Determination of methylmercury in fish using focused microwave digestion following by Cu2+ addition, sodium tetrapropylborate derivatization, n-heptane extraction, and gas chromatography-mass spectrometry

The analytical procedure for analysis of methylmercury in fish was developed. It involves microwave-assisted digestion with alkaline solution (tetramethylammonium hydroxide), addition of Cu2+, aqueous-phase derivatization of methylmercury with sodium tetrapropylborate, and subsequent extraction with n-heptane. The methylmercury derivative was desorbed in the splitless injection port of a gas chromatograph and subsequently analyzed by electron impact mass spectrometry. Optimum conditions allowed sample throughout to be controlled by the instrumental analysis time (near 7 min per sample) but not by the sample preparation step. At the power of 15-30, 45, and 60-75 W, sample preparation time is only 3.5, 2.5, and 1.5 min, respectively. The proposed method was finally validated by the analysis of three biological certified reference materials, BCR CRM 464 tuna fish, NRC DORM-2 dogfish muscle, and NRC DOLT-2 dogfish liver. The detection limit of the overall procedure was found to be 40 ng/g of biological tissue for methylmercury. The recovery of methylmercury was 91.2-95.3% for tuna, 89.3-94.7% for marlin, and 91.7-94.8% for shark, respectively. The detected and certified values of methylmercury of three biological certified reference materials were as follows: 5.34 +/- 0.30 microg/g (mean +/- S.D.) and 5.50 +/- 0.17 microg/g for CRM 464 tuna fish, 4.34 +/- 0.24 and 4.47 +/- 0.32 microg/g for NRC DORM-2 dogfish muscle, and 0.652 +/- 0.053 and 0.693 +/- 0.055 microg/g for NRC DOLT-2 dogfish liver, respectively. It indicated that the method was well available to quantify the methylmercury in fish.     

Formic acid solubilization of marine biological tissues for multi-element determination by ETAAS and ICP–AES

A simple, fast method is described for the determination of Ag, As, Cd, Cu, Cr, Fe, Ni, and Se in marine biological tissues by electrothermal atomic-absorption spectrometry (ETAAS) and Na, Ca, K, Mg, Fe, Cu, and Mn by inductively coupled plasma–atomic emission spectrometry (ICP–AES). Solubilization of the biological tissue was achieved by using formic acid with vortex mixing followed by heating to 50°C in an ultrasonic bath. Once solubilized, the tissues were diluted to an appropriate volume with water for analysis. Aliquots were sampled into a graphite furnace and ICP–AES using a conventional autosampler. The method was validated by use of biological certified reference materials from NRC, DORM-2, DOLT-2, DOLT-3, LUTS-1, TORT-2, and NIST SRMs 1566b and 2976. Simplicity and reduced sample-preparation time prove to be the major advantages to the technique.     

Determination of total mercury in biological samples using flow injection CVAAS following tissue solubilization in formic acid

Total mercury in biological samples was determined by flow injection (FI) cold vapour atomic absorption spectrometry (CVAAS) following tissue solubilization with formic acid. A mixture of potassium bromide and potassium bromate was used to decompose organomercury compounds prior to their reduction with sodium borohydride. A gold amalgam system was used to achieve lower detection limits when required. National Research Council Canada certified reference materials dogfish liver (DOLT-3), dogfish flesh (DORM-2) and lobster hepatopancreas (TORT-2), as well as oyster tissue (NIST SRM 1566b) and mussel tissue (NIST SRM 2976) were used to assess the accuracy of the method. The method of standard additions provided the most accurate results. Limit of detection (LOD) for Hg in the solid sample of 0.001 and 0.01 μg g⁻¹ were achieved with and without amalgamation, respectively. The precision of measurement for 1.6 ng ml⁻¹ methylmercury was 2.7% using the amalgam system.     

