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Study of catechol derivatives of thorium
.I. lnorg. Nud. Chem., 1962, Vol. 24, lap. 821 to 827. Pergamoa Press Ltd. Printed in England
STUDY OF CATECHOL DERIVATIVES OF THORIUM R. P. AGARWAL and R. C. MEHROTRA* Chemical Laboratories, University of Gorakhpur, Gorakhpur, U.P. (India) (Received 12 May 1961 ; in revised form 13 November 1961)
Abaraet--Complexing reactions of thorium with catechol have been studied by potentiometric and preparative methods. In acidic solution the formation of the mono-catechollate derivative only was indicated even in the presence of a large excess of catechol. The equilibrium constant K of the reaction: T h 4+ + C 6 H 4 ( O H ) 2 ~ [ T h ( C 6 H 4 0 2 ) ] 2+ +2H + was found to be: K = 2.98 × 10-s and the formation constant k of the complex was 5.25 x 1017. Potentiometric titrations of thorium nitrate in presence of catechol showed a sharp inflexion at four equivalents of alkali per mole of thorium ion, suggesting the possibility of an electrometric estimation method for thorium. New compounds (NH4)2[Th(C6H402)3]C6H602 and (C~HsN)eTh(C6H602)2 were isolated. IN view of the interesting results reportedO, 2) on the catechol derivatives o f zirconium from our laboratories, it was considered of interest to study similar derivatives of thorium. Although catechol derivatives of thorium have been synthesized by a number of workers, no physico-chemical study appears to have been carried out. In the present investigation, the systems have been investigated by a potentiometric method and formation constant of the thorium mono-catechollate derivative has been determined by calculating the equilibrium constant of the reaction: + OH Th 4 ÷ ~ O H ~ ~ O > T h 2 + q - 2 H
+.
The synthesis of catechol derivatives of thorium has been studied in detail by ROSENnEIM and co-workers,(3) who prepared well defined derivatives with formulae (NH4) 2 [Th(C6H402)3]. 7H20 and (NH4) 2 [Th3(C6H402) 6 (OH)z]. 1OH20. They have also reported an improved method for the preparation of the tri-catechollate derivative, (NH4) 2 [Th(C6H402)3] .5H20. H. SPERL(4) prepared the pyridine complex Th(OC6H4OH)4(CsHsN)2. A similar ammonium compound with the composition (NH4)2H2 [Th(C6H402)4] was obtained by slightly varying the conditions of preparation as described by ROSENnEIM.(3) It was found that the above compound was hydrolysed to (NH4)2 [Tha(C6H402)6(OH)2]. 1OH20 by excess of ammonia on boiling. It was also observed that the pyridine complex Th(OC6H4OH)4(CsHsN)2, reported earlier by SPERL,(4) was converted into (CsHsN)ETh (C6H402) 2 with excess of pyridine. * Present address: University of Rajasthan, Jaipur. tl) R. N. KAPOORand R. C. MErtROTRA,Z. Anorg. Chem. 293, 92 (1957). (z) R. N. KAPOORand R. C. MEHROTRA,Z. ,4norg'. Chem. 293, 104 (1957). (3) A. ROSENHEIM,B. RAIMANNand G. SCt-mNDEL,Z. Anorg. Chem. 196, 173 (1931). (4) R. WEINLANDand H. SPERL,Z. Anorg. Chem. 1.50, 72 0926). 821
822
R . P . AGARWAL and R. C. MEmtcYrgA EXPERIMENTAL
Materials. T h o r i u m nitrate, Th(NO3)4.4H20 employed was of B.D.H. reagent grade quality. Catechol was a Merck product. Potassium permanganate, caustic alkalies and acids were reagent grade products of B.D.H. A stock solution of thorium nitrate was standardized gravimetrically by precipitation with ammonium hydroxide and subsequent ignition of the hydroxide to ThO2. The solution of catechol was made by direct weighing. p H measurements were carried out at room temperature with a Phillips pH-meter (P.R. 9400), standardized against a 0.05 M solution of potassium hydrogen phthalate. Analytical methods. Thorium was estimated as ThO2 by direct ignition of the compound. The alkaline permanganate method¢5) of oxidation of organic compounds was found to oxidize catechol quantitatively to carbon dioxide and water and has been adopted in the present investigation for estimation of catechol. Preparative studies 1. Reaction of thorium nitrate with 3.8 to 4.0 mole of caustic potash in presence of different amounts of catechol In order to test th~ conclusion from potentiometric titrations that thorium monocatechollate is formed in the acidic medium irrespective of the molar ratio of catechol to thorium, thorium nitrate solution was treated with varying amounts of caustic potash (3.8 to 4.0 mole) in the presence of different amounts of catechol. In all the cases a greenish white precipitate having a thorium to catechol ratio approximately equal to one was obtained.
