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A cation-exchange study of the cobalt-nitroso-r salt system
Analytica Chimicu Acta, 63 (1973) 85-94 0 Elsevier Scientific Publishing Company, Amsterdam - Printed in The Netherlands
A CATION-EXCHANGE SYSTEM
STUDY
OF THE COBALT-NITROSO-R
85
SALT
ASHOK MAHAN and ARUN K. DEY Chemical Laboratories, University qf Allahabad. Allababad (India) (Received 16th May 1971)
Nitroso-R salt has been widely used for spectropbotometri~ determinations. Potentiometric stud& on its metal chelaTzs have been made by Dey et ~l.‘*~ andby MakitieYIn the work described here, an ion-exchange technique was used to study the complex formation between cobalt(U) and nitroso-R salt, and the stepwise stability constants were determined. The methods of Fronaeus4 and of Loman and van Daler? were employed; these involve the determination of the vlstribution coefficients of the metal ion in metal-nitroso-R salt systems (in both the absence and the presence of the iigand). Conditional stability constEnts were obtained. THEIORY
The distribution of total metal, nS, between the resin and aqueous phases is given by
4
=40
(1 + 1%; %CLI”>/(I
+ $ p,cLl~)
(1)
where L;),, is the distribution coefficient of metal ions in absence of the ligand, p, the overall stability constant, [Lf the concentration of the Iigand, and the quantity DL is given by the relation = p”,D:L & = +JW&o (2) Here, D,,, and D,, represent the distribution coefficients of the nth complex and the ligand respectively; and ir, the overall stability constant in the resin phase. The function, (1-t 7SWslJKl%
is denoted by F, and the other functions as
FI = (F- WCLI Tz = (4 z- I$)/ CL] (3) where t=n+ 1. The Fronaeus procedure makes use of the functions F; = V&,/t>,) - l)//tLI .F;_= iirnlt+,F; = /3x-0;
(4) (5)
86
A. MAHAN,
G = W,cm,)W, -%Wl - I>+ WM2 G_ = liqLI-di = PI (PI 44) -Uh-D’,)
A. K. DEY
(6) (7)
and the equations,
CC--iJIW F;
= P,W; -KMLl
+%F; -F’s
(8)
=PIF;--F2
(9)
The Loman-van Dalen treatment for the presence of two cationic species was applied, and the following equations were used, F;(X12)-F;(X8)=
[{&(X12)--:(X8)}
+ {&(X12)--D;(X8)}[L]]/F
complex (10)
F;(X12)-FF;(X4)=[{D;(X12)--D;(X4)}+{D;(X12)-DD;(X4))[L]]/F
(11) where X12, X8 and X4 indicate the crosslinking of the Dowex 50W resin used. Extrapolation of the curves [F;(X12) -F;(X8)], and [F; (X12)-FF;(X4)] against [L], to [L] =0, gives the quantities [D;(X12)-DD;(X8)] and [D;(X12)-&(X4)] as limiting values. From eqns. ( 10) and (11)
By plotting rc/against [F; (X12) -F; values of 0; can be obtained.
(X8)]/[F;
(X12) -F;
(X4)],
the difference between
EXPERIMENTAL
Reagents and apparatus Cation-exchange resins. Columns of the cation exchangers Dowex 5OW-X12, -X8, and -X4 were treated with 0.02 M EDTA solution (pH=9+ 1), and washed with water followed by 2 A4 hydrochloric acid. They were then' washed with 2 M sodium chloride until the effluent was neutral to litmus, and finally washed with water until free from chloride. The resins were then filtered on a sintered glass filter, air-dried and stored. Metal and ligand solutions. The cobalt solution was prepared from AnalaR cobalt nitrate by dissolution in water at pH 3.5, and was standardized against EDTA. Nitroso-R salt (B.D.H.) solution was prepared by dissolution in water at pH 3.5. All other chemicals used were of reagent grade. Apparatus. A Leeds-Northrup pH meter, and a Unicam SP 500 spectrophotometer with lo-mm glass cells were employed. A wrist motion Microid flask shaker (Griflin & George) was used for shaking; glass-stoppered lOO-ml Pyrex flasks were used. Experiments were performed at 30” & 2”. Distribution studies The batch equilibration
method was employed;
weighed amounts
of the air-
THE COBALT-NITROSO-R
SALT
SYSTEM
87
dried resin were added to the metal solution (2. 10m4 M) at pH 3.5 and ionic strength 1.0 (NaNO,), and shaken mechanically for 2 h (it was ascertained previously that this time was adequate for equilibration). The resin was then filtered off and the metal ion concentration in an aliquot of the filtrate was determined spectrophotometrically6 after the chelate had been decomposed with bromine. The uptake of metal by the exchanger was calculated from the relation’ iC= [u,(R’-
RS)]/Q
(13)
where R’, R’ and R are the amounts of metal (in pg) per g of resin, per ml of original solution, and per ml of the solution after equilibration with the exchanger, respectively; u0 is the added volume of the original solution (ml) and g is the amount of resin (g); 6 is the swelling factor of the resin. From the data thus obtained, the distribution coefficients (II,) were calculated from the equation D, =
meq. of metal ion/g of resin meq. of metal ion/ml of solution
Determination of swelling factor, 6 The method of Pepper et al.* makes the assumption that the weight and the volume of sorbed solutions are related by their normal densities. koman and van Dalen’ found that this assumption is not valid and considered the volume of a solution in equilibrium with a certain weight of exchanger as:
and the swelling factor, 6, as: 6 = v/v0 = (ue + l& - f&)/f.Yg
(14)
where v is the volume of solution in equilibrium; v0 the added volume of solution; 17~the volume of water in the air-dried resin (determined by drying the samples at 100-l 10” to constant weight); and fz the equilibration volume of the sorbed solution in the exchanger. Therefore, after equilibration of a certain amount (1 g or 5 g) of the resin, a portion of the resin was filtered off and dried thoroughly on a sintered glass filter (constant weight, 6) under strong suction. The crucible and contents were weighed ( Wz) and then dried at 100-l 10” to constant weight ( W3). The resin was then thoroughly washed with water to remove all the adhering sodium nitrate, and dried first by suction and then at 100-110” to constant weight ( W4). The amounts of resin [( W,- WI) in g bone-dry], water [( W,- W,)g/( W,- W,)g resin], and salt [( w, - W)gl(KWl)g resin] were calculated. The amounts of water and salt were then corrected for per gram of resin on an .air-dried basis. The amount of salt was also considered in terms of percentage with respect to water and the specific gravity was read from the specific gravity curve, which was plotted from standard datag. As the mass-and density were known, the volume, fizz, of the sorbed solution present in the re/sincould be calculated. The swelling factors are shown in Table I. For further calculations the 6 values obtained from 5-g portions of the resins were used.