Determination of total mercury and methylmercury in biological samples by photochemical vapor generation

Cold vapor atomic absorption spectrometry (CV-AAS) based on photochemical reduction by exposure to UV radiation is described for the determination of methylmercury and total mercury in biological samples. Two approaches were investigated: (a) tissues were digested in either formic acid or tetramethylammonium hydroxide (TMAH), and total mercury was determined following reduction of both species by exposure of the solution to UV irradiation; (b) tissues were solubilized in TMAH, diluted to a final concentration of 0.125% m/v TMAH by addition of 10% v/v acetic acid and CH₃Hg⁺ was selectively quantitated, or the initial digests were diluted to 0.125% m/v TMAH by addition of deionized water, adjusted to pH 0.3 by addition of HCl and CH₃Hg⁺ was selectively quantitated. For each case, the optimum conditions for photochemical vapor generation (photo-CVG) were investigated. The photochemical reduction efficiency was estimated to be ∼95% by comparing the response with traditional SnCl₂ chemical reduction. The method was validated by analysis of several biological Certified Reference Materials, DORM-1, DORM-2, DOLT-2 and DOLT-3, using calibration against aqueous solutions of Hg²⁺; results showed good agreement with the certified values for total and methylmercury in all cases. Limits of detection of 6 ng/g for total mercury using formic acid, 8 ng/g for total mercury and 10 ng/g for methylmercury using TMAH were obtained. The proposed methodology is sensitive, simple and inexpensive, and promotes “green” chemistry. The potential for application to other sample types and analytes is evident.     

INAA study of Hg, Se, As, and Br irradiation losses from l-cysteine treated and untreated reference materials

U. S. Food and Drug Administration in-house reference material (RM) Cocoa Powder and National Institute of Standards and Technology Standard RMs (SRMs) 1515 apple leaves, 1547 peach leaves, 1571 orchard leaves, 1566a oyster tissue, and 1568a rice flour were co-irradiated together with polyethylene blanks and analyzed for Hg and Se by anticoincidence instrumental neutron activation analysis. The three botanical SRM portions showed a combined Hg recovery of 70 % while the other portions showed a combined Hg recovery of 169 %, indicating that volatile Hg was lost from botanical SRMs and absorbed by the other irradiated portions. Total Hg recovery for all portions was 82 %. Se results showed no evidence of cross-contamination and all results agreed with certified and known values. National Research Council of Canada Certified RMs DOLT-3 dogfish liver, TORT-2 lobster hepatopancreas, and DORM-3 fish protein were separately analyzed either with no treatment or after treatment with l-cysteine solutions followed by drying over magnesium perchlorate. Each set of portions was co-irradiated with polyethylene and treated filter blanks. Analysis of all components of each treated portion irradiation package showed that essentially all Hg was retained within the package. Treated DOLT-3 portions (inorganic Hg content 53 %) showed a tenfold improvement with 99 % Hg retention. Hg retention for DORM-3 (7 % inorganic Hg) was 85 % (a twofold improvement) while retention for TORT-2 (44 % inorganic Hg), was 94 %, similar to that for untreated portions (96 %). Small irradiation losses (≤0.5 %) of volatile species of Se, As, and Br were observed.     

Determination of selenium in marine biological tissues by transverse heated electrothermal atomic absorption spectrometry with longitudinal Zeeman background correction and automated ultrasonic slurry sampling.

A fast, sensitive, and reliable method for determination of selenium in marine biological tissues by electrothermal atomic absorption spectrometry with slurry sampling was developed. Slurries were prepared from fresh and frozen seafood samples that were previously homogenized, dried, and ground; particle sizes <100 microm were taken for analysis. A 3% (v/v) HNO3 solution containing 0.01% (v/v) Triton X-100 was used as slurry diluent. Slurries were mixed on an automated ultrasonic slurry sampler at 20% amplitude for 30 s just before an aliquot was injected into the furnace. The method was successfully validated against the following certified reference materials: NRCC CRM DORM-2 (Dogfish muscle); NRCC CRM TORT-2 (Lobster hepatopancreas); NRCC CRM DOLT-2 (Dogfish liver); and BCR CRM 278 (Mussel tissue), and was subsequently applied to determination of Se in 10 marine biological samples. The influences of the drying procedure (oven-, microwave-, and freeze-drying), matrix modifier amount, mass of solid material in cup, and pipetting sequence are discussed. The limit of determination of Se was 0.16 microg/g and the repeatability, estimated as between-batch precision, was in the range of 4-8%. Se contents in the samples ranged from 0.6 to 2.8 microg/g. The proposed method should be useful for fast assessment of the daily dietary intake of Se.     

Inductively coupled plasma atomic emission spectrometric determination of arsenic in mussel products

Summary A precise, accurate procedure is proposed for the determination of arsenic in mussel products by inductively coupled plasma atomic emission spectrometry. Organic matter is destroyed by dry ashing. Experimental conditions for the determination of arsenic were selected and an interference study was carried out. The methodology developed has a detection limit of 0.1 g g^–1, a relative standard deviation of 3%, and a recovery percentage of 98±2%. The accuracy of the method was checked by analysis of a certified sample of NIST oyster tissue (certified 14.0±1.2 g g^–1; found 14.1±1.1 g g^–1). The proposed procedure was used to analyze real samples of mussel products.     

