Speciation studies of ternary complexes of Co(II), Ni(II), and Cu(II) with 5-Sulfosalicylic acid and 5-Hydroxysalicylic acid in Urea-water mixtures

 

M. Balakrishna^{1, 2}, G. Srinivasa Rao^2*, M. Ramanaiah¹, G. Nageswara Rao³ and B.Ramaraju⁴

¹Department of Chemistry, Aditya Institute of Technology and Management, Tekkali, A.P India-532201

²Department of Chemistry, GITAM Institute of Science, GITAM University, Visakhapatnam, A.P, India-530045

³Professor, Department of Inorganic and Analytical Chemistry, Andhra University, Visakhapatnam, A.P India-530003

⁴Head, Lab and R&D Department, Shan Poornam Metals Sdn Bhd, (485075-P), Plot 34 (No. 1479),

Lorong Perusahaan Maju 6, Kawasan Perindustrian Perai, Fasa 4, 13600 Perai, Pulau Pinang, Malaysia.

 

ABSTRACT:

Speciation of complexes of Co(II), Ni(II) and Cu(II) with 5-Sulfosalicylic acid as primary ligand (L) and 5-Hydroxysalicylic acid as secondary ligand (X) has been studied in various concentrations (0.0-42.47% w/v) of Urea-water mixtures at ionic strength of 0.16 mol L^-1 and at a temperature of 303.0 K. All the titrations were carried out at different relative concentrations (M:L:X = 1.0:2.5:2.5; 1.0:2.5:5.0; 1.0:5.0:2.5) of metal (M) to 5-Sulfosalicylic acid (L) and 5-Hydroxysalicylic acid (X) with NaOH as titrant. The species detected during the refinement were MLXH, ML₂XH and MLX₂H₂ for Co(II), Ni(II) and Cu(II). The extra stability of ternary complexes compared to their binary complexes was understood to be due to electrostatic interactions of the side chains of ligands, charge neutralisation, chelate effect, stacking interactions and hydrogen bonding. The species distribution with pH at different compositions of Urea-water mixtures and plausible equilibria for the formation of species were also presented.

 

 

 

INTRODUCTION:

Speciation analysis, the determination of the concentrations of separate and unique atomic and molecular forms of an element instead of its total concentration in a sample is important in human biology, nutrition, toxicology and in clinical practice^1-3. On the other hand, speciation profoundly influences both the toxicity and bioavailability of an element. Bioavailability of a particular metal depends on its complex chemical reactions of dissolution, binding and complexation with the constituents of the environmental aquatic phase. The role of metal complexes in biological systems is receiving considerable attention in recent years⁴.

The understanding of how the compounds are transported in the blood is essential to evaluate their bioavailability. Serum proteins plays key roles in the pharmacokinetic properties of drugs as they control their distribution and hence their bioavailability⁵. Biological activity describes the beneficial or adverse effects of a drug on living matter. Metal coordination complexes have been widely studied for their antimicrobial activity⁶. The stabilities of mixed chelates are of great importance in biological systems as many metabolic and toxicological functions are dependent upon this stability⁷

 

5-Sulfosalicylic acid (5-SSA) and 5-Hydroxysalicylic acid (5-HAS) has biological importance, due to this reason authors studied on the chemical speciation of these ligands. In the previous investigation, authors studied protonation constants and binary complexes of these bio-ligands in Urea-water mixtures and they have been reported elsewhere^8-10. The extra stability of ternary complexes when compared to their binary complexes has been studied in the present investigation. The study of mixed ligand complex formation is relevant in the field of analytical chemistry. They also play an important role in the field of biological and environmental chemistry¹¹

 

EXPERIMENTAL:

Solutions of 5-SSA, 5-HSA, Co(II), Ni(II) and Cu(II) Chlorides  were prepared by dissolving them in distilled water. Hydrochloric acid (0.05 mol L^-1) is added  to increase the solubility. Urea was used as received. Sodium Chloride (2.0 mol L^-1) is added to maintain the ionic strength in the titrand. Standard methods used to standardize metal solutions¹². To assess the errors that might have crept into the determination of the concentrations, the data were subjected to ANOVA¹³.  The best-fit chemical model for each system investigated was arrived at using a non-linear least squares analysis program MINIQUAD75¹⁴ which exploits the advantage of constrained least squares method in the initial refinement and reliable convergence of undamped, unconstrained Marquardt algorithm Gran plot method ^15,16 is used to determine the strengths of acid and base.

