KINETIC STUDIES ON THE SOLVENT EXTRACTION OF LANTHANUM(III) AND VANADIUM(V) FROM
AQUEOUS SOLUTION WITH CHLOROFORM SOLUTION OF 1-PHENYL-3-METHYL-4-BUTANOYLPYRAZOL-5-
ONE AND 1-PHENYL-3-METHYL-4-STEAROYLPYRAZOL-5-ONE.
A.I.C. Ehirim and M. O. C. Ogwuegbu
Department of Chemistry, School of Science,
Federal University of Technology, PMB 1526, Owerri, Nigeria.
Corresponding author: ikmil2002@yahoo.com. ; Phone: +234-7030561903
ABSTRACT
The rate of recovery of La(III) and V(V) from aqueous solution by chloroform solution of 1phenyl-3-methyl-4-butanoyl pyrazol-5-one (HBPy) and 1-phenyl-3-methyl -4- stearoylpyrazol-
5-one (HSPy) has been investigated at various conditions. Experimental observation shows that the rate is first order and proportional to [M n+
], [HBPy] or [HSPy] and [H
+
]
-1
; where
M n+
= La
3+
and VO
+
2
. The rate constant, k f
has been determined as 9.79min
-1
, 0.28min
-1
,
130.01min
-1 and 0.42min
-1 for La(BPy)
3
, VO
2
(BPy)
2
, La(SPy)
3
and VO
2
(SPy)
2
respectively; while the extraction equilibrium constants, K ex
are -1.64, -1.11, -0.81 and -1.09 for the La(BPy)
3
,
VO
2
(BPy)
2
, La(SPy)
3
and VO
2
(SPy)
2
respectively. The rate determining step is the formation of the first chelate ring of mono-pyrazolonato metal complex; [La(H
2
O)
4
BPy]
2+
and
[VO
2
(H
2
O)
4
(BPy)]
+
; (BPy can be SPy). There is high positive relationship between the rate constants k f of La and V (BPy) and those of their (SPy) homologue, but high negative relationship between their extraction equilibrium constants K ex
, hence HBPy and HSPy can be used as close substitutes in the extraction of the metals studied. The HBPy is a more efficient ligand for the isolation of the metals from their mixture.
Key words : Kinetics, Solvent extraction, Acylpyrazolones, Equilibrium constant, Correlation,
Metals.
INTRODUCTION
Extraction of metals through the process of solvent extraction is a key step in many hydrometallurgical processes. Solvent extraction studies of certain metals have been undertaken by various workers (Ogwuegbu, 1992, Marchetti et al, 2005), using β-diketones, especially 4acyl derivatives of 1-phenyl-3-methylpyrazolone which are known to have several advantages over 2-theonyltrifluoroacetone (HTTA) and its derivatives. For instance, the metal chelate complexes of the 1-phenyl-3-methylpyrazolone have high extracting ability, intense colour of the complex extracts; are highly soluble in most organic solvents and form highly stable neutral metal complexes that are principally hydrophobic. Moreover, the formation of the metal complexes with acylpyrazolones is applied for the separation of elements with similar properties,
1

