Trace determination of chromium in various water types by adsorption differential pulse voltammetry

Summary A new sensitive voltammetric method is presented for the determination of trace amounts of total chromium [Cr(III) and Cr(VI)] in natural waters. The method is based on the preconcentration of the Cr(III)-DTPA complex by adsorption at the HMDE at the potential of –1.0 V. The adsorbed complex is then reduced producing a response with a peak potential of –1.22 V and the peak height of the Cr(III) reduction is measured. The catalytic action of nitrate and bromate ions on the Cr(III)-DTPA reduction has been elucidated using cyclic voltammetry. The adsorption of chromium complexes at the HMDE was investigated using out-of-phase a. c. voltammetry and the potential range of adsorption was determined. Based on these investigations optimal conditions for the determination of the total chromium concentration in the range 20–2,000 ng/l have been established. The determination limit is 20 ng/l and the RSD is 5% for chromium concentrations 200 ng/l.The usefulness and wide scope of this new voltammetric method for reliable and highly sensitive chromium analysis down to the natural ultra trace levels existing in various types of natural waters is demonstrated by determinations of the total dissolved chromium content in river, lake, sea and rain water.

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Spurenbestimmung von Chrom in verschiedenen Wassertypen durch Adsorptions-Differentialpuls-Voltammetrie

Zusammenfassung Eine neue voltammetrische Methode zur Spurenbestimmung von Chrom [als Summe von Cr(III) und Cr(VI)] in natürlichen Gewässern wurde entwickelt. Die Methode beruht auf einer Anreicherung des Cr(III)-DTPA-Komplexes durch Adsorption an der hängenden Queck-silbertropfenelektrode beim Potential –1.0 V. Der adsorbierte Komplex wird anschließend im differentiellen Pulsmodus reduziert und die Peakhöhe beim Peakpotential –1.22 V gemessen. Die katalytische Wirkung von Nitrat- und Bromationen auf die Cr(III)-DTPA-Reduktion wurde mit der cyclischen Voltammetrie geklärt. Die Adsorption der Cr-Komplexe wurde zusätzlich mit der a.c.-Voltammetrie (kapazitive Komponente) untersucht und der Potentialbereich der Adsorption ermittelt. Aufgrund der Untersuchungen wurden die optimalen Bedingungen zur Chrombestimmung im Konzentrationsbereich 20–2000 ng/l festgelegt. Die Bestimmungsgrenze liegt bei 20 ng/l und die relative Standardabweichung beträgt 5% für Konzentrationen 200 ng/l. Die weite Anwendbarkeit der Methode für die zuverlässige und hochempfindliche Analyse von Chromspuren bis zu den natürlichen Ultraspurengehalten in verschiedenen Typen natürlicher Wässer wird an Beispielen der Analyse des gelösten Gesamtgehaltes von Chrom in Flußwasser, Seewasser, Meerwasser und Regenwasser aufgezeigt.

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Dedicated to Prof. Dr. H. Monien on the occasion of his 60th birthday

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Attached from Department of Chemistry of Warsaw University, Poland, within the scope of the joint research project on Eco-toxic Metals in the Environment     

Electrochemical studies and square-wave voltammetric determination of fenofibrate in pharmaceutical formulations

The electrochemical reduction of fenofibrate at a hanging mercury drop electrode (HMDE) was investigated by cyclic voltammetry, square-wave voltammetry, and chronoamperometry. Different buffer solutions were used over a wide pH range (3.0–10.0). The best definition of the analytical signals was found in borate buffer (pH 9.0)–tetrabutylammonium iodide mixture containing 12.5% (v/v) methanol at –1.2 V (versus Ag/AgCl). According to cyclic voltammetric studies, the reduction was irreversible and diffusion controlled. The diffusion coefficient was 2.38×10^–6 cm² s^–1 as determined by chronoamperometry. Under optimized conditions of square-wave voltammetry, a linear relationship was obtained between 0.146–4.96 g mL^–1 of fenofibrate with a limit of detection of 0.025 g mL^–1. Validation parameters such as sensitivity, accuracy, precision, and recovery were evaluated. The proposed method was applied to the determination of fenofibrate in pharmaceutical formulations. The results were compared with those obtained by a published high-performance liquid chromatography method. No difference was found statistically.     

