recrystallization from
ethanol yielded 15.3 g of white crystalline solid, melting at 150°C [Snyder H.R.,
1948].
NH4SCN + NH4Cl
NaOH
Ammonium thiocyanate
+
CO N
CS
H
HN
O
NCSbenzoyl isothiocyanate
O
ClBenzoyl chloride
NH2
CH3
o-toluidine
H3C
HNS
NH2
H3C
1-(2-methylphenyl) thiourea
C6H5COOH +
Benzoic acid
3.3.4 Synthesis of 2-amino-4-methylbenzothiazole In a 250 mL R.B flask, 29.0 g (0.2 mol) of 1-(2-methyl phenyl) thiourea
was added with stirring to 72 mL of thionyl chloride (0.56 mol). Refluxed the
reaction mixture for 2-4 hours at 50-55°C. After 4 hours, the reaction mixture,
which was usually viscous was poured into 2 L of water and stirred for
30 minutes at 60-80°C and filtered. The filtrate was adjusted to pH 8 with 25%
ammonia; 2-amino-4-methyl benzothiazole precipitated out was filtered by
59
suction, washed with water and dried (m.p 132°C). Recrystallization from hot
water yielded 14 g of white flakes melting at 137°C [Papenfuhs Von Theodor,
1982].
1-(2-methylphenyl) thiourea
HNS
NH2
H3C
(i) SOCl2 50-550C
(ii) HydrolysisS
N
NH2
2-amino-4-methylbenzothiazole
CH3
3.3.5 Synthesis of 1-(4-chloro phenyl) thiourea
1-(4-chloro phenyl) thiourea was prepared similarly as described earlier for
the synthesis of α-phenylthiourea except that, instead of 18.6 g (0.2 mol) of
aniline in 50 mL dry acetone, a solution of 26.0 g (0.2 mol) of p-chloro aniline in
50 mL dry acetone was used. The crystalline product of 1-(4-chloro phenyl)
thiourea obtained was filtered and dried. The yield of oven-dried material was
12.5 g. Recrystallization from ethanol yielded 11.0 g of white crystalline solid,
melting at 152-153°C [Snyder H.R., 1948].
C6H5COOH +
Benzoic acid
NH4SCN + NH4Cl
NaOH
+
NH
S
H2N
Cl CO N
CS
Cl
H
HN
O
NCSbenzoyl isothiocyanate
O
ClBenzoyl chloride
NH2
Cl
p-chloroaniline
1-(4-chlorophenyl)thiourea
Ammoniumthiocyanate
60
3.3.6 Synthesis of 2-amino-6-chloro benzothiazole
10 g of 4-chloro phenyl thiourea was dissolved at 20°C in 15 mL of H2SO4
(98%). In the course of 3 hour durations, 1 mL of HBr solution (48% strength)
was added with 30 minutes intervals. The temperature of 45-50°C was
maintained for 1.5 hours, then 65 to 70°C for 6 hours. The solution was cooled to
20°C and 25 mL of methanol was added. The temperature rose to approximately
70°C. The mixture was then cooled to 20°C and filtered using suction. The
filtered residue was washed three times with 15 mL acetone and dried in air.
9 g of 2-amino-6-chloro-benzothiazole sulphate obtained was placed in a solution
of sodium hydroxide (10 g of NaOH in 15 mL of water) and was stirred for
one hour at 70°C. The mixture was then filtered off using suction and the residue
was washed with water and dried. White solid of 2-amino 6-chlorobenzothiazole
melting at 199-201°C was obtained [Steffen Dapperheld, 1994].
NH
S
H2N
Cl
1-(4-chloro phenyl)thiourea.
