Elektrokemijska karakterizacija miješanih kompleksa željeza(III) s glicinom i nitrilotriacetatom i njihova stabilnost u vodenim otopinama

Electrochemical characterization of iron(III)–glycine–nitrilotriacetate (iron(III)-Gly-NTA) mixed ligand complexes and determination of their stability constants and retention time in aqueous solutions (I = 0.1 mol dm–3 in NaClO4, pH = 8.0±0.1 at 25±1 °C), using differential pulse cathodic voltammetry (DPCV), cyclic voltammetry (CV) and direct current (d.c.) polarography with a static mercury drop electrode (SMDE), were performed. Iron(III) concentrations were varied from 5×10–6 to 6×10–4 mol dm–3, NTA total concentrations varied from 2×10–5 to 1×10–3 mol dm–3 and glycine total concentrations were 0.2, 0.02 and 0.002 mol dm–3. Iron(III) redox reaction in the mixed ligand system (by the techniques employed) was found to be a one-electron reversible process. At total concentration ratios of 1 : 800 : 2 for iron(III), glycine and NTA, respectively, the iron(III)-Gly-NTA mixed ligand complexes were dissolved and stable (>18 hours) in the aqueous solution. The complexes were formed either by the addition of NTA into the iron(III) and glycine aqueous solution or by the addition of iron(III) to the mixture of glycine and NTA. Under these conditions, iron(III) hydrolysis was highly suppressed. By fitting of experimental data, the following stability constants for mixed ligand complexes, not found in the literature so far, in 0.1 mol dm–3 NaClO4 aqueous solution were calculated: for iron(III) log β1([FeGlyNTA]) = 27.23±0.69, log β2([Fe(Gly)2NTA]2–) = 30.29±0.77; for iron(II) log β 1([FeGlyNTA]2–) = 14.13±0.43 and log β 2([Fe(Gly)2NTA]3–) = 18.51±0.51.Elektrokemijski su karakterizirani miješani kompleksi željeza(III) s glicinom i nitrilotriacetatom (FeIIIGly- NTA), te su određene njihove konstante stabilnosti i vrijeme zadržavanja u vodenim otopinama (I = 0,1 mol dm–3 u NaClO4, pH = 8,0±0,1 pri 25±1 °C) diferencijalnom pulsnom katodnom voltametrijom (DPKV), cikličkom voltametrijom (CV), te polarografijom s izravnim uzorkovanjem struje na elektrodi s visećom živinom kapi. Koncentracije željeza(III) mijenjane su od 5×10–6 do 6×10–4 mol dm–3, ukupne koncentracije NTA od 2×10–5 do 1×10–3 mol dm–3, dok su ukupne koncentracije glicina bile 0,2, 0,02 i 0,002 mol dm–3. Redoks reakcija željeza( III) u sustavu miješanih liganada (uporabljenim tehnikama) bila je jednoelektronski reverzibilni proces. Pri omjeru totalnih koncentracija 1 : 800 : 2 (željezo : glicin : NTA), FeIIIGlyNTA kompleksi su otopljeni i stabilni (>18 sati) u vodenoj otopini. Kompleksi su formirani kako dodatkom NTA u vodenu otopinu željeza(III) i glicina tako i dodatkom željeza(III) u smjesu glicina i NTA. Pod ovim uvjetima hidroliza željeza(III) je u većoj mjeri spriječena. Primjenom eksperimentalnih podataka izračunate su konstante stabilnosti kompleksa miješanih liganada u 0,1 mol dm–3 NaClO4, koje do sada nisu opisane u literaturi: za željezo(III) log β1([FeGlyNTA]–) = 27,23±0,69 i log β 2([Fe(Gly)2NTA]2–) = 30,29±0,77; za željezo(II) log β1([FeGlyNTA]2–) = 14,13±0,43 i log β2([Fe(Gly)2NTA]3–) = 18,51±0,51

Current Minimum in Differential Pulse Polarography

Current minimum, which sometimes appears as a part of the net response in differential pulse polarography (DPP), was studied in systems characterized by a pronounced IR drop or reactant adsorption. Experimental results obtained on a static mercury drop electrode (SMDE) clearly indicate that this effect is highly influenced not only by solution resistance (or intentionally added resistors) and reactant concentration, but also by both timing parameters (drop time, pulse duration) and electrode surface area. Presentation of the net response along with its components, demonstrates that the current minimum originates from the maximum on dc component, minimum on pulse component or both. In practice, DPP minimum, obtained in measurements with a SMDE, can be treated as an additional diagnostic parameter for the recognition of reactant adsorption or poor experimental conditions (i.e. high resistance within electrode system or low conductivity of the electrolyte medium). (doi: 10.5562/cca2054

