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Spectrophotometric determination of traces of cobalt with 3-hydroxypicolinealdehyde, salicylaldehyde and picolinealdehyde azines
rl~~rrf~ric*u Chirniccr Acfa. 68 ( 1974) 4664G9
466 :c:., Elscvicr Scicntilic
SHORT
Publishing
Company,
Amsterdam
- Printed
in The Nctbcrlands
COMMUNICATION
Spectrophotometric determination of traces of cobalt with 3-hydroxypicolinealdehyde, salicylaldehyde and picolinealdehyde azines
A. GARCIA Deportnwt
(Rcceivcd
DE TORRES.
of A rdyticd 5th June
M. VALCARCEL Clwnistr):
and F. PINO-PEREZ
Fcrcttlty of Scicrtcrs.
Uuiccrsity
it/ Sccilltr.
S~~~illrr(Spciiri)
1973)
The classical reagents’ for determining cobalt either offer high sensitivity at an unfavourable wavelength, e.g. l.lO-phenanthroline (~=3.4. lo4 at 270 nm), or suffer from numerous interferences, e.g. a-nitroso-p-naphthol (E= 2.6. lo4 at 530 nm), bipyridine (E= 1.6. lo4 at 295 nm), dithizone (E= 5.6. lo4 at 542 nm), oxine (E= 7.8 - lo3 at 420 nm), etc. In recent years, numerous spectrophotometric determinations of cobalt have been proposed 2*3 but none of these has entirely solved the problem. In this communication, some acyclic aromatic azines are proposed as analytical spectrophotometric reagents for cobalt. The synthesis and analytical possibilities of 3-hydroxypicolinealdehyde azine (3-OH-PAA), have already been described4. and this reagent is compared with salicylaldehyde (SAA) and picolinealdehyde (PAA) azines, to study the influence of the different bonding atoms which participate in the forming of the cobalt chelates. Reagents. Ethanolic or dimethylformamide (DMF) solutions of 3-hydroxypicolinealdehyde. salicylaldehyde and picolinealdehyde azines were used. The syntheses of these reagents have been described4. All solvents and reagents were of analytical grade. Appnratmz Unicam SP 800, Unicam SP 600~s-2, and Beckman DU spectrophotometers with l.O-cm glass or quartz cells were used, as well as a digital pH meter (Philips, PW 9408) with glass-calomel electrodes. Pwcedure wit/z,3-OH-PAA. To the cobalt solution (440 /cg of Co), add 4 ml of acetate buffer pH 4.5, and 5 ml of 0.02”/;:, (w/v) 3-OH-PAA solution in DMF. Heat the samples in a water-bath at 50” for 15 min, transfer to 25-ml flasks, and dilute to volume with water. Measure the absorbance at 545 and 570 nm against distilled water. Procedure with SAA. To the cobalt solution (75-300 /lg of Co) in a 25-ml flask add 10 ml of 0.05’% (w/v) SAA solution in DMF, and adjust the pH to 8.0 with sodium hydroxide or hydrochloric acid solutions. Dilute to volume with ethanol-water (3 + l), and after 15 min measure the absorbance at 530 and 570 nm against a reagent blank solution prepared similarly.
SHORT
467
COMMUNICATION
Procedure with PAA. To the cobalt solution (75-300 pg of Co) in a 25ml flask add 5 ml of 0.1% (w/v) PAA solution in DMF, and adjust the pH to 7.8 with acid or base, as above. Dilute to volume with water, and measure the absorbance at 400 nm after 1 h, against a reagent blank solution prepared similarly. Reaction
of cobalt( III) am1 3-OH-PAA When dilute cobalt(II) and 3-OH-PAA solutions are mixed, the initial yellow colour (%,uX = 520 nm) slowly changes to a strong violet colour (I.,,,, =545 nm, shoulder at 570 nm), which becomes stable after 12 h (Fig. 1). The variables which could accelerate the process were studied. Temperature has a notable influence: the stable violet colour is obtained in 10 min on heating at 50-60”. 2 I I
I I
3-OH-PAA
I
2
I
SAA
PAA
I I
I 1.5
1.5
I
I
I
I
I
I
I
A
1.0
I
I \ \ \ \
b
a
\
\
400
1 \ \
0.C
I I I I I I I I \
‘1-t
O.!