S
N
NH2
2-amino-6-chloro benzothiazole
Cl(i)Conc.H2SO4 (ii) HBr
(iii)NaOH
3.3.7 Synthesis of 1-(4-bromo phenyl) thiourea
1-(4-bromophenyl) thiourea was also prepared by the similar procedure
described earlier for synthesis of α-phenylthiourea except that, instead of
18.6 g (0.2 mol) of aniline in 50 mL dry acetone, a solution of 26 g (0.2 mol) of
p-chloro aniline in 50 mL acetone was used. The crystalline product of 1-(4-
bromo phenyl) thiourea obtained was filtered and dried. The yield of oven-dried
material was 12 g. Recrystallization from ethanol yielded 11.5 g of white
crystalline solid, melting at 183-184°C [Snyder H.R., 1948].
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C6H5COOH +
Benzoic acid
NH4SCN + NH4Cl
NaOH
Ammonium thiocyanate
+
NH
S
H2N
Br CO N
CS
Br
H
HN
O
NCSbenzoyl isothiocyanate
O
ClBenzoyl chloride
NH2
Br
p-bromo aniline
1-(4-bromo phenyl)thiourea 3.3.8 Synthesis of 2-amino-6-bromo benzothiazole
4-bromo phenyl thiourea (10 g) was dissolved at 20°C in 15 mL of H2SO4
(98%). In the course of 3 hours duration, 1 mL of HBr solution (48% strength)
was added each time at 30 minutes intervals. The temperature was maintained
at 45°C to 50°C for 1.5 hours and then 65 to 70° C for 6 hours at. The solution
was cooled to 20°C and 25 mL of methanol was added. The temperature rose to
approximately 70°C. The mixture was then cooled to 20°C and filtered using
suction. The filtered residue was washed thrice with 15 mL acetone and dried in
air. 2-amino-6-bromo benzothiazole sulphate (9 g) obtained was placed in a
solution of sodium hydroxide (10 g of NaOH in 15 mL of water) and was stirred
for one hour at 70°C. The mixture was then filtered off using suction and the
residue was washed with water and dried. The white solid crystalls of 2-amino-6-
bromo benzothiazole with melting point of 213-217°C was obtained [Steffen
Dapperheld 1994].
NH
S
H2N
Br
1-(4-bromo phenyl)thiourea
S
N
NH2
2-amino-6-bromo benzothiazole
Br
(i)Conc.H2SO4 (ii) HBr(iii)NaOH
62
3.3.9 Synthesis of o-Vanillidine-2-amino benzothiazole Schiff’s Base (VBT) 2-amino benzothiazole (1.5 g), was dissolved in absolute ethanol (15 mL)
and then o-Vanillin (1.10 g) was added to the above solution along with a drop of
piperidine. The mixture was refluxed for 3 hours with stirring and kept aside for
eight hours. The yellowish orange solid obtained was filtered and washed several
times with cold ethanol, dried and recrystallized from carbon tetrachloride to yield
yellow crystalline solid.
Analysis of VBT C15H12N2O2S requires 63.36% C, 4.25% H and 9.85% N; found 63.36% C,
4.24% H and 9.84% N; yellow crystalline solid, m.p 164oC. IR (KBr, cm-1): 3432
(O-H), 1593(C=N), 1255 (C-O). 1H NMR (CDCl3, TMS,δ): 3.9 (s, -OCH3), 6.9
to8.0 (m, aromatic protons), 9.3 (s,-CH=N), 12.49(s,-OH). Mass spectrum M+ ion peak 284.
S
N
NHO OCH3
o-Vanillidine-2-amino benzothiazole (VBT)
3.3.10 Synthesis of Copper Complex of VBT Schiff’s Base Methanolic solution of VBT Schiff’s base (10 m mol) was mixed with
Copper acetate (5 m mol) in methanol (10 mL) solution keeping metal ligand ratio
1:2 with continuous stirring. The mixture was then refluxed for 2 hrs, and then
heated on a water bath till the volume of the solution was reduced to 10 mL. The
brown coloured VBT Copper complex precipitated was filtered and washed with
methanol and acetone. Recrystallized from chloroform and then dried in vacuum.
