Abstract: This invention relates to an apparatus and a method for determining the concentration of a first reactant in a medium. A disc is saturated with a solution containing the first reactant and immersed into a second reactant fluid. A chemical reaction occurs between the first and second reactants to produce a gas which is trapped by the disc. The disc is buoyed to the surface of the second reactant fluid by the gas produced. The process is electronically timed using photocells to start and stop an electronic clock and a digital display displays the time elapsed in tenths of seconds.