Complexing agents determination

In the presence of a metal ion (M " ), a metal chalcogenide phase M2Sen will be precipitated upon exceeding the solubility product of [M and [Se ] (or [HSe ]). The concentration of free metal ions must be controlled by an excess of complexing agent, determining the applicable solubility of the metal and the overall competitive chemical reaction, in order to prevent the formation of sulfite, sulfate, and... 

Matrices Trace Metals Complexing Agents Determination Methods... 

Inorganic heavy metals are usually removed from aqueous waste streams by chemical precipitation in various forms (carbonates, hydroxides, sulfide) at different pH values. The solubiUty curves for various metal hydroxides, when they are present alone, are shown in Figure 7. The presence of other metals and complexing agents (ammonia, citric acid, EDTA, etc) strongly affects these solubiUty curves and requires careful evaluation to determine the residual concentration values after treatment (see Table 9) (38,39). 

The main idea of research is application of accessible, simple and express methods that don t need expensive reagent techniques for analysis of phanuaceutical products based on bischofite. The determination of metal ions such as Mg, Zn, Cu, Fe by complex-formation titrations using a widely applicable chelating agent, EDTA, have been studied as a function of pH, complexing agents and indicators. The analysis consists of four parts ... 

A. Direct titration. The solution containing the metal ion to be determined is buffered to the desired pH (e.g. to PH = 10 with NH4-aq. NH3) and titrated directly with the standard EDTA solution. It may be necessary to prevent precipitation of the hydroxide of the metal (or a basic salt) by the addition of some auxiliary complexing agent, such as tartrate or citrate or triethanolamine. At the equivalence point the magnitude of the concentration of the metal ion being determined decreases abruptly. This is generally determined by the change in colour of a metal indicator or by amperometric, spectrophotometric, or potentiometric methods. 

A similar procedure may also be used for the determination of antimony(V), whilst antimony (III) may be determined like arsenic(III) by direct titration with standard iodine solution (Section 10.113), but in the antimony titration it is necessary to include some tartaric acid in the solution this acts as complexing agent and prevents precipitation of antimony as hydroxide or as basic salt in alkaline solution. On the whole, however, the most satisfactory method for determining antimony is by titration with potassium bromate (Section 10.133). 

In a similar manner, in a solution containing the species Hg2+, HgY2-, MY,n 4)+ and M"+, where Y is the complexing agent EDTA and M"+ is a metallic ion which forms complexes with it, the concentration of the mercury ion is controlled by the stability constants of the complex ions MYhigh stability constant), and the concentration of the metal ions M"+. Hence, a mercury electrode placed in this solution will acquire a potential which is determined by the concentration of the ion M"+. 

Addition of such a complexing agent increases the total number of species present in the solution, affecting thus its viscosity — a phenomenon allowing one to determine the degree of disintegration. The concentration of P as a function of [S] and ofc — the total concentration of the polymers in whatever form, is determined by the stoichiometric equation ... 

Certain surface-active compounds [499], when dissolved in water under conditions of saturation, form self-associated aggregates [39,486-488] or micelles [39,485], which can interfere with the determination of the true aqueous solubility and the pKa of the compound. When the compounds are very sparingly soluble in water, additives can be used to enhance the rate of dissolution [494,495], One can consider DMSO used in this sense. However, the presence of these solvents can in some cases interfere with the determination of the true aqueous solubility. If measurements are done in the presence of simple surfactants [500], bile salts [501], complexing agents such as cyclodextrins [489 191,493], or ion-pair-forming counterions [492], extensive considerations need to be applied in attempting to extract the true aqueous solubility from the data. Such corrective measures are described below. 

Consider the relatively simple case where the chemical reactions of formation and dissociation of the complexes are sufficiently fast so that equilibrium among the complexes, free ions and complexing agent is maintained everywhere in the electrolyte during the electrolysis. The rate of deposition of the metal is then determined by the electrode reaction of the complex MX/, for which the product kc,MXicMX is, at the given potential, the largest for all the complexes. 

Especially sensitive and selective potassium and some other ion-selective electrodes employ special complexing agents in their membranes, termed ionophores (discussed in detail on page 445). These substances, which often have cyclic structures, bind alkali metal ions and some other cations in complexes with widely varying stability constants. The membrane of an ion-selective electrode contains the salt of the determined cation with a hydrophobic anion (usually tetraphenylborate) and excess ionophore, so that the cation is mostly bound in the complex in the membrane. It can readily be demonstrated that the membrane potential obeys Eq. (6.3.3). In the presence of interferents, the selectivity coefficient is given approximately by the ratio of the stability constants of the complexes of the two ions with the ionophore. For the determination of potassium ions in the presence of interfering sodium ions, where the ionophore is the cyclic depsipeptide, valinomycin, the selectivity coefficient is Na+ 10"4, so that this electrode can be used to determine potassium ions in the presence of a 104-fold excess of sodium ions. 

Perez-Pina et al. [805] studied the use of triethanolamine and dimethyl-glyoxime complexing agents in absorptive cathodic stripping voltammetry of cobalt and nickel in seawater. Nickel and cobalt could be determined at levels down to 2 nM and 50 pM, respectively. 

Lieser et al. [628] studied the application of neutron activation analysis to the determination of trace elements in seawater, with particular reference to the limits of detection and reproducibility obtained for different elements when comparing various preliminary concentration techniques such as adsorption on charcoal, cellulose, and quartz, and complexing agents such as dithizone and sodium diethyldithiocarbamate. 

Gardner and Yates [26] developed a method for the determination of total dissolved cadmium and lead in estuarine waters. Factors leading to the choice of a method employing extraction by chelating resin, and analysis by carbon furnace atomic absorption spectrometry, are described. To ensure complete extraction of trace metals, inert complexes with humic-like material are decomposed by ozone [27]. The effect of pH on extraction by and elution from chelating resin is discussed, and details of the method were presented. These workers found that at pH 7 only 1-2 minutes treatment with ozone was needed to completely destroy complexing agents such as EDTA and humic acid in the samples. 

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