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Complex formation of nickel(II) and cobalt(II) with 4-(2-pyridylazo)-resorcinol
456 Elscvicr
SHORT
COMMUNICATIOS
Complex
formation
Publishing
of nickel(lI)
Company,
and cohalt(tI)
Amsterdam
Andyticu Chitnicu Actu -. Printed in The Netherlands
with 4-(2-pyridylazo)-resorcinol
4-( 2-Pyridylazo) -resorcinol ( PAR) was reported in earlier papers’ * ’ as an excellent rcugcnt for spcctrophotometric determination of nickcl( II) and cobalt( II) in alkaline media. In connexion with the use of PAR as a calorimetric reagent for these cations, it is of interest to know the composition and stability of the complexes formed. There is untiertainty concerning the composition of the complexes at pH *8, hence a detailed examination seemed desirable. Previous work’ indicated that ;I I :3 metal-l&and was formed, but more recent work with a pure sample of PAR has proved that in fact the molar ratio of nickel( II) or cobalt( II) ions to l&and is 1 :2 in both slightly acidic and alkaline media. With increasing pH, the absorption maximum shifts toward shorter wavelengths and the molar absorptivity of the complexes increases (~=37200, II,,,,, 520 nrn’ at pH 3.3 and The colour of the I: = 79400, ;I,,,:,x 496 nm at pH 8.0 for the nickel complex). nickel(II)-PAR complex turns from red to orange, while the colour of the cobalt(II)PAR complex does not change visibly (1:=65500, &,,1,X5 IO nm). As will be shown later. this property of nickel(II)-PAR complex and its higher absorptivity is due to splitting off of protons, a fully deprotonated species being formed. The free reagent in alkaline media has its wavelength of maximal absorbance at 485 nm. 4-(2-Py~itlyl~lzo)-~L~so~c,itrol
( Koch-Light).
ized by spectrophotometric titration with a wavelength of 520 nm. The copper(I1) the I : I molar ratio of its PAR complex4.
A lo-’ h4 solution was standnrd1.260. IO-’ M copper( II) at pH 5 and was preferred for titration because of
Nickel( II) rlittwtcj cod cohtclt( II) rtitrtrtc. The titres of stock solutions were controlled by compleximetric titration in the usual way. Solutions of lower concentration were obtained by dilution. The pH was adjusted by adding dilute sodium hydroxide or nitric acid solutions. The ionic strength was maintained at 0.1 with sodium nitratc’*2. Absorbunces were measured on a VSU-1 Universal Spectrophotorneter, and visible spectra were recorded on a Specord u.v.-vis. Spectrophotometer. An L. Seibold pH meter (type GLD) was also used. Inomtigcctiorl
01’ the recrctions trrltl cieterrnincttion i>J the stability
To explain /1ncal.Cltint.
Acfa.
the reactions
62 (I 972)
and
to establish
the stability
cmsttrms
constants
spectro-
SHORT
COMMUNICATION
457
photometrically, methods based on the relationship A = f(pH) were employed, on account of the high stability of the complexes studied. The straight-line portions of the absorbance-pH plots for equimolar solutions, and for solutions containing a slight excess of metal ion or of ligand, lay in the pH range 4-6 for cobalt(II)-PAR complexes and in the pH range 6-8 for nickel(II)-PAR complexes. To represent the corresponding reactions, the fraction of PAR in its ionic forms was calculated in relation to the pH by meanwof the values pK 1 = 2.66, the results are shown in Fig. 1. It is evident that pK,=5.48 and pK,= 12.31’; cobalt(II)-PAR complexes can be formed with H,R or HRspecies. while the HR- anion predominates in the range for nickel(I1) complex formation. Thus, in a solution containing metal ions and PAR in a 1:2 molar ratio, the reactions can bc written : M2+ +2
H,R
= MH+_-nR’;-“)-Z+~z
H+
(I)
M2+ +2
HR-
= MH2_nR’;-“‘-Z+t~
H+
(2)
0.6 -
2
0
Fig.