i.e. lanthanides, coinage metals, actinides, early transition metals, etc (Nishihama et al., 2001).
4-acylpyrazol-5-ones, as modified β-diketones, are able to extract metal ions at lower pH values than open-chain β–diketones (Mickler et al., 1995). Therefore, they offer the possibility of avoiding the pH region where hydrolysis of the metal ions takes place. (Ogwuegbu et al., 1996).
Again, the peripheral positions 1, 3 and 4 in the pyrazolone can be easily changed with different alkyl and other groups in order to vary the electronic and steric features of the acylpyrazolone ligands. The 4-benzoyl derivative of 1-phenyl-3-methylpyrazolone has received much attention in the selective extraction of metal ions from acid solutions (Uzoukwu & Ukeje, 1997). In order to understand the factors that determine selectivity during solvent extraction, the rate and mechanism of the recovery of two metal ions have been examined. The rate of recovery of metal ions from aqueous solution has been postulated to be slow when the charge is large and/ or the radius is small. This has been attributed to the slow substitution of the coordinated water molecules on the metal (Uzoukwu & Adiukwu, 1996). The present study was carried out to measure the rate of recovery of La(III) and V(V) from aqueous solutions and to determine the mechanism that may be involved, besides assessing the correlation that exist between the use of the two ligands in the solvent extraction of the metal ions and the possibility of their use in separating the metal ions studied.
EXPERIMENTAL METHODS
REAGENTS: Analytical grade reagents were used. These included ethylacetoacetate, phenylhydrazine, the corresponding acyl chlorides (butanoyl chloride and stearoyl chloride),
95% ethanol, and different salts of the metals; La(III) and V(V) in their pure forms.
Distilled de-mineralized water was also used.
SYNTHESES OF THE LIGANDS (HBPy and HSPy)
The ligands, 1-phenyl-3-methyl-4-butanoyl pyrazol-5-one (HBPy) and 1-phenyl-3methyl -4- stearoylpyrazol-5-one (HSPy) were synthesized and characterized according to the procedures outlined in the literature (Jensen, 1959; Okafor et al, 1990; Ogwuegbu et al, 1998;
Arinze et al, 2012; Okpareke et al., 2012.).
First, the 1-phenyl-3-methylpyrazol-5-one was prepared from condensation between the phenylhydrazine and ethyl acetoacetate.(Brian, et al, 1989). 50 g (49 ml, 0.384 mol) of ethyl acetoacetate and 40 g (36.5 ml, 0.37 mol) of phenylhydrazine were mixed in an evaporating dish and heated on a water bath in a fume cupboard for 2 hours, stirring from time to time with a glass rod. Heavy syrup was formed and allowed to cool to an extent; then, 100ml of ether was added and stirred vigorously. The syrup, which is insoluble in ether, solidified within 15 minutes. It was filtered and washed thoroughly with ether to remove coloured impurities and recrystallized from hot water. Colourless crystals were formed, with melting point of 127.3 o
C.
15.0 g of the 1-phenyl-3-methylpyrazol-5-one were dissolved in 75 ml dioxane with gentle warming in a 500 ml three-necked round bottom, quick-fit flask equipped with a magnetic
2

stirrer, separating funnel and reflux condenser. 12 g of calcium hydroxide were added to form a paste; followed by drop-wise addition of 10ml butanoyl chloride and stearoyl chloride separately, within 2-5 minutes. The mixture was continuously stirred and gently refluxed for 90 minutes till the yellow calcium complex was formed. It was allowed to cool and the calcium complex decomposed by pouring in 200 ml of 2 M HCl, whereby cream coloured crude crystals precipitated and was recrystallized from an ethanol-water (65% – 35%) mixture slightly acidified to destroy any undecomposed calcium-complex.
The products, 1-phenyl-3-methyl-4-butanoyl pyrazol-5-one (HBPy) and 1-phenyl-3methyl -4- stearoylpyrazol-5-one (HSPy) were characterized through such analytical methods as:
IR, UV-visible, elemental analysis, conductivity, melting points, and colour.
The Infrared spectral measurements were obtained on a Shimadzu 8400 FTIR spectrophotometer. All samples were prepared as transparent KBr pellets using a tenton Carver press; while the UV-visible spectra were recorded on a Pye Unicam SP8-100 spectrophotometer.
They were then used in extracting the V(V) and La(III) ions.
Extraction Procedure
Studies on the rate of recovery of the two metals were carried out by agitating 5 ml of the aqueous solution containing 5 mg/l of the V(V) and La(III) with 5 ml of the various concentrations of HBPy and HSPy (0.01 – 0.05 M) at a fixed pH of 2 for specific time of 30 minutes at 26°C ± 0.5°C. The study was also carried out by agitating the same 5 ml of the V(V) and La(III) solutions at various pH values of 2, 4, 6 and 8 for a specific time of 30 minutes with
5 ml of 0.02 M solution of HBPy and HSPy in chloroform using a mechanical shaker. For the pH studies, the aqueous solutions of the metallic ion were adjusted to different pH values using HCl and Na
2
CO
3 to cover pH 2 to 8. The pH values were measured with a digital pH meter Model pHS-25.
The phases were separated at each extraction process and the concentrations of the V(V) and
La(III) remaining in the aqueous phase were determined spectrophotometrically using a Pye
Unicam SP8-100 spectrophotometer at wavelength of 318.4 nm and 550.1 nm for the V(V) and
La(III) respectively. The metal extracted into the organic phase in each case was found by the difference (Ogwuegbu, 1996).
EXTRACTION EQUILIBRIUM
The extraction of the metal ion (M n+ ) from an aqueous phase using the ligands (HBPy and HSPy) in the organic phase can be treated as follows:
M n+
+ xHBPy
(org)
K ex
M(BPy)n
(org)
+ xH
+
………. (1)
Where K ex
is the extraction equilibrium constant; HBPy can also be HSPy.
K ex
= [M(BPy)n
(org)
].[H
+
] x
/ [M n+
].
[HBPy
(org)
] x
…. (2)
3

