ABSTRACT
Re-evaluation of DPASV procedures for determining low levels of Sb (III) and Sb (V) in solution identified several problem areas, e.g. anomalous ASV behaviour, possible formation of an intermediate valency state during the analytical cycle, and chemical interactions in acidified test solutions containing both valency states. Specific determination of Sb (III) can be achieved using base solutions composed of 0.2M HCl (detection limit 10 nM) or acetic acid/acetate buffer (detection limit 600 nM). For the determination of Sb (V), analysis in 2M HCl is recommended [with response in 0.2M HCl being used to correct for any Sb (III) present].
ABSTRACT
Separation of mixtures containing hypophosphite, phosphite, orthophosphate, chloride, nitrate and sulphate ions has been achieved using a Vydac 300 IC 405 column, refractive index detection and eluents which were 2mM in p-hydroxy benzoic acid or 4mM in p-amino benzoic acid (adjusted to ph 5.5). Calibration curves were linear over the concentration range 3 to 30 mg P/l.; and the precision (RSD for peak heights in mixtures) was < +/- 10%. Using p-hydroxy benzoic acid, detection of eluted peaks by indirect UV absorption was equally satisfactory. In an initial survey, the elution behaviour of nine oxyanions of phosphorus (simple ions and condensed species) was examined, using eluents containing different organic carboxylic acids (six aliphatic and nine aromatic), present at levels of 1 to 5m(M); with the pH adjusted to values between 3 and 8.6. No suitable conditions for separating mixtures containing condensed phosphates were identified.
ABSTRACT
A procedure for determining the "labile" metal content of contaminated sediments (in different chemical environments) has been critically examined. The sediments were extracted overnight with different chemical solutions and the suspensions were analysed by differential pulse anodic stripping voltammetry. The extractants used have been recommended for soil/sediment speciation schemes, and by examination of the suspensions directly in the ASV cell, errors due to re-adsorption of released metal ion were minimized. The existence of different chemical forms of metal was signified by changes in peak shape and position or by the appearance of additional peaks. With complexing agents present the peak size was pH-dependent. The limitations of the ASV/suspension analysis technique have been carefully evaluated by using ten different extractants and seventeen sediments. The "lability" results obtained have been compared with the values obtained from a cation-exchanger transfer procedure. For characterizing the lability behaviour of the metal contents of sediments, preliminary extraction into a minimum of four base solutions is advisable, e.g., 0.02M nitric acid (low-pH labile); hydroxylamine in acetic acid (reducing conditions); acetic acid/acetate buffer (weakly sorbed and carbonate-bound) and 0.05M calcium chloride (exchangeable fraction at natural system pH), where the terms in parentheses describe the character of the fraction. The results are critically compared with those obtained by atomic-absorption analysis of the extracts and with those obtained by an earlier ion-exchange fractionation scheme. The advantages and limitations of the ASY systems are discussed.
ABSTRACT
The labile metal content of sediments can be evaluated by equilibrating sediment suspensions with ion-exchange resins. By use of a sequence of strong-acid and weak-acid cation-exchangers (H(+)- and Na(+)-form) and chelating resins, extraction can be performed at pH values ranging from 2 to 10. The results allow the total metal content to be subdivided into seven categories designated as (i) low-pH labile, (ii) weak-acid labile, (iii) exchangeable and readily desorbed at sediment-suspension pH, (iv) weak-base labile, (v) high-pH labile, (vi) non-labile soluble forms and (vii) detrital metal content. The sediment suspensions are mixed overnight with the different types of exchanger (held in porous containers) and the cations transferred from the sediment are subsequently back-extracted from the resins into 0.05M EDTA (pH 7.5). The EDTA extracts are analysed for Cu, Pb, Zn, Cd, Ca, Mg, Fe and Al. Analysis of the aqueous phase left in contact with the sediment residue gives the amount of non-labile species released. Eighteen sediments, containing various levels of metal contamination, and an effluent dam sludge have been examined by this technique. All the exchangers released Ca and Mg from the sediments, and the H(+)-form exchangers also released Fe and Al. Some of the Fe, Al and to a lesser extent Zn released by the sediment/exchanger interactions was present as non-labile "soluble" species. The advantages and limitations of this "labile metal" fractionation scheme have been considered.
ABSTRACT
Equilibration of sediments with cation-exchangers results in a transfer of loosely bound labile metal species to the exchanger phase. Dissolution of the matrix is also promoted and selectivity rules suggest that some of the cations released (particularly Ca) could effectively compete with metal ions for exchange sites. This potential source of error has been evaluated by studying synthetic mixtures of Ca(2+) and other metal ions (Cu(2+), Pb(2+), Cd(2+), Zn(2+)) and by analysis of two calcium-rich wastes (a calcine and a jarosite). The ion uptake most influenced by calcium competition was that of zinc; uptake of lead was least affected. For minimum error, i.e., optimum transfer of "available" or "labile" metal ion, the level of free Ca(2+) introduced into the solution should not exceed 300 mg/l., and the amount of exchanger added must provide an excess of exchange sites relative to the amount of cations released from the sample. By use of exchangers of different types it is possible to attempt some classification of the labile metal content, e.g., acid-displaced, exchangeable, salts of weak acids.