I. Fraction
6
4
of PAR
present
in various
ionic
forms
PH
10
0
&pending
on
PH.
q.
as kI,R’;
a,.
as H,R;
x2, as HR -; r3, as R2 -.
The number of protons plying the equation:
in its logarithmic
released
during
complex
formation
was examined
by ap-
transformation:
where A is the absorbance at a given pH value, A, is the maximal absorbance (at is the total metal concentration. The straight lines with higher pH value), and Chl slopes U= 1.9 (Ni) and n= 2.2 (Co) confirmed the splitting off of the two protons Anal.
Chirn.
Acra.
62 (1972)
SHORT
2-f 1
4
(Fig.
/ 1
Fig. 2. Loyarilhmic 2.012* IO..” M.
2). Thcreforc
COMMUNICATION
1
5 6 PA plots acoorcling to ccln. (4). csN,= 1.012. IO -’
the following
M2++H2R+HR-
equilibria
= M(HR)R-+2
should
M:
c’,,,=O.973*
IO-”
be valid in pI-I range
M:
C,,Al(=
studied:
H+
(5)
M2’+2HR-=MR;-+2H’
(6) to a The
The red colour of the cobalt(II)-PAR complex is considered to be due monoprotonatcd form, such as Co(HR)R-, in contrast to the orange NiR$-. complexes formed at lower pH value have the composition M( HR),. The formation of two diffcrcnt species in each of these complexing reactions depending on pM is evident from the absorbance spectra (Figs. 3 and 4). The complex formed at lower pH gradually turns into the corresponding monoprotonated or deprotonated species, as the pH is increased. The presence of two different protonutcd spccics agrees with the reactions proposed. Accordingly, three kinds of stability constants arc necdcd to characterize the metal complexes: pi”, /ji” and p2. From the intercepts of the plots (Fig. 2) the equilibrium and stability constants were calculated (Table I). If the dissociation constant of only the hydroxy group o~rl~ to the azo group, PK~~--~, had been used, a higher stability constant would have been obtained for the Co( HR)R complex (log pit’ = 25.6). In order to verify the results established in equimolar solutions, the following equations were applied”: py And.
=
Chim. Acln. 62 (I 972)
[H+12
I(,x;-
+
CH’I
1(-;-’
+
1 (7)
459
A L
1.0
0.5
0 450
400 ~;ig. 3. l>cpcndcncc
of ;Ihsorhancc
spectra
Al; plj: (I) 2X7: (2) 3.03: (3’j 4.90: ( 1 1) X.70: ( 12) x.vs. l’ig. AI:
4. [)cpcll&ncc
pkl: ( I)
2.67:
(2)
(4)
on ~1.1 for nickcl( 5.70: (5) 5.50; (6) _
of ;Ibsorhancc spcctr;r on 2.‘#; (3) .7.30: (4) 3.72:
ptl (5)
II).
5x5;
GO0 t?m
500
Csi = i.OI2* IO ’ Al: C”l.,,H = 2.012. 10. ’ (7) 6.30; (X) h.00: (9) 7.1s: ( IO) 7.80;
for cobalt( II). C‘,.,,==7.78* IO ” AI: C’I*,\R= l.hl()’ 10 ’ 3.95; (6) 4.30; (7) 4.50: (X) 4.87: (9) 5.10: ( IO) 5.65;
( I I) S.85: (I 2) h.40: (13) 7.10.
(8) graphically, The slopes II= 1.7 (Ni) and II = 2.0 (Co) wcrc obtained proved the splitting off of two protons during the reaclions, By means of the equation:
AA
log AAO-AA slopes Arral.
of about Clritn.
Ada.
which
again
= log K,,, + 2 lop C,, -i- 11pH
unity 62 (1972)
were obtained
for complex
formation
with either
metal,
i.e.
