The speciation of arsenic(V) and arsenic(III), by ion-exclusion chromatography, in solutions containing iron and sulphuric acid

Arsenic(V) and arsenic(III) can be separated, by ion-exclusion chromatography, in solutions containing iron and sulphuric acid. Iron is removed by ion-exchange before the speciation of arsenic, with phosphoric acid as the eluent. The separated arsenic(V) and arsenic(III) are measured spectrophotometrically in the ultraviolet region at a wavelength of 195 mn. Arsenic(V) and arsenic(III) can be determined at concentrations > or = 3 mg/1. The relative standard deviations are 1.3% for arsenic(V) and 0.9% for arsenic(III), at the 10 mg/1. level. The time required for the separation of the inorganic arsenic species is 11 min.     

Determination of trace arsenic in hydrofluoric acid by hydride generation and atomic absorption spectrometry

Practical procedures are given for determination of arsenic(III) and (V) in hydrofluoric acid by means of hydride generation and atomic absorption spectrometry. Arsenic(III) can be determined by direct generation of arsine with sodium borohydride in hydrochloric/hydrofluoric acid medium, arsenic(V) being only slightly reduced under the conditions used. For its determination, arsenic(V) has to be prereduced with potassium iodide, and even then its reduction to arsenic(III) and then arsine is far from complete. It is possible to determine it in presence of arsenic(III) by a difference method, but this is recommended only if the As(V)/As(III) ratio is greater than 1. Total arsenic can be determined after oxidation of As(III) and evaporation of most of the hydrofluoric acid. The limit of determination is 5 g/l for arsenic(III) and 0.25 g/l for total arsenic; the relative standard deviation is about 10%.     

Separation of arsenic(III) and arsenic(V) in ground waters by ion-exchange

The predominant species of arsenic in ground water are probably arsenite and arsenate. These can be separated with a strong anion-exchange resin (Dowex 1 x 8; 100-200 mesh, acetate form) in a 10 cm x 7 mm column. Samples are filtered and acidified with concentrated hydrochloric acid (1 ml per 100 ml of sample) at the sample site. Five ml of the acidified sample are used for the separation. At this acidity, As(III) passes through the acetate-form resin, and As(V) is retained. As(V) is eluted by passage of 0.12M hydrochloric acid through the column (resulting in conversion of the resin back into the chloride form). Samples are collected in 5-ml portions up to a total of 20 ml. The arsenic concentration in each portion is determined by graphite-furnace atomic-absorption spectrophotometry. The first two fractions give the As(III) concentration and the last two the As(V) concentration. The detection limit for the concentration of each species is 1 mug l .     

Speciation of arsenic(III) and arsenic(V) by inductively coupled plasma-atomic emission spectrometry coupled with preconcentration system

A novel and simple flow-based method was developed for the simultaneous determination of As(III) and As(V) in freshwater samples. Two miniature columns with a solid phase anion exchange resin, placed on two 6-way valves were utilized for the solid-phase collection/concentration of arsenic(III) and arsenic(V), respectively. As(III) could be retained on the column after its oxidation to As(V) species with an oxidizing agent. The collected analytes were then sequentially eluted by 2 M nitric acid and introduced into ICP-AES. Potassium permanganate was examined as potential oxidizing agent for conversion of As(III) to As(V). The standard deviation of the analytical signals (peak height) for the replicate analysis (n = 5) of 0.5 μg l⁻¹ solution were 3 and 5% for As(III) and As(V), respectively. The limit of detection (3σ) for both As(III) and As(V) were 0.1 μg l⁻¹. The proposed system produced satisfactory results on the application to the direct analysis of inorganic arsenic species in freshwater samples.     

Differential preconcentration of arsenic(III) and arsenic(V) with thionalide loaded on silica gel

2-Mercapto-N-2-naphtylacetamide (thionalide) on silica gel is used for differential preconcentration of μg l^?1 levels of arsenic(III) and arsenic(V) from aqueous solution. In batch experiments, arsenic(III) was quantitatively retained on the gel from solutions of pH 6.5–8.5, but arsenic(V) and organic arsenic compounds were not retained. The chelating capacity of the gel was 5.6 μmol g^?1 As(III) at pH 7.0. Arsenic retained on teh column was completely eluted with 25 ml of 0.01 M sodium borate in 0.01 M sodium hydroxide containing 10 mg l^?1 iodine (pH 10). The arsenic was determined by silver diethyldithiocarbamate spectrophotometry. Arsenic(V) was subsequently determined after reduction to arsenic(III) with sulphite and iodide. Arsenic(III) and arsenic(V) in sea water are shown to be < 0.12 and 1.6 μg l^?1, respectively.     