The Distribution ratio, D, which is the ratio of the concentration of the metal into the organic phase to that in the aqueous phase, is given as:
D = [M(BPy)n
(org)
] / [M n+ ] ……………………………………….. … (3)
Substitution of D in equation (3) into equation (2) gives:
K ex
= D.[H + ] x / [HBPy(org)] x ………………………………..…(4)
D = K ex
.[HBPy(org)] x
/ [H
+
] x
……………………………………….(5)
Taking the logarithms of both sides of equation (5) gives:
Log D = Log K ex
+ log[HBPy
(org)
] x
– log [H
+
] x …………………….(6)
Or; Log D = Log K ex
+ xlog[HBPy
(org)
] – xlog [H
+
]
…………………..(7)
Since pH = -log [H + ]; equation (7) can be re-written as;
Log D = Log K ex
+ xlog[HBPy
(org)
] + xpH ……………………………...(8)
From equation (8), therefore; plots of log D versus log [HBPy
(org)
] and pH are linear, with slopes
(x) equal to the number of moles of ligands involved in the complexation and the number of protons (H
+
) displaced from the acidified aqueous solution of the metal ions under study.
The data on the logarithms of the distribution ratios (D) and the extraction equilibria (K ex
) are presented on table 1.
The results presented in Table 1 show that the distribution ratio, D is a function of x moles of the ligand concentrations as presented in equation (8). The values of x determined from the slopes of the plots of log D versus log [HBPy
(org)
] and log [HSPy
(org)
] as shown on figs. (1) and (2) gave
2.53 for La and 1.8 for V HBPy and 1.91 and 2.91 for the V and La HSPy respectively. Hence, approximately 2 moles each of the HBPy and HSPy ligands reacted with one mole of VO +
2
while
3 moles of the same ligands reacted with La
3+
during the extraction process. This may have indicated that the metal ions existed as hydrated mononuclear species in the aqueous phase while the species extracted into the organic phase were La(BPy)
3
, VO
2
(BPy)
2
for the butanoylpyrazolones and La(SPy)
3
and VO
2
(SPy)
2
for the stearoylpyrazolones.
In the metal ions understudy, there is increase in the extraction process with increase in the concentration of either of the ligands; hence 0.05M of ligands gave the highest extraction while
0.01M gave the least. Again, stearoylpyrazolone proved a better extractant than its butanoyl homologue for the La(III) and V(V) ions at higher concentrations while butanoylpyrazolone is more effective at lower concentrations of the ligands.
The average logarithms of the equilibrium constant (K ex
) values obtained for the metals at the different HBPy concentrations at a constant pH of 2 (Table 1) are -1.64 (La) and -1.11 (V);
4