Species specific isotope dilution for the accurate and SI traceable determination of arsenobetaine and methylmercury in cuttlefish and prawn

Methods based on species specific isotope dilution were developed for the accurate and SI traceable determination of arsenobetaine (AsBet) and methylmercury (MeHg) in prawn and cuttlefish tissues by LC-MS/MS and SPME GC-ICPMS. Quantitation of AsBet and MeHg were achieved by using a ¹³C-enriched AsBet spike (NRC CRM CBET-1) and an enriched spike of Me¹⁹⁸Hg (NRC CRM EMMS-1), respectively, wherein analyte mass fractions in enriched spikes were determined by reverse isotope dilution using natural abundance AsBet and MeHg primary standards. Purity of these primary standards were characterized by quantitative ¹H-NMR with the use of NIST SRM 350b benzoic acid as a primary calibrator, ensuring the final measurement results traceable to SI. Validation of employed methods of ID LC-MS/MS and ID SPME GC-ICPMS was demonstrated by analysis of several biological CRMs (DORM-4, TORT-3, DOLT-5, BCR-627 and BCR-463) with satisfying results.     

Investigation of equilibration and uncertainty contributions for the determination of inorganic mercury and methylmercury by isotope dilution inductively coupled plasma mass spectrometry

The mass fractions of Hg and methylmercury, in two certified reference materials, NIST2710 and DORM-2, have been determined by total and species-specific isotope dilution analysis (IDA), respectively, and uncertainty budgets for each analysis calculated. The mass fraction of Hg in NIST2710 was determined by ID using multicollector sector field inductively coupled plasma mass spectrometry (MC-SF-ICP-MS) whilst the mass fraction of methylmercury in DORM-2 was determined using HPLC coupled with quadrupole ICP-MS.

The extent of equilibration between the spike and the particulate bound mercury compounds was studied temporally by monitoring the ²⁰⁰Hg:¹⁹⁹Hg isotope amount ratio and by determining the total amount of Hg in the liquid phase. For the NIST2710 complete equilibration was only achieved when concentrated HNO₃ in combination with a microwave digestion was employed, and good agreement between the found (31.7±4.0 μg g⁻¹, expanded uncertainty k=2) and certified (32.6±1.8 μg g⁻¹) values was obtained. For DORM-2 complete equilibration of methylmercury between the liquid and solid phases was achieved when using 50:50 H₂O:CH₃OH (v/v) and 0.01% 2-mercaptoethanol as the solvent. Even though only 50% of the analyte was extracted into the liquid phase, complete equilibration was achieved, hence, the found methylmercury mass fraction (4.25±0.47 μg g⁻¹, expanded uncertainty k=2) was in good agreement with the certified value (4.47±0.32 μg g⁻¹).     

Low volume microwave digestion for the determination of selenium in marine biological tissues by graphite furnace atomic absorption spectroscopy

A microwave digestion method for the determination of marine biological tissues has been developed to allow determination of selenium in small sample sizes (< 0.1 g). The benefits of this technique include maintaining concentrates in extracts without the subsequent over dilution encountered when using larger vessels, increased sample throughput and reduced loss of volatile material. Freeze dried biological material (< 0.1 g) and nitric acid (1 ml) were placed into 7 ml screw top Teflon vessels which are completely sealed on capping. Two 7 ml vials were placed into larger 120 ml vessels fitted with a Teflon spacer and 10 ml of distilled water. The effects of microwave power and time, and sample mass on selenium recovery from three marine standard reference materials (NIST SRM 1566a Oyster Tissue, NRCC DORM-1 Dogfish Muscle and NRCC TORT-1 Lobster Hepatopancreas) were examined. The optimum conditions: 600 W, 2 min; 0 W, 2 min; 450 W, 45 min, allowed quantitative recoveries of selenium from these and three other standard reference materials (NRCC DOLT-1 Dogfish liver, NIST RM-50 Albacore tuna and IAEA MA-A-2 fish flesh). Studies on sample mass showed that the analysis of sample masses from 0.025 to 0.1 g gave selenium concentrations within the certified range. Six species of selenium: selenite, selenate, selenomethionine, selenocysteine, selenocystamine, and trimethyl selenonium were added to oyster, dogfish, and lobster tissues. Recoveries were near quantitative for all species (94–105%) except trimethyl selenonium (90–101%).     

