Composition and stability of uranyl complexes of furfuryl thiol

J. inorg, nucl. Chem., 1973, Vol. 35, pp. 941-947.

Pergamon Press.

Printed in Great Britain

COMPOSITION A N D STABILITY OF U R A N Y L COMPLEXES OF F U R F U R Y L THIOL R. S. SAXENA and S. S. SHEELWANT Department of Chemistry, Malaviya Regional Engineering College, Jaipur, India (Received 30 March 1972)

Abstract-Complex formation between uranyl ion and furfuryl thiol has been studied by potentiometric and conductometric titration techniques in 50% ethanol. Two complexes, 1 : 1 and 1 : 2, are formed in the pH range 4-0-6.0 with considerable overlapping, the later complex being more pronounced. The log. Kcab values of these complexes, in presence of 0.1 M. NaCIO4, have been determined at 15°, 25° and 35°C and found to be 7.77, 7.37; 7.75, 7.34 and 7.73, 7.33, respectively. The values of AG, AH and AS calculated at 25°C are -20.50 kcal/mole,- 1.56kcal/mole and +63.56 cal/mole, respectively. INTRODUCTION ORGANIC c o m p o u n d s c o n t a i n i n g s u l p h y d r y l g r o u p s a r e w e l l k n o w n f o r t h e i r t e n d e n c y to f o r m c o m p l e x e s w i t h m e t a l s a n d s e v e r a l w o r k e r s h a v e s t u d i e d t h e i r c o m p l e x a t i o n w i t h v a r i o u s m e t a l s [ 1 - 5 ] . T h e u r a n y l - f u r f u r y l thiol s y s t e m h a s n o t b e e n examined. The compositions of the complexes have now been determined by potentiometric and conductometric titrations and their stability constants are calculated by applying Calvin and Melchior's extension of Bjerrum's method and further refined by various alternative methods such as the correction term method; N

method of solving the equations derived from ~

(h-

n)[A]nfln = 0; l e a s t - s q u a r e

n=O

method and graphical method. The values of thermodynamic functions, AG, A H a n d AS h a v e b e e n c a l c u l a t e d a t 25°C. EXPERIMENTAL Furfuryl mercaptan, obtained from Evan's Chemetics, New York, was of 99% purity. All other chemicals used were of AnalaR (BDH) quality. Freshly prepared solutions were used to avoid any effects of ageing and hydrolysis. pH values were measured on a Cambridge Bench pattern (null deflection type) pH meter associated with glass-calomel electrode assembly. Before and after each series of measurements, the pH meter was calibrated with the standard buffer solution. The conductance was measured by means of an electronic eye type conductometer. A Universal thermostat type U3 (German) was used to maintain the desired temperature. 50% ethanol concentration was maintained throughout the investigation. The experimental procedure involved a series of pH and conductometric titrations of furfuryl thiol (referred herein as FSH) in the absence and presence of various proportion of uranyl ions against standard NaOH solution. Calvin and Melchior's extension of Bjerrum's method was used to determine the stability constants of the complexes formed. The data used in these determinations were taken from the formation curves below pH 6.0 since precipitation occurred above this pH. 1. 2. 3. 4. 5.

N.C. Li and R. A. Manning, J. Am. chem. Soc. 77, 5225 (1955). 1. M. Klotz et al., J. Am. chem. Soc. 80 2920 (1958). J. L. Bear, G. R. Choppin and J. V. Quagliano, J. inorg, nucl. Chem. 25, 513 (1963). R. S. Saxena, K. C. Gupta and M. L. Mittal, Can. J. Chem. 46, 311 (1968). R. S. Saxena, K. C. Gupta and M. S. Mittal, A ust. J. Chem. 21(3), 641 (1968). 941

942

R.S.