A. MAHAN, TABLE
A. K. DEY
I
VALUES
OF
SWELLING
FACTOR,
5
Resin
Ig
59
Dowex SOW-Xl2 Dowex 5OW-X8 Dowex 5OW-X4
0.9926 0.9904 0.999 1
1.0057 1.0006 1.0104
Calculation of free ligand concentration The concentration of the nitroso-R salt available was calculated from the protonation constant: K = [HL]/[H+]
K [H’]
[L-l
for complex
formation
[L-]
= [HL] = [HL],,,
- [L-l
of the undissociated where [HL] and [HL],,, are the equilibrium concentrations ligand and the total concentration of ligand taken, respectively. Thus, log[L-]
= log[HL],,-log(l+K[H+])
The value of the protonation RESULTS
AND
constant,
(15) K, was taken as log K =6.702.
DISCUSSION
The values of the distribution coefficients, DBO, with different resins in the absence of nitroso-R salt were determined; the values found were 14.64, 10.92, and 8.70 for Dowex 5OW-X12, -X8 and -X4, respectively. In each Table, the first column represents the concentration of the nitroso-R salt anion calculated with the help of eqn. (15). Table II gives the values of D&D, for the three resins corresponding to varying concentrations of nitroso-R salt; these values were read from the relevant graphs obtained by plotting the experimentally found values of D&D, against CL]* TABLE
II
VALUES
OF
Nltroso-R (M-IO’)
salt
0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0 5.5 6.0
D&D,
READ
FROM
CURVES
Bowex 5OW-X12
Dowex 5OW-X8
Dowex 50 W-X4
1.60 2.25 3.00 4.00. 5.00 6.75 8.75 11.75 16.50 29.00 56.00 82.50
1.25 1.50 1.75 2.50 3.00 4.00 5.25 7.00 9.75 14.00 23.00 44.00
1.17 1.37 1.60 1.85 2.10 2.45 2.80 3.15 3.55 4.05 4.50 5.05
THE COBALT-NITROSO-R
89
SALT SYSTEM
This was followed by the calculation, as already outlined, of the, functions F;, F$, cfi’, cfi”, and 4”’ (where (6’, c#J”,and 4”’ are the ratios of D&Da for’ the resins 5OW-X12/5OW-X8, SOW-X12/5OW-X4 and 5OW-X8/5OW-X4 respectively), and finally the function F from which the stability constants were determined graphically (eqn. 3). These functions were calculated from the experimental values of D,e and D,, and the quantities Di. The functions F; and Fi were calculated from eqns. (4) and (6) and the values are summarized in Table III. The values of F;, and F&, given in the first row of the Table were read after extrapolation of the plots of F[ or F; against the nitroso-R salt concentration to zero ligand concentration. The values of pi and jlz were determined (eqn. 9) from the plots of F; against Fi as the slope and intercept of the straight lines (Fig. 1). In the case of the Dowex 5OW-X 12 and 5OW-X8 resins, the plots of Fz zs F; passed through the origin, TABLE III TREATMENT
OF THE DATA BY FRONAEUS’
METHOD
(*IO-‘)
(*ro-7)
Fi (X4)
F$ [X12)
(M ‘10’)
(‘20-7)
(.Kr-‘4]
0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0 5.5 6.0
1.20 1.20 1.25 1.33 1.50 1.60 1.91 2.21 2.68 3.44 5.60 10.00 13.58
0.50 0.50 0.50 0.50 0.75 0.80 1.00 1.21 1.50 1.94 2.60 4.18 7.16
0.33 0.34 0.37 0.40 0.42 0.44 0.48 0.5 1 0.54 0.57 0.61 0.63 0.67
1.40 1.44 1.45 1.51 1.65 1.76 2.06 2.36 2.85 3.63 5.84 10.40 14.23
Nitrosa-R
salt
F; (X22)
F; IX81
F; (X8)
FIZfX4.J
(*lo-‘3)
(*IO-‘2)
2.00 2.52 2.50 2.05 2.50 2.80 3.33 4.03 5.00 6.5 1 8.80 13.63 24.72
8.00 9.20 8.20 8.50 9.20 10.10 10.80 11.70 12.50 13.40 14.50 15.40 16.50
a Dowex SOW-X12, -X8 or -X4 resin as indicated.