showing that the ligand is more efficient in the recovery of V(V) than the La(III) from the aqueous solution. This could be as a result of the smaller size of the former.
The values are related to those of the HSPy which are -0.81 (La) and -1.09 (V); but shows that the ligand is more efficient in the recovery of La(III) than the V(V) from their aqueous solutions.
This means that even though the concentrations of the ligands affect the extraction of the metal ions, the alkylation or substituent at the 4-acyl of the ligands may be playing vital roles in determining the trend of the stability M—O bond of metal complexes during extraction process.
Table 1 -- Data on the logarithms of the distribution ratios (D) and extraction equilibria of the metal ions from their aqueous solutions at pH 2 at various concentrations of HBPy and HSPy ligands:
VO
2
(BPy)
2
La(BPy)
3
VO
2
(SPy)
2
La(SPy)
3
Log[HBPy] Log D Log K ex
Log D Log K ex
Log D Log K ex
Log D Log K ex
-2.00
-1.70
-1.06 -1.06
-0.14 -1.01
-1.38 -1.38
-0.79 -1.69
-1.06 -1.06
-0.50 -1.10
-1.38 -1.38
0.00 -0.60
-1.52
-1.40
-1.30
-0.18
0.07
0.21
-1.13
-1.14
-1.19
-0.29
0.10
0.37
-1.72
-1.70
-1.73
-0.10
0.10
0.25
-1.06
-1.10
-1.15
0.33
0.37
0.79
-0.63
-0.84
-0.61
Average log K ex
: VO
2
(BPy)
2
= -1.11, La(BPy)
3
= -1.64, VO
2
(SPy)
2
= -1.09 and La(SPy)
3
= -0.81
Fig.1 - Plot of log of distribution ratio, D of the La and V ions versus log of HBPy concentration at pH 2.
5
Fig.2 - Plot of log of distribution ratio, D of the La and V ions versus log of HSPy concentration at pH 2.

Fig.2 - Plot of log of distribution ratio, D of the La and V ions versus log of HSPy concentration at pH 2.
DEPENDENCE OF EXTRACTION ON pH
The data on the logarithms of the distribution ratios (D) and the extraction equilibria (K ex
) on the extraction of the metal ions at different pH of their aqueous solutions using 0.02 M of the HBPy and HSPy are presented on table 2.
The plots of log D versus pH at a constant concentration of 0.02 M of each of the ligands (HBPy and HSPy) presented in figures 3 and 4 are linear and show inverse relationship, showing that higher acidity favours the extraction; alternatively, there is regression of extraction at higher pH values, probably due to metal hydrolysis. Similar reports have been given elsewhere (Ogwuegbu
& Oforka, 1994; Onyedika, Arinze & Ogwuegbu, 2013).
The slopes of approximately -2.0; {-2.12 for VO
+
2
(BPy)
2
} and -3.0 (-2.83 for La(BPy)
3
} obtained in fig (3) and {-1.86 for VO
2
(SPy)
2 and also -2.83 for La(SPy)
3
}, obtained in fig 4 for the V(V) and La(III) respectively indicate that D is inverse second and third powers respectively dependent on hydrogen ion concentration of the aqueous phase at room temperature and that two and three moles respectively of H
+
ions were released per mole of V(V) and La(III) respectively on the formation of the extractable complexes by the ligands. Again, this confirms the formation of simple metal chelates; VO
2
X
2
and LaX
3
(X = BPy or SPy). Similar metal chelates have been observed by many investigators in the extraction of metals with acylpyrazolones (Pai et al, 2000;
Bhattacharya et al, 2004).
The average values of log K ex
are; La(BPy)
3
= -7.67; VO
2
(BPy)
2
= -7.62, for the butanoylpyrazolones and La(SPy)
3
= -7.57 and VO
2
(SPy)
2
= -7.59, for the stearoyl homologue.
The values for the metals with each ligand are very close and follow the trend: V(V) > La(III) for
6