Study of ammonium molybdate to minimize the phosphate interference in the selenium determination by electrothermal atomic absorption spectrometry with deuterium background correction

The use of ammonium molybdate to minimize the phosphate interference when measuring selenium by electrothermal atomic absorption spectrometry (ETAAS) with deuterium background correction was evaluated. Ammonium molybdate did not produce a selenium thermal stabilization; however, the presence of ammonium molybdate decreased the phosphate interference. The study was carried out with mussel acid digests and mussel slurries. Pd–Mg(NO₃)₂ was used as a chemical modifier at optimum concentrations of 300 and 250 mg l⁻¹, respectively, yielding optimum pyrolysis and atomization temperatures of 1200 and 2100 °C, respectively. A yellow solid (ammonium molybdophosphate) was obtained when adding ammonium molybdate to mussel acid digest solutions. This precipitate can be removed after centrifugation prior to ETAAS determination. Additionally, studies on the sampling of the solid ammonium molybdophosphate (AMP) together with the liquid phase, as a slurry, were also developed. The volatilization of the solid AMP was not reached at temperatures lower than 2500 °C. By this way, phosphate, as AMP, is not present in the vapor phase at the atomization temperature (2100 °C), yielding a reduction of the spectral interference by phosphate. The proposed method was validated analyzing three reference materials of marine origin (DORM-1, DOLT-1 and TORT-1). Good agreement with the certified selenium contents was reached for all cases.     

Methyl mercury determination in environmental and biological reference and other materials by quality control with certified reference materials (CRMs)

Summary Two concentration methods — HCl extraction and extraction followed by water vapour distillation — prior to anion exchange separation and cold-vapour atomic absorption spectrometry were used for the quantification of methyl mercury in a number of reference materials of biological and environmental origin with reference or certified total mercury contents. The applied methods were validated by the analysis of three marine reference materials, certified for methyl mercury. The results obtained in the materials with methyl mercury contents ranging from less than 1 g/kg to a few hundred g/kg showed good agreement between both methods for biological materials, whereas extraction/anion exchange resulted in somewhat too high values for a number of environmental materials. These findings, however, require further confirmation by additional methods and by other laboratories.Dedicated to Professor Dr. Wilhelm Fresenius on the occasion of his 80th birthday     

Determination of total arsenic in serum and packed cellsof patients with renal insufficiency

In order to investigate the arsenic level in serum and packed cells of patients with renal insufficiency, total arsenic (As) concentrations were determined with hydride generation atomic absorption spectrometry (HGAAS) in serum (S) and packed cells (PC) of 31 non-dialyzed patients. The accuracy of the method was tested by the analysis of arsenic in 3 certified reference materials. Patients showed a three-fold increase of arsenic concentrations in serum and a two-fold increase of arsenic in packed cells compared with controls. Patients (n=10) with higher serum creatinine (>2.0 mg/dL), urea (>0.70 g/L) and urinary protein (mean ±SD: 1.12±0.82 g/L) showed higher arsenic concentrations (5.8±3.3 g/L in serum and 18.0±16.7 g/kg in packed cells) compared with those with lower creatinine (<1.6 mg/dL), urea (<0.6 g/L) and urinary protein (mean ±SD: 0.27±0.82 g/L) (n=16, serum arsenic 1.2±1.2 g/L, packed cells arsenic 2.6±1.9 g/kg). The significant differences (both p<0.001) in S and PC-arsenic levels of patients in group I and II implies a relationship between the arsenic level and the degree of chronic renal insufficiency.Dedicated to Professor Dr. Peter Brätter on the occasion of his 60th birthday     

Differential pulse polarographic determination of chromium in gallium arsenide

Summary The proposed method for the determination of Cr in gallium arsenide is based on the catalytic activity of chromium(VI)-DTPA complex in the presence of nitrate ions. After dissolution of gallium arsenide with hydrochloric and nitric acids and volatilization of arsenic, chromium is oxidised to chromate with potassium chlorate in nitric acid. After nitric acid removal, the 0.012 M DTPA — 2.5 M LiNO₃ supporting electrolyte buffered with 0.2 M CH₃COONa — 0.05 M CH₃COOH is directly added to the residue. Differential pulse polarography of this solution provides a detection limit of about 100 ng g^–1, with a relative standard deviation of ±2–5% and a reproducible calibration curve up to at least 1 g of Cr(VI) ml^–1.