0
4
8 +lb7
?2
I
lb
0
I
2
4
f
6
4 gs
I
6
16’
Fig. 1. Plots of function Fi us. F; for the three resins. A, Dowex SOW-X12; B, Dowex SOW-X& C, Dowex SOW-X4.
A. MAHAN,
90 IS-
0
A. K. DEY
16 -
12-
4-
Fig. 2. Plots of [F; -F&,]/[L] SOW-X8: C, Dowex SOW-X4.
us. [F; -F;,]/[L]
C-
for
the three resins. A, Dowex SOW-X12; B, Dowe?
hence the values of pi were also determined .for each resin, from the plots of (I;“z-- &J/CL1 rJs* m - K.M-LIPas the slope of the resulting straight lines (Fig. 2). The corresponding calculated data are given in Table IV. TABLE IV VALUES OF FACTORS (F; - F;,)/[L] Nitroso-R salt (MS 10”
5ow-XI2
0.5 1.0
0.00 0.50 0.87 1.50 1.60 2.36 2.88 3.70 5.00 8.80 16.00 20.46
1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0 5.5 6.0
AND (F; - F;,,)/[L] -i 50&X8 sow-x4
1.25
1.20
1.67 2.03 2.50 3.20 4.20 6.69
11.10
4.00 4.66 4.50 4.40 5.00 5.14 5.25 5.22 5.60 5.45 5.67
0.80 0.50 0.73 1.25 1.44 2.20 2.74 3.62 4.95 8.88 16.36 21.38
2.50 3.20 4.33 5.7 1 7.50 10.00 13.60 I5,63 37.83
2.00 3.30 6.00 8.40 9.30 10.50 11.20 12.00 13.00 13.40 14.10
In treatment of the data by the method of Loman and van Dalen’, the values of the differences in the quantities [O~(X4)-Wr(X12)] and [D;(X4)-Uf);(X8)] were determined from the extrapolations of (+“- l)/[L] and (#“‘- l)/[L] to zero [L] (Table V, Fig. 3). From these values the function II/was calculated (eqn. 12) and from the pIot of II/us. (+“- l)/(+“-#), the quantities [D’,(X4) -&(X12)] and [&(X4)-&(X8)] were determined from the resulting straight line as slope and intercept, respectively (Fig. 3~). The values obtained were l-647* lOi and 6.735. 10i2, respectively. The function F was determined from eqns. (9) and (lo), and then Fr, F2 and the stability constants (pi, p2) from eqn. (3) (Table VI, Fig. 3d).
THE
COBALT-NITROSO-R
Fig. 3. (a) Extrapolation of +h us. (&’ - I)/(#‘--&‘);
TABLE
91
SYSTEM
of (r#P- l)/[L] to [L] = 0; (b) extrapolation (d) plot of function FL 11s.[L].
of (#”
- l)/[L]
to [L] = 0; (c) plot
V
TREATMENT Nitroso-R (M*107)
0.0 2.0 2.5 3.0 3.5 4.0 4.5 5.0 5.5 6.0
SALT
OF
THE
DATA BY THE METHOD 4”’ _ I
cp”-
fg
[LI (-IO-e)
,,,_,,
4.00 5.81 5.52 5.85 6.07 6.83 8.11 12.32 20.8 1 25.56
0.70 1.75 1.71 2.11 2.50 3.05 3.88 4.91 7.47 12.85
salt
2.07 1.93 1.64 1.46 1.11 1.23 1.21 1.14 1.06
OF LOMAN-VAN I
DALEN
*(*IO-=)
- 12.77 - IO.59 -9.50 -8.51 - 8.06 - 6.97 - 6.30 - 5.86 - 5.43
The concentrations of nitroso-R salt were those of the free anion, as calculated from the literature value for the protonation constant of the ligand. Sorption of the nitroso-R salt was not found to be significant under the conditions of the experiments; therefore, direct calculation of the equilibrium concentrations was possible without determining the distribution concentrations. For every system the sorption of even the first cationic complex species was found to be zero in the case of the Dowex 5OW-X12. Thus eqns. (10) and (11) directly gave the value of F as
A. MAHAN,
92 TABLE
VI
VALUES
OF FUNCTIONS
Nitroso-R (M.10’)
salt
2.0 2.5 3.0 3.5 4.5 5.0 5.5 6.0
TABLE VALUES
A. K. DEY
F, F, AND
F;!