S A X E N A and S. S. S H E E L W A N T RESULTS AND DISCUSSION

Stoichiometry The stoichiometry of the reaction between the uranyl ion and furfuryl thiol has been determined by potentiometric and conductometric titrations. Figure 1 represents the pH titrations of solutions containing 4.0 × 10-3 M in FSH (curve 1); 4.0× 10-3M in F S H + 4 . 0 × 10-3M in UO2(NO3)2 (curve 2); 4.0 x 10-3 M in FSH + 2.0 x 10-3 M in U O 2 ( N O 3 ) 2 (curve 3); 4-0 x 10-3 M in FSH + 1-33 x 10-3 M in UO2(NO3)2 (curve 4) and 4.0 x 10-3 M in FSH + 1.0 x 10-3 M in UO2(NO3)2 (curve 5) against 0-1 M NaOH. The abscissa represents the moles of NaOH added per mole of ligand "m". The sudden rise in pH on addition of NaOH to the free ligand indicates that proton of the -SH group is not titrable under the experimental conditions. The addition of an equimolar concentration of urarryl ions greatly alters the shape of the free ligand titration curve because of complex formation (Fig. 1, curve 2).

12"0t

8"0

114/f ¢

0

I-0 2.0 3"0 re, moles of NoOH/mole

4"0 of FSH

Fig. 1. pH titrations of F S H in absence and presence of UO22+ with 0"1 M N a O H . Curve 1 = 4.0 × 10-3 M F S H , Curve 2 = 4.0 × 10-3 M F S H + 4.0 × 10-3 M. UO2(NO3)2, Curve 3 = 4 . 0 × 10-aM F S H + 2 . 0 x 10-3M UO2(NOa)z, Curve 4 = 4 . 0 × 10-aM F S H + 1.33 × 10-3 M UO2(NO3)2, Curve 5 = 4.0 × 10-3 M F S H + 1.0 × 10-3 M UO2(NO3)~.

The fall in the initial pH value clearly shows the displacement of the proton on complexation. Since the extent of the proton displacement depends on the relative affinity of ligand for hydrogen ion and the metal ion, it is obvious from the curve that interaction of the UO2 ion with FSH is sufficient for it to compete with a relatively high concentration of hydrogen ions and hence there is a considerable lowering of the buffer region. The appearance of a precipitate and an inflection at m = 2 suggests the formation of UOz(FS)2 in accordance with the following equation

Uranyl complexes of furfuryl thiol

943

UO~2+ + F S H + 2 O H - ,~ ½UO2(FS)2 + H 2 0 +½ UO2(OH)2.

(1)

T h e titration curve of a solution containing metal ion and ligand in the ratio 1 : 2 offers additional information about the complexation process. An inflection at

m = 1 (Fig. 1, curve 3) corresponds to the formation of UO2(FS)2. Since there is no significant inflection at m = 0.5, the formation of UO2FS + and UO2(FS),, must overlap considerably in accordance with the following equations UO22+ + F S H + O H - ~ UO2FS + -4- H 2 0 UO2FS + + F S H + O H - ~ UO2(FS)2 + H20

(2) (3)

UO,/+ + 2 F S H + 2 O H

(4)

~ UO2(FS)z + 2 H 2 0

i.e, 1 : 1 and 1 : 2 complexes are being formed simultaneously. T h e inflection at m = 0.67 (Fig. 1, curve 4) when the ratio ofligand to metal is 3:1 and a t m ~- 0.5 (Fig. 1, curve 5) when the ratio is 4 : 1 confirm these conclusions.

Conductometric titrations Figure 2 shows the changes in conductance when F S H was titrated against

I

0

'

~

t,

-~ 4"C

~

0

5

q,

I

2"C

O

I

I-0

I

I

2.0

I

3-0

1

[

i.O 2"0 3'0 4'0 m~rnoles of NaOH/rnole of FSH Fig. 2. Conductometric titrations of FSH in absence and presence of UO2 2+ with 0-I M N a O H . Curve I = 4 . 0 x 1 0 - 3 M F S H , Curve 2 = 4 . 0 x 1 0 - ~ M FSH+4.0XI0-~M. UO2(NOa)2, Curve 3 = 4.0 × 10-3 M F S H + 2.0 × 10-3 M UO2(NOa)2, Curve 4 = 4.0 × 1 0 - 3 M F S H + I . 3 3 × 1 0 - s M UO2(NOa)2, Curve 5 = 4 . 0 × 1 0 - s M F S H + I . 0 × 1 0 -3 M UOdNOs)2.