Simultaneous determination of chromium(VI) and chromium(III) by flame atomic absorption spectrometry with a chelating ion-exchange flow injection system

Summary Simultaneous Determination of Chromium(VI) and Chromium(III) by Flame Atomic Absorption Spectrometry with a Chelating Ion-Exchange Flow Injection System A simple method is described for the simultaneous determination of chromium(VI) and chromium(III) in a flow injection system comprising chelating ion-exchange and flame atomic absorption spectrometry. Sampling rates for 2001 and 1 ml sample volumes were 120 and 60 h^–1 (240 and 120 speciations per hour), respectively. Typical relative standard deviations were 0.52% for Cr(VI) (0.50g ml^–1 and 0.67% for Cr(III) (0.10,g ml^–1) and the corresponding limits of detection were 85 ng ml^–1, and 16 ng ml^–1, respectively.On leave from University of Belgrade.     

Investigation for the determination of ultra trace amounts of Co2+ complexed with 1,10-phenanthroline by adsorptive voltammetry

Summary The spontaneous adsorption of the Co²⁺ complex with 1,10-phenanthroline on HMDE was utilized for the development of a voltammetric analytical method employing phase selective alternating current, with first harmonic AC₁ in phase, =0°, voltammetry. The dependence of the analytical voltammetric response on physical and chemical parameters in various supporting electrolytes was investigated in detail. Optimum conditions for Co²⁺ determination were established. The method is very sensitive and selective. The determination limit is 205 ng/l Co²⁺ (adsorption time, t_ads=20 min). The precision and accuracy were evaluated: 1.84 g/l Co²⁺, RSD=3.24%, RE=–0.54%; for 205 ng/l Co²⁺, RSD=4.85%, RE=–3.90%. The results showed good reproducibility and reliabiality. The method was applied to natural waters, wines, mussel tissue and aquatic algae certified reference material.     

Determination of Cd, Co, Cr, Cu, Fe, Mn, Ni and Pb in natural waters, alkali and alkaline earth salts by electrothermal atomic absorption spectrometry after preconcentration by column solid phase extraction

Methods are described for the determination of trace and ultra trace amounts of Cd, Co, Cr, Cu, Fe, Mn, Ni and Pb in natural waters, alkali and alkaline earth salts. Separation and preconcentration of trace metals is achieved by a column solid phase extraction procedure using silica gel modified with derivatives of dithiocarbamates — Na-DDTC (sodium diethyldithio-carbamate and HMDTC (ammonium hexamethylene-dithiocarbamate) as column packing material. The influence of the sorbent preparation procedure on the degree of sorption of the trace analytes is examined for different pH values of the sample solution. Isobutylmethyl ketone (IBMK) is proposed as an effective eluent for quantitative elution of retained metal ions. Optimal instrumental parameters for ETAAS determination of preconcentrated elements in organic eluate are presented. Practical application of sorbents in analysis of natural waters and alkali and alkaline earth salts is demonstrated. Proposed preconcentration procedure combined with ETAAS determination of trace analytes allows the determination of 0.04 g l^–1 Cd, 0.1 g l^–1 Cr, Cu, and Mn and 0.3 g l^–1 Co, Fe, Ni and Pb in natural waters and 1.10^–7% Cd, 3.10^–7% Cr and Mn, 7.10^–7% Co, Ni and Pb and 2.10^–6% Cu and Fe in alkali and alkaline earth salts.     

Speciation of Cr(VI)/Cr(III) by electrothermal atomisation AAS after electrodeposition on a L'vov platform

Summary The pyrolysed graphite L'vov platform of a tube furnace is considered as an electrode for the electrodeposition and speciation of chromium by electrothermal atomisation atomic absorption spectrometry (ETA-AAS). Firstly, a preliminary study of the Cr(VI)/Cr(III) voltammetric behavior at pH 4.70 on a glassy-carbon electrode is carried out. Secondly, the L'vov platform is used as a cathodic macro-electrode for the selective preconcentration of Cr(VI)/Cr(III) on a mercury film. Speciation of Cr(VI)/Cr(III) is carried out on the basis of the electrolysis potential (E_e): at pH 4.70 and E_e=–0.30 V, only Cr(VI) is reduced to Cr(III) and accumulated as Cr(OH)₃ by adsorption on a mercury film; at E_e=–1.80 V both Cr(VI) and Cr(III) are accumulated forming an amalgam with added mercury(II) ions. Once the film has been formed, the platform is transferred to a graphite tube to atomise the element. The reliability of the method was tested for the speciation of chromium in natural waters and it proves to be highly sensitive thanks to the electroanalytical step. In all samples, the Cr(VI) concentration was less than the detection limit (0.15 ng ml^–1), and the concentration of Cr(III) agrees with those of total chromium. The analytical recovery of Cr(VI) added to water samples [3.50 ng ml^–1 of Cr(VI)] was 105±6.2%.     