2. Reaction of thorium nitrate with ammonium hydroxide in presence of excess of catechol (2 a) The above experiment was also repeated with ammonium hydroxide (slightly less than 4 mole) instead of caustic I3otash. In this case as well analysis of the precipitate indicated a thorium to catechol ratio of approximately one. (2 b) A mixture of thorium nitrate (5.88 g, 0.01 g-mole) and catechol (8.8 g, 0.08 g/mole) was dissolved in water (20 mi) and was treated with ammonium hydroxide until faintly ammonical. A little insoluble material separated and was quickly filtered off. The filtrate was heated on a water bath for 5 min and cooled when a white crystalline compound separated. (2 c) In another experiment, more ammonium hydroxide was added to the above reaction mixture (2 b) which was boiled for about 2 min and then cooled. A white crystalline compound separated. The compounds were washed several times with dioxane and finally with water and dried in vacuum (1 mm) for about 6 hr at r o o m temperature (30 ° C).
Analysis. (2a) Th = 52.21, Catechol ----25.34 per cent, Ratio of thorium to Catechol = 0"941. (2b). F o u n d : Th = 3 2 . 4 2 , Catechol = 61.44, NH4 = 5.12 per cent. Calcd for ( N H 4 ) 2 H 2 [ T h ( C 6 H 4 0 2 ) 4 ] o r ( N H 4 ) 2 [ T h ( C 6 H 4 0 2 ) 3 ] ' C 6 H 6 0 2 : Th = 33"04, Catechol = 61.55, NH4 = 5.13 per cent. (2 c) F o u n d : Th = 42.92, Catechol = 40.45, NH4 = 2.24 per cent. Calculated for ( N H 4 ) 2 [ T h 3 ( C 6 H 4 0 2 ) 6 ( O H ) 2 ] ' 1 0 H 2 0 : Th -----43"42, Catechol = 40.47, NH4 = 2"25 per cent. 3. Reaction of thorium nitrate with pyridine in presence of excess of catechol The initial reaction mixture of (2 b) was treated with pyridine, 1 "6 and 2"0 ml in experiments (3 a) and (3 b) respectively. Yellow crystalline compounds were obtained. The compounds were washed with water and dried in vacuum (1 mm) at room temperature (30° C).
Analysis. 3(a) F o u n d : Th = 27.90, Catechol = 52.45, N = 3-35 per cent. Calculated for Th(OC6H4OH)4(C~HsN)2: Th = 28.0, Catechol=52.78, N = 3-38 per cent. ( 3 b ) F o u n d : Th = 38.05, Catechol = 35.89, N = 4.35 per cent. Calculated for ( C s H s N ) 2 T h ( C 6 H 4 0 2 ) 2 : Th = 38.26, Catechol = 35.65, N = 4.62 per cent. (5) G. BOTTGER and R. E. OESPER,Newer Methods of Volumetric Chemical Analysis, p. 55.