944

R . S . S A X E N A and S. S. S H E E L W A N T

N a O H in the absence and presence of UO~ 2+. T h e abscissa represents the moles of N a O H added per mole of ligand "m". T h e titration o f the free ligand (curve 1) shows a repid increase in conductance from the starting point indicating the failure of the -SH group to dissociate, the change in conductance being merely due to additional N a O H . T h e titration curves 2, 3, 4 and 5 show the break at m = 2 when the ratio o f F S H to UO22+ in the solution is 1 : 1; and at m -- 1 when their ratio is 2: 1; at m ~ 0.67 when their ratio is 3 : 1 and at m ~ 0.5 when their ratio is 4: 1. T h e s e results are similar to those obtained by p H titrations and hence agree to the above conclusions. T h e potentiometric and conductometric titrations were also performed directly between F S H and UO22+ using different concentrations of the reactants (Fig. 3).

6"01~5"

~

4

-- 5"0

4"C

3-0

0

I

I'0

I

I

2"0 :3'0 4"0 Volumeof titront added

Fig. 3. Potentiometric and conductometric titrations between FSH and UO2(NOa)2. Potentiometric: Curve 1 = 25 ml of 1.0 mM FSH against 0.01 M UOz(NOa)2;

Conductometric: Curve 2 = 25 ml of 1.0 mM FSH against 0.01 M. UO~(NOa)2, Curve 3 = 25 ml of 0.4 mM. FSH against 0.01 M UOs(NOa)2, Curve 4 = 25 ml of 2.0 mM UO~(NOa)z against 0.1 M. FSH. Curve 1 represents the p H titration o f F S H against UO~(NOa)2. A gradual fall in p H observed till half a mole of UO2 ~÷ has been added, beyond this the p H becomes almost constant indicating that a 1 : 2 complex is formed. T h e breaks in the conductometric titration curves 2, 3 and 4 give results similar to that obtained by p H titration.

Stability constants T h e stability constants o f the complexes have been calculated adopting

Uranyl complexes of furfuryl thiol

945

Bjerrum's method[8] as extended by Calvin and Melchior[6]. The reaction equilibria may be represented by the following expressions: UO2 2+ + FS- ~ UO2FS + UO2FS + + FS- ~ UO2(FS)2 UOz 2++ 2FS- ~ UO~(FS)2"

(5) (6) (7)

The overall stability constant/3 is given by [UO2(FS)2] = KIK2 /3 = [UO22+I[FS_]2

(8)

where KI and K2 are the formation constants of the 1 : 1 and 1 : 2 complexes. The pH titrations of F S H in absence and presence of UO22+ were carried out at three different temperatures (Fig. 4). At any pH the ratio of the horizontal distance between the resulting curves 1-2; 3-4; 5-6 to the total concentration of the metal ions gives so-called "formation function" ~. Concentration of free ligand[A] at various pH's have been calculate from the relation [`4] = [ F S n ] t o t a l -

[UO2FS +] - 2[UO2(FS)2] [H+----!]+ 1 K,

(9)

where Ka, the dissociation constant of FSH, is 7.941 × 10-11. Formation curves for different temperatures were obtained by plotting h vs -log[,4] (Fig. 5). The values of log K1 and log K2 were read directly from these curves at 8 = 0.5 and 1.5, respectively, and were further refined using the following methods: (1) correction term method [7]. (2) method of solving the simultaneous equations derived from Bjerrum's formation function [8]. N

(~-- n) ['4]"13,, = 0.