\
600
\ \
\
‘-500
I I I I \ \
500
Fig. 1. Absorption spcc~ra of Co(iI)-azine and 2 p.p.m. of Co(II): (a) at 2 min; and 10 p.p.m. of Co(H).
600
7oc
i\
‘. 400
300
so0
(nm) against rcagcnt blank (-----). (I) 3-OH-PAA SAA and 10 p.p.m. of’ Co(lI). (III) PAA h
complexes (----) (b) at 12 h. (II)
Oxidants prevent changes in colour, while reducing agents such as ascorbic acid accelerate the change to violet. If an inert atmosphere is used, identical results are obtained. Ultraviolet light does not affect the slow formation of the violet-coloured complex. The optimal pH range is only 4.34.6; this coincides with the zone between the pK, and pK, values of 3-OH-PAA ( i.e. when the pyridine nitrogens are deprotonated and intermolecular hydrogen-bonding exists). Absorbances drop rapidly with change in pH outside these limits. Beer’s law is obeyed between 0.15 and 1.6 p.p,m. of cobalt. The molar absorptivity is 3.04. lo4 1 mole- 1 cm - ’ at 545 nm and 2.66. lo4 1 mole-’ cm-’ at 570 nm. The metal:ligand stoichiometric ratio is 1:3 (Fig. 2). This forced configuration could be the cause of the slow formation of the complex and of the positive influence of the temperature. The formation constant of the complex is K, = 1.7 - lo”, as calculated from the absorbance
’
SHORT
468
COMMUNICATION
0.5 -
o*4,r” /d
--.
*.
P
0.3
/
nm
.y.
/I
0.2 ,-
545
“‘\
\\
jc
Id
‘\
@.
570nm
. . 0. . x=x
0.1 ‘I I/ ,
2
3
-3
5
6
7
8
9
IQ
‘-4 h
I
I 0
0.25
0.5
‘\
0.75
~oH-PAA~ ICOl
Fig. 2. Composition methods.
of the 3-OH-PAA-Co(
II) complex.
by the molar ratio and continuous
vnriations
measurements in the zone of stoichiometric composition of the CLWV~S in Fig. 2. The extraction of the complex was tested for various solvents, but was only successful with benzyl alcohol. The presence of both pyridine nitrogens and hydroxyl groups probably causes the difference between the Co(3-OH-PAA):+ complex and the Co(SAA) and Co( PAA)< + complexes, with regard to both composition and solution characteristics. Reactiott
of kobalr( II) cud SAA When bright yellow solutions of SAA are mixed with dilute solutions of cobalt( iI), a yellow-orange complex is obtained;. the absorption spectra are shown in Fig. 1. At concentrations above 12 p.p.m., a red precipitate of the complex appears. The optimal pH range is 7.8-8. I. The water:organic solvent ratio must be I:3 to prevent precipitation of the ligand. Beer’s law is obeyed in the concentration range 4-l 1 p-p-m. of cobalt(l1); the molar absorptivity at 530 nm is 1356 1 mole- 1 cm-‘. The methods of continuous variations and molar ratio showed a 1: 1 cobalt-ligand ratio; the formation constant is 1.7.10 s. The complex is easily extracted with chloroform. The behaviour of the cobalt-SAA complex is entirely parallel to that of cobalt(I1) oxinate, Co(oxine),. SAA is structurally analogous to oxine, with azine nitrogens in place of pyridine. The ease of extraction and the precipitation in aqueous media imply the existence of an uncharged complex. Since the 1: 1 complex is developed at basic pH, in which the hydroxyl groups are deprotonated, the structure is probably as follows:
Reactiott
oj’ cobaltf II) When cobalt(H)
with PAA and PAA solutions
are mixed
in a partly
organic
solvent,
SHORT
COMMUNICATION
469
a yellow complex is formed (Fig. 1). The optimal pH range for maximal absorbance at 390-410 nm is 7.4-9.5. Beer’s law is obeyed between 3 and 11 p.p.m. of cobalt; the molar absorptivity at 400 nm is 3560 1 mole-’ cm-‘. The cobalt(II)-ligand ratio, given by the continuous variations and molar ratio methods is 1:2; the formation constant is CLJ. 109. The complex is not extracted into organic solvents. According to Stratton and Busch’, picolinealdehyde azine behaves as a tridentate ligand in solution and the complex is positively charged, although in the solid state it has another composition. Its behaviour is analogous to that of the cobalt-terpyridine complex (%,,ll,X=450 nm, optimal pH 7.4-9.8) and differs, from those of the l,lO-phenanthroline and the bipyridine complexes, in which the ligand is bidentate. SpectrophotorlJeric