Adsorptive stripping voltammetric determination of chromium in gallium

The electroanalytical chemistry of trace metals has progressed strongly with the development of cathodic stripping voltammetry (CSV) preceded by adsorption collection of organic metal complexes. A sensitive method for the determination of trace amount of chromium in gallium is described. Gallium is dissolved in sodium hydroxide containing hydrogen peroxide. The method is based on the catalytic activity of nitrate ions on the reduction of Cr(III)TTHA (triethylene tetramine-N,N,N',N',N',N'-hexaacetic acid) complex. The sensitivity of this method is further improved by adsorption preconcentration of Cr(III)TTHA complex at a hanging mercury drop electrode (HMDE). The Cr(III) formed at the electrode surface by the reduction of Cr(VI), which is present in the bulk solution, is immediately complexed by TTHA. The adsorbed complex is then reduced at a peak potential of - 1.26 V, and the peak height of Cr(III) reduction is measured. The determination limit was restricted by the amount of chromium present in the reagent blank solution. The method is suitable for the determination of chromium at level as low as 0.2 mug g(-1) (with about 50 mg of sample) and a relative standard deviation of 15%.     

Cathodic adsorptive stripping voltammetry of 6-thiopurine in absence and presence of some metal ions

Summary Cathodic stripping voltammetry of an adsorbed 6-thiopurine at HMDE was investigated in solutions of varying pH. A rapid and sensitive differential pulse voltammetric method was selected for its trace determination. A method has also been developed for the determination of 6-thiopurine in presence of Cu(II), due to the strong adsorption of the Cu-6-thiopurine complex at the surface of the HMDE and subsequent reduction of the surface-bound complex. A detection limit of 9.9×10^–9 mol/l was achieved in presence of Cu(II) and the slope of the straight line was seven times the slope in absence of Cu(II). Cathodic adsorptive stripping (CAS) voltammetry of 6-thiopurine in presence of Ni(II), Pb(II), Cd(II), and Fe(III) was also investigated. The influence of several operational parameters has been considered. Statistical analysis of the calibration curve data is included.     

Trace Determination of Chromium by Square‐Wave Adsorptive Stripping Voltammetry on Bismuth Film Electrodes

This works reports the use of adsorptive stripping voltammetry (AdSV) for the trace determination of chromium on a rotating‐disk bismuth‐film electrode (BFE). During the reductive accumulation step, all the chromium species in the sample were reduced to Cr(III) which was complexed with cupferron and the complex was accumulated by adsorption on the surface of a preplated BFE. The stripping step was carried out by using a square‐wave (SW) potential‐time voltammetric signal. Electrochemical cleaning of the bismuth film was employed, enabling the same bismuth film to be used for a series of measurements in the presence of dissolved oxygen. The experimental variables as well as potential interferences were investigated and the figures of merit of the method were established. Using the selected conditions, the 3σ limit of detection for chromium was 100 ng L^?1 (for 120 s of preconcentration) and the relative standard deviation was 3.6% at the 2 μg L^?1 level (n=8). Finally, the method was applied to the determination of chromium in real samples with satisfactory results.     

Studies on the voltammetric behaviour of salicyl fluorone and gallium-salicyl fluorone complex

In the NH₄Cl supporting electrolyte, within the pH range from 1 to 5, an irreversible adsorptive reducing wave of salicyl fluorone(SAF) was obtained. The electrode process was verified as follow: On the surface of mercury electrode, the adsorption of SAF obeys Frumkin isotherm.In 0.2 mol/l potassium hydrogen phthalate/HCl buffer solution, at pH 3.0, the sensitive adsorptive complex wave of Ga-SAF was obtained by linear sweep voltammetry. The composition of the electroactive complex was determined as Ga:SAF = 11. The peak height of the complex is proportional to the concentration of Ga(III) in the range of 1.5 × 10^–9 to 6.0 × 10^–7 mol/l, the detection limit is 1.0× 10^–9 mol/l. The proposed method has been applied to the determination of gallium content in aluminium alloys.     