Distribution and speciation of dissolved iron in Jiaozhou Bay (Yellow Sea, China)

The distribution of total dissolved iron (DFe) and its chemical speciation were studied in vertical profiles of the shallow and semi-closed Jiaozhou Bay (JZB, China) during two contrasting periods: summer (July 19th, 2011) and spring (May 10th, 2012). Samples collected from the surface, middle and bottom layers were analyzed by competitive ligand exchange-adsorptive cathodic stripping voltammetry (CLE-aCSV). The mean DFe concentration during the summer period (median 18.8 nM ; average 20.7 nM) was higher than in the spring period (median 12.4 nM ; average 16.9 nM), whereas the spatio-temporal variation in spring was larger than in summer. The DFe-values showed distinct regional and seasonal differences, ranging from 5.6 to 107 nM in spring period and 13.4 to 43.4 nM in summer period. In spring, the highest DFe-values were observed in the eastern coastal region, especially near an industrial area (up to 107 nM), whereas the DFe distribution in summer was relatively even. Due to a tide influence, the vertical variations in the DFe and Lt in both seasons were not significant. On average, the Lt concentration (one class of ligand was estimated in all samples), was higher in spring (35.2 ± 23.4 nM) than in summer (31.1 ± 10.3 nM). A statistically significant correlation was found between Lt and DFe concentrations, it was higher for the summer period than for the spring period. The conditional stability constants (logK′) of organic complexes with iron were weaker in spring (11.7 ± 0.3) than in summer (12.3 ± 0.3). The concentrations of Lt were higher in comparison to DFe in all samples: the average [Lt]/[DFe] ratio in the spring and summer samples was 2.4 and 1.5, respectively. Speciation calculations showed that the DFe in the JZB existed predominantly (over 99.99%) in the form of strong organic complexes in both seasons

Evaluation of Discrete and Passive Sampling (Diffusive Gradients in Thin-films – DGT) Approach for the Assessment of Trace Metal Dynamics in Marine Waters – a Case Study in a Small Harbor

Two complementary approaches, based on discrete and passive samplings (diffusive gradients in thin-films - DGT), supported by the speciation modeling, were evaluated for the assessment of distribution and operational speciation of trace metals (Zn, Cd, Pb, Cu, Ni and Co) within a small marine harbor (Rijeka, Croatia). Concentrations of dissolved metals were relatively low and comparable to, or slightly above those found in coastal Adriatic region. Compared to higher variability of dissolved metal concentrations due to the discrete sampling, smoother temporal distribution was recorded for DGT-labile metals. The percentages of DGT-labile metal concentrations, which reflect their affinity to organic matter, varied among metals with the following order: Cu<Zn≈Co<Ni<Cd≈Pb. DGT-labile metal concentrations predicted by speciation modeling were in a good agreement with the measured ones for Zn, Cd and Ni, while they are underestimated for Pb and Cu, and overestimated for Co. In-situ DGT technique is recommended for the assessment of the water quality status in marine environment. This work is licensed under a Creative Commons Attribution 4.0 International License

Voltammetric Study of Uranyl–Selenium Interactions. Part 2. Uranyl(V) Selenate Complex

Under the influence of recently published papers and some older own results, complex formation in uranyl(VI)–selenium(VI) system was reinvestigated at the ionic strengths of 0.1 and 3.0 mol L–1. Using electrolyzed selenate solutions (and square-wave voltammetry as a measuring technique) two coordination species at the higher electrolyte concentration (log β1 = 1.48 ± 0.01, log β2 = 2.4 ± 0.1) were recognized, in agreement with the literature data based on other experimental methods. For successful interpretation of the voltammetric data, previously unknown weak complex (log β01=0.7 ± 0.1) of the reduction product, i.e. uranyl(V) should also be included in the model. At the lower electrolyte concentration (0.1 mol L–1), formation of complexes other than UO2SeO4 0 cannot be unambiguously confirmed due to the narrow ligand concentration range. Concerning the number of coordination species and their stability constants, seemingly different results that arise from voltammetry and other (independent) experimental methods reflect the fact that the former, in addition to the complexation of initially present metal ion, "sees" the coordination of its reduction product with the ligand of interest, too. When both processes are taken into account during treatment of voltammetric data, such differences disappear. (doi: 10.5562/cca1952