Analysis of VBT Copper Complex C30H22CuN4O4S2 requires 57.18% C, 3.52% H, 10.08% Cu and 8.89% N;
found 57.1% C, 3.5% H, 10.05% Cu and 8.89% N; brown solid, m.p above
260oC, ε.max: 2.87x103 L mol-1 cm-1 (methanol) at 450 nm. IR (KBr, cm-1): 1532
(C=N), 1339 (C-O). 1H NMR (CDCl3, TMS,δ): the signal at 12.49 for –OH is
63
absent in Copper complex and other signals are same as that of the Schiff’s
base(VBT). 3.3.11 Synthesis of o-Vanillidine-2-amino-4-methyl benzothiazole (VMBT) 2-amino-4-methyl benzothiazole (1.66 g) was dissolved in absolute
ethanol (15 mL) and then o-Vanillin (1.10 g) was added to the above solution
along with a drop of piperidine. The mixture was refluxed for 3 hours and kept for
eight hours. The yellowish orange solid obtained was filtered and washed several
times with cold ethanol, dried and recrystallized from carbon tetrachloride to yield
yellow crystalline solid.
Analysis of VMBT C16H14N2O2S requires 64.41%C, 4.73% and 9.39%N; found
64.48% C, 4.7% H and 9.36% N; yellow crystalline solid, m.p 176oC. IR (KBr,
cm-1): 3434 (O-H), 1596 (C=N), 1258 (C-O). 1H NMR (CDCl3, TMS,δ ): 2.76 (s,-
CH3), 3.96 (s,-OCH3), 6.8-7.4 (m, aromatic protons), 9.3 (s,-CH=N), 12.53 (s,-
OH). The –OH signal at 12.53 ppm disappeared upon the addition of D2O to the
solution, indicating that it is an acidic proton. Mass spectrum M+ ion
peak 298.
S
N
NHO OCH3
CH3
o-Vanillidine-2-amino-4-methyl benzothiazole (VMBT)
3.3.12 Synthesis of Copper Complex of VMBT Schiff’s Base Methanolic solution of VMBT Schiff’s base (10 mmol) was mixed with
Copper acetate (5 mmol) in methanol (10 mL) solution keeping metal ligand ratio
1:2 with continuous stirring. The mixture was then refluxed for 2 hrs, and then
heated on a water bath till the volume of the solution got reduced to 10 mL. The
brown coloured VMBT Copper complex precipitated was filtered and washed with
methanol and acetone. Recrystallized from chloroform and then dried in vacuum.
64
Analysis of VMBT Copper Complex C32H26CuN4O4S2 requires 58.39% C, 3.98% H, 9.65% Cu and 8.51% N;
found 58.30% C, 3.94% H, 9.6% Cu and 8.6% N; brown solid, m.p above 260oC, ε.max: 2.94 x103 L mol-1 cm-1 (methanol) at 450 nm. IR (KBr, cm-1): 1542 (C=N),
1375 (C-O). 1H NMR (CDCl3, TMS,δ): The signal at 12.53 for –OH was absent in
Copper complex and other signals were same as that of the Schiff’s base
(VMBT). Mass spectrum M+ ion peak 657. 3.3.13 Synthesis of o-Vanillidine-2-amino-6-chlorobenzothiazole Schiff’s
Base (VCBT) 2-amino-6-chloro benzothiazole (1.84 g, 10 mmol) was dissolved in
absolute ethanol (15 mL) and then o-Vanillin (1.10 g) was added to the above
solution along with a drop of piperidine. The mixture was refluxed for 3 hours with
stirring and kept aside for eight hours. The yellow solid obtained was filtered,
washed several times with cold ethanol, dried and recrystallized from carbon
tetrachloride to yield yellow crystalline solid.
