A method and apparatus for on-line measurement and forecasting of a species concentration while a reaction is taking place, based on the measurement of the electrical conductivity of the reacting volume, and using such measurement to evaluate a first mathematical function relating such value to the concentration of the species in the reacting volume. The evaluation of the first mathematical function requires as inputs: electrical conductivity trajectory, time elapsed, and other significant process variables; it generates as outputs the estimated actual species concentration and at least one forecasted concentration value.