the HBPy ligand and the reverse for the HSPy; which is exactly in uniformity with that of the concentration for the ligands.
Table 2. -- Data on the logarithms of the distribution ratios (D) and the extraction equilibria of the La(III) and V(V) ions at different pH of their aqueous solutions using 0.02 M of the HBPy and HSPy ligands:
VO
2
(BPy)
2
La(BPy)
3
VO
2
(SPy)
2
La(SPy)
3 pH Log D Log K ex
Log D Log K ex
Log D Log K ex log D Log K ex
2 -0.41 -1.01 -0.1 -0.71 -0.50 -1.10
4 -0.79 -5.39 -0.79 -5.39 -0.79 -5.39
0.00
-0.79
-0.60
-5.39
6 -1.19 -9.80 -1.69 -10.30 -0.95 -9.56 -1.38
8 -1.69 -14.29 -1.69 -14.30 -1.69 -14.29 -1.69
-9.98
-14.29
Average log K ex:
: La(BPy)
3
= -7.67; VO
2
(BPy)
2
= -7.62; La(SPy)
3
= -7.57; VO
2
(SPy)
2
= -7.59.
Fig. 3 - Plots of logs of distribution ratio, D of the metallic ions versus pH at 0.02 M of HBPy concentrations.
7

Fig 4.- Plots of logs. of distribution ratio, D of the metallic ions versus pH at 0.02 M of HSPy concentrations.
SEPARATION FACTOR (S.F):
The separation factor between the ions studied; defined as the ratio of the equilibrium constants
(K ex
) of the La/V extracted, with the ligands are 1.48 and 1.35 for the HBPy and HSPy respectively.
It is clear from the results that selectivity among the metals with HBPy and HSPy ligands follows the order: HBPy > HSPy. This trend clearly highlights that the separation of these metals becomes poorer as the extractability increases. The S.F. values observed are comparable with those obtained for Thulium (Tm) and Europium (Eu) using the ligands: 3-phenyl-4-benzoyl-5isoxazolone (HPBI); ( Tm/Eu = 1.24
), 3-phenyl-4-(4-fluorobenzoyl)-5-isoxazolone (HFBPI);
( Tm/Eu =1.55
) and 3-phenyl-4-(4-toluoyl)-5-isoxazolone (HTPI) ( Tm/Eu = 1.80
) (Rani, 2005).
CORRELATION, r, OF K ex
OF EXTRACTION OF HBPy AND HSPy
Pearson’s Product moment correlation coefficient was used to compare the equilibrium constants of the extraction of the metal ions using the HBPy and the HSPy. This is because there are two variables of interest HBPy and HSPy and the data of the variables are both interval in nature
(Nwachukwu, 2007). The correlation coefficient, r value is -1.0, showing that there is perfect negative relationship between them.
8

RATE OF RECOVERY OF THE METALS
The rate of recovery of the metal ions from the aqueous phase was followed as the rate of disappearance of the metal ions, M n+
from the aqueous phase during the solvent extraction, and is represented by the following equation:
-d[M n+
]/dt = k f
[M n+
] a
. [HBPy
(org)
] b
. [H
+
] c …………………… 9; where k f
is the rate constant for the recovery process.
When [HBPy
(org)
] and [H
+
] are present in excess, equation 9 becomes:
-d[M n+ ] /dt = K[M n+ ] a …………………………………10
Substitution of the data in Table 3(a) into the rate law expression for a first order reaction;
K = 2.303/t .log a/a-x (a = initial concentration; a-x = concentration at any given time, t) gave relatively constant values of K (Table 3b), showing that equation (10) is first order, hence the superscript, a =1. Therefore integration of eq. (10) will give:
-log[M n+
] = Kt + C ………………………………………….. 11;
where C is a constant and K= k f
[HBPy
(org)
] b .[H + ] C ……….. 12
Table 3(a) gives the relevant data for the plots of eq. (11) shown on figs. 5 and 6 for the metals studied. Again, linear plots were obtained to further confirm the first order nature of the extraction process.
Table 3(a) – Data on the extent of extraction of the metal ions for one hour at an interval of 10 minutes using the HBPy and HSPy ligand:
La(BPy)
3
VO
2
(BPy)
2
La(SPy)
3
VO
2
(SPy)
2
T(mins) [La
3+
] -log[La
3+
] [VO
+
2
] -log[VO
+
2
] [La
3+
] -log[La
3+
] [VO
+
2
] -log[VO
+
2
]
10 0.38 0.42 0.38 0.42 0.40 0.40 0.39 0.41
20 0.32 0.49
30 0.25 0.60
0.34 0.47
0.30 0.52
0.36 0.44
0.33 0.48
0.34
0.29
0.47
0.54
40
50
60
0.22
0.20
0.17
0.66
0.70
0.77
0.23 0.64
0.15 0.82
0.12 0.92
0.30 0.52
0.26 0.59
0.25 0.60
0.23
0.10
0.08
0.64
1.00
1.10
9