Ion-pair adsorption film colorimetry — A new simple method for ppb level of aluminum ion in water sample

A new, highly sensitive and simple colorimetric method for trace aluminum(III) with 2,2-dihydroxyazobenzene, H₂L, is described, based on the ion-pair adsorption of the anionic Al chelate, [A1L₂ ^–, with crystal violet cation, CV⁺, on the surface of Polyvinylchloride film plasticized with dioctylphthalate. The blue violet species, CV⁺[A1L₂]^–, is enriched onto the transparent film, leading to a remarkable enhancement of the sensitivity, and the detection limits are 3 ng/ml by spectrophotometry and 5 ng/ml by visual colorimetry, respectively. Using spectrophotometer, a linear calibration curve is obtained over the concentration range of 0 to 50 ng/ml of Al. Further, the color system, consisting of red ([A1L₂]^–), yellow (H₂L), and blue violet (CV⁺), gave clear color changes suitable for visual determination of aluminum with an applicable range of 0 to above 3000 ng/ml. The four different color zones are khaki for 0–5 ng/ml, reddish-brown for 5–200 ng/ml, blue violet for 200 ng/ml-3g/ml, and colorless for more than 3g/ml. The proposed method has been successfully applied for the determination of aluminum in tap waters.     

Adsorptive stripping voltammetric determination of cobalt in the presence of dimethylglyoxime and cetyltrimethylammonium bromide

A sensitive procedure for determination of micro-traces of Co(II) by adsorptive stripping voltammetry is proposed. The procedure exploits the enhancement of the cobalt peak obtained by use of the system Co(II)–dimethylglyoxime–piperazine-1,4-bis(2-ethanesulfonic acid)–cetyltrimethylammonium bromide. Using the optimized conditions, a detection limit (based on the 3 criterion) for Co(II) of 1.2×10^–11 mol L^–1 (0.7 ng L^–1) was achieved. The calibration plot for an accumulation time of 30 s was linear from 5×10^–11 to 4×10^–9 mol L^–1. The procedure was validated by analysis of certified reference materials and natural water samples.     

Polarographische Bestimmung von Chrom(III) nach Extraktion als Chrom(III)-oxinat mit geschmolzenem Oxin

Zusammenfassung Nach Extraktion von Chrom(III)-Lösungen mit geschmolzenem Oxin ist die polarographische Bestimmung nach Nitrierung des gebildeten Chrom(III)-oxinats möglich. Die experimentellen Bedingungen für die Nitrierung und die polarographische Bestimmung von 1–25 g Cr/25 ml werden mitgeteilt. Ein kurzer Vergleich mit der spektralphotometrischen Bestimmung als Chrom(III)-oxinat wird gegeben. Das polarographische Verfahren ist um den Faktor 5–10 empfindlicher. Nur Kobalt stört und muß durch eine vorausgehende Extraktion abgetrennt werden.

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Polarographic determination of chromium(III) after extraction as chromium(III) oxinate with an oxine melt

Summary After extraction of chromium(III) solutions with molten oxine the polarographic determination is possible by nitration of the Cr(III) oxinate. Experimental conditions for the nitration and for the polarographic determination of 1–25 g Cr/25 ml are described. A short comparison with the spectrophotometric determination as Cr(III) oxinate is also given. The polarographic method is more sensitive by a factor of 5–10. The only interference is caused by cobalt, which must be separated by a preceding extraction.

Die Untersuchungen wurden in dankenswerter Weise durch Mittel der Deutschen Forschungsgemeinschaft und des Verbands der Chemischen Industrie unterstützt.     