(10)

n=0

For the present case N = 2, 1

gl =

h

[.41 (1--n)+(2--n)K2[,4]

(11)

and 1 Ft+(n--1)KI[A]

K2 = [.41 (2-- h) Kl [,4]

(12)

(3) Least-square method[7]. 6. M. Calvin and N. C. Melchior, J. Am. chem. Soc. 70, 3270 (1948). 7. H. Irving and H. S. Rossotti, J. chem. Soc. 3397 (1953). 8. J. Bjerrum, MetalAmine Formation inAquous Solutions. P. Hassee & Son., Copenhagen (1941).

946

R . S . S A X E N A and S. S. SHEELWANT

I0.0

8.0

IAJI

6-0

4-0 2,0 2.0

I 0

I

2"0

2'0

4'0

I

I

0

0 I

2"0

4.0

i

I

4"0

6"0

NaOH added,

ml

Fig. 4. pH titrations of FSH in absence and presence of UO~ 2+ with 0.1 M NaOH at different temperatures. Curves 1, 3 and 5 = 25 ml containing 8.0 × 10-3 M FSH + 0.1 M NaC10~ + 0.004 M. HCIO4 at 15°, 25 ° and 35°C, respectively. Curve 2, 4 and 6 = 25 ml containing 8.0× 10-aM F S H + 2 . 0 x 10-aM UOz(NO3)2+0'I M NaCIO4+0.004M H CIO, at 15 °, 25 ° and 35°C, respectively.

2"0

C.

~

~

2'0

0'5

O "5

7"0

O(;'5 o

8'5 7-5

I

7"0

i

I

7'5

t

-Log

8-5 I

8"5

[A'I

I

I

9'0

i

B

9"5

Fig. 5. Formation curves: (1) at 15°C; (2) atQ5°C; (3) at 35°C.

h 1)[.4] obtained at various ~ values. This plot is expressed by a straight line. (4) Graphical method[7], by plotting the values of(h -

vs

(2-n)[,4] (h - -

1)

U ranyl complexes of furfuryl thiol

h

947

_ (2 -- n) [AI - -

(n- 1)[AI

-(n- i5 ~ 2 - K I

(13)

with the slope K1K2and intercept --K1. T h e values o f log K~ and log K2 obtained by the various m e t h o d s are summarised in Table 1. Table 1. Methods

1 2 3 4 5 Mean

15°C

25°C

35°C

logKl

logK2

log/3

logKl

logK2

log/3

logKl

logK2

log/3

7.91 7.79 7.70 7.74 7.71 7.77

7.22 7.37 7-43 7-41 7-41 7.37

15.13 15.16 15.13 15.15 15-12 15.14

7.87 7-72 7-66 7-79 7.70 7.75

7-20 7-35 7.41 7.33 7.40 7-34

15-07 15.07 15.07 15.12 15.10 15-09

7.85 7.71 7.64 7.79 7.67 7'73

7"18 7.34 7.39 7.31 7-41 7.33

15"03 15.05 15.03 15.10 15-08 15.06

1. Calvin and Melchior's extension of Bjerrum's method. 2. Correction term method. 3. Solving the equations derived from ~ (a - n) [A] n/3n = 0. rt=0

4. Least-square method. 5. Graphical method.

Thermodynamic functions T h e values of overall changes in free energy (AG), enthalpy (AH) and entropy (AS) a c c o m p a n y i n g complexation have been determined at 25°C with the help o f standard equations [9]. AG is found to be - 20.50 kcal/mole, A H = - 1.56 kcal/ mole and AS = + 63.56 cal/mole. 9. K. B. Yatsimirskii and V. P. Vasil'Ev. Instability Constants o f Complex Compounds. Pergamon Press, Oxford (1960). Acknowledgements - We thank the Ministry of Education, Government of India, for granting research fellowship to one of us (S.S.S.) and to Principal R. M. Advani for providing research facilities.