Analysis of VCBT C15H11ClN2O2S requires 56.52% C, 3.48% H and 8.79% N; found 56.50%
C, 3.49% H and 8.79% N; yellow crystalline solid, m.p 224oC. IR (KBr, cm-1):
3440(O-H), 1596(C=N), 1247 (C-O). 1H NMR (CDCl3, TMS,δ): 3.9 (s,-OCH3), 6.9
to 8.0 (m, aromatic protons), 9.3 (s,-CH=N), 12.38 (s-OH). Mass spectrum M+
ion peak 318.
S
N
NHO OCH3
Cl
o-Vanillidine-2-amino-6-chlorobenzothiazole, (VCBT).
3.3.14 Synthesis of Copper Complex of VCBT Schiff’s Base Methanolic solution of VCBT Schiff’s base (10 mmol) was mixed with
Copper acetate (5 mmol) in methanol (10 mL) solution keeping metal ligand ratio
1:2 with continuous stirring. The mixture was then refluxed for 2 hrs, and then
65
heated on a water bath till the volume of the solution got reduced to 10 mL. The
brown coloured VCBT Copper complex precipitated was filtered and washed with
methanol and acetone. Recrystallized from chloroform and then dried in vacuum.
Analysis of VCBT Copper Complex C30H20 ClCu N4O4S2 requires 51.54% C, 2.88% H, 9.09% Cu and 8.01%
N; found 51.52% C, 2.86% H, 9.0% Cu and 8.0% N; brown solid, m.p above
260oC, ε.max: 2.2x103 L mol-1 cm-1 (methanol) at 450 nm. IR (KBr, cm-1):
1573 (C=N), 1304 (C-O). 1H NMR (CDCl3, TMS,δ): The signal at 12.38 for
–OH was absent in Copper complex and other signals were same as that of the
Schiff’s base
3.3.15 Synthesis of o-Vanillidine-2-amino-6-bromo benzothiazole Schiff’s
Base (VBBT) 2-amino-6-bromo benzothiazole (2.20 g) was dissolved in absolute
ethanol (15 mL) and then o-Vanillin (1.10 g) was added to the above solution
along with a drop of piperidine. The mixture was refluxed for 3 hours with stirring
and kept aside for eight hours. The yellow solid obtained was filtered, washed
several times with cold ethanol, dried and recrystallized from carbon tetrachloride
to yield yellow crystalline solid.
Analysis of VBBT C15H11BrN2O2S requires 49.60% C, 3.05% H and 7.71% N; found
49.58% C, 3.02% H and 7.74% N; yellow crystalline solid, m.p 234oC.
IR (KBr, cm-1): 3432(O-H) 1592 (C=N), 1259 (C-O). 1H NMR (CDCl3, TMS,
δ): 3.15(s,-OCH3), 6.8 to 8.0 (m, aromatic protons), 9.3(s,-CH=N), 12.38
(s,-OH). Mass spectrum M+ ion peak 364.
S
N
NHO OCH3
Br
o-Vanillidine-2-amino-6-bromo benzothiazole (VBBT)
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3.3.16 Synthesis of Copper Complex of VBBT Schiff’s Base Methanolic solution of VBBT Schiff’s base (10 mmol) was mixed with
Copper acetate (5 mmol) in methanol (10 mL) solution keeping metal ligand ratio
1:2 with continuous stirring. The mixture was then refluxed for 2 hrs, and then
heated on a water bath till the volume of the solution got reduced to 10 mL. The
brown coloured VBBT Copper complex precipitated was filtered and washed with
methanol and acetone. Recrystallized from chloroform and then dried in vacuum.
Analysis of VBBT Copper Complex C30H20BrCuN4O4S2 requires 45.73% C, 2.56% H, 8.06% Cu and
22.91% N; found 45.71% C, 2.55% H, 8.01% Cu and 22.89% N; brown solid,
m.p above 260oC, ε.max: 1.53x103 L mol-1 cm-1 (methanol) at 450 nm.