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Differential-pulspolarographische Bestimmung von Chrom in Galliumarsenid

Zusammenfassung Das vorgeschlagene Verfahren beruht auf der katalytischen Aktivität des Chrom(VI)-DTPA-Komplexes in Gegenwart von Nitrationen. Nach Auflösung des Galliumarsenids mit Salz- und Salpetersäure, wird Arsen verflüchtigt und Chrom mit KClO₃/HNO₃ zu Chromat oxidiert. Die Salpetersäure wird entfernt und der Rückstand direkt mit dem Trägerelektrolyt versetzt (0,012 M DTPA — 2,5 M LiNO₃, gepuffert mit 0,2 M CH₃COONa — 0,05 M CH₃COOH). Durch Differentialpulspolarographie dieser Lösung erhält man eine Nachweisgrenze von etwa 100 ng g ^–1. Die relative Standardabweichung beträgt ± 2–5%, die Eichkurve ist reproduzierbar bis 1 g Cr(VI) ml^–1.

     

Bestimmung von Chrom in menschlichem Serum und Plasma mit der flammenlosen Atomabsorptions-Spektrophotometrie

Zusammenfassung Die bis heute in der Literatur mitgeteilten Werte für Chrom im Serum nüchterner gesunder Probanden unterscheiden sich um Größenordnungen. Die in der vorliegenden Arbeit mitgeteilten Untersuchungen betreffen die analytische Problematik der Chrombestimmung mit Hilfe der flammenlosen Atomabsorptions-Spektrometrie, wobei aufgezeigt wird, daß diese Methodik für die Chrombestimmung in biologischem Material ohne großen statistischen Aufwand zu keiner sicheren Beurteilbarkeit und Interpretierbarkeit der gemessenen Werte führt. Mit Hilfe synthetischer Chromkomplexverbindungen werden die Probleme der Standardaddition zur Chrombestimmung untersucht. Unter Berücksichtigung der Blindwerte und der daraus resultierenden Nachweisgrenze sowie Garantiegrenze für Reinheit, wurde Chrom in einem Standardreferenzmaterial (1569 Brewers yeast [U. S. National Bureau of Standards]) im Rahmen eines Ringversuchs zur Chromanalyse sowie im Serum und Plasma von 41 Probanden bestimmt. Für das Referenzmaterial wurde ein Wert von 45±4 mol/kg (2,3±0,2 g/g) ermittelt. Der Referenzwert betrug 41±1 mol/kg (2,12±0,05 g/g). Im Serum wird eine lognormale Verteilung der Chromkonzentration mit den zentralen Parametern ±_M=132,2 nmol/l (0,72,2 g/l) ermittelt. Im Plasma lagen die Werte zwischen 20 und 30 nmol/l (1–1,5 g/l). Aufschluß, Durchführung der Bestimmung sowie die biologische Bedeutung und Interpretation des Parameters Chrom im Serum und Plasma werden eingehend diskutiert.

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Determination of chromium in human serum and plasma by flame-less atomic-absorption spectrophotometry

Summary Concentrations of Cr in serum of overnight fasting volunteers differ over a wide range in the literature. The investigations of the analytical problems of Cr determination by flameless atomic absorption spectrometry underline the necessity of statistical methods for the interpretation of the measurements of Cr in biological matrices. With nine different synthetic chromium complexes the method of standard addition was investigated. Considering the blank values, the limit of detection and the limit of guarantee for purity, Cr was determined in the standard reference material 1569 brewers yeast (National Bureau of Standards) with 45±4 Mol · kg^–1 (2.3±0.2g·g^–1), compared with the reference data 41±1 Mol·kg^–1 (2.12±0.05 g±g^–1). In serumuuuuu we found a lognormal distribution with the central parameters 132.2 nMol ·1^–1 (0.72.2 g·1^–1). In plasma the values were in the range between 20 and 30 nMol·1^–1 (1–1.5 g·1^–1). The ashing process and the determination are demonstrated and discussed in detail.

