In situ determination of oxygen

Gel electrodes have no diaphragm, however during operation in seawater the gel behind the junction hole is partly consumed. In the resulting excavation, small air bubbles or particles are readily trapped when the probe is lowered through the sea surface which block the electrolyte junction and cause a malfunction of the electrode. 

The response times of common glass electrodes (usually 3 s) in in situ applications are generally sufficient with respect to the pH changes. However, in cases of rapid environmental temperature changes, the rather high heat capacities of standard pH electrodes with considerable electrode masses cause a temperature gradient between environmental seawater and inner electrode buffer. This results in undefined temperatures of the electrode surfaces and errors in the electrode potential. 

Precision measurements of the in situ pH below a depth of about 400 m require separate glass and reference electrodes with electrolyte junction and pressure compensation. A pH electrode withstanding IS 000 dbar has been described by Disteche (1959, 1962). Ihe first practicable deep-sea pH electrode was introduced by Ben-Yaakow and Kaplan (1968) and improved by Ben-Yaakow and Ruth (1974). The electrode can stand 6700 dbar hydrostatic pressure and has been tested to a depth of 5000m. The pressure is compensated for by replacing a part of the glass electrode housing by Tygon tube. The accuracy is 0.02 pH units. A correction for a shift of the asymmetry potential (2 mV for 6700 dbar) was included. 

Meanwhile a variety of high-pressure pH electrodes using different modes of pressure compensation have been reported, see, e.g., Dexter and Perkins-Rohrbach (1986) or Queck and Auras (1985). Some commercial CTD probes e.g., Aquatrace by Chelsea Instruments, 55 Central Avenue, West Molesey, Surrey KT8 2QZ, UK) are provided with pH sensors which can operate down to 4000 m and provide accuracies of 0.05 pH units. 

Nearly all sensors used for in situ registration of dissolved oxygen in seawater are of the amperometric membrane covered electrode type introduced by Clark et al. (1953). A de- 

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Sharp, Z.D. 1990 A laser-based microanalytical method for the in situ determination of oxygen isotope ratios of silicates and oxides. Geochimica et Cosmochimica Acta 54 1353-1357. 

Electrolytic loading is the only technique that allows an easy and versatile in situ determination of the hydrogen concentration in a thin film. With an oxygen-free electrolyte it can be used to measure the hydrogen concentration in the films quantitatively and to determine pressure-composition isotherms [91]. At the solid/electrolyte interface the charge transfer reaction is represented by... 

In situ determinations of pH and oxygen in seawater involve two different types of registration registration of temporal variations at fixed locations, e.g., monitoring applications, and registration of spatial variations, i.e., by vertically and/or horizontally moving probes. 

As the reader might have noticed, many conclusions in electrocatalysis are based on results obtained with electrochemical techniques. In situ characterization of nanoparticles with imaging and spectroscopic methods, which is performed in a number of laboratories, is invaluable for the understanding of PSEs. Identification of the types of adsorption sites on supported metal nanoparticles, as well as determination of the influence of particle size on the adsorption isotherms for oxygen, hydrogen, and anions, are required for further understanding of the fundamentals of electrocatalysis. 

From an analysis of their data, Materlik and co-workers were able to determine that for the ex situ case and in the absence of oxygen, the thallium atoms are located at twofold sites at a mean distance of 2.67 0.02 A. For the in situ case and again in the... 

Batley [28] examined the techniques available for the in situ electrodeposition of lead and cadmium in estuary water. These included anodic stripping voltammetry at a glass carbon thin film electrode and the hanging drop mercury electrode in the presence of oxygen and in situ electrodeposition on mercury coated graphite tubes. Batley [28] found that in situ deposition of lead and cadmium on a mercury coated tube was the more versatile technique. The mercury film, deposited in the laboratory, is stable on the dried tubes which are used later for field electrodeposition. The deposited metals were then determined by electrothermal atomic absorption spectrometry, Hasle and Abdullah [29] used differential pulse anodic stripping voltammetry in speciation studies on dissolved copper, lead, and cadmium in coastal sea water. 

The determination of in-situ fluxes of nitrate is comparatively limited. Although there are electrodes for the determination of microconcentration profiles in form of biosensors (Larsen et al. 1996), they are not yet snitable to be nsed on lander systems in the deep sea. The measurement of total nitrate flnxes by benthic chambers in continental slope sediments off California is shown in Figure 3.24 (Jahnke and Christiansen 1989). At this site, nitrate flnxes are directed into the sediment indicating strong denitrification snpported by very low oxygen concentrations in the overlying bottom water. 

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