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This paper describes a method and instrumentation for electrical pH control in aqueous solutions. It is targeted to environmental and industrial on-line chemical and optic fibre sensors measurement, calibration and process control. The method is an alternative to the existing automatic pH controllers based on correction chemical addition 2The on-line measurement of dissolved gaseous species which produce acidic and basic waters is of great concern for environmental monitoring applications. Such measurements are maintained by using preconditioned water samples and ion-selective electrodes or absorption spectrometry. A gas-permeable membrane can be used to allow only the transport of the molecular form of the measured species where the solution pH is controlled to ensure the reversible transition of the species of interest from ionic to molecular form and to allow their permeation through the membrane.An electrochemical instrument for electrolysis-based feedback controlled pH regulation has been attempted. The choice of galvanostatic, potentiostatic and combined methods of modulation electrodes biasing with their speed, settling time and accuracy are discussed. Different feedback control approaches, involving electrochemical and optical sensors, are compared.

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A fiber optic system for water analysis with high transparency in the deep-UV region (lambda >/= 190 nm) is presented. The system consists of special UV-improved silica fibers and a liquid-core waveguide (LCW) as an optical cell. The apertures of both light guides, the silica fiber and the LCW, are matched. The optical losses of the device are investigated experimentally and compared with theory, especially with a standard free-space geometry. The performance of the system with respect to UV absorption spectroscopy is demonstrated for nitrate and chlorine pollution in pure water. For a 203-mm-long LCW the detection limits have been determined to be as low as 22 mug/L for nitrate and 26 mug/L for chlorine.