Thermodynamic Parameters and Stability Constants of Chromium (III) Complexes of Carbohydrazone and Thiocarbohydrazone

Archana Singh¹*, P.Singh² and K.B.S.Chauhan²

¹Department of Chemistry, S.V.College, Bairagarh-462030, India

²Chemical Laboratories, S.S.V.(PG)College, Hapur-245101, India

ABSTRACT:

Stability constants of two ligands, derived from 4- methyl -7- hydroxy – 8- acetyl coumarin with carbohydrazide and thiocarbohydrazide and formation constants of their Chromium (III) complexes have been determined pH metrically at two different temperatures using Calvin and Melchior’s extension of Bjerrum’s method. The stability constants decrease with the increase in temperature. The thermodynamic parameters ΔG, ΔH and ΔS have been calculated. The values of ΔG and ΔH have been found to be negative and ΔS values are positive for both complexes.

KEYWORDS: Stability constant, enthalpy, MHACC, MHACTC

INTRODUCTION:

Carbohydrazone and thiocarbohydrazone has wide chemical and pharmacological applications^1,2. In the present communication we describe the pH metric study of complex formation of Cr (III) with 4- methyl -7- hydroxyl- 8- acetocoumarinyl carbohydrazone (MHACC) and 4- methyl -7- hydroxyl- 8- acetocoumarinyl thiocarbohydrazone (MHACTC).The ligands MHACC and MHACTC were prepared by the condensation of 4- methyl -7- hydroxyl- 8- acetocoumarin³ with carbohydrazide and thiocarbohydrazide . The stability constants were determined pH metrically by Calvin and Melchior’s extension of Bjerrum’s method ⁴ at 30º / 40º ±1ºC.

EXPERIMENTAL:

Preparation of ligands

Ligands MHACC and MHACTC were prepared by using the following method. A solution of 4- methyl -7- hydroxyl- 8- acetocoumarin (0.04 mol) in ethanol was added to refluxing solution of carbohydrazide (0.04 mol) / thiocarbohydrazide (0.04 mol) in aqueous ethanol followed by the addition of fused sodium acetate. The reaction mixture was refluxed for 4-6 hours and then allowed to stand overnight. A solid mass was settled down which was filtered off and dried. The final products were recrystallised from ethanol.

pH –Titration

The pH metric titrations were carried out at two different temperatures (30°/ 40°±0.1°C) with a Phillips pp9040 pH meter as recommended by Zeidler and Fritz^5,6. The instrument was standardized against 0.05M solution of Potassium hydrogen phthalate. The volume of the solution to be titrated was always kept constant (20ml) by mixing with necessary volume of methanol. The following three sets (a), (b) and (c) were prepared for each system and titrated against standard carbonate free potassium hydroxide solution.

Table – 1: Overall Stability Constants at different temperatures and thermodynamic parameters

System	Temp. (^0C)	Log K	ΔG (kJmol^-1)	ΔH (kJmol^-1)	ΔS(JK^-1mol^-1)
Cr-MHACC	30 40	5.03 4.85	-29.18 -29.07	-20.891	8.45
Cr-MHACTC	30 40	4.40 4.25	-25.53 -25.47	-10.075	5.09

(a) 10.0 ml of ligand (0.025M) + 5.0 ml of 1M NaClO₄

(b) 10.0 ml of ligand (0.025M) + 5.0 ml of 1M NaClO₄ + 1.0 ml of metal salt solution (0.005M)

(c) 10.0 ml of ligand (0.025M) + 5.0 ml of 1M NaClO₄ + 2.0 ml of metal salt solution (0.005M)

RESULTS AND DISCUSSION:

The protonation constants were determined by plotting pH values of the solution resulting from the progressive addition of standard alkali to ligand solution of known strength vs nH values. The correct values of pK is obtained as pH where nH = 0.5. In the present study pK values for MHACC and MHCTC were obtained 6.90 and 7.05 respectively at 30°C. From the titration curves at different pH values, different sets of ň ⁷ values were determined and corresponding – log L values were also calculated. The formation curves were drawn and values of stepwise stability constant⁸ LogK₁ and Log K ₂ of the Cr-MHACC and Cr-MHACTC were determined at ň=0.5 and ň=1.5 from the formation curves. Calculations were made with dissociation constants of the ligands MHACC and MHACTC respectively. The values obtained by above methods were found in good agreement. The gradual increase in the value of ň with increase of pH showed that the anionic form of ligands takes part in the formation of the complexes.

The average values of overall stability constants were found to decrease with the increase in the temperature. The thermodynamic parameters i.e. change in free energy (ΔG) , enthalpy (ΔH) and entropy (ΔS) have been calculated and are summarized in table-1. The negative values of ΔG in both cases showed spontaneous reaction between the metal and the ligands. Negative ΔH values showed that the reactions are exothermic in nature. Complexation reactions are entropically favored under the experimental condition; it is supported by the positive values of ΔS.

REFERENCES:

1 C. Duval and N.D. Xuong, Mikrochim.Acta.1956, 747; N.P. Bun-Hoi, T.B. Loc and N.D. Xuong, Bull. Soc. Chim. Fr. 1955,694.

2 R.K. Donovic, F. Pansy, G. Stryker and J. Bernstein, J. Bacteriol., 1950, 59, 667.

3 Mehta R.H., J. Ind. Chm. Soc., 1965,3 12, 574.

4 Calvin N.C. and Melchior M., J Am Chem Soc. 1948, 70, 3270.

5 Zeidler H.Z., Anal. Chem., 1955, 146,251.

6 Sr. Fritz.J., Acid base Titrations in Non Aqueous Solvents, G.F. Smith Chemical Co., Columbus, Ohio, 1952, p-II.

7 Bjerrrum J., Metal Amine formation in aqueous solutions, P. Hesse and Sons, Copenhagen, 1941.

8 Rossotti F.J.C. and RossotH. S., The Determination of Stability Constants, McGraw Hill, New York, 1961.

Received on 24.02.2010

Accepted on 12.05.2010

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Research J. Science and Tech. 2(2): March –April. 2010: 29-30