F(SOW-X8)
F(S0 W-X4)
Average F
F, f *IQ-‘)
FJ-ii?-=)
2.98 3.99 5.18 6.60 9.65 11.35 13.20 16.10
2.91 4.17 5.16 6.42 9.37 11.59 13.55 16.22
2.94 4.08 5.17 6.50 9.5 1 11.47 13.37 16.16
0.97 1.23 1.39 1.57 1.89 2,09 2.24 2.53
3.60 3.92 3.80 3.77 3.64 3.68 3.61 3.80
VII OF STABILITY
Dowex SOW-X12 log P1
1%
7.02 7.03
-
Dowex
82
DJD,
CONSTANTS
b3
Pt
6.53 7.56
(FRONAEUS
50 W-X8 h3
-
(12
METHOD)
Dowex 50 W-X4 m3
6.46 6.85
Pi
1%
Fig. I32
12.48
1 2
= F =
Accordingly, the values of D&Do given in Table II for Dowex SO-Xl2 resin are not rewritten in Table VI, but are considered in the average values of F. The values of the constants obtained for the different resins are given in Table VII. The value of log & obtained by means of Dowex 5OW-X4 resin is 12.48, but, when the other two resins were used, it was not possible to read pz from the curve, since it passed through the origin. The Dowex SOW-X4 resin, being of low cross-linkage, shows the value of &. This value was also determined by the method of Loman and van Dalen: log fli = 6.93, and log /Iz = 13.57. No indication of the formation of a 1:3 complex species of cobalt was obtained by either method. The average values of the stepwise constants obtained by both methods and the corresponding free energies (AF) evaluated for these chelation processes are given in Table VIII. It is interesting to observe that when the cobalt(II)-nit&o-R system was studied spectrophotometrically by Lalor lo, the products of the reaction were found to include a cobalt(II1) complex. This product is formed in both alkaline and acidic media, the oxidation being brought about by oxygen in an alkaline medium, whereas in acidic solution the uncomplexed ligand is the oxidant; the complex in the system was found by L&or to be Co(L),. The present studies indicate that the complex species formed in the cobalt(II)nitroso-R salt system are Co(L) and COG, whereas no direct indication of the
THE
COBALT-NITROSO-R
TABLE
93
VIII
AVERAGE
VALUES
AND
FREE
ENERGIES“
Method
Collsttrrrt
HOEK, log K, lois P2 - AF:’ - AF:) - AF” 11Values
SALTSYSTEM
-
Avercrye
Fronneus
LOI~ICI~I-LXIII Dub
6.9 1 6.24 13.15 9.56 8.33 17.89
6.93 6.64 13.57 9.58 9.18 18.76
6.92 6.44 13.36 9.57 8.75 18.34
ure in kcal mole-‘.
formation of Co(L), was obtained. It is, of course, concluded that both the species present in the system are cationic, since eqns. (lo)-( 12) are followed. Obviously therefore if Co(L), is cationic, the formation of Co(L), is also indicated. The work was supported by the Council of Scientific and Industrial Research, New Delhi. and the financial assistance to A.M. is gratefully acknowledged. SUMMARY
The stability constants of the nitroso-R salt complexes of cobalt(H) were investigated by cation exchange by means of the methods of Fronaeus and of Lomanvan Dalen. The cation exchangers Dowex 5OW-X12, -X8, and -X4 (Na-forms) were employed for the distribution studies. All the measurements were performed at ionic strength 1.0 (NaNO,) and 30 +2”. The constants calculated from the distribution data by both methods were in good agreement. The average values were log K, =6.92, log K, = 6.44, and log /j, = 13.36. The corresponding free energies of the complexation processes were also evaluated.
Une etude est effectuee sur lesconstantes de stabilite des complexes cobalt( II)se1 de nitroso-R en utilisant les methodes de Fronaeus et de Loman-van Dalen avec Cchangeur de cations. (Dowex 5OW-X12, -X8 et -X4 (forme Na)). Toutes les mesures ont CtC effectuees g force ionique de 1.0 (NaNO,), h 30 f 2”. Les valeurs moyennes sont log K1 = 6.92, log Kz = 6.44 et log pz = 13.36. Les energies libres correspondantes sont egalement &al&es. ZUSAMMENFASSUNG
Die Stabilitatskonstanten der Njtroso-R-Salz-Komplexe von Kobalt(I1) wurden durch Kationenaustausch nach den Methoden von Fronaeus und von Lomanvan Dalen untersucht. Fiir die Verteilungsversuche wurden die Kationenaustauscher Dowex 5OW-X12, -X8 und -X4 (alle in der Na-Form) verwendet. Alle Messungen
94
A. MAHAN,
A. K. DEY
wurden bei Ionenstarke 1.0 (NaNO,) und 30+ 2” ausgeftihrt. Die nach beiden Methoden aus den Verteilungsergebnissen berechneten Konstanten stimmten gut tiberein. Die Mittelwerte waren Log K 1=6.92, Log K, = 6.44 und Log pz= 13.36. Die korrespondierenden freien Energien der Komplexbildungsreaktionen wurden ebenfalls berechnet. REFERENCES 1 A. Bancrjee and A. K. Dey. Proc. S_wp. Elecrrode Processes, Joclhpw, 1968, p. 149. 2 S. Mandal and A. K. Dey, Reu. Chim. Miner., 5 (19689 773. 3 0. Makitie, SUO~I. Kemktil., 4 (1968) 31. 4’ S. Fronaeus, Acta C/rem. Scartd., 5 (1951) 859; Soen. Kern.. Tidskr., 65 (1953) 19. 5 H. Loman and E. van Dnlen. J. fuorg. Nucl. C/~e~n., 28 ( 1966) 2037. 6 F. H. Pollard, P. Hanson and W. Genry, Antrl. C’hin~. Acfa. 20 (1959) 26. 7,A. J, Zielen, J. Amer. Cltem. Sot.. 81 (1959) 5022. 8 K. I&‘t%pper. D. Rcichenberg and D. K. Hale, J. Chert~. Sot-., ( 1952) 3 129. 9 Harrdbook of Chemistry md Physics. Chemical Rubber Publishing Co.. Obio, 1956. 10 G. C. Lalor, J. Inorg. Nucl. Chw., 30 (1968) 1925; 31 (1969) 1783.