Study of catechol derivatives of thorium
823
ELECTROMETRIC STUDIES The potentiometric titration of thorium ion with catechol, shown in curves 1 and 2 (Fig. I) indicates chelation of the type: H
T 4+
(])
(J"~OH H
(A) 10-5 =0"0 9'5 9"0 8.5 8.0 7.5 7.0 6.5 6"0 5"5 5"0 4.5 3.2 3.0 2.8 2-3~
0
[
2
3
4
Moles of co*echol odded
FIG. 1.--Curve (1): Potentiometric titration of thorium nitrate (0.05 M, 20 ml) with molar catechol. Curves (2) and (3): Above titrations after the addition of 2 and 4 ml of molar caustic potash to the thorium nitrate solution. This chelation by the donation of a lone pair of electrons from the hydroxy oxygen atoms makes the hydrogen atom more labile and acidic and thus, the lowering of the pH of the system is a direct measure of chelation. The curve (3) (Fig. 1) also supports the above conclusion. The derivative (A) containing the charged complex cation remains in solution. Addition of alkali to the above reaction mixture neutralizes the hydrogen ions produced by chelation which tends to shift the above equilibrium (1) towards right
824
R . P . AOARWAL and R. C. MErmOT~
and thus favours the formation of (A). At higher pH hydrolytic effects in the system also come into play and may be represented: OHOH\ [Cat. Th(OH)] + ,~ [Cat. Th(On)z]
[Cat. Th]2+ \
(2)
\
In acidic medium curves 1, 2, 3, 4 and 5 (Fig. 2) for the potentiometric titrations of thorium nitrate with caustic potash in presence of 1, 2, 3, 4 and 5 moles of catechol respectively, are similar to each other with a common point o f inflexion at four equivalents of alkali indicating the similarity of reactions. Thus in acidic solution the formation of thorium mono-catechollate was indicated irrespective of the amount of catechol present in the system. This was confirmed by the actual isolation of the precipitates. However, beyond the inflexion point, m = 4 where m represents 12
I0
7 ~2L 6
I
0
i
2
I
I
4
I
I
6
I
I
8
I
I0
Moles of KOH added (rn)
FIG. 2.--Curves (0), (1), (2), (3), (4) and (5) represent potentiometrJc tJtrations of thorium nitrate (0.05 M, 20 m]) with caustic potash (1 M) in presence of 0, 1, 2, 3, 4 and 5 ml of catechol (1 M). CMrve (6): same as curve (5) but with ammonium hydroxide in place of caustic potash, m = mole of base added per mole of thorium ion.
Study of catechol derivatives of thorium
825
equivalents of base added per mole of thorium ion, the curves begin to fall short indicating equilibria of the type: OH[Cat. Th(OH)2]
Cat. \ [Cat. Th(OH)3]-
Cat. ~ [Cat. 2Th(OH)]-
\
/
~ [Cat. 3Th]2%
The inflexions at m = 4, 5, 6 in curves (1), (2) and (3) are easily explained by the above equilibria.
II
IO
9
8
Q.
6
5
4
2)] !