Table 3 (b) -- The values of the rate constants, K obtained from the substitution of the kinetic data in table 3(a) into the first order rate expression:
Time (mins) K[La(BPy)
3
]
(min
-1
)
10 0.02745
20
30
40
50
60
0.02232
0.02311
0.02053
0.01833
0.01798
K[VO
2
(BPy)
2
]
3
K[VO
2
(SPy)
2
]
(min
-1
) (min
-1
) (min
-1
)
0.02745 0.02232 0.02485
0.01929
0.01703
0.01942
0.02408
0.02379
0.01643
0.01853
0.01277
0.01833
0.01155
0.01929
0.01816
0.01942
0.03219
0.03055
Fig. 5.– Plot of data on the extent of extraction of the metal ions for one hour at an interval of 10 minutes using the HBPy ligand:
10

Fig. 6.– Plot of data on the extent of extraction of the metal ions for one hour at an interval of 10 minutes using the HSPy ligand:
To obtain the reaction orders, b and c, measurements are made with one of the quantities
[HPy
(org)
] or [H
+
] kept fixed while the other is varied. Thus, when [H
+
] is kept constant, eq. 12 becomes:
Log K
HBPy
= log k f
[H
+
] c
+ blog[HBPy
(org)
] ……………………13; from which b can be determined, as shown in fig.7 and 8.
Similarly, when [HPy
(org)
] is kept constant, eq.12 becomes:
Log K
H
= log k f
[HBPy] b
+ clog[H
+ ] …………………………. 14; from which c can be determined from the slope of a plot of log K
H
against log[H
+
] or –log K
H versus pH, as shown in figs. 9 and 10.
The rate constant for the recovery process, k f
can then be calculated from the intercept of plot of equation 13 or 14 when values of a, b and c are known.
Table 4 gives the relevant data for the plots of eq. 13 shown on figs. 7 and 8 while table 5 gives the relevant data for the plots of eq.14 shown on figs. 9 and 10 for the metals studied.
Again, linear plots were obtained.
Figure 7 shows that the slopes, b, of log K
HBPy
versus log[HBPy
(org)
] for the metals are:1.84 (La) and 1.30 (V) while fig. 8 gives the slopes of the same plot for the HSPy as: 1.82 (La), and
11

1.33(V). Similar plots of -Log K
H
versus pH (figs. 9 and 10) for each of the metal ions had slopes, c, of 0.14 (La) and 0.19 (V) for the HBPy and 0.21 (La) and 0.17 (V) for the HSPy.
The values of k f
for the metals were determined from the intercepts of fig. 7 {0.52 (La) and -
0.23(V)} and those of fig. 8. (0.69 and -0.17 for La and V); and are found to be 9.79min
-1
for
(La(BPy)
3
and 0.28min
-1
for VO
2
(BPy)
2
. The k f
for the HSPy are 130.01min
-1
for La and
0.42min
-1
for V.
CORRELATION, r, OF k f
OF THE EXTRACTION RATE CONSTANTS OF HBPy AND
HSPy
Pearson’s Product moment correlation coefficient was used to compare the rate constants of the extraction of the metal ions using the HBPy and the HSPy; the correlation coefficient, r value is
1.0; showing that there is a perfect positive relationship between their rate constants.
Table 4. – Rate of extraction of the metal ions at different concentrations of the HBPy and HSPy ligands at pH 2 for
30 minutes:
[HBPy] Log
[HBPy]
0.01
0.02
0.03
0.04
0.05
-2.00
-1.70
-1.52
-1.40
-1.30
La(BPy)
3
Rate(K)
0.00067
0.0023
0.0057
0.0093
0.012
Log K K
-3.17
-2.64
-2.24
-2.03
-1.92
VO
0.0013
0.0047
0.0067
0.0090
0.010
2
(BPy)
2
Log K
-2.88
-2.33
-2.18
-2.01
-1.99
La(SPy)
K
3
Log K
0.0013 -2.88
0.005
0.01
0.012
0.014
-2.30
-2.00
-1.93
-1.86
VO
K
2
0.0013
0.004
(SPy)
2
Log K
-2.88
-2.40
0.0073 -2.13
0.0093 -2.03
0.011 -1.97
12