A CATION-EXCHANGE SYSTEM
STUDY
OF THE COBALT-NITROSO-R
85
SALT
ASHOK MAHAN and ARUN K. DEY Chemical Laboratories, University qf Allahabad. Allababad (India) (Received 16th May 1971)
Nitroso-R salt has been widely used for spectropbotometri~ determinations. Potentiometric stud& on its metal chelaTzs have been made by Dey et ~l.‘*~ andby MakitieYIn the work described here, an ion-exchange technique was used to study the complex formation between cobalt(U) and nitroso-R salt, and the stepwise stability constants were determined. The methods of Fronaeus4 and of Loman and van Daler? were employed; these involve the determination of the vlstribution coefficients of the metal ion in metal-nitroso-R salt systems (in both the absence and the presence of the iigand). Conditional stability constEnts were obtained. THEIORY
The distribution of total metal, nS, between the resin and aqueous phases is given by
4
=40
(1 + 1%; %CLI”>/(I
+ $ p,cLl~)
(1)
where L;),, is the distribution coefficient of metal ions in absence of the ligand, p, the overall stability constant, [Lf the concentration of the Iigand, and the quantity DL is given by the relation = p”,D:L & = +JW&o (2) Here, D,,, and D,, represent the distribution coefficients of the nth complex and the ligand respectively; and ir, the overall stability constant in the resin phase. The function, (1-t 7SWslJKl%
is denoted by F, and the other functions as
FI = (F- WCLI Tz = (4 z- I$)/ CL] (3) where t=n+ 1. The Fronaeus procedure makes use of the functions F; = V&,/t>,) - l)//tLI .F;_= iirnlt+,F; = /3x-0;
(4) (5)
86
A. MAHAN,
G = W,cm,)W, -%Wl - I>+ WM2 G_ = liqLI-di = PI (PI 44) -Uh-D’,)
A. K. DEY
(6) (7)
and the equations,
CC--iJIW F;
= P,W; -KMLl
+%F; -F’s
(8)
=PIF;--F2
(9)
The Loman-van Dalen treatment for the presence of two cationic species was applied, and the following equations were used, F;(X12)-F;(X8)=
[{&(X12)--:(X8)}
+ {&(X12)--D;(X8)}[L]]/F
complex (10)
F;(X12)-FF;(X4)=[{D;(X12)--D;(X4)}+{D;(X12)-DD;(X4))[L]]/F
(11) where X12, X8 and X4 indicate the crosslinking of the Dowex 50W resin used. Extrapolation of the curves [F;(X12) -F;(X8)], and [F; (X12)-FF;(X4)] against [L], to [L] =0, gives the quantities [D;(X12)-DD;(X8)] and [D;(X12)-&(X4)] as limiting values. From eqns. ( 10) and (11)
By plotting rc/against [F; (X12) -F; values of 0; can be obtained.
(X8)]/[F;
(X12) -F;
(X4)],
the difference between
EXPERIMENTAL
Reagents and apparatus Cation-exchange resins. Columns of the cation exchangers Dowex 5OW-X12, -X8, and -X4 were treated with 0.02 M EDTA solution (pH=9+ 1), and washed with water followed by 2 A4 hydrochloric acid. They were then' washed with 2 M sodium chloride until the effluent was neutral to litmus, and finally washed with water until free from chloride. The resins were then filtered on a sintered glass filter, air-dried and stored. Metal and ligand solutions. The cobalt solution was prepared from AnalaR cobalt nitrate by dissolution in water at pH 3.5, and was standardized against EDTA. Nitroso-R salt (B.D.H.) solution was prepared by dissolution in water at pH 3.5. All other chemicals used were of reagent grade. Apparatus. A Leeds-Northrup pH meter, and a Unicam SP 500 spectrophotometer with lo-mm glass cells were employed. A wrist motion Microid flask shaker (Griflin & George) was used for shaking; glass-stoppered lOO-ml Pyrex flasks were used. Experiments were performed at 30” & 2”. Distribution studies The batch equilibration
method was employed;
weighed amounts
of the air-
THE COBALT-NITROSO-R
SALT
SYSTEM
87
dried resin were added to the metal solution (2. 10m4 M) at pH 3.5 and ionic strength 1.0 (NaNO,), and shaken mechanically for 2 h (it was ascertained previously that this time was adequate for equilibration). The resin was then filtered off and the metal ion concentration in an aliquot of the filtrate was determined spectrophotometrically6 after the chelate had been decomposed with bromine. The uptake of metal by the exchanger was calculated from the relation’ iC= [u,(R’-
RS)]/Q
(13)
where R’, R’ and R are the amounts of metal (in pg) per g of resin, per ml of original solution, and per ml of the solution after equilibration with the exchanger, respectively; u0 is the added volume of the original solution (ml) and g is the amount of resin (g); 6 is the swelling factor of the resin. From the data thus obtained, the distribution coefficients (II,) were calculated from the equation D, =
meq. of metal ion/g of resin meq. of metal ion/ml of solution
Determination of swelling factor, 6 The method of Pepper et al.* makes the assumption that the weight and the volume of sorbed solutions are related by their normal densities. koman and van Dalen’ found that this assumption is not valid and considered the volume of a solution in equilibrium with a certain weight of exchanger as:
and the swelling factor, 6, as: 6 = v/v0 = (ue + l& - f&)/f.Yg
(14)
where v is the volume of solution in equilibrium; v0 the added volume of solution; 17~the volume of water in the air-dried resin (determined by drying the samples at 100-l 10” to constant weight); and fz the equilibration volume of the sorbed solution in the exchanger. Therefore, after equilibration of a certain amount (1 g or 5 g) of the resin, a portion of the resin was filtered off and dried thoroughly on a sintered glass filter (constant weight, 6) under strong suction. The crucible and contents were weighed ( Wz) and then dried at 100-l 10” to constant weight ( W3). The resin was then thoroughly washed with water to remove all the adhering sodium nitrate, and dried first by suction and then at 100-110” to constant weight ( W4). The amounts of resin [( W,- WI) in g bone-dry], water [( W,- W,)g/( W,- W,)g resin], and salt [( w, - W)gl(KWl)g resin] were calculated. The amounts of water and salt were then corrected for per gram of resin on an .air-dried basis. The amount of salt was also considered in terms of percentage with respect to water and the specific gravity was read from the specific gravity curve, which was plotted from standard datag. As the mass-and density were known, the volume, fizz, of the sorbed solution present in the re/sincould be calculated. The swelling factors are shown in Table I. For further calculations the 6 values obtained from 5-g portions of the resins were used.