cletemiuntiorr of cobalt
The above results show that 3-hydroxypicolinealdehyde azine provides the most favourable analytical reaction with cobalt(I1). The maximal absorption wavelength enables water to be used as a photometric blank, the reaction is very sensitive, and the weakly acidic medium used (acetate buffer) reduces interferences. The optimal concentration range evaluated by Ringbom’s method is 0.3-1.6 p.p.m. of cobalt; the relative error (P=O.OS) of the method is 0.52% at 545 nm and 0.48’%, at 570 nm. The final absorbance remains constant for several hours. The ions Pb(ll), W( VI), Bi(III). As( III), Mo(V1). Se(IV). Zn, Mn( II). Al, La. Ce(IV), Be, Ca, Sr, Rb, Li, PO:-, SCN-, citrate, tartrate, do not interfere when they are present in concentrations 100 times greater (by weight) than that of cobalt. Cadmium, uranyl and oxalate ions interfere in 60-fold amounts. Ag, Hg(I), Hg(II), Pd, Fe(III), 0(I), Cu(II), Ni, S,O$and EDTA interfere when their concentration is six times greater than that of cobalt. REFERENCES 1 2 3 4
DUJU for Culorinwrric A~rcrlysis. Buttcrworths. London. 1963. p. 98. F. Boltz and M. G. Mellon. Amlyfktrl Chcnnktr): 1970; Awl. C/rem.. 42 (1970) IS6 R. F. Boltz and M. G. Mellon. Ard Chem.. 44 (1972) 304 R. Garcia dc Torrcs. M. ValcxxxA nnd F. Pino, Ttlhtc~. 20 (1973) 919. J. Stratton and D. H. Busch, J. Amer. Ckw. Sm.. 82 (1960) 4834.
SI)~cr~oplroror,lutric
D. D. A. S W.
466 :c:., Elscvicr Scicntilic
SHORT
Publishing
Company,
Amsterdam
- Printed
in The Nctbcrlands
COMMUNICATION
Spectrophotometric determination of traces of cobalt with 3-hydroxypicolinealdehyde, salicylaldehyde and picolinealdehyde azines
A. GARCIA Deportnwt
(Rcceivcd
DE TORRES.
of A rdyticd 5th June
M. VALCARCEL Clwnistr):
and F. PINO-PEREZ
Fcrcttlty of Scicrtcrs.
Uuiccrsity
it/ Sccilltr.
S~~~illrr(Spciiri)
1973)
The classical reagents’ for determining cobalt either offer high sensitivity at an unfavourable wavelength, e.g. l.lO-phenanthroline (~=3.4. lo4 at 270 nm), or suffer from numerous interferences, e.g. a-nitroso-p-naphthol (E= 2.6. lo4 at 530 nm), bipyridine (E= 1.6. lo4 at 295 nm), dithizone (E= 5.6. lo4 at 542 nm), oxine (E= 7.8 - lo3 at 420 nm), etc. In recent years, numerous spectrophotometric determinations of cobalt have been proposed 2*3 but none of these has entirely solved the problem. In this communication, some acyclic aromatic azines are proposed as analytical spectrophotometric reagents for cobalt. The synthesis and analytical possibilities of 3-hydroxypicolinealdehyde azine (3-OH-PAA), have already been described4. and this reagent is compared with salicylaldehyde (SAA) and picolinealdehyde (PAA) azines, to study the influence of the different bonding atoms which participate in the forming of the cobalt chelates. Reagents. Ethanolic or dimethylformamide (DMF) solutions of 3-hydroxypicolinealdehyde. salicylaldehyde and picolinealdehyde azines were used. The syntheses of these reagents have been described4. All solvents and reagents were of analytical grade. Appnratmz Unicam SP 800, Unicam SP 600~s-2, and Beckman DU spectrophotometers with l.O-cm glass or quartz cells were used, as well as a digital pH meter (Philips, PW 9408) with glass-calomel electrodes. Pwcedure wit/z,3-OH-PAA. To the cobalt solution (440 /cg of Co), add 4 ml of acetate buffer pH 4.5, and 5 ml of 0.02”/;:, (w/v) 3-OH-PAA solution in DMF. Heat the samples in a water-bath at 50” for 15 min, transfer to 25-ml flasks, and dilute to volume with water. Measure the absorbance at 545 and 570 nm against distilled water. Procedure with SAA. To the cobalt solution (75-300 /lg of Co) in a 25-ml flask add 10 ml of 0.05’% (w/v) SAA solution in DMF, and adjust the pH to 8.0 with sodium hydroxide or hydrochloric acid solutions. Dilute to volume with ethanol-water (3 + l), and after 15 min measure the absorbance at 530 and 570 nm against a reagent blank solution prepared similarly.