Procedure:

Equiptronics (EQ 614 A) pH meter (readability 0.01) was used and calibrated using potassium hydrogen phthalate and phosphate¹⁷. The effects of variations in asymmetry, liquid junction potential, activity coefficient, sodium ion error and dissolved carbon dioxide on the response of glass electrode was accounted in the form of correction factor^{18, 19}. In order to determine stability constants of ternary species, equilibration of glass electrode should be done by titrating strong acid with alkali at regular intervals. These titrations were carried out in Urea-water mixtures (0.0-42.47% w/v) by using a pH meter. The titrand consisted of approximately 1mmol mineral acid in each titration to make a total volume of 50 cm³. The micro burette with graduation of 0.02 ml was used and it is calibrated by the method of vogel²⁰. These titrations carried out with different concentrations of metal ions to ligands 5-SSA (L) and 5-HSA (X) with 0.4 mol L^-1 sodium hydroxide. The concentrations of the ingredients are given in Table 1. The details of the experimental procedure and titration assembly were given elsewhere^21-24

 

 

Table 1. Total initial concentrations of ingredients (in mmol) for mixed ligand titrations in Urea-water mixtures [NaOH] = 0.4 mol L^-1; V₀=50.0 cm³; temp=303 K; ionic strength= 0.16 mol L^-1; mineral acid= 1 mmol. M=Co(II)/Ni(II)/Cu(II); L= 5-SSA; X= 5-HSA.

Solvent % w/v	Co(II)			NI(II)			Cu(II)			TM0:TL0:TX0
	TM0	TL0		TM0	TL0		TM0	TL0
		5-SSA	5-HSA		5-SSA	5-HSA		5-SSA	5-HSA
0.0	0.0991	0.252	0.246	0.0979	0.225	0.228	0.1006	0.225	0.228	1.0:2.5:2.5
		0.504	0.249		0.455	0.225		0.455	0.227	1.0:5.0:2.5
		0.256	0.495		0.222	0.457		0.227	0.459	1.0:2.5:5.0
5.8	0.0991	0.252	0.242	0.0979	0.256	0.246	0.1006	0.254	0.245	1.0:2.5:2.5
		0.504	0.244		0.504	0.249		0.505	0.244	1.0:5.0:2.5
		0.253	0.497		0.257	0.504		0.255	0.496	1.0:2.5:5.0
11.52	0.0991	0.248	0.269	0.0979	0.248	0.264	0.1006	0.242	0.268	1.0:2.5:2.5
		0.499	0.266		0.492	0.265		0.492	0.267	1.0:5.0:2.5
		0.242	0.533		0.244	0.536		0.243	0.539	1.0:2.5:5.0
20.31	0.0991	0.232	0.262	0.0979	0.221	0.262	0.1006	0.228	0.264	1.0:2.5:2.5
		0.457	0.268		0.459	0.268		0.459	0.265	1.0:5.0:2.5
		0.233	0.524		0.228	0.524		0.226	0.525	1.0:2.5:5.0
29.64	0.0991	0.248	0.262	0.0979	0.247	0.269	0.1006	0.244	0.266	1.0:2.5:2.5
		0.496	0.268		0.495	0.262		0.495	0.265	1.0:5.0:2.5
		0.248	0.534		0.242	0.534		0.245	0.537	1.0:2.5:5.0
36.83	0.0991	0.246	0.282	0.0979	0.244	0.283	0.1006	0.244	0.288	1.0:2.5:2.5
		0.491	0.286		0.493	0.284		0.495	0.284	1.0:5.0:2.5
		0.246	0.564		0.245	0.565		0.245	0.566	1.0:2.5:5.0
42.47	0.0991	0.263	0.258	0.0979	0.262	0.258	0.1006	0.265	0.258	1.0:2.5:2.5
		0.537	0.255		0.534	0.257		0.538	0.254	1.0:5.0:2.5
		0.269	0.499		0.265	0.505		0.267	0.502	1.0:2.5:5.0