SHORT
460 one proton
split off. Hence,
M2++2
HR-
the complex
= M(HR)R-
formation
COMMUNICATION
can be written
as:
+H+
(11)
The release of one proton on complex formation can be explained by addition of the second PAR molecule through a coordinate bond”. The values of the molar absorptivity and the wavelength of maximal absorbance of nickel(II)-PAR are constant and do not depend on the M :L ratio are optically identical at higher employed. The species NiR$+ and Ni( HR)RpH, the latter being formed only in the presence of excess of ligand. The data obtained are summarized in Table I. TABLE
I
THE STABILITY CONSTANTS OF THE PAR COMPLEXES OF NICKEL(II) AND COBALT(I1) .._..__ _..-- .._._-.-. .__-._-_-..- _...---_-.. --_- ..-....... . .- .. .._. .--_....-.-_-.._._. .._._-__- ._.__.- . _..._.. _ _ _ ____._
Metal
ion
DeJittccl
I~qrritttol~tr soltrtiott
l’ot1stlItlt
.-.---
_-.....-.__.^-...._..._-....
Ni2 +
1~
rb
-
co2 +
Mettrl CS~‘CSS
._.- ._..-.._ .- . --.. .___~ .._.-.._ ..._..-_._.......... 22.9 (eqn. 4) 21.9 (cqn. 8)
22.3 22.0 22.2 22. I 22. I
IX.8 (cqn. 4) 20.3 (cqn. 7)
18.6 18.G
._.
Ligwd L’S(‘C.SS
_. _
_. -..__...____
17.4
The
value of the stability constant of the cobalt(lI)-PAR complex for solutions containing excess of ligand agrees very well with the value’ log /jZ= 17.1 solution. The stability determined potentiometrically by Geary et ~1.’ 2 in aqueous constant found for the nickel(lI)-DAR complex is lower than that determined by Corsini et al.’ 3 potentiometrically in (1+ 1) water-dioxanc (26.0). The assumption that PAR forms three kinds of metal complex depending on the pH, agrees well with acid-base properties of both the reagent and its metal complexest4* * s. REFERENCES 1 2 3 4 5 6 i’ 8 9 IO
D. Nonova and B. Evtimovu. rlttaf. Chittt. Acttr, 49 ( 1970) 103. B. Evtimova nncl D. Nonova. C. R. Acwl. Bul!/. Sci., 23 ( 1970) I 11 1. D. Nonovu nnd Z. Bojkova. Amt. Uttio. Sofi’tr. 61 (1966/1967) 403. T. Iwamoto, Bdl. Chettt. SW. Jqt...34 (1961) 605. W. J. Gcnry, G. Nick&s and F. I-1. Pollnrd, Atd. Chittt. Acrrr, 26 (1962) L. Sommcr and V. M. Ivanov, Ttrltrtttrr, 14 (1967) 171. R. A. Chnlmcrs, Tdatitrr, 14 (1967) 527. A. Corsini. Ttrhtlttr. I5 (196X) 993. T. Shimizu and K. Ogomi. Tdmttr, 16 (1969) 1527. A. V. Koshcl. K. F. Karlishcvn and 1. A. Shcka. .4tdyricd Chemistry Naukova Dumka, Kiev, 1970,
Atral.
Chim.
Actu.
62 (1972)
575.
trttrl Estrac*tiott
Pro~csscs,
SHORT
S. B. Savvin, L. A. Gribov. V. I. Lebedev and E. A. Likhonina, Zh. Attctl. K/rim.. 26 (1971) W. J. Geary, G. Nickless and F. H. Pollard. Attul. C/tint. Actu. 27 (1962) 71. A. Corsini. I. M. Yih. Q, Fernando and H. Freiscr. Attd. Chettt., 34 (1962) 1090. G. Schwarzenbach and H. Flaschka. Cotttple.~ctttterric Titrutiotts. Methuen. London. 1969. IS J. Kijrbl and R. Piibil, Collrcr. C’zec*/t. Chew. Cottvtnrtt.. 22 ( 1957) 1 122.
11 12 13 14
Anu!.
461
COMMUNICATION
Chim.