SHORT
467
COMMUNICATION
Procedure with PAA. To the cobalt solution (75-300 pg of Co) in a 25ml flask add 5 ml of 0.1% (w/v) PAA solution in DMF, and adjust the pH to 7.8 with acid or base, as above. Dilute to volume with water, and measure the absorbance at 400 nm after 1 h, against a reagent blank solution prepared similarly. Reaction
of cobalt( III) am1 3-OH-PAA When dilute cobalt(II) and 3-OH-PAA solutions are mixed, the initial yellow colour (%,uX = 520 nm) slowly changes to a strong violet colour (I.,,,, =545 nm, shoulder at 570 nm), which becomes stable after 12 h (Fig. 1). The variables which could accelerate the process were studied. Temperature has a notable influence: the stable violet colour is obtained in 10 min on heating at 50-60”. 2 I I
I I
3-OH-PAA
I
2
I
SAA
PAA
I I
I 1.5
1.5
I
I
I
I
I
I
I
A
1.0
I
I \ \ \ \
b
a
\
\
400
1 \ \
0.C
I I I I I I I I \
‘1-t
O.!
\
600
\ \
\
‘-500
I I I I \ \
500
Fig. 1. Absorption spcc~ra of Co(iI)-azine and 2 p.p.m. of Co(II): (a) at 2 min; and 10 p.p.m. of Co(H).
600
7oc
i\
‘. 400
300
so0
(nm) against rcagcnt blank (-----). (I) 3-OH-PAA SAA and 10 p.p.m. of’ Co(lI). (III) PAA h
complexes (----) (b) at 12 h. (II)
Oxidants prevent changes in colour, while reducing agents such as ascorbic acid accelerate the change to violet. If an inert atmosphere is used, identical results are obtained. Ultraviolet light does not affect the slow formation of the violet-coloured complex. The optimal pH range is only 4.34.6; this coincides with the zone between the pK, and pK, values of 3-OH-PAA ( i.e. when the pyridine nitrogens are deprotonated and intermolecular hydrogen-bonding exists). Absorbances drop rapidly with change in pH outside these limits. Beer’s law is obeyed between 0.15 and 1.6 p.p,m. of cobalt. The molar absorptivity is 3.04. lo4 1 mole- 1 cm - ’ at 545 nm and 2.66. lo4 1 mole-’ cm-’ at 570 nm. The metal:ligand stoichiometric ratio is 1:3 (Fig. 2). This forced configuration could be the cause of the slow formation of the complex and of the positive influence of the temperature. The formation constant of the complex is K, = 1.7 - lo”, as calculated from the absorbance
’
SHORT
468
COMMUNICATION
0.5 -
o*4,r” /d
--.
*.
P
0.3
/
nm
.y.
/I
0.2 ,-
545
“‘\
\\
jc
Id
‘\
@.