A. MAHAN, TABLE
A. K. DEY
I
VALUES
OF
SWELLING
FACTOR,
5
Resin
Ig
59
Dowex SOW-Xl2 Dowex 5OW-X8 Dowex 5OW-X4
0.9926 0.9904 0.999 1
1.0057 1.0006 1.0104
Calculation of free ligand concentration The concentration of the nitroso-R salt available was calculated from the protonation constant: K = [HL]/[H+]
K [H’]
[L-l
for complex
formation
[L-]
= [HL] = [HL],,,
- [L-l
of the undissociated where [HL] and [HL],,, are the equilibrium concentrations ligand and the total concentration of ligand taken, respectively. Thus, log[L-]
= log[HL],,-log(l+K[H+])
The value of the protonation RESULTS
AND
constant,
(15) K, was taken as log K =6.702.
DISCUSSION
The values of the distribution coefficients, DBO, with different resins in the absence of nitroso-R salt were determined; the values found were 14.64, 10.92, and 8.70 for Dowex 5OW-X12, -X8 and -X4, respectively. In each Table, the first column represents the concentration of the nitroso-R salt anion calculated with the help of eqn. (15). Table II gives the values of D&D, for the three resins corresponding to varying concentrations of nitroso-R salt; these values were read from the relevant graphs obtained by plotting the experimentally found values of D&D, against CL]* TABLE
II
VALUES
OF
Nltroso-R (M-IO’)
salt
0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0 5.5 6.0
D&D,
READ
FROM
CURVES
Bowex 5OW-X12
Dowex 5OW-X8
Dowex 50 W-X4
1.60 2.25 3.00 4.00. 5.00 6.75 8.75 11.75 16.50 29.00 56.00 82.50
1.25 1.50 1.75 2.50 3.00 4.00 5.25 7.00 9.75 14.00 23.00 44.00
1.17 1.37 1.60 1.85 2.10 2.45 2.80 3.15 3.55 4.05 4.50 5.05
THE COBALT-NITROSO-R
89
SALT SYSTEM
This was followed by the calculation, as already outlined, of the, functions F;, F$, cfi’, cfi”, and 4”’ (where (6’, c#J”,and 4”’ are the ratios of D&Da for’ the resins 5OW-X12/5OW-X8, SOW-X12/5OW-X4 and 5OW-X8/5OW-X4 respectively), and finally the function F from which the stability constants were determined graphically (eqn. 3). These functions were calculated from the experimental values of D,e and D,, and the quantities Di. The functions F; and Fi were calculated from eqns. (4) and (6) and the values are summarized in Table III. The values of F;, and F&, given in the first row of the Table were read after extrapolation of the plots of F[ or F; against the nitroso-R salt concentration to zero ligand concentration. The values of pi and jlz were determined (eqn. 9) from the plots of F; against Fi as the slope and intercept of the straight lines (Fig. 1). In the case of the Dowex 5OW-X 12 and 5OW-X8 resins, the plots of Fz zs F; passed through the origin, TABLE III TREATMENT
OF THE DATA BY FRONAEUS’
METHOD
(*IO-‘)
(*ro-7)
Fi (X4)
F$ [X12)
(M ‘10’)
(‘20-7)
(.Kr-‘4]
0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0 5.5 6.0
1.20 1.20 1.25 1.33 1.50 1.60 1.91 2.21 2.68 3.44 5.60 10.00 13.58
0.50 0.50 0.50 0.50 0.75 0.80 1.00 1.21 1.50 1.94 2.60 4.18 7.16
0.33 0.34 0.37 0.40 0.42 0.44 0.48 0.5 1 0.54 0.57 0.61 0.63 0.67
1.40 1.44 1.45 1.51 1.65 1.76 2.06 2.36 2.85 3.63 5.84 10.40 14.23
Nitrosa-R
salt
F; (X22)
F; IX81
F; (X8)
FIZfX4.J
(*lo-‘3)
(*IO-‘2)
2.00 2.52 2.50 2.05 2.50 2.80 3.33 4.03 5.00 6.5 1 8.80 13.63 24.72
8.00 9.20 8.20 8.50 9.20 10.10 10.80 11.70 12.50 13.40 14.50 15.40 16.50
a Dowex SOW-X12, -X8 or -X4 resin as indicated.