0
2
4 Volume
6 of
8
I0
IJ2
KOH (mOIor)
FIG. 3.---Curves (1), (2) and (3) represent potentiometric titrations of catechol (0.5 M, 20 ml) with molar caustic potash in presence of 5 ml of water, thorium nitrate (0.05 M) and nitric acid (0"2 M). In curves (4) and (5) (Fig. 2) inflexions occur at m ---- 7 and 8 respectively. These appear to be due to the neutralization of free catechol by the extra one or two moles of alkali respectively, but the possibility of further weak chelation by catechol could also be inferred from these observations. To distinguish between these possibilities potentiometric titrations of catechol with caustic potash (curves (1), (2), Fig. 3) in the absence and presence of small amounts of thorium ion were carried out and it
826
R . P . AGARWALand R. C. MEm~o~A
was found that the excess alkali consumed to the inflexion point in the experiment shown in curve (2) compared with that in curve (1) was equal to six moles per mole of the metal ion. This fact together with the inflexions at m = 7 and 8 in curves (4) and (5) (Fig. 2) indicate the probability of the formation of the tri-catechollate only. A compound having catechol to thorium ratio equal to four could only be isolated (preparation 2b) in the presence of a large excess of catechol. In view of the above, this compound appears to have the formula (NH4)2[Th(C6H402)3]. C6H602 rather than (NH4)2H2 [Th(C6H402)4]. EQUILIBRIUM AND FORMATION CONSTANTS
As catechol is prone to be oxidized by atmospheric oxygen, particularly in alkaline media, titrations have been repeated in an atmosphere of nitrogen, curves (l'), (2'), (Y) and (4') (Fig. 4). This also served the purpose of excluding carbon dioxide from the system. In these latter titrations, the ionic strength was maintained relatively constant by using a medium containing 0.1 M potassium nitrate and low concentrations of ligand and metal ion to enable calculations of equilibrium and formation constants.
i2
I0
~,~~ ~4'
8
n-7
i
0
I
2
I
I
4
I
I
6
I
I
!
O
I
I0
I
I
12
f
I
14
I
I
16
f
I
IO
mL of KOH (0.1 M)
Fie. 4.--Potentiometric titrations of thorium nitrate with caustic potash in presence of catecholat 0.1 M ionic strength in nitrogen atmosphere: Curve (19, 5 x 10-3 M in thorium nitrate and 5 x 10-3 M in catechol; Curve (2'), 5 x 10- 3 M in thorium nitrate and I x 10-2 M in catechol; Curve (3'), 5 x 10-3 M in thorium nitrate and 1.5 x 10-3 M in catechol; Curve (4"), 5 x 10-3 M in thorium nitrate and 2 x 10-2 M in catechoL (6> C. F. T w l B ~ r o s , ?. Chem. Soc. 4987 (1957).
Study of cateehol derivatives of thorium
827
As the mono-catechollate derivative alone appears to be formed in the acidic range, it was considered of interest to calculate the equilibrium constant of the reaction given by: K--
[ThA2+][H+] 2 [Th4+][H2 A]
K was calculated from the various concentrations of [ThA2+], [H+], [Th 4+] and [H2A] present in the equilibrium mixture. Over the pH range studied the concentrations of [HA-], [A 2-] and [OH-] were negligible compared to the other ionic species present. The values of p K were calculated at various points on the curves (l') to (4') (Fig. 4) from m --:--o to m ---- 1.5, where m represents moles of base added per mole of the metal ion. It was observed that as the molar ratio o f catechol to thorium in the reaction mixture was increased the constancy of the values of pK improved at the greater values of m. This indicated that in the presence of excess of catechol the reaction (1) is pushed towards right and the hydrolytic effects in the system are gradually reduced. Numerical data obtained for curve (4') are shown below. 0.2 pH 2.82 pK 4-542 Mean pK = 4.526.
MI. KOH
0.4 2.86 4.544
0.6 2.89 4.524
0.8 2.93 4.533
1.0 2.96 4.523
1.4
1.8
3.03 4-526
3.11 4.559
2.4 3.18 4.490
After determining the equilibrium constant of the reaction (I), the formation constant k of the complex ThA2÷ was determined from : K - - , w ere Kol (3"57 x 10 -1°) and k = Ko I K° 2
Ka2 (1"5 x
10-13) 6
represent dissociation constants of catechol. Thus the value of log k ---- 17.72 was obtained. Attempts were also made to calculate the equilibrium and formation constants for other equilibria indicated in the electrometric studies. However, on the alkaline side the hydrolytic effects cause too much disturbance for anything beyond very approximate values to be obtained. Although not investigated from that view point, the titration curves indicate that it would be possible to estimate thorium ions electrometrically with caustic potash when a pronounced inflexion at four equivalents of alkali is obtained.