 

RESULTS AND DISCUSSION:

Modeling of Chemical Speciation:

The best fit models were chosen based on the statistical parameters like χ², R-factor, skewness and kurtosis given in Table 2. The ternary complex species detected are MLXH, ML₂XH and MLX₂H₂ for Co(II), Ni(II) and  Cu(II).

A low standard deviation (SD) value of overall stability constants (log β) indicates the accuracy of these parameters. The small values of U_corr indicate that the models represent the experimental data. For an ideal normal distribution, the values of kurtosis and skewness should be three and zero, respectively. The kurtosis values, in the present study indicate that some of the residuals are nearer to mesokurtic and other form platykurtic patterns. The values of skewness recorded in Table 2 are between -0.93 and 1.89. The suitability of the model is further evident from the low crystallographic R-values recorded.

 

Table 2 Parameters of best fit chemical models of 5-SSA-M(II)-5-HSA-complexes in Urea-water mixtures.  Temperature=303 K, Ionic strength=0.16 mol dm^-3.

% v/v Urea	log βmlxh (SD)			pH-Range	NP	Ucorr x108	χ2	Skewness	Kurtosis	R-factor
	MLXH	ML2XH	MLX2H2
Co(II)
0.0	28.95(16)	31.20(30)	35.91(84)	2.0-11.5	16	41.38	1.75	-0.19	2.70	0.044714
5.8	29.18(17)	31.69(65)	36.11(27)	2.0-11.5	25	95.60	41.92	-0.56	1.87	0.078080
11.52	29.45(46)	31.44(62)	36.42(43)	2.0-11.5	16	89.00	19.00	0.25	2.34	0.110154
20.31	29.12(28)	31.83(55)	36.78(53)	2.0-11.5	21	25.04	41.33	1.28	6.23	0.030884
29.64	29.71(58)	31.97(33)	36.61(79)	2.0-11.5	11	22.59	6.55	0.04	2.70	0.022643
36.83	30.09(35)	32.45(51)	36.91(57)	2.0-11.5	41	92.60	45.17	0.74	4.31	0.053925
42.47	30.52(07)	32.94(45)	37.19(08)	2.0-11.5	09	21.30	29.78	1.89	7.92	0.003331
Ni(II)
0.0	29.24(24)	32.98 (15)	36.56(33)	2.0-11.5	24	70.00	3.33	-0.01	2.44	0.062607
5.8	29.12(25)	32.88(34)	36.77(57)	2.0-11.5	31	94.6	17.55	-0.80	3.22	0.072924
11.52	29.54(33)	32.83(48)	36.91(23)	2.0-11.5	20	14.47	11.20	0.02	2.31	0.031640
20.31	29.46(17)	33.15(17)	37.17(50)	2.0-11.5	17	16.21	29.41	-0.93	6.72	0.025065
29.64	29.89(24)	33.63(46)	37.52(65)	2.0-11.5	13	32.70	5.23	-0.24	2.02	0.047084
36.83	30.07(38)	33.98(26)	37.90(12)	2.0-11.5	12	65.88	17.00	-0.30	1.71	0.057042
42.47	30.53(22)	34.36(61)	38.04(98)	2.0-11.5	21	25.11	4.38	0.37	3.36	0.030534
Cu(II)
0.0	30.63(50)	33.80(50)	37.63(63)	2.0-11.5	16	34.30	13.50	-0.14	1.66	0.041780
5.8	30.72(12)	34.01(14)	37.53(12)	2.0-11.5	27	28.13	19.70	0.42	2.58	0.033661
11.52	30.97(87)	34.47(16)	37.87(20)	2.0-11.5	33	87.60	21.58	-0.29	1.83	0.070568
20.31	30.76(44)	34.55(58)	37.96(96)	2.0-11.5	28	98.8	20.29	0.06	1.97	0.071603
29.64	31.11(22)	34.91(47)	37.29(85)	2.0-11.5	30	64.87	19.73	0.38	3.06	0.029737
36.83	31.61(33)	35.12(74)	37.44(99)	2.0-11.5	20	19.39	25.40	0.03	1.79	0.032369
42.47	31.94(40)	35.61(69)	37.84(25)	2.0-11.5	18	46.00	25.11	0.24	2.55	0.029451