Generation of AsH(3) from As(V) in the absence of KI as prereducing agent: Speciation of inorganic arsenic

It is shown that the potassium iodide to the samples to reduce As(V) to AS(III) is not essential when total inorganic arsenic is determined by molecular spectrophotometry (trapping AsH(3) in Ag-DDTC) or by atomic-absorption spectrometry (if Ar flow-rate and NaBH(4) addition rate are controlled in 6M hydrochloric acid medium). Furthermore, in the presence of low concentration of organic arsenic, a method is reported for the selective determination of inorganic As(III) and As(V), based on the use of citrate/citric acid medium to determine As(III) and hydrochloric acid to determine total inorganic As. As(V) is determined by the difference between total inorganic As and As(III). The interference level of organic arsenic species (monomethylarsenic acid and dimethylarsenic acid) in the determination of total inorganic arsenic and AS(III) in 6M hydrochloric acid and citrate/citric acid medium respectively, is reported in the text. The developed method is applied to determine As(III) and As(V) in spiked, tap and waste waters and in lake sediments.     

Flow potentiometric and constant-current stripping analysis for arsenic(V) without prior chemical reduction to arsenic(III): Application to the Determination of Total Arsenic in Seawater and Urine

Arsenic(V) is reduced to elemental arsenic on a gold-coated platinum-fibre electrode at electrolysis potentials below ?1.60 V vs. Ag/AgCl and subsequently re-oxidized, either by means of a constant current, or chemically, with gold(III) as oxidant. Total arsenic in acidified seawater can be determined by means of electrolysis for 60 s at ?1.80 V vs. Ag/AgCl and subsequent stripping in 4 M hydrochloric acid containing 2.5 M calcium chloride. The detection limit obtained after 60 s of electrolysis (ca. 0.1 μg1^?1) is about ten times lower than that obtained by the electrochemical stripping methods for arsenic(III) reported hitherto. Total arsenic in urine is determined after digestion with nitric acid and hydrogen peroxide.     

Speciation of arsenic in a contaminated soil by solvent extraction

Soil collected from a disused cattle dip in northern New South Wales was studied with the aim of developing an inexpensive, yet effective method for quantitative determination of arsenic(III), arsenic(V) and total organic arsenic in a contaminated soil. Hydrochloric acid extractions were used as a method for removal of the arsenic from the soil in a form suitable for speciation. It was found that the extraction efficiency varied with the ratio of soil to acid, and the concentration of the acid. Arsenic(III), as arsenic trichloride, was selectively extracted into chloroform from a solution highly concentrated in hydrochloric acid. This was followed by back-extraction of the arsenic into water. Total inorganic arsenic was determined in a similar manner after the reduction of arsenic(V) to the trivalent state with potassium iodide. Arsenic(V) was determined by the difference between the results for arsenic(III) and total inorganic arsenic. All analyses for the various arsenic species were performed by hydride generation-atomic absorption spectroscopy; concentrations of total arsenic in the soil were confirmed using X-ray fluorescence spectrometry. It was found that all the arsenic in the soil was present as inorganic arsenic in the pentavalent state. This reflects the ability of arsenic to interchange between species, since the original species in cattle dipping solution is arsenic(III).     

Speciation of arsenic(V) and arsenic(III) by cathodic stripping voltammetry in fresh water samples

A voltammetric stripping procedure is described for the determination of arsenic(V) in a mannitol-sulphuric acid medium. The arsenic is coprecipitated with copper and selenium and reduced to arsine at the hanging mercury drop electrode. Using an accumulation time of 240 s, the detection limit is 0.52 μg L^–1, the determination limit is 0.9 μg L^–1. The method has been applied to the determination of arsenic in water samples. By varying the composition of the supporting electrolyte it is possible to differentiate between arsenic(III) and arsenic(V). As both oxidation states have different toxicological characteristics, the ability to discriminate between both is an distinct advantage of the proposed method. Received: 25 October 1996 / Revised: 7 February 1997 / Accepted: 12 February 1997     

Differential determination of arsenic (III) and total arsenic with L-cysteine as prereductant using a flow injection non-dispersive atomic absorption device