Acknowledgement--Thanksof the authors are due to the Council of Scientific and Industrial Research authorities for providing a junior research fellowship to one of them (R.P.A.)
STUDY OF CATECHOL DERIVATIVES OF THORIUM R. P. AGARWAL and R. C. MEHROTRA* Chemical Laboratories, University of Gorakhpur, Gorakhpur, U.P. (India) (Received 12 May 1961 ; in revised form 13 November 1961)
Abaraet--Complexing reactions of thorium with catechol have been studied by potentiometric and preparative methods. In acidic solution the formation of the mono-catechollate derivative only was indicated even in the presence of a large excess of catechol. The equilibrium constant K of the reaction: T h 4+ + C 6 H 4 ( O H ) 2 ~ [ T h ( C 6 H 4 0 2 ) ] 2+ +2H + was found to be: K = 2.98 × 10-s and the formation constant k of the complex was 5.25 x 1017. Potentiometric titrations of thorium nitrate in presence of catechol showed a sharp inflexion at four equivalents of alkali per mole of thorium ion, suggesting the possibility of an electrometric estimation method for thorium. New compounds (NH4)2[Th(C6H402)3]C6H602 and (C~HsN)eTh(C6H602)2 were isolated. IN view of the interesting results reportedO, 2) on the catechol derivatives o f zirconium from our laboratories, it was considered of interest to study similar derivatives of thorium. Although catechol derivatives of thorium have been synthesized by a number of workers, no physico-chemical study appears to have been carried out. In the present investigation, the systems have been investigated by a potentiometric method and formation constant of the thorium mono-catechollate derivative has been determined by calculating the equilibrium constant of the reaction: + OH Th 4 ÷ ~ O H ~ ~ O > T h 2 + q - 2 H
+.
The synthesis of catechol derivatives of thorium has been studied in detail by ROSENnEIM and co-workers,(3) who prepared well defined derivatives with formulae (NH4) 2 [Th(C6H402)3]. 7H20 and (NH4) 2 [Th3(C6H402) 6 (OH)z]. 1OH20. They have also reported an improved method for the preparation of the tri-catechollate derivative, (NH4) 2 [Th(C6H402)3] .5H20. H. SPERL(4) prepared the pyridine complex Th(OC6H4OH)4(CsHsN)2. A similar ammonium compound with the composition (NH4)2H2 [Th(C6H402)4] was obtained by slightly varying the conditions of preparation as described by ROSENnEIM.(3) It was found that the above compound was hydrolysed to (NH4)2 [Tha(C6H402)6(OH)2]. 1OH20 by excess of ammonia on boiling. It was also observed that the pyridine complex Th(OC6H4OH)4(CsHsN)2, reported earlier by SPERL,(4) was converted into (CsHsN)ETh (C6H402) 2 with excess of pyridine. * Present address: University of Rajasthan, Jaipur. tl) R. N. KAPOORand R. C. MErtROTRA,Z. Anorg. Chem. 293, 92 (1957). (z) R. N. KAPOORand R. C. MEHROTRA,Z. ,4norg'. Chem. 293, 104 (1957). (3) A. ROSENHEIM,B. RAIMANNand G. SCt-mNDEL,Z. Anorg. Chem. 196, 173 (1931). (4) R. WEINLANDand H. SPERL,Z. Anorg. Chem. 1.50, 72 0926). 821
822
R . P . AGARWAL and R. C. MEmtcYrgA EXPERIMENTAL
Materials. T h o r i u m nitrate, Th(NO3)4.4H20 employed was of B.D.H. reagent grade quality. Catechol was a Merck product. Potassium permanganate, caustic alkalies and acids were reagent grade products of B.D.H. A stock solution of thorium nitrate was standardized gravimetrically by precipitation with ammonium hydroxide and subsequent ignition of the hydroxide to ThO2. The solution of catechol was made by direct weighing. p H measurements were carried out at room temperature with a Phillips pH-meter (P.R. 9400), standardized against a 0.05 M solution of potassium hydrogen phthalate. Analytical methods. Thorium was estimated as ThO2 by direct ignition of the compound. The alkaline permanganate method¢5) of oxidation of organic compounds was found to oxidize catechol quantitatively to carbon dioxide and water and has been adopted in the present investigation for estimation of catechol. Preparative studies 1. Reaction of thorium nitrate with 3.8 to 4.0 mole of caustic potash in presence of different amounts of catechol In order to test th~ conclusion from potentiometric titrations that thorium monocatechollate is formed in the acidic medium irrespective of the molar ratio of catechol to thorium, thorium nitrate solution was treated with varying amounts of caustic potash (3.8 to 4.0 mole) in the presence of different amounts of catechol. In all the cases a greenish white precipitate having a thorium to catechol ratio approximately equal to one was obtained.