Ac’lu. 62 ( 1972)
2108.
SHORT
COMMUNICATIOS
Complex
formation
Publishing
of nickel(lI)
Company,
and cohalt(tI)
Amsterdam
Andyticu Chitnicu Actu -. Printed in The Netherlands
with 4-(2-pyridylazo)-resorcinol
4-( 2-Pyridylazo) -resorcinol ( PAR) was reported in earlier papers’ * ’ as an excellent rcugcnt for spcctrophotometric determination of nickcl( II) and cobalt( II) in alkaline media. In connexion with the use of PAR as a calorimetric reagent for these cations, it is of interest to know the composition and stability of the complexes formed. There is untiertainty concerning the composition of the complexes at pH *8, hence a detailed examination seemed desirable. Previous work’ indicated that ;I I :3 metal-l&and was formed, but more recent work with a pure sample of PAR has proved that in fact the molar ratio of nickel( II) or cobalt( II) ions to l&and is 1 :2 in both slightly acidic and alkaline media. With increasing pH, the absorption maximum shifts toward shorter wavelengths and the molar absorptivity of the complexes increases (~=37200, II,,,,, 520 nrn’ at pH 3.3 and The colour of the I: = 79400, ;I,,,:,x 496 nm at pH 8.0 for the nickel complex). nickel(II)-PAR complex turns from red to orange, while the colour of the cobalt(II)PAR complex does not change visibly (1:=65500, &,,1,X5 IO nm). As will be shown later. this property of nickel(II)-PAR complex and its higher absorptivity is due to splitting off of protons, a fully deprotonated species being formed. The free reagent in alkaline media has its wavelength of maximal absorbance at 485 nm. 4-(2-Py~itlyl~lzo)-~L~so~c,itrol
( Koch-Light).
ized by spectrophotometric titration with a wavelength of 520 nm. The copper(I1) the I : I molar ratio of its PAR complex4.
A lo-’ h4 solution was standnrd1.260. IO-’ M copper( II) at pH 5 and was preferred for titration because of
Nickel( II) rlittwtcj cod cohtclt( II) rtitrtrtc. The titres of stock solutions were controlled by compleximetric titration in the usual way. Solutions of lower concentration were obtained by dilution. The pH was adjusted by adding dilute sodium hydroxide or nitric acid solutions. The ionic strength was maintained at 0.1 with sodium nitratc’*2. Absorbunces were measured on a VSU-1 Universal Spectrophotorneter, and visible spectra were recorded on a Specord u.v.-vis. Spectrophotometer. An L. Seibold pH meter (type GLD) was also used. Inomtigcctiorl
01’ the recrctions trrltl cieterrnincttion i>J the stability
To explain /1ncal.Cltint.
Acfa.
the reactions
62 (I 972)
and
to establish
the stability
cmsttrms
constants
spectro-
SHORT
COMMUNICATION
457
photometrically, methods based on the relationship A = f(pH) were employed, on account of the high stability of the complexes studied. The straight-line portions of the absorbance-pH plots for equimolar solutions, and for solutions containing a slight excess of metal ion or of ligand, lay in the pH range 4-6 for cobalt(II)-PAR complexes and in the pH range 6-8 for nickel(II)-PAR complexes. To represent the corresponding reactions, the fraction of PAR in its ionic forms was calculated in relation to the pH by meanwof the values pK 1 = 2.66, the results are shown in Fig. 1. It is evident that pK,=5.48 and pK,= 12.31’; cobalt(II)-PAR complexes can be formed with H,R or HRspecies. while the HR- anion predominates in the range for nickel(I1) complex formation. Thus, in a solution containing metal ions and PAR in a 1:2 molar ratio, the reactions can bc written : M2+ +2
H,R
= MH+_-nR’;-“)-Z+~z
H+
(I)
M2+ +2
HR-
= MH2_nR’;-“‘-Z+t~
H+
(2)
0.6 -
2
0
Fig.
