Low-Dentate Reagents as Titrants in Complexometry: Selection of Promising Reagents

Abstract

The poor selectivity of polydentate ligands (complexones) with respect to metal ions in analyzing samples of complex composition requires the use of masking or separating of interferents. This complicates analytical procedures, increases the duration of analysis, and deteriorates its accuracy. Low-dentate ligands (LDLs) possess higher selectivity. However, the use of LDLs as titrants is hindered by the stepwise character of the reaction, because of which logarithmic titration curves for these reagents exhibit unclear jumps. Nevertheless, some accidentally found low-dentate (LD) reagents proved to be good titrants. A targeted search for LD titrants has not been performed.In the preceding papers, we have derived theoretical equations that allowed us to predict the number, height, and position of jumps in complexometric titration curves even without a computer, based on the tabulated values of cumulative ( β i ) and stepwise ( K i ) stability constants of the forming complexes. It was supposed that metal M and reagent R form either one MR n complex [1] or, more often, a mixture of MR i complexes, where i = 1, 2, … n [2]. The predicted shapes of titration curves were confirmed by the data of the potentiometric titration of corresponding solutions with ion-selective electrodes [2,3]. The criteria proposed in the above works allow us to search for promising LD reagents for developing more selective procedures, as compared to those based on complexones. The aims of this work were, first, to develop and test approaches for the selection of LD titrants using computer databases, second, to find the optimum titration conditions for the selected reagents [4] based on computer simulation, and, third, to compare the precision and selectivity of the corresponding potentiometric titration procedures with those based on titrating metal ions with EDTA solutions. EXPERIMENTAL LD titrants were selected based on the data of the licensed IUPAC Critical Stability Constants Database [5]. This database contains thermodynamic and concentration stability constants for metal complexes with 18000 ligands. It is based on the well-known handbooks by Martell, Smith, Sillen, Perrin, et al. For each of the M-R systems, the database contains K i for complexes with different degrees of saturation. The service of the database allows both the interactive step-by-step information retrieval using multiple descriptors (date of publication, author, method the constant was found, solvent, temperature, etc.) and also combined multifactor retrievals [6].The conditional stability constants ( ) of complexes were calculated by conventional approaches, taking into account the protolysis of M and R at a number of pHs. The equilibrium constants of protolysis reactions were taken from [5]. Calculations were performed using the Complex software [7]. The titration conditions were optimized using the DVK software [4]. For each of the studied systems, we determined the height of the jump ( ± 1% ) in the model titration curve at one constant value of c M and different ...