2. Reaction of thorium nitrate with ammonium hydroxide in presence of excess of catechol (2 a) The above experiment was also repeated with ammonium hydroxide (slightly less than 4 mole) instead of caustic I3otash. In this case as well analysis of the precipitate indicated a thorium to catechol ratio of approximately one. (2 b) A mixture of thorium nitrate (5.88 g, 0.01 g-mole) and catechol (8.8 g, 0.08 g/mole) was dissolved in water (20 mi) and was treated with ammonium hydroxide until faintly ammonical. A little insoluble material separated and was quickly filtered off. The filtrate was heated on a water bath for 5 min and cooled when a white crystalline compound separated. (2 c) In another experiment, more ammonium hydroxide was added to the above reaction mixture (2 b) which was boiled for about 2 min and then cooled. A white crystalline compound separated. The compounds were washed several times with dioxane and finally with water and dried in vacuum (1 mm) for about 6 hr at r o o m temperature (30 ° C).
Analysis. (2a) Th = 52.21, Catechol ----25.34 per cent, Ratio of thorium to Catechol = 0"941. (2b). F o u n d : Th = 3 2 . 4 2 , Catechol = 61.44, NH4 = 5.12 per cent. Calcd for ( N H 4 ) 2 H 2 [ T h ( C 6 H 4 0 2 ) 4 ] o r ( N H 4 ) 2 [ T h ( C 6 H 4 0 2 ) 3 ] ' C 6 H 6 0 2 : Th = 33"04, Catechol = 61.55, NH4 = 5.13 per cent. (2 c) F o u n d : Th = 42.92, Catechol = 40.45, NH4 = 2.24 per cent. Calculated for ( N H 4 ) 2 [ T h 3 ( C 6 H 4 0 2 ) 6 ( O H ) 2 ] ' 1 0 H 2 0 : Th -----43"42, Catechol = 40.47, NH4 = 2"25 per cent. 3. Reaction of thorium nitrate with pyridine in presence of excess of catechol The initial reaction mixture of (2 b) was treated with pyridine, 1 "6 and 2"0 ml in experiments (3 a) and (3 b) respectively. Yellow crystalline compounds were obtained. The compounds were washed with water and dried in vacuum (1 mm) at room temperature (30° C).
Analysis. 3(a) F o u n d : Th = 27.90, Catechol = 52.45, N = 3-35 per cent. Calculated for Th(OC6H4OH)4(C~HsN)2: Th = 28.0, Catechol=52.78, N = 3-38 per cent. ( 3 b ) F o u n d : Th = 38.05, Catechol = 35.89, N = 4.35 per cent. Calculated for ( C s H s N ) 2 T h ( C 6 H 4 0 2 ) 2 : Th = 38.26, Catechol = 35.65, N = 4.62 per cent. (5) G. BOTTGER and R. E. OESPER,Newer Methods of Volumetric Chemical Analysis, p. 55.
