Labile metal determinations

It has been proposed that there may be a close link between the amount of an element available to living matter and the fraction of the total content which is labile (with the lability value being loosely defined as the total, accessible, hydrated ion level). Either the whole or part of the analytical result may be derived from dissociation of labile complex ions or dissolution of moderately soluble compounds. If one or both of these two processes proceed at a relatively slow rate, the magnitude of the lability value becomes time dependent . Conversely, if a complex exchanges ligands fairly rapidly, the amount present in 

Liquid chromatography (column, Plasma emission spectrometry, AAS (flame and 

gel permeation) graphite furnace) fluorimetry, electro-chemical sensors neutron activation analysis 

Gas chromatography Specific detectors (e.g. flame photometric, electron capture, AAS) 

Ion chromatography Conductance, refractive index, indirect UV absorbance electro-chemical sensors 

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The speciation scheme of Batley and Florence requires eight measurements on four samples. After removing insoluble particulates by filtration, the solution is analyzed for the concentration of anodic stripping voltammetry (ASV) labile metal and the total concentration of metal. A portion of the filtered solution is passed through an ion-exchange column, and the concentrations of ASV metal and total metal are determined. A second portion of the filtered solution is irradiated with UV light, and the concentrations of ASV metal... 

On this basis = 0.0170 sec , = 0.645 sec , and K = 0.739 mole.P at 25 °C. The corresponding activation parameters were determined also by Es-penson. By a method involving extrapolation of the first-order rate plots at various wavelengths to zero time, the absorption spectrum of the intermediate was revealed (Fig. 1). Furthermore, the value of K obtained from the kinetics was compatible with that derived from measurements on the acid dependence of the spectrum of the intermediate. Rate data for a number of binuclear intermediates are collected in Table 2. Espenson shows there to be a correlation between the rate of decomposition of the dimer and the substitution lability of the more labile metal ion component. The latter is assessed in terms of the rate of substitution of SCN in the hydration sphere of the more labile hydrated metal ion. 

Because of differing sensitivities and the natural levels of free metal or the anodic scanning voltammetric labile metal, cadmium, and copper in seawater are analysed using a 10 minute plating time, a -1.0V plating potential, and scanning in 6.67 mV/s increments. Zinc determinations can be made on a fresh aliquot of sample to eliminate any possible effects due to Cu-Zn inter-... 

The Ag+ ion is labile. Even with cryptands, which react sluggishly with most labile metal ions, Ag reacts with a rate constant around 10 M s (in dmso). The higher stability of Ag(I) complexes compared with those of the main groups I and II resides in much reduced dissociation rate constants. Dissociation tends to control the stability of most metal cryp-tand complexes. Silver(I) is a useful electron mediator for redox reactions since Ag(I) and Ag(II) are relatively rapid reducers and oxidizers, respectively. Silver(I) thus promotes oxidation by sluggish, if strong, oxidants and catalyses a number of oxidations by S20 in which the rate-determining step is... 

There are a number of alternative approaches to lability measurements, and three which involve different timescales are listed in Table 2.8. The level of labile electro-active metal species present in aqueous samples can be determined using ASV (see Section 2.7.4). The electrochemical response (i.e. peak area values) reflects the hydrated metal ion content together with contributions from any metal complexes present which rapidly dissociate in the diffusion layer (around the mercury cathode). The labile metal is measured either at the natural sample pH, or after pH adjustment (e.g. after addition of pH 5 acetate buffer). 

Electro-active labile metal contents have also been measured by using a combination of electro-deposition and analysis by graphite furnace AAS (Batley and Matousek, 1977). Metals (e.g. Pb, Co, Ni, Cr from seawater) are plated on to a short graphite tube by application of a suitable potential. At the end of the electrolysis period, the graphite cell (plus pre-concentrated metal) is placed in an electro-thermal atomiser attached to an AAS spectrometer, and the element content determined. 

Scarano, G., Bramanti, E. and Zirino, A. (1992) Determination of copper complexation in sea water by a ligand competition technique with voltammetric measurement of the labile metal fraction. Anal. Chim. Acta, 264, 153-162. 

Anodic stripping voltammetry (ASV) is a very sensitive instrumental technique for the measurement of metals in solution. Of particular importance are determinations of reactive or ASV-labile metal concentrations. ASV-labile metal is defined as the fraction of the total metal concentration that is measured under a defined set of ASV and solution conditions. Labilities of metal species in natural water have been related to toxicities. Thus, one objective of speciation analysis carried out by ASV is to find conditions where the ASV-labile fraction is a close approximation to the toxic fraction of a metal. In experimental terms, the ASV-labile metal should be equal to the electroactive fraction of the metal. The latter... 

Stabilized liquid membrane device (SLMD) A water-insoluble organic complexing mixture diffuses to the exterior surface of the sampler through a polymeric membrane Divalent metal ions Preconcentration, in situ sampling, determination of labile metal ions in grab samples Days to several weeks Extraction with acid 73... 

For example, a number of studies have been made on the metal ion-catalyzed hydrolyses of acetyl phosphate and acetyl phenyl phosphate, " but the role of the metal ion in these processes remains uncertain. Investigations of catalysis by exchange labile metal ions e.g. Ca" and Mg") have yielded conflicting results and both the nature and distribution of the kinetically significant species, as well as the positions of bond cleavage, have yet to be determined unequivocally. Chelation, charge neutralization and attack by metal-bound hydroxide have variously been proposed as important factors in acyl phosphate hydrolysis. 

These studies indicate that the sequence of duplexes and the accessibility of bases are the dominant factors that determine the binding pattern of the labile metal ions to DNA, and that the molecular electrostatic potential further affects the metal binding. Of importance for this chapter is that in all of the reported solution and solid-state structures, the 3d transition metal ions coordinate to the N7 of a G nucleobase and complete their octahedral coordination sphere with water molecules or with phosphate groups from adjacent nucleotides within the duplexes. Thus they do not form direct interstrand bridges. 

While most examples of self-assembly involve labile metal ions, a racemic triangle/square mixture synthesized from inert Co ions has been isolated. The triangles and squares, as well as the enantiomers of each respective structure, can be isolated because of kinetic inertness of the metal-ligand interaction. Formation of the square and triangle likely occurs via an irreversible self-assembly process in which the ratio of the triangle to square is kinetically determined. In this and other examples, inert ions have been used in self-assembly for the purpose of isolating kinetic products and potentially elucidating the self-assembly mechanism. 

The technique is particularly suitable for the study of chemical specia-tion, that is, the determination of different physical-chemical forms of the element [39]. Measurements of oxidation states, complexation capacity, stability constants, or labile metal concentration, represent typical stripping speciation experiments. Because of its fundamentally different detection principles, the... 

DGT methods measure a labile metal fraction, which is a subset of the total dissolved metals concentration. The proportion of the dissolved metal fraction determined will depend on the rate and extent of metal complex dissociation in the presence of... 

For the labile zinc determination it is preferable to select a more positive adsorption potential. In this condition the sensitivity for zinc is still about ten times greater than that for nickel and cobalt, so these metals do not interfere unless their concentrations are much greater than usual. 

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