IR (KBr, cm-1): 1538 (C=N), 1304 (C-O). 1H NMR (CDCl3, TMS,δ): The signal at
12.38 for –OH was absent in Copper complex and other signals were same as
that of the Schiff’s base.
3.4 SPECTROPHOTOMETRIC DETERMINATION OF COPPER 3.4.1 Preparation of Copper (II) standard solution Copper (II) solution (1000 mg L-1) was prepared by dissolving a known
amount of Copper (II) sulphate penta hydrate in distilled water, adjusting the pH
of the solution to 1.0 with dilute sulphuric acid and diluting the solution to a liter
with distilled water. The concentration was determined by titrating with EDTA
using 1-(2-Pyridyl azo)-2-naphthol (PAN) as indicator. Working solutions of the
concentrations of 10-5M were prepared by appropriate dilution of standard
solution.
3.4.2 General procedure for the determination of Copper ions using VBT Schiff’s Base
Standard solutions of VBT Schiff’s base of concentration 10-4M were
prepared in A.R methanol. Absorption measurements were made at 450 nm
against the reagent blank from a solution, prepared by 1mL of the VBT Schiff’s
67
base of the concentration 10-4M, 1mL of 0.075 µg mL-1 to 0.9 µg mL-1 of Copper
solution, 2 mL of 5.89 buffer solution and made up to 25 mL with methanol.
3.4.3 General procedure for the determination of Copper ions using VMBT, VCBT and VBBT Schiff’s Bases
Above procedure was repeated with 1mL of the VMBT, VCBT and VBBT
Schiff’s bases.
3.4.4 Determination Copper from the Industrial Effluents The effluent samples were collected from four different industries around
Mysore. Treated effluent solutions were completely evaporated, they were
digested with 10 mL of Conc. HNO3, until all the brown fumes of nitric acid
subside [APHA and AWWA-WPCF, 1995]. The residues obtained were extracted
with dilute acetic acid and made up to 25 mL with redistilled water in a volumetric
flask. The estimations of Copper ions were carried out as described under
general procedure using VBT, VMBT, VCBT and VBBT as analytical reagents
and absorption measurements were made at 450 nm against the reagent blank
and the results are presented in the Table 3.1. The results obtained by the
proposed method agreed with the certified values.
Table 3.1: Results obtained for the determination of Copper from industrial effluents
Industrial effluents
VBT as analytical reagent
(µg mL-1)
VMBT as analytical reagent
(µg mL-1)
VCBT as analytical reagent
(µg mL-1)
VBBT as analytical reagent
(µg mL-1)
AAS method
(µg mL-1 )
Glowtronics 0.026 0.024 0.024 0.022 0.02
Marc batteries 0.024 0.022 0.025 0.0223 0.02
Allcoats 0.031 0.036 0.033 0.0321 0.03
Tritan Valves 0.38 0.356 0.362 0.0359 0.35
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3.5 RESULTS AND DISCUSSION
3.5.1 General Chemical Reactions
S
N
NH2
R
R1
OHC
HO OCH3
S
N
N
HO OCH3
R
R1
S
N
N
Cu O
N
N
SO
R
R1
R
R1OCH3
H3CO
+
Schiff's base Copper complexes
o-Vanillidine 2-amino benzothiazole Schiff's bases
2-aminobenzothiazoleso-Vanilline
[R=H, R1=H] [R=CH3, R1=H] [R= H, R1= Cl] [R=H, R1=Br]
Cu(II) solution
69
3.5.2 IR Spectra of Synthesized Schiff’s Bases and their Copper Complexes IR spectra of the complexes were compared with those of the free ligands
(Table 3.2). The C=N stretching frequencies in the free ligands were observed
around 1562-1573 cm-1. The shift of this band towards a lower frequency
(1538-1542 cm-1) in all Copper complexes suggests that coordination has
taken place through the nitrogen of the azomethine group. The C-O stretching
modes of the phenolimine form of the free Schiff’s bases were found between
1178 and 1259 cm-1. The shift of the phenolic C-O stretch towards higher
frequencies in all Copper complexes suggests Cu-O bond formation. Therefore, it
is believed that chelation of the Copper ions in their Schiff’s base complexes
occurs through Nitrogen and Oxygen. All the IR data of the Schiff’s bases and
their Copper complexes imply that the Schiff’s bases behave as bidentate ligand
and bonded to Copper ion through phenolic oxygen and imino nitrogen of
azomethine [Neelakantan M.A. et al., 2008].