Chelate adsorption for trace voltammetric measurements of iron(III)

Summary A very sensitive electrochemical stripping procedure for trace measurements of iron(III) is described. The chelate of iron with Solochrome Violet RS is adsorbed on the hanging mercury drop electrode, and the reduction current of the accumulated chelate is measured by voltammetry. The adsorption and redox behaviours are explored by cyclic voltammetry. The height of the chelate peak, which is about 0.28 V more negative than the peak of the free dye, is shown to be proportional to the iron concentration. Optimal experimental conditions include a preconcentration potential of –0.40 V, solution pH of 5.1 and a linear scan mode. The sharp chelate peak, associated with the effective interfacial accumulation, coupled with the flat baseline, facilitates measurements at the nanomolar and submicromolar concentration levels using short preconcentration times. The limit of detection after 1 min preconcentration is 0.04 gl^–1 (7 × 10^–10 M), and the relative standard deviation at the 10^–7 M level is 4.7%. The effects of possible interferences, due to coexisting metal ions or organic surfactants, are evaluated. The ability of measuring iron(III) in the presence of iron(II) is illustrated. Actual analyses of sea and tap waters are reported.

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Chelat-Adsorption für voltammetrische Spurenanalyse von Eisen(III)

     

Spectrophotometric determination of microgram amounts of chromium(VI) and total chromium in waste water

Summary A spectrophotometric method has been developed for the determination of chromium in waste water with 5-Br-PADAP (2-(5-brorno-2-pyridylazo)-5-diethylaminophenol). Chromium(III) forms a blue-purple complex at pH 4.7 upon heating. The color is very stable and is highly sensitive for chromium. At 600 nm it obeys Beer's law in the concentration range of 0–15g of Cr(III) per 25 ml. The sensitivity and molar absorptivity of the reaction are 0.00065g of Cr(III) per cm² and 7.93×10⁴1 · mole^–1 · cm^–1, respectively. The continuous variation and molar ratio methods indicate that chromium forms 12 complex with the ligand.

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Spektrophotometrische Bestimmung von Mikrogramm-Mengen Chrom(VI) und Gesamt-Mengen Chrom in Abwasser

Zusammenfassung Eine spectrophotometrische Methode zur Bestimmung von Chrom in Abwasser mit 5-Br-PADAP wurde ausgearbeitet. Chrom(III) bildet mit 5-Br-PADAP bei pH 4,7 durch Erhitzen einen blaupurpurnen Komplex. Die Farbe ist sehr beständig. Bei 600 nm entspricht die Färbung dem Beerschen Gesetz zwischen 0 und 15g Cr(III)/25 ml. Die Empfindlichkeit der Reaktion beträgt 0,00065g Cr(III)/cm² und die molare Absorption 7.93×10⁴ 1 · mol^–1 · cm^–1. Die Untersuchung nach Job ergab für den Komplex das Molverhältnis Cr:Reagens=12.

     

Elimination of SAS Interferences in Catalytic Adsorptive Stripping Voltammetric Determination of Cr(VI) by Means of Fumed Silica

In the work the procedure of chromium(VI) determination by catalytic adsorptive stripping voltammetry (CAdSV) with application of fumed silica, is presented. Two variants of the method are proposed: in the first fumed silica is put directly to the electrolytic cell containing tested solution, in the second the silica is shaken with the sample and next centrifuged. The effectiveness of many surface‐active substances removal from synthetic solutions as well as natural water samples, is studied. In the experiments the fumed silica (Sigma‐Aldrich) of the specific surface area in the range 200–390 m² g^?1 was used. Two types of the working electrodes were applied, i.e., hanging mercury drop electrode (HMDE) and cyclic renewable mercury film electrode (Hg(Ag)FE). In the silica presence i) the relative standard deviation (RSD) for 0.1 μg L^?1 Cr(VI) is <2% (HMDE) and <5% (Hg(Ag)FE), n=7, ii) the detection limits estimated deposition time 20 s were respectively 14 ng L^?1 (HMDE) and 22 ng L^?1 (Hg(Ag)FE). The accuracy of the method was tested by studying the recovery of Cr(VI) from spiked natural water samples.     