Speciation of arsenic in environmental samples gains increasingly importance, as the toxic effects of arsenic are related to its oxidation state. A method was developed for the determination of trace amounts of arsenic (III) and total arsenic by flow injection hydride generation coupled with an in-house made non-dispersive AAS device. The total arsenic is determined after prereduction of arsenic (V) to arsenic (III) with L-cysteine in a low concentration of hydrochloric, acetic or nitric acid. The conditions for the prereduction, hydride generation and atomization were systematically investigated. A quartz tube temperature of 800 degrees C was found to be optimum in view of peak shape and baseline stability. Pb(II), Ni(II), Fe(III), Cu(II), Ag(I), Al(III), Ga(II), Se(IV), Bi(III) were checked for interfering with the 2 microg/L As(V) signal. A serious signal depression was only observed for Se(IV) and Bi(III) at a 150-fold excess. With the above system, arsenic was determined at a sampling frequency of about 1/min with a detection limit (3sigma) of 0.01 microg/L using a 0.5 mL sample. The reagent blank was 0.001+/-0.0003 absorbance units and the standard deviation of 10 measurements of the 2 microg/l As signal was found to be 1.2%. Results obtained for standard reference materials and water samples are in good agreement with the certified values and those obtained by ICP-MS     

A novel method for the estimation of arsenic(V) in organic compounds

A novel method for the determination of arsenic(V) in organic compounds has been developed by reducing combined arsenic(V) to arsenic(III) in aqueous acetic acid medium with zinc dust. In some cases, addition of ethyl alcohol is necessary to dissolve the compound and to keep the arsenic(III) compound in solution. The arsenic(III) is titrated with iodine and the end-point is detected visually with starch as indicator or potentiometrically.     

Determination of As(III) and total inorganic arsenic by on-line pretreatment in hydride generation atomic absorption spectrometry

Summary A method for the on-line prereduction of As(V) was developed in order to determine As(III) and As(V) with the same sensitivity by continuous flow hydride generation. In this procedure, the sample is continuously mixed with concentrated hydrochloric acid and a potassium iodideascorbic acid solution, flows through a heated PTFE-tube and is determined by hydride generation atomic absorption spectrometry in a heated quartz cell. The selective analysis of As(III) is carried out by continuous mixing of the sample with acetic acid and hydride generation. The method allows the rapid determination of inorganic arsenic species at concentrations down to 1 g/l. A manual sample preparation is not required.     

Indirect atomic-absorption determination of ppM levels of arsenic by combustion of an MIBK extract of arsenomolybdic acid

An indirect atomic-absorption method for arsenic has been developed. Arsenic(III) is oxidized to arsenic(V) by iodine, then arsenomolybdic acid is formed and extracted into MIBK from 0.2-1.6M hydrochloric acid. Excess of molybdate is scrubbed from the organic phase, and then the molybdenum in the heteropoly acid is determined by its atomic absorption at 313.3 nm. Silicate and phosphate interfere. A procedure is described for determination of ppM levels of arsenic in water.     

Preconcentration and determination of ultra trace amounts of arsenic(III) and arsenic(V) in tap water and total arsenic in biological samples by cloud point extraction and electrothermal atomic absorption spectrometry

A new approach for developing a cloud point extraction-electrothermal atomic absorption spectrometry has been described and used for determination of arsenic. The method is based on phase separation phenomenon of non-ionic surfactants in aqueous solutions. After reaction of As(V) with molybdate towards a yellow heteropoly acid complex in sulfuric acid medium and increasing the temperature to 55 °C, analytes are quantitatively extracted to the non-ionic surfactant-rich phase (Triton X-114) after centrifugation.To decrease the viscosity of the extract and to allow its pipetting by the autosampler, 100 μl methanol was added to the surfactant-rich phase. An amount of 20 μl of this solution plus 10 μl of 0.1% m/v Pd(NO₃)₂ were injected into the graphite tube and the analyte determined by electrothermal atomic absorption spectrometry.Total inorganic arsenic(III, V) was extracted similarly after oxidation of As(III) to As(V) with KMnO₄. As(III) was calculated by difference. After optimization of the extraction condition and the instrumental parameters, a detection limit (3σ_B) of 0.01 μg l⁻¹ with enrichment factor of 52.5 was achieved for only 10 ml of sample. The analytical curve was linear in the concentration range of 0.02-0.35 μg l⁻¹. Relative standard deviations were lower than 5%. The method was successfully applied to the determination of As(III) and As(V) in tap water and total arsenic in biological samples (hair and nail).     

Determination of arsenic (III) and arsenic (V) in freshwater biological samples from Thailand by solvent extraction and neutron activation

Neutron activation analysis (NAA) methods were employed for the determination of total arsenic, and water soluble As(III) and As(V) compounds in freshwater fish/shellfish and plant samples from Southern Thailand. Total arsenic concentrations varied from 0.05 to 425 mg kg⁻¹. Water soluble arsenic species were separated by solvent extraction using ammonium pyrrolidinedithiocarbamate (APDC)/methylisobutylketone (MIBK) followed by NAA. The water soluble As(III) and As(V) levels varied from 0.07 to 26.4 and 0.03 to 22.9 mg kg⁻¹, respectively. The As(III) and As(V) detection limits were 0.007 for fish/shellfish, 0.005 for As(III) and 0.006 mg kg⁻¹ for As(V) in plants. This separation method allows for the determination of water soluble As(III) and As(V) using commonly available and inexpensive laboratory equipment and chemicals, which can be coupled to a variety of quantification techniques.     