ABSTRACT
The metal content of a series of contaminated sediments has been determined by equilibrating ion-exchange resins of different types (held in porous cages) with aqueous suspensions of the sediments. H(+)-form exchangers took up high proportions of the Cu, Pb, Zn and Cd contents with recoveries depending on whether the acid functional groups were strong. Na(+)-form exchangers took up a smaller, loosely bound, labile fraction. The metal ions held on the exchangers were back-extracted into 0.05M EDTA and determined by flame AAS. Dissolution of sediment components led to the exchangers also taking up large amounts of Ca, Mg, Fe and Al. The exchange-resin technique provides an alternative means of subdividing the metal content of sediments into different "labile" or "available" fractions. Possible advantages include minimal re-adsorption of released metal ions by the sediment phases, retention of only "labile" species (ions or complexes), and a transfer mechanism which may resemble the action of plant roots more closely than chemical extractant processes do.
ABSTRACT
Contaminated sediment samples and wastes were extracted with a series of chemical solutions widely used in soil and sediment analyses. The Cu, Pb and Cd contents of the extracts were then determined by both AAS and ASV, and it was found that not all of the metal ion retrieved was "labile". Differences between selected extraction values were compared with the bonding-mode category values obtained by using a well-known sequential extraction procedure, and it was found that the series and sequential approaches yielded different results for Pb and Cu. The advantages and limitations of ASV monitoring in sediment studies are considered and the inappropriateness of some operationally defined fraction categories is indicated.
ABSTRACT
After stabilization of sulphite by addition of a five-fold excess of formaldehyde, mixtures of sulphite, sulphate and thiosulphate are readily separated by HPLC. For use with Vydac columns (302IC or 300IC) a suitable eluent is 1-3mM phthalic acid (pH 5-6), and the eluted ions may be detected by means of the change in refractive index or the absorbance at 290 nm. With a flow-rate of 2 ml/min, anion elution times are about 2.7 (0.7) min for HOCH(2)SO(-)(3), 7.5 (2.6) min for SO(2-)(4) and 8.4 (5.2) min for S(2)O(2-)(3), the values in parentheses being those for the shorter 300IC column. Detection limits for sulphite and sulphate were around 1 mg/l., with relative standard deviations of 2-3% at higher levels. The sensitivity for S(2)O(2-)(3) was an order of magnitude lower. The proposed method has advantages over earlier procedures based on alkaline eluents.
ABSTRACT
The potential influence of inorganic particulates on the ASV response of < 100-microg/l. levels of Cd, Pb and Cu, at a thin film mercury electrode, was examined by adding various weights of the solids to the acetate background electrolyte solution. Materials added included the hydrous oxides of Mn(IV), FE(III) or Al(III), clay minerals (kaolinite, illite, montmorillonite) and some contaminated sediments. Abrasion of the mercury film was minimized by deaerating the turbid solutions before their transfer to the measuring cell. The hydrous oxides specifically sorbed all three metal ions, resulting in peak-size changes that varied in magnitude with pH. With the clays, only sorption of Pb by illite or montmorillonite was detected. The presence of the solids had little effect on the peak position or half-peak breadth of the Cd or Pb signals, but the Cu peak parameters changed, indicating some hydroxy-species formation at higher pH. Some contaminated sediment samples released a significant fraction of their total metal content into acetate buffer solutions. Shielding the mercury film with a semipermeable membrane had a similar effect to filtering the suspension before analysis, but diffusion equilibrium was only slowly achieved (> 12 hr).
ABSTRACT
The limited ability of added ligands to mask the interference of Al(III) in fluoride determination by means of ion-selective electrode measurements has been re-examined, and an explanation based on competing equilibria developed. When the Al(III) level exceeds the fluoride content, the excess of A1(III) forms colloidal hydrous oxide particles in the pH range 4-9, and this solid strongly sorbs fluoride. Under these conditions, masking ligands have to promote both decomposition of AlF(x), soluble complexes and release of sorbed fluoride by substrate dissolution. The latter is a slow process, particularly with an "aged" gel, and long equilibration periods can be required for total fluoride release. Ion-chromatography studies have shown that the amount of fluoride lost through sorption on the hydrous oxide particles (isolated during the membrane filtration/degassing step) can be quite high. By analogy, preliminary phase separation of natural water samples must also separate sorbed fluoride and soluble fluoride complexes. In the presence of A1(III), accurate analysis for fluoride requires removal of the aluminium, or isolation of fluoride from the matrix, or very careful pretesting of masking efficiencies.
