Complexation of tungsten(VI) with ethylenediaminetetraacetic acid and iminodiacetic acid at different ionic strengths using the Debye-Hückel and specific ion interaction theories

Abstract

The equilibria in systems ethylenediaminetetraacetic acid-W VI and iminodiacetic acid-W VI were studied by a combination of UV Vis spectrophotometric and potentiometric techniques at different ionic strengths ranging from 0.1 to 1.0 mol L -1 (NaClO 4 , 25 °C, pH = 7.5). The ionic strength effect on these complex formation reactions was studied using a Debye-Hückel type equation and the Brønsted-Guggenheim-Scatchard specific ion interaction theory.The large number of applications justifies the intense interest of chemists in the binding ability of complexones as demonstrated by the amount of papers published in the last decades. 1 Complexones are involved in many areas, for example, clinical practice where they are used as contrast agents in computed tomography. 1 There are many reports about the complexation of Mo VI and W VI with amino polycarboxylic acids. 2-6 There is a project (Analytical Chemistry Division of IUPAC, Project No. 2000 003 1 500) about the ionic strength corrections for stability constants using the Brønsted-Guggenheim-Scatchard specific ion interaction theory (SIT), which was completed in 2005, but to our knowledge no reports of the ionic strength dependence of the complexes of W VI with ethylene diaminetetraacetic acid (EDTA) and iminodiacetic acid (IDA) have appeared.Pursuing our systematic study on complexation of dioxovanadium(V), molybdenum(VI), and tungsten(VI) with amino polycarboxylic acids and amino acids, 7-12 we thought it interesting to investigate and compare the dissociation and complexation of EDTA and IDA with tungsten(VI) in NaClO 4 solutions with different ionic strengths using the SIT model and a Debye-Hückel type equation. Tungsten is the third row transition ele ment which is found in a mononuclear form in the active site of a diverse group of enzymes. 12 In the present work, we calculated the stability con stants of W VI -EDTA and W VI -IDA systems in NaClO 4 solutions with an ionic strength (I) ranging from 0.1 to 1.0 mol L -1 at 25 °C. The compositions of the complexes were determined by the continuous variations method and the stability constants were calculated at different ionic strengths at pH = 7.5. Semiempirical methods that were used to simulate the complexation reactions include the Pitzer and SIT models, and these have been shown to be equivalent for all practical purposes. 13 Therefore, in this work the SIT and the Debye-Hückel models were compared to study the ionic strength effects. ExperimentalEDTA, IDA, NaClO 4 , HClO 4 , NaOH, and Na 2 WO 4 were obtained from Merck as analytical reagent grade materials and used without further purification. Dilute perchloric acid solutions were standardized against KHCO 3 . 6 In all experiments, bidistilled water with a specific conductance of (1.3±0.1) μS cm -1 was used.A Horiba pH meter (model D 14 with a sensitivity of 0.01 units) was used for pH measurements. The hydrogen ion con centration was measured with a Horiba combination electrode (model S8720). A 0.01 mol L -1 HClO 4 solution containing 0.09 mol L -1 NaClO 4 ...