570nm
. . 0. . x=x
0.1 ‘I I/ ,
2
3
-3
5
6
7
8
9
IQ
‘-4 h
I
I 0
0.25
0.5
‘\
0.75
~oH-PAA~ ICOl
Fig. 2. Composition methods.
of the 3-OH-PAA-Co(
II) complex.
by the molar ratio and continuous
vnriations
measurements in the zone of stoichiometric composition of the CLWV~S in Fig. 2. The extraction of the complex was tested for various solvents, but was only successful with benzyl alcohol. The presence of both pyridine nitrogens and hydroxyl groups probably causes the difference between the Co(3-OH-PAA):+ complex and the Co(SAA) and Co( PAA)< + complexes, with regard to both composition and solution characteristics. Reactiott
of kobalr( II) cud SAA When bright yellow solutions of SAA are mixed with dilute solutions of cobalt( iI), a yellow-orange complex is obtained;. the absorption spectra are shown in Fig. 1. At concentrations above 12 p.p.m., a red precipitate of the complex appears. The optimal pH range is 7.8-8. I. The water:organic solvent ratio must be I:3 to prevent precipitation of the ligand. Beer’s law is obeyed in the concentration range 4-l 1 p-p-m. of cobalt(l1); the molar absorptivity at 530 nm is 1356 1 mole- 1 cm-‘. The methods of continuous variations and molar ratio showed a 1: 1 cobalt-ligand ratio; the formation constant is 1.7.10 s. The complex is easily extracted with chloroform. The behaviour of the cobalt-SAA complex is entirely parallel to that of cobalt(I1) oxinate, Co(oxine),. SAA is structurally analogous to oxine, with azine nitrogens in place of pyridine. The ease of extraction and the precipitation in aqueous media imply the existence of an uncharged complex. Since the 1: 1 complex is developed at basic pH, in which the hydroxyl groups are deprotonated, the structure is probably as follows:
Reactiott
oj’ cobaltf II) When cobalt(H)
with PAA and PAA solutions
are mixed
in a partly
organic
solvent,
SHORT
COMMUNICATION
469
a yellow complex is formed (Fig. 1). The optimal pH range for maximal absorbance at 390-410 nm is 7.4-9.5. Beer’s law is obeyed between 3 and 11 p.p.m. of cobalt; the molar absorptivity at 400 nm is 3560 1 mole-’ cm-‘. The cobalt(II)-ligand ratio, given by the continuous variations and molar ratio methods is 1:2; the formation constant is CLJ. 109. The complex is not extracted into organic solvents. According to Stratton and Busch’, picolinealdehyde azine behaves as a tridentate ligand in solution and the complex is positively charged, although in the solid state it has another composition. Its behaviour is analogous to that of the cobalt-terpyridine complex (%,,ll,X=450 nm, optimal pH 7.4-9.8) and differs, from those of the l,lO-phenanthroline and the bipyridine complexes, in which the ligand is bidentate. SpectrophotorlJeric
cletemiuntiorr of cobalt
The above results show that 3-hydroxypicolinealdehyde azine provides the most favourable analytical reaction with cobalt(I1). The maximal absorption wavelength enables water to be used as a photometric blank, the reaction is very sensitive, and the weakly acidic medium used (acetate buffer) reduces interferences. The optimal concentration range evaluated by Ringbom’s method is 0.3-1.6 p.p.m. of cobalt; the relative error (P=O.OS) of the method is 0.52% at 545 nm and 0.48’%, at 570 nm. The final absorbance remains constant for several hours. The ions Pb(ll), W( VI), Bi(III). As( III), Mo(V1). Se(IV). Zn, Mn( II). Al, La. Ce(IV), Be, Ca, Sr, Rb, Li, PO:-, SCN-, citrate, tartrate, do not interfere when they are present in concentrations 100 times greater (by weight) than that of cobalt. Cadmium, uranyl and oxalate ions interfere in 60-fold amounts. Ag, Hg(I), Hg(II), Pd, Fe(III), 0(I), Cu(II), Ni, S,O$and EDTA interfere when their concentration is six times greater than that of cobalt. REFERENCES 1 2 3 4
DUJU for Culorinwrric A~rcrlysis. Buttcrworths. London. 1963. p. 98. F. Boltz and M. G. Mellon. Amlyfktrl Chcnnktr): 1970; Awl. C/rem.. 42 (1970) IS6 R. F. Boltz and M. G. Mellon. Ard Chem.. 44 (1972) 304 R. Garcia dc Torrcs. M. ValcxxxA nnd F. Pino, Ttlhtc~. 20 (1973) 919. J. Stratton and D. H. Busch, J. Amer. Ckw. Sm.. 82 (1960) 4834.
SI)~cr~oplroror,lutric
D. D. A. S W.