Gunnar Edwall

Metal-Metal Oxide and Metal Oxide Electrodes as pH Sensors

The pH is one of the most important parameters for char-acterizing the chemical properties of an aqueous solution. The glass eiectrode is by far the most commonly used pH sensor. The determination of pH in special situations, e.g., in vivo applications where the fragility of the glass electrode is a draw-back, requires pH sensors that can easily be miniaturized and built into physically rugged sleeves. Also, for other applications where the volume of solution is very restricted miniaturization of pH sensors is very important. The glass electrode does not respond properly to pH in some corrosive environments (e.g., in hydrogen fluoride [HF] solutions). At present, among alternatives to the hydrogen-selective glass electrode group of sensors, potentiometric metal-metal oxide pH sensors respond to pH, ideally due to an equilibrium involving the metal and its oxide. In the case of metal oxide electrodes, the metal is not involved in the potential-determining reaction. This distinction is of course not clear in many cases because the mechanism giving the pH response is not always unequivocal.

Polycrystalline and monocrystalline antimony, iridium and palladium as electrode material for pH-sensing electrodes

Different ways of making pH-sensing electrodes from monocrystalline or polycrystalline antimony, iridium and palladium have been investigated. Monocrystalline antimony and iridium are superior to the polycrystalline elements with respect to reproducibility between electrodes and stability of the electrode potential over long periods of time. No good palladium/palladium oxide electrode could be obtained by electrochemical oxidation and the thermal preparation method could not take advantage of the properties of the monocrystalline palladium. Therefore, only polycrystalline palladium was used to study this type of electrodes. The different electrodes were compared with respect to the manner of preparation, the pH-response (reproducibility and time response) and the effect that different complexing ligands present in the measuring solutions may have on the electrode response. Also, the redox-response of the electrodes and the effect of different oxygen pressures on the electrode potentials were studied. The monocrystalline antimony electrodes have the best reproducibility and long-term stability but also respond to complexing ligands and to variations in the oxygen pressure. Monocrystalline iridium electrodes can be obtained by continuously cycling the potential between -0.25 and +1.25 V (SCE) in 0.5M sulphuric acid. They do not respond to the complexing ligands tested, and have fairly good long-term stability, but the reproducibility between electrodes is inferior to that of the monocrystalline antimony electrodes. Polycrystalline antimony and iridium electrodes were inferior to the monocrystalline ones. The properties of the palladium electrodes were similar to those of the iridium ones.

An examination of the palladium/palladium oxide system and its utility for pH-sensing electrodes

Abstract Monocrystalline and polycrystalline palladium has been used for the electrochemical examination of the palladium—palladium oxide system in aqueous solutions. Two different methods were used to prepare the electrodes, namely a thermal method of oxidation at high temperature and an electrochemical method. Electrodes prepared by the electrochemical method did not have a good pH response. The properties of the thermally prepared electrodes depend on the temperature at which the oxidation was made. The optimum temperature is 750°C. The temperature range studied was 450–870°C. Electrodes prepared by the thermal method show an almost theoretical slope in the pH range 2.5–8.

Effects of some complex-forming ligands on the potential of antimony pH-sensors.

The E vs. pH relationship for oriented monocrystalline antimony electrodes was studied in various ionic media, containing no buffer or one of several common buffer substances, and the stability of the electrode potential with time was investigated. Several makes of commercially available polycrystalline antimony electrodes were also studied. The results indicate that tris(hydroxymethyl)aminomethane may be used for calibration of antimony electrodes, but most other commonly used buffer substances, e.g., orthophosphate, yield erroneous results. Monocrystalline antimony electrodes are preferable to polycrystalline ones, especially when measurements are made in unstirred solutions. The long-term stability of monocrystalline electrodes is superior to that of polycrystalline ones.

Evaluation of the influence of impurities on the oxygen sensitivity of monocrystalline antimony electrodes

Abstract Measurements are presented which show that the oxygen sensitivity of ultra pure (6N) monocrystalline antimony electrodes presents an improved reproducibility and constancy as compared with monocrystalline electrodes containing impurity inclusions in the electrode surface. The former electrodes also present a higher potential stability. The time to reach a pseudo-stable potential level (drift

Evanescent field refractive index sensor utilizing a narrow fiber Bragg grating and a tunable DBR laser

We have demonstrated an evanescent field refractive index fiber sensor comprising a 42 mm Bragg grating in an etched fiber together with a tunable DBR laser. Characterization of different aqueous sucrose solutions resulted in a resolution of roughly 10 mM sucrose. The sensor in the presented form has a theoretical sensitivity of higher than 10-5 refractive index units (riu) in a refractive index region close to the cladding index of the fiber. However, the technique allows for an even higher sensitivity than 10-6 riu with a proper signal processing scheme.

Cyclic voltammetric studies on a monocrystalline antimony electrode

Abstract Monocrystalline antimony electrodes have been studied by cyclic voltammetry in solutions containing sodium nitrate and one of the following complexing agents: TRIS [Tris (hydroxymethyl) amino methane], orthophosphate, EDTA (ethylenedinitrilo tetraacetic acid) or tartrate at an ionic strength of 0.16 The results have been compared to results obtained in a solution containing only sodium nitrate, which has been shown previously to have no effect on the Sb-electrode potential in the pH range 2–10 which was studied. The results indicate formation of soluble complexes with the ligands studied. Competing reactions seem to take place in the solutions containing orthophosphate (and presumably EDTA). The reactions between Sb(III) and TRIS or orthophosphate seem to be reversible for all practical purposes. The complex formation between Sb(III) and TRIS seems to be weak since the behaviour of the electrode is very similar in TRIS and in pure nitrate solution. TRIS is therefore suitable as a buffer for calibrating antimony electrodes.

Polarization-selective phase modulator: a potential component for interferometric fiber optical sensors?

This paper discusses the potential use of polarisation selective phase modulators, PSPM, in interferometric fiber optical sensors. The principle of the PSPM is shortly reviewed. Two examples of the usefulness ofpolarisation selective phase modulation will be given. The first showing that PSPM introduces a possibility to monitor two measurands, simultaneously, with the same interferometer. In the second, a more detailed look at the consequences of using this concept in fiber optical gyros wiibe presented. A formula is derived describing the suppression ofnoise due to polarisation coupling in the gyro fiber loop. Using a special modulation and detection scheme, the PSPM shows to reduce the noise with the same order of magnitude as a polarizer.

Polarization selective phase modulator in LiNbO/sub 3/

An integrated optical phase modulator which selectively modulates only one mode of polarization is discussed. Selectivity is accomplished by controlling the ratio between the voltages applied to two different electrode sections. A method for determining the correct ratio and evaluating the polarization selectivity of the phase modulation Gamma has been developed. Experimentally, a Gamma of 46 dB for selective modulation in the TM direction and a corresponding value of 40 dB for the TE direction have been obtained. It is also shown that the concept is a possible alternative or complement to polarizers and polarization preserving components in optical sensors.