Study of catechol derivatives of thorium
823
ELECTROMETRIC STUDIES The potentiometric titration of thorium ion with catechol, shown in curves 1 and 2 (Fig. I) indicates chelation of the type: H
T 4+
(])
(J"~OH H
(A) 10-5 =0"0 9'5 9"0 8.5 8.0 7.5 7.0 6.5 6"0 5"5 5"0 4.5 3.2 3.0 2.8 2-3~
0
[
2
3
4
Moles of co*echol odded
FIG. 1.--Curve (1): Potentiometric titration of thorium nitrate (0.05 M, 20 ml) with molar catechol. Curves (2) and (3): Above titrations after the addition of 2 and 4 ml of molar caustic potash to the thorium nitrate solution. This chelation by the donation of a lone pair of electrons from the hydroxy oxygen atoms makes the hydrogen atom more labile and acidic and thus, the lowering of the pH of the system is a direct measure of chelation. The curve (3) (Fig. 1) also supports the above conclusion. The derivative (A) containing the charged complex cation remains in solution. Addition of alkali to the above reaction mixture neutralizes the hydrogen ions produced by chelation which tends to shift the above equilibrium (1) towards right
824
R . P . AOARWAL and R. C. MErmOT~
and thus favours the formation of (A). At higher pH hydrolytic effects in the system also come into play and may be represented: OHOH\ [Cat. Th(OH)] + ,~ [Cat. Th(On)z]
[Cat. Th]2+ \
(2)
\
In acidic medium curves 1, 2, 3, 4 and 5 (Fig. 2) for the potentiometric titrations of thorium nitrate with caustic potash in presence of 1, 2, 3, 4 and 5 moles of catechol respectively, are similar to each other with a common point o f inflexion at four equivalents of alkali indicating the similarity of reactions. Thus in acidic solution the formation of thorium mono-catechollate was indicated irrespective of the amount of catechol present in the system. This was confirmed by the actual isolation of the precipitates. However, beyond the inflexion point, m = 4 where m represents 12
I0
7 ~2L 6
I
0
i
2
I
I
4
I
I
6
I
I
8
I
I0
Moles of KOH added (rn)
FIG. 2.--Curves (0), (1), (2), (3), (4) and (5) represent potentiometrJc tJtrations of thorium nitrate (0.05 M, 20 m]) with caustic potash (1 M) in presence of 0, 1, 2, 3, 4 and 5 ml of catechol (1 M). CMrve (6): same as curve (5) but with ammonium hydroxide in place of caustic potash, m = mole of base added per mole of thorium ion.
Study of catechol derivatives of thorium
825
equivalents of base added per mole of thorium ion, the curves begin to fall short indicating equilibria of the type: OH[Cat. Th(OH)2]
Cat. \ [Cat. Th(OH)3]-
Cat. ~ [Cat. 2Th(OH)]-
\
/
~ [Cat. 3Th]2%
The inflexions at m = 4, 5, 6 in curves (1), (2) and (3) are easily explained by the above equilibria.
II
IO
9
8
Q.
6
5
4
2)] !
0
2
4 Volume
6 of
8
I0
IJ2
KOH (mOIor)
FIG. 3.---Curves (1), (2) and (3) represent potentiometric titrations of catechol (0.5 M, 20 ml) with molar caustic potash in presence of 5 ml of water, thorium nitrate (0.05 M) and nitric acid (0"2 M). In curves (4) and (5) (Fig. 2) inflexions occur at m ---- 7 and 8 respectively. These appear to be due to the neutralization of free catechol by the extra one or two moles of alkali respectively, but the possibility of further weak chelation by catechol could also be inferred from these observations. To distinguish between these possibilities potentiometric titrations of catechol with caustic potash (curves (1), (2), Fig. 3) in the absence and presence of small amounts of thorium ion were carried out and it
826
R . P . AGARWALand R. C. MEm~o~A
was found that the excess alkali consumed to the inflexion point in the experiment shown in curve (2) compared with that in curve (1) was equal to six moles per mole of the metal ion. This fact together with the inflexions at m = 7 and 8 in curves (4) and (5) (Fig. 2) indicate the probability of the formation of the tri-catechollate only. A compound having catechol to thorium ratio equal to four could only be isolated (preparation 2b) in the presence of a large excess of catechol. In view of the above, this compound appears to have the formula (NH4)2[Th(C6H402)3]. C6H602 rather than (NH4)2H2 [Th(C6H402)4]. EQUILIBRIUM AND FORMATION CONSTANTS
As catechol is prone to be oxidized by atmospheric oxygen, particularly in alkaline media, titrations have been repeated in an atmosphere of nitrogen, curves (l'), (2'), (Y) and (4') (Fig. 4). This also served the purpose of excluding carbon dioxide from the system. In these latter titrations, the ionic strength was maintained relatively constant by using a medium containing 0.1 M potassium nitrate and low concentrations of ligand and metal ion to enable calculations of equilibrium and formation constants.
