Thermodynamic study of formation of the complexes of Thorium (IV) with Xylenol orange at different temperatures
Farhoush kiani a*, Sasan Sharifi b, Abbas Ali Rostami a, Ali Niazi b, and Saeid Hagiaghajani b
aDepartment of Physical and Inorganic Chemistry, Faculty of Chemistry, University of Mazandaran, Iran
bChemistry department, Islamic Azad University, Arak Branch, Arak, Iran [email protected]
Abstract
The stability constants of the complexes of thorium(IV) ion with xylenol orange were determined in aqueous solution at 15○C, 20○C, 25○C, 30○C and 35○C and also 0.1 mol dm-3 ionic medium using a combination of potentiometric and spectrophotometric techniques.
Sodium perchlorate was used to maintain the ionic strength. The composition of the formed complexes was determined and it was shown that thorium
(IV) forms two mononuclear 1:1 species with the ligands, of the type ThH(XO) and ThH2(XO)2 in the pH range of study 2-6. The logarithms of the cumulative stability constants, ßxyz, of the complexes,[( Metal ion)x( H +)y( ligand )z ], are log ß111 and log ß122
that were calculated. Also some thermodynamic parameters were calculated at different temperatures.
Keyworlds: Stability constant, Dissociation constant, Thermodynamic parameters, Ionic strength
Results and discussion
The complex MxHyLz (nx+y-z) formed is characterized by its stoichiometry (x: y: z), where M and L represent the metal ion and each ligand, respectively. To determine the stability constant of the complexation or the protonation, equation(1) is defined by ßxyz7
xMn+ + yH+ + zL- = MxHyLz (nx+y-z)+
(1) ßxyz = [MxHyLz(nx+y-z)+]/([M+n]x[H+]y[L]z) (2)
The protonation constants of the xylenol orange have been used for computation of the stability constant, ßxyz , of the metal-ligand. The protonation constants of the xylenol orange have been extensively studied in different kinds of background electrolytes, and the results were reported in the literature. To illustrate, there are xylenol orange compound in solution to four species: H5XO, H4XO, H3XO and H2XO. This assumption was confirmed by offering some other species, as mentioned before, that all were rejected by the computer program4.
Absorbance, A, and –log[H+], were measured for a solution containing Th4+ with a large excess of xylenol orange. Treatment of the spectrophotometric data (each 1 nm) obtained during the titrations as a function of the H+ concentration were conducted on the computer program. Considering the protonation constants of the ligand, in acidic pH the predominant species for complexation are H3XO and H4XO for xylenol orange. In this case the spectrophotometric titration data were analyzed by using the absorbance of Th4+
with ligand at wavelengths in UV range that is given by
A = sTh[Th+] + sc1[ThHXO] + sc2[ThH2XO2] + sH2XO[H2XO]+sH5XO[H5XO] (3)
The average values for various wavelengths, at different temperatures calculated for the stability constants are listed in Table 1.
Table 1. Average values of log ß111 and log ß122 for different wavelengths at different temperatures and ionic strengths, I, 0.1 mol dm-3 (NaCLO4)
T(K) 288.15 293.15 298.15 303.15 308.15
log ß111 8.58 8.13 7.89 7.64 7.42
log ß122 17.09 16.64 16.64 16.49 16.33
Gibs-Helmholtz equation shows that, LnK is a linear function to of 1/T that curves are given in Figure 1. From the slope of this equation, the values of OH, OG and OS are calculated that results are given in the Table 2.
Figure 1. The plot of formation constants of ThHXO versus 1/T
Table 2. OG, OH and OS values for formation of complexes of ThHXO and ThH2XO2
In Figure 2 equilibrium distributions of various species in Th-XO systems are shown as a function pH. The calculations are based on the stability constant values given in Table 1. In the Th-XO system, Figure 2, ThHXO and ThH2XO2 are the dominant species in the pH ≤ 3.4.
3.50E-05 3.00E-05 2.50E-05 2.00E-05 1.50E-05
H5XO
MH2XO2
1.00E-05 5.00E-06 0.00E+00
H4XO
MHXO
H3XO
1.8 2.8 3.8 4.8 5.8
pH
Figure 2. The equilibrium distribution of the species Th-XO system as a function of pH at 25○C in ionic strength 0.1 mol dm-3 sodium per chlorate
8.8 8.6 8.4 8.2 8 7.8 7.6
y = 5126.6x - 9.273 R2 = 0.9859
7.4
7.2
3.20E-03 3.25E-03 3.30E-03 3.35E-03 1lT
3.40E-03 3.45E-03 3.50E-03
-19.636 -77.095
45.486 -27.768
-41.330 ThH2XO2
-42.622 ThHXO
OS J.mol-1.K-1 OH KJ.mol-1
OG KJ.mol-1 compounds
Molar C oncn.L n K
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