A colorimetric analyzer includes a reaction chamber configured to receive a sample and at least one reagent. A measurement cell is operably coupled to the reaction chamber. The measurement cell has an illumination source and an illumination detector spaced from the illumination source such that illumination from the illumination source passes through the measurement cell to the illumination detector. A controller is coupled to the illumination source and the illumination detector. The controller is configured to determine an absorbance of a reducing reagent and store the determined reagent absorbance in the analyzer. The controller is configured to use the determined reagent absorbance to compensate at least one subsequent colorimetric sample measurement.