 

Fig. 1 Variation of magnitude of stability constant (log β) of ternary complexes of 5-SSA-5-HSA with 1/D of Urea: (A) Co(II) (B) Ni(II) (C) Cu(II): (○) logβ _MLXH, (∆) logβ _ML2XH and (□) logβ _MLX2H2,

 

Effect of Solvent on the Stability of Ternary Complexes β:

The dielectric constant is one of the characteristics of liquid. In the present experimental conditions, electrostatic forces are dominating the equilibrium process as there is a linear variation (Fig. 1) of the stability constants of the 5-SSA and 5-HSA complexes of Co(II), Ni(II) and Cu(II) in Urea-water mixtures with reciprocal of dielectric constant (1/D).

 

Stability of Ternary Complexes:

The Δlog K and log X values calculated from binary and ternary complexes are shown in Table 3. The obtained values of Δlog K are in the range from 4.12 to 7.45 and log X values are in the range from 23.02 to 32.58. Both the values found in higher than expected, which accounts for extra stability of the ternary complexes. This extra stability may be due to the hydrogen bonds between the coordinated ligands, charge neutralization, chelate effect and stacking interactions^25-27

 

 

Table 3 Δlog K values of ternary complexes of Co(II), Ni(II) and Cu(II)-5-SSA and 5-HSA in Urea -water mixtures and the corresponding equations used in the calculations

 

 

Effect of Influential Parameters on Stability Constants:

A change in the concentrations of alkali, acid, ligands, metal ions, correction factor and volume affects the magnitude of stability constants. This study was made in the present investigation and the results obtained are given Table 4.

 

 

Table 4 Effect of errors in influential parameters on the stability constants of ternary complexes of Ni(II) with 5-SSA-5-HSA in 11.52% (w/v) Urea-water mixture.

Ingredient	% Error	Log βmlxh(SD)
		1111	1211	1122
Alkali	0	29.54(33)	32.83(48)	36.91(23)
	-5	31.12(74)	33.37(46)	Rejected
	-2	Rejected	35.24(64)	38.72(65)
	2	26.87(54)	36.01(66)	40.69(92)
	5	Rejected	Rejected	41.28(73)
Acid	-5	30.01(41)	35.33(56)	36.12(72)
	-2	29.36(33)	33.27(67)	Rejected
	2	28.97(44)	Rejected	38.28(72)
	5	32.45(62)	Rejected	31.98(93)
5-SSA(L)	-5	31.01(54)	35.10(66)	37.03(63)
	-2	30.41(33)	34.53(14)	36.98(35)
	2	31.18(64)	33.96(48)	37.22(75)
	5	30.66(42)	33.16(69)	39.81(52)
5-HSA(X)	-5	31.52(62)	35.70(58)	37.63(91)
	-2	30.22(43)	34.98(67)	37.08(83)
	2	31.68(38)	33.06(19)	38.12(49)
	5	31.06(48)	34.56(49)	38.81(67)
Metal	-5	30.62(41)	34.13(53)	37.86(42)
	-2	30.06(82)	34.44(76)	37.12(92)
	2	31.13(46)	34.58(72)	36.94(81)
	5	30.46(53)	34.94(49)	37.81(23)
Log F	-5	30.62(72)	33.52(76)	36.82(68)
	-2	30.96(13)	34.89(61)	37.49(37)
	2	30.23(19)	33.94(57)	36.67(31)
	5	30.86(35)	33.47(49)	37.72(37)
Volume	-5	31.02(46)	34.44(63)	36.68(51)
	-2	29.01(59)	34.69(68)	37.66(56)
	2	29.19(78)	34.39(93)	36.73(66)
	5	30.66(84)	34.79(76)	37.14(49)
	-5	30.26(64)	34.19(46)	37.54(19)

