Abstract: A modular reaction vessel cartridge for a photometric reagent analyzer includes a hollow cuvette defining a filling chamber defining an upper orifice through which a first reagent is communicated, a dispensing opening below the upper orifice and through which a second reagent is communicated into the filling chamber, a read chamber below the dispensing opening and fluidically communicating with the filling chamber, and a dispensing control chamber above the read chamber having an exit fluidically communicating with the dispensing opening, defining a receiving opening through which the second reagent is communicated into the control chamber, and a plunger slidably and fluid-tightly disposed in the control chamber and defining therebetween a volume shaped to hold the second reagent such that, responsive to the plunger being moved into the filling chamber, fluid in the control chamber is dispensed into at least the filling chamber without fluid in the cuvette being transferred out from the cuvette.