ABSTRACT
The effect of organic colloids, such as humic and fulvic acids, on the ASV signals of Cd, Pb and Cu in acetate-buffer media has been investigated. Owing to their ability to adsorb metal ions, or form complexes (some sparingly soluble), the presence of the organic acids caused the magnitude of the signal peaks to decrease, the effect increasing with increase in organic acid: metal-ion ratio, and pH (from 5 to 8). Analysis before and after nitration through a 0.45-mum membrane indicated that most of the Cd retained was labile; in the Pb and Cu system less than half of the sorbed metal ion was labile, though a further fraction was displaced in the presence of Chelex 100 resin. The magnitude of the interference effect varied with the molarity of the medium and the presence of diverse anions (e.g. chloride, bromide) in the system. Cu displayed the most anomalous behaviour, with the stripping peak tending to broaden and split, particularly in the presence of humic acids. This behaviour was enhanced when the contact time with the preformed Hg film before the deposition/stripping cycle was increased above 5-10 min. This effect has been attributed to reduction (and subsequent oxidation) of Cu-humate species adsorbed on the mercury film.
ABSTRACT
The ability of a range of electrolyte solutions to release metal ions (Cu, Pb, Cd, Zn) presorbed on two samples of humic acid has been investigated. Though treatment with mineral acid or a chelating agent released a high proportion of the retained metal ion, recoveries were never total. Concentrated salt solutions displaced about 80% of the retained Cd or Zn, and about half of any Cu or Pb held by the organic matter, which indicates that most of the adsorbed metal ion is exchangeable, the extraction efficiency being controlled by competing equilibria. The effect of added clay suspensions was also examined. Analytical procedures for fractionating the total metal content of soils into subgroups have been assessed against the observed extraction behaviour.
ABSTRACT
The effects of pH, electrolyte composition and complex formation on the size and position of the ASV peaks of Pb, Cu and Cd have been systematically evaluated, with an instrument equipped with a mercury thin-film electrode and by applying a linear ramp voltage scan. The peak heights change with pH and the magnitude of the pH effect varies with base electrolyte composition. Anions such as chloride and acetate reduce the signal, as does the presence of excess of ligands such as 2,2'-bipyridyl, NTA and EDTA. Formation of stable chelates (e.g., with EDTA) can lead to total loss of signal, but dissociation of labile complexes can be enhanced by reducing the pH and/or increasing the magnitude of the applied deposition potential, thus producing measurable peaks. The peak potentials vary with pH, and in copper systems there are additional shifts in the presence of citrate, 2,2'-bipyridyl and chloride. With the last two, double peaks are formed and these are attributed to the formation of both Cu(I) and Cu(II) oxidation products. The varied response, particularly in the case of copper, which can follow changes in the base electrolyte composition, supports the need for careful control of the chemical environment in quantitative determination, and raises some queries about the feasibility of using direct ASV for speciation purposes.
ABSTRACT
A critical account is given of the factors affecting preconcentration of traces of lead from solution by adsorption on various substrates. The recovery by various stripping techniques is also considered. Direct determination is recommended in preference to use of stripping methods.
ABSTRACT
The effect of solution variables on the efficiency of tantalum atom formation in hot flames is considered in terms of competing equilibria.
ABSTRACT
In the determination of silver, by atomic-absorption spectroscopy, the presence of small amounts of precipitating agents in the solution to be analysed can result in significant changes in the absorbance at 328.1 nm. The controlling factor is the physical form of the precipitated material. The presence of some complexing agents also causes variable absorption readings, and with ligands such as thiosulphate and cyanide ions the interference can be attributed to secondary reactions associated with the sparingly soluble compounds formed under specific conditions.
ABSTRACT
Seven different types of foodstuff (containing 3-100 ppm Zn) were each analysed for zinc by colorimetry, polarography and atomic-absorption spectroscopy. All the techniques yielded similar values for the zinc content of a given sample, but atomic-absorption spectroscopy had a smaller average coefficient of variation.
ABSTRACT
The interference effects observed in series of absorption studies using nitrous oxide/acetylene or air/acetylene flames are interpreted in terms of the nature of the chemical species present in solution. The elements studied include Nb, Ta, Ti, Y, V, W and Ni, and species shown to influence the atomic absorption include hydrofluoric, phosphoric and sulphuric acids, the ions of Ca, K, Al, Fe and Mn, and compounds such as EDTA and ammonium acetate.
ABSTRACT
A method is proposed for the determination of mg amounts of hydrazine based on the measurement of the volume of nitrogen evolved after oxidation with silver(I) oxide.
ABSTRACT
The reaction of silver(II) oxide with water to yield silver(I) ions and oxygen is an important side-reaction in procedures which use silver(II) oxide as the oxidant. This reaction has been examined in solutions which were 0.01-1M in respect to sulphuric or perchloric acid. The rate of reduction varied directly with the amount of solid added and was a complex function of the concentrations of hydrogen ions, silver(I) ions and anions present. The kinetic results have been used to postulate a mechanism for the reaction.