 

 

Chemical Speciation

Distribution diagrams drawn using the formation constants of the best fit model are shown in Fig. 3 which contain protonated species like MLXH, ML₂XH and MLX₂H₂ for Co(II), Ni(II) and Cu(II).

 

 

M(II) + LH2- + XH3		MLXH3-+ 2H+	4
MLXH3-		MLXH22-+ H+	5
MLXH22-		MLXH3-+ H+	6
M(II) + LH2- + 2XH3		MLX2H6-+ 2H+	7
MLX2H6-		MLX2H52-+ H+	8
MLX2H52-		MLX2H43-+ H+	9
MLX2H43-		MLX2H34-+ H+	10
MLX2H34-		MLX2H25-+ H+	11
MLX2H25-		MLX2H6-+ H+	12
MLX2H2		MLX2H+ H+	13
M(II) +2 LH2- + XH3		ML2XH52-+2 H+	14
ML2XH52-		ML2XH43-+ H+	15
ML2XH43-		ML2XH34-+ H+	16
ML2XH34-		ML2XH25-+ H+	17
ML2XH25-		ML2XH6-+ H+	18
ML2XH2		ML2XH+ H+	19

 

Some typical distribution diagrams in 36.83% Urea-water mixture are shown in Fig. 3.

 

Fig. 2(A), 2(B) and 2(C) shows the formation of Co(II)-5-SSA-5-HSA, Ni(II)-5-SSA-5-HSA and Cu(II)-5-SSA-5-HSA complexes, MLXH, MLX₂H₂ and ML₂XH in the pH range 3.0-11.0, in this range total metal ion participate in bonding. At lower pH the species MLXH has high concentration and it is formed by the reaction of FM with LH₂ and XH₃ (Equilibria 6).

 

As the pH increases the concentration of MLXH is decreased and the concentration of MLX₂H₂ and ML₂XH were increased (Equilibria 11 and 19). At pH value above 8.0, the concentration of MLX₂H₂ species decreases with increasing ML₂XH species indicates that ML₂XH forms through the Equilibria 19.

 

 

Distribution Diagrams:

 

Fig. 2 Species distribution diagrams of ternary complexes of 5-SSA and 5-HSA; (A) Co(II), (B) Ni(II) and (C) Cu(II) in 20.0% vv DMF-water mixture.

Fig. 3 Proposed structures of ternary complexes of 5-SSA and 5-HSA, where S is either solvent or water molecule.

 

 

Structures:

Depending upon the nature of the ligands, the metal ions and the basic chemical knowledge the structures of the ternary complexes were proposed as shown in Fig. 3.

 

CONCLUSIONS:

1.      The ternary metal complex species detected are MLXH, MLX₂H₂ and ML₂XH for Co(II), Ni(II) and Cu(II). Where L = 5-SSA and X = 5-HSA.

2.      The linear variation in the stabilities of ternary complexes with increasing 1/D of the Urea is due to the dominance of electrostatic forces over equilibrium process.

3.      The order of errors in influential parameters are alkali > acid > ligand > metal ion > log F > volume.

 

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Accepted on 07.09.2017        © RJPT All right reserved

Research J. Pharm. and Tech 2017; 10(11): 3681-3686.