Table 3.2: IR Spectra for VBT, VMBT, VCBT, VBBT and their respective Copper complexes
Compound (O-H) cm-1
(C=N) azomethine
cm-1 (C-O) cm-1
(C=N) thiazole ring
cm-1
(C-S-C) cm-1
VBT 3432 1567 1256 1594 762
VBT-Cu 3445 1541 1441 1609 752
VMBT 3434 1572 1178 1596 747
VMBT-Cu 3436 1542 1236 1596 747
VCBT 3648 1573 1247 1699 724
VCBT-Cu 3647 1539 1257 1699 723
VBBT 3438 1566 1259 1592 735
VBBT-Cu 3564 1538 1304 1649 723
70
3.5.3 1H NMR Spectra of Synthesized Schiff’s Bases and their Copper Complexes
1H NMR Spectra of the Schiff’s bases exhibit a multiplet around 6.9 to
7.69 δ for the protons of the aromatic ring. A signal at 2.7δ due to hydrogen of
–CH3 was observed only in VMBT Schiff’s base. The azomethine hydrogen
(-CH=N-) leads to a singlet of integration intensity equivalent to one of the
hydrogen around 9.3 δ. The signal around 3.9 δ was due to the hydrogen of the
–OCH3. Schiff’s bases show a singlet equivalent to a hydrogen around 12.5 δ
corresponding to free –OH. Disappearance of the signal at 12.5 δ upon the
addition of D2O to the solution indicates that, it is an acidic proton. The higher δ
value of the –OH group is attributed to the possible hydrogen bonding in the
Schiff’s bases. Absence of the signals around 12.5 δ were observed in all Copper
complexes indicating the involvement of the –OH group in chelation with Copper.
There was no other appreciable change in signals of the complexes when
compared with the Schiff’s bases [Orhan Buyukgungor et al., 2004].
3.5.4 Absorption Spectra of Synthesized Schiff’s Bases and their Copper Complexes
Under the experimental conditions, absorption spectra of Schiff’s bases
VMBT, VBT, VCBT and VBBT and their corresponding Copper complexes were
scanned at pH 5.89. Absorbance decreased from 370 nm to 500 nm for all the
four Schiff’s bases. Copper complexes of four Schiff’s bases show maximum
absorptions at 450 nm. Hence 450 nm was chosen as the wavelength for the
determination. Absorption spectra of VBT, VMBT, VCBT, VBBT and their Copper
complexes are shown in the Figures 3.1, 3.2, 3.3 and 3.4.
71
0
0.05
0.1
0.15
0.2
0.25
0.3
370 420 470 520 570
Wavelength
Abs
orba
nce
2
1
Figure 3.1: Absorption spectra of VBT and Cu-VBT 1. Absorption spectrum of VBT 2. Absorption spectrum of Cu-VBT
0
0.1
0.2
0.3
0.4
0.5
370 420 470 520 570Wave length (nm)
Abs
orba
nce
1
2
Figure 3.2: Absorption spectra of VMBT and Cu-VMBT
1. Absorption spectrum of VMBT 2. Absorption spectrum of Cu-VMBT
72
0
0.05
0.1
0.15
0.2
0.25
0.3
0.35
370 420 470 520 570
wave length
Abs
orba
nce
21
Figure 3.3: Absorption spectra of VCBT and Cu-VCBT
1. Absorption spectrum of VCBT 2. Absorption spectrum of Cu-VCBT
0
0.05
0.1
0.15
0.2
0.25
370 420 470 520 570
Wave length
Abs
orba
nce
2
1
Figure 3.4: Absorption spectra of VBBT and Cu-VBBT
1. Absorption spectrum of VBBT 2. Absorption spectrum of Cu-VBBT
73
3.5.5 Effect of pH The effects of pH on the absorbance of four Schiff’s base Copper
complexes at 450 nm were studied under different pH conditions. Maximum
absorbance for the complex was obtained at pH 5.89. Hence, pH 5.89 was
selected for the determination of Copper. It is also found that, the addition of
2 mL of buffer solution of pH 5.89 was sufficient for the pH adjustments.
