František Opekar

Detection of hydrogen in air with a detector containing a nafion membrane metallized on both sides

Electrochemical cells with solid electrolytes permit the ,construction of miniature, rugged detectors that cannot be damaged by leaking of liquid electrolytes and corrosion. Some solid substances with ionic conductivity conduct sufficiently at laboratory temperature; zirconium phosphate, Zr(HPO,), . n H,O, and antimonic acid, Sb,O, 2 H,O, are proton conductors and belong to this group of substances. These substances in the form of a disk provided with platinum black electrodes on both sides have been used for the construction of a potentiometric [l] and an amperometric [2] detector of hydrogen and carbon monoxide in air. The present paper points out a possibility of detecting hydrogen in air using a detector with a Nafion@ membrane coated with platinum on both sides. Electrochemical cells of this type with metal electrodes bonded on solid polymer electrolyte membranes (Me/SPE electrodes) are used primarily in technological electrolysis [3] and organic electrosynthesis (see e.g. the references in ref. 4), but they have also been applied to the determination of components in liquid [4] and gaseous [5-71 samples. In all the cases cited, at least one side of the Me/SPE electrode is in contact with an electrolyte solution. The detector described here is in contact on both sides with a gaseous phase alone.

A wall-jet conductometric detector for determination of sulphur dioxide in air after preconcentration in water in an aerodispersion unit

Abstract Sulphur dioxide is preconcentrated from air into a polydispersed aerosol of water. The aerosol condenses on impact on the detector, on the surface of which a film of the condensate is maintained. The conductance of the liquid film is measured with a pair of platinum electrodes. The calibration dependence is nonlinear but corresponds to the theoretical model. The concentration range 0.05–2.2 mg SO2 m−3 was studied. The detection limit was 2 × 10−3 mg m−3; the relative standard deviation for 0.38 mg m−3 was 2.4%. A steady-state response to a change in SO2 concentration was attained within ca. 70 s. Ammonia is the only interferent.

Flow-through coulometric stripping analysis and the determination of manganese by cathodic stripping voltammetry

Abstract A simple flow-through system is described for collection of liquid samples into a closed loop. This arrangement can be used for coulometric determinations by the stripping method without previous calibration. The usefulness of the system is demonstrated on the determination of manganese (ca. 10 -6 M) in acetate buffer by cathodic stripping voltammetry.

Pheumatoamperometric determination of cyanide, sulfide and their mixtures

Abstract A rapid pneumatoamperometric method for quantifying cyanide in the presence or absence of sulfide is described. The gaseous mixture of hydrogen cyanide and hydrogen sulfide is separated by inserting a short chromatographic column packed with silica gel between the reaction vessel used to generate the volatile acids and the porous gold electrode that the detects them. The detection limit is ca. 5 ng cyanide in 2 ml of solution regardless of sulfide content. The detection limit for sulfide is ca. 1.0 ng in 2 ml of solution when cyanide is present and ca. 0.7 ng in absence of cyanide. Both sulfite and nitrite interfere.

Electrochemical stripping determination of traces of copper, lead, cadmium and zinc in zirconium metal and zirconium dioxide

Procedures have been developed for the determination of copper, lead, cadmium and zinc in zirconium metal and zirconium dioxide, at concentrations of 1 ppm or less. Zirconium metal was dissolved in suphuric acid, and zirconium dioxide decomposed under pressure with hydrofluoric acid. Sample solutions were prepared in dilute sulphuric acid. For the stripping determination, the sample solution was either mixed with a complexing tartrate base electrolyte or the pre-electrolysis was carried out in acid solution, with the acid solution being exchanged for a pure base electrolyte (e.g. an acetate buffer) for the stripping step. The stripping step was monitored by d.c., differential pulse and Kalousek commutator voltammetry and the three methods were compared. A stationary mercury-drop electrode can generally be used for all the methods, whereas a mercury-film electrode is suitable only for the d.c. voltammetric determination of copper, lead and cadmium, as pulse measurements with films are poorly reproducible and the electrodes are easily damaged. The relative standard deviation does not exceed 20%. Some samples contained relatively large amounts of copper, which is best separated by electrodeposition on a platinum electrode.

The electrochemical generation of small amounts of hydrogen cyanide.

Hydrogen cyanide was generated by constant-current oxidation of an aqueous solution of potassium thiocyanate at a platinum wire anode. In a solution of 0.1M potassium thiocyanate and 0.01M potassium sulphate at a nitrogen flow-rate of 3.5-5.0 ml sec , the rate of production of HCN was a linear function of the generation current I from 10 to 200 microA. The relative standard deviation for an HCN production rate of 6.07 ng sec (I = 130 microA) was 1.8% and that for 0.92 ng sec (I = 20 microA) was 5.9%. The time required to establish steady-state production after a change in the generation current was 10 min.

Pneumatopotentiometric determination of nanogram amounts of cyanide

Abstract Hydrogen cyanide is liberated from aqueous samples by reaction with sulphuric acid and transferred by a stream of nitrogen to a silver porous membrane electrode. Some HCN passes through the membrane into an alkaline dicyanoargentate solution; the cyanide ion produced causes a decrease in the equilibrium Ag + concentration and the change of potential is related to the amount of cyanide in the sample. The detection limit is 3.0 ng ml −1 cyanide in the injected solution; the relative standard deviation is 0.82% for 17 ng of cyanide. Sulphide interferes (as H 2 S) but can be removed on a lead acetate column.

Amperometric detection of nitrates using a flow-through membrane detector

Abstract A flow-through amperometric detector has been developed based on the electrochemical reduction of nitrates at a silver cathode. The nitric oxide formed is detected by a gold-plated porous membrane electrode placed downstream in the detector. Nitrates were mostly determined in samples containing 5 × 10 −2 M H 2 SO 4 . The sensitivity of the determination is expressed as the slope of the regression straight line was 0.85 nA μ M −1 in the range 5 × 10 −6 to 10 −3 M NO − 3 . The limit of quantitation determined as ten times the peak-to-peak background noise amplitude was 8 × 10 −7 M NO − 3 . The relative standard deviation for the determination of 5 × 10 ∂ and 10 −4 M NO − 3 was 3.6 and 0.8%, respectively. The most important interference in the determination of nitrate is that from nitrite, which exhibits identical behaviour in the determination. A method has been proposed which eliminates the effect of nitrite and permits its simultaneous determination in the presence of nitrate.

A simple laboratory generator for low concentrations of sulphur dioxide

Abstract The generator is based on permeation of sulphur dioxide through silicone rubber tubes immersed in a buffered, thermostated solution of sodium hydrogensulphite. The generator contains three (or more) permeation tubes of different lengths immersed simultaneously in the generation solution. The tubes can be connected stepwise to the carrier gas source, so that three (or more) different concentrations of SO 2 in the carrier gas can be obtained from a single solution. The tested generator produced 1.25–25.4 ng s −1 SO 2 , depending on the hydrogensulphite concentration in the solution. Long-term stability was tested for the production of 10.49 ng s −1 over 50 h; the relative standard deviation was 1.57%. The experimental conditions affecting the production of SO 2 are discussed. An equation is derived for estimating the SO 2 production for various solution compositions and surface areas of the permeation tubes.

Electrochemically-controlled generation of small amounts of carbon monoxide

The generation of carbon monoxide is based on oxidation of carbon at a temperature of 920-950 degrees by electrolytically generated oxygen. The rate of production of CO is a linear function of the oxygen generating current over a range of 5-60 mA and corresponds to the theoretical value. By the method described it is possible to obtain CO concentrations from 0.0075 to 2.5% in an inert gas (nitrogen, argon and helium).