Potentiometric and Thermodynamic Studies of Some Metal Complexes With Carboxy Methyl Mercapto Succinic Acid

Introduction

Mercapto compounds have several application in biological, pharmaceutical and other chemical fields and are well known to form complexes with various metals. Gupta and coworker^1-3 have carried out significant investigation on the electrochemical behavior of several biologically active organo-sulphur compounds and their complexation behavior with metals.

Carboxy methyl mercapto succinic acid has coolant additive with corrosion inhibitor and scale preventive⁴ property. The complex ester of c.m.m.s.a. may be used directly as lubricants⁵, or may be blended with other mineral or synthetic lubricants and various additive and it is also used as adhesive⁶, biodegradable bleach –stabilizers for detergents⁷.In view of wide pharmaceutical and analytical applications of c.m.m.s.a., it seems interesting to study the complexation equilibria of c.m.m.s.a. with some transition metals in dioxane – water mixture. This communication reports the formation , stability constants, and thermodynamic parameters  of  Ni²⁺ , Cu²⁺ ,Mn²⁺, complexes with c.m.m.s.a. employ potentiometric technique. The log K_stab values determined at 25^°,35^°,and 45^°C by Calvin and Melchior’s extension⁸ of Bjerrum’s method⁹.The stability constant have been further determined by correction term¹⁰ and Schroder’s convergence formula¹¹.

Material and Method

All the chemicals used for experiment, such as dioxane, potassium nitrate, nitric acid and c.m.m.s.a.. etc were of analytical grade, Double distilled water was used in preparation of various solutions. All the metal ion solutions were prepared in double distilled water and standardized by using conventional procedures¹².A carbonate free sodium hydroxide was used as a titrate and standardized against oxalic acid. The pH measurements were carried out with 335-Systronic pH meter (accuracy±0.05 units) using glass and calomel electrode. The electrode system was calibrated by using standard buffer solutions of pH 4.00 ,7.00,9.2. The empirical correction to pH meter reading in dioxane medium was corrected according to Van-Uitert and Hass relation¹³.The following sets of titrations were performed under nitrogen atmosphere at ionic strength
µ = 0.1 M (KNO₃)at temperature 25^°,35^°,45^°C in 40%(v/v) dioxane – water mixtures against 0.1 M NaOH .The temperature were controlled by an electrically maintained thermostat.

(i) Free HNO₃ (2.0 X 10 ^-3 M)

(ii) Free HNO₃ (2.0 X 10 ^-3 M) + Ligand (2.0 X 10 ^-3 M)

(iii) Free HNO₃ (2.0 X 10 ^-3 M) + Ligand (2.0 X 10 ^-3 M) + metal ion solution
(4.0 X10 ^-4).

The log K_stab values determined at 25^°,35^°,and 45^°C by Calvin and Melchior’s extension⁸ of Bjerrum’s method⁹.The stability constant have been further determined by correction term¹⁰ and Schroder’s convergence formula¹¹.The thermodynamic parameter for binary complex systems were calculated by Gibb’s Helmholtz  and Isobar equation¹⁵.

Result and Discussion

Identical titration curves were obtained for the different binary system under investigation, according to the sequence described in experimental section. For the sake of brevity only fig (1,2,3) representing formation curves of metal ions have been given.

Metal and ligand stability constant

Calvin and Melchior’s extension of Bjerrum’s⁹ method was used for determining stability constant of the complexes from potentiometric titration data and their values were further determined by Schroder’s Convergence formula ¹¹ and Correction term method¹⁴ the values of stability constants are given in Table-1.

The values of log K₁ and log K₂ at 25^°,35^°,and 45^°C were read directly from the formation curves at n = 0.5 and n=1.5 (fig-1,2,3). Theses values increases with temperature which shows that higher temperature is favourable for the formation of stable complexes and follow the order

Cu²⁺ > Ni²⁺ > Mn²⁺

Which is in agreement with Irving-Williams order of stability ¹⁴

Figure 1: Click here to View figure

 

Figure 2: Click here to View figure

 

Figure 3: Click here to View figure

 

Thermodynamic Functions

The values of overall changes in free energy (∆G),enthalpy (∆H) and entropy (∆S) accompanying complex reactions have been determined at 35^°C with the help of Gibbs –Helmholtz and Isobar equation¹⁵ the values of ∆G, ∆H and ∆S in 40%(v/v)(V/V) dioxane -water mixture are given in Table -2.The negative value of free energy (∆G ) shows that the reaction tends to proceed spontaneously. The values of enthalpy changes are negative indication the exothermic nature of the reaction and the positive values of the entropy changes confirming that the complex formation is entropically favourable.

Acknowledgement

For  the research work of “Potentiometric and thermodynamic studies of some metal complexes with carboxy methyl mercapto succinic acid”

The Authors are thankful to Dr.K.K.Gupta Head of Chemistry Department and Shri Sunil Bhargav Principal government college Kota for providing research facilities.

Table 1: Metal Ligand Stability Constant of Complexes of Cu²⁺, Ni²⁺ and Mn²⁺ With C.M.M.S.A. in 40%(V/V) (V/V) Dioxane-Water Mixture at Different Temperature And Ionic Strength µ = 0.1 M (Kno₃).

METALCOMPLEXES	METHOD	TEMPERATURE
a        Cu2+                   b                                       c             Mean Value                                       a                   Ni2                 b                                       c            Mean Value                                        a                  Mn2+                             b c               Mean Value		25°                                         35°                                        45°logK1    logK2    log ß     logK1   logK2   log ß     logK1   logK      log ß
		4.46     3.17      7.63     4.64   3.32      7.96        4.78    3.47     8.254.39     3.25     7.64      4.57   3.39      7.96        4.72    3.54     8.264.39     3.26     7.65      4.57   3.37      7.94        4.73     3.54    8.274.41    3.23     7.64       4.59   3.36      7.95         4.74     3.52     8.264.32    3.08     7.40      4.46    3.12      7.58          4.60     3.24    7.84 4.24    3.16     7.40      4.40    3.19      7.59          4.59      3.24    7.83 4.34   3.11     7.45       4.47    3.14     7.61           4.63      3.25    7.88 4.30   3.12    7.42      4.44    3.15       7.59          4.59       3.26    7.85 4.06    2.94   7.00      4.18     2.98      7.16          4.32       3.12    7.44 3.95    3.05   7.00      4.09     3.07     7.16          4.22       3.18    7.40 3.94    3.04   6.98      4.08     3.07     7.15         4.21        3.19    7.40 3.98    3.01    6.99    4.12      3.04     7.16         4.25        3.16    7.41

Method a,b and c represent Bjerrum’s Method, Schroder’s convergence formula and Correction Term Method respectively.   

Table 2: Thermodynamic Parameter (∆G), (∆H) and (∆S) Of Complexes Of  Cu²⁺ , Ni²⁺ And Mn²⁺ With C.M.M.S.A. At 35^°C. 

METAL COMPLEXES	∆G =KJMOL-1             (-ve)	∆H= KJMOL-1               (-ve)	∆S =JMOL-1K-1              (+ve)
Cu2+             Ni2+             Mn2+	46.88                                    40.40                                  21.0444.76                                    39.63                                  16.6542.22                                    38.29                                  12.76

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