I. Fraction
6
4
of PAR
present
in various
ionic
forms
PH
10
0
&pending
on
PH.
q.
as kI,R’;
a,.
as H,R;
x2, as HR -; r3, as R2 -.
The number of protons plying the equation:
in its logarithmic
released
during
complex
formation
was examined
by ap-
transformation:
where A is the absorbance at a given pH value, A, is the maximal absorbance (at is the total metal concentration. The straight lines with higher pH value), and Chl slopes U= 1.9 (Ni) and n= 2.2 (Co) confirmed the splitting off of the two protons Anal.
Chirn.
Acra.
62 (1972)
SHORT
2-f 1
4
(Fig.
/ 1
Fig. 2. Loyarilhmic 2.012* IO..” M.
2). Thcreforc
COMMUNICATION
1
5 6 PA plots acoorcling to ccln. (4). csN,= 1.012. IO -’
the following
M2++H2R+HR-
equilibria
= M(HR)R-+2
should
M:
c’,,,=O.973*
IO-”
be valid in pI-I range
M:
C,,Al(=
studied:
H+
(5)
M2’+2HR-=MR;-+2H’
(6) to a The
The red colour of the cobalt(II)-PAR complex is considered to be due monoprotonatcd form, such as Co(HR)R-, in contrast to the orange NiR$-. complexes formed at lower pH value have the composition M( HR),. The formation of two diffcrcnt species in each of these complexing reactions depending on pM is evident from the absorbance spectra (Figs. 3 and 4). The complex formed at lower pH gradually turns into the corresponding monoprotonated or deprotonated species, as the pH is increased. The presence of two different protonutcd spccics agrees with the reactions proposed. Accordingly, three kinds of stability constants arc necdcd to characterize the metal complexes: pi”, /ji” and p2. From the intercepts of the plots (Fig. 2) the equilibrium and stability constants were calculated (Table I). If the dissociation constant of only the hydroxy group o~rl~ to the azo group, PK~~--~, had been used, a higher stability constant would have been obtained for the Co( HR)R complex (log pit’ = 25.6). In order to verify the results established in equimolar solutions, the following equations were applied”: py And.
=
Chim. Acln. 62 (I 972)
[H+12
I(,x;-
+
CH’I
1(-;-’
+
1 (7)
459
A L
1.0
0.5
0 450
400 ~;ig. 3. l>cpcndcncc
of ;Ihsorhancc
spectra
Al; plj: (I) 2X7: (2) 3.03: (3’j 4.90: ( 1 1) X.70: ( 12) x.vs. l’ig. AI:
4. [)cpcll&ncc
pkl: ( I)
2.67:
(2)
(4)
on ~1.1 for nickcl( 5.70: (5) 5.50; (6) _
of ;Ibsorhancc spcctr;r on 2.‘#; (3) .7.30: (4) 3.72:
ptl (5)
II).
5x5;
GO0 t?m
500
Csi = i.OI2* IO ’ Al: C”l.,,H = 2.012. 10. ’ (7) 6.30; (X) h.00: (9) 7.1s: ( IO) 7.80;
for cobalt( II). C‘,.,,==7.78* IO ” AI: C’I*,\R= l.hl()’ 10 ’ 3.95; (6) 4.30; (7) 4.50: (X) 4.87: (9) 5.10: ( IO) 5.65;
( I I) S.85: (I 2) h.40: (13) 7.10.
(8) graphically, The slopes II= 1.7 (Ni) and II = 2.0 (Co) wcrc obtained proved the splitting off of two protons during the reaclions, By means of the equation:
AA
log AAO-AA slopes Arral.
of about Clritn.
Ada.
which
again
= log K,,, + 2 lop C,, -i- 11pH
unity 62 (1972)
were obtained
for complex
formation
with either
metal,
i.e.