Determination of arsenic and antimony in electrolytic copper by anodic stripping voltammetry at a gold film electrode

A simple procedure is described for the determination of arsenic and antimony in electrolytic copper. The copper is digested with nitric acid and copper is separated from arsenic and antimony by passing an ammoniacal solution of the sample through a column of Chelex-100 resin. After digestion with sulphuric acid and reduction to arsenic(III) and antimony(III) with sodium sulphite in 7 M sulphuric acid at 80°C, both arsenic and antimony are deposited at-0.30V and their total is determined by anodic stripping; antimony is then selectively deposited at -0.05 V for anodic stripping. The lower limits of determination are 56 ng As and 28 ng Sb per gram of copper; relative standard deviations (n = 5) are in the ranges 6.1–15.0% for 5.5—0.5 ppm arsenic in copper and 4.1–6.8% for 2.6—0.6 ppm antimony.     

Solvent extraction procedures for the differential determination of arsenic(V) and arsenic(III) species by electrothermal atomic absorption spectrometry

Arsenic(III) can be extracted quantitatively from acidic media with ammonium pyrrolidinedithiocarbamate (APDC) and with diethyldithiophosphoric acid (HDEDTP). Arsenic-(V) can only be extracted after preliminary reduction to the trivalent state. Potassium iodide or a mixture of hydrogensulphite/thiosulphate is recommended. When the extraction is done once with and once without addition of reducing agent, the arsenic(III) and the arsenic(V) contents can be differentiated. Some bottled mineral waters were analyzed. All the arsenic present appears to be in the pentavalent state.     

Determination of the As(III)/As(V) ratio in soil by X-ray absorption near-edge structure (XANES) and its application to the arsenic distribution between soil and water.

The details of a method used to determine the As(III)/As(V) ratio in soil by arsenic K-edge XANES spectroscopy are described. The spectra of mixtures of NaAs(III)O2 and NaH2As(V)O4, conducted for an As(III)/As(V) calibration, were well-fitted by combining normalized spectra of NaAsO2 and Na2HAsO4, where the coefficients multiplied by the normalized spectra were identical to the molar ratio of As(III) and As(V) in the mixtures. XANES spectra of arsenic in soil samples could also be fitted by a linear combination of the spectra of NaAsO2 and Na2HAsO4, which enabled us to estimate the As(III)/As(V) in a soil containing 10.2 mg/kg arsenic. The As(III)/As(V) ratio in the soil was compared with that of a soil solution contacted with the soil determined by HPLC-ICP-MS, showing that As(III) is distributed to water more readily than As(V). The application of the XANES method is important for a better understanding of the behavior of As(III) and As(V) independently in a natural aquifer.     

Selective determination of arsenic(III) and arsenic(V) with ammonium pyrrolidinedithiocarbamate, sodium diethyldithiocarbamate and dithizone by means of flameless atomic-absorption spectrophotometry with a carbon-tube atomizer

The extraction behaviour of arsenic(III) and arsenic(V) with ammonium pyrrolidinedithiocarbamate, sodium diethyldithiocarbamate and dithizone in organic solvents has been investigated by means of nameless atomic-absorption spectrophotometry with a carbon-tube atomizer. The selective extraction of arsenic(III) and differential determination of arsenic(III) and arsenic(V) have been developed. With ammonium pyrrolidinedithiocarbamate and methyl isobutyl ketone or nitrobenzene, when the aqueous phase/solvent volume ratio is 5 and the injection volume in the carbon tube is 20 μl, the sensitivities for 1% absorption are 0.4 and 0.5 part per milliard of arsenic, respectively. The relative standard deviations are ca. 3%. Interference by many metal ions can be prevented by masking with EDTA. The proposed methods are applied satisfactorily for determination of As(III) and As(V) in various types of water.     

Preconcentration and x-ray spectrometric determination of arsenic(III/V) and chromium(III/VI) in water

The speciation of chromium and arsenic in their two common oxidation states is determined by the use of selective preconcentration and energy-dispersive x-ray spectrometry. Chromium(VI) and arsenic(III) are recovered by precipitation with dibenzyldithiocarbamate and filtration. Chromium(III) and arsenic(V) are determined in the filtrate by coprecipitation with hydrated iron(III) oxide. The chromium and arsenic content of each precipitate is determined by use of x-ray spectrometry.