3.5.6 Characteristics of the Copper Complexes
Light yellowish orange solutions of VMBT, VBT, VCBT and VBBT
solutions change to brownish yellow solutions immediately on the addition of
Copper ions at 5.89 pH in methanol solution with maximum absorption at 450 nm
and was stable for at least half an hour.
3.5.7 Calibration and spectral data of Copper Complexes Beer Lambert’s Range, optical Characteristics, such as, molar
absorptivity, Sandell's sensitivity, slope, intercept, correlation coefficient are
shown in Table 3.3. Sandell’s sensitivity (S) represents the number of
micrograms of the determinant per milliliter of a solution having an absorbance
(A) of 0.001 for a path length (l) of 1 cm. Thus S=10-3/a=µg cm-2 where ‘a’ is
specific absorptivity and its value in mL g-1 cm-1 corresponds to the determinant
in a cuvette with an optical length of 1 cm. Also,
a = 1000 x metaltheof wt.at.
ε
ε = (Molar absorptivity) = A/cl, where ‘c’ is the molar concentration of the
determinant and l =1 cm.
A typical plot showed unequivocally, that the stable 1:2 [M-L] complexes
are formed at pH 5.89 (λmax 450 nm). Calibration graph obtained by the
recommended procedure was linear over the range 0.4 to 3.8 µg mL-1 in the final
solution (Figure 3.6).
74
.
0
0.02
0.04
0.06
0.08
0.1
0 0.02 0.04 0.06 0.08
Metal concentration (µg / mL)
Abs
orba
nce
VBT Copper complexVCBT Copper complexVBBT Copper complexVMBT Copper complex
Figure 3.5: Calibration graph obtained for VBT, VMBT, VCBT and VBBT copper complexes
75
Table 3.3: Spectral data for the determination of Copper ions using VBT, VMBT, VCBT and VBBT Schiff’s bases as spectrophotometric reagents
Parameters Cu-VBT Cu-VMBT Cu-VCBT Cu-VBBT
Colour Brownish
yellow
Brownish
yellow
Brownish
yellow
Brownish
yellow
λmax (nm) 450 450 450 450
Stability (minutes) 30 30 30 30
Molar absorptivity,
(εmax) L mol-1 cm-1 2.87x103 2.94x103 2.2x103 1.53x103
Beer Lambert’s Range
(µg mL-1) 0.116-0.81 0.075-0.78 0.12-0.9 0.3-0.9
Sensitivity (µg cm-2) 2.2x10-3 2.14x10-3 2.9x10-3 4.12x10-3
Regression (r2) 0.9996 0.9855 0.9955 0.9995
Slope 1.0616 0.9373 1.0522 1.6713
Intercept 0.00047 0.00365 -0.0049 0.00035
Correlation Coefficient 0.9998 0.9927 0.9978 0.9998
3.5.8 Effect of foreign ions on the determination of metal ions An extensive study was made on the effect of foreign ions on the
determination of Copper in 5.8x10-5 M Copper solution buffered at pH 5.89. The
tolerance amounts of foreign ions are as shown in Table 3.4.
The tolerance limit of foreign ions was taken as that value, which caused
an error of not more than ±5% in the absorbance. Most of the cations do not
interfere with the determination of Copper.