SHORT
460 one proton
split off. Hence,
M2++2
HR-
the complex
= M(HR)R-
formation
COMMUNICATION
can be written
as:
+H+
(11)
The release of one proton on complex formation can be explained by addition of the second PAR molecule through a coordinate bond”. The values of the molar absorptivity and the wavelength of maximal absorbance of nickel(II)-PAR are constant and do not depend on the M :L ratio are optically identical at higher employed. The species NiR$+ and Ni( HR)RpH, the latter being formed only in the presence of excess of ligand. The data obtained are summarized in Table I. TABLE
I
THE STABILITY CONSTANTS OF THE PAR COMPLEXES OF NICKEL(II) AND COBALT(I1) .._..__ _..-- .._._-.-. .__-._-_-..- _...---_-.. --_- ..-....... . .- .. .._. .--_....-.-_-.._._. .._._-__- ._.__.- . _..._.. _ _ _ ____._
Metal
ion
DeJittccl
I~qrritttol~tr soltrtiott
l’ot1stlItlt
.-.---
_-.....-.__.^-...._..._-....
Ni2 +
1~
rb
-
co2 +
Mettrl CS~‘CSS
._.- ._..-.._ .- . --.. .___~ .._.-.._ ..._..-_._.......... 22.9 (eqn. 4) 21.9 (cqn. 8)
22.3 22.0 22.2 22. I 22. I
IX.8 (cqn. 4) 20.3 (cqn. 7)
18.6 18.G
._.
Ligwd L’S(‘C.SS
_. _
_. -..__...____
17.4
The
value of the stability constant of the cobalt(lI)-PAR complex for solutions containing excess of ligand agrees very well with the value’ log /jZ= 17.1 solution. The stability determined potentiometrically by Geary et ~1.’ 2 in aqueous constant found for the nickel(lI)-DAR complex is lower than that determined by Corsini et al.’ 3 potentiometrically in (1+ 1) water-dioxanc (26.0). The assumption that PAR forms three kinds of metal complex depending on the pH, agrees well with acid-base properties of both the reagent and its metal complexest4* * s. REFERENCES 1 2 3 4 5 6 i’ 8 9 IO
D. Nonova and B. Evtimovu. rlttaf. Chittt. Acttr, 49 ( 1970) 103. B. Evtimova nncl D. Nonova. C. R. Acwl. Bul!/. Sci., 23 ( 1970) I 11 1. D. Nonovu nnd Z. Bojkova. Amt. Uttio. Sofi’tr. 61 (1966/1967) 403. T. Iwamoto, Bdl. Chettt. SW. Jqt...34 (1961) 605. W. J. Gcnry, G. Nick&s and F. I-1. Pollnrd, Atd. Chittt. Acrrr, 26 (1962) L. Sommcr and V. M. Ivanov, Ttrltrtttrr, 14 (1967) 171. R. A. Chnlmcrs, Tdatitrr, 14 (1967) 527. A. Corsini. Ttrhtlttr. I5 (196X) 993. T. Shimizu and K. Ogomi. Tdmttr, 16 (1969) 1527. A. V. Koshcl. K. F. Karlishcvn and 1. A. Shcka. .4tdyricd Chemistry Naukova Dumka, Kiev, 1970,
Atral.
Chim.
Actu.
62 (1972)
575.
trttrl Estrac*tiott
Pro~csscs,
SHORT
S. B. Savvin, L. A. Gribov. V. I. Lebedev and E. A. Likhonina, Zh. Attctl. K/rim.. 26 (1971) W. J. Geary, G. Nickless and F. H. Pollard. Attul. C/tint. Actu. 27 (1962) 71. A. Corsini. I. M. Yih. Q, Fernando and H. Freiscr. Attd. Chettt., 34 (1962) 1090. G. Schwarzenbach and H. Flaschka. Cotttple.~ctttterric Titrutiotts. Methuen. London. 1969. IS J. Kijrbl and R. Piibil, Collrcr. C’zec*/t. Chew. Cottvtnrtt.. 22 ( 1957) 1 122.
11 12 13 14
Anu!.
461
COMMUNICATION
Chim.
Ac’lu. 62 ( 1972)
2108.