Abstract: A method and apparatus for quantitatively monitoring a chemical component dissolved in a liquid medium utilize the creation, in a measurement cell, of an interface zone between the liquid medium and an analytical reagent or reference solution. The concentration or amount of the chemical component are determined by the measurement of a value of a measurement parameter in the interface zone, and the derivation from that value of the concentration or amount of the component in comparison with a baseline or reference value measured for the reagent or reference solution alone. Matrix effects and ghosting errors are avoided. The method and apparatus can be used advantageously for on-line optimization and control of industrial, chemical and biological processes, such as chemical reactions in progress, coupling reactions for the synthesis of azo compounds, and the monitoring of in vivo or in vitro biological systems as well as for conventional analytical purposes.