0
4
8 +lb7
?2
I
lb
0
I
2
4
f
6
4 gs
I
6
16’
Fig. 1. Plots of function Fi us. F; for the three resins. A, Dowex SOW-X12; B, Dowex SOW-X& C, Dowex SOW-X4.
A. MAHAN,
90 IS-
0
A. K. DEY
16 -
12-
4-
Fig. 2. Plots of [F; -F&,]/[L] SOW-X8: C, Dowex SOW-X4.
us. [F; -F;,]/[L]
C-
for
the three resins. A, Dowex SOW-X12; B, Dowe?
hence the values of pi were also determined .for each resin, from the plots of (I;“z-- &J/CL1 rJs* m - K.M-LIPas the slope of the resulting straight lines (Fig. 2). The corresponding calculated data are given in Table IV. TABLE IV VALUES OF FACTORS (F; - F;,)/[L] Nitroso-R salt (MS 10”
5ow-XI2
0.5 1.0
0.00 0.50 0.87 1.50 1.60 2.36 2.88 3.70 5.00 8.80 16.00 20.46
1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0 5.5 6.0
AND (F; - F;,,)/[L] -i 50&X8 sow-x4
1.25
1.20
1.67 2.03 2.50 3.20 4.20 6.69
11.10
4.00 4.66 4.50 4.40 5.00 5.14 5.25 5.22 5.60 5.45 5.67
0.80 0.50 0.73 1.25 1.44 2.20 2.74 3.62 4.95 8.88 16.36 21.38
2.50 3.20 4.33 5.7 1 7.50 10.00 13.60 I5,63 37.83
2.00 3.30 6.00 8.40 9.30 10.50 11.20 12.00 13.00 13.40 14.10
In treatment of the data by the method of Loman and van Dalen’, the values of the differences in the quantities [O~(X4)-Wr(X12)] and [D;(X4)-Uf);(X8)] were determined from the extrapolations of (+“- l)/[L] and (#“‘- l)/[L] to zero [L] (Table V, Fig. 3). From these values the function II/was calculated (eqn. 12) and from the pIot of II/us. (+“- l)/(+“-#), the quantities [D’,(X4) -&(X12)] and [&(X4)-&(X8)] were determined from the resulting straight line as slope and intercept, respectively (Fig. 3~). The values obtained were l-647* lOi and 6.735. 10i2, respectively. The function F was determined from eqns. (9) and (lo), and then Fr, F2 and the stability constants (pi, p2) from eqn. (3) (Table VI, Fig. 3d).
THE
COBALT-NITROSO-R
Fig. 3. (a) Extrapolation of +h us. (&’ - I)/(#‘--&‘);
TABLE
91
SYSTEM
of (r#P- l)/[L] to [L] = 0; (b) extrapolation (d) plot of function FL 11s.[L].
of (#”
- l)/[L]
to [L] = 0; (c) plot
V
TREATMENT Nitroso-R (M*107)
0.0 2.0 2.5 3.0 3.5 4.0 4.5 5.0 5.5 6.0
SALT
OF
THE
DATA BY THE METHOD 4”’ _ I
cp”-
fg
[LI (-IO-e)
,,,_,,
4.00 5.81 5.52 5.85 6.07 6.83 8.11 12.32 20.8 1 25.56
0.70 1.75 1.71 2.11 2.50 3.05 3.88 4.91 7.47 12.85
salt
2.07 1.93 1.64 1.46 1.11 1.23 1.21 1.14 1.06
OF LOMAN-VAN I
DALEN
*(*IO-=)
- 12.77 - IO.59 -9.50 -8.51 - 8.06 - 6.97 - 6.30 - 5.86 - 5.43
The concentrations of nitroso-R salt were those of the free anion, as calculated from the literature value for the protonation constant of the ligand. Sorption of the nitroso-R salt was not found to be significant under the conditions of the experiments; therefore, direct calculation of the equilibrium concentrations was possible without determining the distribution concentrations. For every system the sorption of even the first cationic complex species was found to be zero in the case of the Dowex 5OW-X12. Thus eqns. (10) and (11) directly gave the value of F as
A. MAHAN,
92 TABLE
VI
VALUES
OF FUNCTIONS
Nitroso-R (M.10’)
salt
2.0 2.5 3.0 3.5 4.5 5.0 5.5 6.0
TABLE VALUES
A. K. DEY
F, F, AND
F;!