76
Table 3.4: Effect of various cations and anions (0.59 µg mL-1 Copper added)
Ions / Substance
Amount added
(µg mL-1)
Copper found
(µg mL-1) with VBT
Error (µg mL-1)
Copper found
(µg mL-1) with VMBT
Error (µgmL-1)
Copper found
(µgmL-1) with
VCBT
Error (µg mL-1)
Copper Found
(µg mL-1) with
VBBT
Error (µg mL-1)
Cl - 70 0.059 0.0 0.059 0.0 0.059 0.0 0.059 0.0
SO4 2- 103 0.050 -0.009 0.052 -0.007 0.049 -0.01 0.048 0.059
NO3 - 90 0.059 0.0 0.059 0.0 0.059 0.0 0.059 0.0
F - 38 0.063 0.004 0.645 0.586 0.064 0.005 0.067 0.008
PO43-
122 0.068 0.009 0.07 0.011 0.073 0.014 0.072 0.013
DMG 1194 0.050 -0.009 0.049 -0.01 0.050 -0.009 0.052 -0.007
EDTA (0.1M) 0.05mL - - - - - - - -
Thiourea 170 0.0 -0.059 0.0 -0.059 0.0 -0.059 0.0 -0.059
Hg+2 1.25 0.052 -0.007 0.055 -0.004 0.054 -0.005 0.055 -0.004
Pb+2 1.08 0.044 -0.015 0.042 -0.017 0.042 -0.017 0.044 -0.015
Fe+2 2.02 0.052 -0.007 0.054 -0.005 0.054 -0.005 0.053 -0.006
Cd+2 2.12 0.054 -0.005 0.056 -0.003 0.053 -0.006 0.054 -0.005
Zn+2 2.02 0.049 -0.01 0.048 -0.011 0.048 -0.011 0.044 -0.015
Co+2 1.95 0.056 -0.003 0.057 0.002 0.057 -0.002 0.055 -0.004
Ni+2 1.09 0.045 -0.014 0.042 -0.017 0.044 -0.015 0.040 -0.019
77
3.6 CONCLUSIONS 1. Four new Schiff’s bases VBT, VMBT, VCBT and VBBT and their Copper
complexes were synthesized and characterized.
2. These Schiff’s bases react with Copper (II) ions in methanol solution to form
brownish yellow complexes at pH 5.89, with a stoichiometric ratio of
1:2 [Copper to ligand].
3. These Schiff’s bases are selective and specific spectrophotometric reagents
for the determination of Copper. The method is relatively free from
interference with most of the metal ions like Zinc, Cadmium, Cobalt, Lead,
Mercury and Iron.
4. Satisfactory results were obtained when the method was applied to the
determination of Copper in the industrial effluents.
These synthesized Schiff’s bases are very sensitive reagents for the
spectrophotometric determination of Copper ions. Hence, Copper alone can be
estimated from the industrial effluents. For the determination of the other metal
ions from the industrial effluents, we prepared a few organic dyes which contain
two auxochromic groups.
A dye molecule can usually bind a metal ion only if it contains in addition
to the atoms in the auxochromic groups, other electron donating atoms in
such position that together with the metal ion, they can form chelate rings of
five atoms. Such chelate rings are highly stable. The azodyes on formation of
metal complexes undergo greatest change in colour. Therefore literature survey
on heterocyclic azo dyes as spectrophotometric determination of metal ions has
been made and synthesized azo dyes, which are formed by the diazotization
reaction of 2-aminobenzothiazole and its 4-methyl, 6-chloro and 6-bromo
derivatives with 2-amino-3-hydroxy pyridine. These dyes formed purple
coloured complexes with Cu(II), Ni(II), Cd(II), Co(II) and Zn(II) metal ions.
Spectrophotometric determination of these metal ions with above synthesized
azo dyes as reagents have been standardized and procedure has been designed
for the determination of these metal ions from the industrial effluents and are
discussed in the subsequent Chapters.