Usporedna procjena oksidacije amalgama kronopotenciometrijom i voltametrijom

The characteristic features of scanned deposition potential curves constructed from stripping chronopotentiometry (SSCP) and various modes of stripping voltammetry (SSV) are critically evaluated. The strengths and weaknesses of each method for identification of metal ion speciation features and susceptibility to typical interferences are described for conventional (HMDE) and microelectrodes, i.e. irreversibility in the electron transfer reaction, multi-metal resolution, intermetallic compound formation, homogeneous kinetics, induced metal adsorption, and requirement for excess ligand to avoid saturation at the electrode surface during reoxidation. The most advantageous stripping modes are those in which practically complete depletion of the accumulated metal is achieved during the reoxidation step, i.e. SCP with low stripping current and DC-SV with slow potential scan rate. Under these conditions there is a straightforward quantitative relationship between the amount of metal accumulated and the analytical signal. The slow rate of oxidation with these modes renders them practically immune to induced metal adsorption; they have a lower requirement for excess ligand in the sample solution and greater resistance to both irreversibility in the electrochemical oxidation and to interference from intermetallic compounds. Even in the case of nonreversible electrode processes, or for systems limited by complex formation/dissociation kinetics, depletive scanned deposition potential stripping curves allow complexation parameters to be determined from the shift in half-wave deposition potential, analogous to the DeFord-Hume approach for conventional voltammetry. SSCP has greater sensitivity, and provides greater resolution in multi-metal systems than does depletive DC-SSV, while SSV provides useful complementary information in some cases.Kritički su procijenjena karakteristična svojstva pseudopolarograma konstruiranih korištenjem dvaju metoda oksidacije amalgama: kronopotenciometrije i voltametrije. Pseudopolarogram je prikaz ovisnosti najvećeg intenziteta odziva oksidacije amalgama o potencijalu redukcije metalnih iona. Opisane su prednosti i mane obaju metoda pri određivanju raspodjele metalnih kompleksa i njihova podložnost tipičnim smetnjama. Analizirani su odzivi na visećoj živinoj kapi i na mikroelektrodama. Razmatrane su sljedeće pojave: reverzibilnost i brzina prijenosa elektrona, razdvajanje odziva većeg broja metala u živi, stvaranje intermetalnih spojeva u živi, utjecaji kemijskih reakcija koje prethode redukciji metalnih iona ili slijede oksidaciju amalgama, inducirana adsorpcija metalnih iona na površinu živine elektrode i minimalni višak slobodnog liganda potreban da se tijekom oksidacije amalgama svi metalni ioni uz površinu elektrode odmah kompleksiraju. Najbolja metoda oksidacije jest ona kojom se postiže potpuna oksidacija svih metalnih atoma akumuliranih u živi. Taj uvjet zadovoljavaju kronopotenciometrija sa slabom strujom i voltametrija sa sporom promjenom potencijala. Pod tim uvjetima postoji kvantitativna funkcionalna veza intenziteta odziva i količine akumuliranih metalnih atoma. Metode spore oksidacije su imune na induciranu adsorpciju, trebaju manji višak slobodnog liganda, manje ovise o brzini izmjene elektrona i manje im smetaju intermetalni spojevi. Ovim metodama mogu se odrediti konstante stabilnosti metalnih kompleksa i u slučajevima kinetički kontroliranih elektrodnih reakcija i sporih reakcija kompleksiranja ili disocijacije kompleksa. Pokazano je da je kronopotenciometrija osjetljivija od voltametrije i da se njenim korištenjem postiže bolja rezolucija odziva većeg broja metala, ali se voltametrijom mogu dobiti dodatne informacije o istraživanom sustavu

Intraparticulate speciation analysis of soft nanoparticulate metal complexes:The impact of electric condensation on the binding of Cd2+/Pb2+/Cu2+ by humic acids

In aqueous dispersions of soft, charged nanoparticles, the physicochemical conditions prevailing within the particle body generally differ substantially from those in the bulk medium. Accordingly it is necessary to define intrinsic descriptors that appropriately reflect the chemical speciation inside the particle's microenvironment. Herein the speciation of divalent metal ions within the body of negatively charged soft nanoparticulate complexants is elaborated for the example case of humic acid association with Cd(ii), Pb(ii) and Cu(ii). The electrostatic effects are described by a two-state model that accounts for counterion condensation in the intraparticulate double layer shell at the particle/medium interface and Donnan partitioning within the bulk of the particle body. Inner-sphere complex formation is defined by an intrinsic binding constant expressed in terms of local reactant concentrations as controlled by the pertinent electrostatic conditions. For the high particle charge density case (Debye length smaller than charged site separation), three distinct intraparticulate metal species are identified, namely free hydrated ions, electrostatically condensed ions, and inner-sphere metal-humic complexes. For all metal ions studied, the electrostatic contribution to the association of the metal ion with the oppositely charged particle is found to account for a substantial fraction of the total metal bound.</p

Dynamic DGT speciation analysis and applicability to natural heterogeneous complexes

Owing to their inherent heterogeneity, the thermodynamic stability of metal ion complexes with natural ligands is characterised by a distribution, and thus is a function of metal-to-ligand ratio. The kinetic features of such metal complexes are also distributed and can be probed by dynamic speciation techniques. The kinetic regime of the metal complex sample can be manipulated via the metal-to-ligand ratio, and the timescale over which kinetic parameters are actually in effect is defined by the window of the chosen technique. Here we detail the kinetic features of diffusive gradients in thin film (DGT), and show that the range of attainable measurement timescales (t) is rather limited: variation of the gel layer thickness practically allows only one order of magnitude in t to be scanned. The more effective use of DGT to probe the distribution of dynamic metal species in heterogeneous systems is via variation of the metal-to-ligand ratio in the sample solution. Compilation of the literature DGT data for natural waters shows that by assuming a Freundlich isotherm relationship, the degree of heterogeneity is reflected in the measured DGT concentration as a function of metal ion loadin

Biochemodynamic Features of Metal Ions Bound by Micro- and Nano-Plastics in Aquatic Media

A simple model, based on spherical geometry, is applied to the description of release kinetics of metal species from nano- and micro-plastic particles. Compiled literature data show that the effective diffusion coefficients, Deff, for metal species within plastic polymer bodies are many orders of magnitude lower than those applicable for metal ions in bulk aqueous media. Consequently, diffusion of metal ions in the aqueous medium is much faster than that within the body of the plastic particle. So long as the rate of dissociation of any inner-sphere metal complexes is greater than the rate of diffusion within the particle body, the latter process is the limiting step in the overall release kinetics of metal species that are sorbed within the body of the plastic particle. Metal ions that are sorbed at the very particle/medium interface and/or associated with surface-sorbed ligands do not need to traverse the particle body and thus in the diffusion-limiting case, their rate of release will correspond to the rate of diffusion in the aqueous medium. Irrespective of the intraparticulate metal speciation, for a given diffusion coefficient, the proportion of metal species released from plastic particles within a given time frame increases dramatically as the size of the particle decreases. The ensuing consequences for the chemodynamics and bioavailability of metal species associated with plastic micro- and nano-particles in aquatic systems are discussed and illustrated with practical examples