A chemical analyzer analyzes a sample for a target chemical. The sample and reagent are added to a measurement cell until a detectable change is observed. A reference compound, which does not take part in the reaction between the target chemical and reagent, is added to the reagent. By measuring the concentration of the reference compound, the amount of reagent used may be determined. In one embodiment, a mechanism introduces the sample, adds reagent, mixes, cleans, and flushes the measurement cell, which incorporates a longitudinal chamber having inlets for the sample and reagent and an outlet for measured sample/reagent mixtures. A detector located in or adjacent to the chamber between the inlets and outlet and a piston movable in the chamber carries seals such that movement of a piston sequentially opens the inlets and expels the fluid. The analyzer also includes a device that delivers reagent to a measurement cell.