F(SOW-X8)
F(S0 W-X4)
Average F
F, f *IQ-‘)
FJ-ii?-=)
2.98 3.99 5.18 6.60 9.65 11.35 13.20 16.10
2.91 4.17 5.16 6.42 9.37 11.59 13.55 16.22
2.94 4.08 5.17 6.50 9.5 1 11.47 13.37 16.16
0.97 1.23 1.39 1.57 1.89 2,09 2.24 2.53
3.60 3.92 3.80 3.77 3.64 3.68 3.61 3.80
VII OF STABILITY
Dowex SOW-X12 log P1
1%
7.02 7.03
-
Dowex
82
DJD,
CONSTANTS
b3
Pt
6.53 7.56
(FRONAEUS
50 W-X8 h3
-
(12
METHOD)
Dowex 50 W-X4 m3
6.46 6.85
Pi
1%
Fig. I32
12.48
1 2
= F =
Accordingly, the values of D&Do given in Table II for Dowex SO-Xl2 resin are not rewritten in Table VI, but are considered in the average values of F. The values of the constants obtained for the different resins are given in Table VII. The value of log & obtained by means of Dowex 5OW-X4 resin is 12.48, but, when the other two resins were used, it was not possible to read pz from the curve, since it passed through the origin. The Dowex SOW-X4 resin, being of low cross-linkage, shows the value of &. This value was also determined by the method of Loman and van Dalen: log fli = 6.93, and log /Iz = 13.57. No indication of the formation of a 1:3 complex species of cobalt was obtained by either method. The average values of the stepwise constants obtained by both methods and the corresponding free energies (AF) evaluated for these chelation processes are given in Table VIII. It is interesting to observe that when the cobalt(II)-nit&o-R system was studied spectrophotometrically by Lalor lo, the products of the reaction were found to include a cobalt(II1) complex. This product is formed in both alkaline and acidic media, the oxidation being brought about by oxygen in an alkaline medium, whereas in acidic solution the uncomplexed ligand is the oxidant; the complex in the system was found by L&or to be Co(L),. The present studies indicate that the complex species formed in the cobalt(II)nitroso-R salt system are Co(L) and COG, whereas no direct indication of the
THE
COBALT-NITROSO-R
TABLE
93
VIII
AVERAGE
VALUES
AND
FREE
ENERGIES“
Method
Collsttrrrt
HOEK, log K, lois P2 - AF:’ - AF:) - AF” 11Values
SALTSYSTEM
-
Avercrye
Fronneus
LOI~ICI~I-LXIII Dub
6.9 1 6.24 13.15 9.56 8.33 17.89
6.93 6.64 13.57 9.58 9.18 18.76
6.92 6.44 13.36 9.57 8.75 18.34
ure in kcal mole-‘.
formation of Co(L), was obtained. It is, of course, concluded that both the species present in the system are cationic, since eqns. (lo)-( 12) are followed. Obviously therefore if Co(L), is cationic, the formation of Co(L), is also indicated. The work was supported by the Council of Scientific and Industrial Research, New Delhi. and the financial assistance to A.M. is gratefully acknowledged. SUMMARY
The stability constants of the nitroso-R salt complexes of cobalt(H) were investigated by cation exchange by means of the methods of Fronaeus and of Lomanvan Dalen. The cation exchangers Dowex 5OW-X12, -X8, and -X4 (Na-forms) were employed for the distribution studies. All the measurements were performed at ionic strength 1.0 (NaNO,) and 30 +2”. The constants calculated from the distribution data by both methods were in good agreement. The average values were log K, =6.92, log K, = 6.44, and log /j, = 13.36. The corresponding free energies of the complexation processes were also evaluated.
Une etude est effectuee sur lesconstantes de stabilite des complexes cobalt( II)se1 de nitroso-R en utilisant les methodes de Fronaeus et de Loman-van Dalen avec Cchangeur de cations. (Dowex 5OW-X12, -X8 et -X4 (forme Na)). Toutes les mesures ont CtC effectuees g force ionique de 1.0 (NaNO,), h 30 f 2”. Les valeurs moyennes sont log K1 = 6.92, log Kz = 6.44 et log pz = 13.36. Les energies libres correspondantes sont egalement &al&es. ZUSAMMENFASSUNG
Die Stabilitatskonstanten der Njtroso-R-Salz-Komplexe von Kobalt(I1) wurden durch Kationenaustausch nach den Methoden von Fronaeus und von Lomanvan Dalen untersucht. Fiir die Verteilungsversuche wurden die Kationenaustauscher Dowex 5OW-X12, -X8 und -X4 (alle in der Na-Form) verwendet. Alle Messungen
94
A. MAHAN,
A. K. DEY
wurden bei Ionenstarke 1.0 (NaNO,) und 30+ 2” ausgeftihrt. Die nach beiden Methoden aus den Verteilungsergebnissen berechneten Konstanten stimmten gut tiberein. Die Mittelwerte waren Log K 1=6.92, Log K, = 6.44 und Log pz= 13.36. Die korrespondierenden freien Energien der Komplexbildungsreaktionen wurden ebenfalls berechnet. REFERENCES 1 A. Bancrjee and A. K. Dey. Proc. S_wp. Elecrrode Processes, Joclhpw, 1968, p. 149. 2 S. Mandal and A. K. Dey, Reu. Chim. Miner., 5 (19689 773. 3 0. Makitie, SUO~I. Kemktil., 4 (1968) 31. 4’ S. Fronaeus, Acta C/rem. Scartd., 5 (1951) 859; Soen. Kern.. Tidskr., 65 (1953) 19. 5 H. Loman and E. van Dnlen. J. fuorg. Nucl. C/~e~n., 28 ( 1966) 2037. 6 F. H. Pollard, P. Hanson and W. Genry, Antrl. C’hin~. Acfa. 20 (1959) 26. 7,A. J, Zielen, J. Amer. Cltem. Sot.. 81 (1959) 5022. 8 K. I&‘t%pper. D. Rcichenberg and D. K. Hale, J. Chert~. Sot-., ( 1952) 3 129. 9 Harrdbook of Chemistry md Physics. Chemical Rubber Publishing Co.. Obio, 1956. 10 G. C. Lalor, J. Inorg. Nucl. Chw., 30 (1968) 1925; 31 (1969) 1783.