Fig. 7 – Plot of data on the rate of extraction of the La (III) and V(V) ions at different HBPy ligand concentration:
Fig. 8. – Plot of data on the rate of extraction of the metal ions at different HSPy ligand concentration:
13

Table 5 – Rate of extraction (K) of the metal ions at different pH of the aqueous solution of the metal ions at 0.02M of HBPy ligand for 30 minutes:
6
8 pH
2
4
La(BPy)
3
K -Log K
0.0023
0.00067
2.63
3.18
0.00033
0.00033
3.48
3.48
VO
2
(BPy)
2
K -Log K
0.0046
0.0023
2.33
2.63
0.0010
0.00033
3.00
3.48
La(SPy)
3
K -Log K
0.005
0.0027
2.30
2.58
0.0067
0.00033
3.18
3.48
VO
2
(SPy)
2
K -Log K
0.004
0.0023
2.40
2.63
0.0017
0.00033
2.78
3.48
Fig. 9. – Plots of data on the rate of extraction of the metal ions at different pH values at 0.02 M HBPy ligand concentration.
Fig. 10 – Plots of data on the rate of extraction of the metal ions at different pH values at 0.02 M HSPy ligand concentration.
14

MECHANISM:
The steps involved in the extraction of the metal ions from the aqueous phase into the organic phase are as follows:
HBPy
(org)
HBPy
(aq)
…………………………...15
HBPy
(aq)
H
+
+ BPy
-
…………………………16
M n+ + BPy M(BPy) n-1 ………………………17
M(BPy)
M(BPy) n-1
+ BPy
+1 n-1
-
+ BPy
-
M(BPy)
2 n-2 ………………18
M(BPy) n
……..……….
….19
M(BPy) n(aq)
M(BPy) n(org)
….…………………..20
`
Since eqs. 15 and 20 do not involve a pH dependent process, they cannot be considered as rate determining steps. Equation 16 can also be eliminated on the grounds that it does not show first order dependence on the metal ion. Equations 18 and 19 are eliminated on the ground that they show dependence on the square or more of ligand concentrations. That leaves equation 17 which is first order in both the metal ion and ligand concentration. Equation 17, therefore, represents the rate determining step of the extraction process, which is the formation of the first complex between the La (III) and the V(V) ions and the extractant anions, BPy and Spy .
.
Considering that the metal ion is hydrated in solution, probably due to the formation of ion pair with the H
2
O molecules, the equation for the rate determining step can then be rewritten as follows:
VO +
2
(H
2
O)
6
+ BPy slow [VO
2
(H
2
O)
4
(BPy)] + + 2H
2
O ………21; and
[La(H
2
O)
6
]
3+
+ BPy
-
slow [La(H
2
O)
4
BPy]
2+
+ 2H
2
O …………22
NOTE: BPy
-
can also be SPy
-
Thus, the formation of the first chelate ring of mono-pyrazolonato metal complex is the rate determining step. This step probably involves a slow bond-weakening effect exerted by the bidentate HBPy and HSPy on the two water molecules which are trans to each other, leading to their elimination. The other steps are expected to follow in quick succession leading to neutral bis-pyrazolonato complexes of V(V) and tris-pyrazolonato complexes of La(III) which are transferred into the organic phase, in spite of steric factors.
15

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