i2
I0
~,~~ ~4'
8
n-7
i
0
I
2
I
I
4
I
I
6
I
I
!
O
I
I0
I
I
12
f
I
14
I
I
16
f
I
IO
mL of KOH (0.1 M)
Fie. 4.--Potentiometric titrations of thorium nitrate with caustic potash in presence of catecholat 0.1 M ionic strength in nitrogen atmosphere: Curve (19, 5 x 10-3 M in thorium nitrate and 5 x 10-3 M in catechol; Curve (2'), 5 x 10- 3 M in thorium nitrate and I x 10-2 M in catechol; Curve (3'), 5 x 10-3 M in thorium nitrate and 1.5 x 10-3 M in catechol; Curve (4"), 5 x 10-3 M in thorium nitrate and 2 x 10-2 M in catechoL (6> C. F. T w l B ~ r o s , ?. Chem. Soc. 4987 (1957).
Study of cateehol derivatives of thorium
827
As the mono-catechollate derivative alone appears to be formed in the acidic range, it was considered of interest to calculate the equilibrium constant of the reaction given by: K--
[ThA2+][H+] 2 [Th4+][H2 A]
K was calculated from the various concentrations of [ThA2+], [H+], [Th 4+] and [H2A] present in the equilibrium mixture. Over the pH range studied the concentrations of [HA-], [A 2-] and [OH-] were negligible compared to the other ionic species present. The values of p K were calculated at various points on the curves (l') to (4') (Fig. 4) from m --:--o to m ---- 1.5, where m represents moles of base added per mole of the metal ion. It was observed that as the molar ratio o f catechol to thorium in the reaction mixture was increased the constancy of the values of pK improved at the greater values of m. This indicated that in the presence of excess of catechol the reaction (1) is pushed towards right and the hydrolytic effects in the system are gradually reduced. Numerical data obtained for curve (4') are shown below. 0.2 pH 2.82 pK 4-542 Mean pK = 4.526.
MI. KOH
0.4 2.86 4.544
0.6 2.89 4.524
0.8 2.93 4.533
1.0 2.96 4.523
1.4
1.8
3.03 4-526
3.11 4.559
2.4 3.18 4.490
After determining the equilibrium constant of the reaction (I), the formation constant k of the complex ThA2÷ was determined from : K - - , w ere Kol (3"57 x 10 -1°) and k = Ko I K° 2
Ka2 (1"5 x
10-13) 6
represent dissociation constants of catechol. Thus the value of log k ---- 17.72 was obtained. Attempts were also made to calculate the equilibrium and formation constants for other equilibria indicated in the electrometric studies. However, on the alkaline side the hydrolytic effects cause too much disturbance for anything beyond very approximate values to be obtained. Although not investigated from that view point, the titration curves indicate that it would be possible to estimate thorium ions electrometrically with caustic potash when a pronounced inflexion at four equivalents of alkali is obtained.
Acknowledgement--Thanksof the authors are due to the Council of Scientific and Industrial Research authorities for providing a junior research fellowship to one of them (R.P.A.)