Chromogenic reaction of bromopyrogallol red with tri- and hexavalent chromium in the presence of cetyltrimethyl ammonium bromide and its application in Cr speciation

The coordination reactions of bromopyrogallol red (BPR) with tri- and hexavalent chromium in the presence of cetyltrimethylammonium bromide (CTAB) have been studied by absorption spectrometry. Results show that the reactions of Cr(VI) and Cr(III) with BPR in the absence or presence of CTAB have different temperature dependences. The reaction mechanism of Cr(VI) is that Cr(VI) is first reduced by BPR to Cr(III) and then the Cr(III) produced reacts with BPR. Based on the study on the coordination reactions and the effects of surfactants upon them, a simple, rapid, sensitive and accurate method for Cr speciation has been developed. Over the range of 0–8 g Cr(VI) or 0–12g Cr(III) per 25ml final volume, the calibration curve is linear with a detection limit of 3.5 × 10^–7 mol/1 for Cr(VI) or 4.4 × 10^–7 mol/1 for Cr(III).     

Simultaneous and sequential determination of chromium(VI) and chromium(III) by unsegmented flow methods

Summary Several different configurations for simultaneous and sequential photometric speciation of Cr(VI) and Cr(III) based on the reversed flow injection analysis and completely continuous modes are proposed in this paper. The determination of these species at theg · ml^–1 level is achieved with sampling frequencies between 30 and 100 h^–1 and an r.s.d. of less than ±1% for simultaneous methods and of less than ±3% for the sequential method. The proposed methods are suitable for chromium speciation in waters. A simulation of the continuous monitoring of Cr(VI) and periodical of Cr(III) in natural and waste waters has been performed. The most frequent interferents in these types of samples have been investigated.

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Simultane und sequentielle Bestimmung von Chrom(VI) und Chrom(III) durch unsegmentierte Durchflußmethoden

Zusammenfassung Verschiedene Arten der simultanen und sequentiellen photometrischen Cr(III)- und Cr(VI)-Bestimmung nach der Technik der reversed-flow Injektionsanalyse werden beschrieben. Die relative Standardabweichung für die Bestimmung dieser Chromspezies im g/ml-Bereich bei einer Probenfrequenz von 30 bis 100 je Stunde beträgt weniger als +1% für die Simultan- und weniger als +3% für die sequentiellen Methoden. Eine simulierte kontinuerliche Überwachung von Chrom(VI) sowie eine periodische von Chrom(III) in natürlichem Wasser und Abwasser wird beschrieben. Die häufigsten Störungen werden diskutiert.

     

Kinetic Spectrophotometric Determination of Chromium (VI) by Oxidation of Sodium Pyrogallol-5-sulphonate by Hydrogen Peroxide

Summary. A new selective, sensitive, and simple kinetic method is developed for the determination of trace amounts of chromium (VI). The method is based on the catalytic effect of Cr(VI) on the reaction of sodium pyrogallol-5-sulphonate (PS) with hydrogen peroxide. The reaction is followed spectrophotometrically by tracing the oxidation product at 437nm within 1min after addition of H₂O₂. The optimum reaction conditions are PS (1.32·10^–3mol·dm^–3), H₂O₂ (0.32mol·dm^–3), HClO₄ (2.6·10^–3mol·dm^–3) at 25°C. Following this procedure, chromium (VI) can be determined with a linear calibration graph up to 0.25ng·cm^–3 and a detection limit of 0.024ng·cm^–3, based on the 3 criterion. The interference effect of several species was also investigated and it was found that the most common cations and anions do not interfere with the determination. The developed procedure was successfully applied to the determination of Cr(VI) and total Cr in river waters and total Cr in herbal samples.     

Kinetic Spectrophotometric Determination of Chromium (VI) by Oxidation of Sodium Pyrogallol-5-sulphonate by Hydrogen Peroxide

A new selective, sensitive, and simple kinetic method is developed for the determination of trace amounts of chromium (VI). The method is based on the catalytic effect of Cr(VI) on the reaction of sodium pyrogallol-5-sulphonate (PS) with hydrogen peroxide. The reaction is followed spectrophotometrically by tracing the oxidation product at 437nm within 1min after addition of H₂O₂. The optimum reaction conditions are PS (1.32·10^–3mol·dm^–3), H₂O₂ (0.32mol·dm^–3), HClO₄ (2.6·10^–3mol·dm^–3) at 25°C. Following this procedure, chromium (VI) can be determined with a linear calibration graph up to 0.25ng·cm^–3 and a detection limit of 0.024ng·cm^–3, based on the 3 criterion. The interference effect of several species was also investigated and it was found that the most common cations and anions do not interfere with the determination. The developed procedure was successfully applied to the determination of Cr(VI) and total Cr in river waters and total Cr in herbal samples.