An apparatus and related method for optical calibration of spectrophotometers is described. The apparatus is a calibration plate including one or more cuvettes filled with solutions of interest. The cuvettes are sealed to prevent evaporation. The cuvettes also possess a compressible component to allow for expansion of the solution and a bubble control apparatus to ensure that the compressible component does not intersect the beam path. A piece of neutral density glass is optionally included in the apparatus to track optical changes of the solutions over time.