J, Indian Chern. Soc., Vol. 83, September 2006, pp. 922-924 NOTE Effect of buffers and ionic strength on the stability constant of complexes of iron(III) with salicylic acid and its derivatives a b* Sunanda Das and M. C. Chattopadhyaya aDepartment of Chemistry, C. M. P. Degree College, Allahabad- 211 002, Uttar Pradesh, India bEiectrochemical Sensor Laboratory, Department of Chemistry, University of Allahabad, Allahabad-211 002, Uttar Pradesh, India E-mail : sunanda.das@gmail.com, mcc46@rediffmail.com Manuscript received 21 January 2002, revised 13 June 2006, accepted 13 June 2006 Abstract : The 1 : I complex formed by lron(m) with salicylic acid and Its seven derivatives were studied by spectrophotometry and the effect of phthalate-Ha and KCI-HCI buffers on the value of stability constants of I : I complex determined by spec- trophotometry, was evaluated. The thermod,.namic value of stability constant of each system was determined by obtaining the values at different ionic strength and extrapolating them to zero ionic strength. For extrapolation, a new approach was made which gives a straight line curve making It easier to extrapolate at zero ionic strength to get the thermodynamic value thus reducing the uncertainty in extrapolation. Keywords : Stability constants, salicylic acid, lron(m) complexes. Iron(m) forms colored complex with salicylic acid and its derivatives. A number of workers have studied these sys- tems and determined the stability constant of 1 : 1 and higher complexes 1 • Salicylic acid releases proton when it undergoes complex formation with iron(m) and therefore it is necessary to fix the pH. Usually one uses a buffer solu- tion for fixing the pH for making measurements. It has al- ready reported in the literature 2 that citrate, acetate, chloroacetate and phosphate buffers compete with salicylic acid. In the present investigation an attempt has been made to see the effect of buffers like phthalate-HCI and Clark and Lubes KCI-HCI buffer in the determination of stability constant of the complex formed by Fe 111 with salicylic acid and seven of its derivatives. Further in order to determine the thermodynamic value of stability constant the condi- tional stability constant at different ionic strength was ob- tained and then extrapolated to zero ionic strength. A new methodology has been adopted in this work to find out the thermodynamic value using a linear plot. Results and discussion In the lower range of pH ( 1-3), Fe 111 forms I : I complex with salicylic acid and its derivatives. The of these complexes depends on various substituent present in sali- cylic acid ring and the range of A.max varied from 500-560 nm which falls 1n the visible region of the spectrum and therefore the complex can be studied by spectrophotometry. 922 Fe"' ion and ligands were mixed in 1 : 1 ratio and four solu- tions having different total concentrations (4.0-6.4 x 10-4 mol dm- 3 ) were prepared for each system. In each solution 2 ml of HCI0 4 (0.1 mol dm- 3 ), 5 ml of NaCI0 4 ( l M) and 5 ml of phthalate buffer solutions were added. The total vol- ume was made upto 50 mi. The pH of these solutions was measured after recording absorbance at A..n..x and was found to be 2.35 ± 0.05. In order to determine the stability con- stant k 1 ' which is given by k 1 ' = [FeL]/{Fe)(H 2 L], the molar absorptivity was determined for each system using the method described elsewhere 3 • From molar absorptivity, k 1 ' value was determined for each system at different metal- ligand concentrations. For same systems k 1 ' were also de- termined at pH 2.35, adjusted with dil. HCI0 4 solution. Both the results are summarized in Table I. When k 1 ' val- ues determined with buffer, it was observed that consider- able lowering in the value of k 1 ' takes place due to the pres- ence of the buffer which shows that the phthalate buffer competes with ligand. Similar results were also recorded with Clark and Lubes KCI-HCI buffer. Here the enhanced Cl- ion concentration is perhaps responsible for lowering of stability constant value as at high concentration, the Cl- ion also competes with the ligand. If we consider the dissociation of both carboxyl and phenolic group and represent the dissociation constants as