     

Development and application of a simple routine method for the determination of selenium in serum by octopole reaction system ICPMS

The aim of the study was to develop an inductively coupled plasma mass spectrometry (ICPMS) method for robust and simple routine determination of selenium in serum. Polyatomic interferences on ⁷⁶Se, ⁷⁷Se, and ⁷⁸Se were removed by applying an octopole reaction system ICPMS with the reaction cell pressurized with H₂ gas. We developed a novel simple optimization routine for the H₂ gas flow based on a signal-to-noise ratio (SNR) calculation of the selenium signal measured in a single selenium standard. The optimum H₂ flow was 2.9 mL min^–1. The selenium content in serum was determined after a 50-fold dilution with 0.16 M HNO₃ and quantified by using addition calibration and gallium as an internal standard. The method detection limit was 0.10 g L^–1 for ⁷⁶Se and ⁷⁸Se and 0.13 g L^–1 for ⁷⁷Se. Human serum samples from a case-control study investigating if selenium was associated with risk of colorectal adenoma were analyzed. The average selenium concentration for the control group (n=768) was 137.1 g L^–1 and the range was 73.4–305.5 g L^–1. The within-batch repeatability (a batch is ten samples) estimated from 182 replicate analyses was 6.3% coefficient of variation (CV), whereas the between-batch repeatability was 7.4% CV estimated from 361 replicates between batches. The method accuracy was evaluated by analysis of a human serum certified reference material (Seronorm Serum level II, Sero A/S, Norway). There was a fairly good agreement between the measured average of 145±3 g L^–1 (n=36) and the certified value of 136±9 g L^–1. In addition the method was successfully applied for analysis of zinc serum concentrations without further optimization. For the Seronorm certified reference material a value of 911±75 g L^–1 (n=31) for zinc was obtained, which corresponds well to the certified zinc value of 920±60 g L^–1.     

Determination of cadmium in biological samples by inductively coupled plasma atomic emission spectrometry after extraction with 1,5-bis[phenyl-(2-pyridyl)methylene]-thiocarbonohydrazide

A systematic study was made of the optimum conditions for cadmium determination in biological samples by ICP-AES after extraction of the metal into methyl isobutyl ketone containing 1,5-bis[phenyl-(2-pyridyl)methylene]-thiocarbonohydrazide. The maximum volume ratio of aqueous to organic phase was 30 1 for a single-stage extraction of 99–100% of the metal ion. The detection limit was 0.3 ng/ml cadmium, and the calibration was linear from 0.4 to at least 150 ng/ml. No interferences from the elements commonly found in biological materials were observed. A precision of 2.5% (P = 0.05) at the 2 ng/ml level of the metal was achieved. The accuracy of the method was demonstrated by analysis of three standard reference materials, giving concentrations of 1.90 ± 0.43, 27.1 ± 1.4 and 2.63 ± 0.38 /g of cadmium, compared with the certified values of 2.20 ± 0.10, 26.3 ± 2.1, and 2.71 ± 0.15 g/g, respectively.     

Über die katalytische Wirkung einiger Elemente in einer Landolt-Reaktion auf “Brom-Grundlage”; Ultramikrobestimmung von Molybdän(VI)

Zusammenfassung Die Landolt-Reaktion auf Bromgrundlage des Systems Wasserstoffperoxid—Bromid—Ascorbinsäure—Tolidin wird durch Mo(VI), Fe(III), Cu(II), V(V) und W(VI) katalysiert. Auf dieser Grundlage läßt sich Mo(VI) mit Hilfe der Simultankomparationsmethode bei 65° C in einem Konzentrationsbereich von 1 bis 10g/5 ml und bei 50° C in einem Konzentrationsbereich von 0,1 bis 1g/5 ml mit einem relativen Fehler von ±0.5% bestimmen.

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Summary The Landolt reaction on a bromine basis of the system hydrogen peroxide-bromide-ascorbic acid-tolidine is catalyzed by Mo(VI), Fe(III), Cu(II), V(V), and W(VI). On this basis, Mo(VI) can be determined with the aid of the simultaneous comparation method at 65° C in a concentration range from 1 to 10g/5 ml and at 50° C in a concentration range from 0.1 to 1g/5 ml with a relative error of ±0.5%.

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Résumé La «réaction de Landolt, avec brome» du système eau oxygénée—bromure—acide ascorbique—tolidine, est catalysée par Mo-VI, Fe-III, Cu-II, V-V et W-VT. On peut donc doser Mo-VI par la méthode de comparaison simultanée à 65° C dans un domaine de concentration de 1 à 10g/5 ml et à 50° C de 0,1 à 1g/5 ml avec une erreur relative de ±0,5%.

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Die vorliegende Arbeit sollte Herrn Prof. Dr.E. Schulek zu seinem 70. Geburtstag gewidmet werden. Noch ehe die Publikation möglich war, ist Herr Prof.Schulek am 14. Oktober 1964 leider gestorben.