Evaluation and characteristics of a Pb(II) ion-selective electrode based on aquatic humic substances

Combinatorial screening of potentiometric Pb(II) sensors from polysulfoaminoanthraquinone solid ionophore

A potentiometric Pb(II)-selective sensor was fabricated by a combinatorial screening of electrically conducting polysulfoaminoanthraquinone (PSA) nanoparticles as a solid ionophore, ion exchangers (oleic acid (OA) and NaTPB), plasticizers in a polyvinyl chloride (PVC) matrix, membrane thickness, inner filling ion species, and concentration. The membrane sensor with the composition of PSA/PVC/DOP (dioctyl phthalate)/OA (1.0:33:61:5.0) exhibited the best performance, including a slope of 29.3 mV decade(-1) in the concentration range 10(-6.3)-10(-1.6) M, detection limit of 1.6 × 10(-7) M, response time of 16 s, lifetime of five months, and good response reversibility. The proposed sensor has demonstrated good selectivity for Pb(II) over other monovalent, divalent and trivalent interfering ions, and could be used in a pH range of 3.62-5.22. The Pb(II) sensor has been successfully applied for the determination of Pb(II) concentration in real-world samples and also as an indicator electrode for potentiometric titration of lead ions.

Lead(II) ion selective electrodes with PVC membranes based on two bis-thioureas as ionophores: 1,3-bis(N'-benzoylthioureido)benzene and 1,3-bis(N'-furoylthioureido)benzene

Two PVC membrane ion selective electrodes for Pb(II) ion based on two bis-thioureas: 1,3-bis(N'-benzoylthioureido)benzene and 1,3-bis(N'-furoylthioureido)benzene as ionophores, are reported. A first membrane formulated using 1,3-bis(N'-benzoylthioureido)benzene as carrier exhibited a Nernstian response to Pb(II) over a wide concentration range (4.0x10(-6) to 1.0x10(-2)M) with a slope of 31.5+/-1.6 mV/dec. It showed a fast response time (t(90%)=14 s) and could be used for 10 weeks without any divergence in potentials. The membrane formulated using 1,3-bis(N'-furoylthioureido)benzene as carrier exhibited a Nernstian response in the concentration range (5.0x10(-6) to 1.0x10(-2) M), with a slope of 30.0+/-1.3 mV/dec. Its response time was t(90%)=14 s, and it could be used for 14 weeks without any divergence in potentials. The two proposed potentiometric sensors revealed acceptable selectivities for Pb(II) over a wide variety of other metal ions and could be used in a pH range of 2.2-6.0. Both electrodes were assayed in direct potentiometric determination of lead in soils (10-30 mg/kg range) with very good performance (0.99935 correlation coefficient in the comparison against ICP-MS method).

Evaluation of the binding of iron(II) to humic substances derived from a compost sample by a colorimetric method using ferrozine

In evaluating the quality of composts that include humic substances (HSs), the binding of iron(II) (conditional binding constants and binding capacities) are an important criteria. To determine the binding ability, it is necessary to determine the concentrations of free Fe(2+) and Fe(II)-HS complex species present. In the present study, we describe a colorimetric method for determining free Fe(2+) species using ferrozine in an aqueous solution of Fe(2+) and HS. It was found that the colored species in the Fe(2+) and HS mixture represented only free Fe(2+) species. Two water soluble fractions of HS, humic (HA) and fulvic (FA) acids, were isolated from a compost sample, and the conditional binding constants and binding capacities were estimated by colorimetric titration. Differences in the binding of Fe(II) to HA and FA are discussed, based on their structural features, which were characterized by acidic functional group analysis, elemental analysis and solid-state CP-MAS (13)C NMR spectral data.

Bis(2-hydroxyacetophenone)ethylenediimine as a neutral carrier in a coated-wire membrane electrode for lead(II)

A coated-wire ion-selective electrode (CWISE), based on a Schiff base as a neutral carrier, was successfully developed for the detection of Pb(II) in aqueous solution. CWISE exhibited a linear response with a Nernstian slope of 29.4 +/- 0.5 mV/decade within the concentration range of 1.0 x 10(-5) - 1.0 x 10(-1) M lead ion. CWISE has shown detection limits of 5.0 x 10(-6) M. The electrode exhibited good selectivity over a number of alkali, alkaline earth, transition and heavy metal ions. This sensor yielded a steady potential within 10 to 20 s at a linear dynamic range. The electrode was suitable for use in aqueous solutions in a pH range of 2.0 to 5.0. Applications of this electrode for the determination of lead in real samples and as indicator electrode for potentiometric titration of Pb2+ ion using K2CrO4 are reported.

Implications of natural organic matter binding heterogeneity on understanding lead(II) complexation in aquatic systems

A critical analysis of data published for complexation (conditional stability constants and complexation capacities) of lead(II) by natural organic matter (NOM) is presented. The same patterns are observed for the different types of NOM, i.e. isolated humic-type substances, isolated algae- and bacteria derived ligands and cell walls, and filtered whole water systems: apparent stronger binding sites are utilised at lower metal ion loadings, progressively weaker sites contribute to metal complexation at higher loadings. Continuous binding functions should thus be used to adequately describe lead-NOM complexation in natural waters. Two important observations arising from our analysis are that: (i) the binding curves (binding affinity as a function of metal ion loading) for whole natural water samples lie between those for compounds representative of the two main NOM types, viz., isolated humic compounds and biota, and (ii) binding constants commonly used in speciation codes probably underestimate the actual extent of lead(II) binding by NOM at natural concentration levels.